| 大壩對環境所造成的衝擊
作者 勞瑞柏丁格(Lori Pottinger)
土地與水在生態學上是相互連結在我們稱為集水區(watershed) 的自然環境中。從最小的水滴到巨大力量的河流，水可以改變土地形狀、帶走沉積物及溶解物質而流入河道中，在大部分的情況下，最終是流入海洋。所以，河流也算是流經的陸地上的天然產物。石頭、底土的型態，地形及植被的覆蓋數量都是決定河流形狀、大小與流量的因子。
當陸地與河流之間的緊密關係被大壩破壞之後，其結果會影響到整個集水區及依賴河流生存的生物網。在各種干擾或破壞河流的途徑當中，大壩通常有著最直接且深遠的影響，因為大壩所造成的變化會使得河流的循環系統－即河流的水文也跟著改變。
目前全球將近有四萬個大壩阻礙著河流，其中大部分是在過去五十年當中所建造的，超過四十萬平方公里的區域因成為水庫而遭到淹沒，這比辛巴威(Zimbabwe)的面積還大，相當於賴索托(Lesotho)的十三倍大。位於迦納(Ghana)境內的阿卡松波(Akasombo)大壩造成了世界上最大的佛塔(Volta水庫)，面積有八千五百平方公里，淹沒了該國百分之四的國土。美國境內有五千五百個大壩而成為世界上大壩數量第二多的國家，但目前我們已經停止興建大壩，同時也正花大筆的錢來改善現有的一些大壩所帶來的一些問題。
大壩對於環境的影響結果
儘管關於大壩對環境所造成的衝擊之類的文獻報導已經存在有相當長的時間，然而對環境衝擊低估甚至忽略的新的水壩計畫卻一再地被提出。1990年一份關於世界銀行(World Bank)水力發電計劃的內部調查報告顯示有58%的案子是在沒有考慮對河川下游的衝擊情形下就被規劃、興建，甚至這些衝擊可以輕易預測，會造成嚴重的海岸侵蝕、污染及其他問題。
以下是一些關於大壩對於河川環境及依靠河川生存的生物所造成較為嚴重的衝擊，我把焦點擺在可能會影響到橘河(Orange River)分水嶺的數種衝擊，而省略掉在不同的生態情況下大壩會對河流造成的影響。
對河流系統的影響
減少河流的流量會改變河流流經的河川景觀，進而會影響其生態系中的植物相及動物相。一個大壩會讓水中的沉積物堆積，尤其是一些較重的碎石子及鵝卵石，不再攜帶泥沙河流會再經由侵蝕下游河道或河岸、沖刷橋樑底部及其他河岸的結構等途徑，重新獲得這些沉積物。從一個大壩關閉開始的十年內，大壩以下數十公里甚至數百公里的河床會被侵蝕數公尺之深。美國的胡佛大壩(Hoover Dam)在被關閉後九年當中，大壩以下的河床水位降低超過四公尺。河床加深也會影響到河流沿線的地下水層下降，威脅到沖積平原上的植物及當地的水井，而原本不需要灌溉的農地如今也需要了。河床上碎石減少的同時也減少了許多生物如在河流底部碎石間產卵的魚類及其他的昆蟲、軟體動物、甲殼類等無脊椎動物的棲地。河流的棲地及水文環境的改變情形也促成北美(North America)的淡水動物相減少93%。
在亞斯旺高壩(Aswan High Dam)興建以前，每年尼羅河(Nile River)都會將一億兩千四百萬噸的沉積物沖入海洋，另外約有一千萬噸的沉積物沉積在沖積平原及三角洲，如今，有98%的沉積物都留在高壩後面，這造成了埃及沖積平原農業泥土的生產力及厚度下降等嚴重改變。亞斯旺高壩也造成嚴重的海岸線侵蝕，其原因也是失去大壩上方河道沉積物的淤積。另外一個例子則是發生在迦納(Ghana)的佛塔(Volta)河河口，阿卡松波(Akosombo)大壩切斷了佛塔(Volta)河口原有的沉積物供應，連帶地影響到鄰近的托哥(Togo)及貝寧(Benin)，使其海岸以平均每年十到十五公尺的速度向內縮，而一項加強保護托哥(Togo)海岸的計劃平均每公里就花掉了三百五十萬美元。凡是在有大壩阻礙河流沉積物的海岸線，這樣的故事就不斷地發生。
對水文的影響
大壩同時也改變了河川水流的型態，包括減少水體的總體積及改變其季節性變化。衝擊的本質會因大壩的設計、目的、運轉方式等條件的不同而不同。而只要是流量發生變化，整個河流的生態系便會受到衝擊。
在淡水、海水交界的河口處，是一個特別富饒的生態系，全世界的漁獲量將近有80%是來自於這個棲地，而靠的就是大量且適時的營養物質及淡水來源。發生在墨西哥灣(Gulf of Mexico)、黑海(Black Sea)、裡海(Caspian Sea)、加州(California)的舊金山灣(San Francisco Bay)、地中海東部(Eastern Mediterranean)及其他地方海洋漁業快速衰落的情形都與大壩興建及河流改道而造成到達河口的水量改變有關。迦納(Ghana)的佛塔(Volta)河也在興建了調節用的阿卡松波(Akasombo)及柯澎(Kpong)壩之後造成一度在河口相當興盛的蛤工業消失和梭魚及其他運動性魚類大量減少。
洪水的改變
大壩因儲水而延遲且減少了下游的氾濫。河流及沖積平原生態系統對河流的氾濫的週期早已適應，原生植物及動物依賴著這樣的變動來完成其繁殖、孵化、遷徙及其他重要的生活史階段。每年的洪水會增加土地上營養物質的沉積、帶走死水並為重新補足溼地的水源。生物學家普遍認為水壩是所有造成河岸物種快速消失的原因中最具毀滅性的一種。目前世界上已知的八千種淡水物種當中約有20%因此而遭受到物種滅絕的威脅。
沖積平原本身也受到水壩的影響。在南非(South Africa) 澎哥洛(Pongolo)河沖積平原的研究顯示水壩完成之後森林物種的多樣性減低了，另外在肯亞(Kenya)的塔那(Tana)河沿岸的森林也因為一系列水壩的興建減少了大洪水的次數而逐漸消失。
結論
五十年前，美國政府在不是很了解開發水資源會對河流及所有依賴河流生存的物種造成什麼樣負面影響的情形下，便急著進行開發計劃。今天，我們開始「自食惡果」了，包括漁業資源枯竭、被破壞的生態系統、消失的海岸線及其他許多與在河流上建水壩所衍生的問題。我們現在被迫以不同的方法來管理水壩，分配更多的水流給自然環境以嘗試阻止更多因水壩所造成的生態系統破壞，同時也採取昂貴的方法來拯救珍貴的漁業資源。我們甚至準備鉅資拆掉一些特別差的水壩，而且不再興建大壩。
