Difference between revisions of "A 2025 Spring HFE"

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High Flow Experiment- 2025

2025 Spring High Flow Experiment Snapshot

Recommendation Summary

The purpose of this memorandum is for the Planning and Implementation (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 Record of Decision (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).

Under the new HFE protocol, the LTEMP SEIS ROD allows for a year-long accounting period that begins on July 1st each year and includes HFE implementation windows in October-November or March-June. The LTEMP SEIS ROD states that “If sufficient sediment is available in the fall, the PI Team may recommend conducting the fall HFE release or deferring implementation to the spring implementation window. Prior to the spring implementation window, the planning and implementation process will again be used to provide a recommendation on the duration, magnitude, and timing of the spring HFE release.” Furthermore, the LTEMP SEIS states that “if a sediment trigger is met, but an HFE release is not implemented in the fall or spring, rollover of sediment into the next accounting period would be possible.” In the fall of 2024, the Sand Transport/Budget Model results indicated that there was sufficient sediment to conduct a fall HFE or defer implementation to the following spring. After evaluating the LTEMP resources using the best available science and discussions with PI Leadership Team, most PI Leadership Team members recommended deferring to a spring 2025 HFE and no member recommended a fall 2024 HFE. In the spring of 2025, the PI Technical Team reevaluated the Sand Transport/Budget Model and determined that there was sufficient sediment to conduct a spring 2025 HFE or defer implementation to the fall of 2025. 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. A member of the PI team suggested looking at the possibility of a Power Plant Only (PPO) flow. In response is a brief comparison between a spring HFE and PPO flow and the likely affects these flows would have on resources below the dam.

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 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 Record of Decision (ROD) and other binding obligations for Glen Canyon Dam.

Planning/Implementation (PI) Leadership Team Recommendation

The PI Technical Team is recommending to the leadership team to not conduct a High-Flow Experiment (HFE) in the Spring 2025.

Justification for this recommendation is described below.

The LTEMP SEIS ROD states, “If a sediment trigger is met, but an HFE release is not implemented in the fall or spring, rollover of sediment into the next accounting period would be possible.” Given the sediment trigger was met but deferred in fall of 2024, and the current recommendation to forgo a spring 2025 HFE, approximately 522,000 ± 93,000 metric tons of sediment will be rolled over into the new accounting period starting on July 1, 2025.

The PI Technical Team will reconvene in late summer or early fall 2025 to assess resource status and provide a recommendation for a possible fall 2025 HFE.

Justification and Considerations for Recommendation

-12 Mile Slough - Uncertainty surrounding whether a HFE would cause negative effects to the recently modified slough at River Mile -12 mile is a major concern for the PI Technical Team. The modification of the slough to limit Smallmouth Bass spawning habitat is a priority of the Glen Canyon Dam Adaptive Management Program’s Nonnative Strategic Plan. The current construction schedule for the slough is for the modifications to be completed within a few weeks of a proposed Spring 2025 HFE. It unknown whether the stability of these modifications could withstand a HFE and thereby limit Smallmouth Bass from spawning. By not implementing a spring HFE it would provide an opportunity for the slough modifications to stabilize over the summer and provide an opportunity to monitor the effectiveness of the modifications.

Rainbow Trout Fishery – A May 2025 HFE could have negative effects on an already struggling Rainbow Trout fishery. Deferring until fall 2025 would give Young-of-Year Rainbow Trout the opportunity to grow to sizes that could withstand an HFE. Additionally, there would be a reduced effect on economic angling opportunities if a HFE was implemented in the fall vs. the spring.

Hydropower – A spring HFE would have a cost to hydropower. By not running a spring HFE it will limit the cost to hydropower and allow a reevaluation of the resource for the fall HFE implementation window. According to estimates put together by Western Area Power Administration (WAPA), it could cost WAPA near $1,300,000 if a spring HFE would be conducted. This money could remain in the Basin Fund and be allocated towards other planned projects for FY2025.

Dam Maintenance – Currently maintenance is being conducted on the River Outlet Works (ROW). By deferring a spring 2025 HFE it will allow maintenance on the ROWs to continue without the disruption on conducting an HFE. This maintenance would be completed by the time a fall 2025 team process begins.

