9603

• UCREFRP Catalog Number: 9603
• Author: Clayton, R. and A. Gilmore.
• Title: Flaming Gorge Environmental Impact Statement Hydrologic Modeling Study Report - Results of Action and No Action Alternative Analysis.
• USFW Year: 2001.
• USFW - Doc Type: \
• Copyright Material: NO

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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