Difference between revisions of "SEIS Bypass Releases"

2026

After careful consideration of management alternatives and evaluation of all authorized purposes and impacts, DOI made the decision to not implement Cool Mix flows in 2026. Efforts instead focused on expanded rapid response removal actions, additional backwater modifications, evaluation of innovative tools, and advancement of durable structural solutions.

2025

The Cool Mix Alternative was selected with a temperature trigger at RM 30 and an end date of October 20. Cool Mix releases started on August 3 and ran for 2 days. Bypass was stopped flows on August 5 when temperature data from RM 30 came in showing trigger was not yet reached. The trigger was officially reached on August 12 and bypass was re-started then.

2024

The Cool Mix Alternative was implemented from July 9 to November 18, 2024. The temperature trigger was at the Little Colorado River (RM 61) until downstream cooling resulted in warmer release temperatures than at RM 61. At this point the temperature trigger was moved to Glen Canyon Dam releases and the operation continued until release temperatures fell below 15.5 C. A summary report for the 2024 operation can be found at this link.

The United States (US) Department of the Interior (Department), Bureau of Reclamation (Reclamation) has published a Final Supplemental Environmental Impact Statement (SEIS) for the Glen Canyon Dam Long-Term Experimental and Management Plan (LTEMP). The 2024 LTEMP SEIS revises the 2016 LTEMP Record of Decision (ROD) to address two issues:

(1) responding to the threat of smallmouth bass below Glen Canyon Dam; and

(2) incorporating the latest sediment science to assess when high-flow experiments (HFE) should be conducted.

The proposed federal action considered in the 2024 LTEMP SEIS is to revise two aspects of the 2016 LTEMP ROD. The core focus of the 2024 LTEMP SEIS is the evaluation of sub-annual flow options designed to disrupt the establishment of smallmouth bass and other warmwater nonnative, invasive fish below Glen Canyon Dam by limiting additional recruitment, which could threaten populations of humpback chub (Gila cypha). The humpback chub is a species listed as threatened under the Endangered Species Act (ESA). A range of reservoir releases with varying combinations of temperature and release volumes was analyzed to assess their effectiveness in disrupting smallmouth bass spawning and preventing recruiting populations from expanding. Additionally, the 2024 LTEMP SEIS explored changes, using the best available science, to the sediment accounting periods associated with the LTEMP HFE protocol.

The project area encompasses Glen Canyon Dam and the Colorado River downstream of the dam to the inlet of Lake Mead. Lake Powell and Lake Mead are not within the project area. The analysis area may vary depending on the specific being considered. For instance, the cultural resources analysis encompasses a rim-to-rim area of potential effect, while the socioeconomic and hydropower analyses include surrounding counties and communities.

The 2024 LTEMP SEIS supplements the December 2016 LTEMP ROD for the 2016 LTEMP Final Environmental Impact Statement (EIS). The 2024 LTEMP SEIS and ROD were prepared in accordance with the National Environmental Policy Act (NEPA), the Council on Environmental Quality's NEPA regulations (40 Code of Federal Regulations 1500—1508), and Department regulations (43 Code of Federal Regulations 46). The decision made here is based on the Final SEIS filed with the US Environmental Protection Agency (EPA) on May 22, 2024; the EPA published a notice of availability in the Federal Register on May 31, 2024.

For this ROD, Reclamation has identified the Cool Mix Alternative as the preferred alternative for potential flow actions to address smallmouth bass during the summer of 2024. The Cool Mix Alternative would modify Glen Canyon Dam's sub-annual releases with the intent to disrupt the establishment of smallmouth bass below the dam to meet the purpose and need while minimizing impacts on other resources. For potential flow-based actions to address smallmouth bass after 2024 (if warranted based on temperature targets), Reclamation will consider the Cool Mix Alternative along with the other action alternatives described in the 20241-TEMP SEIS, and will make implementation decisions from 2025 to 2027 based on future actual conditions.

