The socioeconomic costs of floods, droughts, and water scarcity in the years 2030 and 2095 are examined under three climate scenarios: continuation of the current climate and two climate-change scenarios based on projections from the respective results of the Canadian and Hadley general circulation models. Measures of the adequacy of water supplies to meet both withdrawal and instream uses under current and future conditions are developed for the 18 major water resources regions and 99 assessment subregions in the conterminous United States. Past and likely future changes in the infrastructure available to control and distribute water, the costs of nontraditional sources of supply, water management practices, conservation opportunities, the nature of the economy, slack in the water supply system, and institutions influencing water use are examined and provide the basis for evaluating the impacts of changes in both climate and non-climate factors on U.S. water resources. The impacts of the climate changes are calculated as the changes in the costs of maintaining the projected no-climate change, non-irrigation offstream water uses with the climate-altered supplies. The costs and benefits are estimated under three alternative management strategies that differ in the protection provided for stream-flows and irrigation. The results support several general conclusions. First, a greenhouse warming could have major impacts on the future costs of floods, droughts, and balancing water demands and supplies. Second, the contrasting hydrologic implications of the Canadian and Hadley climate models indicate that the magnitude as well as the direction of these impacts are uncertain and likely to vary significantly among water resources regions. Third, there are many opportunities to adapt to changing hydrological conditions, and the net costs are particularly sensitive to the institutions that determine how the resource is managed and allocated among users. This report was prepared as part of the Water Assessment Sector Team's contribution to the U.S. National Assessment of the Potential Consequences of Climate Variability and Change for the Nation being conducted under the auspices of the U.S. Global Change Research Program. The climate-change scenarios used in this report were developed for use in the National Assessment.