Fluvial Systems

FLUVIAL SYSTEMS

Compiled by

Okan Tüysüz from different internet sources

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FLUVIAL LANDFORMS AND PROCESSES •

fluvius (L.): river

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the work of rivers, but also the erosion of soil and rock on hillslopes by running

water, particularly in semiarid environments (badlands)

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requires understanding of stream and hillslope hydrology and hydraulics

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hydrological cycle: orderly scheme to systematically examine and analyze the

movement of water through the landscape •

flowing water is the result of net precipitation = total precipitation (input) -

evapotranspiration (output or loss)

Erosional processes on slopes 1.

raindrop impact

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• 2.

force of raindrops on bare soil causing disaggregation of surface soil

rainsplash erosion •

displacement of wet soil by raindrops creating small craters in bare soil

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on slopes the splash is asymmetrical resulting in the progressive downslope displacement of wet soil during intense rain on bare slopes

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sheet wash (rain wash)

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entrainment of loose particles in overland flow

overland flow movement of water over slopes when precipitation intensity exceeds the soil infiltration capacity according to soil porosity and permeability, vegetation, slope gradient, antecedent moisture and seasonal factors (e.g. ice) •

shallow overland flow (sheet flow) on smooth slopes is laminar (layered), so particles can only be displaced but not suspended

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erosion is accentuated by raindrop impact, rainsplash erosion, surging of the overland flow as small vegetation or soil dams break, and by turbulence

4.

rill erosion

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overland flow deepens downslope, reaching a critical depth where laminar flow cannot be maintain and turbulence begins to develop

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turbulent eddies suspend soil particles, creating rills subsurface erosion

piping formation of natural pipes as interflow and baseflow erode macropores and fractures in fine sediments sapping collapse of the roof of a pipe to form a gully

Fluvial landforms on slopes •

rill •

shallow channels eroded by threads of turbulent flow developed in the sheet flow where turbulence and thus entrainment of soil is concentrated

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during storms rills erode headward on the steepest local gradient at cm/minute or faster

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on open slopes they tend to form parallel to one another, converging in hillside hollows to form dendritic patterns

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ephemeral, that is, can be destroyed and recreated during major storms

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terminate at the base of slopes and thus are not part of the regional drainage network

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gully •

first-order stream channels that develop on slopes at the upper reaches of watersheds

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carry ephemeral stream flow

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narrow and steep sided

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persist for years or decades, so more persistent than rills but still not "permanent" features

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agricultural definition: farm machinery can pass through rills but not gullies

RIVER ENERGY & FLOW •

streamflow accounts for 85-90% of total sediment transport to the ocean basins

(glaciers account for 7%) •

2-4% of the total potential energy of running water is converted to mechanical

energy for geomorphic work

A river’s ability to perform geomorphological work—i.e. by eroding its channel and transporting its sediment load, is determined by the amount of energy it possesses.

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The study of river energy and flow is therefore critical as far as the study of fluvial geomorphology is concerned.

1 The Generation and Dissipation of River Energy Fluvial landforms are produced by the different processes rivers undertake. In order to initiate these processes, energy must be generated, and the subsequent work performed will in turn result in the dissipation of the energy.

1.1 Energy Generation A still body of water at any point above sea level has a certain amount of stored energy as a result of its position. This is potential energy and it is available to do work in the river channel. The kinetic energy of a river is caused by its movement and is derived from the potential energy.

1.1.1 Potential Energy The amount of potential energy a river possesses depends on 1. The amount of water present, and 2. The vertical distance of water above the base level •

the greater the amount and the higher the vertical distance, the more energy the river possesses

The potential energy rivers possess actually originates from the sun which evaporates water from the sea enabling it to be deposited at a higher level as precipitation over the land.

1.1.2 Kinetic Energy Kinetic energy is that generated by the flow of the river, which is actually using up the supply of potential energy. The amount of this energy is determined by 1. The volume of the flowing water and 2. Its mean velocity •

or in other words its discharge—since discharge is derived by multiplying the volume and velocity of the flow, an increase in any one of these will mean an increase in the amount of kinetic energy

1.2 The Dissipation of River Energy 6

The energy a river possesses is used up when the river 1. erodes its channel, 2. transports its sediment load, 3. experiences frictional drag i. Along the channel bed and banks, and ii. Between adjacent threads of water flowing at different speeds—e.g. in turbulent flow (Figure 1) which consists vertical and horizontal eddies •

although laminar flow is relatively common in highly viscous fluids, such as lava flows, it rarely occurs in water moving in natural channels

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in stream channels water movement nearly always occurs as turbulent flow, that is, the velocity of flow fluctuates in all direction within the fluid

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water is constantly interchanged in eddies between adjacent zones of flow, and local changes in velocity occur which work against the mean velocity gradient and lead to a loss of energy

It is estimated that under “normal” conditions about 95% of a river’s energy is expended in order to overcome friction. This leaves just 5% to carry out the erosion and transportation of debris.

2 Discharge and River Energy Rivers display considerable variations in energy from place to place and from time to time. This is mainly the result of variation in the rivers’ discharge. The energy a river possesses is determined by its discharge—i.e.

where Q = discharge (usually expressed in m3/s), A = cross-sectional area of the river (in m2), and V = the mean velocity (m/s) 7

2.1 Volume of Water and River Energy The volume of water (related to cross-sectional area) is important since an increase in the amount of water will mean a higher discharge and a more efficient river. This explains why floods can cause so much destruction to human property. In the humid tropical and temperate regions •

the volume of water will increase downstream due to contribution from tributaries

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this will usually lead to a more efficient river downstream—i.e. a high energy river capable of eroding its channel and transporting its load

In the arid regions and in areas with very permeable channels •

the volume of water will actually decrease downstream because of high evaporation rate and high seepage

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this will mean a less efficient river downstream and the effect of a decrease in the river energy will be reflected in its long profile—i.e. a convex profile may be produced (refer to the lecture on “River Long Profile”)

2.2 Velocity and River Energy—the Manning’s Equation When looking at the efficiency of rivers, it is also important to look at their velocities because velocity is a function of discharge which in turn determines how much kinetic energy the river possesses. Factors affecting the velocity of rivers are spelled out by the Manning’s equation

where V = velocity, R = hydraulic radius, S = channel slope, and n = coefficient of roughness/ the Manning “n”

2.2.1 Channel Slope Since stream flow is caused by the force of gravity, a change in the channel’s gradient will affect the amount of energy the stream possesses

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if channel gradient is steep, the change from potential to kinetic energy is rapid and the velocity of the river is high

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conversely, on gentle gradients the velocity is low

2.2.2 Coefficient of Roughness Channel roughness is another factor affecting velocity. Some of Manning’s coefficient of bed roughness is given in table 1 below. Notice that the higher the value the rougher the bed (and therefore the lower the velocity).

Surface

Manning’s n

Very smooth e.g. glass

0.010

Concrete

0.013

Unlined earth drainage channels

0.017

Winding natural channels

0.025

Mountain streams with rocky beds

0.040 - 0.050

Alluvial channels with small ripples

0.014 – 0.024

Alluvial channels with large dunes

0.020 – 0.035

In a downstream direction •

the river channel tends to become smoother because it is more likely that the banks and beds of the river will be made up of clay/ silt/ sand instead of boulders and pebbles

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this reduces the n value leading to a higher V value—i.e. a sinuous course or irregular bed containing large protruding grains or boulder will result in the creation of turbulence which dissipates energy

2.2.3 The Hydraulic Radius The hydraulic radius is the ratio between the area of the cross-section of a river channel and the length of its wetted perimeter, i.e.

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where R = hydraulic radius, A = cross-sectional area, and WP = wetted perimeter (the total length of the bed and bank sides in contact with the water in the channel) Figure 2 shows two channels with the same cross-sectional area but with different shapes and hydraulic radii

Stream A •

has a larger hydraulic radius because of a smaller wetter perimeter—i.e. a smaller amount of water is in contact with the bed and banks of the channel due to a more balanced width depth ratio

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this creates less friction which in turn reduces energy loss and allows greater velocity

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stream A is therefore said to be the more efficient of the two rivers

Stream B •

has a smaller hydraulic radius because of a larger wetted perimeter—i.e. a larger amount of water is in contact with the bed and banks of the channel due to a very large width-depth ratio

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this results in greater friction, more energy loss and reduced velocity

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stream B is therefore less efficient than stream A

in fact, a semi-circle is the ideal shape for the cross profile of a river channel in that it imposes least restriction on stream velocity

2.2.4 Downstream Variation in Stream Velocity With reference to the Manning’s equation •

since S normally decreases (i.e. becomes gentler) in the downstream direction the V value should logically be lower with increasing distance from the source (i.e. a lower velocity)

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however, an increase in R and a decrease in n will compensate for the decrease in S

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the result is that average stream velocity of rivers tend to increase, or at least to remain constant, downstream from the source despite the usual decrease in gradient

The effect of velocity variation in river energy fluctuation is all the more important because a doubling of velocity is likely to result in a four times increase in energy. This is the reason why large rivers or rivers in flood are, in terms of their geomorphological activity, very much more powerful than small streams.

