Center foreground: Oblique view (looking northeast) of a cross section through the geologic model surfaces in the vicinity of the Yakima Fold Belt, Washington.Lower right: Model-generated surficial geology of the Columbia Plateau regional aquifer system, Idaho, Oregon, and Washington.

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Contents-Continued v Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows: °F=(1.8×°C+32). Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows: °C=(°F-32/1.8). *Transmissivity: The standard unit for transmissivity is cubic foot per day per square foot times foot of aquifer thickness [(ft 3 /d/ft 2 ]ft. In this report, the mathematically reduced form, foot squared per day (ft 2 /d), is used for convenience. DATUMS Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 (NGVD29). Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD83).

Although often overlooked, groundwater is increasingly important to all our lives. Groundwater is the Nation's principal reserve of freshwater. It provides one-half of our drinking water and is essential to U.S. food production while facilitating business and industry in promoting economic wellbeing. Groundwater also is an important source of water for sustaining the ecosystem health of rivers, wetlands, and estuaries throughout the country. Large-scale development of groundwater resources with accompanying declines in groundwater levels and other effects of pumping have led to concerns about the future availability of groundwater to meet all our Nation's needs. The depletion of groundwater to satisfy the country's thirst and the compounding effects of recent droughts emphasize the need for an updated status of the Nation's groundwater resources. Assessments of groundwater resources provide the science and information needed by the public and decision makers to evaluate water availability and its effects on the water supply, as well as, to manage and use the water resources responsibly. Adding to this already complex task of resource assessment is the analysis of potential future effects due to climate variability, which can further exacerbate an already challenging situation. The U.S. Geological Survey (USGS) is conducting large-scale multidisciplinary regional studies of groundwater availability, such as this study of the Columbia Plateau Regional Aquifer System. These regional studies are intended to provide citizens, communities, and natural resource managers with (1) improved information and knowledge of the status of the Nation's groundwater resources, (2) how changes in land use, water use, and climate have affected those resources, and (3) tools to forecast how these resources may change in the future. Over time, the findings from these individual regional groundwater assessments of principal aquifers can be scaled up to a national synthesis and scaled down to provide information relevant to issues of local concern. This national scale groundwater assessment directly supports the USGS National Water Census.

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