A 2024 Fall HFE

High Flow Experiment- 2024

2024 Fall High Flow Experiment Snapshot

Recommendation Summary

The purpose of this memorandum is to transmit a recommendation to the Glen Canyon Dam Planning and Implementation (PI) Leadership Team. Once the PI Leadership Team has decided on a recommendation this memo will be forwarded to the Department of the Interior (Department) in accordance with the LTEMP Record of Decision (ROD). 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).

Based on the LTEMP ROD and LTEMP Supplemental Ecological Impact Statement (SEIS) ROD, the PI Technical Team has worked over the past couple weeks to evaluate existing information and data in determining this recommendation regarding a HFE at Glen Canyon Dam. 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.” Sand Transport/Budget Model results indicate that there is sufficient sediment to conduct a fall HFE or defer to implementation to spring. After evaluating the LTEMP resources using the best available science and discussions with PI technical team, most technical team members recommended deferring to a spring HFE and no member recommended a fall HFE. The following provides recommendation language, technical team considerations, and a summary resource impacts for the PI Leadership team to consider.

The Secretary of the Interior and/or her 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. The PI Team incorporated the most current science and data and considered multiple issues with agency experts, as summarized below, in making this final recommendation. Below you will find brief comparisons between a fall and spring HFE and how each HFE would affect other resources.

Planning/Implementation (PI) Leadership Team Recommendation

The leadership team recommends to not conduct a fall HFE and that the Planning & Implementation Team reconvene in spring 2025 to review and provide a final recommendation for a spring High-Flow Experiment using the sediment accounting window from July 1, 2024- June 30, 2025.

Justification:
Because the trigger for an HFE was met in the fall of 2024, and the technical team’s preference is to defer a final recommendation to the spring, the Glen Canyon Dam Adaptive Management Program will plan as if a spring HFE will occur to allow for any preparations to be made. This includes consideration of adjustments to monthly release volumes prior to the spring HFE, allocation of monitoring resources, adjustments to maintenance schedules, and other resource considerations.

Considerations to be discussed during the decision to conduct a 2025 spring HFE.

• If a spring HFE were to occur (April - June), that we plan to utilize water from the spring shoulder months (e.g., March and April) as necessary, to support the HFE volumes.
• Use the most current data available and not rely so heavily on prediction tools.
• Gather the most up to date knowledge of infrastructure status and the results of the current work on the river outlet works (ROW).
• Have a better understanding of resource(s) status, including those laid out in the LTEMP, but also the elevation and reservoir volumes in the spring 2025.
• Have more data available to best explain current monitoring results, impacts on resources, cold water flow intentions, long term tool status as laid out in the Strategic Plan.
• Technical analysis for the final recommendation includes projections of Lake Powell elevations (with and without an HFE) under the most recent 24-month study projections, and includes an updated assumption about monthly holdback volumes based on actual operational guidance.

Future Considerations for the PI Process:

• Future use of the PI Team process as it has been utilized in the past, including the consideration of a fall experiment in the fall and spring experiment in the spring.
• Engage in the PI Technical Team in a timeframe that allows for full consideration and discussion regarding potential resource impacts.
• Public webinars be held by Reclamation to gain feedback regarding experiments being considered.

Resource Considerations

HFE Sand Budget Model

A sand transport/budget model (Wright et al. 2010) was used to predict the mass of sand that would be transported by an HFE and to estimate if a potential HFE would transport more or less sand than had been delivered from the Paria River to the Colorado River since the start of the accounting period on July 1, 2024. Only HFE durations that resulted in a positive sand mass balance were considered, based on the lower uncertainty bound of Paria River sand inputs. Under the LTEMP SEIS, an HFE may be implemented in fall, or deferred to spring. Therefore, we modeled and compared fall and spring implementation. Output of the modeling runs provides the initial recommendation for the duration of the HFEs, given HFE magnitudes based on dam release constraints (Figure 1). The model indicated that sufficient sand is available for a 72-hour HFE in fall, or 60-hour HFE in spring.

