Macroinvertebrate colonization of replacement substrate below a hypolimnial release reservoir

Loss of substrate from occasional high flows below reservoirs on the Trinity River prompted government agencies to replace the gravels to protect the anadromous fisheries resource. Several riffles were restored using 2.5 to to cm gravels. Three of these and two natural riffles, with gravels ranging up to 20 cm, were surveyed over a two-year period to determine the effect of this gravel replacement on the macroinvertebrate fauna. Replaced gravels were too small to support the diversity of organisms found on natural riffle gravels. In addition, temperature alteration by the hypolimnial release reservoir prevented colonization by all but a few organisms able to tolerate such conditions in the riffles below the dam. Large but unstable populations of Simulium, Baetis, and the Chironominae developed below the dam, aided by an abundant periphyton and detrital food supply and lack of competition and predation. It is concluded that habitat diversity through the use of a range of gravel sizes up to the size naturally found in productive riffles should be used in riffle restoration projects in order to increase habitat complexity, species diversity, density, and biomass important in maintaining fishery stocks.