Located downstream from Austin, Texas, the lower Colorado River has felt the impacts of human activity for over 100 years. It currently has diversions, return flows, low water dams, agricultural land uses adjacent to its banks, and other modifications. BIO-WEST was contracted to conduct an instream flow study to develop hydraulic modeling tools and habitat-use criteria for fish in the lower Colorado River. BIO-WEST assessed potential changes in habitat availability under various water-release strategies, using blue sucker (Cycleptus elongatus) as a model. Our data collection for the project included benchmark placement, surveying, collection of river bathymetry and discharge, and collection of depth and velocity validation points.
Since the lower Colorado River basin ecosystem relies on the quality, quantity, and timing of water, BIO-WEST made instream flow recommendations for five categories: subsistence, base, pulse, channel maintenance, and overbank flows. Subsistence flow recommendations represent minimum conditions at which water quality is acceptably maintained and aquatic habitats resemble those found during extreme conditions in a natural setting. The base flow recommendations provide a range of suitable conditions that maintain year-to-year variability and associated ecological functions. Pulse flow recommendations provide a myriad of ecological functions including nutrient and organic matter exchange, limited channel maintenance, flushing, vegetation scouring, and seed dispersal. Channel maintenance flow recommendations provide channel capacity maintenance while flushing accumulated fine sediments from gravel bar and riffle habitats that are important to fishes and scouring accumulated sediments from pool habitats. Overbank flow recommendations inundate low floodplain areas adjacent to the river, thereby providing lateral floodplain and riparian connectivity, floodplain maintenance and nutrient deposition, and recruitment of organic material and woody debris.