Join Date: Dec 2004
RRFW Riverwire Glen Canyon Dam Update
RRFW Riverwire Glen Canyon Dam Update
October 9, 2008
Glen Canyon Dam Operations
The monthly release volume for October 2008 is scheduled to be 743,000 acre-feet. During the months of September and October, releases from Glen Canyon Dam will be steady as described in the Final Environmental Assessment for Experimental Releases from Glen Canyon Dam, Arizona, 2008 through 2012 (EA). The corresponding release rate for the scheduled release volume for October is about 12,083 cubic feet per second (cfs).
Beginning on November 1, 2008 releases from Glen Canyon Dam will likely resume daily fluctuations as described under the 1996 Glen Canyon Dam Record of Decision. The release volume scheduled for November is 600,000 acre-feet which translates to an average daily release of 10,000 cfs. Daily fluctuations about this average will likely be from about 6,500 cfs during the early morning hours to about 12,500 cfs during the afternoon and evening hours.
The water year release volume from Glen Canyon Dam during water year 2008 was 8.978 million acre feet (maf). This volume was based on actual and forecasted inflow and reservoir operation conditions for Lake Powell and Lake Mead under the Equalization Tier of the Interim Guidelines. The 2008 water year ending elevation and storage for Lake Powell was 3626.90 feet above sea level and 14.51 maf respectively. This was a 2.58 maf increase in storage year over year. Lake Powell begins water year 2009 at 59.6% of full capacity.
Under the Interim Guidelines, the August 24-month study projected the end of December, 2008, elevation of Lake Powell to be 3625.75 feet above sea level which set the operational guideline to be the Upper Elevation Balancing Tier for water year 2009. Under the Upper Elevation Balancing Tier, the end of December elevation of Lake Mead was projected in the August 24-month study to be 1110.4 feet above sea level. This condition set the water year 2009 release from Glen Canyon Dam to be 8.23 maf as described in section 6.B.1. However, with this release volume, the August 24-month study projected the 2009 end of water year elevation of Lake Powell to be greater than 3639 feet above sea level, the condition that triggers section 6.B.3 of the Interim Guidelines.
Under section 6.B.3, if the April 24-month study projects the end of water year elevation of Lake Powell to be above the equalization elevation for the current year, then the Equalization Tier should govern for the remainder of the water year. For this reason, the August 24-month study projected 2009 water year release from Lake Powell reflected Equalization Tier objectives. Forecast conditions for the October 24-month study also project that the end of water year conditions will trigger section 6.B.3 of the Interim Guidelines and the Equalization Tier is projected to govern during water year 2009. The actual water year release volume will only be greater than 8.23 maf if the April 2009 24month study projects the end of water year elevation of Lake Powell above 3639 feet above sea level.
Upper Colorado River Basin Hydrology
Precipitation in the basin above Lake Powell was below average during the summer months and has continued through September. Precipitation during June, July and August 2008 was 70%, 65% and 90% of average respectively with precipitation in September measured at 70% of average. The overall precipitation in the Upper Colorado River Basin for water year 2008 will likely be near average (about 101% of normal).
The unregulated inflow to Lake Powell during the April through July 2008 was 8.906 maf (112% of average). The long range outlook for unregulated inflow to Lake Powell for water year 2009 is projected to be 10.8 maf (90% of the 1971-2000 average).
Upper Colorado River Basin Drought
The Upper Colorado River Basin is experiencing a protracted multi-year drought. Since 1999, inflow to Lake Powell has been below average in every year except water year 2005 and 2008.
In the summer of 1999, Lake Powell was essentially full with reservoir storage at 23.5 million acre-feet, or 97 percent of capacity. Inflow to Lake Powell in 1999 was 109 percent of average. The manifestation of drought conditions in the Upper Colorado River Basin began in the fall months of 1999. A five year period of extreme drought occurred in water years 2000, 2001, 2002, 2003, and 2004 with unregulated inflow to Lake Powell only 62, 59, 25, 51, and 49 percent of average, respectively. Lake Powell storage decreased through this five-year period, with reservoir storage reaching a low of 8.0 million acre-feet (33 percent of capacity) on April 8, 2005.
Drought conditions eased in water year 2005 in the Upper Colorado River Basin. Precipitation was above average in 2005 and unregulated inflow to Lake Powell was 105 percent of average. Lake Powell increased by 2.77 million acre-feet (31 feet in elevation) during water year 2005. But as is often the case, one favorable year does not necessarily end a protracted drought. In 2006, there was a return to drier conditions in the Colorado River Basin. Unregulated inflow to Lake Powell in water year 2006 was only 71 percent of average.
Water year 2007 was another year of below average inflow with unregulated inflow into Lake Powell at 68 percent of average. Over the past 9 years (2000 through 2008, inclusive), inflow to Lake Powell has been below average in all but two years (2005 and 2008. Drought conditions have eased in water year 2008 with above average inflows to the main stem Colorado River reservoirs (with the exception of Flaming Gorge and Fontenelle Reservoirs). Reservoir storage in the Colorado River Basin, however, is still below desired levels with the overall Colorado River system storage at the beginning of water year 2009 of 34.0 maf which is 59.3% of capacity.
This release courtesy Rick Clayton, Hydraulic Engineer, Upper Colorado Region US Bureau of Reclamation.
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