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Harnessing the force of falling water may be the world's oldest source of mechanical power. Hydropower currently supplies 10 percent of the nation's electricity and 80 percent of the electricity now produced from renewable resources.

Normally, rain water and melting snow flows by gravity, producing streams, rivers, and lakes. Hydropower facilities intercept the water on its downward path, converting its mechanical energy into electricity. Because the cycle of water evaporating from the heat of the sun and falling back to earth is continuously renewed by the sun's energy, hydropower is often considered a renewable energy resource. However, the construction and operation of hydropower dams impact natural river systems and fish and wildlife. Whether specific hydropower projects create unacceptable environmental damage requires a case-by-case review.

There are several types of hydropower facilities:

  • "Storage" projects impound water behind a dam, forming a reservoir. Water is released through turbine-generators to produce electricity. The water storage and release cycles can be relatively short, for instance, storing water at night for daytime power generation. Or, the cycles can be long, storing spring runoff for generation in the summer when air conditioner use increases power demand. Some projects operate on multi-year cycles carrying over water in a wet year to offset the effects of dry years.

  • "Run-of-river" projects typically use relatively low dams where the amount of water running through the powerhouse is determined by the water flowing in the river. Because these plants generally do not hold back water behind storage dams, they tend to affect upstream water levels and downstream stream flow less than storage projects. Electricity generation from these plants will vary with changes in the amount of water flowing in the river.

  • "Pumped-storage" projects use off-peak electricity to pump water from a lower reservoir to an upper reservoir. During periods of high electrical demand, the water is released back to the lower reservoir to generate electricity. There are only about 40 pumped-storage facilities in the U.S., but some are very large. (Note: the Power Scorecard rates electricity from pumped storage on the basis of the electricity used to pump the water and the impacts of the storage operations.)

What are the environmental impacts?

It is the dams and powerhouse operations essential to hydropower plants that cause the primary environmental impacts. The changes in river conditions and the land and vegetation bordering the water bodies caused by dams and powerhouse turbines may impact fish populations and other wildlife significantly. Even small dams can cause big impacts on to the health of regional fish populations. The impacts of large dams are wide-ranging. The impacts of any dam depend upon many important factors, including the location of the dam, the facility design, the sensitivity of the local environment to effects of the hydropower facility, and steps taken to modify the design and/or operation of each facility to reduce potential impacts.

Many impacts (see list below) can be significantly reduced by changing operations of the dam. For example, installing fish passage systems can reduce impacts on migratory fish; and converting a dam from peaking to "run-of-river" operation can ensure the natural flow of the river remains undisturbed and can adapt the hydropower facility to the unique conditions of each river system.

Because every river and every dam are different, the type and severity of impacts caused by each dam differs. Because these potential impacts are severe, it is important to distinguish the plants that have successfully reduced or eliminated specific impacts from those that have not.

Since 1987 the licensing and review process conducted by the Federal government has more thoroughly addressed environmental impacts. Before 1987 the environmental impacts of facilities were considered inconsistently and sometimes not at all. The Power Scorecard recognizes this change in the quality of environmental review by giving a better environmental rating to projects reviewed since January 1987.

Recently the Low Impact Hydropower Institute has created a Low Impact Hydropower Certification program to identify and reward efforts by dam owners to minimize the impacts of their hydropower dams. The program certifies hydropower facilities with impacts that are low compared to other hydropower facilities based on eight environmental criteria:

  1. river flows
  2. water quality
  3. fish passage and protection
  4. watershed protection
  5. threatened & endangered species protection
  6. cultural resource protection
  7. recreation
  8. facilities recommended for removal

The Power Scorecard recognizes plants that have obtained this "low impact certification" by giving them a better environmental rating.

The following paragraphs outline some of the kinds of environmental impacts hydropower plants can create and measures that can be used to mitigate such impacts. The scope and severity of such impacts vary from facility to facility, and depend on site conditions and the extent to which possible mitigation measures are actually used.

Potential environmental impacts

Hydroelectric facilities disrupt natural river flows.
By diverting water out of the river for power, dams remove water needed for healthy in-stream ecosystems. Stretches below dams may be completely de-watered. By withholding and then releasing water to generate power for peak demand periods, dams may cause downstream stretches to alternate between no water and powerful surges that erode soil and vegetation, and flood or strand wildlife. These irregular releases destroy natural seasonal flow variations that trigger natural growth and reproduction cycles in many species. Peaking power operations can also cause can cause dramatic changes in reservoir water levels - up to 40 feet - that can degrade shorelines and disturb fisheries, waterfowl, and bottom?dwelling organisms.

Dams also slow down the flow of the river. Many fish species, such as salmon, depend on steady flows to flush them downriver early in their life and guide them upstream years later to spawn. Slow reservoir pools disorient migrating fish and significantly increase the duration of their migration.

