The Great Basin is a land defined by water, though water itself is scarce. The "basin" characteristic of this region means that no water in the region ever reaches an ocean, except by human intervention or by evaporation, when water molecules return to the planet-wide hydrologic cycle, or . Any precipitation that falls in the Great Basin, stays in the Great Basin.
In ages past, during the Pleistocene era (3 million years ago to 10,000 years ago), water was abundant as glaciers advanced and retreated in a climate that was an average 8°F cooler than today. Lakes filled many of the basins to a depth of up to 1,000 feet. The shores of massive Lake Bonneville, a large pluvial lake that flooded much of the eastern Great Basin during this time, extended from the Wasatch Mountains in Utah to the Snake Range, in Great Basin National Park. To the west was a smaller lake in Spring Valley. Even farther west sat Lake Lahontan, covering much of western Nevada.
Change began, though, during the Holocene epoch (10,000 years ago) and continued until as recently as 4,500 years ago. The climate warmed significantly, drying up many lakes in the Great Basin, leaving behind small remnants such as the Great Salt Lake in Utah and Pyramid Lake in Nevada. As glaciers melted, the water seeped into the gravel subsurface below, where it remained protected from evaporation. These resevoirs of groundwater, known as aquifers, remain beneath old lake beds.
These aquifers are recharged from surface precipitation. In the Great Basin Desert, however, with less than 10 inches of annual precipitation, there is little to no recharge of these resevoirs. Groundwater can discharge at the surface naturally in the form of springs, or seeps. Water is drawn unnaturally to the surface by the drilling of wells, or large scale groundwater pumping.