Hydro-electric power plants convert the kinetic energy contained in falling water into electricity. The energy in flowing water is ultimately derived from the sun, and is therefore constantly being renewed. Energy contained in sunlight evaporates water from the oceans and deposits it on land in the form of rain. Differences in land elevation result in rainfall runoff, and allow some of the original solar energy to be captured as hydro-electric power (Figure 1).

Hydro power is currently the world's largest renewable source of electricity, accounting for 6% of worldwide energy supply or about 15% of the world's electricity. In Canada, hydroelectric power is abundant and supplies 60% of our electrical needs. Traditionally thought of as a cheap and clean source of electricity, most large hydro-electric schemes being planned today are coming up against a great deal of opposition from environmental groups and native people.

FIGURE 1: Simplified view of the hydrologic cycle in which water is raised by solar energy and can perform work as it falls back to sea level (177K).

History of Hydro Power

The first recorded use of water power was a clock, built around 250 BC. Since that time, humans have used falling water to provide power for grain and saw mills, as well as a host of other applications. The first use of moving water to produce electricity was a waterwheel on the Fox river in Wisconsin in 1882, two years after Thomas Edison unveiled the incandescent light bulb. The first of many hydro electric power plants at Niagara Falls was completed shortly thereafter. Hydro power continued to play a major role in the expansion of electrical service early in this century, both in North America and around the world. Contemporary Hydro-electric power plants generate anywhere from a few kW, enough for a single residence, to thousands of MW, power enough to supply a large city.

Early hydro-electric power plants were much more reliable and efficient than the fossil fuel fired plants of the day. This resulted in a proliferation of small to medium sized hydro-electric generating stations distributed wherever there was an adequate supply of moving water and a need for electricity. As electricity demand soared in the middle years of this century, and the efficiency of coal and oil fueled power plants increased, small hydro plants fell out of favour. Most new hydro-electric development was focused on huge "mega-projects".

The majority of these power plants involved large dams which flooded vast areas of land to provide water storage and therefore a constant supply of electricity. In recent years, the environmental impacts of such large hydro projects are being identified as a cause for concern. It is becoming increasingly difficult for developers to build new dams because of opposition from environmentalists and people living on the land to be flooded. This is shown by the opposition to projects such as Great Whale (James Bay II) in Quebec and the Gabickovo-Nagymaros project on the Danube River in Czechoslovakia.

Hydro-electric Power Plants

Hydro-electric power plants capture the energy released by water falling through a vertical distance, and transform this energy into useful electricity. In general, falling water is channelled through a turbine which converts the water's energy into mechanical power. The rotation of the water turbines is transferred to a generator which produces electricity. The amount of electricity which can be generated at a hydro-electric plant is dependant upon two factors. These factors are (1) the vertical distance through which the water falls, called the "head", and (2) the flow rate, measured as volume per unit time. The electricity produced is proportional to the product of the head and the rate of flow. The following is an equation which may be used to roughly determine the amount of electricity which can be generated by a potential hydro-electric power site:

POWER (kW) = 5.9 x FLOW x HEAD

In this equation, FLOW is measured in cubic meters per second and HEAD is measured in meters.

Based on the facts presented above, hydro-electric power plants can generally be divided into two categories. "High head" power plants are the most common and generally utilize a dam to store water at an increased elevation. The use of a dam to impound water also provides the capability of storing water during rainy periods and releasing it during dry periods. This results in the consistent and reliable production of electricity, able to meet demand. Heads for this type of power plant may be greater than 1000 m. Most large hydro-electric facilities are of the high head variety. High head plants with storage are very valuable to electric utilities because they can be quickly adjusted to meet the electrical demand on a distribution system.

"Low head" hydro-electric plants are power plants which generally utilize heads of only a few meters or less. Power plants of this type may utilize a low dam or weir to channel water, or no dam and simply use the "run of the river". Run of the river generating stations cannot store water, thus their electric output varies with seasonal flows of water in a river. A large volume of water must pass through a low head hydro plant's turbines in order to produce a useful amount of power. Hydro-electric facilities with a capacity of less than about 25 MW (1 MW = 1,000,000 Watts) are generally referred to as "small hydro", although hydro-electric technology is basically the same regardless of generating capacity.

"Pumped Storage" is another form of hydro-electric power. Pumped storage facilities use excess electrical system capacity, generally available at night, to pump water from one reservoir to another reservoir at a higher elevation. During periods of peak electrical demand, water from the higher reservoir is released through turbines to the lower reservoir, and electricity is produced (Figure 2). Although pumped storage sites are not net producers of electricity - it actually takes more electricity to pump the water up than is recovered when it is released - they are a valuable addition to electricity supply systems. Their value is in their ability to store electricity for use at a later time when peak demands are occurring. Storage is even more valuable if intermittent sources of electricity such as solar or wind are hooked into a system.

FIGURE 2: The pumped storage plant showing two stages of operation (83K).

