WSP04917

• File Number: 8220.101.10.D
• Description: Glen Canyon Dam/Lake Powell
• State: AZ
• Basin: Colorado Mainstem
• Date: 12/4/1998
• Author: GCMRC
• Title: Draft Adaptive Contingency Flow Alternatives for Water Year 1999
• Water Supply Pro - Doc Type: Report/Study

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DRAFT ADAPTIVE CONTINGENCY FLOW ALTERNATIVES
FOR WATER YEAR 1999

SUMMARY

Colorado River basin runoff may be at or above normal levels during 1999, as both Lake Powell
and Lake Mead are nearly full and a wet winter may occur. These conditions may result in
surplus water and high flows in the Colorado River ecosystem in lower Glen and all of Grand
Canyon in 1999. The Technical Work Group (TWG) established hydrological and resource
triggering criteria for beachlhabitat-building flows (BHBFs) based on hydrologic and river
resource criteTia. In its role to provide scientific information to the Adaptive Management Work
Group (AMWG), the Grand Canyon Monitoring and Research Center (GCMRC) and cooperating
scientists present this Draft Adaptive Contingency Flow Alternatives Plan for Glen Canyon Dam
releases in Fiscal Year (FY99). This draft plan is designed to provide an outline of information
needs and logistical planning for low, normal or high flows in 1999. With the 1998 State of the
Colorado River Ecosystem report, the high flows triggering criteria, and Reclamation's monthly
inflow prediction report and draft Biological Assessment as background, this draft plan iocludes
a description of possible Water Year 1999 alternative flow scenarios and presents a potential
iterative planning process and schedule.

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Runoff from the upper Colorado River basin in 1999 may consist of:

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1. low or normal inflows;

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2. higher than normal, but manageable. flows;

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3. surplus conditions, with a higher likelihood of spills and possible uncontrolled spills,

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Scenario 1 maintains flows 5 25,000 cfs with no BHBFs, Scenario 2 also maintains normal
ROD flows ( 525,000 cfs) but includes a 2-4 d 44,000 cfs BHBF to increase the residence time of
Colorado River sedimeots contributed by the Paria and Little Colorado rivers, and other
tributaries, in the summers of 1997-98. Under Scenario 3, the flow plan includes one or mOTe
higher flows. The details of these BHBF's are described io the GCMRC memorandum/response
to the TWO request for information on high flows. Details and timing of these high flows is
under discussion within the TWG and AMWG.

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If Scenario 2 or 3 occurs, the draft plan recommends the following schedule:

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1. Review resource criteria based on the 1998 State of the Colorado River Ecosystem Report
(August-December 1998).

[WORKL'iG DRAFT . GCMRC: 12/04/98]