The Geology and History of Mono Lake

Mono Lake Basin was formed by the same geologic processes that shaped the Nevada and Eastern Sierra landscape over the past several million years. Extension, or pulling apart, of the Earth's crust has created north/south trending mountain ranges flanked by deep, long valleys. Later volcanism (around 2.5 million years ago) poured thick accumulations of molten rock (basalt) into the valleys, shaping the landscape and creating natural dams that controlled the flow of water.

The basin began filling with water around ¾ of a million (750,000) years ago, or around the time that the Long Valley Caldera violently erupted to the southeast of the lake. Water sourced from melting ice sheets located on the eastern flank of the Sierra Nevada Mountains flowed downslope and was captured by the basin. During this time in the Earth's history, the region's climate was cooler and wetter, limiting evaporation from the lake and providing abundant precipitation. These climatic conditions likely persisted until the end of the latest ice age (around 11,000 years ago), when Mono Lake reached it's greatest depth of 900 ft. At the termination of this ice age, the climate naturally shifted to drier, warmer conditions and the alpine ice sheets became extinct, cutting off a major supply of fresh water to the basin.

Mono Lake is what geologists refer to as an endorheic basin, meaning that water flows into the lake, but the lake has no outflow. With no outflow, salts accumulate in the lake as water evaporates, making the water body hypersaline (more salty than the ocean) and creating a very unique ecosystem.

Volcanic activity has been ongoing in the Mono Lake Basin for at least the past 2.5 to 3 million years. The most recent volcanic eruptions, which occurred only 400 years ago, are related to the conical cinder cones of Mono Craters to the south of the lake (flanking U.S. Route 395). These 'modern' eruptions formed the Paoha Island in the center of the lake, which is now an important breeding ground for a variety of song and sea birds.

Along the southern shoreline of Mono Lake, large tufa towers or pinnacles rise above the water's surface. These iconic pillars, comprised of precipitated calcium carbonate, formed over thousands of years by the interaction of freshwater springs and the highly alkaline waters of Mono Lake. Today, several of the large tufa towers are nesting sites for predatory Osprey, and bird watching is strongly encouraged. Annual events such as the Mono Basin Bird Chautauqua highlight the region’s remarkable bird populations and migratory patterns.

A small band of the Northern Paiute Tribe known as the Kucadikadi, or pupae eaters, were some of the first humans to inhabit the Mono Lake region. These indigenous people were primarily supported by the lake, obtaining a substantial amount of their nutrition from the larvae of the alkali flies that live in and around the muddy shorelines. The Kucadikadi were exceptional basket weavers and by the 20th century, many of the remaining tribe members relied on the tourist trade for existence.

In the early 1940's, Eastern Sierra runoff water that would have naturally fed Mono Lake was diverted into the Los Angeles Aqueduct system. Without adequate inflow and recharge from the runoff, evaporation quickly caused lake levels to drop and water salinity levels to increase. By 1990, the lake had lost over half of its water volume and salinity levels doubled, creating an ecological disaster and health hazard (due to airborne alkaline dust) for the region.

In 1994, the California State Water Resources Control Board issued orders to protect the Mono Lake Basin in response to outcries by several conservation groups, including the Mono Lake Committee, which played a major role in advocating for water level restoration and long-term ecosystem protection.

Although the lake will likely never fully recover to its pre-diversion levels, lake levels have steadily risen and target lake levels may be reached by the end of the decade. On the positive side, the lowering of the lake exposed the spectacular and photogenic tufa towers that characterize the lake and mesmerize visitors today.

The natural beauty of this desert water body is as diverse as its history. For additional information about the geologic, natural, and early anthropogenic history of the region, visit the Mono Basin National Forest Scenic Area Visitor Center. This facility, located just off of US Route 395 north of Lee Vining, CA., has extensive exhibits informing visitors about the natural and human-influenced history of the lake and seasonal guided group tours of the area are also available.