| Abstract: | The influence of stream regulation on environmental conditions and concomitant alterations of macroinvertebrate community structure was determined from field studies conducted from September 1981-June 1983 at three sites on the upper Colorado River. Site 1 (reference site) was located above Granby and Shadow Mountain Reservoirs, a deep-release storage impoundment, whereas sites 2 (regulated site) and 3 (recovery site) were located 0·4 and 4·0 km, respectively, below the dam. Although macroinvertebrate diversity was reduced at the regulated site compared to both the reference and recovery sites, the number of taxa (43) was considerably higher than values reported from studies of other regulated streams in the Rocky Mountains. Macroinvertebrate mean annual density in the regulated site was twenty times higher than at the reference site and slightly higher than the recovery site. The regulated site was characterized by the absence of heptageniid mayflies, reductions in stoneflies, caddisflies, shredders, and predators, and high densities of Baetis spp., Ephemerella infrequens, chironomids, and non-insect taxa. Many of these faunal changes are attributed to alterations in the temperature regime induced by regulation and to changes in the source and temporal sequencing of organic detritus. Although the number of annual degree days was actually greater below the dam than above the reservoir, other components of the thermal regime was severely altered by regulation. At the regulated site the primary source of coarse organic detritus was autochthonous (decaying algae) with a vernal pulse, in contrast to the typical autumnal pulse of allochthonous leaf litter. There was no evidence that the greater substrate permeability and flow predictability below the dam directly influenced the reduction of species. |
