The slope of a river channel decreases between its source and its mouth, and the longitudinal profile of a well-established stream is thus a smooth curve, concave upward. During a flood, for example, a river has a high velocity and is capable of carrying more sediment than normal. It therefore cuts down into the river bottom, picking up more sediment and decreasing its slope until its velocity matches its sediment load. When the flood ends and the discharge returns to normal, the river rids itself of the excess sediment by redepositing it in the channel, building up its slope until velocity and sediment load are once more in balance. Because baselevel, discharge, and sediment load are all highly variable, this process is never complete.
River channels display a variety of patterns when viewed from the air or on a map.
Generally streams with relatively steep slopes, they carry a considerable load of sand or gravel or both. Meanders are series of rather regularly spaced symmetrical bends that tend to appear in streams with low slopes and channels made of silt and clay. The exact mechanism that produces meanders is unknown, but they are a common feature of many types of flows, including meltwater streams on glaciers and the Gulf Stream, and are probably due to the action of corkscrewlike currents that exist even in straight flows. Irregularly curved river channels also occur. Rivers thus play an important geologic role as the carriers of the products of rock weathering and mechanical erosion.