Table 1. Technical position of Planning and Implementation Technical Team members for consideration of a spring 2025 High-Flow Experiment.

Resource Considerations

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). An HFE duration was considered to have a positive sand mass balance if the lower uncertainty bound of the sand mass balance is positive at the end of the HFE. Multiple scenarios were analyzed (Figure 1), and modeling indicated that with a maximum available release of 41,900 cubic feet per second (cfs), a 60-hour High Flow Experiment (HFE) can be conducted while still ensuring a positive mass sand balance. Other scenarios considered include a Powerplant Only (PPO) flow of 25,000 ft3/s, and a no HFE scenario. These scenarios are consistent with the new protocols associated with the 2024 Long-Term Experimental and Management Plan (LTEMP) Supplemental Environmental Impact Statement (SEIS) Record of Decision (ROD). The modeling results presented below serve as the initial recommendation for the duration of the spring HFE, based on the magnitudes constrained by dam release parameters (Figures 1,2,3).

The following assumptions were made for the Sand Budget Model:

60-hour High-Flow Experiment Magnitude

• Powerplant: 26,900 (8 units)
• Bypass: 15,000 cfs (4 ROW)

96- hour Powerplant Only Magnitude

• Powerplant: 25,000 cfs (8 units)
• Bypass: 0 cfs (4 ROW)

No HFE Magnitude

• Normal operations
• Estimated ~522,000 ± 93,000 metric tons roll over

Flow

• Observed Glen Canyon Dam Releases
• Projected Glen Canyon Dam Hourly Releases from January 24 Month Study
• HFE volume redistributed across the months of March 2025 (49 thousands of acre-feet[kaf]) to be released in May/June

Assessment of Resources

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 HFE and PPO flows on these resources in making the recommendation described here.

In coordination with the US Geological Survey Grand Canyon Monitoring and Research Center (GCMRC), WAPA, Reclamation, the PI Technical Team completed an assessment of key resources impacted or affected by an HFE and PPO flow. 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 spring 2025 HFE or PPO flow using the best available science. This section summarizes the assessment of resources and expected effects of a potential HFE or PPO flow.

Sediment

Spring HFE: Although a spring HFE using sand supplied by the Paria River during the prior summer would result in coarser bed sand, reducing sand concentrations during the HFE and therefore reducing sandbar growth (Topping et al., 2019), the amount of carried-forward sand predicted to be available for an HFE in late May 2025 is sufficient for a 60-hour HFE with a peak of 41,900 cfs. Based on previous sand-enriched HFEs, the proposed May 2025 HFE is predicted to result in general sandbar gains in Marble and Grand canyons above the stages associated with both 8,000 cfs and 25,000 cfs (after U.S. Geological Survey, 2025; Figure 3).

Spring PPO: Based on the surveyed sandbar response during three 30,000–31,000 cfs powerplant capacity releases conducted in 1997 and 2000 (U.S, Geological Survey, 2025), a 96-hour, 25,000 cfs, PPO flow would likely result in net erosion of the high-elevation sand (that is, the sand above the stage associated with 25,000 cfs) in sandbars in both Marble and Grand canyons. This erosion would occur by lateral bank retreat into the existing sandbars. Moreover, based on these same surveys, a 96-hour 25,000 cfs PPO flow would result in either slight net deposition or slight net erosion of sand above the stage associated with 8,000 cfs. These sandbar responses stand in contrast to those expected during a larger sand-enriched HFE, where longer-lasting gains in sandbar volume above the stages associated with both 8,000 cfs and 25,000 cfs have been generally observed (U.S. Geological Survey, 2025). Thus no benefit is expected and negative effects to sediment resources are likely under an HFE with a peak discharge of only 25,000 cfs.

