Rivers are gradational agents. They degrade the high lands and aggrade the low lands. They tend to bring the earth’s surface to a common level nearer to the sea level.
By downward cutting and lateral cutting, the rivers erode the high lands. However, there is a limit to the vertical erosion. This is the sea level and is termed as the base level erosion. The extent of vertical erosion is confined at certain elevati (base level of erosion) which commonly matches with the sea level. Riv erosion takes place by hydraulic action, abrasion, corrosion an attrtion.
Transporation by the river takes place through traction, saltation, suspension, floatation and solution. Higher the gradient, higher is the velocity of the running water. The transporting power increases with increasing velocity. Commonly, if the velocity is doubled, the transporting power is its cube capacity. Thus, with increasing velocity, the erod materials along with the materials from mass wasting are transported down stream by the river. Also a part of the load is carried in solution.
In the plains, the gradient is less, hence the velocity goes down. Besides, any obstruction in the river path by irregularities, hard rocks, vegetation, damming, and curves of meanders – all contribute to the reduction of velocity. When the river meets the sea or lake, the velocity is minimum. For all these reasons, the transporting power of the river is greatly reduced and the load carried by it is gradully deposited as a sedimentary unit.