1 

Considerations for Implementation of a Trout Management Flow 

Experiment

Problem

The Glen Canyon Dam Adaptive Management Program (GCDAMP) is tasked with adaptively 

managing the Glen Canyon Dam to improve resources downstream. The 2016 Long-Term 

Experimental and Management Plan (LTEMP) Environmental Impact Statement (EIS) Record of 

Decision (ROD) and Biological Opinion (BO) are the guiding documents that provide a 

framework for managing those resources related to the operations of Glen Canyon Dam. 

Maintaining a rainbow trout sport fishery at Lees Ferry while preventing movement of this nonnative species downriver where it may negatively affect humpback chub are two seemingly 

opposing objectives that present a challenge for managers and the GCDAMP. One of the 

experiments outlined in the ROD & BO referred to as a trout management flow (TMF) is one of 

the proposed tools designed to reduce the recruitment of rainbow trout and thus preserve that 

balance between maintaining the fishery and protecting humpback chub. 

Purpose

The purpose of this report is to review the current state of science regarding the design, 

implementation, and effectiveness of implementing a TMF experiment as a tool for managing 

rainbow trout. It also includes additional factors that may affect the decision-making process. 

The report also evaluates the conditions under which a TMF experiment is most likely to meet 

intended rainbow trout management and learning objectives. Sources of uncertainty are 

categorized in the document based on the potential to address them through a small-scale fieldbased or laboratory study or through experimental implementation. A list of additional tools that 

has been considered for managing trout is also included (Tables 2, 6). Although 

recommendations for next steps are included, this report is not a decision document, but rather a 

resource to facilitate the process for determining when or if to implement a TMF. TMFs were 

originally designed to manage rainbow trout at Lees Ferry; however, due to the increasing 

population of brown trout at Lees Ferry and the threat they pose to humpback chub, potential 

adjustments needed to apply a TMF to brown trout are described in the Other Considerations 

section. 

Background

Completion of Glen Canyon Dam in 1963 resulted in the delivery of water and hydroelectric 

power to millions of people across the western United States. However, it also altered water 

quality, temperature, nutrients, sediment load, and flow of the Colorado River downstream from

the dam. Hydropower production from the dam results in releases of water volumes which vary 

daily in conjunction with the power needs of the recipients based on the time of day and year. 

These daily flow fluctuations affect the physical conditions of the river resulting in habitat 

modifications that impact early life stages of fish (Korman & Campana 2009). The fluctuations

also cause a vertical and longitudinal shift in wetted nearshore habitat, which can push juvenile 

trout away from preferred habitat leading to slower growth and higher predation rates (Scruton et 

al. 2003). 
2 

Changes to the river from dam operations resulted in cooler summer and warmer winter water

temperatures and reduced turbidity, which contributed to the population decline of native fishes. 

Introduction of non-native fish, along with habitat and water temperature alterations created by 

the dam, have reduced the growth rate and survival of humpback chub (Yackulic et al. 2014). 

Growth rates of humpback chub are slower in colder water making them vulnerable to predation 

for a longer period (Yackulic et al. 2014; Ward & Morton-Starner 2015; Yackulic et al. 2018). 

Juvenile humpback chub survival has the largest impact on population dynamics of the species, 

and survival rates are lower when rainbow trout abundance is higher and water temperature is 

lower (Yackulic et al. 2018). Interactions with non-native fish are another contributing factor in 

the decline of humpback chub (Yard et al. 2011). 

The National Park Service (NPS) introduced brown trout into tributaries downstream from the 

dam in the late 1920s and early 1930s for recreational fishing but have recently increased in 

abundance upstream of Lees Ferry (Runge et al. 2018). Though brown trout are more 

piscivorous than rainbow trout, both species pose a threat to humpback chub, and based on 

stomach content analysis, both species seem to preferentially consume native versus non-native

fish (Yard et al. 2011). From 2014-2016, brown trout populations began increasing in Lees 

Ferry and have continued to increase their population (Runge et al. 2018), reaching ~30% of the 

catch by 2021 (Yard & Dibble, unpublished data). 

Due to the impacts these non-native salmonids may have on recovery of humpback chub, 

effective management tools are essential. Options with minimal impacts on other GCDAMP 

resources are limited. Timing and variation of flows can reduce the abundance of trout 

depending on how flows align with spawning, egg development, and fry emergence (Korman 

and Campana 2009, Korman et al. 2011b, Avery et al. 2015). Trout management flows (TMFs) 

builds on this concept of using flows from the dam to decrease recruitment of juvenile trout to 

the adult population. In addition to flow, prey availability, turbidity-driven feeding efficiency, 

and intraspecific competition are other drivers that can affect rainbow trout growth and 

abundance (Korman et al. 2021). 

When the LTEMP BO and EIS were developed, rainbow trout were the non-native species of 

most immediate concern to humpback chub, and consequently the specific elements of the 

hydrograph for a TMF targeted rainbow trout. As such, this review focuses on rainbow trout, but 

includes how a TMF could be modified to target brown trout (see Summary and Considerations 

section). 

Recreational Rainbow Trout Fishery

Trout thrive in the clear cold water found downstream from Glen Canyon Dam compared to the 

warm and turbid Colorado River that existed prior to its completion. The tailwater stretch of the 

river below the dam was stocked with rainbow trout a year after the dam was completed. 

Initially, releases from the dam fluctuated dramatically, which prevented successful reproduction 

and recruitment owing to daily changes in river stage of 10 feet that alternately dried and scoured 

trout egg nests (redds) and displaced sensitive fry. In the absence of natural spawning and 

recruitment, the rainbow trout population in Glen Canyon was only sustained by continued 

stocking. However, changes to dam management in 1991 reduced the daily fluctuations in stage 

to approximately 3 feet, which allowed natural reproduction and survival of rainbow trout fry to 
3 

occur (McKinney et al. 2001). By 1992, monitoring indicated that natural reproduction was 

occurring, and regular stocking was discontinued after 1998 (Avery et al. 2015). The rainbow 

trout fishery was designated “blue-ribbon” status based on the larger-than-average fish that were 

caught during that time (Avery et al. 2015). The rainbow trout population has fluctuated since 

then, and while regular stocking was discontinued, Arizona Game and Fish Department (AGFD)

still maintains use of stocking as a management tool when catch rates decline below fishery 

management goals stated in the NPS Comprehensive Fisheries Management Plan (CFMP) 

Environmental Assessment (NPS CFMP EA 2013). Stocking of triploid rainbow trout most 

recently occurred in November 2018 (526 rainbow trout) and May and June 2019 (5,956 rainbow 

trout). Catch rates, size, and condition are metrics used to measure the status of the tailwater 

fishery, and to determine whether stocking is necessary (NPS CFMP EA 2013, Rogers 2015). 

Biology & ecology

Rainbow trout spawning occurs in Glen Canyon between February and April with emergence 

ensuing approximately two months after fertilization. Immediately after fry emerge from gravels 

rainbow trout are thought to be most vulnerable to flow variations (i.e., April- June; Korman et 

al. 2011b). Spawning is assumed to be limited in Marble Canyon due to sub-optimal habitat 

conditions (Korman et al. 2012). 

