River systems involve not only a network of stream channels, but the entire drainage basin. Based on the dominant processes operating within them, river systems can be divided into three zones: erosion dominates, sediment trans- sediment production--where port, and sediment deposition(Figure 5.5), It is important to recognize that sediment is being eroded, transported, and
deposited along the entire length of a stream, regardless of which process is dominant within each zone. The zone of sediment production, where most of the water and sediment is derived, is located in the headwater region of the river system. Much of the sediment carried by streams begins as bedrock that is subsequently broken down by weathering, then transported downslope by mass wasting and overland flow. Bank erosion can also contribute significant amounts of sediment. In addition, scouring of the channel bed deepens the channel and adds to the stream's sediment load.