The variously modified flow patterns below dams are considered in relationship to theeffects on ecological factors of importance to the benthic communities of receiving streams.Species composition and diversity are considerably modified by upstream impoundments.Benthic standing crop may be enhanced or reduced, largely depending on the flow regime. Dailyflow fluctuations, if not too sever, may be associated with dense benthic populations as long as arelatively constant seasonal flow pattern is maintained. Little is known regarding subtle, sublethaleffects of dams onlife cycle phenomena and biotic interactions and more data are needed oncurrent preferenda of important fish food species. Any flow regime which significantly reduceshabitat diversity should be avoided. A diverse substrate with silt-free interstices will considerablyreduce deleterious effects of periods of reduced flow, fluctuating flow and high current velocity. Inestablishing flow criteria for benthos, each dam must be considered individually.