We investigated the ecological influence of the Atago weir (Hirose River, Miyagi Pref., Japan) on the riverine benthic faunal community at 21 points in the upstream river section (?1000m) with locally varying hydraulic conditions (flow=-0.08-1.03m/s, depth= 19-179cm). The weir formed a semi-lentic environment (flow =-0.120.27m/s, depth =22179cm) in the upstream section for 700m river length. Silt layer partially emerged in the right bank side of the semi-lentic section. Density (N=33-5317/m2), taxa richness (S=1 - 27 taxa), and Simpson' diversity index (1-D=0.00 - 0.92) of the communities widely varied among the 21 points. The multiple regression analysis indicated that the spatial variations of abundance (N) and species diversity (S, 1-D) were mainly accounted for by physical environmental factors such as flow velocity, depth, and mean grain diameter of the sediment, etc.
