Hydroelectric power converts moving water’s potential and kinetic energy into electricity. Dams hold water in a reservoir; when released, water flows through penstocks and spins turbines (types: Francis, Kaplan, Pelton) connected to generators. Run-of-river systems put turbines directly in flowing rivers without large reservoirs. Pumped-storage stores energy by pumping water uphill to a reservoir during low demand and releasing it to generate power at peak demand. Tidal energy uses tidal flows or barrages to turn turbines. Hydroelectricity makes no air pollution or solid waste during operation, but dam construction can flood habitats, change sedimentation, block fish migration (fish ladders can help), and even cause reservoir-induced seismicity (e.g., Three Gorges Dam). For AP exam focus: describe dam vs. run-of-river, pumped-storage purpose, and environmental trade-offs per EK ENG-3.L.1 and EK ENG-3.M.1. Review the Topic 6.9 study guide (https://library.fiveable.me/ap-environmental-science/unit-6/hydroelectric-power/study-guide/r95V6RauJ8xe1P3Os36x) and Unit 6 overview (https://library.fiveable.me/ap-environmental-science/unit-6); practice questions at (https://library.fiveable.me/practice/ap-environmental-science).

Hydroelectric dams and tidal energy both convert moving water into electricity, but they use different water sources and timing. Hydroelectric dams block rivers to form reservoirs; released water flows through turbines (e.g., Francis or Kaplan) to spin generators. Run-of-river systems use continuous river flow without large reservoirs (EK ENG-3.L.1). Tidal energy captures predictable rise-and-fall or tidal currents—often with tidal barrages or in-stream turbines—using the energy of tidal flows to turn turbines (EK ENG-3.L.2). Environmentally, dam reservoirs can flood land, change habitats, trap sediment, and even induce seismicity; they don’t emit air pollution but are costly to build (EK ENG-3.M.1). Tidal systems have smaller footprints and very predictable output but can affect marine life and local sediment/flow patterns. For AP review, focus on how turbines are driven (river vs. tidal flow) and the key environmental trade-offs; see the Topic 6.9 study guide (https://library.fiveable.me/ap-environmental-science/unit-6/hydroelectric-power/study-guide/r95V6RauJ8xe1P3Os36x) and practice questions (https://library.fiveable.me/practice/ap-environmental-science).

We use water (hydroelectricity) instead of burning coal because hydro converts flowing water’s kinetic and potential energy directly to electricity without combustion—no CO2, sulfur, nitrogen oxides, or ash are produced during generation. Dams or run-of-river systems let water spin turbines (EK ENG-3.L.1), making hydro a renewable, low-air-pollution option. That said, hydro isn’t impact-free: building reservoirs can be costly and change or destroy habitats, cause sedimentation and even reservoir-induced seismicity (EK ENG-3.M.1). Coal burns fossil carbon, releases greenhouse gases and air pollutants, and generates solid waste. For the AP exam, you should be able to describe how turbines/reservoirs produce electricity and list environmental trade-offs (Topic 6.9, Unit 6). For a focused study guide on hydroelectric power, see Fiveable’s Topic 6.9 study guide (https://library.fiveable.me/ap-environmental-science/unit-6/hydroelectric-power/study-guide/r95V6RauJ8xe1P3Os36x) and try practice problems at (https://library.fiveable.me/practice/ap-environmental-science).

Turbines convert the energy of moving water into mechanical rotation that drives a generator. Water stored behind a dam has potential energy; when released it flows through penstocks and gains speed (kinetic energy). That fast-moving water hits turbine blades, making the rotor spin. The rotor turns a shaft connected to a generator, which converts mechanical energy to electricity. Different turbine types suit different “head” (height drop) and flow conditions: Pelton wheels for high-head low-flow, Francis turbines for medium head, and Kaplan turbines for low-head high-flow or adjustable-blade needs. There are also run-of-river setups (no big reservoir) and pumped-storage systems that pump water uphill to store energy. On the APES exam, this fits ENG-3.L (how hydroelectric systems use flowing water) and links to impacts in ENG-3.M (habitat change from dams). For a quick topic review see the Topic 6.9 study guide (https://library.fiveable.me/ap-environmental-science/unit-6/hydroelectric-power/study-guide/r95V6RauJ8xe1P3Os36x). More unit review and practice problems are at the Unit 6 page (https://library.fiveable.me/ap-environmental-science/unit-6) and Fiveable’s practice set (https://library.fiveable.me/practice/ap-environmental-science).

Think of hydroelectric power as converting stored water energy into electricity. A dam holds water in a reservoir, giving it gravitational potential energy. When water’s released, it flows through penstocks and the moving water (kinetic energy) spins a turbine (like a big water wheel). The turbine is connected to a generator; as the turbine shaft turns, magnets and coils inside the generator convert that mechanical energy into electrical energy via electromagnetic induction. Smaller “run-of-river” systems just put turbines in flowing streams; tidal barrages use tidal flows to do the same. Hydroelectricity produces no air pollution while operating, but building reservoirs can be costly and change habitats (CED: ENG-3.L, ENG-3.M). For AP review, see the Topic 6.9 study guide (https://library.fiveable.me/ap-environmental-science/unit-6/hydroelectric-power/study-guide/r95V6RauJ8xe1P3Os36x) and practice problems (https://library.fiveable.me/practice/ap-environmental-science).

Building hydroelectric dams has some big environmental trade-offs. Dams create reservoirs that flood land, permanently changing or destroying terrestrial and riverine habitats and displacing people and wildlife. They alter natural river flow, which disrupts sediment transport (leading to sedimentation upstream and erosion downstream), harms migratory fish (blocks routes unless fish ladders are effective), and changes water temperature and oxygen levels—sometimes causing algal blooms or increased methane from decaying organic matter. Large reservoirs can also trigger reservoir-induced seismicity. Hydropower itself produces no air pollution, but dam construction is costly and causes long-term ecosystem change (see EK ENG-3.M.1 and ENG-3.L.1 in the CED). For AP review, study impacts and mitigation (fish ladders, run-of-river designs, managed flow releases) in the Topic 6.9 study guide (https://library.fiveable.me/ap-environmental-science/unit-6/hydroelectric-power/study-guide/r95V6RauJ8xe1P3Os36x). For broader Unit 6 context, check the unit overview (https://library.fiveable.me/ap-environmental-science/unit-6) and practice questions (https://library.fiveable.me/practice/ap-environmental-science).

Short answer: Hydroelectric plants don’t burn fuel, so they don’t produce typical air pollution or industrial solid waste like coal plants do (this is exactly what the CED says: EK ENG-3.M.1). But that doesn’t mean they’re impact-free. Building dams and reservoirs can flood terrestrial habitats, change river flow and sedimentation, block fish migrations (so fish ladders or bypasses are sometimes needed), and even cause reservoir-induced seismicity. Large reservoirs can also produce methane from decomposing flooded vegetation—a greenhouse-gas concern that’s sometimes overlooked. Run-of-river and tidal systems have smaller footprints and fewer habitat changes. For AP exam framing, mention EK ENG-3.L.1 (how hydro is generated) and ENG-3.M.1 (environmental effects) if asked. For a focused review, see the Topic 6.9 study guide (https://library.fiveable.me/ap-environmental-science/unit-6/hydroelectric-power/study-guide/r95V6RauJ8xe1P3Os36x) and practice questions at (https://library.fiveable.me/practice/ap-environmental-science).

Hydroelectric plants are usually more expensive to build up front than most fossil-fuel plants and many wind/solar projects because dams, reservoirs, and large turbines require heavy civil engineering and materials. However, once built they have low operating costs, long lifespans, and produce no air pollution during operation—so levelized (long-term) costs can be competitive. Small run-of-river projects cost less initially but give less storage and flexibility. Remember the AP CED: construction can be expensive and can change or remove habitats (EK ENG-3.M.1). For exam prep, know the tradeoffs: high capital cost vs. low operating emissions and long-term reliability (ENG-3.L, ENG-3.M). For a concise topic review, check the hydroelectric study guide (https://library.fiveable.me/ap-environmental-science/unit-6/hydroelectric-power/study-guide/r95V6RauJ8xe1P3Os36x). For broader unit review and lots of practice problems, use the unit page (https://library.fiveable.me/ap-environmental-science/unit-6) and practice set (https://library.fiveable.me/practice/ap-environmental-science).