Reclamation has also included adjustments to the high flow experiment (HFE) sediment accounting period and implementation window. These changes consist of adjusting the semiannual sediment accounting period to an annual period, with the option to implement a spring or fall HFE release, or both. 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. [2]

Temperature control

The purpose of the 2024 LTEMP SEIS is for Reclamation to analyze additional flow options at Glen Canyon Dam in response to nonnative, invasive smallmouth bass and other warmwater, nonnative species recently detected directly below the dam. The recent detection of large numbers of young-of-year smallmouth bass suggests spawn-mg is occurring for the first time directly below the dam.

The need is to disrupt the establishment of smallmouth bass below Glen Canyon Dam by limiting additional recruitment, which could threaten populations of the threatened humpback chub below the dam.

HFE Protocol

The 2024 LTEMP SEIS's purpose relative to HFE releases is to consider adjusting sediment accounting periods and HFE implementation windows.

The need is to include the latest scientific information to improve Reclamation's ability to implement HFE releases, as detailed in the 2016 LTEMP EIS. [3]

Cool Mix Alternative

The Cool Mix Alternative aims to maintain a daily average water temperature below 15.5 degrees Celsius (C) (60 degrees Fahrenheit [F]) at the target river mile locations below Glen Canyon Dam where smallmouth bass could spawn. It would involve strategic water releases from both the penstocks and river outlet works, with quantities determined by predicted temperatures. Flows would be triggered when temperatures rise above 15.5C (60F), with variations based on monthly water volumes and conditions.

Cool Mix with Flow Spike Alternative

The Cool Mix with Flow Spike Alternative would maintain daily average water temperatures below 15.5C (60F) at the target river mile locations below Glen Canyon Dam. It would include up to three 8-hour flow spikes, with a maximum flow of up to 45,000 cubic feet per second (cfs), to disrupt spawning in margin habitats. HFE releases could replace flow spikes to maximize sediment benefits. Water releases would vary monthly based on predicted temperatures. Flow spikes and HFE releases would be similar but triggered differently. During a flow spike, peak discharge would be up to 32,000 cfs, moving about 133,000 acre-feet of water over 3 days.

Cold Shock Alternative

The Cold Shock Alternative aims to induce short-duration cold shocks, lowering temperatures to 130C (55.4F) at the target river mile locations below Glen Canyon Dam to disrupt smallmouth bass spawning and rearing. Flows would be activated when temperatures rise above 15.50 (60 F), with cold shocks potentially occurring every weekend for up to 12 weekends, lasting 48 hours each. The goal would be to achieve rapid and sustained cooling of the river to disrupt spawning behavior. Actual release capacities may vary slightly based on operational constraints, and hydropower releases are assumed to be 2,000 cfs. Extreme high-temperature scenarios may limit reaching the target temperatures.

Cold Shock with Flow Spike Alternative

The Cold Shock with Flow Spike Alternative would involve releasing water through the river outlet works for 48 hours to induce a cold shock downstream to the targeted river mile. Up to three 8- hour flow spikes may also be implemented if enough water is available to disrupt spawning in warmer margin habitats. The release quantity would be based on predicted temperatures and operational constraints, which would vary' throughout the year. This alternative would start when daily water temperatures at the Little Colorado River reach 15.5C (60F), with weekly 48-hour cold- shock releases and at least one 8-hour flow spike, lasting up to 12 weeks.

Non-Bypass Alternative

The Non-Bypass Alternative would involve strategic river stage changes along the Lees Ferry reach to disrupt smallmouth bass nests and spawning activities below Glen Canyon Dam. It would include a once-weekly, short-duration, low-flow release followed by a short-duration, high-flow release. The low-flow release would target shallow nesting areas, while the high-flow release would increase water velocities in deeper habitats. These releases would be designed to attenuate by the time they reach the Little Colorado River confluence. Flows would start on Sunday nights, peak on Monday morning, and return to normal by Monday afternoon. The treatment would be repeated weekly based on water temperature projections. This alternative aims to disrupt smallmouth bass spawning down to river mile 61, with adjustments possible during implementation.

