Landforms Created by Rivers

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