An unusual explosion along the Rappahannock River on Feb. 23 defined the day for thousands of onlookers. As planned, at least 650 pounds of explosives blasted a 40-meter-long hole through the bottom of Embrey Dam near Fredericksburg, Va. Immediately, the water began to flow as it hadn’t in about 150 years, since the predecessor to this 6.7-m-tall barrier was constructed. Local, state, and federal officials had determined that the dam, which no longer contributed to the local water supply, was too expensive to maintain and blocked fish movement.
With that blast, American shad, blueback herring, and alewife migrating from the Atlantic through the Chesapeake Bay could move into 106 more miles of potential spawning habitat upstream of the dam. “To finally see this happen, after working on it for 10 years . . . was exciting,” says Alan Weaver of the Virginia Department of Game and Inland Fisheries in Richmond. He identifies himself as a fish-passage biologist.
Embrey Dam’s removal, to be completed by 2006, is part of a nationwide trend to dismantle old dams that are no longer making a significant contribution to water storage, flood prevention, electricity generation, irrigation, navigation, or recreation. More than 500 dams, mostly small ones, have been brought down in the past 3 decades by local, state, and federal agencies. Although reasons for dam removal have included safety concerns and high maintenance costs, environmental restoration has increasingly been used to justify the action.
The proposition seems simple enough: Taking down a dam will return a river to an ecologically healthy state. The remarkably limited scientific evidence on the topic, however, doesn’t entirely support that prediction.
Though scientists agree that the introduction of dams usually affects rivers adversely, recent research has turned up both positive and negative effects of dam removal. In many cases, the limited science that’s available can’t accurately predict what will happen when a particular dam is removed.
“It’s never a simple equation,” says Emily Stanley, a river ecologist at the University of Wisconsin–Madison.
The inherent complexity of river systems and each dam’s unique effect on its waterway lead to the complicated nature of dam-removal science, explains David Hart, a river ecologist from the Academy of Natural Sciences in Philadelphia.
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Researchers are considering both small dams, such as those that dot rivers along the northeastern U.S. coast, and the large ones typical of the Pacific Northwest.
Removing a dam releases the water that has collected behind it in a lake or reservoir. It also releases sediment that decades of slowed water had dropped as it approached the dam. That sediment can replenish riverbanks and beaches downstream, thereby improving river topography. However, sediment can also redistribute pollutants that had been sequestered behind the dam.
Even small dams collect sediments carried from upstream regions. The subsequent reduction in sediment available downstream can lead to shore erosion. Mississippi-watershed dams have reduced by half the natural supply of sediment that used to flow to the Gulf of Mexico, says Hart.
As effectively as dams accumulate sediment, they slow the movement of plant nutrients from run-off of fields treated with fertilizer and manure. “Reservoirs don’t let nitrogen and phosphorus slide down the river,” says Martin Doyle, a geomorphologist from the University of North Carolina at Chapel Hill. In this way, he points out, dams and the reservoirs behind them have the same effect as wetlands, holding back sediment, nitrogen, and phosphorus.
Stanley and other researchers have studied the release of water following the destruction of small dams in Wisconsin, one of the leading states in dam removals. These scientists have observed that nitrogen and phosphorus, which have accumulated in sediments behind dams for up to a century, suddenly head downstream. From Wisconsin, the nutrients go into the Mississippi River and wind up in the Gulf of Mexico.
At five Wisconsin dams, all less than 4.5 m tall, Stanley and Doyle reported in 2002, nitrogen and phosphorus concentrations immediately below the dam were lower than they were upstream of the reservoir. In the July 2003 Water Resources Research, Stanley and Doyle describe what happened after the Rockdale Dam on the Koshkonong River in south-central Wisconsin was breached in 2000.
The scientists used these data in ecological modeling to predict the transport and retention of phosphorus. Their simulations suggested that conversion of the reservoir into a deep, narrow river channel would reduce phosphorus retention.
