The aim of the paper was to find main traits of river ecosystem (river channel morphology, anchorage features and aquatic plants) responsible for occurrence of bivalves from threatened family Unionidae. River channel depth, width, channel slope and sinuosity, substratum type, as well as aquatic plant and bivalve occurrence were studied along 60 transects across the river channel. 4 species of Unionidae occurred mostly in shallow and wide sections of the river, with lesser depth variance, smaller slope, in mud bars adjacent to banks. They were also present on transects with larger layer of mud, and more frequently on transects overgrown with Potamogeton. Results show that main features of river morphology and presence of mud deposits, especially covered with Potamogeton sp., influence clams distribution. These data have direct reference to river management in areas of occurrence of clams from threatened family Unionidae.