A 2025 Fall HFE

High Flow Experiment- 2025

2025 Fall High Flow Experiment Snapshot

DOI Decision

DATE: October 31, 2025 
TO: AMWG, TWG, and Planning and Implementation Team Members
FROM: Wayne G. Pullan, Regional Director, Upper Colorado Basin Region, Bureau of Reclamation 
SUBJECT: Decision Regarding Fall 2025 HFE
Dear AMWG, TWG and Planning and Implementation (PI) Team Members,
I am writing to inform you that Reclamation will not implement a fall HFE from Glen Canyon this year. This decision is based on the current lapse in appropriations, which has created uncertainty concerning necessary resources. Modeling conducted by GCMRC indicates that a sediment trigger will be met next spring based on the current sediment load below GCD and projected monthly release volumes. As such, an HFE will be reconsidered for potential in spring 2026.
I would like to extend my sincere gratitude to the members of the PI Technical and Leadership teams, as well as the Grand Canyon Monitoring and Research Center (GCMRC) for their commitment to ensuring an informed decision could be made.
Wayne G. Pullan, Director
Upper Colorado Basin Region

Recommendation Summary

Based on the 2016 LTEMP Record of Decision (ROD) and 2024 LTEMP Supplemental Environmental Impact Statement ROD, the Glen Canyon Dam Planning and Implementation (PI) Technical Team has worked over the past several weeks to evaluate existing information and data in determining this recommendation regarding a HFE at Glen Canyon Dam.

The Planning and Implementation (PI) Technical Team recommends to the U.S. Department of the Interior to implement a fall High-Flow Experiment (HFE) at 33,000 CFS for 27 hours as described in the attached Tech Team Report.

Additionally, the following items were noted by the PI Technical Team for consideration:

• At this time, Grand Canyon Monitoring and Research Center (GCMRC) has been unable to provide a secondary review of the technical report. The Technical Team recommends that the GCMRC scientists provide a final review of the technical report when they return to work and report any red flags to the Technical Team.
• The fall HFE under consideration is limited in magnitude and duration and could provide a valuable learning opportunity for potential future hydrological scenarios.
• If new evidence is found that a smallmouth bass spawn occurred in the summer of 2025, the possibility that a fall HFE could disperse those small fish downstream should be evaluated.
• This recommendation is based on the understanding by the Technical Team that conducting a fall HFE will not impact the resources or scientific integrity of a potential Cool Mix Flow in 2026.
• Monitoring of the fall HFE will include remote sensing and camera analyses. If staffing is available, physical sandbar monitoring is suggested to evaluate the impacts of this fall HFE.
• We encourage continued communication as developments occur around the fall HFE.

The PI Technical Team’s recommendation regarding implementation of a fall 2025 HFE is based on a careful assessment of resources and best available science.

Background and Purpose of the Recommendation

The purpose of this memorandum is for the PI Technical Team to transmit a recommendation to the Glen Canyon Dam PI Leadership Team. Once the PI Leadership Team has provided a recommendation, this memo will be forwarded to the Department of the Interior (Department) in accordance with the LTEMP ROD (DOI 2016a). The PI Technical Team includes technical representatives from the National Park Service (NPS), U.S. Fish and Wildlife Service (FWS), Bureau of Indian Affairs (BIA), U.S. Geological Survey (USGS) Grand Canyon Monitoring and Research Center (GCMRC), Bureau of Reclamation (Reclamation), Western Area Power Administration (WAPA), Arizona Game and Fish Department (AZGFD), seven Colorado River Basin States (Basin States), and the Upper Colorado River Commission (UCRC).

In the fall of 2024, after evaluating the LTEMP resources using the best available science and discussions within the PI Leadership Team, most PI Leadership Team members recommended deferring to a spring 2025 HFE. One member recommended a fall 2024 HFE. In the spring of 2025, the PI Technical Team reevaluated the Sand Budget Model and determined that there was sufficient sediment to conduct a spring 2025 HFE. Due to the risks of disrupting -12 Mile Slough modifications, the Technical Team ultimately recommended not to implement in the spring of 2025. Accumulated sediment was rolled over to the next accounting period for possible consideration of a fall 2025 HFE. The PI Technical Team incorporated the most current science and data and considered multiple issues with agency experts, as summarized below, in making this recommendation.

In the fall of 2025, critical elevations were predicted for water year 2026 (Most Probable 24-Month Study: September 2025). The Upper Basin Drought Response Operations Agreement (DROA) planning trigger had been met (projections of Lake Powell Elevations less than 3525 ft. within 12 months under most probable hydrology) and the projected likelihood of hitting the IG SEIS 6(e) trigger in the coming months increased. DROA actions can include potential adjustments to monthly releases to Glen Canyon Dam. While no decisions on monthly adjustments or other DROA operations have been made, the potential window for monthly adjustments could begin in December of 2025. Based on the potential for monthly volume adjustments beginning in December 2025, Reclamation recommends consideration of a HFE in November 2025, using the current release anticipated from November 2025 and not re-allocating water from other months. Based on current operating guidance and reservoir levels the River Outlet Works (ROW), or bypass tubes, could be operated up to 3,700 cfs each (total maximum bypass of 14,800 cfs) for a November 2025 HFE. Under this scenario, the November 2025 monthly volume and end of month elevation would not change, but the daily release patterns would change. HFEs do not change annual flow releases.

