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Changing climate is expected to increase both evaporation and precipitation in most areas of the United States. In those areas where evaporation increases more than precipitation, soil will become drier, lake levels will drop, and rivers will carry less water.
Lower river flows and lower lake levels could impair navigation, hydroelectric power generation, and water quality, and reduce the supplies of water available for agricultural, residential, and industrial uses. Some areas may experience both increased flooding during winter and spring, as well as lower supplies during summer. In California’s Central Valley, for example, melting snow provides much of the summer water supply; warmer temperatures would cause the snow to melt earlier and thus reduce summer supplies even if rainfall increased during the spring. More generally, the tendency for rainfall (see climate trends) to be more concentrated in large storms as temperatures rise would tend to increase river flooding, without increasing the amount of water available.
Many federal and state agencies are actively engaged in reducing the nation’s vulnerability to these types of impacts. In the western United States, where freshwater is scarce, a gradual trend toward allowing farmers to sell water is enabling scarce water to be used more efficiently. Along the Mississippi River and other floodplains, the Federal Emergency Management Agency  and others are reviewing structural and land-use measures for reducing vulnerability to floods. Finally, both the Corps of Engineers and the Bureau of Reclamation are developing better ways to manage the federal system of reservoirs in the face of changing climate to meet the competing needs of navigation, hydropower, water supply, recreation, and environmental quality.
Navigation
Climate change could impair navigation by changing average water levels in rivers and lakes, increasing the frequency of both floods during which navigation is hazardous and droughts during which passage is difficult, and necessitating changes in navigational infrastructure. On the other hand, warmer temperatures could extend the ice-free season.
Hydropower
Changes in the flows of rivers would have a direct impact on the amount of hydropower generated, because hydropower production decreases with lower flows. Because of the ambiguous projections of changes in future river flow, studies of the impacts of climate change show ambiguous effects on hydropower production.
Water Supply and Demand
In some parts of the western United States, the most widely discussed potential impact of climate change is the impact on water supply and demand. The potential changes in water supplies would result directly from the changes in runoff and the levels of rivers, lakes, and aquifers.
Environmental Quality and Recreation
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. 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.
Flood Control
Although the impacts of sea level rise and associated coastal flooding have been more widely discussed, global climate change could also change the frequency and severity of inland flooding, particularly along rivers.
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