During the Wisconsin Glaciation, the last glacial period of the Pleistocene Epoch, a large lake covered most of northwest Nevada, extending into southwestern Oregon and northeastern California. Fed by the Truckee, Walker, Carson and Humboldt Rivers (in addition to numerous smaller streams), this pluvial lake, known to geologists as Lake Lahontan, covered 8500 square miles and had a maximum depth of 900 feet; only the mountain ranges of that region rose above the surface of the vast Pleistocene Lake.
The lake owed its existence to the cool, wet climate of the Pleistocene, when mountain glacial meltwaters increased dramatically and when evaporation was reduced by humid air and cloudy skies. The extent of Lake Lahontan reached its maximum at the peak of the Wisconsin glacial melting, some 12,700 years ago, its former boundaries now marked by tufa limestone formations that were deposited in its shallows; then, as the climate began to warm, the lake began to shrink, eventually separating into a maze of smaller basin lakes. Today, most of those lakes have dried up, leaving playas or "sinks," where their feeder streams disappear into the Great Basin desert; Pyramid Lake and Walker Lake are now the only remnants of Lake Lahontan. Pyramid Lake, fed by the Truckee River (which flows down from Lake Tahoe), was the deepest segment of Lake Lahontan. Recently, this lake, NNE of Reno, was found to harbor a remnant population of Lahontan cutthroat trout; thought to have become extinct, it is the largest species of trout native to North America.
While Lake Lahontan, like Lake Bonneville in Utah, formed during the Wisconsin Glaciation, there were predecessors of these vast meltwater lakes during earlier glacial periods of the Pleistocene. Assuming that the current Holocene Epoch is just another warm interglacial period of the Quaternary, the glaciers and the Great Basin lakes will, from the perspective of geologic time, soon return.
