Glen Canyon Dam / Lake Powell
The unregulated inflow volume to Lake Powell in March was 552 thousand acre-feet (kaf) (83% of average). The release volume from Glen Canyon Dam in March was 649 kaf. The end of March elevation and storage of Lake Powell were 3,591.0 feet (109 feet from full pool) and 10.91 million acre-feet (maf) (45% of full capacity), respectively. The reservoir elevation is near the anticipated seasonal low and will soon begin increasing as spring runoff enters the reservoir.
To view the most current reservoir elevation, content, inflow and release, click on: Lake Powell Data.
To view the most current reservoir elevation projections, click on: Lake Powell Elevation Projections.
To view the 2015 progession of snowpack above Lake Powell, click on Lake Powell Snow Chart.
To view the current April to July 2015 inflow forecast relative to past inflows, click on Lake Powell Inflow Forecast
The operating tier for water year 2015 was established in August 2014 as the Upper Elevation Balancing Tier. In the Upper Elevation Balancing Tier the initial water year release volume is 8.23 maf; however, there is the possibility for an April adjustment to equalization or balancing operations to govern for the remainder of the water year. This April 2015 24-Month Study establishes that Lake Powell operations will shift to “balancing releases” for the remainder of water year 2015. Under Balancing, the contents of Lake Powell and Lake Mead will be balanced by the end of the water year, but not more than 9.0 maf and not less than 8.23 maf shall be released from Lake Powell. Based on the most probable inflow forecast, this April 24-Month Study projects a balancing release of 9.0 maf in water year 2015; the actual release in water year 2015, however, will depend on hydrology in the remainder of water year and will range from 8.23 to 9.0 maf. The projected release from Lake Powell in water year 2015 will be updated each month throughout the remainder of the water year. Reclamation will schedule operations at Glen Canyon Dam to achieve as practicably as possible the appropriate total annual release volume by September 30, 2015.
In April, the release volume will be approximately 600 kaf, with fluctuations anticipated between about 7,500 cfs in the nighttime to about 13,000 cfs in the daytime and consistent with the Glen Canyon Operating Criteria (Federal Register, Volume 62, No. 41, March 3, 1997). The anticipated release volume for May is 700 kaf with daily fluctuations between approximately 8,500 cfs and 14,500 cfs . The expected release for June is 800 kaf with daily fluctuations between approximately 10,000 cfs and 18,000 cfs.
In addition to daily scheduled fluctuations for power generation, the instantaneous releases from Glen Canyon Dam may also fluctuate to provide 40 MW of system regulation. These instantaneous release adjustments stabilize the electrical generation and transmission system and translate to a range of about 1,200 cfs above or below the hourly scheduled release rate. Under system normal conditions, fluctuations for regulation are typically short lived and generally balance out over the hour with minimal or no noticeable impacts on downstream river flow conditions.
Releases from Glen Canyon Dam can also fluctuate beyond scheduled releases when called upon to respond to unscheduled power outages or power system emergencies. Depending on the severity of the system emergency, the response from Glen Canyon Dam can be significant, within the full range of the operating capacity of the power plant for as long as is necessary to maintain balance in the transmission system. Glen Canyon Dam typically maintains 27 MW (approximately 800 cfs) of generation capacity in reserve in order to respond to a system emergency even when generation rates are already high. System emergencies occur fairly infrequently and typically require small responses from Glen Canyon Dam. However, these responses can have a noticeable impact on the river downstream of Glen Canyon Dam.
Inflow Forecasts and Model Projections
The April to July 2015 water supply forecast for unregulated inflow to Lake Powell, issued on April 2, 2015, by the Colorado Basin River Forecast Center, projects that the most probable (median) unregulated inflow volume will be 3.75 maf (52% of average based on the period 1981-2010). The forecast decreased by 135 kaf since last month. At this point in the season, there is still uncertainty regarding this year’s water supply and the total inflow to Lake Powell. The spring runoff forecast ranges from a minimum probable of 2.60 maf (36% of average) to a maximum probable of 5.70 maf (80% of average). There is 10% chance that inflows could be higher than the maximum probable and a 10% chance they could be lower than the minimum probable.
