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RRFW Riverwire – Glen Canyon Dam Update
May 27, 2010

Glen Canyon Dam / Lake Powell

The unregulated inflow volume into Lake Powell in May 2010 thus far has been well below the volume forecasted at the beginning of May. The forecasted unregulated inflow for May 2010 for Lake Powell was 1,500,000
acre feet (65% of average). The actual unregulated inflow to Lake Powell thus far in May 2010 (through May 24, 2010) is tracking towards a monthly volume of 1,197,000 acre-feet (52% of average). For this reason, the elevation of Lake Powell has not increased as projected in the May (most probable) 24-Month Study. It is currently projected that by the end of May, the elevation of Lake Powell will be about 2624.2 feet above sea level which is about 2.8 feet below what was projected in the May (most probable) 24-Month Study.

The forecasted June and July unregulated inflows to Lake Powell were revised and increased from what they were at the beginning of May and the overall April through July forecast for unregulated inflow to Powell
actually increased in the mid-month forecast from 5.2 million acre feet (maf) to 5.3 maf (67% of average). A new forecast for June is expected during the first week of June and a new 24-Month Study projection will be
created based on this new forecast.

Releases from Glen Canyon Dam during the month of June will fluctuate each day for power generation between a peak hourly average release of about 12,500 cubic feet per second (cfs), during the morning and afternoon and a daily low hourly average release of 6,500 cfs during the late evening and early morning hours. The release volume scheduled for June is 600,000 acre-feet. The release volume projected for July is 800,000 acre-feet and this will be confirmed in late June.

In addition to the daily fluctuation pattern, instantaneous releases from Glen Canyon Dam also fluctuate to provide approximately 40 megawatts
(approximately 1,100 cfs) of system regulation to maintain stable conditions within the electrical generation and transmission system. This
translates into momentary release fluctuations of about +/- 1100 cfs above or below the hourly average release rate. These momentary fluctuations for regulation are very short lived and typically balance out over the hour. When an unanticipated outage event occurs in the generation system, reserve generation at Glen Canyon Dam can also be called upon up to a limit of 83 megawatts (approximately 2,250 cfs of release) for a duration of 2 hours or less. Under normal circumstances, calls for reserve generation occur fairly infrequently and are for much less than the limit of 83 megawatts.

The May 2010 24-Month Study projects that operation of Glen Canyon Dam in water year 2011 will follow the Equalization Tier of the Interim Guidelines based on a projected shift to Equalization in April 2011. It
should be cautioned however that at this time of year the inflow assumption used in the 24-Month Study for the following water year is based on statistical averages and does not reflect a current projection
from the Colorado Basin River Forecast Center (CBRFC).

Inflow conditions for 2011 are highly uncertain at this point in time. In August 2010, after the inflow conditions for 2010 are more clear, the CBRFC will issue an inflow projection for water year 2011. The August 2010 24-Month Study will be based on this inflow projection rather than statistical averages which
don't account for antecedent conditions.

Upper Colorado River Basin Hydrology

In the Upper Colorado River Basin during water year 2009, the overall precipitation accumulated through September 30, 2009 was approximately 95% of average based on the 30 year average for the period from 1971 through 2000. For water year 2010 dry conditions have persisted.

Estimated percentages of average precipitation for the months thus far in water year 2010 are as follows: October 85%, November 40%, December 130%, January 100% and February 100%, March 90%, April 120%. The overall
estimated precipitation percentage of average thus far in water year 2010 for the Upper Colorado River Basin is 95% of average.

The Climate Prediction Center outlook (dated May 20, 2010) for temperature over the next 3 months indicates that temperatures in the Upper Colorado River Basin are expected to be above average while precipitation over the next 3 months is projected to be near average.

Upper Colorado River Basin Drought

The Upper Colorado River Basin continues to experience a protracted multi-year drought. Since 1999, inflow to Lake Powell has been below average in every year except water years 2005 and 2008. 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 net gains in storage to Lake Powell. As of May 24, 2010 the storage in Lake Powell was 14.1 million acre-feet (58.0 % of capacity) which is still below desired levels while the overall reservoir storage in the Colorado River Basin as of May 24, 2010 is 32.95 million acre-feet (55.4 % of capacity).

RRFW thanks Rick Clayton of the USBOR for his assistance in providing information for this notification.

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