Because trout visually detect prey, they become less effective predators as turbidity increases,

except when prey are abundant (Barrett et al. 1992; Yard et al. 2011; Ward et al. 2016). Since 

turbidity and temperature increase downstream, trout dispersal downstream is limited except 

when abundance and competition are high (typically following large recruitment years), or fish 

condition is poor (Korman et al. 2016; Korman et al. 2021). Rainbow trout that move from Glen 

Canyon to Marble Canyon may be more likely to persist in Marble Canyon when turbidity levels 

are low due to low sediment inputs from the Paria River (Korman et al. 2016; Korman et al. 

2021). Large recruitment events that have led to higher densities of rainbow trout in Glen 

Canyon (e.g., the equalization flow in 2011) have resulted in downstream emigration into Marble 

Canyon and near the confluence of the LCR (Korman et al. 2016). 

Lab trials indicate that relatively small changes in turbidity (25 formazin nephlometric units) can 

significantly alter predation vulnerability of humpback chub from trout (Ward et al. 2016). 

Although primary and secondary production are highest in Glen Canyon compared to downriver, 

growth of rainbow trout is generally lower than downriver (Yard et al. 2016). Density, 

competition, prey size and availability, water temperature and velocity affect the maximum size 

of rainbow trout (Dodrill et al. 2015). Rainbow trout select prey based on width (Dodrill et al. 

2021), and typically prey on midges (chironomid), black flies (simuliid), and Gammarus 

lacustris (amphipod) which are the most abundant prey in the river (Cross et al. 2011). 

Fluctuations in flow affect trout density, recruitment, and the maximum length of adults. Based 

on an analysis of data across tailwaters of the western U.S., releases of high volumes of water 

during early life history stages can decrease rainbow trout recruitment (Dibble et al. 2015). This 

same analysis indicated that large daily variations in flow due to hydropower releases can 

moderately decrease adult rainbow trout size (Dibble et al. 2015). Research suggests that 

increased flow fluctuations during the rainbow trout spawning and incubation period are unlikely 
4 

to be effective at regulating the population because age-0 trout exhibit compensatory survival 

(Korman et al. 2011b). 

Phosphorous concentrations in Glen Canyon are low which affects prey availability. Declines in 

prey availability due to the low phosphorous levels have resulted in poor growth and condition of 

rainbow trout (Korman et al. 2021). Fluctuations in rainbow trout abundance can be seen in 

Figure 1, which shows the population dynamics of this species in Lees Ferry since 2012 (Figure 

1; Table 1). Based on mark-recapture data, rainbow trout dispersal downriver of Lees Ferry is 

low on a per capita basis but is most likely to occur when recruitment is high, or fish condition is 

poor (Korman et al. 2016). Maintaining low rates of dispersal downriver are the most feasible 

way of maintaining low densities near the Little Colorado River to minimize negative effects of 

competition and predation on humpback chub (Korman et al. 2021).

Potential Parasite & Pathogen Concerns

Because TMFs are a population-level management tool, transfer of parasites and pathogens to 

humpback chub is a potential concern. Introduction of pathogens and parasites through stocked 

fish is a concern especially with parasites such as Ich (Ichthyophtirius multifiliis) which have the 

potential to perpetuate throughout the Colorado River (Ward, unpublished data). Though salinity 

levels in the Little Colorado River provide anti-parasite protection to humpback chub, Ich could 

limit larval survival and recruitment of humpback chub in other tributaries (Ward 2012). The 

parasites and pathogens found in rainbow trout tend to be specific to trout or other cold-water

species because the pathogens require a thermal optimum, meaning that there is not a concern for 

transmission to humpback chub due to the difference in thermal habitat preference of the fish. 

However, internal parasites such as Asian tapeworm could be transmitted from non-native warm 

water species such as carp to humpback chub (Ward 2007). 

Figure 1. Abundance of rainbow trout by size class from 2012-2020
5 

Year

225-274 mm

n (95% CI)

>275 mm

n (95% CI)

2015 19,700 (12,200-27,200) 131,200 (61,300-201,000)

2016 7,000 (3,200-10,800) 249,100 (185,100-313,100)

2017 16,100 (8,800-23,400) 106,600 (82,500-130,600)

2018 48,200 (35,900-60,500) 107,800 (88,300-127,300)

2019 64,500 (50,000-79,000) 138,100 (109,600-166,600)

2020 25,400 (14,800-36,000) 84,000 (59,100-108,900)

Trout Management Tools and Experimental Actions

Trout management flows (TMF) are one of the experiments identified in LTEMP as a potential 

tool for managing rainbow trout. The experiment was designed to reduce recruitment of youngof-the-year (YOY) rainbow trout to minimize migration downriver where they may negatively 

impact humpback chub. In the context of TMFs, it is important to note there are other potential 

tools available for managing trout (Table 2). Some of these have been utilized extensively while 

others have been suggested as potential options that would require further evaluation or 

compliance prior to implementation (LTEMP ROD 2016, NPS CFMP EA 2013, NPS NNAS EA 

2018). Due to the unknown efficacy of a TMF, examination of alternate options may be 

warranted. Some of these tools have been evaluated previously while others would require 

careful evaluation prior to implementation. This table includes various options but does not 

imply that implementation would be possible. 

Trout Management 

Tool or Experimental 

Action

Advantages Disadvantages 

Angling regulations  Unlikely to affect downstream 

resources

 May diminish tribal concerns since 

fish will likely be consumed

 Inexpensive 

 May have a negative impact on the 

angling community 

 Catch rates may not be sufficient to 

limit the population

 Populations can rebound quickly 

meaning ongoing efforts would be 

necessary

Chemical controls  Effective at removing non-native fish

 Cost effective

 Could have negative impacts on 

non-target aquatic species in the 

immediate area 

 May require additional compliance 

& permits

 Would only be effective in isolated 

areas such as tributaries or the 

slough 

 Tribal concerns 

Desiccation of redds  Species specific

 May affect recruitment

 Tribal concerns

 Efficacy of managing trout is 

unknown

Table 2. Potential or existing management tools and experimental actions for managing trout listed 

alphabetically and based on their impact to other resources (table created based on information from Bair et 

al. 2017 presentation on management options during Brown Trout Workshop 2017, NPS Non-Native 

Aquatic Species EA 2018 & subsequent ideas by stakeholders). 