The cold-water alternatives have been modeled for cooling effects at river miles 15 and 61. Modeling these locations provides a representation of potential effects on resources at different river reaches. The trigger for implementation would be when observed temperatures exceed 15.5 C (60 F) for 3 consecutive days. Currently real time temperature data exists below Glen Canyon Dam and at Lees Ferry (river mile 0). There are additional gauges at river mile 30 and river mile 61, however these gages do not provide real time data, but can be downloaded remotely. For locations that do not have real time temperature data, the best available models would be used to determine trigger timing. The trigger location for the 15.5 C (60 F) threshold could be anywhere upstream of river mile 61, depending on the smallmouth bass distribution and size class, frequency and efficacy of sampling, or other considerations as determined through the planning and implementation process. Smallmouth bass distribution upstream of river mile 61 would be assessed based on the best available information and considered in the planning and implementation process, with the understanding that smallmouth bass sampling is limited below Lees Ferry and can have limited efficacy in assessing the locations of smaller size fish and of fish located in certain riverine environments where sampling is difficult. Smallmouth bass distribution upstream of river mile 61 will be assessed based on the best available information and considered in the planning and implementation process, with the understanding that smallmouth bass sampling can have limited efficacy in assessing the locations of smaller size fish and of fish located in certain riverine environments where sampling is difficult. (ROD)

For 2024, based on the likelihood of needing to respond to increasing river temperatures, this ROD provides guidance for smallmouth bass flow operations. A cool mix would occur when the average daily temperature at river mile 61 exceeds 15.5 C (60 F) for 3 consecutive days. The temperature data would be determined using real-time stream gage data at the dam and at Lees Ferry and existing downstream models (such as Dibble et al. 2021). The Cool Mix Alterative would be implemented until the mean daily water temperature (without bypass) falls below 15.5 C (60 F) at river mile 61. Temperature and biological monitoring would occur throughout this process to assess effectiveness. Gage data near river mile 61 would be accessed monthly during implementation to confirm temperature targets are being met. (ROD)

The ROD states that in 2024, “The Cool Mix Alternative would be implemented until the mean daily water temperature (without bypass) falls below 15.5°C (60°F) at river mile 61.” Furthermore, it states that in any given year for each alternative, “The trigger location for the 15.5°C (60°F) threshold could be anywhere upstream of river mile 61, depending on the smallmouth bass distribution and size class, frequency and efficacy of sampling, or other considerations as determined through the planning and implementation process.” (2024 SMB Flow Offramp - Decision Memo)

The Cool Mix Alterative would be implemented until the mean daily water temperature (without bypass) falls below 15.5 C (60 F) at river mile 61. (ROD)

Potential off-ramps would be considered based on monitoring data. Off-ramps will occur if any unacceptable adverse impacts on the resources listed in Section 1.3 of the 2016 LTEMP ROD's Attachment B are anticipated. Off-ramps would also occur if the experiments prove to be inefficient at disrupting smallmouth bass spawning and meeting the purpose and need. (ROD)

The expert panel information was provided to the PI Leadership Team who provided the following recommendation for 2024: “Continue Cool Mix-flow to cool to River Mile 61. Stop Cool Mix flows when Glen Canyon Dam penstocks release water temperatures are observed to be at or below 15.5 and anticipated to remain below that threshold as measured by the sonde (NWIS #09379901) immediately below Glen Canyon Dam.” (2024 SMB Flow Offramp - Decision Memo)

• Nonnative Invasive Aquatic Species Page
• Smallmouth Bass Page
• The Temperature Control Page
• 2023 Invasive Fish Species Below Glen Canyon Dam: A Strategic Plan to Prevent, Detect and Respond
• 2019 National Park Service Expanded Non-native Aquatic Species Management Plan
• 2016 Long-term Experimental and Management_Plan (LTEMP) Page
• 2013 National Park Service Comprehensive Fisheries Management Plan

• Glen Canyon Dam Site: Hourly Averages
• Colorado River at Lees Ferry (0)
• Colorado River near river mile 30
• Colorado River above Little Colorado River near Desert View, AZ (61)

