Damming rivers fundamentally alters water temperature and flows in tailwater ecosystems, which in turn affects the presence and abundance of downstream biota. In this study, we quantified the distribution of net-spinning caddisflies Hydropsyche throughout the Colorado River Basin in the western United States to identify the roles of water temperature, flows, and species-specific morphology in determining aquatic species distributions throughout a large arid watershed that has been heavily altered by damming. We predicted that water temperatures would determine Hydropsyche presence and abundance to a greater extent than daily variation in river stage associated with hydropower production. Female Hydropsyche are morphologically adapted for deep water oviposition (egg-laying) which we hypothesized would exempt this group from the deleterious effects of fluctuating flows. To test these hypotheses, we collaborated with more than 200 citizen scientists to collect 2,194 light trap samples across two years and more than 2,000 river km.
Results/Conclusions
Samples contained 16,222 Hydropsyche individuals across six species. Using Bayesian modeling, we found that distributions of the two most widespread species were best predicted by water temperature and stage change. Despite the presence of female morphological adaptations for deep-water oviposition, the abundance of Hydropsyche decreased as diel stage change increased. Environmental flow experiments designed to reduce diel stage changes may improve habitat quality for these ubiquitous and important aquatic insects.