Difference between revisions of "SEIS Bypass Releases"

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−
===Purpose and Need===
+
The proposed updates to the 2016 LTEMP FEIS are a result of the extended period of drought,
 +
aridification, and low runoff conditions in the Colorado River Basin. Despite an above-average
 +
runoff in 2023, the period from 2000 to 2023 is considered one of the driest in over a century and
 +
among the driest in the last 1,200 years (Williams 2022). As Lake Powell's water elevation has
 +
the epilimnion, where most fish reside, has drawn closer to the dam's penstocks. The
 +
reduction in water elevation increases the likelihood of nonnative fish, such as smallmouth bass
 +
(Micropterus dolomieu), being passing through the dam and entering the Colorado River
 +
downstream.
  
−
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 spawning 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.  
+
As Lake Powell's elevations decline, warmer water from the epilimnion is released from the dam,
 +
resulting in increased water temperatures downstream. See Figure I-I for a conceptual
 +
representation of Glen Canyon Dam operations These warmer water conditions facilitate the
 +
reproduction and establishment of warmwater nonnative fish that pose a major threat to federally
 +
listed fish species and other native and sport fish living downstream of Glen Canyon Dam If highly
 +
predaceous smallmouth bass, which specialize in eating other fish, were to establish below the dam,
 +
removal efforts be difficult and expensive with potentially limited success. In 20 years of  
 +
mechanical removal efforts of smallmouth bass the Upper Colorado River Basin upstream of  
 +
Glen Canyon Dam, there has been limited success in reducing smallmouth bass densities to benefit
 +
native fish populations (Dibble et al. 2021, Bestgen and Hill 2016a).
 +
 
 +
In response to these changing conditions, the Secretary of the Interior's acting designee to the Glen
 +
Canyon Dam Adaptive Management Work Group (AMWG) directed Reclamation in August 2022
 +
to identify and analyze operational alternatives at Glen Canyon Dam to disrupt the spawning of
 +
smallmouth bass and other warmwater nonnative fish that pass the dam. [https://gcdamp.com/images_gcdamp_com/d/db/20240500-GCDLTEMP-FinalSEIS-508-AMWD_%282%29.pdf]
  
 
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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 [[Media:LTEMP_SEIS_Cool_Mix_Flow_Summary_Report_508.pdf|'''link''']].  
 
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 [[Media:LTEMP_SEIS_Cool_Mix_Flow_Summary_Report_508.pdf|'''link''']].  
 +
 +
|-
 +
! <h2 style="margin:0; background:#cedff2; font-size:120%; font-weight:bold; border:1px solid #a3b0bf; text-align:left; color:#000; padding:0.2em 0.4em;">Summary</h2>
 +
|-
 +
|style="color:#000;"|
 +
 +
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: <br>
 +
 +
(1) responding to the threat of smallmouth bass below Glen Canyon Dam; and  <br>
 +
 +
(2) incorporating the latest sediment science to assess when high-flow experiments (HFE) should be conducted.  <br>
 +
 +
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. [https://www.usbr.gov/uc/DocLibrary/EnvironmentalImpactStatements/GlenCanyonDamLong-TermExperimentalManagementPlan/20240703-GCDLTEMP-FinalSEIS-RecordofDecision-508-AMWD.pdf]
 +
 +
|-
 +
! <h2 style="margin:0; background:#cedff2; font-size:120%; font-weight:bold; border:1px solid #a3b0bf; text-align:left; color:#000; padding:0.2em 0.4em;">Purpose and Need </h2>
 +
|-
 +
|style="color:#000;"|
 +
 +
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.
 +
 +
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. [https://www.usbr.gov/uc/DocLibrary/EnvironmentalImpactStatements/GlenCanyonDamLong-TermExperimentalManagementPlan/20240703-GCDLTEMP-FinalSEIS-RecordofDecision-508-AMWD.pdf]
  
 
|-
 
|-
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|style="color:#000;"|
  
−
===Cool Mix Alternative (Preferred Alternative) ===
+
===Cool Mix Alternative ===
 
The Cool Mix Alternative aims to maintain a daily average water temperature below 15.5 degrees  
 
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  
 
Celsius (C) (60 degrees Fahrenheit [F]) at the target river mile locations below Glen Canyon Dam  

Revision as of 15:27, 20 October 2025

400px‎

The proposed updates to the 2016 LTEMP FEIS are a result of the extended period of drought, aridification, and low runoff conditions in the Colorado River Basin. Despite an above-average runoff in 2023, the period from 2000 to 2023 is considered one of the driest in over a century and among the driest in the last 1,200 years (Williams 2022). As Lake Powell's water elevation has the epilimnion, where most fish reside, has drawn closer to the dam's penstocks. The reduction in water elevation increases the likelihood of nonnative fish, such as smallmouth bass (Micropterus dolomieu), being passing through the dam and entering the Colorado River downstream.

As Lake Powell's elevations decline, warmer water from the epilimnion is released from the dam, resulting in increased water temperatures downstream. See Figure I-I for a conceptual representation of Glen Canyon Dam operations These warmer water conditions facilitate the reproduction and establishment of warmwater nonnative fish that pose a major threat to federally listed fish species and other native and sport fish living downstream of Glen Canyon Dam If highly predaceous smallmouth bass, which specialize in eating other fish, were to establish below the dam, removal efforts be difficult and expensive with potentially limited success. In 20 years of mechanical removal efforts of smallmouth bass the Upper Colorado River Basin upstream of Glen Canyon Dam, there has been limited success in reducing smallmouth bass densities to benefit native fish populations (Dibble et al. 2021, Bestgen and Hill 2016a).

In response to these changing conditions, the Secretary of the Interior's acting designee to the Glen Canyon Dam Adaptive Management Work Group (AMWG) directed Reclamation in August 2022 to identify and analyze operational alternatives at Glen Canyon Dam to disrupt the spawning of smallmouth bass and other warmwater nonnative fish that pass the dam. [1]

--
--
--

Implementation

2025

The Cool Mix Alternative was selected with a temperature trigger at RM 30 and a projected 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.

Summary

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]

Purpose and Need

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.

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]

Alternatives

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 spawning at river mile 61, with adjustments possible during implementation.

Triggers

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)

Offramps

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)


Links

Temperature Gages

Documents

Presentations and Papers

2025

2024

2023

Other Stuff
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