Effects of a tributary confluence on riverbed environments and macroinvertebrate assemblages in the areas downstream of dams

In river ecosystems, the local effects of large dams on environments and aquatic organisms have been frequently reported. The processes leading to such effects could be generally classified into the following three: a reduction of sediment transport, a modification of flow fluctuations and thermal regime, and an alteration in the energy base. However, the relative importance of each of the processes in determining the changes of organisms remains unknown. The quantitative and comparative understanding of the intensity of variables related to each process is important for prioritizing rehabilitation measures for aquatic organisms along the dammed rivers. This study examined the effects of a dam and of a downstream natural tributary inflow on benthic macroinvertebrate assemblages and their riverbed habitats. In downstream reaches of 10 dams in the Japanese Archipelago, we performed field surveys at stations before and after a tributary joined a mainstream whose upstream section was dammed. As for the environmental variables, fine sediments on riverbed decreased at the station downstream of dams, and the sediment volume tended to recover after the tributary confluence. Macroinvertebrate assemblages also showed compositional shifts over the confluence; for example, burrowers, who prefers sand and/or gravel habitat, increased conspicuously at the stations after the tributary confluence. The results showed that such effects of tributaries varied in magnitude, depending on a size ratio of the tributary to the mainstream, a distance from the dam to the tributary confluence, and a scarcity of fine sediments at the station after the tributary confluence.