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Decreased river flows and higher temperatures could harm the water quality of the nation's rivers, bays, and lakes. In areas where river flows decrease, pollution concentrations will rise because there will be less water to dilute the pollutants; to keep pollution concentrations from increasing sewage treatment plants and other water pollution controls will have to be upgraded — which could cost billions of dollars per year. Increased frequency of severe rainstorms could increase the amount of chemicals that run off from farms, lawns, and streets into the nations rivers, lakes, and bays.
 The amount of dissolved oxygen in the water could also be reduced, effectively suffocating the fish in some areas. Higher water temperatures decrease the solubility of oxygen in water. Moreover, warmer water hastens the rate at which organic pollutants degrade; this degradation exerts a "biochemical oxygen demand" (BOD). The combined effect of lower oxygen solubility and higher BOD will be to reduce the availability of dissolved oxygen, which is critical to the health of aquatic organisms. One study estimated that throughout the southeastern United States, the warmer water temperatures would push dissolved oxygen levels in most rivers to below the 5 ppm necessary to sustain most fish.
Changing climate could also impair water quality in some rivers by reducing the flexibility of the existing water management system. Federal reservoir managers often release water during periods of low flow for environmental purposes. Such releases, however, are at the expense of recreation, water supplies, and hydropower, which benefit from higher water levels. A 1992 study of the Columbia River Basin found that earlier snowmelt would leave less water available in the summer. As a result, reservoirs in the Yakima River sub-basin would occasionally fail to meet even today's minimum release requirements, threatening the salmon fishery with oxygen-depleted water.
Finally, climate change could increase the salinity of some water bodies. Rising sea level (see Coastal Zones) and reduced precipitation would increase the salinity in estuaries Moreover, decreased river flows could lead to excessive concentrations of minerals that run off from farms into various rivers. For example, Nash and Gleick (1993) estimated that salinity from runoff in the Colorado River could rise 15 to 20 percent under various climate model scenarios. Such increases in salinity, as well as the decline in flow, could impair compliance with U.S. obligations to Mexico.
With the rising demand for recreational uses, recreation and environmental quality have been added as objectives (de facto, if not statutory) in the management of the nation's inland waters over the last few decades. Many reservoirs are now encircled by resort developments and marinas. In many cases, a decline in reservoir levels would impair the use of recreational watercraft. If reservoirs are more frequently drawn down during summer droughts, the pleasant shorelines bordering lakefront homes will be transformed more frequently into muddy beaches. Declines in water quality would adversely affect swimming, boating, and other recreational uses of lakes and rivers, and thereby impair property values. The decline in water quality could also adversely affect fish, further reducing recreational benefits.
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