HFE Sand Budget Model Assumptions:

HFE Magnitude

• Powerplant: 26,900 cfs (8 units)
• Bypass: 13,500 cfs (4 ROW)

Flow

• Observed Glen Canyon Dam Releases
• Projected Glen Canyon Dam Hourly Releases from April 24 Month Study
• HFE volume redistributed across the months November 2024 to September 2025

Sediment

• Observed Paria River at Lees Ferry
• Observed Colorado River nr RM30

Fall High-Flow Event Being Considered

• Magnitude - 40,400cfs (All effort will be made to maximize discharge)
• Duration - 72hrs.
• Timeframe – week of November 4th

Spring High-Flow Event Being Considered

• Magnitude - 40,400cfs (All effort will be made to maximize discharge)
• Duration - 60hrs.
• Timeframe - week of May 12th (with flexibility within April-June if needed to achieve magnitude)

In addition to reviewing the sand budget model output, the PI Team assessed the status of the LTEMP resources and the potential effect of an HFE on these resources in making the recommendation described here.

Assessment of Resources

In coordination with the GCMRC, the PI Technical Team completed an assessment of key resources that may be impacted or affected by an HFE. This assessment is based on the current condition of resources and on findings and observations from fall HFEs conducted in 2012, 2013, 2014, 2016, 2018 and spring 2023. Key resources were evaluated relative to the timing, duration, and magnitude of the potential fall 2024 and spring 2025 HFE using the best available science. This section summarizes the assessment of resources and expected effects of the potential HFE. All resources will be reassessed in the spring of 2025 to determine a final recommendation of a spring HFE.

Archaeological and Cultural Resources

Previous research has documented that sandbars are important sand sources that resupply sediment to dunefields and terraces along the Colorado River containing archaeological sites via aeolian transport (Sankey et al. 2018). HFEs replenish these sand sources, and through subsequent wind transport of HFE sediment, help to offset erosion at archaeological sites caused by rainfall-runoff and wind deflation. A deferred spring HFE in 2025 would not pose any direct additional risks to archaeological and cultural resources. However, if deferred to the spring, modeling indicates that a spring HFE would result in 30% less HFE sediment deposition and coarser sediment deposition, both of which would be potentially less beneficial to maintaining and sustaining archaeological sites in situ over the long term.

Natural Processes (Flow Regime)

A deferred spring HFE in 2025 would not pose any direct additional risks to Natural Resources (Flow Regime). It is hypothesized that a spring HFE would mimic a more natural flow disturbance (pre-dam peak flow) then a fall HFE. A more natural flow regime could benefit ecological resources within the river system.

Natural Processes (Aquatic Food Base)

A deferred spring HFE in 2025 would not pose any direct additional risks to Natural Resources (Aquatic Food Bases).

Humpback chub (Gila cypha) and Native Fish

A deferred spring HFE in 2025 would not pose any direct additional risks to Humpback Chub or Native Fish. In the PI Technical team discussions, it was suggested that a spring HFE may be more aquatic ecosystem benefits 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

A deferred spring HFE would not pose any consequential risks to hydropower. If a spring HFE (60 hours in May) would be conducted it would cost between about $0.9 and $1.4 million, assuming that water will be taken from earlier months (March/April). However, if water volumes for a spring HFE are taken out of summer months (after the HFE, June/September) the cost may increase substantially, initial estimates are roughly 50% increase, but WAPA has not specifically modeled the 60-hour spring HFE under this scenario. A potential HFE in June has not been fully modeled yet. Note these values are preliminary and subject to additional modeling.

Basin Fund current status is below WAPA’s target balance but at a level that will likely support operations through FY2025. The basin fund has significant risks beyond FY2025, due primarily to capital expenditures for aging infrastructure and the Smallmouth Bass experimental bypass flows from 2025 through 2027. These expenditures could jeopardize the Basin Fund if no other funding source is identified.

Recreational Experience

A deferred spring HFE would not pose any direct effects to the recreational users. The HFE should benefit recreational users through improvements to camp areas and vegetation reduction in the near shore habitat. However, it should be noted that a fall HFE was predicted to build larger sandbars than a spring HFE and therefore have greater recreational benefit than a spring HFE.

