Aim for FiveEnvironmental Science

Unit 6 · Topic 6.9

6.9 Hydroelectric Power

Hydroelectric power turns the energy of flowing or falling water into electricity, usually at dams, and tidal power taps the rise and fall of the tides. It is renewable, reliable and clean to run, but dams flood land, block fish, trap sediment and transform the river downstream.

Key terms

  • dam
  • reservoir
  • run-of-the-river
  • tidal energy
  • fish ladder

How hydroelectric power works

A dam blocks a river and backs water up into a reservoir, an artificial lake. Water at the top has gravitational potential energy (stored energy from its height). When it is released, it rushes down a large pipe called a penstock, gaining kinetic energy, and spins a turbine that turns a generator. The higher the drop and the more water flows, the more electricity the dam makes.

Because operators can open and close the gates, a reservoir dam can raise or lower its output quickly to match demand, which makes it a useful partner for wind and solar. Some dams also provide flood control, irrigation water and recreation.

Run-of-the-river and tidal power

A run-of-the-river system diverts part of a river's natural flow through turbines with little or no reservoir. It floods far less land and disturbs the river less, but output rises and falls with the river's flow, so it is less steady.

Tidal energy uses the movement of water as tides come in and go out. Some systems use a barrage, a low dam across a bay or estuary that lets water through turbines as the tide changes; others place underwater turbines in strong tidal currents. Tides are perfectly predictable, but only a few coastlines have large enough tidal ranges, and barrages can change the estuary's habitat.

Environmental and social costs of dams

  • Flooding: the reservoir drowns forests, farmland, towns and habitat. China's Three Gorges Dam displaced more than a million people.
  • Blocked fish migration: dams stop fish such as salmon from swimming upstream to spawn. Fish ladders, a series of small stepped pools that fish can jump up, help some species get past.
  • Trapped sediment: the river slows in the reservoir and drops its sediment there. The reservoir slowly fills with silt, and downstream river beds, floodplains, deltas and beaches lose the sediment and nutrients that used to rebuild them.
  • Changed water downstream: released water can be colder or warmer, lower in oxygen and different in timing from natural flow, which disrupts downstream ecosystems.
  • Greenhouse gases: plants flooded in a new reservoir decompose without oxygen and release methane, especially in warm tropical reservoirs.
  • Evaporation: a large, still reservoir loses water to evaporation, which matters in dry regions.

Worked examples

Try each one yourself first, then open the solution.

  1. Example 1Calculator allowed

    How many homes can a dam supply?

    A small hydroelectric dam generates 450,000 megawatt-hours (MWh) of electricity per year. An average home uses 10,800 kilowatt-hours (kWh) per year. How many homes can the dam supply? (1 MWh = 1,000 kWh)

    Show the solution
    1. Step 1: Put both numbers in the same unit. 450,000 MWh × 1,000 kWh/MWh = 450,000,000 kWh (4.5 × 10⁸ kWh) per year.
    2. Step 2: Divide by the use per home: 4.5 × 10⁸ kWh ÷ 10,800 kWh per home ≈ 41,667 homes.
    3. Step 3: Round down, since you can't fully supply part of a home: 41,666 homes, or roughly 42,000.

    Answer: About 41,666 homes (roughly 42,000).

  2. Example 2

    Trap: 'no emissions' and 'no impact' are different claims

    A dam supporter says: 'Hydropower is clean, so building the dam has no environmental downside.' Give two specific downsides and one way to reduce one of them.

    Show the solution
    1. Step 1: Hydropower makes no air pollution while running, but building a dam reshapes the whole river.
    2. Step 2: Downside 1: the reservoir floods habitat and can displace people.
    3. Step 3: Downside 2: the dam blocks migrating fish such as salmon from reaching spawning grounds upstream.
    4. Step 4: Mitigation: build a fish ladder or fish elevator so fish can pass the dam (or choose a run-of-the-river design that floods less land).

    Answer: For example: habitat flooding and blocked fish migration; a fish ladder reduces the impact on migrating fish.

Common mistakes

  • Calling hydropower 'impact-free'. It is clean to run but has major habitat, sediment and fish impacts.
  • Mixing up MWh and kWh in calculations. Convert everything to the same unit before dividing.
  • Saying dams increase sediment downstream. They trap sediment, so downstream areas get less.

On the exam

  • Questions often ask for an ecological impact of a dam and a way to reduce it. Fish ladders for blocked migration and sediment loss downstream are reliable answers.
  • Unit-conversion calculations with kWh and MWh are common in the math free-response question.

Connected topics

Videos

Check yourself

4 questions on 6.9 Hydroelectric Power. Pick an answer to see if you got it, and why.

A large hydroelectric dam is built across a river where salmon swim upstream each year to spawn. A reservoir forms behind the dam, flooding a forested valley. Downstream, the river's delta begins to shrink.

Described scenario

Question 1 of 4

Which statement best explains why the delta downstream is shrinking?

Question 2 of 4

Which addition to the dam would most directly reduce its harm to the salmon population?

Question 3 of 4

A run-of-the-river hydroelectric plant sends part of a river's flow through turbines without building a large reservoir. Compared with a large dam, which is the most likely trade-off?

Question 4 of 4

A new hydroelectric dam floods a large area of tropical forest. Which statement best explains why the reservoir may release greenhouse gases for years afterward?

0 of 4 answered