研究顯示：拆除水壩會衍生新問題

Old, obsolete dams are being removed from rivers and streams across the country - good news for wildlife, but in some cases, the removals create unforeseen problems, find researchers from the University of Wisconsin-Madison.

Emily Stanley, a river ecologist at UW-Madison's Center for Limnology, has found that dam removal allows not just fish and canoes, but also damaging nutrients, to flow through the water system. Results of the study, which focused on dam removal sites along the Baraboo River and Koshkonong Creek in Wisconsin, appear in the August 2002 issue of the journal "BioScience."

The nation's network of dams, some of which date back to the mid-1800s, generate power and help control floods. But they also transformed ecosystems by blocking the movement of organisms, worsening the water quality and altering downstream flow and channel formation. Today, there are almost 4,000 dams in Wisconsin, and several million can be found throughout the United States. Fewer than 60 rivers in the country retain more than 62 miles of free flowing channel.

"Many of the dams are getting old," said Stanley. "Time has taken its toll on these structures and transformed them from productive sites of commerce to safety risks." Tearing them down, rather than restoring them, may seem like the best option. In fact, 120 dams were razed last year in the United States, and, over the years, more than 60 have been torn down in Wisconsin.

"Very few quantitative studies have been done on the effects of dam removal," Stanley said. "It's surprising how little we actually know about how the system will respond." Over the past two years Stanley collected data at dam sites, both before and after their removal, along the Baraboo River and Koshkonong Creek in Wisconsin. She says her findings show that removing dams allows excess nutrients that run off from the land to drift downstream, where they then can empty into lakes and oceans.

"When the nutrients used to fertilize crops enter these systems, they end up fertilizing them, too," said Stanley. Too much phosphorous and nitrogen in the water can create algae blooms, which turn the water green and can starve other plants and animals of oxygen. Dams prevent most of these nutrients from flowing downstream. Each structure forms a reservoir of water behind it that often fills up with sediments, which carry the nutrients.

At one site, Stanley and her co-author Martin Doyle from the University of North Carolina found that the reservoir along the Koshkonong Creek retained 15 to 20 percent of the total amount of phosphorous carried downstream. "The sediment trapping ability of reservoirs means that topsoil and nutrients lost from farm fields are now stored behind dams," they wrote. After removal, the nutrient poured into the water system: Phosphorous concentrations downstream jumped from 0.3 to 2.7 milligrams per liter. Two years later, the amount has decreased, but it is still about 30 percent greater than the amount found in the water entering the former reservoir.

Based on these findings, Stanley concludes, "Removing dams may not be the best way to manage how rivers handle nutrients." "There are always going to be tradeoffs, and what we want to do is maximize the gains," she added. "When the day is done, I'd much prefer to see the dams go."