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CHAPTER I - PURPOSE OF AND NEED INTRODUCTION As Lake Powell began to fill behind Glen Canyon Dam, water depth increased over the penstock intakes and release temperatures dropped. Deep, cold-water releases (about 8- IODC) from Glen Canyon Dam are believed to be causing serious negative impacts to native and endangered fish which need warm water to reproduce. The evaluation of temperature control was an element of the preferred alternative in the Operation of Glen Canyon Dam Environmental Impact Statement (USDI 1995) and contained in the Fish and Wildlife Service's (FWS) reasonable and prudent alternative in their jeopardy opinion under Section 7 of the Endangered Species Act. The proposed temperature control alternative would include modification of the penstocks to allow warmer releases and post-project monitoring and testing to refine its operation. There are several factors believed to have caused the decline of native fish in the Grand Canyon. With regard to the operation of Glen Canyon Dam, it appears logical that the cold- water releases have added to the decline of native (warmwater) fish and that native fish would benefit from efforts to return the river to a more natural, warmwater release pattern during certain times of the year. There are however, interactions between the river's primary productivity, native and non-native fish, and the habitat that are ecologically linked. An important part of the proposed temperature control alternative would be to test and refine the operation of the temperature control device through the Glen Canyon Adaptive Management process. A post- project monitoring and testing program would be developed as part of the Glen Canyon Dam Adaptive Management process. Monitoring would be conducted by the Grand Canyon Monitoring and Research Center. The Canyon Dam as Lake Powell fill. results of the tests and monitoring would be reviewed through the adaptive management process. The Glen Canyon Adaptive Management Work Group would recommend further tests and refinements in operations to the Secretary of the Interior Glen Canyon Discharge Temperatures Obselled During Filling of Reser.oir 25 Q: l!! 20 :l ii! 8. 15 E .. f0- G> 10 ~ Data from 1965-1973 (July-Sept) 5 .0 60 80 '00 '20 '.0 '60 Depth of Penstocks (feet) '80 200 Figure 1 - Release temperatures below Glen Reclamation is considering two alternatives. The proposed temperature control alternative would implement temperature controls at Glen Canyon Dam. The second alternative is the no-action alternative. This environmental assessment will look at each of these alternatives.