Does dewatering a river alter the life histories of its emerging aquatic invertebrate adults?

Intense water withdrawal in agricultural areas is capable of removing much of the in-stream flow, resulting in increased water temperatures in some arid-land river systems. Accelerated growth and development of aquatic invertebrate larvae has repeatedly been shown in warmer temperatures, suggesting that the timing of adult insect emergence may be significantly altered by water withdrawals. To examine the magnitude and nature of the influence of water withdrawals on aquatic invertebrate life histories, emergence from an arid-land river in northeastern Oregon was monitored continuously for 10 weeks during the summer of 2006. Sample sites were located above and below a series of agricultural water diversions, which created a gradient of decreasing discharge in both time and space. Reductions in average daily discharge between the lowermost and upstream control sites averaged 89% and at times exceeded 97%. Daily mean water temperature at the lowermost site averaged 4.6C higher than the control, with occasional spikes of up to 7C. Despite the dramatic alterations in temperature and discharge, peak emergence for all but a few taxa did not occur appreciably earlier at sample sites exposed to reduced flows and elevated temperatures. A resilient invertebrate community and possible differences in emergence timing finer than the sampling resolution are potential explanations for this unexpected result.