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FLUVIAL SYSTEMS

Compiled by

Okan Tüysüz

from different internet sources

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FLUVIAL LANDFORMS AND PROCESSES

• fluvius (L.): river

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

water, particularly in semiarid environments (badlands)

• requires understanding of stream and hillslope hydrology and hydraulics

• 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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• force of raindrops on bare soil causing disaggregation of surface soil

2. rainsplash erosion

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

• on slopes the splash is asymmetrical resulting in the progressive

downslope displacement of wet soil during intense rain on bare slopes

3. sheet wash (rain wash)

• 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

• 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

• turbulent eddies suspend soil particles, creating rills

5. 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

• 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

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

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

network

• gully

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

watersheds

• carry ephemeral stream flow

• narrow and steep sided

• persist for years or decades, so more persistent than rills but still not

"permanent" features

• 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

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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

• in stream channels water movement nearly always occurs as turbulent flow, that is,

the velocity of flow fluctuates in all direction within the fluid

• 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)

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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

• 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

• 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

• 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

Concrete

Unlined earth drainage channels

Winding natural channels

Mountain streams with rocky beds

Alluvial channels with small ripples

Alluvial channels with large dunes

0.010

0.013

0.017

0.025

0.040 - 0.050

0.014 – 0.024

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

• 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

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

velocity

• 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

• 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)

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

S

• 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

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

channels

• 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)

• 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

• 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

• 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

• 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

• 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

• In steepened valley sections, the rapidly flowing water causes accelerated erosion, with

the result that the steepened section migrates upstream

• 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

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

• 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

• if they are angular they may become wedged between other fragments

• such movements are referred to as traction

• 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

• 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

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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

• 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

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

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

• as a stream leaves a canyon, drainage becomes distributary; these wider, shallower,

lower-gradient streams have less transport capacity

• also water infiltrates the coarse bed materials, losing transport capacity

• debris flows may occur with increased sediment concentration

• 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

• 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.

30

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

• 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

31

• the result is that while the concave banks tend to retreat, the flatter convex banks

gradually advance with the accumulation of more sediments

• 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

• 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

• 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

• 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

32

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

• 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

• hence, along the margins of the river channel there is a sudden decrease in the

competence velocity and capacity

• this causes deposition to occur, beginning with sediment of coarser calibre because of

their higher settling velocity

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

formation of levees

33

FLUVIAL SYSTEMS:

Three ‘end-member’ fluvial styles are:

• Braided River systems

• 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 (upward-

fining; 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.

34

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:

35

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.

36

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

37

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.

38

Anastomosed river system in southeastern Saskatchewan. Notice the multiple channels

and interchannel wetlands.

39

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

40

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

41

• 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.

42

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

43

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)

44

• 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

45

• 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.

46

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

47

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

48

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

49

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

50

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.

51

• 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

52

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 long-

term 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)

53

• 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

54

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.

55

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

56

• 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

57

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

58

• 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

59

60

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

61

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)

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• derived from the Greek

dendron, a tree, this

pattern comprises 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 some

resemblance to trellised

patterns

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• 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,

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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

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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)

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• 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

69

• 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

70

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

71

• 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

74

75

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 fourth-

order stream, plus numerous third-, second- and

first-order segments, is referred to as a fourth-

order 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 first-

order 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

79

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

80

• 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.

81

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