4.3 Watersheds

What is a watershed?

A watershed (or drainage basin) is the land area that drains all precipitation and runoff to a common outlet, such as a river, lake, or bay. Watersheds are separated by divides, which are ridges of high land. Everything within a watershed — soil, plants, land use, and pollution — affects the water quality flowing into its outlet.

How water moves through a watershed

Precipitation becomes surface runoff when it flows over land into streams and rivers, or infiltration, soaking into soil to recharge groundwater. Factors like slope, vegetation cover, soil permeability, and impervious surfaces (pavement) determine how much water infiltrates versus runs off. More impervious surface means more runoff, faster flow, and greater erosion and flooding risk.

Human impacts and management

Urbanization, deforestation, and agriculture increase impervious surfaces and reduce vegetation, increasing runoff, erosion, and nonpoint source pollution (fertilizers, pesticides, sediment). Because everything upstream affects everything downstream, watersheds are managed as connected systems — using buffer strips, wetlands, and land-use planning to reduce pollutant loads and protect downstream water quality.

Key terms

Watershed

The land area that drains all precipitation and runoff to a common outlet, such as a river or lake.

Divide

A ridge of high land that separates two adjacent watersheds.

Infiltration

The process of water soaking into the soil, recharging groundwater.

Surface runoff

Water that flows over land into streams and rivers rather than infiltrating the soil.

Nonpoint source pollution

Pollution from diffuse sources (e.g., fertilizer or sediment runoff) rather than a single identifiable point.

Try an AP-style question

What is a watershed?

1. A shed where rainwater is collected
2. The deepest part of a lake or pond
3. All land that drains into one body of water
4. An underground layer of stored water