2.2.5 Urbanization and Its Effects on Stream Velocity To reduce the risk of flooding, urban drainage systems are often designed to get rid of storm run-off in as short a time as possible. As shown in Figure 3, these artificial drainage systems are usually straight, smooth and semi-circular in shape giving the drains a very high R and low n values (notice the lower Manning’s n for concrete as compared to winding natural channels in table 1).

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According to Manning’s equation, this will lead to a very high velocity of flow and therefore ensuring the urban areas rapid clearance of overland flow.

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FLUVIAL PROCESSES The processes which allow rivers’ slope, width, depth, bed sediment, load and roughness to adjust are erosion, transportation, and deposition—i.e. processes which respond to the increase or decrease in energy which the discharge provides.

1 River Erosion By the process of erosion a river can deepen, widen or even lengthen its channels and thus adjust its efficiency to suit the workload to be done.

1.1 Types of River Erosion Erosion is carried out in various forms such as 1. corrasion/ abrasion, 2. hydraulic action, 3. attrition, and 4. solution

1.1.1 Corrasion/ Abrasion Mechanical abrasion or corrasion is the most common type of river erosion •

it occurs when coarse and angular fragments of hard rocks are released, rolled and dragged along the channel floor, thus slowly wearing away exposed rock outcrops rather like sandpaper

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this process is responsible for much of the downcutting that creates and deepen channels

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it is likely to be most effective in upland channels in times of flood when the large angular fragments of bedload can be activated upon the exposed rock of the bed

In fast flowing rivers with strong eddy motions, one extreme form of abrasion, known as pothole drilling can occur •

eddy motions will, by localized erosion , create a shallow depression

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when fragments of load/pebbles are trapped in these hollows, turbulent eddies produced by currents will swirl them around to drill smooth depression, termed potholes, into the bed rock (Figure 1)

1.1.2 Hydraulic Action Hydraulic action is another form of river erosion •

in the middle and lower courses of rivers where bedrock is less likely to be exposed to abrasion o the river is more likely to attack its channels, and in particular its banks, carrying out lateral erosion o the sheer force of flowing water is sufficient to dislodge particles or fragments of unconsolidated material, which may lead to bank collapse, especially on the outsides of bends where the current collides with the banks (e.g. on the outside of a meander bend)

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where bed rock is exposed o the hydraulic power exerted by rapid river flow may also lead to the fragmentation of bedrock in the channel particularly where joints and bedding planes are present and have been opened up by localized corrasion or chemical attack An exaggerated form of the process, known as cavitation, occurs where turbulence is

extreme. When the bubbles in the water collapse the resultant shock waves will hit and slowly weaken the river banks.

1.1.3 Attrition Another form of erosion that takes place in river channels attacks the load itself rather than the channel •

attrition of the suspension and bedload, as fragments collide with each other in motion, causes particles not only to become rounded but also to decrease in size downstream

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this, of course, has its impact on the efficiency of the channel downstream and helps to reduce the need for steep gradients in the lower course

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1.1.4 Solution Solution occurs continuously and is independent of river discharge or velocity. It is related to the chemical composition of the water, e.g. the concentration of carbonic and humic acid. Limestone is especially susceptible to this form of erosion but many chemical compounds are also soluble, especially in their weathered state, and thus a wide range of rocks may be vulnerable.

1.2 River Erosion in Rock-Cut Channels and Alluvial Channels In considering processes of river erosion, it is helpful to distinguish between 1. erosion of bedrock, and 2. the removal of loosely compacted sediments which have been laid down temporarily

1.2.1 Erosion in Rock-Cut Channels Some rivers flow in rock-cut channels. As stated above, processes such as corrasion and pothole drilling are dominant in this type of channel.

1.2.2 Erosion in Alluvial Channels In alluvial channels, erosion involves the washing away of incoherent sediments, comprising the channel floor and banks, by the hydraulic force of the flowing water. In the case of river banks •

erosion concentrated at and below the water surface will cause undercutting and collapse of the upper face of the bank (Figure 2), particularly if this has been weakened by wetting during a previous high flow

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bank erosion also performs the important function of entraining within the river material which has been released by weathering of the valley slopes and transported down to the river by rainwash and mass movements

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1.3 The Components of River Erosion River erosion is commonly seen to have three main components 1. vertical downcutting, 2. lateral erosion, and 3. headward erosion

1.3.1 Vertical Downcutting Vertical downcutting is characteristic of fast flowing rivers with a large bed load of coarse, hard particles •

aided with the high velocity of flow, these coarse bed load are used to abrade and pot hole the channel floor, which is thus lowered relatively rapidly

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when eventually neighbouring potholes are joined together, a rock-walled gorge is formed The rate of downcutting may increase with 1. uplift or 2. fall in sea-level

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either of the above may lead to the formation of deep gorges or deep and narrow V shaped valleys especially if slopes are formed of hard rocks that resists weathering

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this means that the relative rate of downcutting will be faster than slope recession (Figure 3)

1.3.2 Lateral Erosion Lateral erosion occurs when a river swings to one side, thus causing bank erosion— i.e. when rivers meander. Where the channel impinges on the base of the valley-side slope, erosion of the solid rock may lead to the formation of meander cliff. It is widely believed that lateral erosion is most active either 1. where the river is transporting a large sediment load, or 2. when short-lived floods occur under desert conditions lateral erosion can also be especially pronounced in those river sections which are braided

1.3.3 Headward Erosion Headward erosion is active either 1. at the head of the river, or 2. at points where the river long profile is locally steep—i.e. at knickpoints

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for example, rivers in limestone terrain may emerge from underground as springs, which are eating back into the hill slope and extending the valley headwards

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In steepened valley sections, the rapidly flowing water causes accelerated erosion, with the result that the steepened section migrates upstream

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in its most spectacular form, headward erosion of this form is associated with waterfalls as shown in Figure 4, particularly where these are formed by a hard cap rock overlying weak strata

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vigorous vertical erosion is concentrated in the plunge pool at the base of the fall

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enlargement of the plunge pool can undermine the face of the waterfall so that periodic collapse of the hard rock will cause it to retreat upstream

2 River Transport River transportation refers to the downstream movement of sediment either along the channel bed (bedload or traction load) by the processes of dragging, rolling and saltation (jumping) of particles, or within the body of flowing water (suspended load and dissolved load).

2.1 Types of River Transport The sediment load a river carries can be transported in a number of ways depending on their sizes, i.e. 1. bedload by traction and saltation, 2. suspended load by suspension, and

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3. dissolved load by solution (Figure 5)

2.1.1 Transportation of Bed Load Large rock fragments tend to roll or slide along the stream bed •

if they are rounded in shape they may move comparatively easily, but

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if they are angular they may become wedged between other fragments

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such movements are referred to as traction

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traction is usually most important near the source of a stream where river channel and valleyside gradients are steep, and valleyside slopes are capable of delivering coarse debris to stream channels Smaller grains may be transported by saltation under the hydraulic force of the

moving water •

in this case, they are lifted bodily from the stream bed by turbulence and then they fall back again a short distance downstream Together, the particles moving close or along the bed of the river by traction and

saltation are referred to as the bed load.

2.1.2 Transportation of Suspended Load Some particles, such as silt and clay, are small enough to be held up by turbulence within the water, and form the suspended load. The greater the turbulence and velocity the larger the quantity and size of particles which can be picked up. The material held in suspension usually forms the greatest part of the total load and the amount increases towards the river’s mouth.

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2.1.3 Transportation of Dissolved/ Solution Load Water flowing within a river channel contains acids (e.g. carbonic acid from precipitation). If the bedrock is readily soluble, like limestone, it is constantly dissolved in the running water and removed in solution. These will form the dissolved load.

2.2 Sediment Entrainment The amount, type and size of load rivers transport varies. This is the result of the difference in amount of discharge possessed by the rivers, geology, climate, etc.

2.2.1 Erosion Velocity, Competence Velocity and Settling Velocity Velocity is a very important factor regulating the erosion, transport and deposition of particles 1. the critical tractive force or erosion velocity is the drag needed to initiate particle movement o as water flows over a particle, it is subjected to drag, a force affecting the upstream face and top of the particle o if the drag is sufficiently powerful, the inertia of the particle will be overcome, and it will begin to roll or slide 2. the competent velocity is the lowest velocity of flow at which particles of a particular size, resting loosely on the channel floor, are set in motion o as the size of the particle increases, the competence velocity also increases o the erosion velocity must therefore be increased if movement is to occur 3. the mean fall/settling velocity is the velocity at which particles of a given size become too heavy to be transported and so will fall out of suspension and be deposited

2.2.2 Velocity Variation and Transportation When river velocity increases, such as during floods •

both the total load and the maximum size of the particles being moved will increase because the competence velocity of the large sized particles are now reached

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particles of sand and silt (and boulders and gravels under extreme conditions) which at lower velocities are subjected to traction will begin to saltate or even become temporarily suspended—i.e. they become suspended load along with fine clay particles When river velocity decreases, such as during the later stages of a flood

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the total load and the maximum size of the particles being moved will decrease because the coarser and then the finer sediments will be deposited, as the competent velocity is no longer attained This shows that the amount of bed load and suspended load of rivers is not fixed. The

proportion of these load vary with fluctuating velocity—i.e. an increase in the river velocity will increase the proportion of suspended load and reduce that of the bedload, vice versa.

