The northern San Luis Drainage Basin, which includes the northern closed basin of San Luis Valley and contributing watersheds of the, Sangre de Cristo and San Juan Mountains.(Fig. 1), is a high mountain basin encompassing about 8,400 sq km of south-central Colorado. The basin is located 210 km southwest of Denver, along the Rio Grande Rift Zone, a zone of Cenozoic normal faulting, volcanic activity, and high heat flow (Chapin, 1971). The basin is bounded on the east and west by the surface water divides of the Sangre de Cristo and San Juan Mountains, respectively, on the north by Poncha Pass, and on the south by a shallow ground- and surface-water divide north of the Rio Grande River.

The aquifers of San Luis Valley have undergone rapid development since the discovery of artesian water in 1887. A definition of the sources of ground-water recharge to these aquifers is necessary for the rational management of both the developed and potential resources of the valley. More important perhaps is that the ground-water recharge system must be understood before the relations between aquifer development and flow in the Rio Grande can be defined and used to meet Colorado's obligations to New Mexico under the 1938 Rio Grande Compact. Although several studies have discussed the hydrogeology of portions of the valley (Siebenthal, 1910; Powell, 1958), the ground-water chemistry of the valley (Klein, 1971; Glanzman, 1972; Emery and others, 1973), and the general hydrology of the valley (Emery and others, 1969, 1971, 1972, 1973), this is the first such study to focus specifically on the ground-water recharge system of the northern San Luis Basin.

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