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. I NTRODUCTI ON . Results of a dam rehabilitation feasibility study of Rio Grande Dam are presented in this report. The dam, completed in 1914, is located in Hinsdale County, about 20 miles southwest of Creede in southwestern Colorado. Rio Grande Reservoir, with a storage capacity of 52,192 acre-feet, is the only main stem reservoir on the Rio Grande in Colorado. The drainage basin includes 162 square miles with elevations ranging from 9400 feet at the dam to over 13,000 feet along the Continental Divide. The San Luis Valley Irrigation District (SLVID) operates Rio Grande Reservoir. Water released from the reservoir is primarily used for irrigation of about 45,000 acres of land under the Farmers Union Canal in the San Luis Valley. Water is also stored in the reservoir by agreement with the State Engineer of Colorado and the Rio Grande water users for the purpose of aiding Colorado in adminis- tering the Rio Grande Compact. Incidental use is made of the reser- voir for recreation and fish and wildlife. A Phase I Inspection Report under the National Dam Safety Program was prepared by Bruns, Incorporated in 1978. The spillway was found to be seriously inadequate to pass the probable maximum flood as required for dams rated as "large" and "high hazard" under the Corps of Engineers classification system. Dam stability was considered to be questionable. It was recommended that a thorough engineering investigation be conducted to establish the exact inadequacies of the dam. The Rio Grande Dam rehabilitation feasibility study was initiated through an advance to the SLVID provided by the Colorado Water Conservation Board. W. W. Wheeler and Associates, Inc. and Chen and Associates were engaged by the SLVID to conduct the study. Wheeler made an evaluation of flood hydrology and determined the capability of the existing dam facilities to pass the design floods. Geotechnical . -1-