Watershed Boundaries

Watershed boundaries are determined by elevation, or the height of the land. The boundaries are the highest points or ridges that surround a body of water. The boundary is called a drainage divide, and it may be a hill, a ridge, a mountain, or any other landscape feature that enables a difference in elevation.

The map shows the major continental drainage divides in North America. The red and yellow ones are the most prominent. What landscape feature do you think is associated with these divides?

The divides correspond to large mountain ranges. The great divide shown in red is located along the Rocky Mountains. The yellow divide is marked by the Appalachian Mountains.

Some watersheds are very large. For example, the Continental Divide separates the United States into two enormous watersheds. Rain falling west of the Continental Divide drains into the Pacific Ocean; rain falling east of the Continental Divide drains into the Atlantic Ocean. Along the way, however, the water spends some time in smaller watersheds. It might start in the watershed of a small creek that eventually joins a larger river. The river eventually flows into the ocean. While most watersheds eventually flow toward an ocean, some watersheds do not. Watersheds that do not drain to an ocean are called endorheic watersheds.

Almost all watersheds are divided into smaller watersheds. This map shows how the larger Mississippi River watershed is divided into several smaller watersheds, such as the Ohio River watershed.