A watershed is the area of land where all of the water that is under it (groundwater) and the water that drains off of it (precipitation) goes into the same place. Watersheds come in all shapes and sizes, and cross county, state and even national boundaries. Wherever you are, you're in a watershed.
Managing lands would be simpler had political boundaries matched watershed boundaries. Even early explorer John Wesley Powell (the first European to travel through the Grand Canyon) suggested that state boundaries in the western United States be drawn on watershed divisions to minimize water conflicts. Since most boundaries do not, understanding watersheds is critical for land managers, because the effects of upper watershed activities are often felt on the middle and lower watersheds.
Great Basin National Park is part of two Environmental Protection Agency drainage areas. The east side of the mountain range is part of Hamlin-Snake Valleys (USGS cataloging unit 16020301) and the west side is part of Spring-Steptoe Valleys (USGS cataloging unit 16060008). Both of these drainage areas are part of the Great Basin hydrologic area, which has no surface connection to the ocean. All water that falls within the Great Basin stays within the Great Basin unless influenced by humans.
For park projects, the park has been further delineated into 25 watersheds. Due to the steepness of the terrain on the west side of the park, the 12 found on this side are much smaller than the 13 found on the east side of the Snake Range.
More information on watersheds can be found at the Environmental Protection Agency’s Surf Your Watershed website.