Archaeological and Cultural Resources

Spring HFE: Previous research has documented that sandbars are important sand sources that resupply sediment via aeolian transport to dunefields and terraces along the Colorado River containing archaeological sites (Sankey et al. 2018). High-Flow Experiments of sufficient magnitude under sediment-enriched conditions can replenish these sand sources, and through subsequent wind transport of HFE sediment, can help to minimize and offset erosion at archaeological sites caused by rainfall-runoff and wind deflation. In Glen Canyon, upstream of the Paria River, an HFE with a magnitude of 41,900 ft3/s over 60 hours is likely to erode terrace scarps located immediately adjacent to the river margins, which could potentially impact archaeological sites embedded within those terraces. In Marble Canyon and Grand Canyon, however, where sediment is resupplied via the Paria River and other tributaries, an HFE with a magnitude of 41,900 ft3/s over 60 hours is likely to rebuild and expand many sandbars that serve as source areas for wind-blown sand (see sediment section below), increasing the potential for the bars to resupply sediment to archaeological sites and thereby reducing erosion and increasing protection for archaeological and cultural resources within the Grand Canyon.

Spring PPO: A springtime flow of 25,000 ft3/s without supplementary bypass is likely to erode and reduce the size of many sandbars that serve as source areas for wind-blown sand (see sediment section below), thereby reducing the potential for the bars to resupply sediment to archaeological sites and increasing the erosion potential at archaeological sites in the long run. This expected outcome contrasts with that of a spring HFE with bypass, which is more likely to rebuild high elevation sand bars and thereby reduce erosion and increase protection for archaeological and cultural resources within the Grand Canyon.

Natural Processes (Flow Regime)

Spring HFE: A spring HFE in 2025 could benefit ecological resources within the river system as a spring HFE would mimic a more natural flow disturbance (pre-dam peak flow) and more closely align with the goals for this resource (Palmquist et al. 2024). It would not pose any direct additional risks to Natural Resources (Flow Regime).

Spring PPO: A spring Powerplant Only flow in 2025 would not pose any direct additional risks to natural resources (Flow Regime). A spring PPO flow with no bypass does not mimic a more natural flow disturbance (pre-dam peak flow), because the peak flow is not large enough to be similar to a natural flood peak (Palmquist et al. 2024).

Natural Processes (Aquatic Food Base)

Spring HFE: A spring 2025 HFE would not pose any direct additional risks to natural processes (Aquatic Food Base) and may benefit the aquatic food base. Flood disturbance can help maintain native biological diversity in rivers including that of aquatic invertebrates, which represent the food base for fish and other wildlife. For example, a national synthesis of flow and biological data from over 700 streams and rivers in the lower 48 states found that intact and healthy communities of native aquatic invertebrates were most often present where flood disturbance still occurred, and where flood timing was seasonally appropriate (i.e., similar to the natural condition; Carlisle et al. 2017).

Spring PPO: A spring PPO flow in 2025 would not pose any direct additional risks to Natural Processes (Aquatic Food Base). Although a spring pulse flow with no bypass would mimic the natural timing of flow disturbance in the Colorado River, the peak flow would be small compared to natural flood peaks and is therefore less likely to enhance diversity of native aquatic invertebrates (Carlisle et al. 2017).

Humpback chub (Gila cypha) and Native Fish-Spring

HFE or PPO: A spring 2025 HFE or PPO flow would not pose any direct additional risks to Humpback Chub or Native Fish. In the PI Technical Team discussions in the fall of 2024, it was suggested that a spring HFE may have more aquatic ecosystem benefits due to historic peak timing. Humpback Chub recruitment in Western Grand Canyon was high in 2023, but there have been other high recruitment events in years without spring HFEs (evidence is needed to further test this hypothesis).

Hydropower and Energy

Spring HFE: A spring HFE in 2025 would not pose any direct additional risks to Hydropower and Energy. Some additional costs would be incurred due to the need to purchase replacement power. A spring HFE (60 hours in May) is estimated to cost near $1,300,000 via GTMax.

Spring PPO: A spring PPO flow (96 hours in May) is estimated to cost near $81,000 via GTMax.

Recreational Experience

Spring HFE: A spring HFE would likely have direct positive effects to recreational users. The HFE should benefit recreational users through improvements to campsite areas and vegetation reduction in the near shore habitat.

Spring PPO: A spring PPO could pose direct negative effects to recreational users by the lateral erosion of high elevation sand (above 25,000 cfs) reducing campsite areas and increasing the potential for vegetation encroachment.

