Hydraulic Geometry of Streams and Stream Habitat Assessment

Quantification of the relationship between stream discharge and the availability of suitable aquatic habitat requires detailed information on the values of local depths and velocities as they vary through riffles and pools. Conventional flow models are inadequate to supply the needed information. A basin model is presented which provides a new approach to basinwide flow modeling for stream habitat evaluation. Streams within a basin exhibit similar patterns of development, varying in scale with the drainage area. Average width, depth, and velocity increase in a predictable way with drainage area and discharge. Hydraulic geometry relations define the average values of width, depth, and velocity for any stream in the basin. Local variations of depth and velocity about the average are evaluated from probability distribution models developed from field measurements made throughout riffle and pool sequences. The basin flow model provides the needed hydraulic data to apply the Instream Flow Incremental Methodology (a state‐of‐the‐art methodology for stream habitat evaluation) of the U.S. Fish and Wildlife Service.