Abstract
The dam construction has substantial impacts on the downstream environment even from the beginning of construction stages. The stage of impounding water is supposed to have the largest effects on the downstream environment. Monthly observation was carried out during the trial impoundment of a large concrete dam, including water temperature, organic matter fluxes, epilithic chlorophyll a, and macroinvertebrate communities both at the upstream and the downstream water of the impoundment. Both before and after the impoundment, the epilithic chlorophyll a contents and fine particulate organic matter (FPOM) fluxes variations were insignificant As the impoundment volume increased the downstream water temperature substantially increased in comparison to upstream water temperature. Coarse particulate organic matter (CPOM) fluxes (gDW/10min) in the downstream became lower than that of the upstream even with one day of retention period. Chironomidae, Stenopsychidae, Simuliidae, Tipulidae and Hydropsychidae substantially enhanced immediately after the impoundment started, whereas, Taeniopterygidae and Chloroperlidae were declined. It was evident that the trial impoundment has a substantial effect on the downstream biota even from the low volume of water.