The impounded water behind the dam forms Folsom Lake, with a normal maximum pool of and a surcharge capacity of , for a total capacity of . The original capacity was , but it has been reduced somewhat due to sedimentation. At its maximum elevation of , the reservoir covers , with of shoreline. The dam and reservoir control runoff from an area of , or 87.6% of the American River watershed. The average amount of runoff entering the reservoir is , forcing the release of for flood control.