Dams and Reservoirs


Dams are structures that restrict the flow of water in a river or stream. Both streams and rivers are bodies of flowing surface water driven by gravity that drain water from the continents.

Once a body of flowing surface water has been slowed or stopped, a reservoir or lake collects behind the dam. Dams and reservoirs exist in nature, and man-made water control structures are patterned after examples in the natural word.

Many lakes are held back by rock dams created by geologic events such as volcanic eruptions, landslides or the upward force of Earth that creates mountains. Humans and beavers alike have discovered how to modify their natural environment to suit their needs by constructing dams and creating artificial lakes.

Dams are classified into four main types: gravity, embankment, buttress, and arch.
  • Gravity dams: Gravity dams are massive earth, masonry (brick or stone work), rock fill, or concrete structures that hold back river water with their own weight. They are usually triangular with their point in a narrow gorge (deep ravine). The Grand Dixence dam in the Swiss Alps is the world’s tallest gravity dam.
  • Embankment dams: Embankment dams are wide areas of compacted earth or rock fill with a concrete or masonry core that contains a reservoir, while allowing for some saturation and shifting of the earth around the dam, and of the dam within the earth.
  • Buttress dams: Buttress dams have supports that reinforce the walls of the dam and can be curved or straight. Buttresses on large modern dams, such as the Itaipu dam in Brazil, are often constructed as a series of arches and are made of concrete reinforced with steel.
  • Arch dams: Arch dams are curved dams that depend on the strength of the arch design to hold back water. Like gravity dams, they are most suited to narrow, V-shaped river valleys with solid rock to anchor the structure. Arch dams, however, can be much thinner than gravity dams and use less concrete.

Dams in history

Humans have used dams to trap and store fresh water in reservoirs for more than 5,000 years. Although water is ultimately a plentiful, renewable resource on Earth (Earth is after all "the water planet"), fresh water is scarce or only seasonally available in many regions. Left unregulated, the rivers and streams that provide humans’ most essential natural resource are often hazardous to human life and too unpredictable to provide a constant source of fresh water.

The ancient civilizations of Egypt, Assyria, Mesopotamia, and China grew and prospered in part because construction of dams and reservoirs allowed for irrigation (watering) of arid (extremely dry) lands, control of seasonal floods, and water storage during dry weather. If Earth’s streams and rivers are veins that support human survival, dams are valves that regulate the flow of water through those vessels.

Humans today depend on dams to store water for irrigation, drinking water, and flood control just as they did in the ancient Middle East. Mesopotamians and Sumerians used weirs (low dams built across streams or rivers) and channels (passage for water) to irrigate the land between the Tigris and Euphrates Rivers, called the Fertile Crescent, about 6,500 years ago.

Earthen dams that hold drinking and irrigation water in reservoirs for small towns and farms around the world today resemble the earliest known remains of dams. Archeologists estimate that a rock weir and series of small dams and reservoirs near the modern-day town of Jawa in Jordan were constructed about 5,000 years ago.

Systems of aqueducts (artificial channels for conveying water) and canals (man-made watercourses designed to carry goods or water) like those constructed during the Roman Empire (1500-2000 years ago ) carry water from reservoirs to modern farmlands and cities.

Dams and reservoirs have a second important use beyond water storage and regulation of river flows. They can be used to generate hydropower, one of humans’ oldest, simplest, and cleanest forms of renewable and reusable energy. Water that is held in a reservoir above the elevation of the river downstream has stored energy called hydrologic potential.

When water is released through the dam from the reservoir, its motion can be used to turn a wheel that can then power a mill or an electrical generator. The farther the water falls, the more energy it releases. Water scientists and engineers use the height of the reservoir surface, called the hydraulic head, to estimate the amount of potential energy stored behind a dam.

The technology to harness the mechanical power of falling water is almost as old as that for water storage and flood control. Ancient Sumerians and Egyptians used waterwheels with buckets on their blades, called norias, to dip water from streams or rivers.

