How does flowing water make electricity, and what does a dam do to a river?
Topic 6.9 Hydroelectric Power: describe how hydroelectric and tidal power generate electricity and evaluate their benefits and drawbacks.
A focused answer to APES Topic 6.9, covering how hydroelectric dams and tidal power generate electricity, the benefits (renewable, low emissions, reliable) and drawbacks (habitat disruption, sediment, displacement) of damming rivers, and a worked hydro power calculation.
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What this topic is asking
The College Board (Topic 6.9) wants you to describe how hydroelectric and tidal power generate electricity and evaluate their benefits and drawbacks.
How hydro and tidal power work
Benefits
Drawbacks of large dams
Why this matters
Hydroelectric power connects Unit 6 to watersheds and the water cycle (Units 1 and 4). It is the textbook case of a clean-energy source with heavy local ecological costs, so the AP exam uses it to test whether you can weigh low emissions against habitat disruption, sediment trapping and displacement. A common free-response prompt gives a proposal to dam a river and asks you to argue both for it (renewable electricity, flood control, water storage, no air emissions) and against it (lost habitat, blocked fish migration, trapped sediment, displaced communities), then justify a recommendation. The strongest answers name a specific ecological mechanism, such as sediment starvation of a downstream delta or the loss of a salmon run, rather than just asserting that dams are harmful.
Try this
Q1. Identify the natural force that drives tidal power. [1 point]
- Cue. The rise and fall of ocean tides, caused by the Moon's gravity.
Q2. Explain one ecological drawback of a large hydroelectric dam. [2 points]
- Cue. A dam blocks migrating fish and traps sediment behind it, so fish cannot reach spawning grounds and downstream habitats and deltas are starved of sediment, degrading the river ecosystem.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2021 (style)4 marksSection II (FRQ). (a) Describe how a hydroelectric dam generates electricity. (b) Identify one environmental advantage of hydroelectric power. (c) Identify two environmental drawbacks of building a large dam. (d) Explain how tidal power differs from conventional hydroelectric power.Show worked answer →
A 4-point FRQ on hydroelectric and tidal power.
(a) Describe (1 point): water held behind a dam is released through turbines; the moving water spins the turbines, which drive generators to produce electricity.
(b) Identify (1 point): hydroelectric power is renewable and emits almost no carbon dioxide or air pollutants during operation.
(c) Identify (1 point): any two of habitat loss from flooding the reservoir, blocking fish migration, trapping sediment, altering downstream flow and temperature, or displacing people.
(d) Explain (1 point): tidal power uses the rise and fall of ocean tides (driven by the Moon's gravity) to turn turbines, rather than river flow held behind a dam.
Markers reward the release-through-turbines mechanism, the low-carbon advantage, two valid dam drawbacks, and the tidal-versus-river distinction.
AP 2018 (style)1 marksSection I (multiple choice). A major ecological drawback of large hydroelectric dams is that they: (A) emit large amounts of carbon dioxide while running (B) block fish migration and trap sediment (C) use a nonrenewable fuel (D) cannot store water. Justify your choice.Show worked answer →
A 1-point MCQ on hydroelectric power. The answer is (B).
Large dams block migrating fish such as salmon and trap sediment behind the wall, starving downstream habitats and deltas. Hydroelectric power emits little carbon dioxide while running (A), uses renewable flowing water not a fuel (C), and reservoirs are built to store water (D). The trap is forgetting that a low-carbon source can still cause major ecological harm by altering the river.
Related dot points
- Topic 6.1 Renewable and Nonrenewable Resources: distinguish renewable from nonrenewable energy resources and explain why the distinction matters for sustainability.
A focused answer to APES Topic 6.1, covering the difference between renewable and nonrenewable energy resources, examples of each, the idea of potentially renewable resources, and why the distinction matters for sustainability, with a worked depletion calculation.
- Topic 6.12 Wind Energy: describe how wind turbines generate electricity and evaluate the benefits and drawbacks of wind power.
A focused answer to APES Topic 6.12, covering how wind turbines convert wind into electricity, onshore and offshore wind, the benefits (renewable, low emissions, low operating cost) and drawbacks (intermittency, location, wildlife, noise) of wind power, and a worked wind farm output calculation.
- Topic 4.6 Watersheds: define a watershed, describe the factors that affect its characteristics, and explain how land use changes runoff and water quality.
A focused answer to APES Topic 4.6, covering the definition of a watershed, divides, the factors that shape watershed behavior (area, slope, vegetation, soil), runoff versus infiltration, and how land use affects flooding and water quality, with a worked runoff comparison.
- Topic 1.7 The Hydrologic (Water) Cycle: describe the processes of the water cycle and explain how human activities alter the storage and movement of water.
A focused answer to APES Topic 1.7, covering evaporation, transpiration, condensation, precipitation, runoff, infiltration and groundwater, and how deforestation, paving and irrigation alter the cycle, with a worked water-budget calculation.
- Topic 6.4 Distribution of Natural Energy Resources: explain why energy resources are unevenly distributed and the consequences of that uneven distribution.
A focused answer to APES Topic 6.4, covering why fossil fuels and renewable resources are unevenly distributed across the globe, how geology and geography determine availability, and the economic and political consequences of that uneven distribution, with a worked import dependence calculation.
Sources & how we know this
- AP Environmental Science Course and Exam Description — College Board (2020)