Mono Lake ( ) is a saline soda lake in Mono County, California, formed at least 760,000 years ago as a terminal lake in an endorheic basin. The lack of an outlet causes high levels of salts to accumulate in the lake which make its water alkaline. The desert lake has an unusually productive ecosystem based on brine shrimp, which thrive in its waters, and provides critical habitat for two million annual migratory birds that feed on the shrimp and alkali flies (Ephydra hians). Historically, the native Kutzadika'a people ate the alkali flies' pupae, which live in the shallow waters around the edge of the lake. When the city of Los Angeles diverted water from the freshwater streams flowing into the lake, it lowered the lake level, which imperiled the migratory birds. The Mono Lake Committee formed in response and won a legal battle that forced Los Angeles to partially replenish the lake level. Geology Mono Lake occupies part of the Mono Basin, an endorheic basin that has no outlet to the ocean. Dissolved salts in the runoff thus remain in the lake and raise the water's pH levels and salt concentration. The tributaries of Mono Lake include Lee Vining Creek, Rush Creek and Mill Creek which flows through Lundy Canyon. The basin was formed by geological forces over the last five million years: basin and range crustal stretching and associated volcanism and faulting at the base of the Sierra Nevada. Five million years ago, the Sierra Nevada was an eroded set of rolling hills and Mono Basin and Owens Valley did not yet exist. From 4.5 to 2.6 million years ago, large volumes of basalt were extruded around what is now Cowtrack Mountain (east and south of Mono Basin); eventually covering and reaching a maximum thickness of . Later volcanism in the area occurred 3.8 million to 250,000 years ago. This activity was northwest of Mono Basin and included the formation of Aurora Crater, Beauty Peak, Cedar Hill (later an island in the highest stands of Mono Lake), and Mount Hicks. Mono Lake is believed to have formed at least 760,000 years ago, dating back to the Long Valley eruption. Sediments located below the ash layer hint that Mono Lake could be a remnant of a larger and older lake that once covered a large part of Nevada and Utah, which would put it among the oldest lakes in