Work of Rivers as an Agent of Erosion

The geological work performed by rivers may be placed under three heads: Erosion, Transport and Deposition. Rivers are active agents of erosion, especially in times of floods. They carry away much material and redeposit some part of it farther downstream, the rest being transported to the sea. Some matter goes into solution in the river water. The finer particles remain in suspension and the coarse roll along the bed during floods and hence perform the work by abrading the channel of river.

The work of erosion, performed by rivers, results in the widening and deepening of their valleys during their course of development. Stages of youth, maturity and old age may be distinguished in the history of river. There is a steep sided valley of a youthful stream the broader valley and more deeply dissected landscape of mature river system and the flat meandering course of a river in the stage.

Meanders

When a river has cut down nearly to its base level, it flows more slowly with a reduced gradient and begins to swing side by side of its valley. The energy imparted to the load of sediments which it carries is consumed in the widening of the valley by lateral erosion, and the course of the rivers develops a series of big looped curves called meanders. The length of the loop when fully formed is about sixteen times the width of the stream. On the concave side of a meander the bank is undercut and eroded while deposition is on the convex side.

River Deposits

The general term given to deposit laid down by rivers is alluvium, though this is often restricted to the finer material such as silt and mud as distinct from gravels and larger fragments. After heavy rains, the velocity of river may be trebled; larger boulders which would not be moved under normal conditions of flow are shifted with an intermittent motion by stream is spate. Transported sediments are dropped by a stream whenever its velocity is checked. A river emerging from a mountain on to a flatter ground, such as the edge of a plain, builds up a heap of detritus known as alluvial cone. The material deposited is mainly sand and gravel, partly rounded during transport. The finer particles being carried on down to the sea. In the lower courses of a mature river the finer alluvium is spread out to form an alluvial flat. This is subject to periodic flooding and a fresh layer of alluvium is deposited at each flood.

Water Falls

These are performed where a stream in a youthful stage flows over rocks of different hardness. A hard rock is worn away less rapidly than a soft, with a result that a river’s gradient is increased where it crosses a ledge of hard rock, softer material below the resistant layer is undercut by the water, leaving an overhanging ledge over which the stream falls.

Deltas

A river entering a body of still water such as a lake or sea drops much of its load of sediments as its velocity is reduced and forms a delta which is gradually built forward into the still water. The building of a delta proceeds as sediments is deposited at a river’s mouth. The stream than subdivides and flows through the water on either side of the obstacle which it has made. Further deposition takes place along these distributions and after further barriers have been made, the streams branch again. In this way by repeated bifurcation and sedimentation, the deltaic deposits cover a large area which has a roughly triangular like the Greek latter ∆ as is the case of Nile delta, Ganges and Mississippi.