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Figure 1. Moisture sources to the Colorado River basin. precipitation with elevation. Cold frontal systems produce substantial amounts of snow above about 5,000 feet and rainfall at lower elevations in the Rocky, Uinta, and Wind River Mountains, which constitute the headwaters of the Colorado River and its principal tributary, the Green River. These storms build snowpacks that melt in the late spring, providing runoff to the Colorado River. Warm winter storms, which may tap moisture from the tropical Pacific Ocean, may produce rainfall on snowpacks, resulting in high runoff and floods on major rivers. The frequency and moisture content of frontal systems are strongly affected by atmospheric circulation patterns and sea-surface temperature (SST) of the tropical and North Pacific Oceans. Moisture delivered to the Colorado River basin during summer typically is a mixture of moist air from

Hydrologic data were collected in Washington and Iron Counties, Utah, from 1995 to 1997 to better understand the hydrologic system. Data from earlier years also are presented. Data collected from wells include well-completion data, water-level measurements, and physical properties of the water. Data collected from springs and surface-water sites include discharge and physical properties of the water. Selected water samples collected from ground-and surface-water sites were analyzed for isotopes, chlorofluorocarbons, and dissolved gases. INTRODUCTION This report contains hydrologic data collected in Washington and Iron Counties, Utah, from 1995 to 1997, as well as data from earlier years. The study area is in the southwestern corner of Utah and includes all of Washington County west of the Hurricane Fault and the southern part of Iron County. The area is about 1,900 ry mi and includes both the Basin and Range and the Colorado Plateau physiographic provinces described by Fenneman (1931). Population in this area has increased by more than 160 percent from 1980 through 1995 (Utah State Data Center, 1991, 1997). Both full-time and winter residents are attracted to the area, known as "Utah's Dixie," because the climate is warmer there than in the rest of the State. The greatest challenge to further growth is the limited water resources because the climate is so dry. The largest constraint on development of additional residential areas and commercial activities is water supply. Surface water in the area drains from the central Virgin River drainage basin and is fully appropriated. The need for increased groundwater withdrawals prompted the Utah Department of Natural Resources, Division of Water Rights, to initiate a 4-year study of the groundwater resources of the area in cooperation with the U.S. Geological Survey and the Washington County Water Conservancy District. Data were collected to better understand the hydrologic system in the area and to assess the effects of increased groundwater withdrawals on groundwater levels, discharge from springs, surface-water flows, and water quality. This report documents hydrologic data collected as part of the central Virgin River basin area groundwater study. Groundwater data were collected from existing wells and springs and from new wells completed by private owners, developments, and municipalities during 1995-97. For comparison, this report also provides data from earlier years, some of which were published previously by Cordova (1972, 1978) and Budding and Sommer (1986).

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