Previous observations had shown that, for a few years after dam removal, the increased water flow washes out accumulated sediments and plant nutrients, and downstream water quality suffers. That effect eventually subsides as the channel through the former reservoir becomes wider and more capable of hanging onto nutrients.
“By opening up a dam, you’re not only releasing nutrients that are stored, you’re also removing the potential [of sediments] to retain nutrients,” Doyle says. He hastens to add that he’s not suggesting that more Wisconsin dams be retained or built for the sake of improving Mississippi River or Gulf of Mexico water quality. Nutrient release is “just a side effect to dam removal that hasn’t really been thought out,” says Doyle.
Since the 1970s, scientists and public officials have grown more aware of the possibility of contaminated reservoir sediments. In addition to plant nutrients, contaminants such as heavy metals and polychlorinated biphenyls (PCBs) can accumulate behind dams. In 1973, removal of the Fort Edwards Dam on the Hudson River near Albany, N.Y., caused an environmental disaster. Tons of PCBs were released, and eventually New York State closed the river to fishing.
The state removed 180,000 cubic yards of PCB-laden sediments in 1977 and 1978, and, in 1983, the U.S. Environmental Protection Agency declared a portion of the river a federal Superfund site.
In the early 1990s, plans to remove several small dams along the Blackstone River in Massachusetts were abandoned after tests found reservoir sediments contaminated with heavy metals.
More recently, the planned 1999 removal of the 4-m-high Oak Street Dam on Wisconsin’s Baraboo River was delayed for a year after the discovery of coal tar upstream of the dam. Coal tar came from the coal-gasification process used across the Midwest in the late 1800s. Alliant Energy, the owner of the dam, funded cleanup of the contaminant.
Environmental contaminants that may be in sediments behind dams is “one of the hot-button issues in dam removal,” says Hart.
While some scientists focus on water conditions after dam removals, others look to the fish in the river. “From a lot of people’s points of view, this is what it’s all about,” says Stanley.
Dams’ detrimental effects on fish have been obvious for decades. In fact, Idaho dynamited the 10-m-tall Sunbeam Dam in Custer County in the early 1930s strictly to improve the annual sockeye salmon run up the Salmon River.
Besides several species of Atlantic and Pacific salmon, American shad, steelhead, white sturgeon, and other fish are born in rivers, mature in the ocean, and then return to freshwater to spawn. Despite special fish ladders and other bypass devices, dams alter the habitat, impede fish migrations, kill many young, seaward-migrating fish in hydroelectric turbines, and expose species to predation that they wouldn’t typically encounter in a free-flowing river.
For instance, in 1880, the annual salmon harvest in Maine rivers and streams tallied about 10,000 fish. However, only about 800 salmon returned to those waterways in 2002, according to Lewis S. Incze, an oceanographer at the University of Southern Maine in Portland.
Incze notes that dams are only partly to blame for the current low number of salmon. Industrial pollution, high seas fisheries, and climate also play a role.
However, a committee of the National Research Council in Washington, D.C., recommended in January that removal of several dams in the Penobscot River, the last major spawning ground for salmon in Maine, should take priority over other Atlantic salmon-recovery activities.
“To the extent that you can open up more habitat, you are simply increasing the opportunity for this fish to survive,” says Incze, who was on the committee.
Anecdotal evidence supports the idea that dam removal helps fish immediately. For instance, within months after the 1999 destruction of the 7-m-tall Edwards Dam on the Kennebec River near Augusta, Maine, people sighted striped bass, alewife, Atlantic salmon, and Atlantic sturgeon upstream of the dam site. That was the first time in more than 150 years that migrating fish had access to the 17 miles of river above the dam site.
Doyle finds that hard evidence of dam-removal effects on fish is sparse. In the upcoming Geomorphology, he and Stanley say that the speed with which fish populations recover after dam removal remains “relatively unknown.”
