Evaluating the effects of hydropeaking and a selective withdrawal structure on primary and invertebrate secondary production in the Colorado River using artificial streams

Water temperature and velocity are dominant physical factors affecting lotic ecosystems.  A selective withdrawal structure (SWS) that is proposed for Glen Canyon Dam will increase release temperatures by 5 ºC, but changes in important ecosystem processes are difficult to predict without measurements on system-specific taxa.  Further, no studies have evaluated whether daily changes in water velocity associated with hydropeaking affect ecosystem processes.  We evaluated these aspects of river regulation by studying the response of algae and invertebrates placed in 18 artificial streams to a fully-factorial experiment involving two treatments: one treatment mimicked the daily changes in velocity due to hydropeaking from Glen Canyon Dam (a constant-velocity control, low daily fluctuations, and high daily fluctuations) and the second treatment simulated release temperatures below Glen Canyon Dam with a SWS (10 ºC control and 15 ºC).  Temperature had a positive effect on primary production.  Temperature also had a positive effect on the instantaneous growth rates of simuliid larvae, but not amphipods.  In contrast, daily fluctuations in water velocity had no effect on primary or secondary production.  These results indicate that a SWS will likely lead to increases in the production of important food resources for native fish in the Colorado River.