Sediment

The best course of action given the published peer-reviewed sediment science would be to conduct a fall HFE. Without additional sand inputs, deferring to a spring HFE would decrease sandbar building. A fall HFE would allow for the greatest sandbar building for FY2025 within the Colorado River below Glen Canyon Dam compared to a spring HFE. The Mueller and Grams (2021) Sandbar Model was used to predict sandbar volume following HFE implementation in fall vs. spring. The model predicts that sandbars built during a November 2024 HFE would be ~ 30% larger than those produced during an HFE conducted in April 2025, relative to sandbar volume if no HFE were run (Figure 2). This comparison is based on the June 30, 2025, sandbar volume, and therefore takes into account the sandbar erosion that is likely to occur between November 2024 and June 2025. This assumes no additional sediment input between now and April. Roughly 50,000 Mg of sand would be needed to increase the spring HFE duration from 60 hours to 72 hours; an analysis of historical Paria River sand loads indicates that this amount of sand input during the November to March period occurs around 40% of the time. Deferring the HFE to spring would result in coarser bed sediment, reducing sand concentrations during the HFE and therefore reducing sandbar growth. Additionally, deferral may affect the grain size of the fine sediment that will be deposited by the HFE, which will affect the longevity of the resulting HFE deposits, the nutrient content of those deposits, the types and abundance of plants that will subsequently grow on those deposits, and the habitat that will result (Topping et al. 2019).

Figure 2. Modeling for Sandbar Building for the proposed fall 2024 High Flow Experiment and a spring 2025 High Flow Experiment. Subject to minor revision.

Rainbow Trout (Oncorhynchus mykiss) Fishery

A deferred spring HFE would not pose any direct additional risks on the Rainbow Trout fishery. If a spring HFE were to be conducted it could have a temporary and direct effect of the HFE on angling access that are described in the recreation section. There would be no changes to angling access if a fall HFE is conducted. It is hypothesized that the effects to Rainbow Trout may be neutral to slightly positive to spring, dependent on exact timing.

Nonnative Invasive Species - Green Sunfish (Lepomis cyanellus)

A deferred spring HFE 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.

Nonnative Invasive Species- Brown Trout (Salmo trutta)

A deferred spring HFE 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. There is potential of a fall HFE leading to upstream movement of Brown Trout (Healy et al. 2023).

Nonnative Invasive Species – Smallmouth Bass (Micropterus dolomieu)

A deferred spring HFE 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. Additionally, entrainment is likely not a risk due to cool water flows and high elevations in Lake Powell

Nonnative Invasive Species – Other Fishes

A deferred spring HFE would not pose any direct additional risks from other Nonnative Invasive Species. A 2025 spring HFE does not pose unacceptable risks to endangered and/or native fishes. Additionally, entrainment is likely not a risk due to cool water flows and high elevations in Lake Powell.

Riparian Vegetation

A deferred spring HFE would not pose any direct additional risks from Nonnative Invasive Species. 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, willows, and ashes; however, the loss of fine sediment over the winter may have negative impacts on marsh/obligate wetland species, which have already declined in the last several decades. A spring HFE is unlikely to significantly impact riparian vegetation resources over the long term and no unacceptable risks are anticipated.

Water Delivery - Monthly, Daily, and Hourly Releases

A deferred spring HFE would not pose any direct additional risks to Water Delivery. A spring HFE does not pose unacceptable risk to Lake Powell elevations. A deferment to the spring would allow for additional time for planning water delivery.

Water Quality

A deferred spring HFE would not pose any direct additional risks to Water Quality. The bypass tubes withdraw water from deeper in the reservoir than the penstocks. Releasing water from the bypass tubes during an HFE in general temporarily cools water temperatures and increases dissolved oxygen concentrations.

Slough

A deferred spring HFE is not expected to cause any direct additional risks to the slough modification. Depending upon Slough Environmental Assessment decision and construction schedule, slough modification may be completed before this spring HFE.

Dam Maintenance and Operations

Timeframe suggested is based on dam maintenance schedule that allows for maximum discharge at the dam. This includes the use of eight penstocks and four river outlet works. Current maintenance schedule is targeting the week of May 12th, but there is greater uncertainty that far into the future.

Safety Considerations: Recreational Safety

Not needed at this time. We will address this issue in the spring.

Safety Considerations: Research and Monitoring

Not needed at this time. We will address this issue in the spring.

COMPLIANCE

Notification of the potential for a LTEMP sediment-triggered fall or spring HFE was communicated to Glen Canyon Dam Adaptive Management Program (GCDAMP) stakeholders at the GCDAMP AMWG meeting in August, 2024, though the final decision 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 and decision to defer until a spring HFE. Representatives from the Basin States participated in the development of this recommendation.

On October 9, 2024, 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.

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