| 大壩對河流產生直接的物理方面及生物方面的影響。最重要的影響在於河流流量減少或有所改變，這會影響河流流經地區的生態系統和景觀。水壩會留住沈澱物，尤其是很重的沙礫和鵝卵石。河流沒有了沈澱物這個重負，就會更容易侵蝕下游河床河岸、破壞橋樑和其他河岸建築來重新夾帶沈澱物。尤其典型的是河床下游的水壩在首次關閉的十年中被侵蝕了數公尺，所造成的破壞會在水壩下擴展到數十公里，甚至數百公里。河床的加深還會降低河流沿岸地下水的水平面，威脅到沖積平原的植被和當地的井，要求在以前不需要灌溉的地方灌溉作物。河床改變了，在河底產卵的很多魚類和無脊椎動物諸如昆蟲、軟體動物和甲殼類動物等就會失去不少棲息地。
亞斯文大壩竣工以前，尼羅河每年夾帶著約1億2400萬噸的沈澱物流入大海，還在沖積平原和三角洲上留下將近一千萬噸的沈澱物，這樣就讓土壤補充了維持生命必須的營養物質。如今，98%的沈澱物都留在了大壩裏。結果，土壤的生產力和深度下降，埃及的沖積平原農業還受到了其他影響。結果，數以萬計家庭的生活遭到了破壞。亞斯文水壩還導致了海岸受到了嚴重侵蝕，這是另一個因河流築堤沈澱物減少而引起的問題。這個問題的另一個例子在加納伏特加河河口沿岸。Akosombo水壩切斷了伏特加河入海口的沈澱物來源，也影響了鄰國多哥和貝寧，現在這兩國的海岸正以每年10-15公尺的速度遭到侵蝕。一項加強多哥海岸的工程每保護一公里海岸就得耗費350萬美元。水壩留住了河流沈澱物，海岸就會發生同樣的問題。南非奧蘭治河河內的魚群已經受到了Gariep和Vanderkloof 水壩的嚴重影響。這些水壩甚至減少了河裏的水流，防止了洪水，魚兒們失去了這一遺傳性的牢固線索，就不產卵了。
對水文的影響：水壩還改變了河水流量的模式，既減少了水流總量，又改變了其季節變化。影響主要取決於水壩的設計、目的以及運做等等。河水流量發生變化，會影響河流生態的所有部分。
河流的入海口是淡水、海水交彙的地方，這裏的生態系統特別豐富。世界上捕到的魚相當部分是來自於這些棲息地，這取決於淡水水流和營養物質的總量和時間選擇。水壩和入海口河流改道是導致墨西哥灣、黑海、里海、加利福尼亞的舊金山灣、東地中海以及一些非洲入海口海洋漁場突然減少的主要原因。Akosombo水壩和Kpong水壩對加納的伏特加河的制約，使該河入海口曾經繁榮一時的蚌蛤業因此消失，梭魚和其他魚也嚴重減少。尚比西河上的水壩嚴重損害了莫三比克沿海的對蝦捕撈，使沖積平原的上游漁業就此消失。
對洪水的影響：水壩裏貯存的水延緩和減少了下游的洪水。河流和沖積平原的生態系統與河流的洪水周期密切相關。土生土長的植物和動物依靠洪水周期來繁殖、孵化、遷徙以及其他重要的生命迴圈步驟。一年一度的洪水在土地上積起營養物質，沖通閉塞停滯的水渠，爲沼地補充水分。一般生物學家認為水壩是導致河流生物迅速消失的許多弊病中危害性最大的。世界上已知的八千種淡水生物中約有２０％正瀕臨滅絕。
洪泛區本身也受水壩影響。對Pongolo河洪泛區的一項研究顯示，攔截以後森林物種的多樣性程度降低。肯亞的塔恩河正因一系列的攔截造成的高洪減少而漸漸消失。
版權歸屬 國際河網 環境信託基金會(陶俊 譯，李堅、陳維立審校)
中英對照全文：http://news.ngo.org.tw/issue/water/
issue-water-irn00122501.htm
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Large dams directly impact rivers in a variety of physical and biological ways. The most significant impact is the reduction or alteration of a river's flow, which affects the river ecosystem and the landscape through which the river flows. 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 riverbed and banks, undermining bridges and other riverbank structures. Riverbeds downstream of dams are typically eroded by several metres within a decade of first closing a dam; the damage can extend for tens or even hundreds of kilometres below a dam. 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. Altering the riverbed reduces habitat for many fish that spawn in the river bottoms, and for invertebrates such as insects, mollusks and crustaceans.
Before completion of the Aswan High Dam, the Nile River carried about 124 million tons of sediment toward the sea each year, depositing nearly 10 million tons on the floodplain and delta, thus replenishing the soil with vital nutrients. Today, 98% of that sediment gets trapped behind the dam. The result has been a drop in soil productivity and depth, among other serious changes to Egypt's floodplain agriculture. As a result, the livelihoods of many thousands of families have been destroyed. 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 metres per year. A project to strengthen the Togo coast has cost US$3.5 million for each kilometre protected. The story is the same on other coastlines where dams have stoppered a river's sediments. And in South Africa's Orange River, fish populations have been significantly affected by the Gariep and Vanderkloof Dame. The dams even out the river flow and prevent the floods, and without this genetically entrenched cue, the fish do not spawn.
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 so percent of the world's fish catch comes from these habitats, which, depend on the volume and timing of fresh water flows and nutrients. The alteration by dams and diversions of the flows reaching estuaries 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 a number of estuaries in Africa. The regulation of the Volta River in Ghana by the Akosombo 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. Dams on the Zambezi have seriously harmed the coastal prawn fishery in Mozambique, and have also depleted floodplain fisheries upstream.
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 life cycle 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.
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