2-D fish habitat modeling for instream flow assessment

The Texas Water Development Board has developed a two-dimensional (2-D) fis h habitat modeling procedure for estimating instream flow needs. The proced ure makes use of a 2-D hydrodynamic model, a Geographical Information Syste m (GIS), and an optimization scheme to suggest alternative values for instr eam flows. After the selection of representative and/or critical stream seg ments, fish surveys are conducted to determine habitat preferences of nativ e species. A 2-D hydrodynamic model is developed, based on bathymetric, sta ge-discharge, and calibration data. Results of hydrodynamic modeling at var ious now rates are combined with substrate and habitat maps in a GIS. Spati al queries based an fish preferences are conducted to determine suitable ha bitat at each flow rate, allowing the development of habitat versus now rel ationships for each species. Using these relationships, habitat features of alternative instream flow strategies can be compared. A genetic algorithm based optimization package is used to aid in the search for instream flow s trategies during normal, dry, and drought conditions. Examples from the app lication of this assessment procedure to Sandies Creek within the Guadalupe River Basin, TX are presented.