Downstream variations of benthic macroinvertebrate communities on the River Rede below Catcleugh Reservoir, impounded in 1905, are related to a simple index of regulation magnitude, related to hydrological, water quality and trophic changes, and variations of channel morphology. The river is divided into five sectors on the basis of network morphometry and channel form. Channel change to flow regulation is confined to a short (1.6 km) reach immediately below the dam within which channel width and conveyance have been reduced by 53 and 75%, respectively. Downstream, siltation has occurred, but it is patchy and average levels of sediment finer than 2 mm are less than 10%. The main faunal responses — increased abundance and number of taxa — are also confined to the first sector below the dam. Ephemeroptera are shown to be the best describers of the downstream pattern. Variations of the fauna between riffles within sectors are shown to relate to site-specific hydraulic conditions, especially bed shear stress.