These impacts can, at times, be mitigated by technological and operational enhancements to the hydro project - e.g., minimum flow turbines, re-regulating weirs, and pulsed operation at peak efficiency. Impoundments can be managed to create new upstream and downstream habitat for fish species and to provide minimum discharges and improved habitat during seasonal or annual drought conditions.

Hydropower may alter river and riverside habitat.
Construction of a dam can flood riverside lands, destroying riparian and upland habitats. Construction of a dam can also convert river habitat into a lake-like reservoir, threatening native populations of fish and other wildlife. Warm, slow moving reservoirs favor predators of naturally occurring species. Dramatic changes in reservoir water levels, described above, can degrade shorelines and disturb fisheries, waterfowl, and bottom-dwelling organisms.

Dams alter water quality.
Impoundments can cause changes and variation in temperature or the amount of dissolved gases in the river.

Surface temperatures in the reservoir may rise when the flow of the water is slowed. If water is released from the top of the dam, this warmer water may increase river water temperature down stream. Cooler downstream temperatures may result when cool water is released from the bottom of a reservoir. Such altered conditions can affect the habitat, growth rate, or even the survival of fish and other species.

For hydropower projects with intakes located deep in the reservoir, water with low dissolved oxygen (DO) levels released to the river downstream may harm aquatic habitat in the river and contribute to other water quality problems. Applying mitigating technologies can improve dissolved oxygen levels.

Water sometimes passes over a spillway, rather than through the turbines. As water plunges into the pool at the base of the dam, too much air can be trapped in the water, creating "gas supersaturation," a condition that in some fish species fosters something called lethal gas bubble disease. This can be mitigated by installing structures to keep fish away from such areas.

A dam or a powerhouse can be a significant obstacle to fish migration.
Ladders or lifts can be installed to pass certain fish species upstream, though multiple dams on a river reduces the success rate of these fish passage devices. Fish migrating downstream can become disoriented, bruised, stressed, or mortally injured from contact with turbines or other parts of the facility. Bypass systems can improve survival rates for migrating juveniles. When fish are trucked or barged around the dams, they may experience increased stress and disease and decreased homing instincts. Survival rates for fish passing through large turbines vary but may approach 90-95 percent. In the case of multiple dams along a river these effects can significantly harm migrating populations of important, sensitive juvenile fish populations.

Impoundments also slow down the flow velocities of rivers. Slow reservoir pools may disorient migrating fish, increase the duration of their migration, which in turn may increase their mortality rate.

The steep decline in salmon populations in the Pacific Northwest and California is perhaps the best known negative environmental impact associated with hydroelectric facilities. Although several factors have affected this decline - including commercial fish harvests, habitat degradation, and artificial fish hatcheries - hydropower dams have contributed significantly. The causes for these declines and the best strategies for restoring these important fisheries are currently the subject of a major public policy debate.

Hydropower projects can impede the natural flow of sediments.
Flowing water transports sediment. When the flow velocities are reduced in an impoundment, sediment drops out and collects on river and reservoir bottoms, where it can affect habitat for fish spawning. The loss of sediment downstream can degrade in stream habitat and cause the loss of beach at the mouth of the river. The deposited sediment also may contain chemical or industrial residues from upstream sources. Dams may block and concentrate contaminated sediment in the impoundment. Dredging is used in some cases, though it is costly and may raise questions regarding disposal of the dredged material. Various flushing and piping techniques are available for moving non-contaminated sediment downstream.


Additional Information:

See also Water Use, Water Quality and Land Impacts Issue Papers for more information on hydropower impacts.

American Rivers http://www.amrivers.org/index.php?module=HyperContent&func=displayview&shortname=riverconservation

Union of Concerned Scientists: "How Hydroelectricity Energy Works" http://www.ucsusa.org/clean_energy/renewable_energy/page.cfm?pageID=82

Low Impact Hydropower Institute http://www.lowimpacthydro.org

Hydropower Reform Coalition http://www.hydroreform.org

Idaho National Engineering and Environmental Lab (INEEL)Hydropower Program http://hydropower.inel.gov/

National Hydropower Association http://www.hydro.org/

U.S. Dept. of Energy - Energy Efficiency and Renewable Energy Network/Hydro Links Page http://www.eren.doe.gov/RE/hydropower.htm

Foundation for Water and Energy Education (FWEE) http://www.fwee.org/hpar.html

Association of State Dam Safety Officials http://www.damsafety.org/

Bureau of Reclamation Hydropower Program http://www.usbr.gov/power/

Hydro Research Foundation http://www.hydrofoundation.org/

Northwest Power and Conservation Council - "Guide to Major Hydropower Dams of the Columbia River Basin" http://www.nwcouncil.org/library/2004/2004-1/default.htm

The United States Society on Dams http://www2.privatei.com/~uscold/

Wisconsin Valley Improvement Company http://www.wvic.com/hydro-works.htm

World Commission on Dams http://www.dams.org

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