O Level Notes : Geography - landforms and landscape processes - River Processes

RIVER EROSION

There are three types of river erosion and these are:

(i) Head ward erosion

This is when the river cut back towards the source. This takes place at the source of the river. The river wears back towards its source thereby increasing its length.

(ii) Lateral erosion

Rivers wear away their banks thereby increasing their width.

(iii) Vertical erosion

Rivers wear away the bed leading to increased depth of the channel

 Types of River Erosion

A- head ward erosion

B- lateral erosion

C- vertical erosion

These are the three types of river erosion but they occur through different processes.

Processes of river erosion

(i)Abrasion

It is an erosional process whereby the load carried by the river brushes and polishes the banks and bed thereby wearing the channel away. It is also called corrasion. The particles carried by the water have the sandpaper effect on the bed and banks. The bigger the particles i.e. pebbles and boulders, the higher the rate of abrasion.

(ii) Solution

The solvent particles and channel parts dissolve in water. They are eroded through dissolving in water. Some water contains weak acids such as carbonic and humic acids, these acids increases the rate of dissolving by channel and sediments.

(iii)Attrition

This process leads to particles carried by the river breaking into small particles. The carried sediments carried hit and collide against each other. In so, doing they break down into smaller particles. Angular rocks become increasingly rounded due to collision and wearing away.

(iv) Hydraulic action

It is the impact of the flowing water which hits the banks removing and eroding gravel, sand and silt. Banks of the river collapse due to the impact of the water. It is the weakest and least effective form of erosion. See fig.

(v) Cavitation

Is linked to hydraulic action because it involves the Impacts of flowing water. The flowing water hits the banks and causes bubbles. The bubbles of collapsing air causes shock waves which weakens the rock on the banks.

River Erosion Process

The different erosional processes show the different ways through which either the load or river channel is eroded. River erosion involves the wearing away of rocks and soil found along the river bed and banks and also the breaking down of the particles carried by the river.

Features formed by river erosion

1. Waterfalls and Rapids

A rapid is a point where water flows rapidly due to a sharp break on the bed of the river. Rapids grow to become waterfalls. These features are common on the upper course of the river but may occur in any part of the river. Rapids and waterfalls occur where a resistant rock, overlays a less resistant rock though they may occur under other scenarios. The resistant rock resists erosion while as the less resistant rock below is eroded faster. Fig 2.10 shows the waterfall. The resistant rock remains uneroded to form the waterfall. The resistant rock remains protruding while the soft rock has been eroded hence water falls from the resistant rock down to the foot where the soft rock is still being eroded. The distance between where there was the overhang and the foot of the waterfall continue increasing as erosion of the soft rock progresses. That waterfall might have started as a small rapid.

 Waterfalls and rapids

As shown in the diagram above, rapids will continue growing due to rapid erosion of the weaker rock to form a waterfall. Fig 2.11 below shows a waterfall where an overhang falls to let the water fall from the remaining resistant rock. The waterfall can also form where the layers of resistant rock are vertical or inclined to the less resistant layer.

The Development of Rapids and Waterfalls

Where there is a resistant rock lying vertically besides the less resistant layer, the resistant rock remains protruding due to rapid erosion of the less resistant part to form a rapid. Continued erosion of the less resistant part leads to the development of the rapid into a waterfall. The same process happens where there is an inclined resistant rock.

 Victoria Falls

The Victoria Falls was formed when water flowing on the Zambezi plateau eroded a weaker part formed by a fault fracture filled with sandstone. The Victoria Falls is formed on the Zambezi plateau which contains faults on a basalt rock. The faults are filled with soft sandstone which was then eroded faster to allow water to fall down into the gorge. Erosion was active along the part marked XXXX... creating a gorge into which the mighty Zambezi falls.

1. Plunge pools

A plunge pool is a large depression found at the foot of the waterfall. The water falling from the waterfall hits sediments at the foot of the waterfall. The water hitting the foot of the waterfall experiences eddies which is a swirling motion. When it does so, the sediments are forced to turn and as they turn, they brush  against  the  river  bed  causing  corrasion. The  bed  is  worn  away  and depression at the foot of the water fall develops.

1. Pot holes

These are small depressions found on the river bed. They are formed by abrasion.

 Pot holes

 The load carried by the river brushes against the bed, wearing it away to produce these potholes.

1. V- shaped valleys

These are caused by rapid vertical erosion on the upper course of the river. In the upper course, the river flows down steep gradient hence velocity is high. Lateral erosion is slow than vertical erosion since the bed is hit by the water falling from high gradient. The banks wear away slowly leading to incision by the rivers to form V-shaped valleys.

1. Interlocking Spurs

V-shaped valleys.

The river winds to avoid resistant rocks and highlands. The flow is faster at the bends and erosion of the sides is experienced. The resistant rock layers remain as projections or Spurs interlocking. The concave banks remain standing as river cliffs while the opposite convex develop into a slip off slope due to little erosion.

1. Steep sided valleys

Interlocking spurs.

These are common in the upper course of the river. Vertical erosion is faster than lateral erosion. Rapid down cutting produces narrow steep sided valleys.

1. Gorges/ canyons

These are deep steep sided valleys with the stream flowing at the bottom. They are a result of rapid down cutting by the river.

