Ubiquitous hydrological alterations - dam construction and associated water diversion, exploitation of groundwater aquifers, stream channelization, and intercatchment water transfer - are producing global-scale effects on the environment. The articles in this special issue of BioScience highlight the cumulative effects of hydrological alterations associated with dam and reservoir development. Such information is critical for deciding whether, when, and where the next major hydrological project will be built; it can also warn us about impending environmental impacts. The articles in this issue are based on presentations at the 1998 symposium. The series has the following objectives: to synthesize as much information as possible on large-scale environmental effects of dams and reservoirs, to identify knowledge gaps and research needs to improve our understanding of global-scale effects, and to produce currently available information in a form that is readily accessible to policymakers. http://www.ucpressjournals.com/journal.php?j=bio