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0(11271 Draft Task 7 Technical Memorandum . . Reservoir Contents Williams Fork Reservoir remains relatively full over the study period and on only two occasions briefly drops below 60,000 af (Figure 15). The potential to exchange water up the Yampa River was occasionally limited by either the natural flows or the minimum flow targets specified in the model. This places more burden on Elkhead Reservoir to meet the Craig area demands than occurred in Scenario IV. The result was an increase in the draw on Elkhead Reservoir and greater fluctuations in storage levels !ban in Scenario IV. ENVIRONMENTAL CONSIDERATIONS An important issue in formulating near and long term water development projects and strategies as part of the Feasibility Study was the consideration of development effects on fisheries and other wildlife. Of particular concern were streamflows predicted for the Yampa River mainstem in habitat areas of !be Threatened and Endangered species. The ability of !be Juniper Project contemplated draft, as converted to an instream flow right at Juniper Canyon, to maintain the historical hydrograph of the Yampa River was specifically evaluated. The influence of administration of the Juniper instream flow right slightly modified the historical 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. During the spring and summer, the instream flow right frequently calls out junior direct flow diversions. Many of these juniors have access to various reservoir storage contracts. After reservoirs are drawn on to supply these called out juniors, stream flows in Juniper Canyon may be diminished in the following time steps, as the reservoirs refill, ahead of the instream flow. The effect is a slight decrease in flows in !be spring and early summer mon!bs. Model results from Scenarios I through V indicate only minor differences between scenarios with respect to instream flows, and that selection of the most feasible development alternative is not possible based on instream flow conditions alone. It is evident that even the long-term (year 2040) basin demands were are small compared to the average annual yield of the Yampa River at Maybell and Deerlodge Park. The degree to which reservoir levels fluctuate has important biological implications. The ability of a reservoir to act as a water supply to perimeter wetlands and other plant species 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. As with the evaluation of instream flow predictions in the model Scenarios I through V, 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. Fisheries The evaluation of the effects of each scenario on aquatic biology used separate factors for reservoirs and rivers. The reservoir evaluation used two main criteria, average annual change in reservoir elevation and average annual change in reservoir area. Reservoir fluctuations can limit reproductive success, establishment of vegetation in littoral areas and productivity of benthic invertebrates and fish. All of these factors can be limiting to the size of fish populations. More stable water levels are generally better for establishing productive fisheries. For !be comparison of alternative scenarios, the scenarios with less change in reservoir elevation and area were judged better than the scenarios wi!b more fluctuation in elevation and area. 16 :to .-"