In the river channel the current is fast moving, but where the river spreads over its banks on to the surrounding land (the flood plain) the current slows down and mud and very fine sand are deposited as the water leaves its channel. The coarsest sediment is dropped first.ĭuring a flood, however, river water moves at different speeds. More usually the material is deposited en route as the river current slows up.

Where there is a sudden change in gradient and therefore water speed, as when a river leaves the mountains and runs out onto a plain, nearly all the sediment will be dropped, forming an alluvial fan. Coarser material is rolled along the bed.Īs the river enters its more gentle slopes, some of its sediment is dropped. Eddies bounce the sand from the bottom and it is carried downstream a small distance by the current before it falls to the bottom again. The finest particles are carried in suspension, kept up by the turbulent motion of the water. This is why most erosion and transportation occurs during floods. The faster the stream flows the larger the fragments it can carry. During the process the stones are broken up into smaller pebbles, so that the boulders in the upper course of the stream provide the sand grains that are present in the lower course. Erosion mainly takes place at this stage by stones banging into the stream bed and sides and so wearing them away. Repeated implosions cause part of the rock to be sucked into the stream and carried away. Much of the babble of a brook is the sound ofimplosions. Sometimes the pressure is so low that a vacuum is formed on a small part of the stream bed: as the eddy changes this vacuum implodes. During normal flow little physical erosion takes place, but during flood the movement of water becomes turbulent and this causes eddies which in turn cause rapid changes in the pressure on ther ocks. The ability of a stream to erode mechanically is closely related to its speed. Weak acids such as carbonic and humic acids in the water help to decompose limestone and other rocks. Stream water erodes in two ways: chemically and physically. Erosion, transportation, and deposition are the main work of a river and most rivers can be divided into three sections: an upper course in which erosion dominates a middle one where transportation occurs and a lower one where deposition takes place. The stones and sand formed by erosion of the rock are transported downstream and are finally deposited at the mouth of the river. May be deposited to form a delta (Fig 1). Toward the river mouth, the flow becomes more sluggish and meandering prominent: The valley is broader, the flow less swift, and meandering more common. In the middle part of its course the gradients become less steep,Īnd lateral (sideways) erosion becomes more important than downcutting. Variations in the hardness of the rocks over which it runs may result in Near the source a riverįlows swiftly, the rocks and other abrasive particles eroding a steep-sided A river is a body of inland water that flows downhill under gravity in a natural channel into the sea, a lake,

Delta meander river