Abstract
There has been an increasing effort to construct dams across rivers to control the flow of the streams and secure water storage, and river damming has become one of the most prominent human impacts on the freshwater ecosystem. Nonetheless, the effect of river damming on the river ecosystem is only partially understood in Ethiopia. This study is aimed to assess the ecological effect of small dams on macroinvertebrate assemblages and the water quality of the Koga River ecosystem. A total of 15 sites; five from upstream, five from the dam, and five from downstream of the Koga River was sampled for macroinvertebrates and water quality parameters. The sampling was done from September to November 2016. A total of 40 families of macroinvertebrates were recorded, among which Coenagrionidae, Belostomatidae, Naucoridae, and Physidae were the most abundant. The biodiversity of macroinvertebrates was significantly higher in the downstream location of Koga Dam, where there was an attenuated sediment load to the river. Among the functional feeding groups, the percentage of filterer-collectors was higher in the upstream locations, whereas scraper families were higher in the downstream locations of the dam. Vegetation cover, turbidity, and pH were identified as the most important water quality factors delineating the pattern of macroinvertebrate community structure in the river system. Turbidity and orthophosphate concentrations were higher in the upstream sampling locations. The average sediment layer thickness was higher on the upstream side of the dam. The results suggest that sediment adversely affects the macroinvertebrate assemblage. Higher concentrations of sediment and phosphate were noted in the upstream location of the dam. The water quality (turbidity and nutrient concentrations) of the stream was affected by River Damming by influencing the sediment and nutrient dynamics of the river. Therefore, planning and implementation of an integrated watershed and dam management system are suggested to prolong the useful lifespan of the dam and sustain its ecological integrity.