Environmental Costs of Hydroelectricity

The Glen Canyon Dam on the Colorado River. Matthew Micah Wright/Lonely Planet/Getty

Hydroelectricity is a significant source of power in many regions of the globe, providing 24% of the global electricity needs. Brazil and Norway rely almost exclusively on hydropower. In the United States, 7 to 12% of all electricity is produced by hydropower; the states which depend the most on it are Washington, Oregon, California, and New York.

Hydropower is when water is used to activate moving parts, which in turn may operate a mill, an irrigation system, or an electric turbine (in which case we can use the term hydroelectricity).

Most commonly, hydroelectricity is produced when water is held back by a dam, led down a penstock through a turbine, and then released in the river below. The water is both pushed by pressure from the reservoir above and pulled by gravity, and that energy spins a turbine coupled to a generator producing electricity. The rarer run-of-the-river hydroelectric plants also have a dam, but no reservoir behind it; turbines are moved by the river water flowing past them at the natural flow rate.

Ultimately, the generation of electricity relies on the natural water cycle to refill the reservoir, making it a renewable process with no input of fossil fuel needed. Our use of fossil fuels is associated with a multitude of environmental problems: for example, the extraction of oil from tar sands produces air pollution; fracking for natural gas is associated with water pollution; and the burning of fossil fuels produces climate change-inducing greenhouse gas emissions.

We therefore look to sources of renewable energy as clean alternatives to fossil fuels. However like all sources of energy, renewable or not, there are environmental costs associated with hydroelectricity. Here is a review of some of those costs, along with some benefits.


  • Barrier to Fish. Many migratory fish species swim up and down rivers to complete their life cycle. Anadromous fish, like salmon, shad, or Atlantic sturgeon, go upriver to spawn, and young fish swim down river to reach the sea. Catadromous fish, like the American eel, live in the rivers until they swim out to the ocean to breed, and the young eels (elvers) come back to freshwater after they hatch. Dams obviously block the passage of these fish. Some dams are equipped with fish ladders or other devices to let them pass unharmed. The effectiveness of these structures is quite variable, but improving.
  • Changes in Flood Regime. Dams can buffer large, sudden volumes of water following spring melt of heavy rains. That can be a good thing for downstream communities (see Benefits below), but it also starves the river from a periodic influx of sediment, and prevents the natural high flows from regular re-countering of the river bed, which renews habitat for aquatic life. To recreate these ecological processes, authorities periodically release large volumes of water down the Colorado River, with positive effects on the native vegetation alongside the river.
  • Temperature and Oxygen Modulation. Depending on the design of the dam, water released downstream often comes from the deeper parts of the reservoir. That water is therefore much the same cold temperature throughout the year. This has negative impacts on aquatic life adapted to wide seasonal variations in water temperature. Similarly, low oxygen levels in released water can kill aquatic life downstream, but the problem can be mitigated by mixing air into the water at the outlet.  
  • Evaporation. Reservoirs increase a river’s surface area, thus increasing the amount of water lost to evaporation. In hot, sunny regions the losses are staggering: more water is loss from reservoir evaporation than is used for domestic consumption. When water evaporates, dissolved salts are left behind, increasing salinity levels downstream and harming aquatic life.
  • Mercury Pollution. Mercury is deposited on vegetation long distances downwind from coal-burning power plants. When new reservoirs are created, the mercury found in the now submerged vegetation is released, and converted by bacteria into methylmercury. This methylmercury becomes increasingly concentrated as it moves up the food chain (a process called bio-magnification). Consumers of predatory fish, including humans, are then exposed to dangerous concentrations of the toxic compound.
  • Methane Emissions. Reservoirs often become saturated with nutrients coming from decomposing vegetation or nearby agricultural fields. These nutrients are consumed by algae and microorganisms which in turn release large amounts of methane, a powerful greenhouse gas. This problem has of yet not been studied enough to understand its true extent.


    • Flood control. Reservoir levels can be lowered in anticipation of heavy rain or snowmelt, buffering the communities downstream from dangerous river levels.
    • Recreation. Large reservoirs are often used for recreational activities like fishing and boating.
    • Alternative to Fossil Fuels. Producing hydroelectricity releases a lower net amount of greenhouse gases than fossil fuels. As part of a portfolio of energy sources, hydroelectricity allows greater reliance on domestic energy, as opposed to fossil fuels mined overseas, in locations with less stringent environmental regulations.

    Some Solutions

    Because the economic benefits of older dams wane while the environmental costs mount, we have seen any increase in dam decommissioning and removal. These dam removals are spectacular, but most importantly they allow scientists to observe how natural processes are restored along the rivers.  

    Much of the environmental problems described here are associated with large-scale hydroelectric projects. There is a multitude of very small scale projects (often called “micro hydro”) where judiciously placed small turbines use low-volume streams to produce electricity for a single home or a neighborhood. These projects have little environmental impact if properly designed.