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<br />Chapter II Description of Alternatives 10 <br /> <br />inexpensive alternative to existing designs. The consensus of those attending the scoping <br />meetings was that the risks were a bit too high to invest an estimated $45 to $150 million in a <br />traditional selective withdrawal structure. Fortunately, the value planning work done by an <br />interdisciplinary team from Reclamation uncovered an alternative design which would use <br />much of the existing structure of the dam to achieve temperature control at a cost of$15 <br />million, yet retain the functionality of a traditional selective withdrawal structure. <br /> <br />The intakes to the dam would be modified to draw water from either the existing penstock <br />intake or through a surface intake at the top of the existing trashrack. Traditional designs <br />usually provide multiple gates or sliding weirs to control intake location and temperature for <br />each individual penstock. This facility would have only two fixed openings (cold and <br />warm). Each of the eight penstocks could be individually set to select either cold or <br />warm(er) water with the new gate system. Blending of water between the penstocks would <br />be used to adjust the resulting downstream (outflow) temperature. <br /> <br />Because the upper intake elevation would be fixed and about 40 feet of submergence would <br />be required to operate the upper intake. warmwater releases would only be made when the <br />reservoir water surface is between 3,700 feet (full) and 3,670 feet. Under present conditions <br />of water development, computer projections of the Colorado River system would show the <br />water surface elevation in Lake Powell to be within this range about 85 out of 100 years <br />during the May-October season. Statistics aside, it can be expected that an extreme dry <br />period would cause operations to be postponed. On occasion, the Colorado River has <br />experienced near decade-long dry periods which could impact the operation. Fortunately, if <br />the reservoir is severely drawn down by an extended drought, the lower intakes will begin to <br />draw some warm water. <br /> <br />Warm water in the reservoir is usually available beginning in the late spring through fall <br />turnover and peaks in the late summer. Release temperatures from the dam would be ramped <br />up to about a 15 oC and then limited at that temperature to prevent significant impacts to the <br />"blue-ribbon" trout fishery below the dam. Once these limits are reached, cold water would <br />be blended from one or more of the eight penstocks to prevent impacts to trout fishery. The <br />goal of this management scheme is to increase water temperatures in the lower portions of <br />the river for native, warmwater fish while maintaining near optimal conditions for the <br />"upstream" cold-water trout fishery in the Lees Ferry reach below the dam. Releases of <br />150C below the dam should be near optimal for the coldwater fishery and promote better <br />growth rates. Then, as the water flows downstream, it will warm. By the time the water <br />reaches the Little Colorado River (76 miles below the dam), temperatures should be <br />sufficiently warm to meet the thermal needs of native, warmwater fish. <br /> <br />Role of Adaptive Management in Operations - Adaptive management is an approach to <br />natural resource policy that embodies a simple imperative: policies are experiments; learn <br />from them. In order to live we use the resources of the world, but we do not understand <br />nature well enough to know how to live harmoniously within environmental limits. Adaptive <br />management takes that concept seriously, treating human intervention in natural systems as <br />experimental probes. Its practitioners take special care with information. First, they are <br />