Table 1. The fall population estimate (n) of adult (>225mm) rainbow trout at Lees 

Ferry from 2015-2020 with 95% confidence intervals.
6 

High flow experiments

(fall) 

 Depending on timing may increase the 

sand index 

 Would improve beaches for 

recreational use

 Diminished tribal concerns 

 May cue brown trout to migrate

upstream to Glen Canyon which 

could increase the number of 

brown trout that spawn

 May improve brown trout egg 

survival & recruitment 

 May negatively impact the aquatic 

food base depending on the timing 

 Any adjustments to the timing 

prescribed in LTEMP would 

require environmental compliance 

and permits 

 Efficacy of managing trout is 

uncertain

 May increase costs to generate 

electricity

High flow experiments 

(spring) 

 May reduce brown trout recruitment

depending on the timing 

 May improve the condition of the 

rainbow trout fishery 

 May improve the food base 

depending on the timing 

 May increase turbidity to benefit 

native species

 Could improve aquatic habitat 

 May negatively affect the food 

base depending on the timing 

 Any adjustments to the timing 

prescribed in LTEMP would 

require environmental compliance 

and permits 

 Efficacy of managing trout is 

uncertain

 Depending on timing may 

negatively impact the sand index 

 May increase costs to generate 

electricity

Large-scale fishing with 

gill nets 

 Cost effective

 Likely to have population level 

impacts on competition & density 

 Diminished tribal concerns since fish 

would likely be consumed 

 Could be accomplished quickly 

 Would require additional 

compliance & permits

 May not be effective given flow & 

habitat conditions 

 Efforts would need to be ongoing 

to account for how quickly 

rebounding trout populations 

 May conflict with NPS regulations

Mechanical removal

(electrofishing, various 

nets, etc)

 Less impacts to non-target species

 Limited impact to rainbow trout 

fishery

 Tribal concerns 

 Populations can rebound very 

quickly meaning efforts would 

need to be ongoing 

 Expensive and time & labor 

intensive 

 Angler concerns with effect on 

fishery

Rainbow trout stocking  Stocking with individuals that could 

outcompete brown trout or are less 

vulnerable to whirling disease could 

cause a decrease in brown trout 

populations

 Efficacy is highly uncertain.

 Rainbow trout may be more likely 

to move downstream under high 

densities and negatively impact 

humpback chub

Spawning site 

disruption 

 Species-specific

 Reduced risk of trout moving 

downriver 

 Efficacy of managing trout is 

unknown 

 Conflicts with tribal concerns 

 Labor and time intensive
7 

Temperature control 

device

 Could change the temperature to 

affect the rainbow trout fishery, 

humpback chub and other native or 

nonnative species 

 Efficacy is uncertain

 Would require appropriate 

environmental compliance and 

permitting

 The cost would be very high ($15-

150 million) 

 Depending on the temperature it 

may impact native species

Trout management 

flows 

 May reduce the density of rainbow 

trout resulting in an increase in size 

(reduced competition) 

 May positively impact humpback 

chub (reduced predation and 

competition) 

 May positively affect the food base 

depending on timing 

 May negatively impact the rainbow 

trout fishery

 May negatively impact sand 

resources

 May negatively affect food base 

depending on timing 

 Tribal concerns

 The cost to generate electricity 

could be significant

 Efficacy is unknown

Turbidity enhancement  May reduce trout predation on 

humpback chub 

 Diminished tribal concerns 

 May be expensive depending on 

mechanism to increase sediment

 May reduce gross primary 

productivity/food base

TMF Experimental Design

Experimental Theory

The experimental theory behind a TMF is that peak discharge of water from the dam is used to 

push YOY trout into higher elevation near-shore shallow water environments, and then the water 

level is dropped rapidly with the objective of stranding fry and fingerling-sized trout (<6 months 

old) in the shallows. Ideally, the reduction in recruitment would lead to lowered densities of 

trout and may result in an increase in the growth rate and size of adult rainbow trout which is 

more desirable to anglers. Presumably the reduction in density of rainbow trout in Glen Canyon 

would lead to decreased competition, and thus less movement downriver into Marble Canyon or 

the confluence of the Little Colorado River where they may negatively impact humpback chub. 

During this TMF, as described in the LTEMP ROD, flows are raised above normal operating 

levels for a prolonged period, and then dropped rapidly to a lower level. Although the TMF was 

designed to be implemented during periods of high rainbow trout recruitment, it could also be 

implemented to determine its efficacy as a tool for managing trout and evaluating the impact on 

humpback chub survival. The TMF would likely need to be implemented multiple times to be 

most effective, and there is flexibility to implement it up to 3 cycles per month from May 

through August.

LTEMP identifies specific options for dam operations (including hourly, daily, and monthly 

release volumes) including experiments that could be implemented to improve conditions and 

minimize adverse impacts to resources downriver. Release volumes fluctuate based on 

hydroelectric power demand, but the limits for the elements of base operations (minimum and 

maximum flows, daily range, ramp rates, etc.) are specified (LTEMP ROD page 30; B-3). 

Prior to 1991, large hourly variations in flow caused by power loads were normal; during this 

time, the rainbow trout population was only maintained by stocking. Recruitment success 

appeared to increase when flows were controlled and steady (Korman et al. 2012). Fluctuations
8 

in flow may limit survival rates of juvenile rainbow trout by displacing them from preferred 

habitats, depleting food, and causing an increase in stress, energetic costs, and predation 

(Korman & Campana 2009). Floods or high flow releases from the dam are known to affect the

survival rates of incubating life stages and juvenile rainbow trout (Korman et al. 2011a). Once 

more stable flows were incorporated into dam management in 1991, the rainbow trout population 

became self-sustaining (Avery et al. 2015). 

Juvenile trout tend to utilize low angle shoreline for rearing likely due to lower depth and water 

velocity compared to high angle shorelines (Korman et al. 2011b). Rainbow trout growth is

affected by high fluctuating flows because there is an energetic cost associated with young trout 

maintaining position in fluctuating environments (Korman and Campana 2009). Fluctuations in 

flow, even for a short period, can result in dewatering of spawning habitat, which can cause 

increased mortality of eggs. Fluctuations can also cause displacement of juvenile fish from 

shallow low-velocity habitats along the shoreline that are most conducive to efficient foraging 

(Korman et al. 2011b). Survival of trout in response to a fluctuating flow may depend on the 

timing of changes in flow variability relative to early life stage shifts in use of low (cobble bars) 

and high angle (talus slopes) shorelines (Korman et al. 2011b). Habitat use of early life stages of 

trout also varies by season meaning that the timing of flow fluctuations can impact survival 

(Yard et al. 2016). 

Recruitment could be limited by increasing flow variation in late spring or summer when age-0 

trout utilize near-shore shallow-water environments (Korman et al. 2011b). Adults tend to shift 

towards deeper water during increases in discharge (Pert and Erman 1994), meaning they are less 

susceptible to stranding and are less likely to be directly affected by TMFs.

Triggering Mechanism

TMFs are an experimental management tool designed to control production of trout at the 

recreational trout fishery in Glen Canyon and limit emigration downriver where they may 

negatively impact humpback chub. The LTEMP BO specifies that TMFs may be implemented 

when observed or predicted rainbow trout recruitment levels are high (Table 3). Rainbow trout 

recruitment is the metric for determining when a TMF should be implemented because 

downriver dispersal of trout is most likely to occur when recruitment is high. Rainbow trout are 

most likely to be vulnerable to TMFs from May through July, but recruitment cannot be forecast 

from field data reliably until July. By July, only 50% of rainbow trout age-0 remain in low-angle 

habitat, where they may be more vulnerable to stranding during a TMF (Yard & Korman, ARM 

2020). In other words, the timeframe when recruitment forecasting can most reliably be 

determined occurs in the middle of the window when TMFs can be implemented, thus affecting 

our ability to effectively utilize field data to forecast recruitment. However, a model has been 

developed which utilizes soluble reactive phosphorous and spawner abundance to predict 

rainbow trout recruitment (Yackulic, unpublished data). Although model inputs can always be 

improved and may need to be adjusted to account for the effect of increasing numbers of brown 

trout on rainbow trout recruitment, the existing model provides a basis for predicting 

recruitment. 