Environmental Impacts

Hydro-electric power plants have many environmental impacts, some of which are just beginning to be understood. These impacts, however, must be weighed against the environmental impacts of alternative sources of electricity. Until recently there was an almost universal belief that hydro power was a clean and environmentally safe method of producing electricity. Hydro-electric power plants do not emit any of the standard atmospheric pollutants such as carbon dioxide or sulfur dioxide given off by fossil fuel fired power plants. In this respect, hydro power is better than burning coal, oil or natural gas to produce electricity, as it does not contribute to global warming or acid rain. Similarly, hydro-electric power plants do not result in the risks of radioactive contamination associated with nuclear power plants.

A few recent studies of large reservoirs created behind hydro dams have suggested that decaying vegetation, submerged by flooding, may give off quantities of greenhouse gases equivalent to those from other sources of electricity. If this turns out to be true, hydro-electric facilities such as the James Bay project in Quebec that flood large areas of land might be significant contributors to global warming. Run of the river hydro plants without dams and reservoirs would not be a source of these greenhouse gases.

The most obvious impact of hydro-electric dams is the flooding of vast areas of land, much of it previously forested or used for agriculture. The size of reservoirs created can be extremely large. The La Grande project in the James Bay region of Quebec has already submerged over 10,000 square kilometers of land; and if future plans are carried out, the eventual area of flooding in northern Quebec will be larger than the country of Switzerland. Reservoirs can be used for ensuring adequate water supplies, providing irrigation, and recreation; but in several cases they have flooded the homelands of native peoples, whose way of life has then been destroyed. Many rare ecosystems are also threatened by hydro-electric development.

Large dams and reservoirs can have other impacts on a watershed. Damming a river can alter the amount and quality of water in the river downstream of the dam, as well as preventing fish from migrating upstream to spawn. These impacts can be reduced by requiring minimum flows downstream of a dam, and by creating fish ladders which allow fish to move upstream past the dam. Silt, normally carried downstream to the lower reaches of a river, is trapped by a dam and deposited on the bed of the reservoir. This silt can slowly fill up a reservoir, decreasing the amount of water which can be stored and used for electrical generation. The river downstream of the dam is also deprived of silt which fertilizes the river's flood-plain during high water periods.

Bacteria present in decaying vegetation can also change mercury, present in rocks underlying a reservoir, into a form which is soluble in water. The mercury accumulates in the bodies of fish and poses a health hazard to those who depend on these fish for food. The water quality of many reservoirs also poses a health hazard due to new forms of bacteria which grow in many of the hydro rivers. Therefore, run of the river type hydro plants generally have a smaller impact on the environment.

The Future of Hydro-Electric Power

The theoretical size of the worldwide hydro power is about four times greater than that which has been exploited at this time. The actual amount of electricity which will ever be generated by hydro power will be much less than the theoretical potential. This is due to the environmental concerns outlined above, and economic constraints. Much of the remaining hydro potential in the world exists in the developing countries of Africa and Asia. Harnessing this resource would require billions of dollars, because hydro-electric facilities generally have very high construction costs. In the past, the World Bank has spent billions of foreign aid dollars on huge hydro-electric projects in the third world. Opposition to hydro power from environmentalists and native people, as well as new environmental assessments at the World Bank will restrict the amount of money spent on hydro-electric power construction in the developing countries of the world.

In North-America and Europe, a large percentage of hydro power potential has already been developed. Public opposition to large hydro schemes will probably result in very little new development of big dams and reservoirs. Small scale and low head hydro capacity will probably increase in the future as research on low head turbines, and standardized turbine production, lowers the costs of hydro-electric power at sites with low heads. New computerized control systems and improved turbines may allow more electricity to be generated from existing facilities in the future. As well, many small hydro electric sites were abandoned in the 1950's and 60's when the price of oil and coal was very low, and their environmental impacts unrealized. Increased fuel prices in the future could result in these facilities being refurbished.


Hydro-electric power has always been an important part of the world's electricity supply, providing reliable, cost effective electricity, and will continue to do so in the future. Hydro power has environmental impacts which are very different from those of fossil fuel power plants. The actual effects of dams and reservoirs on various ecosystems are only now becoming understood. The future of hydro-electric power will depend upon future demand for electricity, as well as how societies value the environmental impacts of hydro-electric power copmpared to the impacts of other sources of electricity.

Written by Stuart Baird, MEng., MBA.


Philip Raphals, "The Hidden Cost of Canada's Cheap Power", New Scientist, February 15, 1992.

Geoffrey P. Sims, "Hydroelectric Energy", Energy Policy, october 1991.

Eric M. Wilson, "Small-scale Hydroelectricity", Energy Policy, october 1991.

Michael Brower, Cool Energy: The Renewable Solution to Global Warming, Union of Concerned Scientists, 1990.

Solar Energy Research Institute, The Potential of Renewable Energy, An Interlaboratory White Paper, SERI/TP-260-3674, Golden Colorado, 1990.

Peter Kakela, Gary Chilson & William Patric, "Low-Head Hydropower for Local Use", Environment, January/February 1984.

To purchase a copy of this Fact Sheet, please visit the ICLEI Web Store where they are available for $3.50 (US) each or $50 (US) for the set of 30. We accept VISA and MasterCard through our secure server.