This report describes the occurrence, availability, and quality of ground-water and related surface-water resources in Catron County, the largest county in New Mexico. The county is located in the Lower Colorado River Basin and the Rio Grande Basin, and the Continental Divide is the boundary between the two river basins. Increases in water used for mining activities (coal, mineral, and geothermal), irrigated agriculture, reservoir construction, or domestic purposes could affect the quantity or quality of ground- water and surface-water resources in the county. Parts of seven major drainage basins are within the two regional river basins in the county--Carrizo Wash, North Plains, Rio Salado, San Agustin, Alamosa Creek, Gila, and San Francisco Basins. The San Francisco, Gila, and Tularosa Rivers typically flow perennially. During periods of low flow, most streamflow is derived from baseflow. The stream channels of the Rio Salado and Carrizo Wash Basins are commonly perennial in their upper reaches and ephemeral in their lower reaches. Largo Creek in the Carrizo Wash Basin is perennial downstream from Quemado Lake and ephemeral in the lower reaches. Aquifers in Catron County include Quaternary alluvium and bolson fill; Quaternary to Tertiary Gila Conglomerate; Tertiary Bearwallow Mountain Andesite, Datil Group, and Baca Formation; Cretaceous Mesaverde Group, Crevasse Canyon Formation, Gallup Sandstone, Mancos Shale, and Dakota Sandstone; Triassic Chinle Formation; and undifferentiated rocks of Permian age. Water in the aquifers in the county generally is unconfined; however, confined conditions may exist where the aquifers are overlain by other units of lower permeability. Yields of ground water from the Quaternary alluvium in the county range from 1 to 375 gallons per minute. Yields of ground water from the alluvium in the Carrizo Wash Basin are as much as 250 gallons per minute for short time periods. North of the Plains of San Agustin, ground-water yields from the alluvium in the San Agustin Basin range from 1 to 10 gallons per minute. Irrigation wells completed in the Quaternary bolson fill produce as much as 975 gallons per minute immediately east of the county. Water from the bolson fill in the Plains of San Agustin has specific-conductance values generally ranging from 180 to 3,300 microsiemens per centimeter at 25 degrees Celsius (microsiemens/cm). Yields from the Gila Conglomerate generally range from 2 to 5 gallons per minute. Water samples from two springs from the Gila Conglomerate have specific conductances of 289 and 381 microsiemens/cm. The Tertiary Datil Group is present in the Carrizo Wash, San Agustin, San Francisco, and Gila Basins. The Datil Group commonly is unconfined, but may be confined at depth. Water levels of wells completed in this unit range from 60 to 1,260 feet below land surface. Wells completed in the Datil Group typically yield 1 to 15 gallons per minute. Specific conductance of water from the Datil Group ranges from 210 to 820 microsiemens/cm. The Tertiary Baca Formation in the Carrizo Wash Basin produces 5 to 20 gallons per minute in stock wells. Water from wells completed in the Baca Formation has specific-conductance values ranging from 312 to 752 microsiemens/cm. Aquifers in Cretaceous rocks are present in the Carrizo Wash, North Plains, and Rio Salado Basins. The potential yield from wells completed in Cretaceous rocks in northwestern Catron County is from 1 to 122 gallons per minute. Specific conductance of water from Cretaceous rocks ranges from 210 microsiemens/cm in the Moreno Hill Formation to 4,490 microsiemens/cm in the Mancos Shale. Yields of ground water from Cretaceous rocks in the Carrizo Wash Basin typically range from 1 to 100 gallons per minute; in an artesian well, however, the yield in the main body of the Creta