High discharge can cause a river to flood in its middle and lower courses. If this happens, as the water overflows its banks,the velocity will fall.

Deposition will occur because the competence of the river is reduced. Firstly, the coarsest material will be deposited first, forming small raised banks called levees along the sides of the channel.

Further floods can increase the size of these banks and further deposition on the bed of the river also occurs, meaning the river now flows at a higher level than the floodplain.

This is why levees are sometimes strengthened and their heights are increased.

On the Mississippi River in USA for example, strengthening of the levees began in 1699. By the 1990's, the length of engineered levees was 3,200km