By 2,500 years ago, waterwheels drove grain mills and pumped water from wells in the Greek and Roman Empires. During the late Middle Ages, water mills in the industrial centers of Germany and Italy ground grain, pulped wood for paper, spun silk for textiles, pounded metal, tanned hides, and crushed ore (mineral deposits) from mines.

During the Industrial Revolution of the nineteenth century, British civil and mining engineers constructed 200 dams taller than 49 feet (15 meters, which is about the height of a five-story building) to store water for Britain’s rapidly growing cities and to provide hydropower for mining and transport of coal, the energy source that powered industrialization.

Modern dams

Today’s dams and reservoirs provide many of the same benefits to humans and rely on the same basic technology as they did in ancient times. However, the size and complexity of modern water control and structures and systems would have astounded ancient Greeks and nineteenth century engineers.

In developed nations like the United States, all of the major rivers have been dammed and almost every river system has been altered by humans. Worldwide, there were over 45,000 dams taller than 49 feet (15 meters) in 150 countries at the end of the twentieth century. Today, dams hold water for irrigation, control flooding along rivers, provide water for cities, and generate about one-fifth of the world’s electricity.

In the countries with the most dams—China, the United States, and the nations of the former Soviet Union—engineering has given humans almost complete control over the rivers. In fact, one of the main reasons humans can no longer depend on hydropower to meet rising electricity needs is that there are very few large rivers left on Earth to be dammed.

Dams are, by nature, destined to fail. A river erodes (wears away) and deposits sediment (particles of sand, silt, and clay) along its path from where it originates to the ocean in an attempt to create a constant slope (slanting contour of the land) called a graded profile. When a dam, natural or otherwise, blocks a river, the river adjusts to a new pattern of erosion and deposition in an attempt to return to its graded profile.

In essence, the river attempts to remove the obstacle; reservoirs fill with sediment, and downstream erosion cuts under dams. Dams built before the 1930s were constructed with little knowledge of how rivers work or how structures can be designed to resist failure. One in ten dams built in the United States before 1930 has collapsed.

In 1889, more than 2,200 people were killed when the earthen embankment above Johnstown, Pennsylvania failed and the town was flooded. By the 1930s, use of concrete and metal in dams, arched designs, and an understanding of rivers allowed engineers to build safer, stronger dams. The new technology also led to an era of construction of ever-larger dams that has lasted until the present.

Environmental and social implications of superdams

Three Gorges Dam
Three Gorges Dam

The world’s largest dams are massive structures over 492 feet or 150 meters tall (more than three times the height of the Statue of Liberty) that hold back reservoirs that cover a total land area about the size of Nebraska and Kansas combined.

Construction of more than 300 super dams since the early twentieth century has created both benefits and problems for people living nearby The economic, social, and environmental costs of major dams like the Grand Coulee dam on the Columbia River in the United States, the High Aswan dam on the Nile in Egypt, the Itaipu dam on the Parana River between Brazil and Paraguay, the La Grande dams in Canada, and the Three Gorges Dam across the Yangtze River in China are extremely high and could possibly, according to many geologists, exceed the long-term benefits of the projects.

Problems associated with very large dams are now becoming apparent to geologists. According to the World Commission on Dams (WCD), between 30 and 60 million people, mostly poor farmers and people in India and China, have been displaced by large hydropower projects. Irreplaceable natural, archeological, and historical sites are drowned beneath huge reservoirs.

Drowned vegetation contaminates reservoir water and fish. Dams like Hoover and Glen Canyon dams on the Colorado River in the United States, or the Aswan High Dam on the Nile, disrupt river systems so large that the ecology (living environment) of an entire region has to adjust.

Downstream, agricultural lands may lose their fertility, water quality is poor, and natural ecosystems (interactions between living organisms and their environment) are harmed. Coastal erosion results when rivers no longer replenish deltas (land area before river enters larger body of water) with sediment.

Many environmental groups, scientists, and even some governments have begun to seek solutions to the problems presented by large dams. Decreasing the size and number of new dams, discovering new energy alternatives, managing river flows to counteract harmful environmental and social effects, and even removing some dams have all been considered.

With the new goal of using dams and reservoirs to create a sustainable human and natural environment, modern and ancient water management technology combined could serve well far into the future.