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<br />00130~ <br /> <br />. <br /> <br />. <br /> <br />DRAFT <br /> <br />TASK 9 TECHNICAL MEMORANDUM <br /> <br />INTRODUCTION <br /> <br />The objective of Task 9 of the Yampa River Basin Alternatives Feasibility Study was to <br />complete additional evaluations necessary to support the preliminary project recommendations <br />made by the Technical Steering Group and reviewed with the Public Advisory Group. The <br />purpose of these evaluations was to further understand the requirements, costs and impacts of <br />the recommended alternative project. Results of these evaluations are presented below. <br /> <br />ADDITIONAL EVALUATIONS <br /> <br />Refinement of Elkhead Creek Hydrology <br /> <br />The existing Elkhead Dam and Reservoir is located on Elkhead Creek approximately <br />five miles upstream of Colorado State Highway 40. Task 4 of the Feasibility Study included a <br />preliminary estimate of the inflow hydrology to Elkhead Reservoir. This estimate was based <br />on standard techniques for developing synthetic hydrology from information on basin . <br />characteristics and extension of incomplete records through regression techniques. The set of <br />inflows produced through this effort was then used in the basin model to evaluate several <br />different potential water development scenarios. Model inflows represent depleted hydrologic <br />conditions, i.e. they account for historical depletions associated with irrigation practices and <br />other water uses in the area, and are not meant to represent virgin hydrologic conditions. <br /> <br />In Task 9 of the Feasibility Study, the procedures and assumptions for developing inflow <br />hydrology to Elkhead Reservoir were revisited. Measurements of basin areas were double <br />checked and gage accuracy and regression equations reviewed. In addition, a more thorough <br />search of previous related studies was made in an effort to provide comparisons of estimated of <br />hydrologic conditions on Elkhead Creek. Particular attention was also given to water rights <br />along Elkhead Creek and how these rights might affect the storable yield of the Creek under <br />the existing Elkhead Reservoir water rights and the proposed enlargement. An enlargement at <br />Elkhead was assumed to have a priority data identical to the Juniper Project rights, i.e. 1954.. <br /> <br />Development of Elkhead Reservoir Inflow Hydrology <br /> <br />There are four USGS stream gages located on Elkhead Creek. Three of these gages are <br />located above the site of the existing Elkhead Reservoir: gage 9244500 (Elkhead Creek near <br />Clark); gage 9245000 (Elkhead Creek near Elkhead) and gage 9245500 (North Fork Elkhead <br />Creek near Elkhead). A fourth gage is located immediately above the confluence with the <br />Yampa River (gage 9246500). Inflow hydrology was developed for Elkhead Reservoir using <br />information from gages 9245000 and 9245500 which are both located above the reservoir. The <br />method used to develop the basin model inflows was as follows: <br /> <br />I <br /> <br />,~",-;,,~ <br />