AP Environmental Science 4.6 - Watersheds

Main Ideas

What Is a Watershed?

A watershed is an area of land whose water drains into a shared body of water, such as a river, lake, or stream. Its boundaries are largely determined by slope: rain falling on opposite sides of a ridge may flow into different watersheds.

A watershed can be thought of as a large funnel, with gravity carrying runoff toward its central body of water.

What Affects Runoff and Water Quality?

• Vegetation: Plant cover slows rainwater, helps it soak into the ground, and can reduce runoff and erosion.
• Slope: Steeper slopes generally make runoff move faster, allowing it to carry more soil and debris into waterways.
• Soil permeability: More permeable soil allows greater infiltration and groundwater recharge. Less permeable soil tends to produce more surface runoff.
• Human activity: Mining, agriculture, and clear-cutting can introduce pollutants or loosen soil, which runoff may carry into rivers and other bodies of water.

The Chesapeake Bay Watershed and Ecosystem Services

The Chesapeake Bay watershed covers a six-state region whose rivers and streams ultimately drain into the bay. The bay is an estuary, where fresh water mixes with salt water. Its tributaries carry nutrients and sediment into habitats such as salt marshes.

These productive habitats benefit people by providing:

• Tourism and recreation, including hiking, canoeing, and kayaking.
• Water filtration: Wetlands and estuaries can trap pollutants and help recharge groundwater.
• Wildlife habitat and seafood: These habitats include breeding grounds for crabs and other shellfish.
• Storm protection: Wetlands and estuaries can absorb or buffer surges of water during large storms.

Human Impacts on the Chesapeake Bay Ecosystem

Nutrient Pollution and Eutrophication

Excess nitrogen and phosphorus can cause eutrophication, a rapid increase in algae often called an algal bloom. The bloom blocks sunlight and harms submerged plants. When those plants die, bacteria decompose them and consume oxygen.

Oxygen depletion can kill aquatic organisms and create hypoxia—very low oxygen levels—or a dead zone that supports little life.

The main nutrient sources discussed include:

• Wastewater treatment plant discharges, including occasional releases during flooding or overflow.
• Agricultural runoff, including manure from concentrated animal feeding operations and synthetic nitrate fertilizers.

Other Pollutants

Endocrine disruptors include some medications that pass through people and may not be fully removed during wastewater treatment.

Sediment pollution can result from clear-cutting, urbanization, and farming. It can:

• Cloud water and reduce photosynthesis.
• Increase water temperature by increasing sunlight absorption.
• Cover rocky stream bottoms and the small crevices used as habitat by crustaceans and other macroinvertebrates.

Strategies to Reduce Nutrient and Sediment Pollution

• Plant cover crops: Grow plants between harvest crops to hold soil in place, reduce runoff, and take up excess nitrates.
• Improve manure management: Use better-contained manure lagoons with lining and higher banks or berms, or remove manure more often and convert it into fertilizer pellets.
• Create riparian buffers: Plant vegetation along rivers and other waterways to slow runoff, absorb water, trap pollutants, and keep sediment out of the water.
• Upgrade septic systems: Improve septic tanks and drain fields so wastewater drains and filters through the ground more gradually, reducing overflow and contamination of nearby surface water.
• Use biological waste removal: Employ microbes that consume waste and use nutrients, reducing the amount released into rivers.

Speakers and Sources Featured

• Presenter/narrator: Jordan Dischinger-Smedes, as identified by the channel metadata. No other speakers are apparent in the subtitles.
• Examples and visuals discussed: A watershed-boundary diagram; the Chesapeake Bay watershed and salt-marsh habitats; algal-bloom imagery; wastewater treatment and agricultural runoff illustrations; sediment-affected stream imagery; and diagrams of pollution-reduction strategies.