The northern San Luis Valley is a high mountain valley encompassing about 4,000 sq km (1,500 sq mi) of south-central Colorado and is located 210 km (130 mi) southwest of Denver (Fig. 1). The valley is situated along the Rio Grande Rift, a zone of Cenozoic normal faulting, volcanic activity, and high heat flow (Chapin, 1971). The valley is bounded on the east and west by the Sangre de Cristo and San Juan Mountains, respectively, on the north by Poncha Pass, and is bounded in part on the south by a shallow ground and surface water divide north of the Rio Grande.
Previous studies of the northern San Luis Basin can be grouped into two categories: (1) geologic studies of portions of the Sangre de Cristo and San Juan Mountains, such as those by Knepper (1974), Peel (1971), Bolyard (1956), Johnson (1969), Larsen and Cross (1956), Steven and others (1974), and Huntley (1976); and (2) general hydrologic studies of San Luis Valley, such as the work of Siebenthal (1910), Powell (1958), Emery and others (1969, 1971, 1972, 1973, 1975), and Huntley (1976). This report results from an attempt to integrate geologic and hydrologic data of the valley in order to evaluate mechanisms of groundwater recharge to the basin.