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Diversion of River and River Piracy

Diversion of channels in meandering rivers: Accumulation and possible preservation of river channel deposits can occur only if the river changes its position in some way, either by shifting sideways, as above, or if the channel changes position on the floodplain, a process known as avulsion. When a river avulses part of the old river course is completely abandoned and a new channel is scoured into the land surface. Returning now to processes of channel sedimentation, recall that the table of classification and sub-facies of alluvial systems differentiates active and abandoned channel types. The sedimentary processes of channels just described may be terminated abruptly by switching or avulsion of the channel course. This can happen in several ways. A sinuous river channel can ultimately meander back on itself to short circuit its flow by necking, as it is termed. Another type of channel diversion occurs when a river has raised itself up above the floodplain by levee building to the extent that its floor is above the level of the floodplain. Channel diversion of this type can often occur on a huge scale with resultant catastrophic flooding and loss of life. Both types of channel switching are characteristic of meandering river systems. In some situations the lower end of the abandoned channel may still open out as a backwater into the main channel. After a time, however, this may become blocked to form an isolated ox-bow lake, as it is picturesquely termed, lying within the floodplain away from the present active channel. Diversion of Channel due to crevasse splays: Sometimes New channels (Crevasses) cut in levee my become so wide that even after flood subsides, river does not flow along the old river bed but along the crevasses. Diversion of Channel due to braiding of channel: Braided rivers give rise to channel bars made up of coarse gravelly material which blocks the course of river to make it divert its channel. Channel abandonment occurs by channel bars enlarging until they block the course of active channel. Diversion of Channels in case of river capture: Headward erosion of channel gullies may result into capture of previously active channels, which is called river capture. Abandoned channel may open out as a backwater into the main channel.