2.2.3 The Hjulstrom Curve Further complications in the process of sediment transport are revealed by the Hjulstrom Curve (Figure 6). The Hjulstrom Curve shows two important points 1. for particles of greater than 0.5 mm in diameter, competent velocity increases with grain size, and for particles less than 0.5 mm in diameter, competent velocity increases with decreasing grain size

o this means that sand particles can be picked up at relatively low velocities, whereas gravel and also fine silt and clay particles are more difficult to entrain o in the former instance, higher velocities are needed because of the weight of the particles and 21

o in the latter instance, high cohesiveness and electrical bonding means that higher velocities are needed to dislodge the smaller particles 2. the velocity required to maintain particles in suspension is less than the velocity needed to pick them up o for very fine clays, the velocity required to maintain them is virtually nil, meaning that material picked up by turbulent tributaries and lower order streams can be kept in suspension by a less turbulent, higher order main river o for coarser particles, the boundary between transportation and deposition is narrow, indicating that a relatively small drop in velocity is sufficient to cause sedimentation

2.2.4 River Capacity and River Competence A distinction should be made between river capacity and river competence 1.

the capacity of a stream refers to its ability to transport a particular volume of

sediment o it varies with the third power of the stream’s velocity o thus, if the stream’s velocity doubles, its capacity increases by 23 or 8 times 2.

more important than capacity is the competence of the river, which refers to its

ability to transport particles of sediment of various sizes and weights o it varies with the sixth power of the river’s velocity o this means that if its velocity doubles, the river can transport particles 26 or 64 times heavier than before o this is because fast-flowing streams have greater turbulence and are therefore better able to lift particles from the stream bed

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2.3 Changes in the Amount and Nature of Sediment Load Downstream The amount of sediment load increases downstream because of 1. contribution by tributaries, and 2. continued feeding in of weathered material from the valley sides (Figure 7) The size of the individual sediment particles tend to become more rounded and of finer calibre in a downstream direction due to 1. attrition, and that 2. gentler valleyside slopes are only able to deliver material of finer calibre to the river channel

3 River Deposition Deposition of sediment takes place when the river becomes incompetent either because 1. there is a sudden input which in effect overloads the river, or where 2. there is a loss of energy When the river no longer has the competence or capacity to carry all of its load, deposition will occur. So starting with the largest particles, material begins to get deposited. Deposition occurs where 1. a river broadens out and therefore has a larger wetted perimeter which, assuming the volume of water remains constant, results in increased friction and a reduction of velocity, 2. a river enters the sea or a lake and therefore velocity is lessened (due to sharp change in gradient),

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3. discharge is reduced following a period of low precipitation or when excessive percolation takes place (as is common in deserts), 4. the river is shallower on the outside of a meander (i.e. the convex bank), or 5. the load is suddenly increased, e.g. by debris from a landslide

3.1 Features Associated with Deposition Features associated with deposition, both within and outside the margins of the channel, are described below

3.1.1 Pools and Riffles The pools and riffle sequence is found both in the straight as well as meandering channels as shown in Figure 8 •

when a river has a bedload of mixed calibre, such as a mixture of sand and gravel, it

tends

to

have

an

undulating bed, consisting of alternating

pools

(deeper

areas floored by sand) and riffles (accumulations of gravel forming shallower areas) •

a riffle is therefore, a localized concentration of coarse sediment which has been deposited temporarily from the bed load.

3.1.2 Alluvial Fans As shown in Figure 9, in upland areas with steep-sided valleys •

tributary streams flow into the main valleys along very steep gradients and, at times, they can be heavily loaded with sediment derived from their valleyside slopes

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when they reach the main valley floor there is a sudden decrease in their velocity (and thus energy) because of the sudden drop of gradient

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deposition may then result in the production of an alluvial fan, which is a cone-shaped mass of alluvium with its apex at the point where the stream leaves the mountain

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segment of a low-angle cone with its apex at the mouth of a canyon

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convex in cross-section, slightly concave in long-profile

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as a stream leaves a canyon, drainage becomes distributary; these wider, shallower, lower-gradient streams have less transport capacity

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also water infiltrates the coarse bed materials, losing transport capacity

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debris flows may occur with increased sediment concentration

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adjacent alluvial fans coalesce to produce a peidmont plain at the base of mountain fronts

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Delta •

similar morphology to an alluvial fan but deposition results from sharp reduction in velocity as a stream enters standing water

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also tends to include finer sediments and turbidity currents Alluvial fans: require high relief and an adjacent low-lying area for collection of

sediment. They generally occur in areas of active tectonism where a high gradient stream crosses into an open valley and flow becomes less constrained. Commonly alluvial fans are associated with faults. A.) Morphologic elements of fans: Upper fan (proximal part): characterized by a small number (1-3) of deeply incised channels. Upper fan deposits are usually poorly sorted and generally contain no sedimentary structures other than imbrication. Debris flow deposits are common. Middle fan: numerous shallower channels with longitudinal bars separating channels. Midfan facies contain both streamflow and debris flow sediments. Sheetfloods are common and their deposits consist of gravel/sand couplets that display both planar and trough cross bedding. Imbrication in stream channels is common. Lower fan (distal part): braided channel patterns similar to middle fan but with smaller channels and less dense braiding. Distal fan deposits are largely sand and silt deposits of sheetflood origin, with thin conglomerate layers. Low angle cross stratification and trough cross stratification are common; distal fan deposits may be similar to braided fluvial deposits and commonly interfinger with basinal sediments (playa lake deposits, fluvial deposits, etc.) B.) Depositional processes and classification of alluvial fans: Blair and McPherson (1994, Journal of Sedimentary Research, section A, p. 451-489) review the historic classification of fans, describe sedimentation processes characteristic of fans, and present their own classification of alluvial fans into two broad types: debris flow dominated fans (type 1) and sheet flow dominated fans (type 2). 1) Type 1 fans (debris flow dominated): Debris flow dominated fans occur in areas with a ready source of mud (i.e., areas with exposed fine-grained sedimentary or volcanic sections) and show interbedding of fine and coarse material that reflects the extreme ranges in flood magnitudes on these fans.

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2) Type 2 fans (sheet flood dominated): Sheet flow dominated fans tend to form in areas of perennial flow and formerly were considered "wet fans" or "humid fans". These tend to be better sorted and display a more uniform grain size throughout than debris flow dominated fans. C.) Evolution of alluvial fans through time: Alluvial fan facies vary considerably between debris flow dominated and sheet flow dominated fans and depends also on the age (stage) of the fan: Typically the early stages of a fan (stages 1 and 2 of Blair and McPherson, 1994) are dominated by rock avalanche and rock-slide deposits resulting from very high depositional slope angles characteristic of "mature" talus or colluvial cones. Stage 2 fans are dominated by coarse gravelly debris flows (for type 1 fans) or sheetfloods or incised channel flows (for type 2 fans). Stage 3 fans contain cobbly, pebbly, and sandy debris flows (type 1 fans) or sheetflood deposits and incised channel flows (type 2 fans). D.) Stacking patterns associated with alluvial fans: Common upward thickening and coarsening in alluvial deposits is caused by progradation or outbuilding of fan. Below are some alluvial fan photos.

Photo of a couple of talus cones at Ubehebe Crater near Death Valley. These would be considered to be in the stage 1 of development, in terms of the classification scheme presented by Blair and McPherson (1994)

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An alluvial fan near Oshin Nuur in western Mongolia. Notice that the fan occurs at the mouth of a mountain canyon and most of the fan is currently inactive, as suggested by the dissected fan surface. The active part of the fan is at the far left.

On-the-ground view of the active part of the Oshin Nuur fan. Note the topographically-higher inactive fan surface in the background. The person is behind a train of weakly imbricated boulders. This boulder train may be the winnowed remnants of a debris flow levee. Flow was right to left.

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Debris-flow dominated fans near Mammoth Hot Springs in northern Yellowstone Park, Wyoming. The cliff in the background is made of Cretaceous shale that supplies debris flows on the fan surface with mud.

Aerial shot by Martin Miller of the Badwater fan in Death Valley. Note the road traversing around the fan. The white deposits surrounding the fan are evaporites on the basin floor the distal (lower) parts of the alluvial fan interfingering with these basinal deposits.

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Series of coalesced alluvial fans (termed a bajada) on the west flanks of the Panamint Range, eastern California. Photo by Martin Miller. Notice the distributary channels on these fans. Eolian deposits cover much of the basin floor and the distal parts of the alluvial fans making up the bajada interfinger with these deposits.

Alluvial fan in the mouth of a tributary of the Kızılırmak River, Kargı-Sinop road.

