A 2026 Spring HFE - Revision history

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== 2026 Spring High Flow Experiment Snapshot ==

=== DOI Decision ===

DATE: June 5, 2026  
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 Spring 2026 HFE 
Dear AMWG, TWG and Planning and Implementation (PI) Team Members,   
I am writing to notify you that Reclamation will not implement a spring High Flow Experiment (HFE) from Glen Canyon this month. This decision reflects the mixed support expressed by members of the PI Leadership Team. In accordance with the revised HFE protocol, the sediment mass balance will roll over into the next accounting period beginning July 1, 2026, and an HFE will be reconsidered for potential implementation in fall 2026.
I would like to extend my sincere appreciation to the PI Technical and Leadership Teams, as well as the Grand Canyon Monitoring and Research Center (GCMRC), for their dedication and contributions in supporting a well-informed decision.   
Consideration of a cool mix flow remains pending, and we anticipate sharing a decision in the coming weeks. Thank you for your patience as we work through these important and complex issues.   
Wayne G. Pullan, Director   
Upper Colorado Basin Region   

=== Recommendation Summary ===

=== Background and Purpose of the Recommendation ===

== Resource Considerations ==

===High-Flow Experiment (HFE) Sand Budget Model===

===Monitoring Plan===

==Assessment of Resources consistent with the 2016 LTEMP ROD and 2024 LTEMP SEIS ROD (all resources were evaluated for a fall HFE)==

===[https://gcdamp.com/index.php?title=GCDAMP_Sediment Sediment]===

===[https://gcdamp.com/index.php?title=CULTURAL Archaeological and Cultural Resources]===

===[https://gcdamp.com/index.php?title=ECOSYSTEM Natural Processes (Flow Regime)]===

===[https://gcdamp.com/index.php?title=FOOD_BASE Aquatic Food Base]===

===[https://gcdamp.com/index.php?title=Humpback_Chub_Page Humpback chub (Gila cypha)] and [https://gcdamp.com/index.php?title=FISH Native Fish]===

===[https://gcdamp.com/index.php?title=HYDROPOWER Hydropower and Energy]===

===[https://gcdamp.com/index.php?title=RECREATION Recreational Experience]===

===[https://gcdamp.com/index.php?title=FISHERY Rainbow Trout (Oncorhynchus mykiss) Fishery]===

===[https://gcdamp.com/index.php?title=Nonnative_Invasive_Aquatic_Species Nonnative Invasive Species]===

'''[https://gcdamp.com/index.php/Green_Sunfish_Page Green Sunfish (Lepomis cyanellus)]'''

'''[https://gcdamp.com/index.php/Brown_Trout Brown Trout (Salmo trutta)]'''

'''[https://gcdamp.com/index.php/Smallmouth_Bass_Page Smallmouth Bass (Micropterus dolomieu)]'''

'''[https://gcdamp.com/index.php?title=Nonnative_Invasive_Aquatic_Species Other Nonnative Invasive Fishes]'''

===[https://gcdamp.com/index.php?title=Riparian_Vegetation Native Riparian Vegetation]===

===[https://gcdamp.com/index.php/GCDAMP_Glen_Canyon_Dam Water Delivery - Monthly, Daily, and Hourly Releases]===

===[https://gcdamp.com/index.php?title=WATER_QUALITY Water Quality]===

===[https://gcdamp.com/index.php/The_-12_Mile_Slough -12 Mile Slough]===

===[https://gcdamp.com/index.php/GCDAMP_Glen_Canyon_Dam Dam Maintenance and Operations]===

===Other LTEMP Experimental Flows (i.e., [https://gcdamp.com/index.php/SEIS_Bypass_Releases Smallmouth Bass Flows])===

== GCDAMP and LTEMP PA Consultation ==

==Conclusion ==

== SUPPORTING DOCUMENTS ==

== ADDITIONAL INFORMATION==

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@@ Line 53: / Line 53: @@
 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 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.
-Spring HFE and No HFE
+===Spring HFE and No HFE ===
 Over the past several weeks the PI Technical Team considered two primary options: conducting a spring HFE or not conducting an HFE and reconvening the PI Team in fall 2026 to review and provide a recommendation for a fall HFE using the sediment accounting window from July 1, 2024 - November 30, 2026.
@@ Line 63: / Line 63: @@
 The PI Technical Team’s recommendation to conduct/not conduct a spring HFE was by a consensus/majority vote (Table 1).  The PI Team identified the following resource concern as a significant reason to implement/not implement a spring HFE flow:
 Table 1. Technical position of Planning and Implementation Technical Team members for consideration of a spring 2026 High-Flow Experiment.
@@ Line 102: / Line 103: @@
 |}
-High-Flow Experiment (HFE) Sand Budget Model
+===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 for the entire accounting period on July 1, 2024-June 30, 2026. Only HFE durations and magnitudes that resulted in a positive sand mass balance were considered (Figure 12). 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.

