Generally, rivers, streams, and creeks can be defined as surface-water runoff flowing in a natural or modified channel. Runoff results either from excessive precipitation which cannot infiltrate the soil, or from groundwater where the water table intersects the surface of the ground. Drawn by gravity to progressively lower levels and eventually to the sea, the surface runoff organizes into a system of channels which drain a particular geographic area.
The drainage system serves as a transportation network for nature's leveling process, selectively eroding materials from the higher altitudes and transporting the materials to lower elevations where they are deposited. A portion of these materials eventually reaches the sea where they may form beaches, dunes, or splits.
Distinct Stages Exhibited by Rivers
Typically, a river exhibits several distinct stages as it flows from the headwaters to the mouth. In the upper reaches where the gradient is steepest, the hydraulic action of the flowing water results in a net erosion of the stream bed and a V-shaped cross section, with the stream occupying all or most of the valley floor.
Proceeding downstream, the gradient decreases and the valley walls become gentler in slope. A point is eventually reached where erosion and deposition equalize and the action of the stream changes from vertical cutting to lateral meandering. As the lateral movement continues, a floodplain is formed, over which the river meanders and upon which materials are deposited during floods. Finally, when the river enters a body of standing water, the remaining sediment load is deposited.
Extensive human use is made of rivers, including transportation, recreation, waste and sewage dumping, and for drinking water. Rivers are dammed for the production of electric power, diked for flood control, and withdrawn for the irrigation of crops. Many of these activities directly affect the natural hydraulic functioning of the streams and rivers as well as the biology of the watercourses (see Use Activity Policies and Regulations).