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Introduction oversplash and the flood storage/conveyance capability needs to be increased to handle a larger inflow design flood than was originally considered. During the Phase I National Dam Safety Program inspection in 1979 (USACOE, 1980) the dam's service spillway was identified as having an inadequate capacity to handle the Probable Maximum Flood (PMF). The original PMF used for design, 15,900 cfs, was judged "conservative" in 1972. During the Phase I inspection 7 estimates of the PMF calculated it to lie between 13,299 cfs and 58,704 cfs, with the largest number being used for dam safety purposes. This finding, and the inference that physical modifications to the dam/reservoir would be needed to provide for safely passing the PMF, motivated the CDOW to complete their own PMF study. That study, which was completed in 1984 (CDOW, 1984), estimated a PMF of 37,387 cfs, a number which was subsequently approved by the State Engineers office. With the PMF issue temporarily resolved, several different options to safely pass the PMF were studied (M-K, 1985a) and (M-K, 1985b). The option of an auxiliary spillway in the left abutment was determined to be infeasible, although a design for this option was apparently prepared and submitted to the State for approval in 1986 (M-K, 1986). In the interim, interest was generated in the idea of enlarging the dam and reservoir as a raw water supply for the City of Craig and a new study was completed (M-K, 1987) addressing this subject. Neither the auxiliary spillway nor any of the options of enlargement (M-K, 1987) were pursued to the implementation stage. In an apparent attempt to consolidate the need to provide an enlarged spillway with Craig's desire to have an additional raw water source, the City, in 1990, entered into an agreement to purchase (actually, to trade) the CDOW's land, facilities and water rights at Elkhead for an equivalent value over a period of five years. As a part of refining the spillway capacity and enlargement issues the City completed several additional studies, a meteorologic report by Crow (which was apparently not directly utilized on this project) and one which revised the PMF once again (Harza, 1991a), this time to 40,600 cfs. Several other studies (Harza, 1991b,c,d) addressed various aspects of the enlargement and spillway issues, but have not directly led to the adoption of an implementation plan. The Yampa River Basin Alternatives Feasibility Study (Hydrosphere, 1993) was ongoing during 1990-1993 and reached a conclusion that the Elkhead Dam/Reservoir enlargement could meet short term basin water storage needs. The present study is addressing this possible enlargement, and with it the inadequate spillway issue, to a greater level of detail. ORGANIZATION OF REPORT This report is organized into six chapters followed by several technical appndices. The chapters describe the work carried out under the tasks listed above. Chapter 2 discusses water rights issues and reservoir operations studies. Chapter 3 discusses engineering investigations carried out in area of the existing Elkhead Reservoir. Chapter 4 presents preliminary design recommendations for the enlargement of the reservoir. Chapter 5 provides a preliminary schedule for the permitting, engineering, and construction activities associated with the reservoir enlargement and discusses the financial requirements of these activities. Chapter 6 contains a Preliminary Environmental Assessment of the reservoir enlargement project. Appendix A contains an annoted bibliography of information sources consulted during the study. 1-6