3.1.3 Point Bars and Flood Plain Formation by Lateral Accretion In a meander •

vigorous undercutting tends to occur along the concave banks and eroded material slumps into the river

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some of this sediment is transported to the opposite convex bank or the next convex bank on the same side of the river to form a point bar

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•

the result is that while the concave banks tend to retreat, the flatter convex banks gradually advance with the accumulation of more sediments

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the original valleyside slopes are then replaced by almost level point bars deposited by rivers which become the river’s flood plain as shown in Figure 10

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in this case, the flood plain is the result of lateral accretion

3.1.4 Flood Plain Formation by Vertical Accretion Flood plains may also form by vertical accretion when river overflows its banks during floods as shown in Figure 11a

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as the floodwater, highly charged with sediment, extends over the adjacent plain, silt in suspension settles out due to a decrease in velocity, leading to vertical accretion

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sometimes deposition, especially of the coarse particles, is greatest along the margins of the river channel, giving rise to natural levees shown in Figure 11b

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•

when the river floods it may cover the whole of the flood plain

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however, the water still flows fastest along the line of the river channel because here the hydraulic radius is greater than in the case of the shallow flood water that covers the flood plain

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hence, along the margins of the river channel there is a sudden decrease in the competence velocity and capacity

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this causes deposition to occur, beginning with sediment of coarser calibre because of their higher settling velocity

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the accretion of coarse sediment just beyond the banks will eventually lead to the formation of levees

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FLUVIAL SYSTEMS: Three ‘end-member’ fluvial styles are: •

Braided River systems

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Meandering River systems

•

Anastomosed River systems

A) Braided rivers: low sinuosity (channel length/valley length = small); characterized by many channels separated by bars or small islands , high but sporadic water discharge, little vegetation, and easily eroded banks. Best developed in distal parts of alluvial fans, on glacial outwash plains, and in mountainous reaches of river systems where slope angles are high and coarse-grained sediment is plentiful. The abundance of sediment characteristic of braided fluvial systems results in the capacity of stream usually being exceeded except during peak flow conditions. Braiding results as the stream leaves behind those sizes of particles that it is incapable of transporting for a given stage. Deposition of coarser bedload causes mid-channel bars to form. During non-flood stage, only finest sediment is typically moved by stream. Braided stream deposits are typically coarse grained, with poorly developed channels and abundant cross-bedded and current-rippled bar deposits. Fining upward sequences are common though are not as prominent or as well developed as in meandering systems. B) Meandering Streams are characterized by single, highly sinuous channel with cohesive banks. Meandering streams form on lower slope gradients than braided systems; they commonly form downstream of braided fluvial systems and upstream of delta systems. Morphologic elements of meandering fluvial systems are controlled by the development of helical flow as water moves towards inside of meander on bottom, carrying sediment across stream channel and up sloping bank of adjacent point bar Deposits include channel sediments (principally lag deposits); point bars (upwardfining; with large dunes on lower part, ripples on upper part); natural levees (thickest and coarsest near channel bank); fine-grained flood basin deposits; and sandy crevasse splay deposits interbedded with floodplain fines.

33

C) Anastomosed Stream systems are volumetrically minor by comparison with meandering and braided systems, but their distinctive style warrants a separate category. Anastomosed streams are characterized by low gradients and low stream power Lateral migration of channels is minimal, producing isolated channels bounded by fine-grained floodplain deposits, both of which aggrade vertically rather than accrete laterally. Channels evolve through crevassing rather than lateral migration. D) Three main types of fluvial bar deposits: 1) Longitudinal bars (mid-channel bars). Deposited during waning flow competency. Oriented with long axes parallel to stream. Internal structure: massive or crude horizontal bedding; transport under upper flow regime conditions. Composed mainly of gravel or sand and gravel. 2) Linguoid and transverse bars: oriented transverse to flow and particularly characteristic of sandy braided streams. Tend to have steep avalanche face and display tabular cross sets or large scale trough cross sets. Transverse bars tend to be more sandy than longitudinal bars. 3) Lateral bars: very large bars that develop in areas of relatively lower energy along sides of channel. River migration results in lateral bar progradation and creation of lateral accretion sets and upward-fining sequences. E) Stacking patterns associated with fluvial systems. Typically, fluvial systems result in upward-fining sequences. In the case of meandering river systems upward-fining results from lateral migration of channels and deposition of lower energy deposits (higher parts of the point bar) on top of higher energy deposits associated with the channel itself. In the case of braided river systems, upward-fining can result from progradation of any bar type (longitudinal, transverse, lateral) and associated decrease in energy associated with shallower flow. Anastomosed systems are generally regarded to aggrade vertically and may not contain the well developed upward fining typically associated with most fluvial deposits. Here are a few photos of river systems and their deposits:

34

Photo of the Wood River meandering river system in southern Alaska. Notice the abundant mud suspended in the river typical of mud-dominated fluvial systems that result in highly sinuous channels. The oxbow lake in the background was produced by meander cut-off and channel abandonment. Note also the flooded chute channels on the inside of the meanderbends.

An example of some mud-dominated stream deposits in western Mongolia. These deposits probably formed from a meandering river system and consist of three main channel sandstone complexes separated by fine-grained overbank deposits that locally contain coal. The person for scale is standing on one such coal bed.

35

A close up of an incised channel margin in an inferred fluvial deposit. Jurassic Badaowan Formation, Turpan-Hami basin, western China. Notice that the channel sandstone is incising into fine-grained muds and silts of presumed overbank origin.

A shot of some lateral accretion surfaces from the Book Cliffs in central Utah. The lateral accretion surfaces are the left-dipping beds in the middle of the photograph and reflect channel migration from right to left.

Rio Puerco in southeastern Utah during low flow. Most of the river system is exposed and consists of a series of braids (longitudinal and transverse bars) with current rippled tops

36

and a mud drape. This is a good example of a braided stream in which the sediment supply has greatly exceeded the capacity of the stream.

Braided river system showing a well developed longitudinal bar. Flow is towards the camera. Note the presence of several other longitudinal bars in the background and the presence of multiple channels, characteristic of braided systems.

Inferred braided river deposit in Upper Jurassic strata from western China. Notice the multiple stacks of sandstone holding up the cliff. Each of the sandstone units is pervasively cross-bedded and some contain bar foresets several meters in relief.

37

Anastomosed river system in southeastern Saskatchewan. Notice the multiple channels and interchannel wetlands.

38

CHANNEL PLAN FORMS Three types of channel patterns are generally recognised 1. straight, 2. meandering: •

sinuous single thread, the most stable and efficient channel geometry (least variable energy distribution) to conduct water and sediment over any surface (e.g. supraglacial streams

•

formed and maintained by erosion of banks and deposition on point bars

3. braided •

multiple thread, superimposed meandering channels as discharge and sediment load vary seasonally and diurnally, e.g. semiarid and proglacial streams

•

bars reforms during flood stage, deposition during falling stage that splits subsequent flow

•

different hydraulic geometry at different stages

39

Photo: Meandering river

Photo: Braided river Rivers are rarely straight for lengths of more than ten times their average width. If they do •

they are probably flowing down steep slopes or

40

•

they are strongly influenced by the presence of zones of weakness, such as joints or faults, in the underlying rocks

1 River Meanders All three types of channel plan forms may be seen along a single river but meandering channels are most common.

1.1 Sinuosity Ratio How sinuous (or meandering) a river is can be expressed by the sinuosity ratio •

this is the ratio between distance along the centre line of the valley, and the distance along the channel—i.e. it shows the extent to which the meander deviates from a straight line

•

Figure 2 shows that a 1:1 ratio indicates a straight course while a 1:4 ratio denotes a well developed meandering course

•

a river is said to be meandering when its sinuosity ration exceeds 1:1.5

1.2 Geometric Features of Meanders Figure 3 shows the various geometric features of a meandering stream.

41

Meanders are usually symmetrical, their form relatively consistent wherever they occur. Such consistency is supported by the mathematical relationships that have been shown to exist between their various dimensions. For example, the wavelength of a fully developed meander is dependent on three major factors 1. channel width •

the meander wavelength is usually 7-10 times as great as channel width, and

•

points of inflexion (cross-over points) are usually 5-7 channel width apart, measured along the channel

2. discharge •

there has been much debate in the past about the particular discharge to which meander wavelength is related

•

bankful (Figure 4) seems the most likely, with a suggested relationship between wavelength and the square root of bankful discharge

3. nature of the bed and banks •

Schumm’s investigations have revealed that the lower the percentage of clay-silt in the bed and banks the greater meander wavelength is likely to be

42

1.3 Theories of Meander Formation There is an extensive literature on the origin of meanders suggesting a range of possible mechanisms. Although the actual mechanisms by which meanders develop are understood in general terms, they still pose difficult problems in details. Two theories of meander formation will be discussed here

1.3.1 Early Theory of Meander Formation It was once thought that the initial sinuosity of the channel, from which meanders are developed, was due to chance irregularities which deflected the stream from one side of the channel to the other. When this happens •

alternate deflections would initiate, by way of bank erosions, a slightly winding channel

•

at the concave banks on the meander banks, where the thalweg is diverted against, there will be erosion and the formation of bluffs (as shown in Figure 5, thalweg is the line tracing the deepest water and which coincides with the line of maximum velocity)

•

however, after the water is being dragged cross the river bed to the convex banks, there will be a loss of energy due to frictional drag, leading to deposition of sediment load which will accumulate to form point bars

•

with continuous erosion at the outer banks and deposition at the inner banks, the meander amplitude (and hence sinuosity) will increase over time leading to the formation of a well developed meander (Figure 6)

43

•

when the river becomes too sinuous the cutting of the meander necks during high discharge will straighten the river channel resulting in the formation of ox-bow lakes

One of the weaknesses of this early theory of meander formation lies in the fact that the development of meanders is assumed to be the result of chance irregularities. Study of the geometry of meanders, as expressed by dimensions such as meander wave length, meander amplitude and radius of curvature has revealed that meanders are very regular forms (as illustrated above) and as such, are not likely to be the outcome of random causes.