@@ Line 130: / Line 130: @@
 *Begin ramp-down from bypass at 2:00am on June 24 (Wednesday)
 *Complete HFE by 9:00 am on June 24
 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.
@@ Line 240: / Line 248: @@
 The NPS Comprehensive Fisheries Management Plan completed in cooperation and coordination with AZGFD (NPS 2013), and the AZGFD Management Plan (AZGFD 2015) establish objectives for the rainbow trout fishery at Lees Ferry. Two of those objectives are to maintain angler catch rates of ≥ 1 rainbow trout per hour, and fish condition ≥ 1 in the summer. In addition, metrics have been established to monitor trends in LTEMP goals (Fairly et al. 2025), including angler catch rates and age-1+ abundance of rainbow trout (i.e., spring abundance). After a population crash of rainbow trout in the winter of 2015-2016, the population started rebounding with increased young of the year production in 2017-2018, however that did not result in an increase of catchable fish, and overall abundance has been decreasing since 2018 (Figure 4). Currently, angler catch rates are well below the 1 fish/hour threshold, and abundance of age-1+ rainbow trout is the lowest on record, indicating LTEMP goals are not currently being met for the rainbow trout fishery (Healy et al. 2025).

@@ Line 22: / Line 22: @@
 SUBJECT: Decision Regarding Spring 2026 HFE <br>
 Dear AMWG, TWG and Planning and Implementation (PI) Team Members,  <br>
-I am writing to notify you that Reclamation will not implement a spring High Flow Experiment (HFE) from Glen Canyon this month. This decision reflects the mixed support expressed by members of the PI Leadership Team. In accordance with the revised HFE protocol, the sediment mass balance will roll over into the next accounting period beginning July 1, 2026, and an HFE will be reconsidered for potential implementation in fall 2026.
+'''I am writing to notify you that Reclamation will not implement a spring High Flow Experiment (HFE) from Glen Canyon this month.''' This decision reflects the mixed support expressed by members of the PI Leadership Team. In accordance with the revised HFE protocol, the sediment mass balance will roll over into the next accounting period beginning July 1, 2026, and an HFE will be reconsidered for potential implementation in fall 2026.
 I would like to extend my sincere appreciation to the PI Technical and Leadership Teams, as well as the Grand Canyon Monitoring and Research Center (GCMRC), for their dedication and contributions in supporting a well-informed decision.  <br>
 Consideration of a cool mix flow remains pending, and we anticipate sharing a decision in the coming weeks. Thank you for your patience as we work through these important and complex issues.  <br>

← Older revision		Revision as of 19:00, 19 August 2026
Line 65:		Line 65:

	Table 1. Technical position of Planning and Implementation Technical Team members for consideration of a spring 2026 High-Flow Experiment.		Table 1. Technical position of Planning and Implementation Technical Team members for consideration of a spring 2026 High-Flow Experiment.
−
−	Entity DOI-BOR Recommendation/Position Support DOI-NPS DOI-FWS Support Support DOI-USGS DOI-BIA Abstain No Vote DOE-WAPA Arizona Support Support California Colorado Abstain Support Nevada New Mexico Abstain Support Utah Wyoming Support Oppose AZGFD UCRC Support Oppose

	{\| class="sortable wikitable" style="text-align:center;"		{\| class="sortable wikitable" style="text-align:center;"

@@ Line 27: / Line 27: @@
 Wayne G. Pullan, Director  <br>
 Upper Colorado Basin Region  <br>
 === Recommendation Summary ===
 === Background and Purpose of the Recommendation ===

