A river can be viewed as an open system; see G. M. Kondolf and H. Piégay, eds (2003). The fluvial system is a process-response system that includes the morphologic component of channels, floodplains, hill slopes, and the cascading component of water and sediment. The system changes progressively through geologic time as a result of normal erosional and depositional processes, and it responds to changes of climate, base level, and tectonics.

A fluvial system can be divided into three zones:

1. Zone 1 is the drainage basin, watershed, or sediment-source area. This is the area from which water and sediment are derived. It is primarily a zone of sediment production, although sediment storage does occur there in important ways.

2. Zone 2 is the transfer zone, where, for a stable channel, input of sediment can equal output.

3. Zone 3 is the sediment sink or area of deposition (delta, alluvial fan).

These three subdivisions of the fluvial system may appear artificial because sediments are stored, eroded, and transported in all the zones; nevertheless, within each zone one process is dominant. From this viewpoint, a river system is supplied with water, sediment of specified sizes, and potential energy. The principles that govern the fluvial system include hydraulic principles that govern the transport of water and sediment, constraints imposed by external variables, and time (S. A. Schumm, 1977).