1.3.2 Revised Theory of Meander Formation Langbein and Leopold have managed to provide further explanation for the development of meanders by suggesting that meanders represent the ultimate aim of a river to equalise its energy expenditure along the length of the river. In other words, rivers meander so that their energy is expended at a uniform rate along the channel to obtain a uniform energy grade line in a channel containing pool and riffle sequences. The links between pools and riffles and meanders seem unquestionable, not least because the spacing of meanders along the channel’s course appears to be equivalent to that of pools and riffles (i.e. 5 – 7 times channel widths). At those sections of the river where riffles occur •

there is a high expenditure of energy because the water is shallow (as shown in Figure 7 and at point B in Figure 8) and more heat energy is generated

44

•

to balance this loss of energy, these particular sections of the river will have to be straight so that there will be no deflection or helicoidal flow of water (which will mean even more energy loss)

•

this accounts for the existence of the crossover

points—i.e.

the

straight

portions of a meandering river At those sections of the river where pools occur •

the water is deep and there will be little loss of energy (as shown in Figure 7 and at point A in Figure 8)

•

to get rid of the excess energy, the river develops bends so that the water at these particular sections of the river will be deflected to form helicoidal flows

•

helicoidal flows will cause more energy loss at these particular locations when erosion occurs and when more heat friction is produced as water is dragged across the river bed and sides as shown in Figure 10

•

this in turn accounts for the existence of “bends” at areas where pools are found

It is because of the alternating arrangement of riffles and pools along the river channel that the river gets the alternating arrangement of straight channels and “bends”, which together make up a meandering channel. Therefore the formation of meanders can be attributed to the presence of the pool and riffle sequence and the river’s tendency to develop a uniform energy grade line along its length. 45

2 Braided Rivers A braided channel consists of a mass of diverging and intertwining threads of water separated by ever-changing islands of sediment (Figure 11).

2.1 Main Features of a Braided Channel The three main features of braided channels are 1. strong but localised bank erosion, which tends to widen the channel as a whole, 2. low elongated unvegetated bars of sand and gravel, and 3. vegetated islands which normally stand above water level

2.2 Formation of Braided Channels The formation of braided channels is associated with various conditions and processes.

2.2.1 Conditions for Braiding Three conditions are necessary for the formation of braided channels 1. unstable flow regime associated with flow that is markedly seasonal 2. the transportation of large bed loads, as in periglacial regions where large amounts of coarse debris are produced by active freeze-thaw weathering on slopes 3. channels with banks composed of incoherent sands and gravels, which are easily eroded during high discharge, thus adding further to the sediment load

46

Braiding is therefore common in 1. semi-arid regions where there is •

a great deal of rock waste, and

•

only a low rainfall and stream discharge

2. channels of meltwater streams flowing from glaciers where •

much rock waste is supplied as the glacier melts, and

•

the stream’s discharge varies in relation to the rate of melting of the glacier

2.2.2 The Braiding of Rivers When the above three conditions are satisfied, the formation of braided rivers will take place as illustrated below •

during periods of high discharge o large amounts of sediment are entrained as bed load o the banks of the channel may be undermined so that they collapse into the channel therefore causing the development of a wider channel

•

during periods of lower discharge o the sediment load will accumulate at certain points within the channel to give sand and gravel banks o in the formation of these mid-channel bars, the coarse bed load is the first to be deposited o this material forms the nucleus of bars which grow downstream as the flow velocity is reduced and finer sediment accumulates o with further decreases in discharge the water level progressively falls and the

bars

are

gradually

exposed (Figure 12)

o the production of a shallower channel by deposition within the channel, together with

47

channel widening by bank collapse, tends to lead to a lower hydraulic radius (due mainly to an increase in the wetted perimeter) o the width/depth ratios may even exceed 300 in certain cases and to compensate for this, a braided river channel is usually steeper than, for example, a meandering channel •

some of the sand and gravel accumulations will be washed away during subsequent floods, but others will grow and become colonised by vegetation o these will then become more stable o the plants will assist the trapping of more sediment, and the bars will eventually become islands that are rarely inundated o however, the islands can rarely survive for long, since they are composed of loose sediments and can suffer bank erosion by flowing water in the anabranches on either side

2.3 Characteristics of Braided and Meandering Channels: a Comparison Compared to meandering channels, braided rivers are characterised by 1. a higher stream power and flow velocity 2. a larger sediment load and sediment size 3. a higher width-depth ratio and channel gradient 4. a higher proportion of bed load (as compared to mix or suspended load) 5. a less stable channel with the constant formation and destruction of mid-channel bars

48

RIVER LONG PROFILE By plotting a line graph of a river’s height above base level against the distance from its sources, one obtains an impression of a river’s long or longitudinal profile.

1 Profile of Equilibrium W. M. Davis proposed that •

in the early stages of development, valley profiles are irregular, reflecting the influence of factors such as i. variations in the initial slope over which the river begins to flow ii.differences in rock type, and iii. the occurrence of structural features such as faults

•

however, such irregularities are smoothed away by river erosion, to give a smooth graded profile over a long period of time as shown in Figure 1

•

this is also referred to as the profile of equilibrium

49

1.1 The Graded River Long Profile Although some individual valleys are characterised by irregularities, such as channel steepenings, rapids or even waterfalls, it has been widely observed that many develop a smooth concave-up profile with a short, steep upper section and an elongated, gentle lower section as shown in Figure 2.

However, in arid regions and areas where the rock is particularly permeable, the equilibrium profile may be one that is convex in appearance. This is mainly the result of the decrease in discharge downstream due to evaporation and percolation. Some authorities have attempted to show that these long-profiles approximate to mathematical curves in which the change in gradient down valley is even and progressive.

1.2 The Graded River Associated with the concept of profile of equilibrium is that of the graded river. The condition of grade is attained when the energy of the river is used up in the movement of the water and the sediment load, so that none is available for erosion—thus, the graded profile represents a slope of transportation. In other words, when a river long profile is graded, the slope of the channel is adjusted so that there is no net gain (erosion) or loss (deposition) of sediment from the reach.

50

•

according to Small, such a balance between energy and work cannot exist at an instant of time, but is an average condition existing over a period of time o for example, a river channel may be scoured during a brief flood, but following the flood the scoured areas may again be infilled by channel deposits

•

over geologic time, slope and channel characteristics adjust to provide, with the available discharge, just the velocity required to transport the sediment supplied from the basin

•

analogous to a railway grade

•

no excess, erosion or deposition, i.e. only to maintain the channel morphology

•

a change in any of the controlling factors will cause a displacement of the system in a direction that will tend to absorb the effect of the change (dynamic equilibrium)

•

independent factors o discharge, sediment from the basin and base level (potential energy) o determined by climate and geology, i.e. external to the fluvial system o a change in any one of these controlling factors results in an adjustment of the stream channel by degradation or aggradation towards a new longitudinal profile, also manifest in plan view by a change in meander pattern and locally in terms of cross-sectional (hydraulic) geometry

•

semi-dependent factors o channel width, depth, roughness, velocity, pattern and load grain size o depend on the controlling factors but also some self-regulation (dependence on each other)

•

dependent factors o slope of the water surface o the final adjustment, responds to the semi-dependent variable, cannot change abruptly like the other factors

•

rate of adjustment

51

o depends on resistance of the bed materials and amount of energy, i.e. mass (Q) and relief (base level) o thus fastest adjustments with large Q and adjustable materials o grade can exist locally in alluvial channels bounded by barriers such as resistant rock (waterfalls) or a landslide (e.g. Battle Creek valley) o grade extends to the entire profile as the barriers are eliminated o

1.3 River Self Regulation Rivers are open systems—systems that receive inputs of matter and energy from its environment and produces outputs that return to the environment. They •

are sustained by inputs of o water (from precipitation, slope run-off and springs), and o sediments (from slope weathering and channel erosion),

•

experience outputs of o water (into lakes or seas) and o sediments (by channel deposition and the formation of deltas)

Such systems have the capability to achieve a steady state/dynamic equilibrium where the inputs and outputs are balanced. There is therefore no tendency for progressive longterm change to occur. However, if any of these is disturbed (e.g. when sediment input increases or discharge decreases), the state of equilibrium will be upset and the river will have to try to reestablish equilibrium by self regulation. For example 1. when there is an increased sediment input (such as from the valley slopes after mass movement) •

the river will try to reestablish equilibrium by changing its channel slope

•

for instance, since a greater energy is needed to transport this increased sediment load, it will deposit part of the load (Figure 3)

