研究顯示:拆除水壩會衍生新問題 | 台灣環境資訊協會-環境資訊中心

研究顯示:拆除水壩會衍生新問題

2002年09月05日
ENS報導,蕭玉珠譯,莫聞、蔡麗伶審校

美國,威斯康新州,麥迪遜,2002年9月5日 (ENS) -目前美國有多條河流上的老舊廢棄水壩正進行拆除工程,就野生物而言,此為一正面訊息;但美國威斯康辛大學麥迪遜校區的研究人員指出,在某些方面,拆除水壩會產生不可預知的問題。

威斯康新─麥迪遜大學淡水生物學研究中心的河川生態學家史坦利(Emily Stanly)發現,拆除水壩雖可使魚類和獨木舟在河川中暢行無阻,但也會使危害性的養份流入河川系統之中。這項研究的採樣來自威斯康新州內的巴伯若河(Baraboo River)及哥許哥儂溪(Koshkonong Creek)中的水壩,結果發表於2002年8月的《生物科技》(BioScience)期刊中。

美國的水壩網路起源於18世紀中期,以發電和幫助控制水流量為主,但興建水壩會阻礙河川中的生物遷移,惡化水質,變更河流流量及河道,因而改變生態系統。今日,在威斯康新州有將近4千座水壩,總計在美國境內一共有數百萬座水壩,而全國仍保有62英哩以上自由流通水道的河流不到60條。

史坦利指出:「許多水壩正逐漸老化中,隨著時間的洗禮,水壩的結構損壞,使得它們由具經濟效益的商業場址,搖身一變為對人類安全造成危害的建築。」許多人認為,與其去修復水壩,拆除水壩不失為一最好的方案,事實上去年一年在美國有120座水壩遭拆毀,這幾年內,威斯康新州已拆除60座個的水壩。

史坦利研究指出:「有關水壩拆除後所帶來的影響研究相當稀少,令人驚訝的是,實際上我們並不清楚生態系統對水壩拆除會產生何種反應。」在過去兩年內,史坦利於威斯康新州內的巴伯若河及哥許哥儂溪沿岸,在即將拆除的水壩場址收集資料,並於拆毀後再進行一次調查。根據她的調查顯示,拆除水壩將使得過量養分隨著陸上逕流流入河川,最後被水流帶入湖泊或海洋之中。

史坦利指出:「用來施肥的養份進入水系之後,水系本身的養分也因此而提高。」如此將造成水中的磷、氮含量過高而形成藻華,使水呈現藻色,且讓其他水中動植物產生缺氧狀態。水壩可阻礙絕大部份的營養物沿著河川向下游漂流,每一水庫結構形成一蓄水池,並使含有養份的淤泥留在水池中。

史坦利和另一位共同撰寫此報告的北卡羅來納大學科學家馬汀道爾(Martin Doyle)發現,哥許哥儂溪沿岸的水庫阻擋15%至20%的磷向下漂流。他們在報告中指出:「水庫的泥沙淤積效應,等於讓農田所流失的表土層及養份儲存在水庫中。」在拆除水壩後,養份傾注入水系統中,河川下游的磷質濃度由原本的每公升0.3毫克躍升至每公升2.7毫克。兩年後,此種情形已減緩,但和先前流入水庫的水流相比,磷濃度仍高出30%。

根據這些研究調查結果,史坦利表示:「就河川中的養分流向管理而言,拆除水壩也許不是最好的辦法。」接著她補充道:「事情總是有好有壞,我們所要做的,就是盡量拉大正反兩面效果的差距。當這一天到來時,我會樂見水壩被拆除。」

Dam Removal Can Create New Problems
MADISON, Wisconsin, September 5, 2002 (ENS)

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."