RIVER DEPOSITION

It is the process whereby materials carried by the river are dropped. The material carried by the river is called the load. This load is deposited when the river loses its energy. Stream energy is lost due to:

• Loss of velocity due to falling gradient.
• The channel being chocked with sediments.
• The channel getting wider hence friction with banks is increased.
• The river entering the sea.
• Low discharge caused by drought and change of seasons.
• river meandering and flooding

Deposition occurs in such a way that large particles are deposited just close to the channel because these cannot be carried further away from the channel. Features formed by deposition include:

(i) Levees

These are ridges of sediments deposited just outside the channel. During river flooding, water overfills the channel and some of the load is thrown outside the channel. The biggest particles are deposited first just outside the channel. Smaller ones are deposited later when velocity decreases hence these are found on top of courser sediments. Some small sediments are carried further away from the banks of the channel into the flood plain.

Development of Levees

In diagram (a) before flooding the water is flowing within the channel but during flooding as shown by diagrams (b) the flooding water overfills the channel to occupy the floodplain. When water overflow the banks onto the floodplain, it transports some materials outside the channel. Bigger particles are deposited just outside the channel because of their weight. Lighter sediments accumulate a distance away from the channel. In diagram(c) after the flood the deposits are left behind forming a ridge of these deposits as levees. 

(ii) Flood plain

These are gentle sloping surfaces on the sides of the channel. The flood plain is the surface on the sides of the channel which is covered by water during flooding. During flooding, the river deposits alluvial deposits on the flood plain. The flood plain may contain features such as marshes, alluvium, meanders and ox-bow lakes.

Depositional Features

In some cases, the flood plain contains fertile alluvial deposits hence stream bank cultivation is practiced. They also contain alluvial minerals like gold where panning is common.

(iii) Deferred tributaries

The formation of levees creates a ridge which prevents tributaries from joining the major river at a given point. The tributaries are delayed or deferred from joining the major river only to do so downstream.

(iv) Swamps/ marshes

These are areas of stagnant water occupied by water loving vegetation. These are found on the flood plains. Tributaries deferred from entering the main river forever may end up spreading their water on the flood plain forming wetlands.

(v) Bluffs and point bars

These are well pronounced slopes marking the edge of the flood plain. They are steep and sometimes are found outside the bend of a meander. Point bars are found on the convex bend of the meander. Water hits the concave part with more energy causing active erosion which leads to the formation of  bluffs but on the convex part there is less energy and deposition takes place forming point bars.

(vi) Braided channels

This is when the channel is divided into multiple channels flowing downstream by deposits. These diverging channels will later converge downstream. The small islands formed by the deposits dividing the channel called braids. It is also formed when a river meets resistant rocks.

  Braided channel

Braiding occurs when the river deposit sediments within the channel. This is due to the river suddenly getting wider. When the river suddenly gets wider and shallow its energy is reduced due to increased friction with the banks and the bed. However, it should be noted that braids do not always only occur where the river suddenly gets wider. Even when the channel maintains its width, the presence of an obstacle, for example, a rock or stump at the middle of the channel may trap the load being transported. The sediments accumulate around the obstacle to form braids.

(vii) Delta

A delta is a sediment and silt deposited landform formed by rivers in the lower course where they enters the sea. A delta is formed at the lower course of the river where the river enters the sea or lake. When the river enters the sea,  deposition occurs at the mouth because:

• The channel gets wider hence water is distributed over a wider area reducing its energy.

• Fresh water in the river reacts with salty sea water and a reaction called flocculation occurs causing particles carried by the river to coagulate and settle at the bottom of the water.
• Flowing river water faces resistance from stationary sea water. Reduced velocity as the river enters the sea leads to deposition.
• At the lower course, the river is chocked with its load hence it becomes weaker.

As a result, different types of deltas are formed. These include:

(a) Bird's foot

This delta is formed where the river's distributaries at the river mouth flow further into the sea. This is caused by weaker tidal currents at the mouth. The result is that these distributaries produce a delta which looks like the foot of a bird (fig 1.19). A good example is Mississippi River where it enters the sea. Such type of a delta is formed by fine silt. The distributaries may be bordered by levees. It also has few distributaries which are long.

(b) Cuspate delta

Moderate currents do not favour development of distributaries at the mouth of the river. A delta without the distributaries is therefore formed and is called cuspate delta, for example,  Medjerda River delta in Tunisia. Cuspate deltas are triangle shaped.

(c)  Estuarine

(d) Arcuate Delta

Types of deltas

It  is  a  arc  shaped    or  bow  shaped  delta  formed  when  water  is  as  dense  as the sea. it is formed by sand and gravel. Examples of arcuate delta are Ganga Delta, Niger and Nile Delta

(viii) Ox-bow lake

There are some features which are not solely attributable to erosion only but to both erosion and deposition. For such features to form there must be both erosion and deposition taking place. For example, Ox- bow lakes. An Ox- bow lake is a C- shaped or U- shaped feature formed when active erosion at the neck of the meander cuts off the meander and take a straight course leaving the meander separated from the main channel. On the meandering segment of a river, erosion is active at the neck of the meander. With time that active meander will break through the neck enabling the river to avoid meandering but to take a straight course. The meander is left out without supply of water from the main river hence it may end up drying.

Formation of Ox-Bow Lake

TRANSPORTATION

River transportation is the process where by rivers move their load downstream. Rivers transport their load through different processes. These are:

• Suspension

Fine and light materials float in water and they are carried downstream.

• Solution

Some materials simply dissolve in water and are transported whilst dissolved. This usually make the water murky.

• Saltation

This is also referred to as bouncing. Particles are picked and dropped repeatedly all the way down the stream. Small pebbles and stones are carried downstream this way.

• Traction

Heavier boulders which cannot be completely lifted from the river bed are rolled downstream. Hence this process is also referred to as rolling.

River transportation processes