Project Watershed Central New York

The bed of a stream or river or creek is the physical confine of the normal water flow. The lateral confines (channel margins) during all but flood stage are known as the stream banks. In fact, a flood occurs when a stream overflows its banks and partly or completely fills its flood plain. As a general rule, the bed is that part of the channel below the "normal" water line, and the banks are that part above the water line; However, because water flow varies, this differentiation is subject to local interpretation. Usually the bed is kept clear of terrestrial vegetation, whereas the banks are subjected to water flow only during unusual or infrequent high water stages, and therefore might support vegetation much of the time.

The descriptive terms right bank and left bank always apply from the perspective of looking downstream (in the direction the current is going).

With small streams in mesophytic regions, the condition of the stream bed is strongly responsive to conditions of precipitation runoff. Where conditions are natural, either with native grassland or native forest, most small streambeds are stable, rich with organic matter and exhibit little scour. These support a rich biota. Where conditions produce unnatural levels of runoff, such as below roadways, these streambeds will usually exhibit a great amount of scour, often down to the bedrock, and banks will be undercut. This greatly increases watershed erosion and results in thinner soils upslope from the stream bed.