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.
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.