6.9 - Hydroelectric Power

Learning Objectives

I. Hydroelectricity Basics and Generation Systems

A. Fundamentals

• Hydroelectricity comes from the kinetic energy of moving water.

• Moving water spins a turbine, which powers a generator to produce electricity.

• It is the largest single renewable source of electricity in the world, about 14% of global electricity in 2025. Wind and solar combined now make more (about 17% in 2025), and they keep growing (Ember).

• The biggest producers are China (by far, about 1,350 TWh in 2024), Brazil, Canada, and the United States. China's Three Gorges Dam on the Yangtze River is the world's largest power station (22,500 MW).

B. Water Impoundment (Dams)

• This is the most popular way of generating hydroelectricity.

• A dam stores water, creating an artificial lake called a reservoir.

• Controllable power: Dams let humans control the flow of water through the intake channel, which controls the rate of electricity production.

C. Alternative Systems

• Run-of-river systems: Use a smaller dam that diverts water through a man-made channel running parallel to the river.
• Less impactful on the surrounding ecosystem because it does not flood the area or block sediments from moving downstream.
• Drawback: It generates less electricity and can fluctuate with river water levels, for instance during low rainfall or snowmelt.

• Tidal energy: Uses ocean tides, which move in and out predictably, to spin an underwater turbine and generate electricity.
• Drawback: Only available in coastal areas close to the ocean.
• How it works: A tidal barrage is a low dam across a bay or river mouth; water flows through turbines as the tide comes in and as it goes out. Tidal stream turbines sit on the seafloor where tidal currents are strong, like underwater wind turbines.
• Tides are predictable (unlike sun or wind), but only a few coastlines have big enough tides. The two largest tidal plants are Sihwa Lake in South Korea (254 MW, opened 2011) and La Rance in France (240 MW, running since 1966).

II. Environmental and Ecological Drawbacks of Dams

1. Habitat loss and ecosystem change (upstream):
• Flooding behind the dam displaces ecosystems like forests and wetlands.
• Rivers, which are typically fast-moving and shallow, get converted into lakes (reservoirs), which are completely different ecosystems.

2. Sedimentation and nutrient deprivation:
• Sediment builds up behind the dam, trapping sand, silt, and dead organic matter.
• Upstream impact: That buildup covers the rocky stream bed, destroying important spawning habitat for fish and habitat for macroinvertebrates (aquatic insects). Sediment buildup also makes the dam less efficient.
• Downstream impact: The dam deprives downstream ecosystems of sediments and their essential nutrients like phosphorus and nitrogen, which normally flow along the bottom of the water. Wetlands especially depend on that fertile organic matter.

3. Water quality changes:
• Water behind the dam gets warmer because it sits still and has a higher surface area, so it absorbs more sunlight.
• Warmer water holds less oxygen, which supports fewer aquatic species.

4. Greenhouse gas and pollution:
• The flooded organic matter in the reservoir decomposes in anaerobic (low oxygen) conditions, which produces methane, a potent greenhouse gas.
• Dam construction causes a temporary increase in fossil fuel emissions.
• Increased evaporation happens because of the reservoir’s large surface area.

5. Disruption of fish migration:
• Dams physically block the migration of species like salmon.
• Real example: Four dams on the Klamath River (Oregon and California) blocked salmon for about a century. After they were removed in 2023 and 2024, the largest dam removal in U.S. history, Chinook salmon swam past the old dam sites and spawned upstream within weeks.

III. Economic and Social Impacts

• Costs: Initial construction is very expensive. Human homes and businesses have to be relocated out of the flooded reservoir zone. Expensive dredging (using large cranes to remove sediment) is needed to maintain efficiency.
• Real example: The Three Gorges Dam's reservoir flooded towns and farmland along the Yangtze, and about 1.3 million people had to move.

• Benefits: Dams create many jobs for construction, operation, and maintenance. They are also a big source of tourism and recreation dollars through boating and fishing.

• Flood control: Dams control the seasonal flooding of many rivers. That flood prevention lets humans build settlements closer to rivers, though it negatively affects downstream wetland ecosystems.

IV. Migration Solutions

• The most common solution to migration disruption is the fish ladder.
• Fish ladders are a series of cement steps or pools that let migratory species like salmon swim or jump over the dam to reach upstream spawning grounds.
• Benefits: Maintains the fish population, benefits their predators like bears, eagles, and osprey, and preserves salmon as a food source for humans.

• A more fun alternative is the salmon cannon, a large tube that catches fish and shoots them over the dam.

V. Core Benefits of Hydroelectricity

• Clean power: No greenhouse gas emissions like $CO_2$ are released while the electricity is being produced.

• No air pollutants: No SOx, NOx, or particulate matter released during generation, which improves local air quality.

• Reliability: It is a reliable, consistent source of electricity because water can be stored and the rate of production controlled.