The objectives of this investigation are as follows: 1) review the existing hydrogeologic data for the San Luis Valley, the Great Sand Dunes Monument, and Medano Creek; 2) review the surface-water data that have been collected on Medano Creek; 3) collect or review previously collected water-level data obtained in the area of lower Medano Creek and correlate them with the surface-water data; 4) conduct constant-head permeameter tests on sand samples collected near the ground surface along lower Medano Creek; 5) produce a conceptual model of lower Medano Creek; and 6) produce a numerical model of lower Medano Creek that will predict the effect that a lowering of the regional water table could have on the terminus of flow of Medano Creek. The complex hydrogeologic conditions under lower Medano Creek have been approximated with three homogeneous and anisotropic layers. A complex system of confining layers is represented by a single low-hydraulic-conductivity layer in the middle. Numerical-modeling results suggest that the location of the terminus of flow in Medano Creek will recede significantly in response to a lowering of the regional water table, possibly by as much as 21,000 feet (6,400 meters) if the regional water table is lowered 150 feet (46 meters). These results indicate the qualitative effect that a lowering of the regional water table would have on lower Medano Creek, but they cannot be considered to be precise quantitative predictions. The results should be regarded with caution due to the paucity of data available.

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