Rainbow Trout (Oncorhynchus mykiss) Fishery

Spring HFE or PPO: If a spring HFE or PPO were to be conducted it could have a temporary and direct effect on angling access. The longer the HFE or PPO lasts, the greater the disruption to angling and guiding for the Rainbow Trout fishery during their peak time of year. Rainbow Trout are most vulnerable during the eggs stage and for a couple of weeks after emergence when they are transitioning to exogenous feeding. Peak hatch date occurs early May making early June peak emergence date. High velocities have been found to reduce survival of recently emerged trout and salmon fry. A May-timed HFE would have a negative impact on Rainbow Trout. This is especially a concern due to the declining Rainbow Trout fishery since 2014 (USGS-GCMRC Personal Communication). Studies of natural floods on recently emerged fry have been completed in smaller streams/creeks and its uncertain whether the higher velocities of an HFE are substantive enough to sweep fry downstream and increase their mortality.

Drift data from 2008 (Rosi-Marshall et al. 2010) showed that the spring HFE initially reduced the drift of trout food (midges, black files, and scuds) after the flood, but then caused recovery 4-6 weeks later of midges and black flies to much higher levels than pre-flood abundance. Even if HFE velocities are not large enough to directly impact recently emerged fry, a spring HFE in May would reduce food availability during a critical time for feeding (trout and salmon fry only have a few weeks to feed on their own after the yolk sac is absorbed.).

Nonnative Invasive Species:

Green Sunfish (Lepomis cyanellus) -Spring HFE or PPO: A spring HFE or PPO would not pose any direct additional risks from Green Sunfish dispersal. Green Sunfish have become more common and widely distributed in the Grand Canyon and high flows in the spring may also contribute to dispersal, so the risk to Humpback Chub is similar with or without a spring HFE or PPO.

Brown Trout (Salmo trutta) -Spring HFE or PPO: A spring HFE or PPO would not pose any direct additional risks from Brown Trout. No unacceptable exacerbation of risk to brown trout is expected and no unacceptable risks are anticipated for a spring HFE or PPO.

Smallmouth Bass (Micropterus dolomieu) -Spring HFE or PPO: A spring HFE or PPO would not pose any direct additional risks from Smallmouth Bass. A spring HFE does not pose unacceptable risk to exacerbating the spawning, migration, and/or establishment of Smallmouth Bass below Glen Canyon Dam. Smallmouth Bass minimal spawning temperatures (~16oC) are not predicted to be reached until early July and adult Smallmouth Bass are capable of withstanding swift currents thus limiting the potential for spawning and distribution downstream. Additionally, entrainment is likely limited, but still possible, due to cool water flows and high elevations in Lake Powell preventing the epilimnion, where most nonnative fish species including Smallmouth Bass reside, from interacting with the intake of the penstock. Currently Lake Powell is near 3560 acre-feet and predicted to be near 3575 acre-feet by June according to the latest CRMSS projection. Fish entrainment is thought to be at high risk when elevations reach 3540 acre-feet or below (Eppehimer et al. 2024).

Other Nonnative Invasive Fishes -Spring HFE or PPO: A spring HFE or PPO would not pose any direct additional risks from other Nonnative Invasive Species. A 2025 spring HFE or PPO does not pose unacceptable risks to endangered and/or native fishes. Additionally, entrainment is likely a low risk due to cool water flows and high elevations in Lake Powell preventing the epilimnion, where most nonnative fish species reside, from interacting with the intake of the penstock.

Native Riparian Vegetation

Spring HFE: A spring HFE would not pose any direct risks to native riparian vegetation. Spring HFEs have greater potential to benefit regeneration of native species from seed, as they correspond more 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, so also promotes unique native riparian plant communities (Palmquist et al. 2025). Further, higher annual peak flows accompanied by sand deposition may be maintaining more non-vegetated surfaces, which is desirable for recreation. No unacceptable risks are anticipated, and there are opportunities for promoting desirable riparian vegetation conditions.

