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Glen Canyon Dam/Smallmouth Bass 

Environmental Assessment Update

January 26, 2023
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Intro to GCD/SMB EA

Purpose:

The Bureau of Reclamation has determined that an 

Environmental Assessment is necessary to pursue 

implementation of flow options to respond to 

smallmouth bass detections in the Colorado River below 

Glen Canyon Dam.
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Intro to GCD/SMB EA

Need:

To respond to the threat of smallmouth bass establishment, this 

EA identifies various GCD flow options designed to disrupt and 

help prevent SMB from spawning in the Colorado River between 

Glen Canyon Dam and the confluence with the Little Colorado 

River. A mix of water releases from both the GCD penstocks and 

bypass tubes would be needed to cool the river below 16 degrees 

C from GCD to the LCR confluence, which is the temperature at 

which smallmouth bass initiate spawning behaviors.
Modified from Bureau of Reclamation

3700 ft

3490 ft

3370 ft

3524 ft

Full Pool

Current Elevation

Minimum Power Pool 

Dead Pool
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Alternatives

 No Action

 Proposed Action with Flow Options

• Option A: Cool Mix

• Option B: Cool Mix with Flow Spikes

• Option C: Cold Shock

• Option D: Cold Shock with Flow Spikes
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Penstock discharge Jet tube discharge Total discharge

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Flow Option A: Cool Mix

• Water is released from both penstocks and bypass tubes to maintain a 

daily average water temperature below 16°C from below the dam to 

the Little Colorado River (LCR). 

• This would be initiated whenever temp at LCR is 16°C.
Preliminary information – do not distribute or cite

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Penstock discharge Jet tube discharge Total discharge

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Flow Option B: Cool Mix with Flow Spikes

• Water is released from both penstocks and bypass tubes to maintain a daily 

average water temperature below 16°C from below the dam to the Little 

Colorado River (LCR). In addition, up to three 36-hour flow spikes would be 

added between late-May and mid-July if sufficient water was available. 

• This would be initiated whenever temp at LCR is 16°C.
Flow Option C: Cold Shock

• Once a week for at least 48 hours, switch to the minimum amount of water 

released through the bypass tubes required to create a cold shock all the way 

down to the Little Colorado River (LCR).

• Minimum of 12 weeks starting when daily water temperatures near the LCR 

approach 16°C

Preliminary information – do not distribute or cite

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Penstock discharge Jet tube discharge Total discharge

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– 

Flow Option D: Cold Shock with Flow Spike 

• Once a week for at least 48 hours, switch to the minimum amount of water 

released through the bypass tubes to create a cold shock from the dam down 

to the Little Colorado River (LCR). In addition, up to three 36-hour flow spikes

would be added between late-May and mid-July if sufficient water was

available. 

• Minimum of 12 weeks starting when daily water temperatures near the LCR 

approach 16°C Discharge (CFS) 

35000 

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Penstock discharge Jet tube discharge Total discharge 

Preliminary information do not distribute or cite 
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GCD/SMB EA Process

• Timeline/Key Dates:

• Dec 1 – Information session

• Dec 15 - Stakeholder Written Input due (Comments from 

11 stakeholders)

• Early Feb- Draft EA for public review (14 day review)

• Early-Mid April - Post Final EA
Questions?

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Glen Canyon Dam Key Elevations
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Problem

• Sustained drought conditions

•Low lake elevations

•Increased risk of entrainment 

•Warmer release temperatures

• Suitable for reproduction/growth of smallmouth bass
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Why worry about smallmouth bass?

• Smallmouth bass have caused a decline 

in native fishes throughout the west

• Upper Basin

•Highly piscivorous - highest predatory 

threat in the upper basin

•Highly fecund 

• Females have lots of eggs

• Males guard nests which results in higher 

survival rates of young

•Considered a high-risk species
Smallmouth bass capture locations in 

the Grand Canyon prior to 2022

*Spatial distribution of effort differs through time
Besides enough spawners, what other 

conditions are required for SMB to 

establish?

