Long-term dynamics of an invertebrate assemblage downstream from a large dam

A century of hydrologic data (1895-1999) and 50 yr of aquatic macroinverteb rate assemblage data (1947-1999) were examined for two tailwater reaches of the Green River downstream from Flaming Gorge Dam in northeastern Utah, US A (40 degrees 54' N, 109 degrees 25' W). One reach was located upstream of an intermittent tributary, and the other downstream. The purpose of the stu dy was to chronicle long-term dynamics and the effect of partial thermal re storation on invertebrate assemblages. The immediate hydrologic effect of t he dam was a large decrease in annual maximum daily discharges, water tempe ratures, and sediment transport. Upstream of the intermittent tributary, ma croinvertebrate genera declined from > 70 to < 30, and the mean macroinvert ebrate density increased from 1000 to 10 000/m(2) after dam closure. In 197 8, a multilevel water intake structure was installed on the dam. Mean summe r water temperatures increased from 6 degrees to 12 degreesC, and the numbe r of annual degree days warmed from 2340 to 3200, which was similar to pre- dam conditions; but the rate and timing of warming remained different than before the dam. In contrast to an expected increase in taxon richness, the number of taxa routinely collected after partial thermal restoration was si milar to or lower than that observed before thermal restoration. Downstream of the intermittent tributary, taxon richness was about twice that found u pstream after dam closure, and the mean annual per-sample taxon richness in creased-from 7.6 to 11.0 following partial thermal restoration. In both rea ches, invertebrate densities were near 10 000/m2 before and after thermal r estoration. The lack of an appreciable increase in insect taxon richness upstream from an intermittent tributary following partial thermal restoration is likely d ue to the combined effects of three factors: (1) the competitive dominance of insect taxa by amphipods, (2) low rates of immigration and colonization, and (3) low reproductive success of insects due to a few degrees differenc e in the water temperatures between the regulated river and natural streams in the area. These results suggest that we should not only evaluate tradit ional habitat attributes, but biological interactions as well, when determi ning or monitoring the effects of river regulation an aquatic biota.