river

river Large, natural channel containing water that flows downhill under gravity. A river system is a network of connecting channels. It can be divided into tributaries, which collect water and sediment, the main trunk river and the dispersing system at the river's mouth where much of the sediment is deposited. The discharge of a river is the volume of water flowing past a point in a given time. The velocity of a river is controlled by the slope, its depth, and the roughness of the river bed. Rivers carry sediment as they flow, by the processes of traction (rolling), saltation (jumping), suspension (carrying), and solution. Most river sediment is carried during flood conditions, but as a river returns to normal flow, it deposits sediment. This can result in the erosion of a river channel or in the building up of flood-plains, sand and gravel banks. All rivers tend to flow in a twisting pattern, even if the slope is relatively steep, because water flow is naturally turbulent. Over time, on shallow slopes, small bends grow into large meanders. The current flows faster on the outside of bends, thereby eroding the bank, while sedimentation occurs on the inside of bends where the current is slowest. This causes the curves to exaggerate forming loops. Rivers flood when their channels cannot contain the discharge. Flood risk can be reduced by straightening the channel, dredging sediment, or making the channel deeper by raising the banks. See also delta; levée; oxbow lake