Stream valleys, channels, and floodplains form complicated systems that evolve through time in response to changes in sediment supply, precipitation, land use, and rates of tectonic uplift affecting a drainage basin. Stream channels serve to convey flow during normal periods, whereas floodplains accommodate flow above the bankfull stage (floods) that occurs with frequencies inversely proportional to their magnitude. Bankfull stage is defined as the discharge at which the water level is at the top of the channel. Any further increase in discharge will cause the water to spill out of the channel and inundate the adjacent floodplain. Flood frequency studies of streams throughout the world show a remarkably consistent one to twoand-one-half year recurrence interval for bankfull discharge in most streams, averaging about one and a half years, meaning that small floods are relatively common events.

Stream channels are classified...