Rivers Notes

Formation of floodplains When there is heavy rain, a river may overflow its banks, causing land on either side of the river to be flooded. When the water recedes, the river deposits the materials it is carrying on the land, leaving behind alluvium on the river banks and river beds to form a flat, fertile floodplain.

Formation of a Waterfall • •

A river flows across rocks of different resistances, eroding the less resistant rocks more rapidly and causing a change in the gradient of the river course. Over time, the river plunges from a great height to hit the river bed below with tremendous force. A waterfall forms.

Traction is

a transpiration process that involves the rolling and sliding of large particles like pebbles, boulders and gravels along the river bed when the river has enough energy to move downstream.

Corrasion

is the wearing down of a river bed and banks by the grinding action of rock fragments carried by the river.

Formation of Oxbow Lakes • •

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Erosion, transportation and deposition results in the formation of a meander. The meander develops into a pronounced loop and eventually meets as deposition and erosion continue to take place along the inner and outer banks. The sediments deposited will eventually dam up the water in the cut-off, forming an oxbow lake.

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With constant erosion along the outer bank of two consecutive meanders, the two meanders move closer to each other, forming a loop. The loop becomes more distinct and is separated by a narrow neck of land. This neck gets narrower with continual erosion and deposition and the two meanders eventually meet. The sediments deposited will eventually dam up the water in the cut-off, forming an oxbow lake.

Roughness of a River Channel

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A rough channel reduces the speed of the river because of obstacles (e.g. rocks, boulder, underwater vegetation) on the river bed or/and an uneven river bed, resulting in high friction. A small channel increases the speed of the river because of low friction

Wetted Perimeter • •

If the wetted perimeter is large, more water comes into contact with the channel, generating a lot of friction and slowing the speed of the river A small wetted perimeter causes the speed of the river to be high because there is less friction

Valleys •

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In the upper course of a river, the gradient of the land is very steep. Thus, the river has sufficient energy to erode the rocks in its path and cut deep into the channel, causing vertical erosion. V-shaped, narrow and steep-sided valleys are eventually formed. In the middle course of a river, the river flows with less energy and cuts less deeply into its channel since its gradient is not as steep as in the upper course. Lateral erosion occurs more then vertical erosion, thus the sides of the valley in the middle course tend to be wider than the upper course. In the lower course, the valleys are wide because the gradient of the land is gentle. Lateral erosion occurs instead of vertical erosion, giving rise to a broad, flat valley.