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

INTRODUCTION

In summer 1998, both Lake Powell and Lake Mead filled, and Glen Canyon and Hoover dams
operated at near full pool stages, In addition to present reservoir conditions, long-range climate
forecasts indicate that precipitation in the Colorado River basin may be at or above normal levels
during 1999. Because of the increased likelihood that Water Year 1999 will produce at or above
normal runoff into nearly full reservoirs (surplus conditions), the possibility exists that flows
from GCD may exceed 25,000 cfs in 1999. The Secretary of the Interior's (Secretary) direction
and the Record of Decision (ROD, October 1996) prohibit fluctuating hydropower plant releases
from Glen Canyon Dam (GCD) under flows ~25,000 cubic feet per second (cfs). Flows above
this level can be used to avert emergency situations, anticipated spillsl, or to accommodate high
forecasted inflows; however, such flows are to be steady, not fluctuating, releases.

In spring 1996, an experimental Beach/Habitat-building Flow (BHBF) from GCD was initially
tested. The BHBF consisted of a controlled constant release from GCD of 45,000 cfs for seven
days during late March and early April. This level of flow was achieved through a combination
of releases from the powerplant and bypass jet tubes. NeitheT of the two spillways at GCD were
used during the BHBF.

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The initial BHBF was conducted as an experiment to study planned flooding as a management
tool for restoration and preservation of Colorado River ecosystem resources. The primary goals
of the experimental flood were to learn about sediment transport under high flows and to
mobilize and redeposit channel-stored sand as higher-elevation bars. Additiooal objectives
included preservation of cultural resources, providing more natural flood-disturbance processes
to Tiparian vegetation, rejuvenation of other important channel features such as backwater
habitats, and flushing non-native fishes from critical areas in downstream reaches. Of the eight
objectives generally listed in the GCD Environmental Impact Statement (GCD-EIS), the BHBF
achieved five that were mostly related to sediment transport and deposition. Subsequent releases
from GCD dUTing Water Years 1997 and 1998 have eroded sandbars and redistributed sand back
into eddies. Exceptional sediment inputs from the Paria River occurred in the summers of both
1997 and 1998, and sediment supplies appear to be acceptable for another BHBF.

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The goal of adaptively managing the Colorado River is to use the best scientific information to
plan and accomplish preservation and enhancement of Colorado River ecosystem resources, and
to mitigate the undesirable impacts of GCD releases on those resources. Because releases from
Lake Powell above 25,000 cfs may be required in Water Year 1998 as the result of high runoff,
early planning efforts aimed at preparing fOT managing these flows are recommended.
Guidelines have been developed through the Adaptive Management Program (AMP) and under
the Grand Canyon Protection Act (GCPA), GCD-EIS and ROD for formulating daily flow
regimes for GCD. The AMP includes the Adaptive Management Work Group (AMWG), the

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I A spill is defined as any dam release in excess of power plant capacity (33,200 cfs),

[WORKING DRAFT 2 . GCMRC: 12/04/98] 3