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2 All major elements of the Green River System are represented in the Green River model and some elements are more accurately represented than others, The Green River below Flaming Gorge Dam is divided into three sections, known as reaches, in the Flow Recommendations. All three of these reaches are represented in the Green River model. Reach One extends from the tailrace of Flaming Gorge Dam to the confluence with the Yampa River. Reach Two extends from the confluence of the Green River with the Yampa River to the confluence of the Green River with the White River. Reach Three extends from the confluence of the Green River with the White River to where the Green River meets the Colorado River. The flows for Reaches One and Two are more accurate in the model than those for Reach Three. This is because the effects of the White and Duchesne Rivers on the flows in the Green River are not fully understood. At this point, these river systems have not been adequately modeled to determine how they will be regulated and developed in the future. For this reason, results for Reach Three have not been included in this report. The Green River model routes natural flows (river flows that do not include human interferences such as depletion and regulation), referred to as input hydrology, through the reservoir system on the Green River (Fontenelle and Flaming Gorge Reservoirs). A monthly natural flow database was developed for the Upper Colorado River Basin for use as input hydrology for CRSS. The input hydrology for the Green River model was selected from this database. A period of record was selected that had the most complete natural flow dataset available for the Upper Green River Basin, This period begins in January 1921 and ends in December 1985 (65 calendar years). The natural flow data is being extended to 1995, however, this work has not yet been completed. The Green River model will be reevaluated with this additional data for the Final Flaming Gorge EIS. The initial conditions of the Green River model were selected to be the state of the Green River system in January of 2002 as described in the 2000 Annual Operating Plan (AOP) for Colorado River System Reservoirs, The 2000 AOP was based on the August 2000 run of the 24- Month Study Operational Model of the Colorado River. The Green River model runs for 39 years to December 2040. Beyond 2040, estimated depletion schedules for water users represented in the model were considered too speculative to be useful. Depletion schedules were updated to reflect water development forecasts produced by the Upper Colorado River Commission (1999), Given the uncertain nature of water development schedules far into the future, and the fact that the model predicted reservoir elevations that appeared stabile in the distant future, ending the model run in 2040 was reasonable. Different hydrologic scenarios, referred to as input traces, are routed through the Green River model. Each trace is one set of 39 years of natural flows, Because the input hydrology for the model is based on historic hydrology, all the input traces have a high probability of occurring in the future. No single input trace, or set of input traces, has a higher probability of occurring then any of the other input traces. The Index Sequential Method (ISM) was used to construct 65 input traces for the Green River model from the natural flow dataset selected, The ISM involves