@@ Line 285: / Line 285: @@
 ===[https://gcdamp.com/index.php?title=Riparian_Vegetation Native Riparian Vegetation]===
+A spring HFE is not expected to pose any direct additional risks to native riparian vegetation, however a June-timed HFE followed by relatively steady flows could lead to germination of tamarisk in addition to native species. Spring HFEs (March – May) have the greater potential to benefit regeneration of native species from seed, as they correspond closely with the natural flow regime and germination period of some native species, such as cottonwoods and willows. Maintenance of sandbars and sediment resources maintains unique habitats for native riparian plant species, and therefore also promotes unique native riparian plant communities (Palmquist et al. 2025). Higher annual peak flows that inundate the floodplain accompanied by sand deposition may maintain more non-vegetated surfaces, which is desirable for maintaining natural river processes and recreation. Spring HFEs may also be increasing native species richness and contributing to maintaining more native plant species cover than nonnative cover (Palmquist and others, 2026). Tamarisk seed throughout the summer and could germinate with a June HFE. Historically, higher summer daily fluctuating flows eroded seedlings from the shoreline. If fairly stable flows follow a June HFE, it is possible that tamarisk seedlings could establish in addition to native species. No unacceptable risks are anticipated, however, and there are opportunities for promoting desirable riparian vegetation conditions.
 ===[https://gcdamp.com/index.php/GCDAMP_Glen_Canyon_Dam Water Delivery - Monthly, Daily, and Hourly Releases]===
+The spring HFE considered by the PI Team would occur in June without changing planned monthly release volumes near 460kaf according to the April 24-Month Study Webinar on April 17, 2026. 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 (projections of Lake Powell Elevations less than 3525 ft. within 12 months under most probable hydrology) was met. The projected likelihood of hitting the Interim Guidelines (IG) Supplemental Environmental Impact Statement (SEIS) 6(e) trigger (DOI 2024a) increased in the winter months due to low snowpack in the Upper Colorado basin. The LTEMP SEIS 6(e) requires operational adjustments in the Upper Colorado River basin when Lake Powell is projected to drop below elevation 3525 ft within 12 months, triggering modified monthly release volumes rather than changes to the annual release. In April, IG LTEMP 6e triggers were met based on the latest April Colorado River Mid-term Modeling System (CRMMS) results presented on April 17, 2026. Based on the monthly volume adjustments in December 2025 and proposed volume adjustments in April 2026, Reclamation recommends consideration of a HFE in June 2026, using the current proposed release (~ 460 kaf) anticipated from June 2026 and not reallocating water from other months. Any HFEs proposed in the spring of 2026 do not change annual flow releases. A spring HFE is expected to change elevation on a daily/hourly basis since required movement of water is restricted to the month of June. The End-of-Month elevation would remain the same as estimated without an HFE. If there is a reduction it will be temporary and will equalize by the end of June.
 ===[https://gcdamp.com/index.php?title=WATER_QUALITY Water Quality]===
+A spring HFE is not expected to pose any direct adverse effects to water quality either in Lake Powell or in the Colorado River. Glen Canyon Dam River Outlet Works (ROWs) withdraw water from deeper in the reservoir than the penstocks. As such, releasing water from the ROWs during an HFE temporarily reduces water temperatures, increases dissolved oxygen concentrations, and increases concentrations of dissolved nutrients. During an HFE, water temperatures in the Glen Canyon reach can be around 1-2°C cooler compared to normal operations when the bypass tubes are not in use. Dissolved oxygen concentrations in the Glen Canyon reach during an HFE increase because the bypass tubes effectively aerate the water. Biologically available phosphorus concentrations have been increasing in the last year and are currently ~0.003 to 0.008 mg/L; this represents a moderate value relative to historical values. By releasing water from lower in the reservoir using the bypass tubes, an HFE would temporarily increase phosphorus concentrations; phosphorus concentrations are higher near the bypass tube elevation compared to the penstock elevation. For example, the 2018 HFE increased Glen Canyon’s biologically available phosphorus concentration by ~ 0.001 mg/L from 0.003 to 0.004 mg/L. However, this effect of elevated phosphorus would be during the duration of the HFE, and an HFE is unlikely to cause any longterm changes in phosphorus concentrations or loading to the Colorado River. Effects to downstream resources of an HFE arising from changes in phosphorus would therefore be negligible.
 ===[https://gcdamp.com/index.php/The_-12_Mile_Slough -12 Mile Slough]===
