Today about 800,000 dams operate worldwide, 45,000 of which are large—that is, greater than 15 meters tall. Most were built in the past century, primarily after World War II. Their benefits are clear. Hydroelectric power makes up 20 percent of the globe’s electric supply, and the energy is largely clean and renewable, especially when contrasted with other sources. Dams control flooding, and their reservoirs provide a reliable supply of water for irrigation, drinking and recreation. Some serve to help navigation, by stabilizing flow.
Their costs are obvious as well. Dams displace people and as a result have become increasingly controversial in the developing world [see “The Himba and the Dam,” by Carol Ezzell; Scientific
American, June 2001]. The structures ruin vistas, trap sediments (needed for deltas, riverbanks and beaches), stymie migratory fish and destroy ecosystems in and around waterways.
Conservationists have a long history of opposing dams: John Muir tried to block the dam in Yosemite’s Hetch Hetchy Valley; Edward Abbey’s novel The MonkeyWrench Gang targeted Arizona’s Glen Canyon Dam for guerrilla demolition. In recent years, as the downsides of dams have become more widely recognized, groups made up of several interested parties—utility officials, regulators, policymakers, conservationists, native peoples, researchers and the public—have fought to decommission aging dams.
In the U.S., where hydropower dams must be relicensed every 30 to 50 years, the rate of dam removal has exceeded the rate of construction for the past decade or so. In the previous two years alone, about 80 dams have fallen, and researchers following the trend expect that dams will continue to come down, especially small ones. Although the U.S. is currently leading the effort, it is not alone. France has dismantled dams in the Loire Valley; Australia, Canada and Japan have also removed, or are planning to remove, dams.
Clear successes have driven much of this activity. In 1999 engineers took apart the Edwards
Dam on Maine’s Kennebec River after a long battle waged by environmentalists culminated in the Federal Energy Regulatory Commission’s denial of a renewal permit. Within years, biologists observed with some surprise the return of scores of striped bass, alewives, American shad, Atlantic salmon, sturgeon, ospreys, kingfishers, cormorants and bald eagles. They also found that the water became well aerated and that populations of important food-chain insects such as mayflies, stoneflies and caddisflies grew.
In the Loire Valley, the story is similar. Salmon were abundant in the 19th century—about 100,000 would migrate each year—but by 1997, only 389 were counted making the trip. Despite the incorporation of fish ladders and elevators, the eight dams along the Loire and its major tributaries—as well as their turbines and pumps—had decimated the salmon population. Nongovernmental organizations, including the European Rivers Network, led a campaign to bring the salmon back. In response, the French government decommissioned four of the dams—two in 1998, one in 2003 and one in 2005. Within a few months of each dam removal, five species of fish, Atlantic salmon and shad among them, began to reestablish their historic migratory pathways.
In most places where dams have been eliminated, the stories of the Kennebec and the Loire have been repeated. Water clarity and oxygen levels increase as flow comes back, and aquatic insects thrive again. Warm stagnant water runs from behind the dam along with the fish, such as nonnative carp, that love it. As the water moves freely, its temperature falls and cold-loving fish species, such as trout, proliferate or return. The carp population, which tends to squeeze out others, dwindles, sometimes disappearing completely.
People, in addition to flora and fauna, return to enjoy the rivers. Biologists have observed these benefits from Wisconsin—one of the U.S. leaders in small dam removal—to New South Wales in Australia. Even restoring some water to rivers without removing a dam has had positive effects.
Biologists have also recorded unexpected problems. The release of sediments trapped behind a dam’s walls can choke waterways, muddying the environment and wiping out insects and algae, which are important food for fish. This wave of turbidity can also eliminate habitat for sessile filter feeders, such as freshwater mussels. Sometimes the mud that had been held back by the structures is rife with contaminants. When engineers removed the Fort Edward Dam on the Hudson River in 1973, concentrations of PCBs rose in downstream fish and remained high for many years; even today the striped bass fishery remains closed because of high levels of PCBs.
Sediments that are not washed downstream can become problematic as well. As they dry out, they may provide fertile ground for potentially noxious exotic plants whose seeds they harbored. Eurasian reed canary grass—which homogenizes wetlands by outcompeting native plant species—grew explosively after Wisconsin’s Oak Street Dam fell, even though restoration scientists had seeded the area with native prairie plant species.
In some cases, dams have blocked invasive species from moving upriver and into zones above the dam. The dam at Fossil Creek, for example, halted the advance of exotic fish such as bass and sunfish, creating a sanctuary above the structure for imperiled southwestern fish, including headwater chub and speckled dace. The reservoir also provided habitat for a locally threatened species, the lowland leopard frog.
And dam removal can pose dangers for people living nearby. In places where flood control is crucial, government organizations have had to devise safety strategies before dams could come down.
Sediments stuck behind dams are proving crucial variables when dams are taken down. Often the biggest issue facing managers is how to contend with what can be a massive accumulation of dirt and debris. Because of the legacy of releasing PCBs downstream in the Hudson River, scientists now routinely test these materials for toxicity. If the sediments contain high levels of pollutants, the cost of removing them—especially from remote locations—has to be weighed against the ability of the waterway to wash them away.
If the sediment load is very high and the river’s flushing capacity low, engineers might opt to remove the dam in stages, allowing small amounts of sediment to be released at a time. Sometimes engineers build channels through reservoirs, planting vegetation to stabilize sediments or placing physical barriers such as rocks or temporary fencing to hold the dirt in place.
From the Science Magazine: