Two-dimensional fish habitat modeling for instream flow assessment in Texas

Determining the instream flow required to maintain fish habitat within regulated streams and rivers in Texas is a very important, though difficult task. The public continues to place growing demands on limited water resources in the form of withdrawls for agricultural, hydroelectric, municipal and industrial uses. At the same time, the public maintains a keen interest in maintaining the health of aquatic ecosystems. In order to balance these competing demands, it is necessary to develop methods to estimate fish habitat as a function of stream flow.; A method to quantify the variation in fish habitat with stream flow was developed for the Texas Water Development Board. The method makes use of recently available technology, including two-dimensional hydrodynamic modeling software and a Geographic Information System (GIS). A genetic algorithm (GA), a robust optimization scheme, was used to search for potential management schemes. The result was a fish habitat modeling tool suitable for assessment of instream flow requirements at potential reservoir sites in Texas.; Instream flow assessment procedures based on this approach were demonstrated on a proposed reservoir site on Sandies Creek within the Guadulepe River Basin, TX. Biological data collected from the impacted stream was incorporated into the model. Two-dimensional hydrodynamic behavior of the stream was modeled with the software package RMA-2. Results were entered in a GIS developed in ArcView and manipulated in order to determine fish habitat variation with flow. Natural habitat conditions and effects of alternative management schemes were evaluated and a GA was used to search for improved management schemes. Management alternatives included schemes with fixed monthly flow rates and others with specified monthly pass-through flow rates.; After examining alternatives, an instream flow scheme specific to Sandies Creek was recommended. The GA proved powerful enough to find alternative fixed monthly flow schemes. However, due to limited biological understanding of the fish species of Sandies Creek, it was difficult to evaluate the environmental value of these schemes. In the end, a pass-through flow scheme with a value of 20 cfs for the entire year was found to minimize environmental risk. Due to hydrologic conditions at the site, this scheme requires only modest flow volumes throughout the year.; Results of this procedure were compared with default criteria currently used by the State of Texas. These criteria are used to make decisions in situations where site-specific studies have not yet been completed. Although these criteria are intended to overestimate environmental needs, they were found to be inadequate to maintain fish habitat on Sandies Creek.; The instream flow assessment technique developed for this study appears well suited for application to other sites and states.