Glen Canyon Dam Operations
The monthly release volume in July 2008 will be scheduled to be 865,000 acre feet. For the first two weeks of July, weekday releases will average about 13,800 cfs with afternoon peaks to about 17,500 cfs and off peak lows to about 9,500 cfs. Saturday and Sunday releases will average about 13,000 cfs with afternoon peaks to about 17,250 cfs and off peak lows to about 9,500 cfs. After July 14, weekday releases will average about 14,500 cfs with afternoon peaks to about 18,000 cfs and morning lows to about 10,000 cfs.
Monthly releases are being managed to prepare for a steady flow experiment to be conducted in September and October of 2008. The release volume for August will be adjusted in the range from 840 KAF to 890 KAF in order to target a release volume of 700 KAF for September which would result in steady flows of approximately 12,500 cfs. The release volume for September may be adjusted if necessary to achieve a projected Lake Mead elevation of 1105 feet above sea level by the end of water year 2008. The release volume for October will be adjusted to match the steady flow conditions that occur in September.
Releases from Glen Canyon Dam for the remainder of water year 2008 will be governed by the Equalization Tier of the Interim Guidelines for the Operation of Lake Powell and Lake Mead (Interim Guidelines). Under the Equalization Tier, the water year annual release volume can be above 8.23 million acre-feet (maf). For the July 2008 24-Month Study, the controlling Equalization objective for water year 2008 is an end of water year Lake Mead elevation of 1,105 feet above sea level. To achieve this objective, the water year annual release volume from Glen Canyon Dam will be controlled as practicably as possible to achieve an end of water year elevation at Lake Mead of 1105. The July 2008 24-month study projects the annual release volume from Glen Canyon Dam that would accomplish this objective to be 8.965 maf which equates to an equalization volume (volume in excess of 8.23 maf) projected to be 735,000 af. These projected values, as well as the monthly release volumes, for the remaining months of water year 2008 will be adjusted as conditions change.
Inflows to Lake Powell in early June increased to over 75,000 cfs with the elevation of Lake Powell increasing at nearly 1 foot per day. The current elevation of Lake Powell (July 6, 2008) is 3,632.64 feet above sea level. The Castle Rock Cut became passable in early June for the first time in over 5 years. The elevation of Lake Powell will likely reach its peak for water year 2008 by early August at approximately 3635 feet above sea level.
Upper Colorado River Basin Hydrology
Precipitation in the basin above Lake Powell was below average in June (70% of average). The precipitation above Lake Powell in March and April was below normal at 60% of normal over the 2 month period and was average in May (100% of average). The overall precipitation in the Upper Colorado River Basin for water year 2008 so far is 104% of normal. Temperature conditions in June were below normal to begin the month but ended the month above normal.
The unregulated inflow to Lake Powell in May was 3,612,000 acre-feet (117% of normal). This was 188,000 acre-feet below the level forecasted in early June. The April through July unregulated inflow forecast for Lake Powell has been decreased from 9.2 maf (issued for June update) to 8.8 maf (issued for July update) which is 111% of 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 one.
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 8 years (2000 through 2007, inclusive), inflow to Lake Powell has been below average in all but one year (2005). Drought conditions have eased again in water year 2008 with projected inflows to the main stem Colorado River reservoirs at or above normal. Reservoir storage in the Colorado River Basin, however, is still below desired levels with the overall Colorado River system storage (above Lake Mead) projected to be about 58% of capacity at the end of water year 2008.
Reservoir storage in Lake Powell and Lake Mead has decreased during the past 8 years but is projected to increase by the end of water year 2008. Current reservoir storage in Lake Powell is 62 percent of capacity. Storage in Lake Mead is 46 percent of capacity.
This release courtesy Rick Clayton, Hydraulic Engineer, Upper Colorado Region US Bureau of Reclamation