4.6 - Watersheds
Learning Objectives
The main objective is to describe the characteristics of a watershed and to explain how human activities within a watershed directly affect the quality of the receiving body of water. The associated skill is explaining relationships between environmental concepts and models.
I. Watershed Definition and Characteristics
A watershed is an area of land where all the rainfall or runoff drains into a single, central body of water. That central body can be a stream, river, or lake.
A. Determining Watershed Boundaries
* Watersheds are primarily determined by slope.
* A watershed acts like a giant funnel of land where gravity pulls all surface runoff downward.
* A ridge line or high point in the land determines the boundary. A drop of water landing on one side of the ridge flows into one watershed, while a drop landing on the other side flows into a different one.
That boundary line is called a divide. Neighboring watersheds share a divide, and small watersheds (a creek’s) nest inside bigger ones (the river it flows into). The biggest divide in North America is the Continental Divide along the Rocky Mountains: rain that falls on the west side ends up in the Pacific Ocean, while rain on the east side flows toward the Atlantic or Arctic Oceans.
B. Factors Affecting Runoff and Infiltration
The characteristics of the land inside the watershed determine how fast water runs off and whether it infiltrates, meaning seeps into the ground.
Factor | Effect on Runoff/Infiltration | Explanation |
Vegetation | Increases infiltration | Plant roots absorb water, anchor the soil, and make the ground more permeable, so more water soaks in instead of running off. |
Slope | Increases runoff velocity | A greater slope increases velocity, giving the runoff more energy to pick up sediments and debris and carry them into the central body of water. |
Soil Permeability | Affects the rate of infiltration vs. runoff | Highly permeable soil lets water infiltrate and recharge groundwater. Less permeable soil increases surface runoff. |
C. Size, Shape, and Land Cover
- Area: All the land that drains to the same outlet. A bigger area collects more rain, so its river carries more water, along with pollution from a bigger region. Example: the Mississippi River watershed covers about 1.2 million square miles, roughly 41% of the lower 48 states, and drains all or part of 31 states and 2 Canadian provinces.
- Length: How far water travels from the far end of the watershed to the outlet. In a long watershed, rain from upstream takes days to arrive, so floods downstream build up later and last longer, and water has more time to soak in or pick up pollutants on the way.
- Vegetation Types: Forests and wetlands soak up the most water and release it slowly, keeping streams clear. Grassland soaks up less, and plowed fields and pavement the least, so those watersheds send fast, muddy runoff into streams.
Practice calculation: A storm drops 2.0 cm of rain on a watershed. If 25% of the rain runs off into the river (the rest soaks in or evaporates), how much water reaches the river?
If the same watershed were mostly paved and 75% of the rain ran off, the river would get , three times as much. That’s why paving over a watershed makes floods worse (5.10).
II. The Chesapeake Bay Watershed and Ecosystem Services
The Chesapeake Bay Watershed is one of the largest watersheds in America, spanning a six-state region where all rivers and streams eventually empty into the Chesapeake Bay.
- Size: About 64,000 square miles, covering parts of Delaware, Maryland, New York, Pennsylvania, Virginia, and West Virginia, plus all of Washington, D.C. About 18.6 million people lived in it in 2023, so what happens on farms in Pennsylvania and streets in D.C. shows up in the bay.
- Estuary Definition: The Chesapeake Bay is an estuary, a unique habitat where saltwater and freshwater mix. Estuaries collect a lot of nutrients and sediments from all of their tributary rivers.
- Ecosystem Services: Highly productive habitats like salt marshes provide valuable ecosystem services with real monetary benefits to humans:
- Tourism/Recreation: Revenue from tourism, including hotels, tours, kayaking, and canoeing.
- Water Filtration: Wetland roots are highly effective at trapping pollutants and recharging groundwater with clean water, which cuts down on the money humans have to spend on water filtration.
- Habitat/Provisioning: Estuaries are the breeding ground for animals like crabs and shellfish that get harvested. Without this habitat, seafood availability plus the associated jobs and restaurant revenue would drop.
- Storm Protection: These areas act as overflow or surge protection during large storms.
III. Human Impacts: Nutrient Pollution and Hypoxia
Human activities directly determine water quality in a watershed. A major impact is nutrient pollution, the excess introduction of nitrogen and phosphorus.
The Eutrophication Cycle
1. Nutrient Input: Excess nitrogen and phosphorus, which are often limiting nutrients, enter the water.
2. Algae Bloom: This causes an explosion of algae growth called eutrophication.
3. Sunlight Blockage: The algae bloom forms a layer that blocks sunlight from reaching the plants beneath the surface.
4. Oxygen Consumption: When those submerged plants die, bacteria break them down. That’s an aerobic process, so it consumes large amounts of the oxygen other organisms need.
5. Hypoxia / Dead Zone: The lack of oxygen kills organisms, creating a positive feedback loop where more death consumes more oxygen. The result is hypoxia (very low oxygen levels), or a dead zone, where the water can’t support much life anymore.
Sources of Nutrient Pollution
- Sewage Treatment Plants: These plants process wastewater but can’t always remove all the nutrients, so some gets through. When heavy rain floods these facilities, they may release raw sewage directly into the water, spiking nutrient levels.
- Agriculture: Runoff from CAFO manure lagoons, which contain animal waste, is rich in nitrogen and phosphorus. Synthetic nitrate fertilizers used on agricultural fields also run off into waterways.
IV. Other Pollutants and Solutions
A. Other Major Pollutants
- Sediment Pollution: Activities like clear cutting, urbanization, or farming loosen soil, so sediments get carried into rivers easily. This sedimentation can decrease photosynthesis rates, make the water warmer because of lower albedo, and cover up rocky stream bottoms. Those rocky crevices are crucial habitats for small crustaceans and macroinvertebrates.
- Endocrine Disruptors: Chemicals that interfere with hormones, such as medications that pass through the human body and aren’t fully removed by wastewater treatment plants.
B. Solutions to Watershed Pollutants
Solutions generally target either agriculture or human sewage.
Strategy | Target | Mechanism |
Cover Crops | Agricultural runoff | Plants grown between main crop seasons that anchor the soil, keeping sediments and fertilizer from running off into water. They also use up excess nitrates. |
Riparian Buffers | General runoff | Planting vegetation in the riparian areas alongside a river creates a giant filter or sponge. The roots absorb water and trap pollutants before they reach the water. |
Manure Management | Agricultural waste | Improving manure lagoons with better lining and higher banks, or processing waste into fertilizer pellets, reduces the risk of runoff. |
Septic Tank Upgrades | Human sewage | Optimizing the drain field of a residential septic tank so sewage releases slowly lets the ground filter it naturally, preventing contamination of nearby surface waters. |
Biological Waste Removal | Human sewage | Using microbes (bacteria) to eat and use up nutrients in sewage before the water gets released into the river. |
Big picture
A watershed works like a giant conveyor belt for water and everything dissolved or suspended in it. Just like the health of a city shows up in the trash piled at the end of its sanitation line, the quality of a central body of water like the Chesapeake Bay is a direct, cumulative reflection of every human and natural action happening anywhere upstream in that funnel of land.