Utilizing Side-Imaging Sonar and GIS to Quantify Impacts of Reservoir Water-Level Fluctuations on Littoral Habitat and Recreational Access

In addition to water supplies, flood control, and hydroelectricity, reservoirs provide opportunities for fishing and aquatic recreation.  Increasing water demands and drought suggest maintaining quality fisheries and recreational access will be difficult.  In 2012, the Brazos River Authority partnered with the Texas Parks and Wildlife Department to investigate effects of water-level fluctuations on fish habitat and recreational access in 11 reservoirs.  We used reservoir bathymetry and side-imaging sonar data within geographic information systems to quantify relationships between reservoir water levels, littoral habitat, and recreational access.  Littoral area decreased 30 to 81% with 2- to 4-m reductions in pool elevation in most reservoirs.  Aquatic vegetation and coarse substrate (e.g., gravel and cobble) availability declined 50% or more with 2- to 3-m decreases in pool elevation.  Woody habitat (e.g., downed and standing timber) availability was variable and less sensitive to changes in water levels.   Half of all boat launches were dewatered with 2- to 3-m water-level reductions; additional declines of 1- to 5-m resulted in total loss of access.  In cooperation with the Brazos River Authority, results were used to develop reservoir-specific threshold elevations that maintain quality fish habitat and access.