Watersheds (College Board AP® Environmental Science): Revision Note

Characteristics of watersheds

What is a watershed?

• A watershed is an area of land where all precipitation and surface water drains into a common body of water, such as a river, lake, or ocean

  • Also known as a drainage basin

  • A watershed collects and channels water through a network of connected streams that all drain to the same outlet

• Watersheds vary in size, from small creeks to massive river basins like the Mississippi River Watershed

Insert image
Image caption: A watershed is the entire area of land where all rainfall and surface water drains to a common outlet. Its boundary follows the high ridges that separate one drainage area from another.

Key characteristics of a watershed

Area

• The total land surface area that drains into a water body

• Larger watersheds collect more water

  • This influences regional water flow

• Example: The Amazon River Basin is the largest watershed in the world, covering about 7 million km²

Length

• The distance from the watershed’s highest point to its outlet (where water drains into a larger body)

• Longer watersheds typically have a more complex drainage network with more streams feeding the main river

Slope

• The gradient or steepness of the land within the watershed

• Affects water speed, erosion, and sediment transport

  • Steep slopes cause rapid runoff and higher erosion potential

  • Gentle slopes allow more infiltration and slower runoff

• Example: Mountainous watersheds have steep slopes, leading to flash flooding

Soil type

• Determines water infiltration, soil water retention, and erosion potential

  • Sandy soils: High permeability, low water retention, higher erosion risk

  • Clay soils: Low permeability, high water retention, more prone to surface runoff

  • Loamy soils are an ideal mixture of sand, silt and clay, so are ideal for plant growth and water absorption

• Example: Watersheds with clay-heavy soil may experience more surface runoff and erosion

Vegetation

• The type and density of plants affect water retention, erosion control, and infiltration

  • Dense vegetation slows runoff, improves groundwater recharge, and reduces erosion

  • Deforested or barren areas have higher erosion and reduced water retention

Watershed divides

• The boundaries separating one watershed from another

  • Typically found along ridges or high elevations where water flows in different directions

• Example: The Continental Divide in North America determines whether water flows to the Atlantic or Pacific Ocean

Human activities and water quality

• Whatever happens on the land within a watershed eventually affects the water quality at its outlet

  • Pollutants, sediments, and excess nutrients are washed by rainfall into the stream network and carried downstream

  • This is why watersheds are often used as management units when protecting water quality

• Major land-use impacts covered elsewhere in the course:

  • Clearcutting removes the trees that anchor the soil, raising sedimentation and stream temperatures (see The Effect of Clearcutting on Forests)

  • Mining of ores releases heavy metals, acid mine drainage, and tailings into local waters (see Mining of Ores)

  • Agricultural practices such as tilling and fertiliser use cause nutrient runoff and eutrophication downstream (see Environmental Impacts of Agricultural Practices)