A stream bed is the channel bottom of a stream or river or creek; the physical confine of the normal water flow. The lateral confines (channel margins) during all but flood stage are known as the stream banks or river banks. In fact, a flood occurs when a stream overflows its banks and flows onto its flood plain. As a general rule, the bed is that part of the channel just at 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 perhaps infrequent high water stages, and therefore might support vegetation some or 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). The nature of any stream bed is always a function of the flow dynamics and the local geologic materials influenced by that flow. With small streams in mesophytic regions, the nature of the stream bed is strongly responsive to conditions of precipitation runoff. Where natural conditions of either grassland or forest ameliorate peak flows, streambeds are stable, possibly rich with organic matter, and exhibit minimal scour. These streams support a rich biota. Where conditions produce unnatural levels of runoff, such as occurs below roads, the streambeds will exhibit a greater amount of scour, often down to bedrock, and banks may be undercut. This process greatly increases watershed erosion and results in thinner soils upslope from the stream bed as the channel adjusts to the increase in flow.
