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  3. Rivers By original usage, a river is flowing water in a channel with defined banks. Modern usage includes rivers that are mu

Rivers By original usage, a river is flowing water in a channel with defined banks. Modern usage includes rivers that are mu

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问题                                          Rivers
    By original usage, a river is flowing water in a channel with defined banks. Modern usage includes rivers that are multichanneled, intermittent, or ephemeral in flow and channels that are practically bankless. The concept of channeled surface flow, however, remains central to the definition.
    Rivers are nourished by precipitation, by direct overland runoff, through springs and seepages, or from meltwater at the edges of snowfields and glaciers. The contribution of direct precipitation on the water surface is usually minute, except where much of a catchment area is occupied by lakes. River water losses result from seepage into shallow or deep aquifers (沙石含水层) and particularly from evaporation. The difference between the water input and loss sustains surface discharge or streamflow. The amount of water in river system at any time is but a tiny fraction of the Earth’s total water; 97 percent of all water is contained in the oceans and about three-quarters of fresh water is stored as land ice; nearly all the remainder occurs as groundwater. Lakes hold less than 0.5 percent of all fresh water, soil moisture accounts for about 0.05 percent, and water in river channels for roughly half as much, 0.025 percent, which represents only about one four-thousandth of the Earth’s total fresh water.
   Water is constantly cycled through the systems of land ice, soil, lakes, groundwater (in part), and river channels, however. The discharge of rivers to the oceans delivers to these systems the equivalent of the water vapour that is blown overland and then consequently precipitated as rain or snow, e., some 7 percent of mean annual precipitation on the globe and 30 percent of precipitation on land areas.
    The historical record includes marked shifts in the appreciation of rivers, numerous conflicts in use demand, and an intensification of use that has rapidly accelerated during the 20th century. External freight trade became concentrated in estuarine ports (河口港) rather than in inland ports when ocean-going vessels increased in size.
   Demand on open-channel water increases as population and per capita water use increase and as underground water supplies fall short. Irrigation use constitutes a comparatively large percentage of the total supply.
    Present-day demands on rivers as power sources range from the floating of timber, through the use of water for cooling, to hydroelectric generation. Logging in forests relies primarily on flotation during the season of melt-water high flow. Large power plants and other industrial facilities are often located along rivers, which supply the enormous quantities of water needed for cooling purposes. Manufacturers of petrochemicals, steel, and woolen cloth also make large demands. Hydroelectric power generation was introduced more than 100 years ago, but the majority of the existing installations have been built since 1950.
    The ever-increasing exploitation of rivers has given rise to a variety of problems. Extensive commercial navigation of rivers has resulted in much artificial improvement of natural channels, including increasing the depth of the channels to permit passage of larger vessels. In some cases, this lowering of the river bottom has caused the water table of the surrounding area to drop, which has adversely affected agriculture. Also, canalization, with its extensive system of locks and navigation dams, often seriously disrupts riverine ecosystems.
    An even more far-reaching problem is that of water pollution. Pesticides and herbicides are now employed in large quantities throughout much of the world. The widespread use of such biocides and the universal nature of water makes it inevitable that the toxic chemicals would appear as stream pollutants. Biocides can contaminate water, especially of slow flowing rivers, and are responsible for a number of fish kills each year.
    In agricultural areas the extensive use of phosphates and nitrates as fertilizers may result in other problems. Entering rivers via rainwater runoff and groundwater seepage, these chemicals can cause eutrophication (水体加富过程). This process involves a sharp increase in the concentration of phosphorus, nitrogen, and other plant nutrients that promotes the rapid growth of algae in sluggish rivers and a consequent depletion of oxygen in the water. Under normal conditions, algae contribute to the oxygen balance in rivers and also serve as food for fish, but in excessive amounts they crowd out populations of other organisms, overgrow, and finally die due to the exhaustion of available nutrients and autointoxication (自身毒物). Various species of bacteria then begin to decay and putrefy the dead algal bodies, the oxidation of which sharply reduces the amount of oxygen in the river water. The water may develop a bad taste and is unfit for human consumption unless filtered and specially treated.
    Urban centres located along rivers contribute significantly to the pollution problem as well. In spite of the availability of advanced waste-purification technology, a surprisingly large percentage of the sewage from cities and towns is released into waterways untreated. In effect, rivers are used as open sewers for municipal wastes, which results not only in the direct degradation of water quality but also in eutrophication.
    Still another major source of pollutants is industry. Untreated industrial chemical wastes can alter the normal biological activity of rivers, and many of the chemicals react with water to raise the acidity of rivers to a point where the water becomes corrosive enough to destroy living organisms. An example of this is the formation of sulfuric acid from the sulfur laden residue of coalmining operations. Although upper limits for concentrations of unquestionably toxic chemicals have been established for drinking water, no general rules exist for the treatment of industrial wastes because of the wide variety of organic and inorganic compounds involved. Moreover, even in cases where a government-imposed ban checks the further discharge of certain dangerous substances into waterways, the chemicals may persist in the environment for years. Such is the case with PCBs (多氯化联苯). Since PCBs cannot be broken down by conventional waste-treatment methods and are degraded by natural processes very slowly, scientists fear that these compounds will continue to pose a serious hazard for decades to come. PCBs have been found in high concentrations in the fatty tissues of fish, which can be passed up the food chain to humans. An accumulation of PCBs in the human body is known to induce cancer and other severe disorders.
    As noted above, many industrial facilities, including nuclear power plants, steel mills, chemical-processing facilities, and oil refineries, use large quantities of water for cooling and return it at elevated temperatures. Such heated water can alter the existing ecology, sometimes sufficiently to drive out or kill desirable species of fish. It also may cause rapid depletion of the oxygen supply by promoting algal blooms.
Urban centers located along rivers result in the fact that ______.
选项
答案rivers are used as open sewers for municipal wastes
解析 文章第十自然段第一句和第三句说明此点。事实上，河流成了沿其河道建造的城市的排污明沟。
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