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What is a Watershed?

Watersheds comprise the basis of organization for Master Watershed Steward activities and for the information presented on this site. The following is a description of the physical concept of a watershed.

What is a watershed?

Watershed characteristics.A watershed encompasses all of the land which drains to a single water body or point on the landscape. Topography drives watershed boundaries as water flows downhill. A watershed boundary is defined by connecting the highest points surrounding the area drained. A watershed is bounded by ridgelines, mountains, and hills. An animation of how watershed boundaries are delineated.

Watersheds range in scale from square meters to hundreds of square miles. Large watersheds are made up of many smaller subwatersheds, delineated by ridges and high points on the landscape. Smaller watersheds are nested within larger watersheds.

Colorado River watershed.

Watershed boundaries are natural, following the topography of the landscape. Very often, political boundaries of all types cross watershed boundaries – parts of several major watersheds are encompassed within a state; several states comprise a major watershed. The Colorado River drains approximately 637,000 square kilometers (246,000 square miles), drawing water from 7 states in the U.S. and 2 Mexican states!

On a smaller scale, the creek on your property may drain a few acres.

The animation below demonstrates how to delineate a watershed using a topographic map. Click on the button to begin the animation. Once it has begun, the animation will run automatically.

All contents copyright 2007. Arizona Board of Regents.