Mono Lake, a striking water body set against the dramatic backdrop of the Eastern Sierra Nevada mountains, represents a unique intersection of natural history and human engineering. This ancient, saline soda lake is a closed system (an endorheic basin), meaning water flows in but has no natural outflow. The lack of an exit ensures that dissolved salts and minerals accumulate over time, making the water highly alkaline and roughly twice as salty as the ocean. This unusual chemistry supports a specialized, productive ecosystem of brine shrimp and alkali flies, which in turn sustain millions of migratory birds. The complexity of the lake’s water source is defined by its natural geography, which has been profoundly altered by the needs of a distant metropolis.
The Mono Basin Natural Sources of Inflow
The reservoir of fresh water that originally feeds Mono Lake originates high above the basin, primarily from the annual snowpack of the Sierra Nevada. As winter snow melts during the warmer spring and summer months, it generates the vast majority of the lake’s natural inflow, accounting for approximately three-quarters of the total incoming volume. This snowmelt is efficiently captured by the Mono Basin watershed, a funnel-shaped drainage area stretching across 780 square miles. The average total inflow to the lake is estimated to be around 230,000 acre-feet per year under natural conditions.
Five major perennial streams carry this frigid Sierra runoff down the steep eastern escarpment into the lakebed. Rush Creek and Lee Vining Creek are the two largest contributors, delivering the bulk of the surface flow. Mill Creek is the third largest, while Parker Creek and Walker Creek also contribute significant volumes of water.
These creeks flowed through expansive riparian zones and formed delta habitats before merging with the lake’s saline waters. The natural flow regime involves stable, low flows in the winter, which then swell into powerful surges during the peak snowmelt period. Direct precipitation falling onto the lake’s surface and groundwater seepage contribute a smaller, secondary input to the lake’s overall water budget.
The Los Angeles Aqueduct System and Diversion
The natural flow of these Sierra Nevada streams was fundamentally interrupted in the early 20th century by an ambitious water infrastructure project designed to supply a rapidly growing city. In 1941, the Los Angeles Department of Water and Power (LADWP) completed the Mono Basin Extension, integrating the basin’s water into the existing Los Angeles Aqueduct system. This extension involved the construction of the Mono Craters Tunnel, which redirected the freshwater away from the lake.
The diversion infrastructure intercepted the waters of four major tributary streams: Rush Creek, Lee Vining Creek, Parker Creek, and Walker Creek. Water was captured at diversion dams, most notably at the Grant Lake Reservoir on Rush Creek, and routed south through a system of tunnels and canals. This allowed for the transport of Mono Basin water over 300 miles to Los Angeles, utilizing gravity for much of the journey.
The environmental consequence of this diversion was immediate, as the rate of evaporation from the lake soon exceeded the remaining inflow. Between 1941 and 1982, the lake level dropped by approximately 45 feet, exposing over 15,000 acres of lakebed. This drop doubled the water’s salinity, stressed the ecosystem, and turned former islands, which were nesting sites for California Gulls, into peninsulas accessible to land predators like coyotes.
Legal Mandates and Current Restoration Flows
Decades of litigation over the diversions led to a series of landmark decisions that altered the legal status of the lake’s water. The most significant was the 1994 ruling by the California State Water Resources Control Board, Decision 1631 (D-1631). This decision established a public trust mandate, recognizing the necessity of restoring and protecting Mono Lake and its tributary streams.
The ruling set a target water elevation for the lake, known as the “management level,” at 6,392 feet. This level is the minimum required to protect the lake’s ecosystem, reduce salinity, and re-establish a protective moat around the California Gull nesting islands. To achieve this, D-1631 imposed strict limits on the amount of water LADWP could export, establishing minimum required stream flows for the diverted creeks.
The water currently flowing into Mono Lake is a managed combination of natural runoff and legally mandated releases. LADWP must release sufficient water into the lower sections of Rush, Lee Vining, Parker, and Walker Creeks to meet the required stream flows and contribute to the lake’s gradual rise toward the 6,392-foot goal. While the water still originates as Sierra Nevada snowmelt, the volume reaching the lake is only a fraction of the historical natural input, dictated by the regulatory framework of D-1631.