52

•

this will result in an increase in the river slope and velocity downstream from the site of the deposition, which will in turn lead to an increase in the river energy for the transport of the extra load

2. when there is an increase in the water input (e.g. due to climatic changes) •

the river will erode its banks so that the development of a very wide and shallow channel will increase the wetted perimeter

•

this will in turn cause a decrease in the hydraulic radius and velocity of the river leading to the formation of an inefficient river

•

in this way, the increase in stream energy from the increased discharge will be offset

It is through these self regulation mechanisms that rivers are able to achieve a state of equilibrium and this is reflected in the way the rivers’ long profile adjust to changes in discharge, sediment and channel characteristics in the downstream direction. One piece of evidence supporting the channel slope adjustment hypothesis is that •

former valley profiles, as indicated by the longitudinal gradients of river terraces, have virtually the same angle as the present channel (this cannot be coincidental)

•

after the uplift which led to the incision of the channel and the abandonment of the terrace, the river was able to restore its former gradient, which is that required to handle the discharge and sediment load of the river

53

2 Reasons for a Concave Up River Long Profile Modern research has shown that even irregular long-profiles can be graded—these comprise individual reaches, in each of which the steady state exists. Even so, the concave-up form is very common and its formation can be explained as a response to the following controlling factors 1. the normal increase in mean velocity downstream enables the sediment load to be carried over a progressively gentler slope (explain using the Manning’s Equation) 2. the reduced median grain size of the sediment load in a downstream direction has a similar effect (explain using the Hjulstrom Curve) 3. the increased efficiency of the river channel (larger hydraulic radius, and reduced influence of roughness) also enables the river to transport sediment more easily in a downstream direction The shape of a river’s long profile therefore basically depends upon the relationship between the river’s discharge and velocity (i.e. the ability to perform the work of transportation) and the sediment load that is being transported •

a tendency seems to exist for each part of a river’s course to adjust itself so that it is just able to transport a certain amount of sediment of a certain caliber

•

when the total energy of the river is just sufficient to transport its water content and its load of sediment, the river is said to be in equilibrium

3 Factors Affecting River Long Profile Other than the changes in discharge, sediment size and channel characteristics downstream, factors such as geological structure and changes in sea-level can also influence rivers’ long profile.

3.1 Faulting The geological structure of the area a river flows through will influence the river’s long profile. Faulting can cause the development of irregular river long profiles due to the formation of waterfalls. Subsequent river regulation to such irregularities is discussed below.

54

3.2 Rock Type and the Development of Waterfalls The existence of rocks with different resistance and characteristics will affect the development of long profile in two ways. Different rock resistance will induce different rates of river downcutting and the development of waterfalls. This will produce an irregular river long profile as shown in Figure 4. By self regulation, the river will work towards the development of a graded profile by the following processes •

in the steep reaches the river would tend to have a surplus of energy over and above that needed to transport its water and sediment content

•

it would therefore erode its channel and thus decrease the channel’s slope

•

in the reaches with gentler gradients, the river’s energy would tend to be insufficient to transport its load of sediment, so deposition would occur, so as to steepen the channel slope

55

•

erosion at the upper reaches and deposition at the lower reaches will eventually reduce the differences in the river’s gradient as shown in Figure 5

3.3 Rock Type and Its Influence on Sediment Calibre Different rock types tend to release sediment of a certain calibre, depending on its mineral composition and the spacing of joints and bedding planes. Where the sediment is coarse, the channel gradient will need to be steep for transport to occur and vice versa. This is one reason why graded profiles, developed across a series of different rock types, do not necessarily show a progressive decrease in slope down valley. As is illustrated in Figure 6 •

a tributary stream from a nearby area, for example, may be carrying a load of very coarse caliber down a steep gradient (e.g. because the weathered end product of the rock in the area is coarse)

•

this is likely to cause the trunk stream to steepen its profile below the confluence

•

further downstream the coarse calibre load may be comminuted (reduced to smaller fragments), so that the trunk stream’s gradient gradually flattens

56

Shales or clays tend to produce a sediment load of very fine calibre, and limestones produce very little visible load. Hence, one might expect a river to have a low gradient when passing over such rocks. Sandstones, on the other hand, produce coarser, more resistant sediments, so one would expect the long profile to be steeper. One would not expect to see very sudden changes in gradient, however, since a bed load of sandstone fragments can be carried a considerable distance downstream before the fragments are comminuted by attrition.

3.4 Changes in Base Level Irregularities in rivers’ long profiles may also occur as a result of a change in their base level, which is the level below which it is impossible for them to lower their channels by erosion. The base level for streams that flow into the sea is sea level. For streams flowing into enclosed lakes that have no outlet the lake surface is the base level. Most rivers flow eventually into the sea, and sea level (their base level) may change from time to time. These changes may be 1. positive—a relative rise of sea level, or 2. negative—a relative fall in sea level Such changes of sea level may be of either eustatic or isostatic origin 1. an eustatic change of base level occurs when the level of the sea changes 2. an isostatic change of base level occurs as a result of a change in the level of the land

3.4.1 Effects of a Positive Change in Base Level A rise in sea level will •

flood the lower parts of river courses, turning them into estuaries

•

this will reduce the gradient of the lower reaches of the river

•

the resulting decrease in velocity will lead to extensive alluviation of lower valley courses when rivers gradually fill these estuaries with sediment

57

•

this will produce alluvial flats resembling river flood plains (but their origin is quite different) and the process will continue till a new long profile is achieved

3.4.2 Effects of a Negative Change in Base Level A fall in sea level can have a much greater effect upon relief •

in this case, the extension of the river’s course over the former sea bed may have a steeper slope than the lower course of the river

•

a fall in sea level and the exposure of the sea bed will thus increase the gradient and the velocity of the river

•

the excess energy generated will in turn result in the erosion of the oversteepened reaches, known as knickpoints (Figure 7)

•

this will induce headward erosion which will migrate upstream

•

the result is the successive steepening of each section of the river and the eventual lowering of the whole profile till a new equilibrium profile is reached

•

this influence also extends along the tributaries

•

a succession of sea level falls will produce a series of knickpoints, separated by gentler graded reaches related to former high sea levels

58

59

DRAINAGE DEVELOPMENT Geomorphologists have devoted much time and effort to the study of drainage systems and their evolution because 1. a drainage system is a major feature of the physical landscape and the form of the system, especially the orientation and spacing of its component streams, does much to determine the essential character of the landscape, and 2. evolutionary studies of drainage systems may afford valuable information about the denudational history of an area

1 Descriptive Studies of Drainage Systems—Drainage Patterns Stream networks can be classified according to the nature of the geometrical patterns which they form on the surface of the ground. These patterns are related to 1. the mode of origin of the stream system, and 2. the topographical and geological characteristics of the land surface upon which the network has evolved •

all streams in adjustable materials will from a dendritic (tree-like, branching) network

•

all other channel networks (radial, trellis, rectangular, distributary, annular) result from structural control

There is an almost infinite variety in the patterns formed by drainage systems but it is useful to make a classification of some of the more obvious ones. •

deterministic explanation o represents the movement of water with the least expenditure of energy; least path length o governed by conservation of energy in an open system

60

o acute junctions involve least rate of work (power) expended and therefore neither erosion (excess energy) of deposition (insufficient energy to transport the load) •

probabilistic explanation

•

branching networks are high probable random structures o they can be produced from a random walk model (using random number, dice or a bingo machine) o this, however, is only a explanation of the form and not the origin (e.g. headward erosion or progressive intersection of channels)

1.1 Subparallel Patterns These are perhaps the simplest patterns of all and comprise a series of streams which run approximately parallel to each other (Figure 1) •

subparallel patterns are especially characteristic of areas of uniform dipping rocks such as

1. the dip-slopes of cuestas or 2. areas which have

recently

been

exposed by regression of the sea •

in both cases, geological conditions and/or the time factor have not yet permitted the development of a more complex pattern by the process of adjustment to structure

•

a subparallel pattern is therefore essentially an “initial” drainage pattern

1.2 Dendritic Pattern These are very common patterns, and are mostly associated with areas of uniform lithology, horizontal or very gently dipping strata, and low relief (as on an extensive clay plain)

61

•

derived from the Greek dendron,

a

pattern

tree,

comprises

this a

multitude of small branch streams which join each other, usually at fairly acute angles, to nourish a large

trunk

stream

(Figure 2) •

the ramifying tributaries in the dendritic pattern whose position and direction are entirely fortuitous are often described as insequent streams

•

the actual “closeness” of the pattern of the pattern will vary a great deal, depending on

1. the permeability of the underlying rock, 2. the amount and nature of the precipitation, and 3. the time factor (i.e. how “old” the rivers are-headward erosion add tributaries to stream so that older rivers are less “open”)

1.3 Trellised Patterns These (refer to Figure 3) are again common, particularly in areas 1. of well-developed cuestas where the main dip-slope streams run broadly at right angles across alternately resistant and unresistant rock outcrops 2. affected by subparallel Jura-type folding

1.4 Rectangular Patterns Rectangular pattern is shown in Figure 4 •

these

show

resemblance

to

some trellised

patterns

62

•

however, whereas in the latter the main and tributary streams join approximately at right angles, in a rectangular pattern the individual streams may themselves show marked angularities of course

•

these are invariably the result of geological controls (i.e. well-defined lines of weakness such as faults or joints) along which the streams have extended their course by headward erosion

1.5 Radial Patterns These comprise streams diverging from a present or former high point (refer to Figure 5), and are most usually associated with dome structures such as batholiths, volcanic cones and domes created by folding •

the initial pattern is a simple one, but the gradual exposure of less resistant rocks within the core of the dome or cone will lead to the modification of the radial streams

•

in the case of a dome of sedimentary

rocks,

prolonged denudation may reveal a series of concentric rock outcrops, the weaker of which will favour growth of tributaries •

thus a “ring-like” element will be added to the initial pattern giving rise to annular drainage (Figure 6)

1.6 Centripetal Patterns These are formed by a series of streams converging on a central lowland from surrounding highlands. They are characteristic of many desert areas,

63

where downfaulted blocks give rise to basins of internal drainage.

