Small, low-head diversion dams are capable of withdrawing much of the flow of a river, often resulting in increased water temperatures and habitat loss. Accelerated growth and development of aquatic invertebrates has been demonstrated in warmer temperatures, suggesting that the timing of insect emergence and adult body size may be significantly altered by water withdrawals.  To examine the influence of water withdrawals on aquatic invertebrate life histories, timing of summer emergence and biomass was monitored continuously for 10 weeks on the Umatilla and Walla Walla rivers in arid northeastern Oregon.  Multiple sample sites were located along gradients of decreasing discharge and increasing water temperatures caused by successive diversions on each river.

Despite an 89% reduction in discharge and up to 7˚C increase in maximum water temperatures from upstream to downstream, emergence timing differed for only 3 of ­­­9 taxonomic families.  Elevated temperature was correlated with reduced body size of adult Petrophila confusalis, despite no change in emergence timing.  In laboratory experiments, higher temperatures led to earlier emergence for Helicopsyche borealis, though this relationship not observed in the field.  Total reach biomass of emerging insects was lower at sites of high withdrawal due to decreased wetted width.