As specified in the LTEMP ROD, TMFs could also be implemented early in the LTEMP period

to learn more about the effectiveness of the flow as a management tool. Other flows listed in the 
9 

LTEMP EIS that may influence the decision to implement a TMF include equalization flows and 

high flow experiments (HFE). Equalization flows are thought to cause a subsequent increase in 

rainbow trout (Avery et al. 2015) which might also impact the decision to implement a TMF. 

Implementation may also follow a spring HFE which is expected to increase rainbow trout 

recruitment by stimulating the food base and thus trout production (LTEMP ROD page 53). 

Regardless, potential short-term unacceptable impacts to other resources would need to be 

considered annually in the decision to implement a TMF (LTEMP ROD page 39). 

Subsequent TMFs would only be implemented if the initial TMF proved to be successful in 

reducing trout recruitment in Glen Canyon and there were not long-term unacceptable impacts to 

other resources such as increased costs or population level effects to species of concern or 

population level increases in non-native fish. Ideally, if a TMF was successful, it would reduce 

the number of times mechanical removal would be needed. 

 Table 3. LTEMP considerations for implementing a TMF (from LTEMP ROD page 39, B-12) 

Experimental 

Treatment

Trigger & Primary 

Objective Replicates Duration

Annual 

Implementation 

Considerations

Long-term Offramp 

Conditions

Action if 

Successful

Trout 

management 

flows

Trigger: Predicted 

high trout 

recruitment in the 

Glen Canyon reach 

Objective: Test 

efficacy of flow 

regime and survival 

of humpback chub

Implement as 

needed when 

triggered after 

consultation with 

tribes; test may be 

conducted early in 

the 20-year period 

even if not 

triggered by high 

trout recruitment 

Implemented

up to 3 

cycles per 

month, in as 

many as 4 

months 

(MayAugust)

Potential shortterm 

unacceptable 

impacts on 

resources listed in 

Section 1.3

TMFs have little 

or no effect on 

trout recruitment 

after at least

three tests; or 

long-term

unacceptable 

adverse impacts 

on the resources

listed in Section 

1.3 are observed

Implement as

adaptive treatment

triggered by

predicted high 

trout recruitment 

in Glen Canyon, 

taking into

consideration 

Tribal concerns

Hydrograph Elements for TMF implementation

The 2016 LTEMP ROD includes normal operating restrictions for operation of the dam (Table 

4). There are also certain guidelines for implementing a TMF described in the ROD. A TMF

may be implemented up to three periods per month from May through August and feature 

repeated fluctuation cycles that consist of relatively high flows (e.g., 20,000 cubic feet per 

second [cfs]) sustained for a period of time (potentially ranging from 2 days to 1 week) that 

would inundate low-angle nearshore habitat and prompt young fish (specifically rainbow trout) 

to move into the shallows along the channel margins (Table 4). The high flows would be 

followed by a rapid drop to a very low flow (e.g., 5,000 cfs to 8,000 cfs) within normal 

operations (Table 4) which would potentially strand YOY trout, and depending on the time of 

year, possibly expose eggs, and prevent them from hatching. The low flow would be maintained 

for a period of less than a day (e.g., 12 hr) to limit adverse effects to the food base (Table 4). A 

low flow during the day is expected to have the best chance of exposing stranded fish to direct 

sunlight and heat, thereby increasing mortality. If the low flow was timed during the day, 

operating restrictions specify that flows must be ≥ 8,000 cfs from 7 am – 7 pm. Up-ramp rates 

for the TMF would be the same as the limit for this alternative overall (i.e., 4,000 cfs/hr). The 

down-ramp from peak to base would be over a single hour (e.g., 15,000 cfs/hr for a drop from 

20,000 cfs to 5,000 cfs). 
10

Table 4. LTEMP requirements for normal operations (LTEMP ROD page 30; B-3) 

Implementation 

timeframe

# of 

cycles

per 

month

Peak 

magnitude Minimum flow Ascending ramp rates

Descending 

ramp rates

Maximum 

daily flow 

range* 

May to August ≤3 ≤ 25,000 for 

normal 

operations 

for 2 days to 

1 week

5,000-8,000 cfs 

for < 1 day (e.g. 

12 hr)

(5,000 from 7 

pm-7am and 

8,000 from 7 am7pm)

≤4,000 cfs ≤ 2,500 cfs

(can exceed 

this for a 

TMF)

10 x monthly

volume (in kaf) 

June to Aug., 

and 9 x monthly 

volume in other 

months not to 

exceed 8,000 cfs

*The maximum daily flow range is based on the monthly release volumes which vary depending on the 

flow parameters determined during the August 24-month study for the next water year. 

In addition to the TMF described in the LTEMP ROD, a more recent hypsometric analysis 

conducted by GCMRC includes two additional alternatives for the hydrograph (Giardina et al. 

2022). The first is to implement a TMF during years when equalization between Lake Powell 

and Lake Mead is required such as last occurred in 2011. Rainbow trout recruitment would be 

expected to be higher during equalization years (Avery et al. 2015). During equalization years, 

flows are higher and steadier than normal operations through spring and summer which provides 

sufficient time for age-0 trout to colonize the newly wetted habitat before the flows are dropped 

back to normal operational levels. The second alternative is referred to as “go low, not high.” In 

this alternative, flows are rapidly decreased from the normal peak flow to lower than the normal 

minimum flow with the flow dropping to as low as ≈ 5,000 cfs which results in stranding in 

algae and submerged vegetation (Giardina et al. 2022). Downstream effects are minimized by 

using a short-duration low flow period followed by a rapid increase to a higher flow (Giardina et 

al. 2022). Wave attenuation dynamics predict that the rapid increase to a higher-level flow 

would move downriver so quickly that it would catch up with the lower flow released earlier in 

the day (Giardina et al. 2022). The potential impact to other resources such as downstream 

habitats utilized by humpback chub or recreation could be minimized with this strategy (Giardina 

et al. 2022). 

Monitoring & Effectiveness

If TMFs are determined to be effective for controlling trout numbers while minimizing impacts 

to other resources, they may be deployed as an adaptive experimental treatment triggered by 

estimated trout recruitment. The efficacy would be measured by a reduction in trout numbers

after accounting for compensatory survival1 of remaining fish, an increase in the size and body 

condition of the rainbow trout at Lees Ferry, and either increased or stable humpback chub 

survival. Ongoing long-term research and monitoring programs that collect trout and humpback 

chub data (Trout Recruitment and Growth Dynamics (TRGD) project, Juvenile Chub Monitoring 

(JCM)-east, JCM-west and LCR sampling) would help inform these metrics. It is unknown if 

the flow modifications would result in stranding and mortality of juvenile humpback chub and

depending on the timing it may overlap with periods when larval humpback chub are found in 

nearshore habitats. 

