Library Index » Science Encyclopedia » Aquatic Species and Their Environments - Water Pollution—many, Many Causes, Dams, Water Diversion—the Aral Sea, Overfishing—too Many Boats, Not Enough Fish

Aquatic Species and Their Environments - Dams

river salmon water rivers

Some 100,000 dams regulate America's rivers and creeks. Of the major rivers in the lower 48 states (those more than 600 miles in length), only the Yellowstone River still flows freely. In fact, University of Alabama ecologist Arthur Benke notes that it is difficult to find any river in the United States that hasn't been dammed or channeled.

Dams epitomized progress, American ingenuity, and humankind's mastery of nature. In North America, more than 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. Worldwide, dams now collectively store 15 percent of Earth's annual renewable water supply. Figure 5.7 illustrates the primary uses of U.S. dams.

The very success of the dam-building endeavor accounted, in part, for its decline. By 1980 nearly all the FIGURE 5.3
Biomagnification of mercury in the food chain
FIGURE 5.4
Trends in number of lake acres under advisory for various pollutants, 1993–2002
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.

Where Have All the Rivers Gone?

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 FIGURE 5.5
Total number of fish consumption advisories, 2002
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.

Numerous species of salmon are in decline, at least partly due to the effects of dams. In the Pacific Northwest in particular, most experts estimate that native salmon will be gone in 25 years. 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 with high salmon mortality during both downstream and upstream migrations.

There are ongoing debates regarding dam management throughout the Pacific Northwest. In April 2002, for example, American Rivers, the National Wildlife Federation, and other fisheries and conservation groups initiated a lawsuit against the Grant County Public Utility District (PUD) in eastern Washington State over the management of two dams on the Columbia River. The NWF is charging that dam mismanagement is responsible for the continued decline of chinook and steelhead salmon, both of which are protected under the Endangered Species Act. Some 32 percent of juvenile salmon migrating downstream towards the ocean are killed at the dam. Many adult salmon are also dying as they swim upstream to spawning grounds. Fishermen are particularly outraged at the collapse of salmon runs, since they have been required to limit their catch in the hope of population recovery. There are a total of eight dams on the Columbia River, all of which must be surmounted successfully for salmon to complete their migrations.

Tearing Down Dams?

In November 1997, for the first time in U.S. history, the Federal Energy Regulatory Commission ordered the Edwards Dam 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 FIGURE 5.6
Percentage of lake acres and river miles under advisory, 1993–2002
prevailed. The 160-year old dam produced 1 percent 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.

Conservation and fisheries interests 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 over 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 will, however, budget $390 million over the next ten years 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. China now accounts for more than one-fourth of the big dams under construction, and China, Japan, South Korea, and India together account for more than half.

The Three Gorges Dam on the Yangtze River in China (see Figure 5.8) will be the largest dam in the world when it is completed in 2009. It will be 6,600 feet—over a mile—wide and over 600 feet high. The creation of a FIGURE 5.7
Primary purpose or benefit of dams
water reservoir upstream of the dam will flood 13 cities and countless villages, and displace well over 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.

Although the long controversy surrounding the Three Gorges dam did not prevent its erection, it has perhaps raised awareness within China regarding some of the destructive impacts of other proposed dams. In 2003 China's Environmental Protection Agency and the Chinese Academy of Sciences announced their opposition to plans to dam the Nu River, a World Heritage Site that has been described as the "Grand Canyon of the Orient." The proposed series of 13 dams would affect over 7,000 plant FIGURE 5.8
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. This dam will likely result in the extinction of numerous unique species, including the Yangtze River dolphin. (AP/Wide World Photos)
species and 80 rare and endangered animals, in addition to requiring over 50,000 people to be relocated. Many of these are farmers and herders from ethnic minorities in China.

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