Ancient Lake Lahontan was an enormous endorheic lake that once covered much of northwestern Nevada and portions of northeastern California. At its peak approximately 15,000 years ago, the lake had a surface area of about 8,000 square miles (2,070,000 hectares). The depth of the lake was approximately 800 feet (240 meters) at present day Pyramid Lake, and 500 feet (150 meters) at the Black Rock Desert.

Climate change around the end of the Pleistocene epoch led to a gradual dessication of ancient Lake Lahontan. Modern day remnants include Pyramid Lake, Walker Lake and Winnemucca Lake in Nevada, and Honey Lake in California. The ancient shoreline is evidenced by tufa formations throughout the region.

Modern Lake Lahontan is a man-made reservoir maintained by the Truckee-Carson Irrigation District (TCID). It is located between Fallon, Nevada and Carson City, Nevada. It is directly fed by the Carson River, and by diversions from the Truckee River.