| 各種各樣的人類活動都會影響流域，從而傷害依賴流域生存的人類和其他生物。以下描述了一些流域所面臨的最常見問題。無論壓力程度或大或小，這些問題已經存在於世界各地了。
砍伐森林：如前所述，原生樹種可以保護流域免遭土壤流失，免於洪水，提供許多動物陰涼處和棲息地，通過植物的根系疏通雨水，滲入土壤，從而幫助補充地下水。當原生樹種遭到砍伐時，整個集水區都會深受其害。由於過度地砍伐森林，許多非洲的流域已經受到破壞，包括林波波河流域，已經失去了99%的原始植被；還有尚比西河流域，已經失去了43% 的森林；還有奧倫治河，幾乎失去所有的植被。（資料來源：《Watershed of the World》，請見資料來源，獲取更多資訊。）事實上，奧倫治和林波波河流域位列世界上原始森林損失最大的前十處集水流域之列。
森林的砍伐尤其盛行在那些依靠木材作燃料來獲取能量的鄉村地區和公有地。以木材做燃料這個問題與貧窮和性別之間有著極大的關係。砍伐樹木以供燃料對生態體系的運作與生物的多樣性有負面的影響。一些研究發現，按目前的消耗速度，到2020年，南非天然的公有林就會全部消失。
商用林場和外來植物：以非洲商業林場的情況為由，大片土地改種生長快速的樹種，比如松樹和桉樹(或譯為油加利樹)，徹底地改變了許多流域的水文。這些生長快速的樹種比原生樹種消耗更的水，它們甚至可以導致曾經終年不斷水的溪流每年都會有很長一段時間完全乾涸。例如，由於最近幾年興起的商用林場，每一年Oliphants河流域都要流失5600萬立方公尺的土壤（1995年的估計值）。1971年的研究表明，南非的Mariepskop實驗林的年平均降雨量一直保持自然變化， 但從1935年到1964年，它卻保持穩定，但是該地區河盆流失的水土減少到了90%。唯一的改變是流域內興起了越來越多商業林場。
入侵南非的外來植物的額外年用水量估計多達33億立方公尺，這個數量約是該國年平均溢流水量的6.7%。《南非土地的退化》一書（Hoffman, Todd, Ntshona和Turner）中提到，即使對外來植物現有的發展速度採取保守估計，這個數字也可能在未來的15年中增為兩倍。商業林場對流域的影響包括：土壤流失，溢流水量減少，生物的歧異度的減低，管理成本的增加，因為許多外來樹種具有天生的侵略性，土壤會發生化學變化，鄉村經濟經歷了改變，失去了牧地、藥用植物、水和建築材料。
非政府已開始著手控制外來林材農場的管理和激增。他們的其中一項措施--"保護水資源"專案已著手清除開普敦（Cape Town）市周圍的外來植物。與建造大壩的成本相比，該專案取得一單位水的花費只有建大壩的6%
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-irn00111301.htm
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All kinds of human activities can impact catchments, and therefore harm the people and animals that depend on catchment functions for their livelihoods and lives. The following describes some of the most common catchment threats. These kinds of stresses have occurred, to varying degrees, all over the world.
Deforestation: As described in the previous section, nonplantation forests protect catchments from erosion and flooding, provide shade and habitat for many animal species, and help replenish groundwater by funneling rain through the soil with their root systems. When indigenous forests are removed, the entire catchment suffers. Many African catchments have been damaged by severe deforestation, including the Limpopo, where 99 percent of the original forest cover has been lost; the Zambezi, with a loss of 43 percent of its forests; and the Orange, which has lost nearly 100 percent of its vegetative cover. (Source: Watershed of the World, see Resources for more information). In fact, the Orange and Limpopo are in the world's top 10 catchments with highest percentage of original forest loss.
Deforestation is especially prevalent in rural and communal areas in the region where people rely on fuelwood for their energy needs. Fuelwood problems are inextricably linked to issues of poverty and gender. Fuelwood deforestation has negative impact on ecosystem functions and biodiversity. Some studies show that at current consumption rates, South Africa's natural communal woodlots will be virtually gone by the year 2020.
Timber Plantations and Alien Plants: A related issue has to do with to catchments from commercial forestry Africa, large areas of indigenous with fast growing plantation tree species such as pine and eucalyptus, which has drastically altered the hydrology of many catchments. These fast-growing species consume much greater quantities of water than indigenous species, and have even resulted in once-perennial streams becoming completely dry for long periods each year. For example, the Oliphants River catchment has experienced a reduction in runoff of 56 million cubic metres a year due to forestry plantations in recent years (1995 estimate). A 1971 study revealed that the mean annual rainfall in South Africa's Mariepskop forestry station experienced natural variation but remained steady between 1935-64, while runoff decreased up to 90 percent in the region's river basin. The only change was the progressive establishment of alien tree plantations in the catchment.
The total additional volume of water used by invading alien plants in South Africa is estimated to be more than 3,300 million cubic metres of water per annum, which amounts to about 6.7 percent of the nation's mean annual runoff. Even using conservative estimate on the current expansion rate of the alien-plants invasion, these figures are likely to double within the next 15 years, according to the book Land Degradation in South Africa (Hoffman, Todd, Ntshona and Turner). Impacts of timber plantations on catchments include increased soil erosion, reduced water runoff, reduced biodiversity, increased maintenance costs due to the invasive nature of many species, chemical changes to the soil, the transformation of rural economies, and the loss of grazing lands, medicinal plants, water and building materials.
The South African government has begun to take steps to control the management and proliferation of alien-tree farms. One of its efforts, the "Working for Water" program, has cleared alien vegetation around Cape Town. Per unit of water gained, this work costs just 6 percent of what it would cost to build a new dam.
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