• 2024 Glen Canyon Dam Long-Term Experimental and Management Plan Final Supplemental Environmental Impact Statement (SEIS)
• 2024 Record of Decision (ROD) for the Glen Canyon Dam Long-Term Experimental and Management Plan (LTEMP) Final Supplemental Environmental Impact Statement (SEIS)
• Modeling Impacts Glen Canyon Dam Operations Colorado River Resources
• 2023 Glen Canyon Dam/ Smallmouth Bass Flow Options Draft Environmental Assessment

2026

• 2026 Cool Mix Flow Decision for Glen Canyon Dam
• 2026 Cool Mix Tech Team Report
• Response of Smallmouth Bass and Other Species to Cool Mix
• Cool Mix Experiment Update - Hydropower

2025

• Financial Analysis of the Smallmouth Bass Flows implemented at the Glen Canyon Dam during 2025
• Informational Memo Reclamation Summary of 2025 PI Leadership Team Recommendation
• Attachment 1: Final Summer 2025 SMB Recommendation and Technical Memo
• Attachment 2: WAPA-CRSP - Minority Report 2025 SEIS bypass
• 2024 & 2025 Coolmix Cost Impacts to Hydropower
• Cool-Mix 2025
• Two Years of Cool-Mix Flows
• LTEMP Experiments Update
• LTEMP SEIS Update
• LTEMP Flow Experiments
• Response of Smallmouth Bass and other species to cool mix

2024

• 2024 LTEMP SEIS Cool Mix Flow Summary Report
• Financial Analysis of the Smallmouth Bass Flows implemented at the Glen Canyon Dam during Water Year 2024
• News Release: Reclamation discontinues cold water releases from Glen Canyon Dam
• 2024 Off Ramp Memo Final
• 2024 SMB Flow Offramp - Decision Memo
• Arizona Republic: Cold water shots into the Colorado River slow a bass invasion in the Grand Canyon
• WAPA navigates costs of countering smallmouth bass
• Reclamation begins cold water flows to disrupt spawning of nonnative fish below Glen Canyon Dam
• Reclamation finalizes environmental process to combat nonnative fish below Glen Canyon Dam

2023

• Glen Canyon Dam/Smallmouth Bass Environmental Assessment Update

Cool Mix does not prevent entrainment of smallmouth bass through Glen Canyon Dam and it does not cool spawning and nursery habitats that are disconnected from the mainstem (e.g. the Dam Slough at -15 mile). Fish can and will migrate from the tailwater downstream into Grand Canyon and into areas where temperature control at Glen Canyon Dam cannot reach. If habitat in Grand Canyon is in fact suitable for smallmouth bass establishment (see the Pine et al. 2025 - Yackulic et al. 2026 discussion on the Smallmouth Bass Page), continued entrainment of juvenile and adult smallmouth bass and the availability of spawning and nursery habitat like the Dam Slough at -15 mile poses similar risks to humpback chub as allowing warmwater releases from Glen Canyon Dam.

Entrainment

Eppehimer et al. (2025) found that maintaining Lake Powell elevations above 3590 ft (1094 m) would likely minimize propagule pressure from the reservoir and create downstream conditions that minimize smallmouth bass population growth. Smallmouth bass propagule pressure increases dramatically as reservoir elevation declines below 3530 ft (1076 m). Breton et al. (2015) found that eliminating entrainment from source reservoirs was the only way to reduce smallmouth bass populations to zero in Little Yampa Canyon.

Water temperatures in backwaters and sloughs

The Rapid Response Tech Team reported on 4/22/26 that bluegill and green sunfish had recently spawned in the Dam Slough at river mile -15. Minimum spawning temperatures for bluegill and green sunfish are 17 C and 19 C, respectively. Releases from Glen Canyon Dam during this period were between 10 and 11 C. This shows the ineffectiveness of Cool Mix at cooling spawning and nursery habitats that are exposed to solar heating and isolated from the river and the need to modify these habitats in order to keep them from becoming source areas for invasive fish.

Downstream movement of smallmouth bass

Hawkins et al. (2009) found that smallmouth bass in the Yampa River moved long distances both upstream and downstream within short timeframes. They found this high mobility contributes to rapid basin-wide dispersal and complicates nonnative predator suppression efforts aimed at protecting native endangered fish.