The Bill Williams area includes about 3,200 mi² in Mohave, Yavapai, and Yuma Counties in west-central Arizona. The west half of the area is in the Basin and Range lowlands water province, and the east half is in the Central high-lands water province (see index map). The Basin and Range lowlands province generally is characterized by high mountains separated by broad valleys filled with deposits that commonly store large amounts of ground water. The Central highlands province consists mostly of rugged mountain masses made up of igneous, metamorphic, and well-consolidated sedimentary rocks that contain little space for the storage of ground water except where highly fractured or faulted. A few small valleys between the mountains contain varying thicknesses of water.-bearing deposits. The area is drained by the Bill Williams River and its major tributaries-the Big Sandy River and the Santa Maria River. Many reaches of the Big Sandy and Santa Maria Rivers and their major tributaries are perennial; the flow is sustained by ground-water discharge (Brown and others, 1978, sheet 2). In the Bill Williams area most of the water used is from ground water, although a small amount of surface water also may be diverted. About 18,000 acre-ft of ground water was withdrawn in 1979 (U.S. Geological Survey, 1981). About 17,000 acre-ft was used for the irrigation of 5,200 acres, and the rest was used for domestic, stock, and public supplies. Most of the irrigated land is in Skull Valley and along lower Kirkland Creek and the Bill Williams River. Only selected wells are shown on the maps in areas of high well density. The hydrologic data on which these maps are based are available, for the most part, in computer-printout form and may be consulted at the Arizona Department of Water Resources, 99 East Virginia, Phoenix, and at U.S. Geological Survey offices in: Federal Building, 301 West Congress Street, Tucson, and Valley Center, Suite 1880, Phoenix. Material from which copies can be made at private expense is available at the Tucson and Phoenix offices of the U.S. Geological Survey.