Landforms Created by Rivers

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Waterfalls occur when there is a sudden interruption in the course of a river

These interruptions may be the result of earth movements, sea level changes or a change in rock type

In the case of a waterfall forming as a result of a change in rock type, the river will have met a band of softer, less resistant rock after flowing over a band of relatively hard, resistant rock

The river flows over the hard resistant rock and cascades into the plunge pool below

The plunge pool is deepened and widened through the process of hydraulic action

Splashes created as the river hits the water in the plunge pool erode the soft rock at the back of the waterfall through the process of hydraulic action

The erosion of the soft rock at the base of the waterfall means that the harder rock on top is undercut

This means that an overhang of hard rock is left unsupported

The weight of the river on the top layer of hard rock will put stress on the overhang

As a result, the overhang will eventually collapse

As the overhang falls into the plunge pool, rocks will collide with other rocks

They will break up into smaller pieces in the process known as attrition

Some of these smaller pieces of rock will ricochet into the back of the waterfall dislodging any loose material and eroding it further, this process is known as abrasion

The entire process is likely to be repeated several times causing the waterfall to retreat upstream

A deep sided gorge is left as a result of the waterfall retreating back towards its source

The Niagara Falls on the border of Canada/America are retreating by approximately one metre a year

As a river approaches its mouth, it usually flows over flatter land and develops increasingly large bends known as meanders

At this stage in its journey the river is eroding the landscape laterally

When the water reaches a meander most of the water is directed towards the outside of the bend

This increases the velocity and depth of the water at this point

The river therefore has more energy to transport material in suspension

This material will erode the outside of the bank through the process of abrasion

The bank will be undercut and a river cliff will form

Meanwhile, as there is less water on the inside of the bend, there is a decrease in velocity and depth

As the river loses energy, it begins to deposit some of its load

The deposited material builds up to form a gently sloping slip-off slop

Continual erosion on the outside of the bend results in the neck of the meander getting wider until, during a flood, the river cuts completely through the neck

This means a separate channel is formed

The majority of the water will now flow through the new channel as it is the most direct way of reaching the sea

However, some water will continue to take the old route

As less water is now travelling around the meander, it has less energy and so more deposition will occur

In particular, deposition will occur at entrances to the old bend

The deposited material will form a bar

Over time the material will block off the old meander so no more water can enter it

The deposited material will be slowly colonised as plants will begin to grow and new land will eventually form

The original meander will remain in the landscape as a crescent shaped area of water known as an ox-bow lake

This lake will eventually dry up except during periods of heavy rain

Waterfalls

Ox-Bow Lakes