Environmental flow protections are crucial to the conservation of freshwater biodiversity in dam-regulated river systems. Nevertheless, the implementation of environmental flows has lagged far behind the pace of river ecosystem alteration. The vast number of dams now in operation and the substantial resources required to modify their operations suggest that a systematic approach is needed to identify rivers potentially benefiting from environmental flows. Here, we develop and apply a screening framework that is focused on the selection of dams that warrant environmental flows on the basis of evidence of hydrologic alteration and ecological impairment. Our approach is illustrated for California, where 181 (of an initial 1400+) dams are identified as high-priority candidates to improve environmental flows for native fish conservation. Given the rapid declines of freshwater biodiversity and pervasive alteration to rivers, this systematic screening approach should be useful for guiding the strategic implementation of environmental flows.