A nationwide need for information characterizing hydrologic conditions in mined and potential mine areas has become paramount with the enactment of the Surface Mining Control and Reclamation Act of 1977. This report, one in a series covering the coal provinces nationwide, presents information thematically by describing single hydrologic topics through the use of brief texts and accompanying maps, graphs, or other illustrations. The summation of the topical discussions provides a description of the hydrology of the area. Area 54, in north-central Colorado and south-central Wyoming, is 1 of 20 hydrologic reporting areas of the Northern Great Plains and Rocky Mountain coal provinces. Part of the Southern Rocky Mountains and Wyoming Basin physiographic provinces, the 8,380-square-mile area is one of contrasting geology, topography, and climate. This results in contrasting hydrologic characteristics. The major streams, the North Platte, Laramie, and Medicine Bow Rivers, and their principal tributaries, all head in granitic mountains and flow into and through sedimentary basins between the mountain ranges. Relief averages 2,000 to 3,000 feet. Precipitation in the mountains may exceed 40 inches annually, much of it during the winter, which produces deep snowpacks. Snowmelt in spring and summer provides most streamflow. Precipitation in the basins averages 10 to 16 inches annually, insufficient for sustained streamflow; thus, streams originating in the basins are ephemeral. Streamflow quality is best in the mountains where dissolved-solids concentrations generally are least. These concentrations increase as streams flow through sedimentary basins. The increases are mainly natural, but some may be due to irrigation in and adjacent to the flood plains. In the North Platte River, dissolved-solids concentrations are usually less than 300 milligrams per liter; in the Laramie and the Medicine Bow Rivers, the concentrations may average 500 to 850 milligrams per liter. However, water-quality stations on the Laramie and the Medicine Bow Rivers are farther removed from the mountain sources than the stations in the North Platte drainage. Because of the semiarid climate of the basins, soils are not adequately leached. Consequently, flow in ephemeral streams usually has a larger concentration of dissolved solids than that in perennial streams, averaging 1,000 to 1,600 milligrams per liter. Aquifers containing usable ground water are combined into three groups: (1) consolidated and unconsolidated non-coal-bearing Quaternary and Upper Tertiary deposits, (2) Mesozoic and Paleozoic sedimentary rocks, and (3) Lower Tertiary and Upper Cretaceous sedimentary rocks containing coal. These aquifers are used for municipal, domestic, irrigation, and stock supplies. Well yields range from about 5 to 1,000 gallons per minute, and depend on type of aquifer, saturated thickness, and degree of fracturing. The best quality ground water usually comes from the non-coal-bearing Quaternary and Upper Tertiary rocks or the Mesozoic and Paleozoic rocks; often it is dominated by calcium and bicarbonate ions. The coal-bearing formations have a large variability in water chemistry; dominant ions may be bicarbonate or sulfate and sodium, calcium, or magnesium. Dissolved-solids concentrations are generally larger than in the former two groups. The U.S. Geological Survey operates a network of hydrologic stations to observe the streamflow and groundwater conditions. This network currently includes 31 surface-water stations and 35 observation wells; information is available for many other sites observed in the past. Data available include rate of flow, water levels, and water quality; much of the data are available in published reports or from computer storage through the National Water Data Exchange (NAWDEX) or the National Water Data Storage and Retrieval System (WATSTORE). Five formations of Late Cretaceous and early Tertiary age contain coal.