This report presents the results of a county-wide ground-water appraisal of Williams County, a mostly agricultural county of more than 36,000 people that is undergoing gradual commercial and industrial development. Most of the County's ground water is in the 80-to 320-foot thick cap of unconsolidated glacial sediments. The underlying Mississippian and older bedrock units are mostly Wisconsin till containing discontinuous lenses of sand and gravel. Two end moraines that cross the County form low northeast-southwest-trending ridges. Ground moraine covers the rest of the County except for fine sand and silt lacustrine sediments in the southeastern corner.The water-bearing sand and gravel bodies appear to be thickest and most widespread in the end moraines and thinnest and more localized in the lacustrine sediments. A generally productive (up to 1,000 gallons per minute) zone of sand and gravel and broken, weathered rock is present in places at the contact of the unconsolidated sediments and the shale. A study of well logs and aquifer tests shows that well yields of 500 gallons per minute are possible over all but the southeastern corner of the County. Transmissivities range from 2,800 to more than 64,300 feet squared per day. Storage coefficients that range from 0.0001 to 0.00038 indicate confined to semiconfined conditions. A gently southeast-sloping water-level surface was identified by measuring water levels in an 87-well network. A potentiometric-surface map constructed from these water-level measurements shows a fairly consistent gradient of 10 to 30 feet per mile across the County, which indicates that the unconsolidated sediments, on a large scale, act as one aquifer. Ground water flows toward the southeast. The recharge area for the ground-water system includes Williams County, and the area just to the northwest of Williams County, whereas the discharge areas are mainly the streams within and to the southeast of the County. Water quality in the unconsolidated sediments was evaluated through the analysis of samples from 48 wells. The predominately calcium magnesium bicarbonate type water generally is suitable for most uses, but is hard and high in iron. The median pH is 7.6, the median specific conductance is 600 microsiemens per centimeter, the median iron concentration is 1.4 milligrams per liter, and the median hardness (as CaCO3) is 290 milligrams per liter. Water in the southeastern corner of the County contains more sodium than elsewhere in the County. Seasonal variations in the ground-water quality are small. Analysis of four samples showed the water quality of area streams at base flow to be very similar, although slightly more dilute and less hard than the ground water.