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. General circulation models suggest that some regions of the United States may have more rainfall during the wet season, which would increase river and Lake levels. Moreover, increased flooding could occur even in areas that do not become wetter: 1) earlier snowmelt could worsen spring flooding while diminishing summer water availability; 2) some climate models suggest wetter winters and drier summers; 3) the need to ensure summer/drought water supplies could lead water managers to keep reservoir levels higher, thereby limiting the capacity for additional water retention during unexpected wet spells; 4) warm areas generally have a more intense hydrologic cycle and thus more rain in a severe storm; and, 5) many areas may receive more intense rainfall. With the exception of earlier snowmelt, however, it is also possible that these processes will become more benign as climate changes, at least in some areas.
The most flood-prone communities of the United States are at least partly protected by levees and reservoir flood-storage capacity. However, as the Mississippi/Missouri River floods of 1993 illustrate, the systems are designed to prevent the relatively frequent, moderately destructive floods, for example those with at least a 1 percent chance of occurring in any given year. These systems are overwhelmed and almost completely ineffective, however, against the rare flood that is more devastating than the flood the system was designed to handle. Although the 1993 flood has not been causally linked to global warming, it also illustrates another limitation of case studies, which generally focus on a shift in average conditions and assume that variability is otherwise unchanged: we can not rule out a completely unexpected change in weather patterns, we merely lack the basis for determining what these changes might be.
Because of the cost and environmental impacts of flood-protection structures, many parts of the United States rely on land-use regulations to prevent flood damages. Almost one thousand communities along rivers and lakes are part of the National Flood Insurance Program, which effectively precludes construction in most 100-year non-coastal floodplains. Changing climate may shift floodplain boundaries, so construction may be taking place today in areas
that will turn out to be within the floodplain in the coming decades.