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The nationwide need for hydrologic information characterizing conditions in mined and potential mined areas has become critical with the enactment of the Surface Mining Control and Reclamation Act of 1977. This report is designed to be useful to surface mine owners, operators, and others by presenting existing hydrologic information and by identifying sources of hydrologic information. A brief text with an accompanying map, chart, graph, or other illustration presents general hydrologic information for each of a series of water-resources-related topics. Summation of the topical discussions provides a description of the hydrology of the area. Area 49 encompasses about 10,700 square miles in southeastern Montana and north-central Wyoming, near the center of the Northern Great Plains Coal Province. The land surface typically is characterized by rolling uplands dissected by steep-walled valleys and primarily is drained by the Yellowstone River, the Tongue River, and Rosebud Creek and their tributaries.

Water-level data were collected in the Gallatin Valley to provide a basis for evaluation of the configuration of the ground-water surface and water-level change. Water-level records collected during a 2-year study of the area and historical water-level data for 121 wells in the Gallatin Valley are summarized in the report. Well-depth and primary-aquifer data are included. The locations of the wells are shown on a map at a scale of 1:275,500.

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