儘管現在要在美國或是其他水壩密度較高的國家進行會傷害河流的計劃已經是相當困難，然而我們的水利工業及銀行機構如同殺蟲劑在已經被原產國禁用很長的一段時間後化學工業依然將它輸出到其他國家一樣，卻還是繼續將這項已經廢棄不用的科技輸出到國外，。
在關於水壩的研討會上，現在談論的都是圍繞在尋找可開發的「新市場」及如何將水壩賣給持懷疑態度大眾的新方法。
目前新南非政府有機會從世界上過去五十年錯誤的河流開發經驗中，發展一套水資源政策，那就是允許公民社會參與決策過程。長遠來看，這樣的方法可能是唯一不會讓非洲最珍貴的資源之一－河流變少的方法。
原文詳見: http://www.irn.org/basics/impacts.shtml
版權歸屬 International River Network， 環境信託協會 (陳均輝 譯，梁明煌審校)
中英對照全文詳見：http://news.ngo.org.tw/issue/water/
issue-water-irn00052202.htm |
|
Lori Pottinger
Land and water are ecologically linked in a natural system called a watershed. From the smallest droplet to the mightiest river, water works to shape the land, taking with it sediment and dissolved materials that drain to watercourses and, in most cases, eventually to the sea. So, too, is the river a product of the land it inhabits--the type of rock and soil, the shape of the land, and the amount of vegetation are some of the factors that determine the river's shape, size and flow.
When these ties between the land and the river are broken by a large dam, the consequences are felt throughout the watershed, as well as by the web of life it supports. Of all the ways to tamper with or harm a river, a large dam usually has the most immediate and far-reaching effects because of the huge changes it causes to river hydrology--its very circulation system.
Some 40,000 large dams, most of which were built in the past 50 years, now obstruct the world's rivers. More than 400,000 square kilometers--an area larger than Zimbabwe, and 13 times the size of Lesotho--have been inundated by reservoirs worldwide. The world's largest impoundment, the 8,500 sq.km. Volta Reservoir behind Ghana's Akasombo Dam, flooded 4% of that nation's land area. In the United States, whose 5,500 large dams make it the second most dammed country in the world, we have stopped building large dams, and are now spending great amounts of money trying to fix the problems created by existing dams.
The Environmental Consequences of Big Dams
Although the impacts of large dams have been well documented for some time now, in case after case, new ones are proposed whose environmental impacts are downplayed or even ignored. A 1990 internal survey of World Bank hydroelectric dam projects showed that 58% were planned and built without any consideration of downstream impacts, even when these impacts could be predicted to cause massive coastal erosion, pollution and other problems.
