Mono Lake is a large, shallow saline soda lake in Mono County, California, formed at least 760,000 years ago as a terminal lake in a endorheic basin. The lack of an outlet causes high levels of salts to accumulate in the lake. These salts also make the lake water alkaline.
This desert lake has an unusually productive ecosystem based on brine shrimp that thrive in its waters, and provides critical nesting habitat for two million annual migratory birds that feed on the shrimp.
The human history of Mono Lake is associated with its productive ecosystem. The native Kutzadika'a people derived nutrition from the larvae of the alkali flies that live in the lake. When the city of Los Angeles diverted water from the lake, it lowered the lake level, which imperiled the migratory birds. The Mono Lake Committee formed in response, winning a legal battle that forced Los Angeles to partially restore the lake level.
Mono Lake formed because it lies in 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 created 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 erupted around what is now Cowtrack Mountain (east and south of Mono Basin); eventually covering 300 square miles (780 km2) and reaching a maximum thickness of 600 feet (180 m). 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 North America. At its height during the most recent ice age, the lake may have been 900 feet (270 m) deep. Prominent old shore lines, called strandlines by geologists, can be seen above Lee Vining and along volcanic hills northeast of the current lake.
Currently, Mono Lake is in a geologically active area at the north end of the Mono–Inyo Craters volcanic chain and is close to Long Valley Caldera. Volcanic activity continues in the Mono Lake vicinity: the most recent eruption occurred 350 years ago, resulting in the formation of Paoha Island. Panum Crater (on the south shore of the lake) is an excellent example of a combined rhyolite dome and cinder cone.