Effects of an experimental flood on invertebrates in the Colorado River

Glen Canyon Dam (GCD) has altered the flow regime of the Colorado River, increasing daily variability and dampening seasonal variability in discharge. We measured the effects of an experimental flood (March, 2008) on invertebrate assemblages downstream of GCD. We hypothesized that increased discharge would reduce invertebrate densities and biomass in the short-term (i.e., weeks) due to increased scour and drift. Secondarily, we hypothesized that a more productive and higher quality post-flood algal community would lead to increased invertebrate growth rates, offsetting the effects of increased discharge on invertebrate biomass in the longer-term (i.e., months). We measured benthic invertebrate abundance and biomass of dominant invertebrate taxa at 3 sites for 2 years prior to the flood, one day before, and 1, 5, 15, 30, and 60 days after the flood. Further, we measured invertebrate drift in the tailwaters of GCD during the flood to examine export. Our results demonstrate that while invertebrates, in particular the exotic New Zealand mud snails, were exported in the drift, reductions in benthic assemblages were temporary and within annual variation. Our data corroborate earlier findings that experimental floods do not have negative consequence for the predominantly non-native invertebrate taxa inhabiting the Colorado River.