+A spring HFE would not pose any adverse effect to the slough modification. Construction of the slough channelization by the NPS and Reclamation finished June 2, 2025. This project channelized the slough at river mile –12, and vegetation restoration and monitoring will be done in following years. The project reduced suitable habitat for reproduction of smallmouth bass and other warm water high-risk species at this location and reduced the likelihood of downstream expansion and establishment in Glen, Marble, and Grand Canyons. Details of the slough modification efforts can be found here: ParkPlanning - Warmwater Nonnative Fish Management at the -12-Mile Slough. An evaluation of the modified cobble bar and channel was conducted in May 2025 and confirmed that it should withstand an HFE. Glen Canyon National Recreation Area considers an HFE to be the final contour shaping event prior to active restoration of vegetation (Personal Communication Bud Fazio). Some vegetation growth is returning to the slough site naturally, but further vegetation restoration of the slough has been postponed in anticipation of a potential spring HFE. In order to maintain a cool flow through the modified slough a minimum flow of 6,800cfs has been suggested by National Park Service. This would allow the slough modification to maintain its goal of cooler, flowing water to help limit nonnative spawning and refuge. Current projects of would put maintaining that flow at risk.
 ===[https://gcdamp.com/index.php/GCDAMP_Glen_Canyon_Dam Dam Maintenance and Operations]===
+A spring HFE is not expected to pose any direct adverse effects to dam maintenance or operations at Glen Canyon Dam as long as bypass releases are adjusted to avoid cavitation. The April 2023 HFE resulted in cavitation to the bypass tubes and provided guidance for future releases under low reservoir conditions (DOI 2024d). In 2024-2025 the bypass tubes were recoated to protect the bypass tube for future operations. The bypass tubes have been used since that recoating and evaluation has shown they have mainatined operational. Furthermore, an electrical outage occurred on units 3 and 4 at the Glen Canyon Dam on September 25, 2025. It is unlikely that units 3 and 4 will be available for use during the HFE implementation window. Due to the maintenance issue, the PI Technical Team considered an updated magnitude for a spring HFE that reflected the decreased release capacity. The new conditions considered were a magnitude of 32,400 cfs and up to a 96-hour duration. After Glen Canyon ROW operating restrictions (DOI 2024d), the estimated maximum available flow for each bypass is 3,150 cfs (total maximum bypass = 12,600 cfs). Further investigations of dam maintenance are ongoing, but a spring HFE is not expected to have any significant effects to dam maintenance or dam operations.
 ===Other LTEMP Experimental Flows (i.e., [https://gcdamp.com/index.php/SEIS_Bypass_Releases Smallmouth Bass Flows])===
+A spring HFE is not expected to pose any direct impacts on the ability to conduct Smallmouth Bass Flows in the summer of 2026. The LTEMP SEIS modeled for the possibility of both an HFE and Smallmouth Bass Flow occurring in the same Water Year. Impacts to post-HFE sandbars would depend on the Smallmouth Bass Flows implemented and therefore would be further analyzed dependent on the HFE recommendation, and which Smallmouth Bass Flow may be considered. In 2024 and 2025, cool-mix flows did not appear to impact sandbar building since volumes of water released was the same as normal operations. It has been suggested that other Smallmouth Bass Flows (e.g., nonbypass flows) could have an impact to the sandbars but uncertainty remains. In Planning and Implementation Team discussions there were concerns that a spring HFE could influence the availability of daily flows both before and after the spring HFE within the month of June. An HFE would result in redistribution of the daily releases to accommodate an HFE. These adjustments will depend on the duration of the HFE. The flows before and after the HFE would be lower with an HFE compared to without an HFE. This may influence potential SMB flow operations but this was not explored in detail at this time. Further discussion on this may occur in the Planning and Implementation meeting as the team gathers more information.
 == GCDAMP and LTEMP PA Consultation ==
+On March 3, 2026, the required 30-day advance notification and offer for consultation were sent electronically to the Tribes and Parties to the LTEMP Programmatic Agreement regarding the potential for a High-Flow Experiment. A follow-up notification will be sent electronically to the Programmatic Agreement signatories, including Tribes, following the Department’s decision regarding the potential High-Flow Experiment. As of March 6, 2026, no tribes have asked for a consultation.  Notification of the potential for a LTEMP sediment-triggered spring HFE was communicated to Glen Canyon Dam Adaptive Management Program (GCDAMP) stakeholders on April 9, 2026, through the GCDAMP AMWG/TWG webinar (See Appendix I). A follow-up email will be distributed to the GCDAMP stakeholders informing them of the final recommendation.
 ==Conclusion ==
+Determining whether to recommend an HFE required coordination and effective communication among the technical staff of multiple agencies. The PI Technical Team relied heavily on and would like to acknowledge the contributions of staff from GCMRC, Reclamation, and WAPA in making this recommendation. The PI Technical Team has thoroughly evaluated the issues discussed above and has taken into consideration the information and analyses included in the 2016 LTEMP FEIS, and ROD and the 2024 LTEMP SEIS ROD. The PI Technical Team’s recommendation regarding implementation of a spring 2026 HFE is based on a careful assessment of resources and best available science. By a consensus/majority vote, the PI technical Team does/does not recommend the Department of the Interior conduct a High-Flow Experiment in the spring of 2026.
 == SUPPORTING DOCUMENTS ==

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 == SUPPORTING DOCUMENTS ==
+*[[ Media:2026_Spring_HFE_Tech_Report.pdf | 2026 Spring HFE Tech Report]]
-−
 == ADDITIONAL INFORMATION==