Stream valleys are the most abundant and wide spread landforms on the continents. A river system consists of a main chanel and all of the tributaries that flow into it. A high degree of order exists among the various elements of a river system. A river is a network of connecting channels through which water, precipitated on the surface, is collected and funneled back to the ocean. The major downcutting of stream channels is accomplished by abrasion, dissolution, and hydraulic action.
Drainage systems grow by headward erosion, which commonly results in stream piracy.
Slope retreat causes valleys to grow wider and is associated with downcutting and headward erosion. It results from various types of mass movements. As a river developes a low gradient, it deposits part of its load on point bars, on natural levees, and across the surface of its flood plain. Sediment is transported if the velocity of the stream exceeds the settling velocity of the sediment particles. The capacity of running water to erode and transport sediment is largely dependent on stream velocity, although characteristics of the channel are also significant.