The following are some of the more serious environmental impacts of dams on rivers and the life they support. I have concentrated on the kinds of impacts that might affect the Orange River watershed, leaving out other major dam-caused problems that have affected rivers under different ecological circumstances.
Effects on River Systems
Reducing the flow of water from a river changes the landscape it flows through, which in turn can affect the ecosystem's flora and fauna. A dam holds back sediments, especially the heavy gravel and cobbles. The river, deprived of its sediment load, seeks to recapture it by eroding the downstream channel and banks, undermining bridges and other riverbank structures. Riverbeds are typically eroded by several meters within a decade of first closing a dam; the damage can extend for tens or even hundreds of kilometers below a dam. Within nine years of closing Hoover Dam in the US, the riverbed below the dam had lowered by more than 4 meters. Riverbed deepening will also lower the groundwater table along a river, threatening vegetation and local wells in the floodplain and requiring crop irrigation in places where there was previously no need. The depletion of riverbed gravels reduces habitat for many fish that spawn in the gravelly river bottom, and for invertebrates such as insects, molluscs and crustaceans. Changes in the physical habitat and hydrology of rivers are implicated in 93% of freshwater fauna declines in North America.
Before the Aswan High Dam, the Nile River carried about 124 million tons of sediment to the sea each year, depositing nearly 10 million tons on the floodplain and delta。 Today, 98% of that sediment remains behind the dam. The result has been a drop in soil productivity and depth, among other serious changes to Egypt's floodplain agriculture. The Aswan Dam has also led to serious coastal erosion, another problem stemming from the loss of sediments in a dammed river. Another example of this problem is along the mouth of the Volta River in Ghana. Akosombo Dam has cut off the supply of sediment to the Volta Estuary, affecting also neighboring Togo and Benin, whose coasts are now being eaten away at a rate of 10-15 meters per year. A project to strengthen the Togo coast has cost US$3.5 million for each kilometer protected. The story is the same on coastline after coastline where dams have stoppered a river's sediments.
Hydrological Effects
Dams also change the pattern of the flow of a river, both reducing its overall volume and changing its seasonal variations. The nature of the impacts depends on the design, purpose and operation of the dam, among other things. All parts of a river's ecology can be impacted by changes to its flow.
A river's estuary, where fresh water meets the sea, is a particularly rich ecosystem. Some 80% of the world's fish catch comes from these habitats, which depend on the volume and timing of nutrients and fresh water. The alteration of the flows reaching estuaries because of dams and diversions is a major cause of the precipitous decline of sea fisheries in the Gulf of Mexico, the Black and Caspian Seas, California's San Francisco Bay, the Eastern Mediterranean and others. The regulation of the Volta River in Ghana by the Akasombo and Kpong dams has led to the disappearance of the once-thriving clam industry at the river's estuary, as well as the serious decline of barracuda and other sport fish.
Changes to Flooding
The storage of water in dams delays and reduces floods downstream. River and floodplain ecosystems are closely adapted to a river's flooding cycle. The native plants and animals depend on its variations for reproduction, hatching, migration and other important lifecycle stages. Annual floods deposit nutrients on the land, flush out backwater channels, and replenish wetlands. It is generally recognized by biologists that dams are the most destructive of the many abuses causing the rapid disappearance of riverine species. About 20% of the world's recognized 8,000 freshwater species are threatened with extinction.
The floodplain itself is also affected by dams. Studies on the floodplain of the Pongolo River in South Africa has shown a reduction in diversity of forest species after it was dammed. And forests along Kenya's Tana River appear to be slowly dying out because of the reduction in high floods due to a series of dams.
Conclusions
Fifty years ago, the United States rushed into a water development program with little understanding of the negative impacts it would have on its rivers and all who depend on them. Today, we are beginning to "pay the piper" in depleted fisheries, damaged ecosystems, receding coastlines and many other problems linked to the damming of our rivers. We are now being forced to manage our dams differently, allocating more flow to the environment in an effort to stop further dam-related destruction of ecosystems and taking other costly steps to save valuable fisheries. We are even preparing to take down some particularly bad dams, at enormous expense. And we no longer build big dams.
Although it has now become very difficult to build destructive river projects in the US and many other highly dammed countries, our hydro industry and financial institutions continue to export this obsolete technology, much in the same way the chemical industry continued to export pesticides long after they had been banned in the country of origin
At dam conferences, the talk these days always centers around finding "fresh markets" to exploit and new ways to sell dams to a skeptical public.
The new South Africa has the opportunity to devise a water policy that builds on what the world has learned in the past fifty years of unchecked river development, and that involves civil society in the decision-making process. In the
long term, such an approach is the only one that doesn't diminish one of Africa's most treasured resources--its rivers. |
