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<br />OOJ327 <br /> <br />Draft - Do Not Cite <br /> <br />3 <br /> <br />August 2003 <br /> <br />2 METHODS <br /> <br />The Western Area Power Administration (Western) proposes that a hydrologically based <br />triggering criterion be established for the Aspinall Unit, to identify years when a spill would <br />occur and the volume of water available for spills, which could then be released in a pattern to <br />achieve FWS flow recommendations in the lower Gunnison River. This trigger criterion would <br />be based on forecasted April through July inflows to Blue Mesa Reservoir. Under Western's <br />proposal, the timing, magnitudes, and durations of flows associated with spilled water would be <br />flexible, but limited to the volume identified as being at risk of spill. Using water that is already <br />likely to bypass Aspinall Unit power plants would reduce impacts to power generation. <br /> <br />2.1 RELEASE SCENARIOS EVALUATED <br /> <br />For our evaluation, six different release scenarios were evaluated (fable 1). Four of these <br />(designated as Western A, B, C, and D) are based on Western's proposal and differ in the inflow <br />volume that would trigger a spill (Western B and D use a 900,000 ac-ft inflow trigger; <br />Western A and C use a 950,000 ac-ft trigger) and the way spilled water would be released. <br />Western A, B, and C would achieve the highest one-day peak possible either with existing ramp <br />rate restrictions (Western A and B) or without them (Western C). Under the Western D scenario, <br />the peak release would be limited to 10,000 cfs, but would be extended for as many days as <br />needed to release all excess water. No ramp rate restrictions were applied to the Western D <br />scenario. The effects of these scenarios were compared to the effects of historical flows for the <br />period 1975 through 2000 (gage scenario) and flows that would have occurred over this same <br />time period using the u.s. Bureau Reclamation's (Reclamation's) current operating rules. <br />Release patterns under each scenario (minus flow diverted at the Gunnison Tunnel) are shown in <br />Figures 1 through 6). <br /> <br />'2.2 FLOW MODELING <br /> <br />For each release scenario, flow and sediment transport in the lower Gunnison River were <br /> <br />"'f:Jf' n ,evaluated at two locations: the United States Geological Survey (USGS) Delta gage (gage <br />..,. '<..l(\~i \D~ . <br />W~ -\e..,v\ ~ <br />~t ~O l<;) ~~~) ''l"(.t.~'V\~ <br />B': ~ ", ~i1'^-~u..\(.. v~o.w..r\~ <br />~ :qfjO~1. r;,~~t~\...) .No;'f~~~ <br />1) l. t'. \b\~; \~T: . I <br /> <br />\A1> r, ~ <br /> <br />\-\\5 ttrr\c" <br /> <br />~ <br /> <br />tu..-.t\{~\ (l~0-]\"~ <br />,,~ <br /> <br />