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Reservoir Site Characterization In addition to this mapping, a comprehensive effort to identify and collect all available , site area mapping was conducted. Most of this information proved to be of marginal value because it was either outdated, insufficient in coverage,'or limited in accuracy. All the sources which were identified are tabulated in the annotated bibliography. This other mapping was utilized in two important ways. First, the USGS mapping of Routt County and Moffat County from 1975 and the USGS 15' quadrangle maps of 1971 and 1973 provided the drainage basin maps for the hydrologic evaluation. Second, all of the maps which covered at least some of the dam site and downstream area were used in the downstream fluviomorphologic analysis. The maps were modified to a common scale basis with common physical landmarks used as overlay register points. The temporal changes in the Elkhead Creek channel plan location could be examined in this manner and quantitative conclusions reached about channel movement. Unfortunately, only limited quantitative horizontal dimensions could be made in this manner and vertical elevations with an accuracy of greater than 2' are not available to make precise degradation conclusions. OPERATIONAL AND FLOOD HYDROLOGY Background Research of available documentation reveals essentially no pre-construction information on either operational hydrology (how the dam/reservoir was expected to regulate seasonal inflows during its lifetime) or extreme event hydrology (rare, short duration, high instantaneous peak flows associated with distinct meteorologic events). Operational hydrology is used to determine the water storage capacity necessary to meet, in this case, raw water supply demands; such operational modeling is described in another section of this report. Rare event hydrology is used to size the components of the project which are needed to prevent the overtopping failure of the dam, thereby providing protection of the infrastructure investment in the dam and providing public safety. This rare event or flood hydrology is the subject of this section of the report. Fortunately, since the construction of the existing dam, several Probable Maximum Flood (PMF) evaluations have been completed. These provide a basis for the generation of the remaining rare event hydrologic information needed for the present study. Operational Hydrology I In order to carry out the operational studies of existing and proposed reservoirs in the Phase I Study, monthly inflow data sets were developed for numerous points in the basin using a regional analysis approach. This analysis was based on records from USGS streamflow gages and on estimates from previous studies of water yields (e.g., M-K, 1987; Tipton and Kalmbach, 1980). This approach utilized existing stream flow data to derive physical water yield models (regression equations) which accepted tributary area, average elevation, and aspect as independent variables. These regression models predicted annual stream flows which were then disaggregated into monthly flows based on distribution patterns observed over the gaged periods. Inflow files comprising monthly values for the entire 1930 to 1982 study period were developed. The veracity of regional models was tested by applying them to selected sub- basins for which a substantial period of record of gaged flows were recorded. Favorable comparisons between predicted and recorded flows for these sub-basins validated this approach. 3-10