RRFW Riverwire – Glen Canyon Dam Update
July 11, 2020
Current Status
Last Updated: July 10, 2020
The unregulated inflow volume to Lake Powell during June was 1,453 thousand acre-feet (kaf) (54 percent of average). The release volume from Glen Canyon Dam in June was 650 kaf. The end of June elevation and storage of Lake Powell were 3610.62 ft (89 feet from full pool) and 12.79 maf (53 percent of full capacity), respectively.
The Department of the Interior is conducting the fourth 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, macroinvertebrate flows (bug flows), trout management flows, and low summer flows. Collaborative discussions among technical experts resulted in a decision to begin this third consecutive year of the bug flow experiment on May 1 and continue through August 31, 2020. 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.
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Macroinvertebrate Release Information |
|||||
|
Month |
Release Volume (af) |
Maximum Daily Fluctuation (cfs) |
Weekday Maximum (cfs) |
Weekday Minimum (cfs) |
Weekend Release (cfs) |
|
May |
630,000 |
2,525 |
11,665 |
9,135 |
9,890 |
|
June |
650,000 |
6,500 |
14,565 |
8,065 |
8,815 |
|
July |
750,000 |
7,500 |
16,030 |
8,530 |
9,280 |
|
August |
835,000 |
8,000 |
17,880 |
9,880 |
10,630 |
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 two 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. Although every effort will be made to match the design of the experiment described above, Reclamation will continue to exercise the operational flexibility described in the LTEMP ROD.
To view the most current reservoir elevation projections, click on: https://www.usbr.gov/uc/water/crsp/studies/images/PowellElevations.png
To view the most current monthly release projections, click on: https://www.usbr.gov/uc/water/crsp/studies/images/PowellRelease.png
To view the 2020 progession of snowpack above Lake Powell, click on: https://www.nrcs.usda.gov/Internet/WCIS/basinCharts/POR/WTEQ/assocHUC2/Upper%20Colorado%20Region.html
To view the current inflow forecast relative to past inflows, click on: https://www.usbr.gov/uc/water/crsp/studies/images/PowellForecast.png
Current Operations
The operating tier for water year 2020 (September 2019 through October 2020) was established in August 2019 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 2020 will be governed by the Upper Elevation Balancing Tier. With an 8.23 million acre-foot (maf) release from Lake Powell in water year 2020, the April 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 above 1,075 feet. Therefore, in accordance with Section 6.B.1 of the Interim Guidelines, Lake Powell will continue to release 8.23 maf through the remainder of water year 2020.
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 28 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 July 1, 2020, by the Colorado Basin River Forecast Center, projects that the most probable (median) unregulated inflow volume this year will be 6.59 maf (61 percent of average). There is significant uncertainty regarding this 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 April forecast ranges from a minimum probable of 6.73 maf (62 percent of average) to a maximum probable of 11.24 maf (104 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 July 24-Month Study https://www.usbr.gov/uc/water/crsp/studies/24Month_07.pdf projects Lake Powell elevation will end water year 2020 near 3,602.03 feet with approximately 11.94 maf in storage (49 percent of capacity). Note that projections of elevation and storage for water year 2020 have some uncertainty at this point in the season. Projections of end of water year 2020 elevation and storage using the minimum and maximum probable inflow forecast from April 2020 are 3,604.01 feet (12.14 maf, 50 percent of capacity) and 3,636.08 feet (15.55 maf, 64 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 2020 will be 8.23 maf under all scenarios.
Upper Colorado River Basin Hydrology
Upper Colorado River Basin regularly experiences significant year to year hydrologic variability. During the 20-year period 2000 to 2019, 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 2000-2019 is the lowest 20-year period since the closure of Glen Canyon Dam in 1963, with an average unregulated inflow of 8.76 maf, or 81 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 2000-2019 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 2020 unregulated inflow to Lake Powell is projected to be 6.59 maf (61 percent of average).
At the beginning of water year 2020, total system storage in the Colorado River Basin was 31.64 maf (53 percent of 59.6 maf total system capacity). This is an increase of 3.64 maf over the total storage at the beginning of water year 2019 when total system storage was 28 maf (47 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 53 percent of capacity at the beginning of water year 2020. Based on current inflow forecasts, the current projected end of water year total Colorado Basin reservoir storage for water year 2020 is approximately 29.21 maf (48 percent of total system capacity). The actual end of water year 2020 system storage may vary from this projection, primarily due to uncertainty regarding this season’s runoff and reservoir inflow.
This release courtesy the Bureau of Reclamation
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