Hydrologic information and analyses are needed to aid in decisions to lease Federally owned coal and to prepare the necessary environmental assessments and impact-study reports. This need has become even more critical with the enactment of the Surface Mining Control and Reclamation Act of 1977. This report, which is one of a series covering the Nation's coal areas, presents information thematically by describing single hydrologic topics through the use of brief texts and accompanying maps, graphs, or other illustrations. The topics encompass the complete physical and hydrologic setting of the area. Area 58, located primarily in west-central Colorado, is in the Southern Rocky Mountain and Colorado Plateau physiographic provinces. The area consists of 13,132 square miles of diverse geology, topography, and climate. The diversity results in a broad array of hydrologic characteristics. The Colorado River is the principal river draining the region. The headwaters of the Colorado and most of its tributaries originate in the mountains forming the eastern boundary of the area. This boundary also is the Continental Divide. The U.S. Geological Survey operates a network of hydrologic stations to measure the streamflow and waterquality conditions. This network currently includes 97 stations collecting streamflow data and 12 stations collecting water-quality data. Information is available for many surface-and groundwater sites measured in the past. Data available include rate of flow, water levels, and water quality. These data are available from the U.S. Geological Survey either in published reports or from computer storage through the National Water-Data Exchange (NAWDEX) or the National Water-Data Storage and Retrieval System (WATSTORE). All coal mines in the basin but one are underground. Production from these mines during 1982 totaled almost 1.0 INTRODUCTION 7.7 Objective Report Summarizes Available Hydrologic Data Existing hydrologic conditions and sources of information are identified to aid leasing decisions and preparation and appraisal of environmental impact studies and mine-permit applications. Hydrologic information and analysis are needed to aid in decisions to lease Federally owned coal and to prepare the necessary environmental assessments and impact study reports. This need has become even more critical with the enactment of Public Law 95-87, the "Surface Mining Control and Reclamation Act of 1977." This Act requires an appropriate regulatory agency to issue mining permits based on the review of permit application data to assess hydrologic impacts. This need is partly fulfilled by this report, which broadly characterizes the hydrology of Area 58 in west central Colorado and eastern Utah, a part of the Northern Great Plains, and Rocky Mountain Coal Provinces (fig. 1.1-1). This report is one of a series that describes coal provinces nationwide. MONTROSE

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Hydrologic data were collected from 35 wells and 82 springs in the San Miguel River basin from 1977 to 1979. Depth to water was measured for 22 wells and discharges were measured for 53 springs. Chemical analyses for water samples collected from 19 wells and 21 springs indicated larger dissolved solids concentrations in bedrock water samples than in alluvial water samples. Drillers' records obtained from the Colorado State Engineer's Office for 86 wells indicated generally larger yields from wells completed in alluvium than in bedrock.

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