1 The natural mortality rate decreases when the population falls below a certain level to account for the lower 

population size. 
11

Near the LCR confluence, under current temperature releases, negative impacts to YOY 

humpback chub would likely be minimal because spawning occurs in the LCR, fish that leave 

the LCR at smaller sizes are already expected to have extremely low survival, and most YOY 

leave in July or later when they are relatively large and capable of moving to avoid stranding

(Yackulic et al. 2014). Warming mainstem conditions near the LCR could lead to increased 

survival of smaller fish that leave the LCR earlier in the season or potentially lead to mainstem 

spawning. Either of these conditions would increase the potential for stranding effects. In 

addition, mainstem spawning most likely occurs in the western Grand Canyon (Van Haverbeke 

et al. 2017), so early life stages of humpback chub may already be vulnerable to stranding though 

changes in flow decrease with increasing distance from the dam. On the other hand, a TMF is 

expected to benefit humpback chub by reducing rainbow trout and possibly other non-native 

predators, while not affecting existing habitat used by humpback chub. Either way, the impact of 

the TMFs on humpback chub and trout would be evaluated.

If TMFs prove to be effective in controlling trout production and emigration, regular 

implementation of TMFs may need to include variable timing to prevent adaptation of the 

population to specific timing (e.g., increase in recruitment by fall-spawning rainbow trout) 

(LTEMP ROD page 52, B-25) or to account for compensatory survival.

Summary 

TMFs are intended to reduce the recruitment of rainbow trout within Glen Canyon. Higher 

recruitment leads to increased competition and lower condition of rainbow trout in the Lees 

Ferry recreational fishery which increases the likelihood of downriver dispersal and competition 

with and predation on humpback chub. Although implementation of a TMF may reduce 

recruitment of trout, it may negatively impact other resources by increasing energy generation

costs, increasing beach erosion, conflicting with tribal concerns with the taking of life, and 

potentially stranding native fish species. There are also various design aspects that need to be 

determined prior to implementation. In addition, rainbow trout populations have remained low, 

and brown trout populations have increased drastically in Glen Canyon since 2016. Brown trout 

predation on humpback chub is now of more concern than rainbow trout, and because brown 

trout can outcompete and prey on rainbow trout, they are a threat to maintaining a viable 

recreational fishery in Glen Canyon. However, TMFs were not originally designed to target 

brown trout.

Other Considerations

Brown trout

Background

Brown trout were originally introduced into several of the Colorado River tributaries in the 

1920s and 1930s. Since 2010, annual control efforts have been conducted in the Bright Angel 

Creek tributary to reduce the number of brown trout to reduce the risk of predation to humpback 

chub (Healy et al. 2020). The LCR is the most important tributary for humpback chub spawning 

and growth, and annual monitoring is conducted to ensure that recovery efforts are effective. 

Brown trout numbers have been increasing in Glen Canyon over the last several years (Table 5), 

which is due to successful reproduction and recruitment of locally hatched fish into the spawning 

class in combination with immigration from downstream reaches (Runge et al. 2018). This 

increase in brown trout population has likely led to increased competition with rainbow trout. 
12

Brown trout spawn from October to December with emergence occurring approximately two

months later; young fry are most vulnerable to fluctuations in flow following emergence (Runge 

et al. 2018). Based on a synthesis of data across tailwaters in the western U.S., high flow 

velocity during winter months when fry are growing and rearing can decrease brown trout 

recruitment (Dibble et al. 2015), possibly due to displacement or high energetic costs that 

decrease survival. Even though adult brown trout may not be directly affected by high flow 

volume, ultimately, flow can affect recruitment thereby altering fish density, which plays a role 

in determining adult size (Dibble et al. 2015). 

Although diets of brown trout within their native range are dominated by aquatic invertebrates, 

in areas outside their native range such as the Colorado River, their diet switches to include fish 

which also allows them to grow larger in size (Budy et al. 2013). Their feeding strategy is 

flexible and allows them to feed on large numbers of small prey or small numbers of large prey 

(Sanchez-Hernandez and Cobo 2015). This is concerning not only due to the negative impact on 

native fish such as humpback chub, but it also means that they pose a threat to rainbow trout in 

Lees Ferry. Brown trout condition is higher than rainbow trout in Lees Ferry, which is likely due to 

their piscivorous diet as well as differences in their morphology (Yard & Korman, 2020 ARM). 

In 2020, the number of small adult brown trout (200-349 mm) in Glen Canyon was estimated to 

be 7,000 and the number of large adults (>350 mm) was estimated as 4,300 (Table 5; Yackulic

unpublished data). The population has been steadily increasing since 2015 (Table 5). Though 

most salmonids are not effective predators until they are > 300 mm, brown trout are piscivorous 

at smaller sizes and are tolerant of warmer water temperatures, making them more of a threat to 

humpback chub and other native fish (Ward and Morton-Starner 2015). However, brown trout 

are less effective predators of humpback chub as turbidity increases (Ward et al. 2016). 

Year

>200-349 mm

n (95% CI)

>350mm

n (95% CI)

2015 1,400 (900-2,100) 1,400 (900-2,000)

2016 2,000 (1,500-2,600 1,300 (900-1,800)

2017 3,300 (2,500-4,400) 1,600 (1,100-2,100)

2018 4,200 (2,200-6,500) 3,400 (1,400-5,600)

2019 6,100 (4,900-7,300) 4,200 (3,200-5,200)

2020 7,000 (5,800-8,400) 4,300 (3,300-5,400)

Tools for managing brown trout

Many of the potential tools available for managing brown trout are the same as the ones for 

rainbow trout (Table 2). There are a few additional tools that might be applied specifically for 

brown trout (Table 6). For example, the National Park Service is currently evaluating an 

incentivized harvest program that rewards anglers for catching and retaining brown trout at Lees 

Ferry with the objective of reducing the population to an acceptable level. However, preliminary 

results from the program suggest an insignificant effect on overall population abundance due to 

low harvest rates (https://www.nps.gov/glca/planyourvisit/brown-trout-harvest.htm). 

Table 5. The fall population estimate (n) of adult (>200 mm) brown 

trout at Lees Ferry with 95% confidence intervals from 2015-2020.
13

Trout Management 

Tool or Experimental 

Action

Advantages Disadvantages 

Brown trout stocking

(Trojan Y chromosome) 

 Stocking with YY female brown trout 

would result in all XY male offspring

(prevent reproduction) 

 Efficacy is highly uncertain

 May still have negative effects on 

rainbow trout & humpback chub 

 Temporary increase in adult brown 

trout due to stocking 

 Increased abundance may cause 

outmigration of brown trout into 

Marble Canyon

Incentivized harvest  Species-specific

 May result in an increase in size of 

rainbow trout 

 May alleviate tribal concerns 

assuming fish are consumed

 May be difficult to convince 

anglers to retain brown trout 

 May not be effective at reducing 

the population 

Vegetation (aquatic) 

removal

 Removing habitat assumed to be used 

by larval brown trout may result in 

decreased recruitment

 Limited impacts to other resources

 Efficacy is highly uncertain

 May require additional compliance 

 May be expensive 

 May push other non-native fishes 

downstream

Considerations for adapting hydrograph

The current design of the TMF experiment is unlikely to be effective for reducing recruitment of 

brown trout because the timeframe the experiment can be implemented (May through August) is 

after the two-month window when early life stages of brown trout are most vulnerable to flow 

(February through April). However, recent research suggests that larval brown trout may not 

utilize nearshore habitats but instead retreat to vegetation in deeper waters (Dibble, unpublished 

data). Because ambient temperatures are also typically lower February through April, survival 

rates of brown trout fry that have been stranded in shallow pools by low flows may be 

sufficiently high to prevent a population level effect on adult abundance. If this occurs, 

additional cycles/replicates of the TMF may be necessary to account for potentially higher 

survival rates due to lower temperatures. Although air and water temperatures are warmer by 

May when TMFs can currently be implemented (meaning mortality rates of stranded fish might 

be higher than February through April), YOY brown trout are sufficiently large and capable 

swimmers by May making them much less vulnerable to stranding during low flows. Thus, 

implementation of TMFs to limit brown trout recruitment would likely be most effective if 

implemented from February through April to coincide with emergence (Giardina et al. 2022), yet 

current compliance only allows for implementation from May through August. Additional 

environmental compliance would be needed to allow for TMF implementation in February, 

March or April when it is likely to be most effective at reducing brown trout.