1.7 Deranged Patterns These are essentially “initial” patterns and occur where there has been insufficient time for drainage integration.

2 Drainage Evolution The initiation and subsequent evolution of any drainage system are determined by 1. the nature of the surface (mainly relief) on which the streams begin to flow •

these will naturally follow the lines of steepest gradient

•

in other words, their courses will be consequent upon the form of the land surface, and such streams are accordingly referred to as consequents

2. the geological structure of the area •

the geological structure will affect the later development of the rivers

•

the most significant feature in this context will be the appearance and growth of subsequent streams, which by the process of headward erosion will extend along lines of geological weakness such as clay and sand outcrops, fault-lines, major joints and anticlinal axes

•

the closeness of the relationship between stream courses and lines of geological weakness may afford some measure of antiquity of a drainage system—i.e. if there is little adjustment of the stream pattern to structure the drainage may be extremely “youthful” and vice versa

2.1 Consequent Streams It has been stated that the only factor determining the course of a consequent stream is the slope of the land surface on which that stream develops. Clearly the initial consequent pattern will vary greatly in its complexity, depending on the degree of irregularity of the “initial surface”.

2.1.1 Uplift and the Development of Consequent Streams

64

If the “initial surface” is produced by the uplift and gentle tilting of say, a plain of marine erosion, the result will probably be a series of nearly parallel streams as shown in Figure 7. •

in this case, the geological structure planed off by the sea seemed to have included rock strata with different resistance, yet these would exert little or no influence on the course of the consequent streams

•

in this instant, the drainage pattern would be discordant to structure

2.1.2 Folding and the Development of Consequent Streams If the initial land surface is produced by more complicated earth movements, leading to the formation of a folded geological structure, the drainage pattern will tend to develop along different lines (refer to Figure 8)

65

•

the largest streams will flow along the synclinal axes to form the “primary” or longitudinal consequents

•

smaller tributary streams, known as “secondary” or transverse/lateral consequents will drain the flanks of the anticlines and join the primary consequents to give a “fish-bone” pattern

2.2 Subsequent Streams The courses of subsequent streams are more often related to geological structure. They develop as tributaries of the original consequent streams and tend to excavate valleys in the outcrops of weaker rocks or along zones of weakness caused by faults or the joint pattern.

2.2.1 Folding and the Development of Subsequent Streams Folding may create a number of parallel anticlines and synclines occupied by consequents (longitudinal and lateral) as shown in Figure 8 above. The influence of the anticlines and synclines on drainage evolution however does not stop here •

lateral consequents may deepen their channel (as shown in Figure 9a) and erode headwards till the crest of the anticline is reached

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•

as shown in Figure 9b, this in turn encourages headward erosion by the streams and tributaries near and along the crest of the anticline where folding has structurally weakened the rock (refer to Figure 10)

•

this will lead to the formation of combes, or stream eroded hollows, in the crests of the ridges

•

since the anticlinal streams will be more active than the synclinal rivers, the anticlinal vales will be rapidly enlarged and the ridges reduced

•

eventually, longitudinal subsequents will develop along the anticlinal axes as shown in Figure 9c

•

meanwhile, vertical erosion by the consequent stream flowing along the synclinal valley has been hindered by the presence of the resistant rock layer at the synclines (Figure 10)

•

as a result, longitudinal subsequents that erode into softer underlying beds may be able to lower their valley to below the level of the neighbouring longitudinal consequents (Figure 9d)

•

erosion by the subsequent streams will therefore result in the formation of “inverted relief” with anticlinal valleys and synclinal ridges (i.e. inverted because the

topography

will

be

the

inverse

of

the

geological

structure)

2.2.2 Uplift and the Development of Subsequent Streams In Figure 11a, a consequent stream flows transverse to the outcrops of alternate layers of weak and resistant rocks on a former sea bed that has been exposed due to uplift and tilting. Figure 11b shows the possible development of the drainage pattern •

subsequent streams, at right-angles to the original consequent, have excavated parallel strike vales in the weak rock layers

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•

these vales are asymmetrical in cross profile, having steep scarp slopes where the ground surface cuts across the resistant layer

•

the subsequent streams have two sets of tributaries o resequent streams flow in the same direction as the dip of the strata and o the obsequent streams flow from the scarp slopes, in the opposite direction to the dip of the strata

•

uplifting and tilting has in this case resulted in the development of a trellis drainage pattern

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•

these resequents and obsequents should not be confused with the lateral consequents shown in Figure 8 -resequents and obsequents cannot exist until there are subsequent streams

•

as was illustrated above, subsequents are structurally guided streams which are continually being added to the initial consequents

•

with the development of subsequents over time, the drainage system will therefore be characterized by an ever increasing degree of adjustment to structure when these streams take advantage of structural weaknesses (e.g. joints and weak rocks) to erode headward and become entrenched

2.3 River Capture/Piracy •

When drainage pattern becomes more closely adjusted to structure with the

development of subsequent streams, the consequents may loose their dominance and may even be captured by newly developing subsequents. Drainage progressively or abruptly diverted from one basin to another as a stream is beheaded by headward erosion

2.3.1 River Capture in Alternating Layers of Weak and Hard Rocks The process of river capture is shown in Figure 12 •

Figure 12b shows a uplifted sea bed with alternate layers of resistant chalk and weak clay

•

two consequent streams flow across the area excavating valleys in both the clay and the chalk

•

consequent B is the more vigorous and proceeds to develop a tributary along the outcrop of clay (Figure 12c)

•

this subsequent stream extends its source headwards until it reaches consequent A, which it “captures”, so that the upper part of A’s course is diverted and becomes a tributary of consequent B

•

a right-angled bend (an elbow of capture) is created (Figure 12a)

•

the valley along the clay outcrop is deepened

•

the former lower course of consequent A is now a dry gap (wind gap) at the crest of the chalk scarp and a small misfit or underfit stream (misfit because this stream

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is clearly too small to have been responsible for the creation of the valley that it now occupies) •

the misfit stream is sometimes referred to as a beheaded or victim stream since its headwaters have been diverted to consequent B (the pirate stream) by the river capture that has taken place

2.3.2 Pre-requisite for the Occurrence of River Capture For river capture to take place, the pirate stream must be incised to a level substantially lower than its victim. All rivers possess an appreciable gradient downstream towards the sea, which acts as a common base-level for fluvial erosion

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•

in streams flowing over similar rocks, and having similar catchment areas, this gradient may not vary greatly from one stream to another

•

thus the height of consequent B in the above instant should not be greatly different from consequent A

•

moreover, if river capture is to occur, the subsequent flowing from consequent B to A must also have a downstream gradient

•

in these circumstances, capture and diversion of consequent A by consequent B is an impossibility

•

therefore for capture to take place, one stream must obtain a very great erosional advantage deriving from particular relief or geological conditions that are by no means encountered in all areas

3 The Development of Discordant Drainage Patterns Both antecedent and superimposed drainage are discordant to structure. The basic difference between the two is that in the former the rivers are actually older than the structures they cross (this is why this type of rivers are called antecedent rivers), whereas in the later the rivers are by definition a good deal younger than the underlying folds and faults.

3.1 Antecedent Drainage After the initiation of a consequent stream pattern, earth movement may lead to a substantial alteration of the original geological structure nourishing that drainage •

if rapid and violent, these movements might succeed in entirely disrupting the existing river system, and give rise to a wholly new consequent pattern, intimately related to the form and orientation of the new structures

•

if the movements are more protracted and gentle, the original drainage may be able to

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maintain its form by incision into the new structures as they develop •

such a phenomenon is known as “antecedent drainage” (Figure 13)

3.1.1 A Case Study of the Development of Antecedent Drainage Antecedent drainage occurs when an established river, existing before orogenic uplift, keeps pace by erosion with the elevating upland and this cuts a gorge through the mountain chains •

antecedent drainage is exemplified in the Himalayas where the Indus and the Brahmaputra Rivers rise in the less mountainous areas to the north and both flow south through gorges in the main Himalayan chains

•

as an example of the depth of these gorges, the floor of the Indus in Kashmir is only 1000 meters above sea level

•

thus the river eroding vertically has kept pace with about 5000 meters of uplift during the Tertiary orogency when the formation of the Himalayans began

3.2 Superimposed Drainage As consequent streams initiated on a certain geological formation or structure vertically corrade their courses, they may in time erode through an unconformity and encounter an older and substantially different structure. However, the streams cannot adapt their courses immediately to conform with the new geological conditions. Adjustment to structure will certainly proceed in due course, but in the meantime the old consequent directions will persist and, even after the rocks of the overlying structure have been entirely removed, will often be easily recognisable because of their discordant relationship with the newly exposed structure. This phenomenon is known as “superimposed drainage”.

