Pyramid Lake occupies a large, north-south oriented endorheic graben wholly within the Pyramid Lake Paiute Indian Reservation at the western margin of the Great Basin Desert. It is the largest remnant of pluvial Lake Lahontan, which at maximum Pleistocene development was about 22,300 km². Presently, Pyramid Lake is the deepest terminal saline lake in the western hemisphere. Modern climate is typical of a mid-latitude steppe with low annual precipitation, most occurring during winter. Pyramid Lake receives about 85% of its annual water input from Truckee River, whose origin is Lake Tahoe in the Sierra Nevada mountains to the southwest. The lake is slightly saline and moderately alkaline, about 5 g l^-1 salinity and 23 meq l^-1 alkalinity. Order of cation abundance is: Na+ > K+ >= Mg2+ > Ca2+, and among anions: Cl- > HCO₃^- > CO₃^2- >SO₄^2- . Dissolved orthophosphate concentrations are moderately high ( >= 70 mg P m^-3), while dissolved inorganic nitrogen concentrations are low ( <= 100 mg N m^-3), yielding low N:P ratios, indicative of potential nitrogen limitation.

Lakewide precipitations of calcium carbonate ("whitings") occur irregularly (29), and massive calcium carbonate "tufa" deposits surround the lake. One of these tufa formations provides the lake's name (3). Anaho Island is the only major island, and hosts large nesting colonies of colonial birds, most notably, white pelicans (Pelecanus erythrorhynchos). Primary productivity is low for a saline lake, although moderate (about 500 mg m^-2 day^-1) compared with freshwater lakes (10). Large summer-autumn blooms of the nitrogen fixing cyanobacterium, Nodularia spumigena, occur regularly and contribute large amounts of nitrogen to the lake's annual nitrogen budget (1, 2).

Pyramid Lake was first recorded by Anglo Americans in 1844, who found the Native Americans consuming two endemic fishes, cui-ui (Chasmistes cujus) and Lahontan cutthroat trout (Salmo clarki henshawi). The Pyramid Lake Paiute Indian Reservation was established in 1859, following a brief period of armed conflict (3). Disputes over land, water and fisheries rights continued between Paiutes and Anglos and persist today as water rights and water quality litigation between the Pyramid Lake Paiute Tribe and upstream water users (4). Derby Dam was constructed in 1905 on the Truckee River, 56 km upstream from Pyramid Lake. About 50% of the annual discharge to the lake was subsequently diverted out of the catchment basin for agriculture. Pyramid Lake's elevation declined 26 m between 1905 and 1967 and its salinity increased from about 3.7 g l^-1 to its present level. Pyramid Lake supported a nationally renown sport fishery for Lahontan cutthroat trout prior to the 1940's and retains the world's sport-fishing size record for this species. However, as the lake level declined a large delta formed at the terminus of Truckee River, preventing spawning migrations of cui-ui and cutthroat trout. This delta and Derby Dam impeded upstream migration resulting in demise of the Lahontan cutthroat trout in 1938 (5). Lahontan cutthroat trout were reintroduced in the 1950's and a trophy sport fishery reestablished, although all recruitment is presently by artificial propagation. As a result of declines in both species, cui-ui were federally listed as endangered and Lahontan cutthroat trout as threatened. Major recovery efforts for both species are currently underway. Additional environmental concerns include maintaining lake water quality as populations upstream in the Truckee River watershed (primarily Reno and Sparks, Nevada), rapidly expand (Q).