BOARD00148 ‎(2)‎

• Board Meeting Date: 3/12/2007
• Description: WSP Section - Colorado River - DEIS - Development of Lower Basin Shortage Guidelines and Coordinated Management Strategies for Lake Powell and Lake Mead, Particularly Under Low Reservoir Conditions
• Board Meetings - Doc Type: Memo

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executive Summary

Environmental Consequences

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("balance contents"). This alternative does not include a mechanism for the storage and
delivery of conserved system and non-system water in Lake Mead. The existing lSG
would be extended through 2026.

ES.1.4.5 Reservoir Storage Alternative
The Reservoir Storage Alternative was developed in coordination with the cooperating
agencies and other stakeholders, primarily Western and the NPS. This alternative would
keep more water in storage in Lake Powell and Lake Mead by reducing water deliveries
and by increasing shortages to benefit power and recreational interests. This alternative
includes larger, more frequent shortages that serve to conserve reservoir storage;
coordinated operations of Lakes Powell and Mead determined by specified reservoir
conditions (more water would be held in Lake Powell than under the Basin States
Alternative); and an expanded mechanism for the storage and delivery of conserved
system and non-system water in Lake Mead. The existing ISG would be terminated after
2007.

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ES.2 Summary of Potential Environmental Effects

ES.2.1 Methodology
Hydrologic modeling of the Colorado River system was conducted to determine the potential
hydrologic effects of the alternatives. The modeling provides projections of potential future
Colorado River system conditions (i.e., reservoir elevations, reservoir releases, river flows)
for comparison of those conditions under the No Action Alternative to conditions under each
action alternative. Due to the uncertainty with regard to future inflows into the system,
multiple simulations were performed in order to quantify the uncertainties of future
conditions and as such, the modeling results are typically expressed in probabilistic terms.

The hydrologic modeling also provides the basis for the analysis ofthe potential effects of
each alternative on other environmental resources such as recreation, biology, and electrical
power. The potential effects to specific resources are identified and analyzed for each action
alternative and are compared to the potential effects to that resource under the No Action
Alternative. These comparisons are typically expressed in terms of the relative differences in
probabilities between the No Action Alternative and the action alternatives.

ES.2.2 Hydrologic Resources

ES.2.2.1 Reservoir Storage
Lake Powell. Under the No Action Alternative and the action alternatives, the elevations of
Lake Powell are projected to fluctuate between full and lower levels during the period of
analysis (2008 through 2060). At the 90th percentile Lake Powell end-of-July elevations
values, the action alternatives and the No Action Alternative are projected to be similar
over the period of analysis.

At the 50th percentile Lake Powell end-of-July elevation values, the action alternatives
and the No Action Alternative are projected to be similar during the period of 2008
through 2015. During the period of2016 through 2026, the Reservoir Storage Alternative

Draft EIS - Colorado River Interim Guidelines for
Lower Basin Shortages and Coordinated Operations for
Lake Powell and Lake Mead

ES-7

February 2007