ABSTRACT
A one-year field study has been made of the 8301-km2 basin of the Little Wabash River in Illinois to determine instream flow needs for aquatic life. The stream system structure follows Horton’s laws fairly well. Three reaches of about 1·5 km each were selected for detailed study of hydraulics, hydrology, and aquatic life. These reaches were selected to be representative of the entire stream system. The analytical tool used at each reach was the IFG Incremental Methodology developed by the Instream Flow Group of the U.S. Fish and Wildlife Service. Detailed measurements of depth, velocity, and bed material were taken at each reach. The methodology relates the area of fish habitat calculated from these inputs based on preference curves of various fish species and life stages. Hydraulic geometry equations are given to allow calculation of stream discharge, cross-sectional area, width, depth, and velocity at any location in the stream system. Results show the amount of available habitat as related to streamflow and as the amount of habitat varies throughout the various months of the year. The results are considered meaningful and useful for managing streamflow for instream uses.
