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I I I I I I I I I I I I I I I I I I I 00043(\ Executive Summary Canyon; was evaluated to characterize the impact on the annual hydro graph of the Yampa River. The modeled influence of the Juniper instream flow right was to slightly modify the annual temporal distributions of flow on the Yampa River. The instream flow right had the effect of slightly increasing flows during the low flow fall and early winter months. Junior demands which were called out by the instream flow right (in scenarios II through V) asserted an increased draw on basin reservoirs, which resulted in a slight decrease in Juniper Canyon flows in the spring and early summer months when the reservoirs filled. . Instream flow targets were modeled in two other locations on the Yampa River besides Juniper Canyon. These instream targets were located immediately below Stagecoach Reservoir. and below the City of Craig. The Stagecoach target reflects current operating procedures of the reservoir. The instream flow target (a constraint really) below Craig was modeled only in Scenario V to help protect this river reach from the depletive effects of operating an exchange out of Williams Fork Reservoir. No instream flow targets were modeled below Elkhead ReserVoir, Steamboat Lake or the Williams Fork Reservoir. Model results from Scenarios I through V indicated relatively little variation between development scenarios (III through V) with respect to instream flows in Juniper Canyon and other mainstem reaches. Selection of the recommended development alternative was not possible based on instream flow considerations alone. Reservoir Fluctuations The degree to which reservoir levels fluctuate has important biological and recreational implications. The ability of a reservoir to act as a water supply for perimeter wetlands depends greatly on seasonal rises and declines in the water surface elevation. In addition, reservoir fish populations can be affected by the degree and frequency of reservoir operations. There were relatively minor differences in reservoir fluctuations between the modeled scenarios. Hence, as with the evaluation of instream flow predictions, it was not possible to select the best water development project in the Yampa Basin based solely on the effects of operations of reservoirs on fisheries and wetlands. Development of new reservoirs would generally have more impact to existing wetlands than reservoir enlargements. Project Costs Preliminary cost estimates were generated for development of storage capacity at all five candidate reservoir sites, although only the Elkhead and Stagecoach enlargements and the Williams Fork project were modeled in the operating studies. These cost estimates were intended to provide only comparative information between alternative projects. A greater amount of information on physical characteristics was available for the Stagecoach and Elkhead sites than the other sites.. To allow comparison of projects on an equal basis, several simplifying assumptions were applied to normalize cost estimating parameters. All estimated costs reference the Engineering News Record Cost Construction Index No. 4777 (January, 1991). These figures were not intended to represent discrete, absolute numbers. The preliminary cost estimate for the enlargement of Elkhead Reservoir was based on a new reservoir with a total capacity 52,000 af; total costs of this dam raise and associated work were estimated to be in the range of $15 - $20 million. The maximum reasonable reservoir enlargement size at the existing Stagecoach Reservoir is 52,000 af and the preliminary costs of this dam raise and associated work was estimated to be in the range of $6 - $10 million. The S-18