Laserfiche WebLink
<br />OD2438 <br /> <br />Sediment supply thus became one of the primary driving forces behind ecological <br />recommendations for changing powerplant operations. <br /> <br />After the passage of the GCP A, the thinking of some sediment experts began to change, primarily <br />as the result of the hypothesis that the sediment rating curves below the dam were not static with <br />time. Additional thought was also being given to the location of stored sediment in the canyon <br />and the mechanisms for moving sediment from the channel bottom to eddy areas. Extensive <br />modeling by the sediment researchers changed to a great degree the way in which transport <br />mechanisms were viewed. The long term balance of sediment in the Grand Canyon continued to <br />be an important issue in these discussions. <br /> <br />Sediment researchers now believe that flood flows counteract the possible adverse impacts that <br />fluctuations have on beach erosion, thus rebuilding the deposits that would eventually slough back <br />into the eddies, regardless of the nature of the powerplant operations. Some suggested that more <br />frequent floods could allow higher levels of fluctuations. <br /> <br />The Agreement Contained in the 1996 AOP <br /> <br />With this evolving positive view towards spills, a desire for a test of the GCDEIS Beach Habitat <br />Building Flow was expressed by the Transition Work Group beginning in 1994. This request for <br />a purposeful powerplant bypass was strongly opposed by the Basin States because of the <br />GCDEIS language triggering such bypasses, claiming a violation of the 1968 Colorado River <br />Basin Project Act provision of avoiding anticipated spills, interpreted as powerplant bypasses. <br />This opposition created an impasse that blocked such a test. <br /> <br />Additional discussions between members of the Transition Work Group and the Basin States <br />resulted in a proposal for a modification of the GCDEIS preferred alternative, that of moving <br />Beach Habitat Building Flows (BHBF) from years oflow reservoir conditions (when spills would <br />not be required for hydrologic reasons) to years of high reservoir conditions and high inflows. <br />Thus a BHBF would occur in years when there was an expectation of having a hydrologically <br />induced spill. This agreement was institutionalized in the 1996 Annual Operating Plan for the <br />Colorado River, signed by the Secretary of the Interior in December 1995. A subsequent BHBF <br />test was conducting in April 1996, confirming the hypothesis that high flows could rebuild <br />sandbar deposits. In December 1996, the GCDEIS Record of Decision was signed by the <br />Secretary of the Interior and included this modification of the preferred alternative. <br /> <br />Options and Impacts of Using Spillway Gate Extensions <br /> <br />This section addresses key questions raised earlier in this report which combined raise the issue, <br />"Should the extensions be installed?". <br /> <br />The need to reduce the frequency of powerplant bypasses <br />