Spring PPO: A spring PPO flow is unlikely to pose any direct risks to native riparian vegetation but will likely have fewer potential benefits than an HFE using bypass. Spring HFEs in general have greater potential to benefit regeneration of native species from seed, as they correspond more closely with the natural flow regime and germination period of some native species, such as cottonwoods and willows. However, the peak of the PPO flow lies within the area of the shoreline that can be affected by daily fluctuations and thus erosion. Any seedlings that germinate from a spring pulse may be subsequently eroded (Bejarano et al. 2020; Porter and Kearsley 2001). Maintenance of sandbars and sediment resources maintains unique habitats for native riparian plant species, so also promotes unique native riparian plant communities (Palmquist et al. 2025). Thus, experimental flows that promote the most sandbar building and maintenance also promote the most riparian plant habitats. The lack of a benefit and chance of harm to sediment resources, then also has a chance of harm for unique riparian plants. The spring disturbance flow in 2021 did not result in any measurable change to vegetated area, so a PPO flow is unlikely to maintain or increase camping area (through a decline in vegetated area). No unacceptable risks are anticipated. Few to no benefits are expected, but research surrounding this kind of flow would provide more information on how summer pulse flows alter riparian plant communities (Butterfield et al. 2023; Butterfield and Palmquist 2024).

Water Delivery - Monthly, Daily, and Hourly Releases

Spring HFE or PPO: Water delivery for water year 2025 would not be impacted by a spring HFE. Lake Powell elevations will remain within acceptable risks within any duration spring HFE. Monthly distributions would change. A total of 49 kaf would be redistributed from March to June to accommodate a potential HFE or PPO.

Water Quality

Spring HFE: A spring HFE would not pose any direct adverse effects to water quality either in Lake Powell or in Colorado River. Glen Canyon Dam ROWs withdraw water from deeper in the reservoir than the penstocks. As such, releasing water from the ROWs during an HFE temporarily cools 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 colder compared to normal operations when the bypass tubes are not in operation. 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 long-term 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.

Spring PPO: A spring HFE would not pose any direct adverse effects to water quality either in Lake Powell or in Colorado River since it would mimic normal operations.

Slough

Spring HFE or PPO: Slough work started February 27, 2025, and is currently underway. This project will channelize the slough at river mile -12 below Glen Canyon Dam to prevent reproduction of Smallmouth Bass at this location and reduce the likelihood of downstream expansion and establishment in Glen, Marble, and Grand Canyons. Estimated time to completion is early May. If there are additional schedule delays it could conflict with the May window for an HFE. Additionally, it is unclear to what extent an HFE or PPO immediately following the completion of the slough work will impact the newly formed channel. The newly formed channel is designed to withstand a high-flow experiment in the long-term. However, due to the limited time available for the newly formed channel to stabilize after completion, there could be a short-term risk of damage, which may limit the effectiveness of the slough modification in preventing Smallmouth Bass recruitment. There will be monitoring throughout the summer to help evaluate the stability of the newly formed channel through normal operations and future experiments.

Dam Maintenance and Operations

Spring HFE or PPO: Timeframe suggested is based on dam maintenance schedule that allows for maximum discharge at the dam. This includes the use of eight penstocks and potentially four river outlet works depending on HFE or PPO. Current maintenance schedule is targeting the week of May 3rd for ROW #3 to be complete, but uncertainty still exists and schedules could change. ROW #4 would begin maintenance after an HFE or PPO has been completed. There could be potential delay and cost for demobilization in ROW #4 maintenance if an HFE or PPO could not be implemented shortly after the completion of ROW #3.

SUPPORTING DOCUMENTS

• Spring 2025 HFE Recommendation and Technical Memo (4/7/2025_DRAFT)
• P&I Team Presentation 3/20/25

COMPLIANCE

Notification of the potential for a LTEMP sediment-triggered spring HFE was communicated to Glen Canyon Dam Adaptive Management Program (GCDAMP) stakeholders on April 10, 2025, through the GCDAMP TWG meeting though the final recommendation of a deferment to a spring HFE has not yet been communicated to stakeholders. A follow-up email will be distributed to the GCDAMP stakeholders informing them of the recommendation.

On March 5, 2025, 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.

ADDITIONAL