Library Index » Science Encyclopedia » Threats to Aquatic Environments - Dams, Freshwater Diversion And Use, Water Pollution Poses A Threat, Sediment—good And Bad, Air Pollution Affects Water Quality

Threats to Aquatic Environments - Dams

river water species salmon

Dams have affected rivers, the lands abutting them, the water bodies they join, and aquatic wildlife throughout the United States. Water flow is reduced or stopped altogether downstream of dams, altering aquatic habitats and drying wetlands. Some rivers, including the large Colorado River, no longer reach the sea at all, except in years of unusually high precipitation. Keeping enough water in rivers is especially difficult in the arid West. Of the major rivers in the lower forty-eight states (those more than 600 miles in length), only the Yellowstone River still flows freely. In fact, University of Alabama biologist Arthur Benke, the editor of Rivers of North America (Elsevier, 2005), notes that it is difficult to find any river in the United States that has not been dammed or channeled. According to Benke, "All human alterations of rivers, regardless of whether they provide services such as power or drinking water supply, result in degradation. The only exception is when we try to restore them."

Dams have epitomized progress, American ingenuity, and humankind's mastery of nature. In North America, more than two 200 major dams were completed each year between 1962 and 1968. Dams were promoted for their role in water storage, energy generation, flood control, irrigation, and recreation.

The very success of the dam-building endeavor accounted, in part, for its decline. By 1980 nearly all the nation's best-suited sites—and many dubious ones—had been dammed. Three other factors, however, also contributed to the decline in dam construction: public resistance to the enormous costs, a growing belief that dams were unnecessary "pork-barrel" projects being used by politicians to boost their own popularity, and a developing awareness of the profound environmental degradation caused by dams. In 1986 Congress passed a law requiring the U.S. Bureau of Reclamation to balance issues of power generation and environmental protection when it licenses dams.

The U.S. Army Corps of Engineers maintains the National Inventory of Dams (NID) at the Web site As of 2006 the inventory included approximately 76,000 dams throughout the country. Dams are listed that are at least six feet tall or hold back at least fifteen acre-feet (nearly five million gallons) of water. Dams are built for a variety of purposes. The most frequent purposes listed in the NID database are recreation, fire protection, stock or small farm pond creation, flood control, and storm water management. As shown in Figure 4.1 these purposes account for 70% of all purposes listed.

Although only a small percentage of dams listed with the NID produce hydroelectric power, these dams tend to be the largest in size and affect large watersheds. Figure 4.2 shows the components of a typical impoundment dam producing hydroelectric power. These structures provide numerous challenges to aquatic species, besides impeding water flow and migration paths. Turbines operate like massive underwater fans. Passage through running turbine blades can result in the death of many small aquatic creatures unable to escape their path. Some modern FIGURE 4.1 Purposes of U.S. dams Adapted from "Dams Data," in National Inventory of Dams, U.S. Army Corps of Engineer, U.S. Army Topographic Engineering Center, 2006, (accessed January 31, 2006) FIGURE 4.2 Impoundment hydropower plant "An Impoundment Hydropower Plant Dams Water in a Reservoir," in Renewable Energy Sources in the United States: Hydropower, U.S. Department of the Interior, National Atlas of the United States, January 24, 2006, (accessed January 31, 2006)hydroelectric dams include stair-like structures called fish ladders that provide migrating fish a path to climb up and over the dams.

The Snail Darter

The snail darter, a small fish species related to perch, was at the center of a dam-building controversy during the 1970s. The U.S. Fish and Wildlife Service (FWS) listed the snail darter as endangered in 1975. At the time it was believed to exist only in the Little Tennessee River, and this area was designated as critical habitat for the species. That same year, the Tellico Dam was near completion on the Little Tennessee River, and the filling of the Tellico Reservoir would have destroyed the entire habitat of the snail darter. A lawsuit was filed to prevent this from happening. The case went all the way to the Supreme Court, which ruled in 1978 that under the Endangered Species Act (ESA) species protection must take priority over economic and developmental concerns. One month after this court decision, Congress amended the Endangered Species Act to allow for exemptions under certain circumstances.

In late 1979 the Tellico Dam received an exemption and the Tellico Reservoir was filled. The snail darter is now extinct in that habitat. Fortunately, however, snail darter populations were later discovered in other river systems. In addition, the species has been introduced into several other habitats. Due to an increase in numbers, the snail darter was reclassified as threatened in 1984.

The Missouri "Spring Rise" Issue

During the 2000s a heated debate has surrounded the issue of water flow on the Missouri River. The U.S. Fish and Wildlife Service determined in 2000 that existing water flow patterns—managed to create a steady depth for barge traffic—were endangering three listed species: the pallid sturgeon (a fish) and two bird species, the piping plover and least tern. The FWS argued that increased water flow in the spring—a "spring rise"—was necessary for sturgeon spawning. In addition, it called for less water flow in the summer, which is necessary for exposing the sandbars used by the bird species as nesting grounds. The FWS and the Army Corps of Engineers, which manages the flow of water on the Missouri River, implemented a water management plan beginning with the 2003 season. The issue has been extremely controversial in the Midwest, with environmentalists, recreation interests, and upper-basin officials favoring a spring rise, and farmers, barge interests, and Missouri leaders opposed.

