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2 EIS impact analyses, the upramp and maximum flow criteria were found to be overly conservative for the long term. With the concurrance of the Cooperating Agencies, the preferred alternative identified in the final EIS reflected the changes requested by Western. Concern was raised by the environmental community that the changes had not gone through a rigorous enough scientific review prior to inclusion in the final EIS preferred alternative. The objective of this document is to highlight the scientific. information upon which the determination of the impact of the changes were made and to present more detail on the analyses presented in the final EIS. The analyses described in this document are based on existing information, interpolation of GeES research flow results, and on additional model simulations with the revised operational criteria. The relationship between riverf10ws and sediment resources is the most important element in defining the ecosystem of the Colorado River in Glen and Grand Canyons. Sediment is therefore the key indicator resource of ecosystem response because nearly all canyon resources are strongly linked to sediment. There would have to be a significant change in the long-term sand storage before the changes in the preferred alternative could make a significant difference to downstream resources. II. BACKGROUND The Glen Canyon Dam Interim ODeratinQ Criteria (interim flows) were implemented by Reclamation in .November 1991 after three months of testing, considerable deliberation by the Glen Canyon Dam cooperators, and recommendations made by the GCES Scientific Coordination Group. The purpose of the interim flows was to operate the dam in a conservative manner with the goal of avoiding adverse impacts to downstream resources in Glen and Grand Canyons, especially sediment which was and is believed to be the resource most sensitive to dam operations. The time period for the interim flows was defined to be from the end of the Research Flows (July 1991) until implementation of the Record of Decision. The interim flow criteria were based on information from GCES Phase I and some preliminary information from GCES Phase II. No specific studies were conducted on the particular parameter levels (see Table 1). The criteria were purposely designed to be conservative for protection of the natural and cultural resources. The specific objectives were: (I) Store sediment in the channel of the Colorado River (2) Minimize the erosion of the beaches in the.Grand Canyon (3) Minimize the impacts to the biological resources . (4) Protect the cultural resource areas being impacted by erosion Specific operating criteria at Glen Canyon Dam included: ramping rates, maximum flows, minimum flows, and allowable daily fluctuations. The criteria were set conservatively because for the majority of the GCES Phase II technical studies, only preliminary, non-peer reviewed, results were available. It was intended that the interim flows result in a net storage of