3.2.1 A Case Study of the Development of Superimposed Drainage In late Cenozoic times, uplifting of sedimentary rocks in the Lake District had resulted in the formation of a dome and an associated radial pattern of drainage (Figure 14) •

as the cover of sedimentaries is removed over 30 million years during the Tertiary era, the much older rocks beneath were exposed (Figure 14b)

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•

with insufficient time for adjustment, the radial pattern is superimposed into these rocks (rocks which the radial pattern is quite compatible as illustrated in Figure 14c)

•

thus, today, the three parallel bands of Borrowdale Volcanics, Skiddaw Slates and Silurian rocks which compose the Lake District provide a geological structure discordant with the pattern of rivers and lakes which drain them

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DRAINAGE BASIN ANALYSIS Most earlier discussions of drainage patterns considered their development in relation to the structure and the lithology of the underlying rocks. During the last twenty years, the subjective approach to the description of drainage patterns has been largely supplanted by more objective—and far more useful—techniques of study. These form the basis of what is known as drainage morphometry, which is an important branch of quantitative geomorphology. This quantitative analysis of drainage networks has been developed 1. to enable comparisons to be made between different drainage basins 2. to enable relationships between different aspects of the drainage pattern of the same basin to be formulated as general laws, and 3. to define certain useful properties of drainage basins in numerical terms

1 Stream Order Analysis The first step in the morphometric analysis of drainage basins is to apply the technique known as “order designation”.

1.1 Strahler’s Method of Stream Analysis Strahler’s system of stream analysis is probably the simplest and most used system. His stream ordering method is given below •

on a detailed topographical map, the very smallest headwater tributaries of the basin are identified and designated as first-order streams

•

where two streams join, a second-order segment is formed

•

where two second-order streams unite, a third order segment results and so on (Figure 1)

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If Strahler’s method of stream ordering is used, it follows that the trunk stream of the basin, through which all the discharge of the basin finds its outlet, is the stream segment of the highest order. In fact, the drainage basin itself is designated after the highest order stream segment that it contains—thus a basin containing a fourthorder stream, plus numerous third-, second- and first-order segments, is referred to as a fourthorder drainage basin.

1.1.1 Strength and Weakness of the Method The strength of the method lies basically in its simplicity and ease of application which have commended its widespread use in the 1960s. One of the weaknesses of this method is that •

the stream order number does not reflect its relationship with channel size and capacity

•

for example, a fourth-order stream will have a capacity far in excess of four times that of a first-order stream

•

this is a limitation because one purpose of stream ordering is to provide an index of scale and also to indicate the discharge which can be produced by a particular network

1.2 Shreve’s Method of Stream Analysis R. L. Shreve tried to remedy this with another method of stream ordering. This consist of •

adding the rank numbers of the two streams contributing to a junction to arrive at the rank

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number of the stream below the junction (Figure 2) •

thus, at any point on a drainage network the order of a stream is given by a number which represents the total number of the first-order streams which have contributed to it

1.2.1 Strength and Weakness of the Method One advantage of using Shreve’s method over Strahler’s is that if we assume that firstorder streams are of approximately the same magnitude and that discharge is neither lost nor gained from any source other than the tributaries (which is not true), than the Shreve number is roughly proportional to the discharge in the segment of a stream to which it refers. The disadvantage of his method is that a large number of stream orders will be defined and there will be gaps in the sequence.

1.3 Law of Stream Number From the study of streams ordered by the Strahler system certain general tendencies or “laws” may be derived. For example 1. the law of stream number states that within a drainage basin, a constant geometric relationship exists between stream order and stream number •

if the logarithm of the number of streams in each order is plotted against stream order the points lie approximately on a straight line

•

the property is known as the law of stream numbers, which states that the number of stream segments of each order form an inverse geometric sequence with order number as shown in Figure 3

2. if the logarithms of the mean lengths of the stream segments of different orders are plotted against stream order, the result is usually an approximately straight line—this is the law of stream lengths •

Figure 4 shows that the higher the stream order the longer the mean stream length (a positive relationship)

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3. the law of basin areas follows the same general pattern

•

Figure 5 shows that the higher the stream order the greater the mean drainage basin area (a positive correlation)

1.4 The Bifurcation Ratio This is a relationship between the number of streams of one order and those of the next higher order. It is obtained by dividing the number of streams in one order by the number in the next highest order.

1.4.1 Calculation of the Bifurcation Ratio-an Example The calculation of the bifurcation ratio can be done for streams ordered according to Strahler’s method •

if there are o 26 first-order streams, o 6 second-order streams, o 2 third-order streams, and o 1 fourth-order stream

•

the bifurcation ratio for the first- and second-order streams will be 26/6 = 4.33

•

the bifurcation ratio for the second- and third-order streams will be 6/2 = 3.00

•

the bifurcation ratio for the third- and fourth-order streams will be 2/1 = 2.00

•

the bifurcation ratio for the whole basin will therefore be

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4.33+3.00+2.00 3 = 3.11 The bifurcation ration in drainage basins not greatly distorted by geological factors usually varies between values of 3.0 and 5.0.

2 Drainage Density This is a useful measure of the frequency and spacing of streams within the drainage basin because 1. it reflects the extent to which the landscape is cut into by valleys 2. it also reflect to some degree the amount of run-off that a basin generates, since the total channel capacity needs to be sufficient to cope with normal discharges, otherwise flooding will be frequent and widespread Drainage density, usually abbreviated to Dd and expressed in km channel length per km2 of basin area is calculated by dividing the total stream length within a basin by the total basin area—i.e.

The calculation of the drainage densities allow comparisons to be made between area and area. For example, it becomes possible to compare the drainage density •

between a very rainy and a very dry region or

•

between areas of permeable and impermeable rocks o it was found that the values usually range from less than 5 km/km2 on permeable sandstones, to extreme values of more than 500 km/km2 on unvegetated clay “badlands”

2.1 Problems Associated with the Calculation of Drainage Density Some of the problems associated with the calculation of drainage density are 1. in areas with distinct wet and dry seasons

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•

surface drainage may assume the form of intermittent streams

•

the wet season Dd will therefore have a higher value as compared to the dry season value

2. in limestone or chalk terrains •

the Dd values will be very low

•

however, the presence of numerous dry valleys occupied by streams in the recent geomorphological past, points to a higher value for Dd o for such landscapes, it may be useful to calculate both Dd in the strict sense of the term and also the valley density

2.2 Factors Controlling the Values of Drainage Density The Dd values are controlled by many factors (especially factors that influence infiltration and overland flow).

2.2.1 Time In the early stages of development, the drainage network may be very “open” and the rivers may be widely spaced (low Dd). However, as a result of the formation and growth of tributaries, a closer and more dense network may be formed (higher Dd).

2.2.2 Rock Type Impermeable rocks tend to give rise to more overland flow which will in turn lead to a higher drainage density (especially when the rock is weak and easily eroded). Permeable rocks such as limestone, on the other hand will give lower drainage density since most of the water will percolate downwards so that little surface erosion or channel formation will take place.

2.2.3 Total Annual Precipitation A high annual precipitation may mean more discharge and therefore a higher drainage density, vice versa.

2.2.4 Vegetation Vegetation will serve to enhance infiltration so that little surface run-off is generated.

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Dense vegetation cover is therefore usually associated with lower drainage density values.

2.2.5 Relief Steep slopes will generate more run-off than gentle slopes and will therefore have higher drainage densities.

2.2.6 Rainfall Intensity High intensity rainfall may lead to the generation of Horton overland flow and areas which experiences heavy rainfall (e.g. convectional rain) will therefore have higher drainage density values.

2.2.7 Infiltration Capacity of the Soil Permeable soil is associated with low drainage density and vice versa.

2.3 Drainage Density in the Different Climatic Regions The highest values of Dd usually occur in semi-arid regions where 1. the rainfall intensity is often high, 2. the soils are usually baked by the sun so that they become rather impermeable 3. the lack of vegetation cover means the frequent generation of overland flow •

very high drainage density in this region therefore appears to result from the prevalence of surface run-off and the relative ease with which new channels are initiated

In the humid mid-latitudes •

rainfall is generally low in intensity and occur throughout the year

•

Dd values are therefore markedly lower than in semi-arid regions

In the humid tropics •

Dd values are intermediate between temperate and semi-arid values because although heavy rainfall tends to generate run-off, this is countered by the dense forest cover and the permeable deep regolith

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