Research Updates

Recruitment forecasting & modeling

Recent research suggests that condition of rainbow trout in the fall is a good predictor of sexual 

maturation rates for the following winter and spring (Korman et al. 2021). When factors support 

Table 6. Tools and experimental actions that may be used to manage brown trout populations (table created 

based on information from Bair et al. 2017 presentation on management options during Brown Trout 

Workshop 2017, NPS Non-Native Aquatic Species EA 2018 & subsequent ideas by stakeholders). 
14

increased growth rates (i.e., prey availability, water temperature, etc.), fish condition is high (≥ 1: 

mass of an individual relative to the average mass of others of the same length), and the 

likelihood that the trout will reach sexual maturity and spawning activity increases. 

Understanding how condition affects sexual maturation and recruitment may help develop a 

more reliable method for forecasting recruitment which is the metric for determining when to 

implement a TMF. Recruitment forecasting can most reliably be determined using field data in 

July which is in the middle of the window when TMFs can be implemented. 

The current nutrient model is perhaps a better method of predicting rainbow trout recruitment 

and it uses soluble reactive phosphorous in Lees Ferry as a predictor. Predictions of rainbow 

trout recruitment could potentially be improved by examining the amount of soluble reactive 

phosphorous during the previous year and the in-flows into Lake Powell in the current year. 

Recruitment estimates for rainbow trout are calculated based on the soluble reactive phosphorous 

inputs into Lake Powell from the current year and the number of rainbow trout >150 mm based 

on catch per unit effort data from the previous year (Yackulic, unpublished data). The model 

estimates could provide enough accuracy to aid with management decisions. However, the 

rainbow trout recruitment estimates may be below the model predictions if brown trout 

populations continue to increase and suppress rainbow trout.

Literature review & hypsometric analysis

GCMRC recently completed a literature review that examines the key factors affecting fish 

stranding and includes recommendations to guide the timing and flow rate for designing a TMF, 

based on existing literature (Giardina et al. 2022). The report also includes a hypsometric 

analysis to predict stranding risk for age-0 trout in Glen Canyon under a range of TMFs, using 

existing bathymetric data, flow, and habitat suitability models. The study examines stagedischarge relationships and hydraulic model predictions of depth and velocity. 

Although the literature review and hypsometric analysis focused on rainbow trout, the key 

findings below may apply to rainbow trout and brown trout (Giardina et al. 2022): 

 The greater the difference between the high and low flow elements of a TMF cycle, the 

greater the stranding risk.

 A steady flow of 16,000 cfs followed by a rapid drop to a low flow of 3,000 cfs would 

also be effective at stranding trout and may have less impacts to generation of electricity

and sediment than a high flow of 25,000. 

 Stranding risk is predicted to increase with decreases in the low flow element of the 

TMF.

 Compensatory survival can occur after a TMF meaning that repeated cycles are needed to 

be effective. 

 Repeated cycles may also be necessary to account for the difference in size of susceptible 

trout due to the prolonged emergence period.

 If a TMF hydrograph design utilizes an increase from normal operations to a high flow 

prior to dropping to a low flow, the duration of time that the flow needs to remain high 

for fish to colonize the newly wetted habitat is unknown; the best available evidence 

indicates this process of colonizing newly wetted shorelines may take several weeks. 
15

Brown Trout Early Life History 

A project to examine the early life history of brown trout similar to one that was conducted for 

rainbow trout was included in the FY21-23 Triennial Budget and Work Plan. The data obtained 

from this project will increase understanding of early life stage vital rates for brown trout, assess 

hatch and swim-up dates to identify when brown trout emerge from gravel redds, and identify 

habitat use in low vs. high-angle shoreline habitat to understand stranding vulnerability to 

experimental flows, including TMFs. Initial results from this project suggest that there is 

uncertainty whether larval brown trout are utilizing nearshore habitats during their early life 

history meaning they may not be susceptible to nearshore stranding resulting from a TMF. 

During the spring disturbance flow in March 2021, flows were dropped to 4,000 cfs followed by 

a high flow of approximately 20,000 cfs. Since the low flow coincided with peak brown trout 

emergence it provided a unique opportunity to collect additional data on age-0 brown trout 

susceptibility to flows. However, since 2021 was the first year of the brown trout early life stage 

study (BTELSS), baseline data for comparison to a year that lacked a spring disturbance flow 

cannot be collected until FY22. Thus, it may be a few years before the effectiveness of the 

spring disturbance flow at reducing brown trout survival can truly be evaluated.

Trout Recruitment, Growth & Population Dynamics

The trout recruitment, growth and population dynamics (TRGD) project is ongoing and provides 

a long-term dataset and information on rainbow and brown trout populations in Lees Ferry. 

TRGD project data is used to determine the effects of experimental flows on trout recruitment, 

growth rates of juvenile and adult trout, and dispersal of age-0 trout from Glen Canyon to Marble 

Canyon. In addition, the TRGD project aims to understand other key factors (besides flow 

manipulation) that control the abundance and growth of trout populations, including fish density, 

prey availability, and nutrient concentrations from Lake Powell. An improved understanding of 

other ecosystem drivers that benefit the rainbow trout fishery could lead to the identification of 

non-flow policies that may limit downstream dispersal of rainbow trout and potentially control

brown trout populations. 

Bioeconomic model

A bioeconomic model to assess the impact of removing rainbow trout under different scenarios 

on adult humpback chub abundance has been developed to assist with management decisions 

(Bair et al. 2018, Donovan et al. 2019). The model is based on general assumptions in the 

LTEMP EIS and suggests that to cost-effectively meet humpback chub abundance goals, TMFs 

should be considered when rainbow trout recruitment is high (>1,000,000), rainbow trout 

populations in Marble Canyon and the JCM-east Reach (RM 63.45-65.2) are high (>290,000 and 

>2,000), and the humpback chub population is low (<7,000) (Donovan et al. 2019, Bair, 

unpublished data). Even when humpback chub populations are low, it is more cost effective to 

conduct mechanical removals than implement a TMF unless rainbow trout recruitment and 

populations in Marble Canyon and the JCM-east reach are high (Figure 2). One year of 

mechanical removals of rainbow trout at the LCR is estimated to cost $450,000 based on 

$75,000 for every month of removal (personnel, removal equipment, logistics, etc.) and 6 months 

of trips (Bair et al. 2018). The cost of implementing a single TMF is estimated as $450,000 (EIS 

Appendix K page K-114). 
16

Although the model was developed and applies specifically to rainbow trout it could be updated 

to incorporate brown trout. Brown trout parameters would have to be incorporated into the 

existing model, and the margin of error may be much greater than that for rainbow trout since 

there are many unknowns, but it would provide an idea of the targets for and the feasibility of 

removing brown trout. 

