|Abstract or Summary
- 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 water temperature of up to 4.6°C from upstream to downstream, emergence timing was unaltered for H. borealis, P. confusalis, and G. traviatum. However, in a laboratory experiment, higher
temperatures led to earlier emergence for H. borealis. With the exception of Hydroptilidae, emergence timing was not correlated with discharge or water temperature for all 8 taxonomic families on each river, and the timing of total emergence remained similar among sites. Similarly, water temperature was not significantly correlated with reduced adult body size of
these species, though female P. confusalis were 31% smaller at the warmest sample site.
Total emerging biomass was reduced at sites subject to high water withdrawals, particularly on the more heavily diverted Umatilla River.
The holometabolous life histories of the three tested species and the occurrence of
water withdrawals for decades prior to this study are possible explanations for the observed lack of response to seasonal alterations in discharge and water temperature. These results suggest that the life histories of some aquatic invertebrates in Western rivers may be resistant to severe water withdrawals, but important ecological effects may exist through the reduced
export of emerging biomass into the riparian ecosystem.