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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conflicts with U,S, Fish and Wildlife Service Biological Opinion and cultural resource
management may exist with this hydrograph,

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Possible Hydrograph Design in Response to Scenario 3: Hypotheses derived from 1996
BHBF results related to sediment and flow indicate that very high flows should be implemented
as early as possible to mitigate impacts on sediment supplies, Based on the state of the sediment
stored in the mainstream channel, as well as other non-physical parameters, a hydrograph with a
very brief (20 to 48 hours), high-magnitude flow could be implemented (60,000 to 100,000 cfs, if
the spillways were utilized), Such a flood flow should be conducted prior to initiating high
steady flows, to quickly mobilize and redeposit sediment on bars at high elevations throughout
the system, This high-energy period would also quickly coarsen the bed-stored sediments
remaining in the channel. The high flow is likely to be followed by extended periods of varied
steady high flows in the range of 25,000 to 45,000 cfs, as required to accommodate expected
inflows without further use of the spillway(s), Hence, large transfers of water could be made in a
fashion that preserves some fine sediment supplies,

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If Scenario 3 occurs, the prime objectives are the protection of Glen Canyon and Hoover Dams
and preservation of physical, biological, and cultural resources, Conflicts with the FWS
Biological Opinion and cultural resource management exist under this hydrograph, However,
such a hydrograph may:

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1. Re-form backwater nursery habitats for use by fish;

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2. Rework rapids and fans that have aggraded at tributary confluences; and

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3. Remove unwanted vegetation (especially tamarisk) which has colonized during the 1997-1998
Water Years.

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Such a scenario would attempt to minimize use of the spillways, and maximize the use of the
powerplant and the jet tubes.

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Potential Hydrograph Planning, Scheduling and Implementation

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Preliminary evaluation of recent, exceptional tributary inflows in the Marble Canyon reach
indicates that sediment availabily is good. Recent monsoons have resulted in large sediment
inputs into the Marble Canyon reach through high flows from the Paria and Little Colorado rivers
as well as other ungaged tributaries. Additional monitoring of sediment storage is required to
determine current system status.

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Given these conditions and assuming Runoff Scenario 2 occurs, the following hydro graph may
be considered. This hydrograph is considered as a baseline for beginning discussions of resource
impacts and a base scenario which can be modified as actual conditions develop and additional

[WORKING DRAFT 2 . GCMRC: 12/04/98] 7

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