• Sufficient food

•Infrequent high turbidity

•Low velocity gravel/cobble habitat

• Suitable water temperature (primary driver)
Temperature Related to Smallmouth Bass

Figure courtesy of GCMRC
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Timeline of Smallmouth Bass in Glen Canyon Reach

• Smallmouth bass detected in -12 mile slough on June 30th

• Chemical treatment in slough in September 2022

• Smallmouth bass detected throughout Lees Ferry reach over 

course of September/October 2022

• Primarily juveniles detected

• Indicates not established

• Removal efforts following science plan initiated in October 2022 

• Serve to buy some time
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May AMWG - Directive from 

Secretary’s Designee

Develop 2-4 operational alternatives that could 

help prevent cool- and warmwater invasive fish 

establishment, while minimizing potential 

adverse effects to other resources. Operational 

alternatives that are not within the scope of the 

LTEMP ROD may be proposed, but would require 

additional NEPA, ESA, and NHPA compliance.
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Directive from August AMWG Meeting

NEPA Compliance for Operational Flexibilities to Address Nonnative 

Fish – The second of the non-native fish actions is to task Reclamation with 

developing a project management plan that includes a budget and schedule 

for initiating a NEPA process associated with operational 

alternatives /actions to disadvantage SMB and other non-native fish, 

which may require further refinement from GCMRC. We ask that the 

schedule be aimed at completing a NEPA decision document in 

time for possible implementation in the late spring/early 

summer of 2023. I encourage Reclamation to analyze the degree to which 

such compliance can be tiered off the LTEMP FEIS and ROD. It will be 

important to maintain a focused scope for this effort and to avoid inclusion of 

ancillary actions and issues to ensure the process can meet a possible 

spring/summer 2023 implementation. This NEPA analysis must not become 

a vehicle for addressing the range of concerns about the LTEMP FEIS 

and ROD but should rather give us possible tools that we can implement in a 

timely manner to address the non-native fish challenges we are currently 

facing. I propose that the project management plan, be shared with the 

GCDAMP partners by October 14, 2022.
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Process for Developing Flow Options

• SMB task force – started meeting in January/February 2022

• Provided preliminary data to start process presented during 

April TWG

• SMB Ad Hoc Group formed after May AMWG 

• Tasked with developing strategic plan

• Provided input during development of flow options

• GCMRC conducted the modeling and developed flow options 

• Initial conversations for flow options during SMB task force

• Refined during SMB Ad Hoc group process
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Considerations for Developing Flow Options

•Maintenance schedule

•Hydrology

•Bypass tube coating

•Regulatory – maintain 1,500 cfs

•Conduct flows within normal operations (up-ramp, 

down-ramp, etc)
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Operational/Regulatory Constraint 

Considerations in Model 

• 1,500 cfs minimum discharge always released through penstocks 

• 25,000 cfs maximum discharge released through penstocks

• 15,000 cfs maximum discharge released through bypass tubes 

• Bypass tubes can be operated at 1⁄2 tube increments 

• (i.e., 1,750 CFS, assuming 3,500 CFS per outlet tube) to minimize the bypass 

required to reach a particular target temperature

• Follow normal operations as described in LTEMP:

• Maximum up-ramp rates of 4,000 cfs

• Maximum down-ramp rates of 2,500 cfs

• Minimum total discharge of 8,000 cfs during the day (7am-7pm) 

• Minimum total discharge of 5,000 cfs during the night (7pm- 7am) 
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General Modeling Assumptions

• Target fish is smallmouth bass

• Downriver warming estimated with Dibble et al. (2021) model 

• ≥16°C is required for SMB to initiate spawning

• Cold shocks may disrupt SMB spawning behavior

• <13°C most likely to be effective

• Flow spikes can disrupt spawning in margin habitats
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Hydrograph Assumptions

• All options can be implemented at 740 kaf (forecast for June 2023)

• Water temperatures through penstocks is 18°C

• Water temperature through bypass tubes is 11°C

*All hydrographs are samples only and need to be updated 

based on the conditions at the time the flow is implemented
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Flow Options

• Option A: Cool Mix

• Option B: Cool Mix with Flow Spikes

• Option C: Cold Shock

• Option D: Cold Shock with Flow Spikes
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Temperature & Velocity

•Combination of cold water and flow spikes are most 

likely to be successful at preventing establishment

•Temperature is the primary driver of spawning

•Changes in velocity with flow (>0.3 m/s) can cause 

nest abandonment but not prevent it completely

• SMB can spawn multiple times in a season
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Potential Success of Flow Options

• Option A: Cold Mix – high certainty of prevention under 

most conditions except the potential for spawning in 

warmer margin habitats

• Option B: Cold Mix with Spike Flow - high certainty of 

prevention under most conditions

• Option C: Cold Shock – less certainty than options A or B 

but good chance of prevention under most conditions

• Option D: Cold Shock with Spike Flow – less certainty than 

option A or B but higher than option C because it will also 

disrupt margin habitats.

*Success of any of these options depends on available hydrology

https://www.usbr.gov/uc/progact/amp/amwg/2022-08-18-amwg-meeting/20220818-Yackulic_Op_Alts_AMWG_508.pdf
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Stakeholder Written Input

•Email your written input to Sarah Bucklin 

sbucklin@usbr.gov

by December 15th
Questions?