Mono Lake is a saltwater lake that supports a large population of shrimp and has two islands that serve as resting and nesting areas for several bird species. The lake is fed by snowmelt from the Sierra Nevada via five streams. In 1940, the Department of Water and Power of the City of Los Angeles (DWP) was granted a permit to divert freshwater from four of these streams. As a result, the water level of the lake dropped, a third of its surface area dried completely, and one of the islands became a peninsula. This gave land predators access to the birds’ nests, forcing the nesting birds to abandon the island. The National Audubon Society (Audubon) (plaintiff) sued to enjoin DWP from further diverting the flow into Mono Lake, arguing that the lake, island, and shores were protected by a public trust. The Superior Court of Alpine County granted summary judgment for DWP, holding that the public-trust doctrine provided no independent basis for enjoining the diversion of water. Audubon petitioned the California Supreme Court for a writ of mandate.