Figure 2. The conditions under which mechanical removal and a combination of trout management flows and 

mechanical removal are cost-effective actions to meet humpback chub abundance goals. At a humpback chub 

population of approximately 8,000 it is most cost-effective to conduct mechanical removal (when other conditions 

warrant action). It does not become cost-effective to implement trout management flows until the humpback chub 

population falls below 6,500, the rainbow trout population in the juvenile monitoring reach is greater than 2,800, and 

the number of rainbow trout in Marble Canyon exceeds 300,000 adults and 1,000,000 recruits (Bair unpublished 

data).

Model to examine turbidity

Development of a model to examine how dam operations could be adjusted to increase turbidity 

may be helpful in determining whether it could be used to improve conditions for humpback 

chub and decrease efficiency of sight-feeding predators such as rainbow trout in Marble Canyon 

and near the LCR. As turbidity increases, reactive distance (feeding efficiency) decreases. Since 

higher reactive distance (i.e., lower turbidity) is positively related to rainbow trout growth, 

survival, and reproduction (Korman et al. 2021), and more rainbow trout are associated with 

lower juvenile humpback chub survival (Yackulic et al. 2018), an increase in turbidity could 

reduce predation. As such, a turbidity model could evaluate different scenarios and provide 

inputs for the bioeconomic model to aid decision making on dam operations. 

Sources of Uncertainty 

Though research over the last few years has helped provide clarity regarding some of the key 

questions (Table 7), there are still many elements for which answers might not be obtained until 

full implementation of a TMF.
17

Flow design

Although the LTEMP ROD provides parameters for base operations, it does not specify the exact 

design for the TMF but rather provides limits within which the TMF must be conducted. The

design for implementing a successful TMF is unknown other than it must fall within the 

following limits: 

• The maximum peak discharge of the TMF cycle must be below 25,000 cfs, but the 

threshold and duration that would be most effective at luring trout into the shallow water 

habitats is unknown. 

• The time interval between reaching the maximum flow and initiating the flow recession 

is also a key uncertainty. 

• The downward ramp rate can exceed the operational limitation of 2,500 cfs/hr, and while 

the hypsometric analysis suggests that faster rates optimize stranding, the specific down 

ramp rate needed is uncertain (Giardina et al. 2022). 

• The minimum flow must be between 5,000-8,000 cfs depending on the time of day, but 

the minimum discharge at the peak of the TMF cycle is also unknown. The hypsometric 

analysis predicts that a low flow of 3,000 cfs would be most effective at stranding.

• The duration of the flow regardless of whether the flow starts out high or low is 

unspecified.

• Although there can be up to 3 cycles per month from May through August, the number of 

TMF cycles and the timing during that period (only in May and June, May, June & July, 

etc.) that are needed to account for compensatory survival and the extended emergence to 

have the desired affect are unknown. 

• High recruitment is the trigger for a TMF, but it cannot be estimated using field data until 

the timeframe when the TMF is meant to be implemented. The current recruitment 

model uses the amount of soluble reactive phosphorous in the spring to estimate 

recruitment. The model could be modified to predict soluble reactive phosphorous in the 

spring by using the observed amount of soluble reactive phosphorous from the prior 

winter or fall. This would allow trout recruitment to be forecasted prior to the window 

when a TMF would need to be implemented. 

• It is unknown exactly when the TMF cycle should be implemented but it is assumed to be 

most effective in May and June when peak rainbow trout emergence occurs and young 

trout are most vulnerable to flow changes (Yard & Korman, ARM 2020). If targeting 

brown trout it would likely need to occur between February and April. 

• Based on the hypsometric analysis, the TMF could be designed with a “go high, then 

low” flow (as specified in the LTEMP ROD), an equalization flow, or a “go low, not 

high” approach (Giardina et al. 2022). Predicted stranding risk would vary depending on 

the approach selected and may have different impacts on other resources depending on 

the strategy (Giardina et al. 2022).

Key elements

There are several key elements that are uncertain; some of these can be resolved with study while 

others may not be known until a TMF is implemented (Table 7). 
18

Table 7. Key Uncertainties and Possibility for Resolving Uncertainties with Experimentation (Adapted

from discussions that occurred by a stakeholder group during a series of meetings in 2019).

Key Elements Questions

Elements that can be resolved with study

(lab-based, models, or by examining 

historical data)

 Are YOY brown trout susceptible to flow manipulations?

 Are YOY brown trout utilizing near-shore habitat where 

they would be susceptible to flow variations?

 What is the shortest duration of the low flow to be 

effective? 

 What time of day is most effective?

Elements that can be resolved with small 

scale field testing

 What is the effectiveness of a TMF on different size 

classes?

Elements that can be resolved with full 

implementation

 Once the flow has dropped, what proportion of the 

population is removed?

 What flow recession rates are effective at stranding?

 What is the duration of the high flow that needs to be 

maintained for fish to colonize?

 How many cycles are needed to account for 

compensatory survival or extended emergence?

 What is the accuracy of recruitment forecasting?

Elements included in FY21-23 triennial 

budget & work plan (from Project H; TWP 

2021-2023) that will help inform TMFs

 Understand the early life history stages of YOY brown 

trout in Glen Canyon

 Determine hatch and swim-up dates to identify when 

brown trout are likely to be emerging from gravel redds

 Identify habitat preferences for low angle and high angle 

nearshore habitat to understand whether brown trout are 

susceptible to flow manipulation.

 Determine the effects of flows on recruitment of YOY 

rainbow and brown trout in Glen Canyon, growth rate of 

juveniles & adults, and dispersal of YOY trout from Glen 

Canyon

 Understand the key elements of flow in the design of a 

TMF

 Is it necessary to implement a high flow followed by a 

drop or would a rapid drop from normal flow be 

sufficient to reduce YOY numbers?

 Understanding how trout condition affects sexual 

maturation and recruitment, which may help develop a 

more reliable method for forecasting recruitment 

Impacts to other resources

TMFs were designed specifically to reduce recruitment of rainbow trout in the Glen Canyon 

reach to reduce the likelihood that they will move downstream and negatively impact humpback 

chub. However, the impacts of the flow on humpback chub are uncertain and the most effective 

flow components for designing the hydrograph are unknown. It is possible that social, 

recreational, environmental, and electricity generation costs may outweigh the uncertain benefits 

of TMFs. The effectiveness of implementing a TMF relies on the assumption that rainbow trout 

are the primary factor limiting humpback chub recruitment and adult abundance, and that 

implementation will impact rainbow trout population dynamics. However, other factors such as 
19

water temperature and food availability may be just as or more important in influencing 

humpback chub growth and survival rates (Yackulic et al. 2018). 

The extent to which brown trout utilize nearshore habitat was originally thought to be minimal

during the timeframe when they would most likely be susceptible to stranding (Yard & Korman, 

Annual Reporting Meeting 2020). However, brown trout numbers in Glen Canyon have been 

increasing and a project in the FY21-23 Triennial Work Plan includes a project to examine this 

more closely. Larval fish collected from shoreline habitats in 2021 and during the first four trips 

in 2022 were all identified as rainbow trout, suggesting that brown trout may not be utilizing 

shoreline habitats in early life stages in the same way as rainbow trout fry (Dibble, unpublished 

data). This means that a steady flow designed to lure fry into the nearshore environment to 

strand fish upon flow reduction may not work for this species since they are utilizing alternate, 

and perhaps offshore habitats. 

