Area 50 includes the entire Powder River Basin and the upstream parts of the Belle Fourche and Cheyenne River basins a total of 20,676 square miles. The information in this report will be useful to Federal agencies in leasing and management of Federal coal lands, to surfacemine owners, operators and others preparing permit applications, and to regulatory authorities. The report represents a summary of the results to date of the waterresources investigations of the U.S. Geological Survey, conducted in cooperation with State and other Federal agencies. Groundwater data water levels, well yields, and water quality are available for about 3,000 wells in Area 50. Water levels in 800 wells completed in the Wyodak-Anderson coal were collected during 1981 by mining companies and others, resulting in a compilation of water-level data for the coal and overburden in the mine areas of Campbell County, Wyoming. Groundwater supplies suitable for stock and domestic use can be developed from wells less than 500 feet deep in most of the area. Deeper wells generally are needed to obtain larger yields. The Madison Limestone usually is considered where large supplies are sought; however, because the permeability of the formation is principally secondary, large-yield wells are not always obtained. Recharge to aquifers occurs mainly in the outcrop areas near their southern, western, and eastern extents. Small quantities of water flow northward out of the area through the Madison and Lower Cretaceous aquifers. Movement in the coal aquifers is complex. Eighty-four percent of the wells and springs sampled yield water with dissolved-solids concentrations greater than the recommended national secondary drinking-water standard of 500 milligrams per liter. Nearly all sources sampled, however, yield water suitable for livestock. Manganese and iron concentrations in water from some aquifers are large enough to be objectionable for domestic supplies because of taste and staining problems. The chemical quality of the ground water is controlled by the solubility of the minerals that comprise the rocks, the reactions that occur as water moves through the rocks, the water temperature and pH, and the length of time during which the water is in contact with the rocks. This report represents a summary of the results of water-resources investigations to date in northeastern Wyoming and southeastern Montana. The U.S. Geological Survey has collected hydrologic data and made interpretive hydrologic studies in the area for decades. Much of the work has been done in cooperation with State and other Federal agenciesparticularly the Wyoming State Engineer, Wyoming