Glen Canyon Dam
(Last Updated: September 17, 2018)
The Department of the Interior is exploring the possibility of a High Flow Experiment from Glen Canyon Dam to be conducted in November 2018. High Flow Experiments (HFE) below Glen Canyon Dam are driven by weather, sediment inputs, and other resource conditions, in accordance with the Long-Term Experimental and Management Plan (LTEMP).
When sediment conditions during the summer and fall meet specific thresholds as described in the LTEMP, a fall HFE can occur. Fall HFEs can be scheduled to occur anytime during the months of October and November; however, under the LTEMP, HFEs have historically always occurred in November. Our best preliminary data and model runs indicate we now have enough sediment input from the Paria River for an HFE to occur at Glen Canyon Dam. The HFE Technical Team will be meeting to begin planning for a possible HFE and reviewing the status of resources and potential resource impacts.
The HFE will likely begin on November 5, 2018. The duration of the HFE is dependent upon further data collection, modeling, and analysis prior ot the end of October. A final determination on the timing, magnitude and duration of a potential fall 2018 HFE will likely be made in mid- to late-October.
The April to July 2018 unregulated inflow to Lake Powell was 2.6 maf (36 percent of average). The unregulated inflow in August was 11.2 thousand acre-feet (kaf) (2 percent of average). The is the second lowest unregulated inflow to occur in August. The lowest occurred in August 2002 when the volume was negative 50 kaf. Unregulated inflow is a calculated value that attempts to eliminate changes in upstream reservoir storage and evaporation. Negative unregulated inflow indicates that the upstream reservoir storage is being used to keep the river wet and without upstream storage the river would be dry. The release volume from Glen Canyon Dam in August was 900 kaf. The end of August elevation and storage of Lake Powell were 3,597 feet (103 feet from full pool) and 11.2 million acre-feet (maf) (47 percent of full capacity).
To view the 2018 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 2018 was established in August 2017 as the Upper Elevation Balancing Tier. The April 2018 24-Month Study established that Lake Powell operations will be governed by balancing for the remainder of water year 2018. 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 August 24-Month Study projects a balancing release of 9.0 maf in water year 2018. The projected release from Lake Powell in water year 2018 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, 2018.
The operating tier for water year 2019, established by the August 2018 24-Month Study, is the Upper Elevation Balancing Tier. Under this Tier the initial annual water year release volume is 8.23 maf but there is potential for an April 2019 adjustment to equalization or balancing releases. Based on the current forecast, an April adjustment to balancing releases is projected and Lake Powell is currently projected to release 9.0 maf in water year 2019. This projection will be updated each month throughout the water year.
In September, the release volume will be approximately 671 kaf, with fluctuations anticipated between about 7,600 cfs in the nighttime to about 13,640 cfs in the daytime and consistent with the Glen Canyon Dam, Record of Decision on LTEMP (dated December, 2016). The anticipated release volume for October is 625 kaf with daily fluctuations between approximately 7,070 cfs and 12,700 cfs. The expected release for November is 625 kaf, unless an HFE is implemented. Once the timing, magnitude and duration of an HFE has been determined, a release volume will be available.
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 1,200 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 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 2019 unregulated inflow to Lake Powell, issued on August 1, 2018, by the Colorado Basin River Forecast Center, projects that the most probable (median) unregulated inflow volume next year will be 8.1 maf (75 percent of average). There is significant uncertainty regarding next season’s snow pack development and resulting runoff into Lake Powell. The forecast ranges from a minimum probable of 4.8 maf (44 percent of average) to a maximum probable of 15.6 maf (144 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, the August 24-Month Study projects Lake Powell elevation will end water year 2018 near 3,586 feet with approximately 10.51 maf in storage (43 percent of capacity) and water year 2019 near 3,581 feet with approximately 10.07 maf in storage (43 percent of capacity). Note that projections of elevation and storage for water year 2019 have significant uncertainty at this point in the season. Projections of end of water year 2019 elevation and storage using the minimum and maximum probable inflow forecast are 3,566 feet (8.8 maf, 36 percent of capacity) and 3,648 feet (17.0 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 2019 is projected to be 9.0 maf under the most probable and maximum probable inflow scenarios and 8.23 maf under the minimum probable inflow scenario.
Upper Colorado River Basin Hydrology
The Upper Colorado River Basin regularly experiences significant year to year hydrologic variability. During the 18-year period 2000 to 2017, 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 18 years. The period 2000-2017 is the lowest 18-year period since the closure of Glen Canyon Dam in 1963, with an average unregulated inflow of 8.76 maf, or 81percent 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 2000-2017 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 2017 unregulated inflow volume to Lake Powell was 11.9 maf (110 percent of average), the fourth year to be above average. Under the current most probable forecast, the total water year 2018 unregulated inflow to Lake Powell is projected to be 5.25 maf (48 percent of average). At the beginning of water year 2018, total system storage in the Colorado River Basin was 32.9 maf (55 percent of 59.6 maf total system capacity). This is an increase of 2.7 maf over the total storage at the beginning of water year 2017 when total system storage was 30.2 maf (51 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 a low of 50 percent of capacity at the beginning of water year 2005. 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 total Colorado Basin reservoir storage for water year 2018 is approximately 29.0 maf (48 percent of total system capacity). The actual end of water year 2018 system storage may vary from this projection, primarily due to uncertainty regarding this season’s runoff and reservoir inflow. Based on the April minimum and maximum probable inflow forecasts and modeling, the range of end of water year 2018 total system capacity is approximately 27.8 maf (47 percent of capacity) to 31.0 maf (52 percent of capacity), respectively.
Please contact the Operations Group via e-mail at ResourceMgr@usbr.gov for additional information.
Last Updated: 9/18/18