This report is one of a series designed to characterize the hydrology of drainage basins within coal provinces, nationwide. Area 51 (in the Rocky Mountain Coal Province) includes all or part of the Shoshone, Bighorn, Greybull, Wind, and Popo Agie River drainage basins - a total of 11,800 sq mi. Area 51 contains more than 18 million tons of strippable bituminous coal and extensive deposits of subbituminous coal, in the arid and semiarid basins. The report represents a summary of results of water resources investigations of the U.S. Geological Survey, some of which were conducted in cooperation with State and other Federal agencies. More than 30 individual topics are discussed in brief texts that are accompanied by maps, graphs, photographs , and illustrations. Primary topics in the reports are physiography, resources and economy, surface-water quantity and quality, and groundwater. (USGS)