4.6 Watersheds

A watershed (also called a drainage basin) is the whole area of land where rain and snowmelt drain to the same outlet, like a river, lake, bay or ocean. Every piece of land is part of some watershed, including your school and home.

Watersheds are separated by drainage divides, which are high points like ridges or hilltops. Rain falling on one side of a divide flows into one watershed; rain falling on the other side flows into another. The Continental Divide in the Rocky Mountains separates water flowing toward the Pacific from water flowing toward the Atlantic and the Gulf.

Small watersheds nest inside larger ones. A creek's watershed is part of a river's watershed, which is part of an even larger one. The Mississippi River watershed drains all or part of 31 U.S. states, plus parts of two Canadian provinces.

• Area: larger watersheds collect more water and deliver larger floods downstream.
• Length: longer watersheds take more time for water to reach the outlet.
• Slope: steep slopes make water run off faster, with more erosion and quicker flooding; gentle slopes give water more time to soak in.
• Soil: sandy, permeable soils let water infiltrate; clay or compacted soils produce more runoff.
• Vegetation: plants slow runoff, hold soil in place, absorb water and filter pollutants. Forested watersheds usually deliver cleaner, steadier flow than bare or paved ones.

Because all the water in a watershed ends up in the same place, pollution added anywhere can reach the outlet. Fertilizer from farms, manure from livestock, oil and metals from roads, sediment from construction and sewage overflows all travel downstream.

The Chesapeake Bay watershed covers about 64,000 square miles across parts of six states and Washington, D.C. Nutrient runoff from farms and cities throughout that huge area feeds algal blooms and low-oxygen zones in the bay. Likewise, fertilizer washing down the Mississippi River helps create a large low-oxygen dead zone in the Gulf where the river empties every summer.

Clearing forests, paving land and draining wetlands all increase runoff and erosion and reduce water quality. Dams change how water and sediment move. Protecting a watershed often means keeping natural vegetation along streams (riparian buffers), restoring wetlands, controlling erosion on farms and construction sites, and reducing fertilizer use. Because watersheds cross town, state and even national borders, protecting them usually requires cooperation among many communities.