| 如上所述，人可以對流域產生很大的影響。我們不但比起其他的生物用了更多的水，人類個人和集體均造成流域重要的變化--有些是最河川有益的，有些卻會造成嚴重傷害。整體而言，我們將流域改變地太多了，而使它們不再像往常一樣，具有很多有用的功能，來保護、維持我們的生活空間。下列數點描述某社區所遭遇的可能情況，因為這些改變帶來的未預料的結果：他們的流域不能再像過去一樣處理洪水了。
▲砍伐森林降低了土壤的貯水能力，大大增加了泥土的流失到小溪流、河川。
▲被砍伐了森林的斜坡會造成越來越嚴重的水土流失、淤泥增加，改變河床和河岸的形狀，從而導致洪水期的變化。
▲農民和都市規劃者排乾了溼地的水，使流域失去了的天然"海綿"，而它們原本是吸收那些流失的泥土和雨水的最佳機制。
▲由於城市化的結果，道路被鋪平、建築物越來越多，空曠地帶越來越小。結果水土的流失量急劇增加，因為屋頂和鋪平的道路使得水不能滲透到地裏。
▲由於可用土地面積減少，人們與自然爭地，使土地問題增加，越來越的人在衝積平原上建造房屋，增加了多建水壩防範洪水的壓力。
▲最後，人們建造水壩控制洪水並作他用，流域就被永遠地改變了。衝積平原不再像以前一樣發揮對生物社區的生態功能，而且越來越多的人搬到了易遭洪水侵襲的地方。他們認為水壩可以免遭洪水的襲擊。水壩減少了洪水發生的頻率，但卻不能防止最大、最具危害性的洪水發生。結果就是：洪水造成了比以前更大的損失。
人們以集體的行動決定興建水壩來防洪，卻造成更具大生態危機。不幸的是這些損失太常見，人們已經見怪不怪。好消息是人類也能成為流域的看護者--所需的僅是知識和改變的決心。個人的舉動能真正的改變現狀、添磚加瓦。接下來的一部分描述了人們改變流域的一些糟糕情況，緊接著說明了我們能積極看護流域的方法。
版權歸屬 國際河網IRN 環境信託基金會(陶俊譯，陳維立審校)
中英對照全文：http://news.ngo.org.tw/issue/water/issue-water-irn00110601.htm
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People can have a great impact on the health of a catchment, as described above. Not only do we use more water than other creatures, but we make major changes to catchments individually and collectively - some of which are beneficial, and some of which can do serious harm. In general, we have modified catchments so much that they no longer perform many of the useful functions that protect and support our communities. The following scenario describes a typical series of events undertaken by a community, with the unintended result that their catchment can no longer handle flood waters the way it once did:
▲ Deforestation reduces the soil's ability to hold as much water, greatly increasing runoff into local streams and rivers.
▲ Increased siltation from deforested slopes changes the shape of river bed and banks, which can lead to changes in the flood regime.
▲ Farmers and city planners drain wetlands, thus removing the catchment's natural "sponges" which absorb run-off and rain.
▲ Urbanization leads to paved roads, more buildings and less open space. The result is a dramatic increase in runoff, since rooftops and paved roads prevent water from infiltrating.
▲ As land pressures increase, more people build permanent structures in the river's floodplain, thus increasing pressure to try to stop floods with dams.
▲ Finally, a dam is built for flood control and other uses, and the catchment is permanently changed. The floodplain no longer serves the ecological function it once did for the community, and more people move into the most flood-prone lands, believing they will be protected from all floods. The dam reduces the frequency of floods, but does not prevent the biggest, most damaging floods from occurring. The result: more expensive damages from floods than ever before.
It takes a lot of actions by a lot of people to do this much damage, but unfortunately it is all too common. The good news is that humans can also be great caretakers of their catchments - all it takes is knowledge and the will to make a difference. Individual actions add up to real change. The next section describes some of the ways people have changed catchments for the worse, followed by sections describing ways we can make positive changes.
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