When a dam is built across a river it reshapes whole ecosystems. Upstream, a reservoir floods terrestrial habitat (loss/change of habitat, EK ENG-3.M.1), forcing plants and animals out. Downstream, altered flow regimes change water temperature, dissolved oxygen, and sediment transport—that can reduce food and spawning habitat for fish. Migratory species (like salmon) are blocked from reaching spawning grounds unless fish ladders or bypasses are installed, and turbines can injure or kill fish. Sedimentation behind the dam traps nutrients and can starve downstream ecosystems; reservoir drawdowns also expose shorelines and change wetland habitat. Reservoirs can increase disease vectors and sometimes trigger reservoir-induced seismicity. Fish ladders and run-of-river designs help but don’t fully fix impacts. For AP exam prep, link this to EK ENG-3.L.1 (how dams generate hydroelectricity) and EK ENG-3.M.1 (environmental effects). Review Topic 6.9 on Fiveable (https://library.fiveable.me/ap-environmental-science/unit-6/hydroelectric-power/study-guide/r95V6RauJ8xe1P3Os36x) and practice questions (https://library.fiveable.me/practice/ap-environmental-science).

You can’t just put turbines in every river because of physical, ecological, and economic limits. Hydroelectric (ENG-3.L) needs enough water flow and “head” (height difference) to spin turbines efficiently—small streams often don’t provide that year-round. Building dams or even run-of-river turbines can change habitats, block fish migration, cause sedimentation and reservoir-induced seismicity, and be very expensive (ENG-3.M). Some rivers are protected, used for navigation, or seasonal (dry parts in summer), so power output would be unreliable. Also, cumulative impacts on ecosystems and communities make widespread installs unsustainable. For AP exam revision, focus on differences between dam/reservoir systems and run-of-river, plus environmental trade-offs (see the Topic 6.9 study guide on Fiveable: https://library.fiveable.me/ap-environmental-science/unit-6/hydroelectric-power/study-guide/r95V6RauJ8xe1P3Os36x). For practice questions, check Fiveable’s practice set (https://library.fiveable.me/practice/ap-environmental-science).

Tidal energy uses the rise and fall (and flow) of ocean tides to turn turbines, while “regular” hydroelectric power usually uses river water stored behind dams or in run-of-river systems to spin turbines (CED EK ENG-3.L.1–2). Key differences: tidal systems harness predictable tidal cycles (usually two high and two low tides daily) and often use tidal barrages, underwater tidal turbines, or tidal stream arrays; hydroelectric dams create reservoirs and rely on gravitational water release. Both generate electricity without direct air pollution, but impacts differ: dams flood upstream habitats and cause sedimentation and reservoir-induced seismicity, while tidal projects can alter estuary ecology, change sediment transport, and affect marine wildlife (fish interactions). For AP exam focus, know types (dam/reservoir, run-of-river, tidal barrage) and environmental trade-offs (EK ENG-3.M.1). For a quick review, see the Topic 6.9 study guide (https://library.fiveable.me/ap-environmental-science/unit-6/hydroelectric-power/study-guide/r95V6RauJ8xe1P3Os36x) and try practice problems (https://library.fiveable.me/practice/ap-environmental-science).

