Cool Mix Alternative (Preferred 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.
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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)
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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)
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