Figure 4.--Diagram of the three main geomorphic channel units: (A) pool--a part of the stream with reduced velocity, commonly with water deeper than surrounding areas; (B) riffle--a part of the stream where the water flows swiftly over completely or partially submerged obstructions to produce surface agitation; and (C) run--a slow moving, relatively shallow body of water with moderately low velocities and little or no surface turbulence (modified from Bisson and others, 1982).