As determined in the August 2014 24-Month Study, and documented in the 2015 Annual Operating Plan, Lake Powell’s operations in water year 2015 will be governed by the Upper Elevation Balancing Tier. In this tier, the initial water year release volume is 8.23 maf, however, there is the potential for an April adjustment to equalization or balancing releases in April 2015. This April 2015 24-Month Study projects the end of water year elevation at Lake Powell to be above 3,575 feet and the end of water year elevation at Lake Mead to be below elevation 1,075.0 feet. Therefore, in accordance with Section 6.B.4 of the 2007 Interim Guidelines, Lake Powell operations will shift to “balancing releases” for the remainder of water year 2015. Under Section 6.B.4, the contents of Lake Powell and Lake Mead will be balanced by the end of the water year, but not more than 9.0 maf and not less than 8.23 maf shall be released from Lake Powell.
Based on the April most probable inflow forecast, the annual release volume from Lake Powell during water year 2015 is projected to be 9.0 maf. Under the minimum probable inflow scenario, the water year release is projected to be 8.9 maf. Under the maximum probable inflow scenario, the release is projected to be 9.0 maf. There 10% chance that inflows will be lower than the current minimum probable forecast, potentially resulting in lower releases. If inflows are less than the minimum probable forecast, the water year 2015 annual release could be as low as 8.23 maf. If inflows are greater than the current forecasted maximum probable inflow, the annual release will be 9.0 maf. The projected release from Lake Powell in water year 2015 will be updated each month throughout the remainder of the water year.
Based on the current forecast, the April 24-Month Study projects Lake Powell elevation will end water year 2015 near 3,583 feet with approximately 10.24 maf in storage (42% capacity). Projections of elevation and storage still have significant uncertainty at this point in the season, primarily due to uncertainty regarding spring runoff and the resulting inflow to Lake Powell. Under the minimum probable inflow scenario, updated in April, the projected end of water year elevation and storage are 3574 feet and 9.45 maf (39% capacity), respectively. Under the maximum probable inflow scenario, updated in April, the projected end of water year elevation and storage are 3603 feet and 12.02 maf (49% capacity), respectively. Modeling of projected reservoir operations based on the minimum and maximum scenarios will be updated again in August.
Upper Colorado River Basin Hydrology
The Upper Colorado River Basin regularly experiences significant year to year hydrologic variability. During the 15-year period 2000 to 2014, however, the unregulated inflow to Lake Powell, which is a good measure of hydrologic conditions in the Colorado River Basin, was above average in only 3 out of the past 15 years. The period 2000-2014 is the lowest 15-year period since the closure of Glen Canyon Dam in 1963, with an average unregulated inflow of 8.39 maf, or 78% of the 30-year average (1981-2010). (For comparison, the 1981-2010 total water year average is 10.83 maf.) The unregulated inflow during the 2000-2014 period has ranged from a low of 2.64 maf (24% of average) in water year 2002 to a high of 15.97 maf (147% of average) in water year 2011. The water year 2014 unregulated inflow volume to Lake Powell was 10.381 maf (96% of average), which, though still below average, was significantly higher than inflows observed in 2012 and 2013 (45% and 47% of average, respectively). Under the current most probable forecast, total water year 2015 unregulated inflows to Lake Powell is projected to be 7.18 maf (66% of average), and ranges from a minimum probable inflow of 5.9 maf (55%) and maximum probable inflow of 9.4 maf (87%).
At the beginning of water year 2015, total system storage in the Colorado River Basin was 30.0 maf (50% of 59.6 maf total system capacity). This is nearly the same as the total storage at the beginning of water year 2014 which began at 29.9 maf (50% of capacity). Since the beginning of water year 2000, total Colorado Basin storage has experienced year to year increases and decreases in response to wet and dry hydrology, ranging from a high of 94% of capacity at the beginning of 2000 to a low of 50% of capacity at the beginning of water year 2014. One wet year can significantly increase total system reservoir storage, just as persistent dry years can draw down the system storage. Based on current inflow forecasts, the current projected end of water year 2015 total Colorado Basin reservoir storage is approximately 27.4 maf (46% of capacity). The actual end of water year storage may vary from this projection, primarily due to uncertainty regarding this season’s runoff and resulting reservoir inflow. Based on the April minimum and maximum probable inflow forecasts and modeling the range is approximately 26.1 maf (44%) to 29.5 maf (50%), respectively.
Updated April 15, 2015
Email comments/inquires to: ResourceMgr@usbr.gov