WSP10995

• File Number: 8220.101.09
• Description: Glen Canyon Dam/Lake Powell
• State: AZ
• Basin: Colorado Mainstem
• Date: 12/21/1994
• Author: USDOI-USFWS
• Title: Final Biological Opinion-Operation of the GlenCanyon Dam as the Modified Low Fluctuation Flow Alternative of the Final Environmental Impact Statement-Operation of Glen Canyon Dam
• Water Supply Pro - Doc Type: Biological Opinion

The annual volume of water released is influenced by regional hydrology and legal requirements.
In general, the types of water years and annual discharge in million acre-feet (mat) are classified
as: Low - e.g. 8.23 maf, 1989 (the minimum discharge); Moderate - e.g. 13.6 maf, 1987;
and High - e.g. 19.3 maf, 1985.

Low water years are considered normal and.likely to occur. more than 50% of the time. Full
pool for Lake. Powell is defined as 1,128 meters (m) and 24:3. ma( of storage. Flood flows
(flows greater than 45,000 cfs) are estimated to occur for the No Action Alternative at a rate of
1 in 40 years. .

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Monthly volumes of releases vary depending on the type of water year and the Annual Operating
,Plan. Under other than lrigh water conditions, the greatest monthly releases have historically
occurred during peak energy requirement periods (summer and winter). Daily and hourly
discharge rates are primarily a function of power generation requirements. Daily fluctuations
are generally highest during low volume years (power is generated during high use periods of
day and reduced at night to save limited water supply) and decrease as yearly release volumes
increase (essentially extending peak period throughout day and into night).

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The MLFF is similar to the original interim operating criteria with the exceptions that it does
not include the financial exception criteria allowed under Interim Flows but would include
habitat maintenance flows, increased flood protection, higher maximum flows, and increased up-
ramping rates (Table 1.). EIS elements common to all alternatives also would be included.

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Table 1. Modified Low Fluctuating Flow Alternative I

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Monthly Minimum Habitat Maximum Daily Ramp
Release release Maint. Flow release Fluctuation rate
Vol. (ac-ft) (cfs) (CfS)2 (cfs) (cfs/24 hr) ( cfs/hr)
Low 8,000 30,000 to 25,000 5,000 4,000 up
< 600,000 between 33,200 Exceeded 1,500 down
7am & 7 pm during hab. ..,
5,000 night maint flows
Medium same same same 6,000 same
600,000 to
800,000
High same same same 8,000 same
>800,000

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I Description of preferred alternative from January 1994 Draft EIS and Reclamation's
June 17, 1994, memorandum (U.S. Bureau of Reclamation 1994).

2 Steady flows with minor fluctuations of:t 1,000 cfs.

15 December 1994 rmal biological opinion 2':!1-93-F-167

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