Based on an anticipated November monthly release volume of 500 thousand acre-feet (kaf), the volume available for a November 2025 HFE was estimated at 75 kaf. Using this estimate the PI Technical Team initially considered an HFE with a magnitude of 40,000 cfs, based on availability of all eight penstock units and four ROWs, and a duration of 22 hours (Figure 3). However, on September 25, 2025, an electrical outage occurred on units 3 and 4 at the Glen Canyon Dam. At the time of this technical memo, the outage is being assessed. Based on current information, it is unlikely that units 3 and 4 will be available for use during the HFE implementation window. Due to the maintenance issue causing decreased release capacity, the PI Technical Team considered a fall HFE with a magnitude of 33,000 cfs and a duration of 27 hours.

The PI Leadership team will consider the PI Technical Team’s recommendation and develop a PI Leadership Team recommendation to be submitted to the Secretary of the Interior. The Secretary of the Interior or his Designee will consider the recommendation of the PI Leadership Team but retains sole discretion to decide how best to accomplish operations and experiments in any given year pursuant to the LTEMP ROD and other binding obligations for Glen Canyon Dam.

Fall HFE and No HFE

Over the past several weeks the PI Technical Team considered two primary options: conducting a fall HFE or not conducting an HFE and reconvening the PI Team in spring 2026 to review and provide a recommendation for a spring HFE using the sediment accounting window from July 1, 2024 - June 30, 2026.

The fall HFE under consideration is for a duration of 27 hours at a peak magnitude of 33,000 cfs, based on available water, sediment resources, and maximum possible Glen Canyon Dam release.

The PI Technical Team’s recommendation to conduct a fall HFE was by a majority vote.

Resource Considerations

High-Flow Experiment (HFE) Sand Budget Model

The Sand Budget Model of Wright et al. (2010) was used to project instream sand mass balance in upper and lower Marble Canyon under multiple flow scenarios. Reclamation analyzed the resulting mass balance to estimate how a potential HFE would transport sand delivered from the Paria River to the Colorado River since the start of the accounting period on July 1, 2024. Only HFE durations and magnitudes that resulted in a positive sand mass balance were considered (Figure 1-2). An HFE duration was considered to have a positive sand mass balance if the lower uncertainty bound of the sand mass balance was positive at the end of the HFE.

Two primary scenarios were analyzed:

• A fall HFE with a magnitude of 33,000 cfs and duration of 27 hours (Figure 1).
• A no fall HFE scenario (Figure 2), where the PI Team in would reconvene in spring 2026

Both scenarios are consistent with the new revised HFE protocols associated with the 2024 LTEMP SEIS ROD (DOI 2024a). Modeling results presented below serve as the initial recommendation for the duration of the fall HFE, based on the magnitudes constrained by dam release parameters (Figures 1-3). The following assumptions were made for the Sand Budget Model for Option 1 and are subject to change:

• Estimated Available Powerplant: ~ 18,200 cfs (6 units)
• Estimated Available Bypass: ~ 14,800 cfs (4 River Outlet Works, ROW)
• Total Volume Bypassed: ~ 35,000 ac-ft (including both ramp up and ramp down)
• Estimated Peak Release = ~ 33,000 cfs (adjusted for uncertainties)

Potential Ramp Rate (Experimental Design):

• Ramp-up from base releases at 4,000 cfs/hr at approximately 7:00 am on Wednesday, November 12, 2025 (all times Mountain Standard Time) until reaching powerplant capacity (~18,200 cfs)
• Open first bypass tube at 10:00 am on November 13
• Ramp-up from powerplant capacity to full bypass (~33,000 cfs) at one full bypass tube (~3,500 cfs) per hour in 4 hrs
• Stay at peak release (~33,000 cfs) for 27 hrs
• Ramp-down from peak release to base releases beginning at half bypass of 1,750 cfs/hr until reaching powerplant capacity and then decreasing at 2,500 cfs/hr

The above ramp rate recommendation results in the following release schedule at Glen Canyon Dam:

• Begin ramp-up from 6,500 cfs at 7:00 am on November 12 (Wednesday)
• Reach powerplant capacity at approximately 10:00 am on November 12
• Open bypass tubes at approximately 10:00 am on November 12
• Reach full bypass at 2:00 pm on November 12
• Begin ramp-down from bypass at 5 pm on November 13 (Thursday)
• Complete HFE (back to 9,000 cfs) at 5 am on November 14 (Friday)

Figure 1. Hydrograph and modeled sand mass balance for proposed fall 2025 High-Flow Experiment (Option 1) Subject to minor revision.