The few scientific studies available indicate that while dam removal helps some fish species, it hurts others. In some cases, the losers are species that people find undesirable. In a 1997 study, fishery biologist Paul Kanehl of the Wisconsin Department of Natural Resources and his colleagues kept tabs on fish populations after the removal of Woolen Mills Dam on the Milwaukee River at West Bend, Wis.
The city took down the 4.3-m-high structure in 1988, and carp declined in the former reservoir while smallmouth bass increased. Carp is a bottom feeder that’s not native to Wisconsin and is “not well loved in this part of the world, ” says Stanley. Not a tasty fish, carp muddies the water, making it unsuitable for the much-preferred smallmouth bass, a “great sport fish,” she says.
Other scientific studies have shown that fish communities may change for the better during the years following dam removal. Matthew Catalano, a fishery biologist from the Wisconsin Department of Natural Resources, studied three dam removal sites on the Baraboo River. He collected fishes during the year before each dam was removed and continued for 2 to 5 years. The dams were destroyed between 1998 and 2001.
Within about 2 years, fish populations “appear to recover” at the site of the former reservoir, he says. After the Waterworks Dam was removed in 1998, for example, the number of native species, such as darters and smallmouth bass, rose from 11 to 26. Between 3 and 5 years after removal, the number of native species fluctuated little, varying between 24 and 26. Moreover, the proportion of fish species, such as carp and white sucker, that tolerate poor habitats decreased from 42 percent before removal to 2 to 15 percent afterward.
In Pennsylvania, native fish also thrived at a dam-removal site. Hart and his colleagues did an extensive study in Manatawny Creek near Pottstown, Pa. The reservoir formed by the late-1700s dam had been dominated by lake and pond fish, such as golden shiners and goldfish. Within a year after removal of the 2-m-high dam in 2000, these species declined in favor of river species, such as shield darters and longnose dace, Hart and his colleagues reported in 2002. These river fish, which are adapted to free-flowing water, had been present both upstream and downstream of the dam.
However, the area immediately downstream of the dam site had reduced populations of river fish for up to 3 years after dam removal—presumably from the effects of released sediment, Hart says. Overall, he concludes, the conversion of the impoundment to a free-flowing system “has been good for many native fish, but downstream fish have not yet rebounded from the effects of dam removal.”
Another effect of some dams has been to protect biodiversity by limiting the spread of invasive species, such as sea lampreys within the Great Lakes regions. Problems might arise if such dams are removed.
There are trade-offs in dam removal. “Whether it’s better or not [for fish] just depends on your values about what species you’d like to see” in a river, Hart says.
Animals other than fish can be affected by dam removal. Conversion of a reservoir to a free-flowing river can change the array of birds that the water, with its fish, attracts. After elimination of Edwards Dam, for example, once-rare bald eagles and great blue herons became common in that area of the Kennebec River.
Mollusks can also experience both negative and positive effects when a dam goes down and its reservoir drains. Stanley reports in an upcoming Hydrobiologia that water-filtering mussels both above and below the Rockdale Dam suffered after the structure came down. Many mussels in the reservoir were stranded when the water level dropped, while some of the mussels downstream were smothered by released sediments.
Stanley says that she expects mussel populations that survive the initial trauma to rebound as hitchhiking larvae take advantage of enhanced fish movements.
Vegetation also changes at a converted dam site but doesn’t necessarily return to the river’s original assortment. As a reservoir empties, both native and invasive plants can establish themselves in the newly exposed sediments.
At some sites in Wisconsin, for example, exotics such as reed canary grass and unpleasant, aggressive weeds such as stinging nettles have taken advantage of the abundant nutrients from exposed sediments.
“Dam removal isn’t going to be a silver bullet for stream ecosystems,” says Doyle.
Hart in turn cautions against using any one dam-removal experience as a model for another. “We cannot easily generalize from the handful of studies that have been done,” he says. “Many site-specific factors influence the ecological changes that occur when a dam is removed.”