Mono Lake, situated in California’s Eastern Sierra, is an ancient, unique saline lake. It formed in an endorheic basin over 760,000 years ago, meaning water flows in but does not flow out, leading to the accumulation of salts and minerals. This geological characteristic results in hypersaline (saltier than the ocean) and alkaline waters, supporting a distinct ecosystem. The lake’s striking tufa towers, formed by calcium-rich freshwater springs interacting with the alkaline lake water, add to its natural beauty. Mono Lake also serves as a significant habitat for millions of migratory birds, providing a crucial resting and feeding stop along the Pacific Flyway.
The Great Water Diversion
In 1941, the Los Angeles Department of Water and Power (LADWP) began diverting water from the freshwater streams that feed Mono Lake. This action was part of an extension of the Los Angeles Aqueduct system, designed to supply water to the growing city of Los Angeles. The diversions significantly reduced the natural freshwater inflow into the lake. Mono Lake’s water level dropped over the next four decades, falling by approximately 45 vertical feet and losing half its volume by 1982. This reduction in water volume led to a nearly doubled salinity concentration, altering the lake’s delicate chemical balance.
Ecological and Environmental Fallout
The increased salinity and decreased water levels had severe consequences for Mono Lake’s ecosystem. The lake’s unique inhabitants, such as brine shrimp and alkali flies, which form the base of the food chain, were significantly impacted. Their productivity declined as the water became saltier, threatening the food supply for vast numbers of birds.
The shrinking lake devastated migratory bird populations, particularly the California Gulls. As the water level dropped, Negit Island, a historical nesting site for thousands of gulls, became connected to the mainland by a land bridge. This connection allowed predators like coyotes to access the nesting colonies, leading to significant nesting failures and forcing gulls to abandon the site.
The receding water also exposed vast areas of the lakebed, leading to the emergence of tufa towers that were once submerged. These exposed lakebed sediments, rich in alkali, became a source of toxic dust storms on windy days. These dust storms caused significant air quality issues, with particulate matter violating federal Clean Air Act standards, making it one of the worst air pollution sources in the nation.
The Fight for Restoration
The ecological crisis at Mono Lake spurred a significant public and legal response. In 1978, a group of concerned individuals, including students who had conducted ecological studies of the lake, founded the Mono Lake Committee. This environmental organization began advocating for the lake’s protection and restoration.
The Mono Lake Committee, alongside the National Audubon Society, initiated legal action against LADWP in 1979. These lawsuits argued that the water diversions violated California’s Public Trust Doctrine. This doctrine asserts that certain natural resources, like navigable waters, are held in trust by the state for the public’s benefit, encompassing values such as recreation, aesthetics, and ecology.
In a landmark 1983 decision, the California Supreme Court ruled that the Public Trust Doctrine applied to Mono Lake, even if it meant reconsidering established water rights. This ruling established that the state had an obligation to protect the lake’s public trust values. The court’s decision was a turning point, emphasizing that the public’s interest in natural resources needed to be balanced against water supply needs.
Path to Recovery
The legal and advocacy efforts culminated in a significant decision by the California State Water Resources Control Board. On September 28, 1994, the Board adopted Decision 1631, which mandated a minimum elevation for Mono Lake to ensure its ecological health. The target elevation was set at 6,392 feet above sea level, a level deemed necessary to sustain the ecosystem and mitigate environmental problems.
This decision imposed limitations on LADWP’s water diversions from Mono Lake’s tributary streams. It also required the city to implement stream rewatering programs to restore the damaged aquatic habitats. The State Water Board’s ruling was designed to balance Los Angeles’s water needs with the protection of Mono Lake’s public trust resources.
Mono Lake has been gradually recovering since the 1994 decision, with its level rising by approximately 8 feet. However, it remains below the mandated 6,392-foot target, with the lake elevation around 6,383 feet as of August 2025. Ongoing monitoring and efforts continue, as the lake’s full recovery is dependent on sustained water management and natural precipitation.