Figure 2. Hydrograph and modeled sand mass balance under a potential deferring to a spring 2026 HFE (Option 2). Red shaded areas on Option 2 indicate where previous HFEs were triggered but not implemented. The green shaded area on Option 2 indicates where a potential spring HFE would be implemented in May and June if triggered. Subject to minor revision.

Figure 3. Monthly release volume distribution based on modeling for Reclamation’s September 2025 24-Month Study.

The purpose of HFEs conducted in the context of the LTEMP ROD is to determine if sandbar building during HFEs exceeds sandbar erosion during periods between HFEs, such that sandbar size can be increased or maintained over the 20-year period of the LTEMP. In addition to reviewing the sand budget model output, the PI Technical Team assessed the status of the LTEMP resources and potential effects of an HFE on these resources in making the recommendation described in this document.

In coordination with GCMRC, WAPA, and Reclamation, the PI Technical Team completed an assessment of key resources impacted or affected by an HFE. This assessment is based on current resource conditions and findings from fall HFEs conducted in 2012, 2013, 2014, 2016, 2018 and a spring HFE conducted in 2023. Key resources were evaluated relative to the timing, duration, and magnitude of a potential fall 2025 HFE using the best available science. This section summarizes the assessment of resources and expected effects of a potential HFE.

Monitoring Plan

GCMRC developed a science plan for the LTEMP that describes a program of monitoring and research activities that support ongoing information needs associated with implementation of the LTEMP and associated experiments like HFEs (Vanderkooi et al. 2017). This approach relies on water quality, sediment, aquatic biology, and other resource monitoring and research projects funded in the GCDAMP Fiscal Year (FY) 2025-2027 Triennial Budget and Work Plan (DOI 2024b). These projects inform the effect of future HFEs on the downstream resources of Glen, Marble, and Grand Canyons. Projects from the Triennial Work Plan (TWP) specific to monitoring HFEs are further discussed below.

Project A, Streamflow, Water Quality, and Sediment Transport and Budgeting in the Colorado River Ecosystem, and Project B, Sandbar and Sediment Storage Monitoring and Research, are essential components to implementation of the HFE Protocol under LTEMP because the protocol calls for high flow releases from Glen Canyon Dam whenever a specified minimum amount of fine sediment delivered from the Paria River is exceeded. Under Project A, the sediment-transport measurements needed to trigger and evaluate HFEs are collected. Project B supports the direct measurements of the volume of fine sediment, especially sand, that is stored on the bed of the Colorado River, in its eddies, or at higher elevation along the river’s banks; these measurements allow assessment of the effectiveness of the HFE Protocol. A substantial accomplishment of these programs in previous work plans was the development of web-based interfaces to serve sediment transport and water quality data, calculate fine sediment mass balances (see https://www.gcmrc.gov/discharge_qw_sediment/), and to serve photographs of approximately 50 sandbars located from Lees Ferry to Diamond Creek (see https://www.usgs.gov/apps/sandbar/). The latter data allow stakeholders to evaluate the effects of controlled floods implemented under the HFE Protocol.

As described in the HFE Protocol Environmental Assessment (EA; DOI 2011) and the LTEMP FEIS (DOI 2016a), the potential HFE planned for fall 2025 would not be an isolated event, but a component of a longer-term experiment to restore and maintain sandbars with multiple high flows over a period of several years. The monitoring data that are needed to assess the outcome of this multi-year experiment include annual sandbar monitoring at selected long-term monitoring sites, periodic monitoring of changes in sand storage in the river channel, and measurements of sandbar size at more than 1,000 sites based on aerial photographs that are collected periodically. These activities are described in detail in the TWP (DOI 2024b). It is also important, however, to evaluate the sandbar building response of each high flow to assess whether the sandbar building objectives are being achieved incrementally. This evaluation will be based on sites that are monitored by remotely deployed digital cameras and repeat topographic surveys of sites that will occur in fall 2025.

GCMRC scientists have installed digital cameras that capture 5 images every day at 43 sandbar monitoring sites throughout Marble and Grand Canyons between Lees Ferry and Diamond Creek. The images acquired by these cameras will be used to evaluate both the magnitude and spatial distribution of sandbar building caused by the HFE (Grams et al. 2018). They will also be used to assess the rate of post-HFE sandbar erosion. Because the remote cameras are monitoring the same sites that are monitored by the annual surveys and the same sites that were monitored during the previous high flows, it will be possible to evaluate sandbar-building effectiveness of the planned HFE relative to the previous HFEs.

Project D will continue monitoring for changes in sediment storage in dunefields covering archaeological sites. An HFE implemented in 2025 would provide GCMRC and NPS the opportunity under their collaborative LTEMP vegetation management project to address a critical uncertainty about the potential combined effects of HFEs and riparian vegetation management.

GCMRC will also collect data on water quality (including nutrients), native and nonnative fishes, aquatic invertebrates, riparian plants, and other resources, as described in the TWP (DOI 2024). Project G, Humpback Chub Population Dynamics throughout the Colorado River ecosystem, continues long-term monitoring in Grand Canyon and will provide information about potential response of humpback chub (Gila cypha) to a fall HFE.

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