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Executive Summary Simulated flows at Maybell are generally higher than those under "future without project" conditions though still somewhat lower than under current conditions. The median flows equal or exceed the values in the preliminary flow recommendations. Late season flows in dry years are significantly higher than the future without project case and exceed current conditions in September. Spring runoff flows at Maybell are reduced by about 1-2% due to the refilling of the enlarged Elkhead Reservoir. The median and dry-year flows under this model scenario are also shown on Figure S-1. SITE CHARACTERISTICS Geotechnical Conditions Previous geotechnical studies of the existing dam site were reviewed in detail and new site investigations were carried out in the summer of 1993. These new investigations included borings and test pits in the areas of the new embankment and spillways, as well as in the existing dam embankment, and geophysical surveys along the proposed dam axis. Test pits were also dug at a number of potential borrow sites in the reservoir area. Based on the results of these surveys and tests, it was concluded that foundation conditions were suitable for the contemplated embankment and spillways and that suitable materials for the embankment fill were available in the reservoir area. Seismic analysis indicated that design ground accelerations would not induce liquefaction in these construction materials. Hydrology and Sediment Movement Operational hydrology for Elkhead Reservoir was developed as part of the Phase I Study These data represent monthly inflows over the 1930-82 study period and were developed using regional analysis methods. These methods relied principally on records from two stream gages upstream on Elkhead Creek and from the Maybell gage. Average annual inflow to the reservoir over this study period was estimated to be 64,500 acre-feet. A study by others completed in 1991 estimated the Probable Maximum Flood (PMF) at the reservoir to be 40,600 cfs. The State Engineer has indicated that this estimate is still considered valid and it was not revised in the present study, though recently a statewide review of PMF criteria has been initiated. A runoff analysis was completed which estimated the 100- year rainfall flood peak at the reservoir to be 3,232 cfs. Limited sediment sampling of reservoir inflows and outflows was conducted as part of the present study. This sampling suggests that all bed load and roughly 95 % of suspended load is trapped in the reservoir, even though noticeable suspended material reaches the spillway during runoff periods. The estimated sediment capture rate is 81 of/yr and an allowance for 7,500 acre-feet was recommended for sediment accumulation over the 100-year life of the reservoir. S-6