Salmon and Dams

Numerous species of salmon are in decline, at least partly due to the effects of dams. Salmon have an unusual life cycle that involves a migration from freshwater habitats to oceans and back. Hatching and the juvenile period occur in rivers, followed by a long downstream migration to the ocean, where individuals mature. Adult salmon eventually make an arduous, upstream return to freshwater habitats, where they spawn (lay their eggs, burying them in gravel nests) and then die. Dams are associated FIGURE 4.3 Flow regime for the Columbia River, 1916, 1966, and 1980 D.L. Bottom, C.A. Simestad, J. Burke, A.M. Baptista, D.A. Jay, K.K. Jones, E. Casillas, and M.H. Schiewe, "Figure 83. River Flows (Cubic Meters per Second) for 1916, 1966, and 1980," in Salmon at River's End: The Role of the Estuary in the Decline and Recovery of Columbia River Salmon, U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Marine Fisheries Service, Northwest Fisheries Science Center 2005, (accessed February 13, 2006)with high salmon mortality during both downstream and upstream migrations.

Scientists are increasingly learning of the importance of estuaries to juvenile salmon in the Pacific Northwest. Estuaries are areas where freshwater meets and mixes with ocean water. Juvenile salmon pass through estuaries on their downstream trip to the sea. Prior to excessive damming of the Columbia River system, spring and summer floods (called freshets) would have spread juvenile salmon throughout the estuaries into various marshes and natural channels. From there the salmon would make their way to the sea. Flow regulation has dramatically changed the river flows in this area and limited the amount of estuary habitat available to the salmon. Figure 4.3 shows daily river flows recorded in 1916, 1966, and 1980. The flows in 1916 were highly variable on a seasonal basis, reflecting the natural effects of melting snows and heavy spring rains. Damming the river dampened these seasonal changes and virtually eliminated freshets from occurring. This result has been beneficial for residents and farmers living along the river, but detrimental to the habitat of the Pacific salmon.

Tearing Down Dams?

In November 1997, for the first time in U.S. history, the Federal Energy Regulatory Commission ordered a dam removed when the Edwards Dam was ordered FIGURE 4.4 The Three Gorges Dam, currently under construction on the Yangtze River in China, will be the largest dam in the world when it is completed. (AP/Wide World Photos.)removed from the Kennebec River in Augusta, Maine, to restore habitats for sea-run fish. The dam's owner, Edwards Manufacturing, appealed the decision, but the federal government prevailed. The 160-year old dam produced 1% of Maine's electricity. Normal river conditions were achieved at the site within days of water release. Environmentalists viewed the removal of the dam as a boon to both aquatic species and the terrestrial species that feed on them.

Conservationists and fisheries have also argued for the removal of four dams on the Snake River in the Pacific Northwest to allow salmon runs to recover. The issue was extremely contentious, with more than 8,700 people attending public hearings on the debate and over 230,000 written comments submitted. The Army Corps of Engineers announced in February 2002 that the dams would not be removed, citing the fact that they produce $324 million in electricity and water with operating costs of only $36.5 million. The Corps did agree, however, to budget $390 million over a ten-year period to improve salmon survival, including trucking juvenile salmon around the dams. This decision represented the culmination of nearly ten years of debate regarding the Snake River dams.

Foreign Dams

As the era of big dams faded in North America, construction increased in Asia, fueled by growing demand for electricity and irrigation water. The Three Gorges Dam on the Yangtze River in China (see Figure 4.4) will be the largest dam in the world when it becomes operational in 2009. It will be 6,600 feet—more than a mile—wide and over 600 feet high. The creation of a water reservoir upstream from the dam will flood thirteen cities and countless villages, and displace more than a million people. In addition, the dam will disrupt water flow and increase water pollution, threatening unique species such as the Yangtze River dolphin, one of only five freshwater dolphin species in the world.

The Yangtze River dolphin was placed on the Endangered Species List in 1989 and is at extreme risk of extinction, with only 150 individuals remaining. Other species likely to be threatened or wiped out altogether include the Chinese sturgeon, the Chinese tiger, the Chinese alligator, the Siberian crane, the giant panda, and countless species of fish, freshwater invertebrates, and plants. Several U.S. agencies provided much technical assistance in planning the Three Gorges Dam. However, U.S. government involvement ceased due to a challenge under the Endangered Species Act, which prohibits government activity detrimental to listed species. The main part of dam construction has been completed, and filling of the Three Gorges Dam began in June 2003. However, generators have yet to be installed as of 2006.

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