Environmental laws like the Clean Water Act, which regulate the quality of water, are increasingly being used to regulate the quantity (streamflow) of water as well. Recently the federal Clean Water Act has been revealed as a potential obstacle to water suppliers who need to withdraw water or to construct water supply projects. Until recently, the question of securing an adequate supply of raw water was a matter governed not by environmental laws but by other areas of law, called "water rights law" or "riparian law." This paper discusses two recent court cases which show to what extent the environmental laws can be used to limit or prevent the withdrawal of water, usually in the name of protecting fish or wildlife. In both cases, a federal permit was needed to build a dam, and in both cases the permit became a vehicle for imposing conditions that either restricted or prevented the withdrawal of water.