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CHAPTER II NEEDS AND RESOURCES Water supply The 83-square-mile Plateau Creek watershed has a mean yield of about 11,500 acre-feet per year. From an evaluation of simulated flows on Plateau Creek for the years 1928 to 1973, it was found that a 15,000- acre-foot reservoir would have filled during a single spring runoff 12 out of 46 years and would have taken up to 5 years to fill during the 1953-57 and 1961-65 dry periods. Once full, the only depletion would be from evaporation and reservoir seepage, The estimated makeup water for the unit would be around 500 acre-feet per year. Topography and geology An overall elevation difference of about 1,000 feet between the forebay and the afterbay would provide an average power head of about 780 feet. The distance between the forebay inlet and the afterbay outlet is about 4,400 feet, The length-to-head ratio of 5.6:1 is well within recommended ratios for pumped-storage projects. The geologic formations at the proposed site are expected to be similar to those at the nearby McPhee Dam, and thus suitable for the proposed forebay and afterbay reservoirs and other project features. Formations at the damsite for Plateau Creek afterbay are solid sandstone and might be suitable for a concrete arch dam. The only identified geologic problem is a lack of stability along the edge of the mesa where the Campbell forebay reaervoir would be located. This restriction would limit the maximum size of the unit to 500 MW if Campbell forebay were utilized in the plan. Power transmission The connection to the Colorado River Storage Project grid would be through the Colorado-Ute Electric Association's proposed 345-kV trans- mission line between the Aspinall and Shiprock substations. That line would follow the corridor of the existing 230-kV line that passes near the unit area. Colorado-Ute and the Western Area Power Administration have recently received approval to upgrade the line to a single 345-kV line, If the line is not upgraded by Colorado-Ute and Western, at least the portion of the line between the unit and the Shiprock substation would need to be upgraded as part of the unit, Constraints The possible lack of nearby construction materials is the only sig- nificant constraint on the unit. Construction on the Dolores Project has shown that materials are not readily available in the vicinity. The recommended site appears to lack zones two and three pervious material and aggregate for the proposed dams. Reworked Mancos Shale, apparently plentiful in the area or within a short haul distance, could be used as zone one impervious materials. The only known sources of aggregate are in the upper reaches of Beaver Creek and on Ormiston 6