Glen Canyon Dam Update - July 8, 2011

Glen Canyon Dam Lake Powell

The unregulated inflow volume to Lake Powell for June 2011 was 5.40 maf (175% of average). This was below the forecasted volume for June, which was 6.1 maf (198% of average) but was still the third wettest June on record since the closure of Glen Canyon Dam. The observed inflow to Lake Powell peaked on June 11th and 12th at just over 96,000 cfs. Inflows are now declining but are still averaging about 80,000 cfs (as of July 5, 2011). The forecasted unregulated inflow volume for July was increased from 3.30 maf to 3.53 maf (226% of average) which would be the 3rd wettest July for Lake Powell since the operation of Glen Canyon Dam began in 1963.

The reservoir elevation of Lake Powell has increased significantly so far this runoff season. On April 9, 2011 the elevation of Lake Powell was 3609.7 feet above sea level which was the lowest elevation observed so far in 2011. Since that time the elevation of Lake Powell has increased by 42.7 feet to 3652.4 feet on July 5, 2011. The elevation is projected to continue to rise and should peak near 3660 by the end of July.  The last time Lake Powell’s reservoir elevation was at this level was in October of 2001 (over 10 years ago) near the beginning of the recent drought. 

Current Dam Operations

Releases from Glen Canyon Dam are approximately 24,400 cfs which is very near the full capacity of the powerplant.   This release rate includes reserving enough generation capacity for up to 100 MW of reserve generation and 40 MW of system regulation.  As the elevation of Lake Powell increases, the capacity of the powerplant will change and operation of Glen Canyon Dam will be adjusted daily to maximize release volumes.  It is anticipated that the release volume for July will be approximately 1,465 kaf.  The actual release volume for July will largely depend on generation unit efficiencies that occur throughout the month and could be higher or lower than this estimated release volume.

While the release rate over the next several months is likely to be steady, the instantaneous releases from Glen Canyon Dam may fluctuate somewhat to provide 40 MW of system regulation.  These instantaneous release adjustments maintain stable conditions within the electrical generation and transmission system and result in momentary release fluctuations within a range of about 1100 cfs above or below the targeted hourly release rate.  The momentary fluctuations for regulation are very short lived and typically balance out over the hour. 

Spinning and non-spinning reserve generation is also maintained at Glen Canyon Dam.  In order for Colorado River Storage Project (CRSP) powerplants to participate in the electrical generation and transmissions system, these powerplants must maintain a level of generation capacity available in reserve to assist the local control area for when unanticipated generation outages occur. The current CRSP powerplant reserve requirement is 100 MW (equivalent to approximately 2,675 cfs of release from Glen Canyon Dam).  When an electrical outage occurs within the control area, CRSP powerplants can be called upon to provide up to 100 MW of additional generation for up to 2 hours.  Under normal circumstances, calls for reserves infrequent and for much less than the required 100 MW.  Because Glen Canyon Powerplant is the largest facility of the CRSP powerplants, most of the CRSP reserve requirement is maintained at Glen Canyon Dam.

Annual Operations-Coordinated Operation of Lake Mead and Lake Powell under Interim Guidelines for Water Year 2011

In August of 2010, the 24-Month Study was used to project the January 1, 2010 elevations of Lake Powell and Lake Mead.  Based on these projected elevations and pursuant to the December 2007 Record of Decision on Colorado River Interim Guidelines for Lower Basin Shortages and the Coordinated Operations of Lake Powell and Lake Mead (Interim Guidelines), the operating tier for water year 2011 was selected to be Upper Elevation Balancing.  Operation of Glen Canyon Dam under Upper Elevation Balancing can result in annual releases as low as 7.0 maf to as high as 13 maf or greater depending on system conditions.  The operational outcome of the Upper Elevation Balancing Tier is largely dependent on system conditions at the end of the water year that are projected in the April 24-Month Study.

The April 2011 24-Month Study projected the end of water year elevation for Lake Powell would be above 3643 feet which is the Equalization Level for 2011.  For this reason, pursuant to the Interim Guidelines, Equalization will govern the operation of Glen Canyon Dam for the remaining months of water year 2011. For more information on the Interim Guidelines click

Current Inflow Forecasts and Model Projections

The Water Supply forecast for Lake Powell (April through July Unregulated Inflow Volume) is 12.0 maf (151% of average) and the water year unregulated inflow to Powell for 2011 is projected to be 16.2 maf (135% of average).  The unregulated inflow forecasts for Lake Powell over the next 3 months are as follows: July-3,530 kaf (226% of average); August-950 kaf (155% of average): September-670 kaf (141% of average).  These forecasts were last updated on July 1, 2011.  Incorporating these new forecasts, the projected most probable unregulated inflow for water year 2011 is now 16.2 maf (131% of average). This is the median projection for water year 2011.  There is a 50% chance that the actual volume could be higher and there is a 50% the actual volume could be lower than this projected volume.

The June 24-Month Study projects a Lake Powell WY 2011 annual release volume of 12.44 maf.  Due to recent increases to the inflow forecast for Lake Powell, Equalization may not be fully achieved by the end of the water year.  The projected Lake Powell releases will be updated each month to reflect changing hydrology in order to achieve the operation specified by the Equalization Tier.

The July 2011 24-Month Study will published on July 8, 2011 and will be available here:

Updated elevation projections for Lake Powell through water year 2011 and 2012 based on the most recently published 24-Month Study are maintained at:

Upper Colorado River Basin Hydrology

The Upper Colorado River Basin has experienced a protracted multi-year drought since early 2000.  During this drought, the inflows to Lake Powell have been below average in every year except water years 2005, 2008 and likely 2011.  In the summer of 1999, Lake Powell was close to full with reservoir storage at 23.5 million acre-feet, or 97 percent of capacity.  During the next 5 years (2000 through 2004) unregulated inflow to Lake Powell was well below average.  This resulted in Lake Powell storage decreasing during this period to 8.0 million acre-feet (33 percent of capacity) which occurred on April 8, 2005.

During 2005, 2008 and 2009, drought conditions eased somewhat with near or above average inflow conditions and net gains in storage to Lake Powell.  This year (2011) will likely be another above average inflow year.  As of July 5, 2011 the storage in Lake Powell was approximately 17.52 million acre-feet (72.0 % of capacity) which is still below the desired operating level for this time of year.  The overall reservoir storage in the Colorado River Basin as of July 5, 2011 is approximately 37.48 million acre-feet (63.0 % of capacity).

RRFW thanks Rick Clayton of the USBOR for this information.