Clear, cold water tumbles from snowcapped peaks and alpine fields at 13,000 ft (4,000 m) down the precipitous eastern escarpment of California's Sierra Nevada. The water feeds semi-arid, sagebrush–covered Mono Basin and, at the basin's heart, majestic Mono Lake. This is a salt lake with an area of 60 square miles (155 km²) at an elevation of 6,400 ft (2,000 m), and mountain waters have flowed to it for at least the past half million years, making it one of the oldest lakes in North America.

Mono Basin lies in the rain shadow of the Sierra Nevada; shielded from moist Pacific air masses to the west, it has a semi-arid climate. Being surrounded by higher ground, it also has no natural outlet. Over the millennia, the major water loss from Mono Lake has been by evaporation, a process that removes pure water and leaves dissolved salts behind, and the water in...