Think of a turbine like a water-powered windmill. Water behind a dam has gravitational potential energy because it’s high up in a reservoir. When the dam releases water, that potential energy converts to kinetic energy as water flows down a pipe (the penstock) toward the turbine. Fast-moving water hits the turbine blades, pushing them and causing the turbine to spin. The turbine is connected to a generator, which converts that mechanical rotation into electricity. Two things control how much power you get: the flow rate (how much water per second) and the head (how far the water falls). Engineers choose blade shapes (Francis, Kaplan, Pelton) to match different heads and flows: Kaplan for low head/high flow, Pelton for high head/low flow, Francis for mid-range. Run-of-river plants skip large reservoirs and use the river’s flow to spin turbines directly. Tidal setups do the same idea but use predictable tidal flows. This aligns with the CED EK ENG-3.L points; review the Topic 6.9 study guide for diagrams and exam connections (https://library.fiveable.me/ap-environmental-science/unit-6/hydroelectric-power/study-guide/r95V6RauJ8xe1P3Os36x). For more practice, check unit resources (https://library.fiveable.me/ap-environmental-science/unit-6) and practice questions (https://library.fiveable.me/practice/ap-environmental-science).

Pros: Hydroelectric power produces electricity without air pollution or fuel waste and is a reliable, dispatchable renewable source (dams + turbines, run-of-river, pumped-storage). It gives large amounts of low-carbon power and can store energy (pumped storage) for grid balancing—useful to reduce CO2 from fossil fuels (good AP connection: ENG-3.L.1). Cons: Building dams is costly and changes habitats—flooding reservoirs displaces people and terrestrial ecosystems, blocks fish migration (salmon), and alters sediment flow, which lowers downstream fertility and changes river morphology (CED: EK ENG-3.M.1). Reservoirs can cause reservoir-induced seismicity and create methane from decomposing biomass. Smaller run-of-river setups and fish ladders reduce impacts but don’t eliminate them. For a focused review on AP-style points and examples (Three Gorges, fish ladders, sedimentation), check the topic study guide (https://library.fiveable.me/ap-environmental-science/unit-6/hydroelectric-power/study-guide/r95V6RauJ8xe1P3Os36x). For broader unit review and practice problems, see the unit page (https://library.fiveable.me/ap-environmental-science/unit-6) and practice set (https://library.fiveable.me/practice/ap-environmental-science).

Big dam turbines typically sit at large hydroelectric plants where a dam creates a reservoir. Water stored at higher elevation is released through penstocks; the high pressure and volume spin big Francis/Kaplan or Pelton turbines to drive generators (CED EK ENG-3.L.1). That storage lets dams produce steady, high-capacity power and even do pumped-storage for load balancing. Small river (run-of-river) turbines are placed directly in flowing streams without large reservoirs. They rely on kinetic flow rather than stored potential energy, so they’re lower-head and lower-capacity and produce variable output tied to river flow. Environmentally, big dams change habitats, sedimentation, and can cause reservoir-induced seismicity and habitat loss (CED EK ENG-3.M.1), while small turbines have smaller footprints and less flooding but can still affect fish passage and local flow. For AP review, remember EK ENG-3.L.1 describes both approaches; see the Topic 6.9 study guide (https://library.fiveable.me/ap-environmental-science/unit-6/hydroelectric-power/study-guide/r95V6RauJ8xe1P3Os36x) and more unit resources (https://library.fiveable.me/ap-environmental-science/unit-6). For extra practice, try problems at (https://library.fiveable.me/practice/ap-environmental-science).

Even though hydroelectric dams don’t make air pollution, people oppose new dams for several big environmental and social reasons that match the CED: building dams floods land and changes habitats (loss of terrestrial and riverine ecosystems), blocks fish migration unless mitigations like fish ladders are added, and traps sediment (sedimentation) which hurts downstream soils and deltas. Reservoirs can produce methane from decomposing organic matter, and large reservoirs have caused reservoir-induced seismicity and forced human displacement (Three Gorges is a classic example). Dams also change stream flow and water temperature, which harms aquatic species. These impacts show why the AP CED warns about habitat change after dam construction (EK ENG-3.M.1). For more exam-aligned review on hydroelectric power, check the Topic 6.9 study guide (https://library.fiveable.me/ap-environmental-science/unit-6/hydroelectric-power/study-guide/r95V6RauJ8xe1P3Os36x), Unit 6 overview (https://library.fiveable.me/ap-environmental-science/unit-6), and practice problems (https://library.fiveable.me/practice/ap-environmental-science).