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).