Glen Canyon Dam
(Last Updated: July 2, 2018)
The Department of the Interior is conducting the first experimental flow at Glen Canyon Dam since implementing its Long-Term Experimental and Management Plan (LTEMP). The goal is to provide enhanced habitat for the lifecycle of aquatic insects that are the primary food source for fish in the Colorado River.
Experiments under LTEMP consist of four different flow regimes: high flows, bug flows, trout management flows, and low summer flows. Collaborative discussions among technical experts resulted in a decision to begin this first experiment on May 1 and continue through August 31, 2018. It will slightly modify the schedule and flow rates of water releases from Lake Powell through Glen Canyon Dam, Arizona. The normally scheduled monthly and weekly release volumes will not be affected.
Flows during the experiment will include steady weekend water releases with routine hydropower production flows on weekdays that include normal hourly changes in release rates. Those steady weekend flows are expected to provide favorable conditions for aquatic insects to lay and cement their eggs to rocks, vegetation, and other materials near the river’s edge. Steady weekend flows will be relatively low, within four inches of typical weekday low water levels. It is unlikely casual recreational river users will notice the changes in water levels.
Insects expected to benefit from this experiment are an important food source for many species of fish, birds, and bats in the canyon. Beyond expected resource benefits, this experiment will also provide scientific information that will be used in future decision making.
The unregulated inflow volume to Lake Powell during June was 883 thousand acre-feet (kaf) (33 percent of average). The release volume from Glen Canyon Dam in June was 760 kaf. The end of June elevation and storage of Lake Powell were 3,610 ft (90 feet from full pool) and 12.7 maf (52 percent of full capacity), respectively. The reservoir is projected to decline each month this water year with some reprieve during the smaller than normal spring runoff period.
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 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.
To view the Bug Flow planned release graph for June, click on Bug Flow hourly release Graph.
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 June 24-Month Study projects a balancing release of 9.0 maf in water year 2018; the actual release in water year 2018, 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 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.
In June, the release volume will be approximately 760 kaf, with fluctuations anticipated between about 8,850 cfs in the nighttime to about 16,450 cfs in the daytime and consistent with the Glen Canyon Dam, Record of Decision (dated December 2016). The anticipated release volume for July is 860 kaf with daily fluctuations between approximately 10,180 cfs and 18,180 cfs. The expected release for August is 900 kaf with daily fluctuations between approximately 10,500 cfs and 18,500 cfs.
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 April to July 2018 water supply forecast for unregulated inflow to Lake Powell, issued on June 4, 2018, by the Colorado Basin River Forecast Center, projects that the most probable (median) unregulated inflow volume will be 2.8 maf (39 percent of average based on the period 1981-2010). The forecast decreased by 200 kaf since last month. 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 2.34 maf (33 percent of average) to a maximum probable of 3.37 maf (47 percent 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 2017 24-Month Study, and documented in the 2018 Annual Operating Plan, Lake Powell’s operations in water year 2018 will be governed by the Upper Elevation Balancing Tier. Starting with an 8.23 million acre-feet (maf) release from Lake Powell in water year 2018, the April 2018 24-Month Study projected 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, Lake Powell operations shifted to balancing releases 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 June most probable inflow forecast, the annual release volume from Lake Powell during water year 2018 is projected to be 9.0 maf. Under the minimum probable inflow scenario, the water year release is projected to be 9.0 maf. Under the maximum probable inflow scenario, the release is projected to be 9.0 maf. There is 10% chance that inflows will be lower than the current minimum probable forecast, potentially resulting in lower releases. However, inflows to Lake Powell so far this water year have been sufficient enough to preclude any release less than 9.0 maf. If inflows are greater than the current forecasted maximum probable inflow, the annual release will be 9.0 maf.Based on the current forecast, the June 24-Month Study projects Lake Powell elevation will end water year 2018 near 3,597 feet with approximately 11.47 maf in storage (47 percent 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 3,589 feet and 10.7 maf (44 percent capacity), respectively. Under the maximum probable inflow scenario, updated in April, the projected end of water year elevation and storage are 3616 feet and 13.3 maf (55 percent 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 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: 7/2/18