Title: The Effects of Water Withdrawals on the Emergence Timing, Adult Body Mass, and Total Emerging Biomass of Aquatic Insects from two Northeast Oregon Rivers. Abstract approved:

Small low-head diversion dams are capable of removing much of the flow of a river, often resulting in increased water temperatures and habitat loss. Warmer temperatures have been shown to accelerate aquatic invertebrate growth and development, and discharge reductions can reduce instream habitat, suggesting that water withdrawals may alter both the timing of aquatic insect emergence and the total emerging biomass from a river. To examine the influence of water withdrawals on the timing and magnitude of aquatic insect emergence, we monitored emergence continuously for 9-10 weeks during the summer of 2006 on the Umatilla and Walla Walla rivers of northeastern Oregon. Multiple sample sites were located along gradients of decreasing discharge and increasing water temperatures caused by successive diversions on each river. The emergence timing and adult body size of three holometabolous species (Helicopsyche borealis, Petrophila confusalis, and Glossosoma traviatum) were compared to discharge, water temperature, and other physical habitat variables. The emergence timing of entire taxonomic families was also compared to these variables, and the total emerging biomass was estimated for each site. Despite reductions in discharge of up to 93 % and increases in average wate