Last Updated: October 15, 2020
The unregulated inflow volume to Lake Powell during September was 47 thousand acre-feet (kaf) (11 percent of average). The release volume from Glen Canyon Dam in September was 602 kaf. The end of September elevation and storage of Lake Powell were 3595.98 ft (104 feet from full pool) and 11.37 maf (46 percent of full capacity), respectively.
To view the most current reservoir elevation projections, click on: Lake Powell Elevation Projections.
To view the most current monthly release projections, click on: Lake Powell Release Projections.
To view the 2020 progession of snowpack above Lake Powell, click on Lake Powell Snow Chart.
To view the current inflow forecast relative to past inflows, click on Lake Powell Inflow Forecast.
The operating tier for water year 2021 (September 2020 through October 2021) was established in August 2020 as the Upper Elevation Balancing Tier, consistent with Section 6.B of the Interim Guidelines. Consistent with Section 6.B of the Interim Guidelines, Lake Powell’s operations in water year 2021 will be governed by the Upper Elevation Balancing Tier. With an 8.23 million acre-foot (maf) release from Lake Powell in water year 2021, the October 2020 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 1,075 feet. Therefore, in accordance with Section 6.B.4 of the Interim Guidelines, an April adjustment to balancing releases is projected to occur and 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 October 24-Month Study projects a balancing release of 9.0 maf in water year 2021.
In October, the release volume will be approximately 640 kaf, with fluctuations anticipated between about 6,988 cfs in the nighttime to about 12,758 cfs in the daytime, and consistent with the Glen Canyon Dam, Record of Decision (dated December 2016). The anticipated release volume for November is 640 kaf with daily fluctuations between approximately 7,021 cfs and 12,781 cfs. The expected release for December is 640 kaf.
In addition to daily scheduled fluctuations for power generation, the instantaneous releases from Glen Canyon Dam may also fluctuate to provide 40 megawatts (mw) of system regulation. These instantaneous release adjustments stabilize the electrical generation and transmission system and translate to a range of about 1,100 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 currently maintains 30 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 forecast for water year 2020 unregulated inflow to Lake Powell, issued on September 1, 2021, by the Colorado Basin River Forecast Center, projects that the most probable (median) unregulated inflow volume this year will be 7.9 maf (73 percent of average).
There is significant uncertainty regarding next season’s snowpack development and resulting runoff into Lake Powell. Reclamation updates the minimum and maximum probable forecasts four times a year: January, April, August and October. The October forecast for water year 2021 ranges from a minimum probable of 5.17 maf (48 percent of average) to a maximum probable of 16 maf (148 percent of average) with a most probable water year unregulated inflow forecast of 7.9 maf (73 percent of average). There is a 10 percent chance that inflows could be higher than the current maximum probable forecast and a 10 percent chance that inflows could be lower than the minimum probable forecast.
Based on the current forecast of 7.9 maf unregulated inflow, the October 24-Month Study projects Lake Powell elevation will end water year 2021 near 3,576.06 feet with approximately 9.6 maf in storage (39 percent of capacity). Note that projections of elevation and storage for water year 2021 have significant uncertainty at this point in the season. Projections of end of water year 2021 elevation and storage using the minimum and maximum probable inflow forecast from October 2020 are 3,563.36 feet (8.58 maf, 35 percent of capacity) and 3,649.33 feet (17 maf, 70 percent of capacity), respectively. Under these scenarios, there is a 10 percent chance that inflows will be higher, resulting in higher elevation and storage, and 10 percent chance that inflows will be lower, resulting in lower elevation and storage. The annual release volume from Lake Powell during water year 2021 is projected to be 9.0 maf under the October most probable scenario, and 8.23 maf under the October minimum and maximum probable inflow scenarios.projects Lake Powell elevation will end water year 2021 near 3,584.12 feet with approximately 10.30 maf in storage (42 percent of capacity). Note that projections of elevation and storage for water year 2021 have some uncertainty at this point in the season. Projections of end of water year 2021 elevation and storage using the minimum and maximum probable inflow forecast from August 2020 are 3,567.54 feet (12.14 maf, 37 percent of capacity) and 3,656.19 feet (18 maf, 74 percent of capacity), respectively. Under these scenarios, there is a 10 percent chance that inflows will be higher, resulting in higher elevation and storage, and 10 percent chance that inflows will be lower, resulting in lower elevation and storage. The annual release volume from Lake Powell during water year 2021 is projected to be 9.0 maf under the September most probable scenario, and 8.23 maf under the August minimum and maximum probable inflow scenarios.
Upper Colorado River Basin Hydrology
Upper Colorado River Basin regularly experiences significant year to year hydrologic variability. During the 21-year period 2000 to 2020, 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 4 out of the past 19 years. The period 2000-2020 is the lowest 21-year period since the closure of Glen Canyon Dam in 1963, with an average unregulated inflow of 8.62 maf, or 80 percent 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 2000-2020 period has ranged from a low of 2.64 maf (24 percent of average) in water year 2002 to a high of 15.97 maf (147 percent of average) in water year 2011. In water year 2018 unregulated inflow volume to Lake Powell was 4.6 maf (43 percent of average), the third driest year on record above 2002 and 1977. Under the current most probable forecast, the total water year 2021 unregulated inflow to Lake Powell is projected to be 7.9 maf (73 percent of average).
At the beginning of water year 2021, total system storage in the Colorado River Basin was 28.88 maf (48 percent of 59.6 maf total system capacity). This is a decrease of 2.77 maf over the total storage at the beginning of water year 2020 when total system storage was 31.64 maf (53 percent 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 percent of capacity at the beginning of 2000 to the now current level of 48 percent of capacity at the beginning of water year 2021. Based on current inflow forecasts, the current projected end of water year total Colorado Basin reservoir storage for water year 2021 is approximately 27.27 maf (46 percent of total system capacity). The actual end of water year 2021 system storage may vary from this projection, primarily due to uncertainty regarding this season’s runoff and reservoir inflow.