The California Water Project (CWP) consists of a series of water storage and transportation facilities. The water which will be used by the CWP is initially stored behind dams on the Sacramento River and, as needed, released into the Sacramento-San Joaquin Delta. CWP then diverts a quantity of this water from the Delta and directs it into canals and aqueducts which will carry it south. Water is drawn from the Delta by the Delta Pumping Plant and deposited in the northern terminus of the California Aqueduct. Salt waters from the Pacific Ocean find their way into the Delta and degrade the Delta waters. D has proposed the construction of a 42-mile Peripheral Canal along the eastern edge of the Delta area, which would avoid any mixing of the water from the north with the saline water of the Delta. Depositing water in the proposed canal would carry high quality water directly to the Tracy and Delta Pumping Plants. Sierra (Ps) commenced the present action alleging that present and proposed diversions of water from the Delta degraded the quality of Delta water and that such diversion violated §10 of the Rivers and Harbors Appropriation Act of 1899. Ps sought to enjoin further operation or construction of water diversion facilities until the consent of the Army Corps of Engineers was obtained as required by the Act. The District Court held that Ps could avail themselves of a 'private cause of action' to enforce §10 of the Act. The Court of Appeals agreed that a private cause of action to enforce the Act existed. The Supreme Court granted certiorari.