Both rainbow and brown trout preferentially prey on native fishes including humpback chub

compared to non-native fishes (Yard et al. 2011). However, rainbow trout are a desired species 

in the Glen Canyon reach due to the GCDAMP resource goal of a recreational sport fishery. 

Rainbow trout are also more susceptible to trout management flows during the timeframe that is 

specified in LTEMP and the habitats that early life stages occupy. Brown trout are perceived as 

a bigger threat when abundance is high because they are more piscivorous than rainbow trout. 

Tribal Perspectives

The tribes that are part of the GCDAMP shared the following perspectives related to TMFs. 

This section is not a substitute for formal consultation but is meant to provide a deeper 

understanding of the tribal perspective and relationship to life and the Grand Canyon. Tribal 

concerns are an important component of a decision to implement a TMF. 

Hopi (submitted by Jakob Maase)

Although the Hopi support removal of non-natives to protect native species, they do not support 

the taking of life for no purpose. Consumption of the removed fish by humans or eagles or 

another bird or animal would be consistent with Hopi values. Hopi are stewards of the land and 

recognize the interconnectedness of the ecosystem and are sensitive to the effects of each 

resource on the other. A TMF would be inconsistent with Hopi values because the larval fish 

that would be removed during the flow would not be used for another purpose.

Southern Paiute Consortium (submitted by Daniel Bulletts)

The Southern Paiutes have a deep spiritual, physical, and emotional connection to the Grand 

Canyon and the Colorado River. To place something there that is foreign to the ecosystem is not 

only wrong but will have some long-lasting negative affect over time and affect all that live and 

thrive in the Grand Canyon. Trout management flows conflict with our cultural connection to 

the Grand Canyon and Colorado River.

The Southern Paiute Consortium perspective related to trout management flows is simple. The 

more you try to manage and control the worse things will get. If the trout was meant to be in the 

Colorado River system Mother Nature will help that along but if they are not, then they will 
20

leave. Southern Paiutes believe that all life is special and has its place in the world and Mother 

Nature takes care of all life in her own special ways.

Pueblo of Zuni (adapted from Runge et al. 2015)

The Grand Canyon is the place of Zuni emergence into this current world at a place called 

Chimik’yana’kya dey’a, also known as Ribbon Falls in Bright Angel Canyon. The natural 

environment that Zuni people saw at Emergence became central to traditional Zuni culture. In 

fact, all of the plants that grow along the stream from Ribbon Falls to the Colorado River, and all 

the birds and other animals, springs, minerals and natural resources located in the Grand Canyon 

and its tributaries, have a central place in Zuni traditional cultural practices and ceremonial 

activities. The confluence of the Little Colorado and Colorado Rivers is understood to be a 

spiritual umbilical connection between the Pueblo of Zuni and the Grand Canyon that is 

facilitated through the union of the Zuni River with the Little Colorado and the Colorado Rivers. 

The confluence is also held by the Zuni people to be an extremely important and sacred place 

because of its abundance of aquatic and terrestrial life that simultaneously expresses and 

represents the fertility of nature. 

The Colorado River is a particularly important place to the Zuni people because it was the 

location of an important historical event. This historical event was conveyed to Frank Hamilton 

Cushing, an American Anthropologist, by the Zuni in the late nineteenth century and is 

summarized below to convey the deep, intense, and remarkable significance that the Colorado 

River and the aquatic life within it indelibly hold for the Zuni people. 

“Shortly after Emergence, men of the Bear, Crane, and Seed clans strode into the red waters of the 

Colorado River and waded across. The men of the clans all crossed successfully. The women travelling 

with them carried their children on their backs and they waded into the water. Their children, who were 

unfinished and immature (because this occurred shortly after Emergence), changed in their terror. Their 

skins turned cold and scaly and they grew tails. Their hands and feet became webbed and clawed for 

swimming. The children fell into the swift, red waters. Some of the children became lizards, others turned 

into frogs, turtles, newts, and fish. “The children of these clans were lost to the waters. The mothers were 

able to make it to the other side of the river, where they wailed and cried for their children. The Twins 

heard them, returned, and advised all the mothers to cherish their children through all dangers. After 

listening to the Twins, those people who had yet to pass through the river took heart and clutched their 

children to them and safely proceeded to the opposite shore. “The people who successfully made it out of 

the river rested, calmed the remaining children, and then arose and continued their journey to the plane 

east of the two mountains with the great water between.” (Cushing, 1896, 1920, 1988; as summarized in 

Dongoske and Hays-Gilpin, 2016).

As a consequence of this historical event, all aquatic life is recognized by present day Zunis to 

be descendants of those Zuni children who were lost to the waters, thus creating a strong and 

lasting familial bond to all aquatic life and a fundamentally important stewardship responsibility. 

The animals, including all aquatic life, birds, plants, rocks, sand, minerals, and water in the 

Grand Canyon convey special meaning and have significant material and spiritual relationships 

to the Zuni people. To needlessly take life causes an imbalance in the natural world, and also 

disturbs the harmony and health of the spiritual realm and the Zuni peoples. As such, the Zuni 

people believe that actions that occur in the Colorado River in Grand Canyon directly affect the 
21

Zuni people; for example, they believe that removing non-native fish may result in the demise of 

Zuni youth and pregnant women.

Navajo Nation (submitted by Erik Stanfield)

The overall ethic of Navajo Nation in relation to resource management is to find balance and 

beauty with the natural environment by avoiding invasive management practices that conflict with 

the paramount tribal value of maintaining natural and self-sustaining processes. This ethic also 

includes the avoidance of introducing and/or maintaining non-native populations of any species. 

TMFs conflict with tribal values in their support and maintenance of non-native species since these 

flows are not intended to mimic natural flows/processes but are to limit the spread of rainbow trout 

below Lees Ferry, and not an attempt to reduce or eliminate them. This activity does not support a 

natural process, highlights a conflict in balanced environmental management by actively 

supporting the existence/management of non-natives, and promotes an incoherent management 

strategy in the Colorado River. 

Generally, TMFs are not in line with Navajo Nation values or with the natural process (and other 

compatible) LTEMP goals because of the narrow focus of the activity. It is, however, recognized 

TMFs may be important in providing some short-term protection for native species and in 

observing their efficacy as a management technique for potential use with other non-natives in 

the future. 

Hualapai (submitted by Carrie Cannon) 

Hayiŧađa, the Hualapai name for the Colorado River, is a descriptive terminology that refers to 

the water as the backbone or spine. Hualapai ancestral land base encompasses 7 million acres of 

northwestern Arizona bound by the Colorado River to the north and west, the Little Colorado 

River to the east, and the Bill Williams and Santa Maria Rivers to the south. Although the 

Hualapai ancestral land base once included a far greater region, our present-day Reservation 

encompasses 1 million acres which includes the mid-point of the Colorado River from river mile 

165 to river mile 273 as the northern boundary. The Hualapai have had extensive relationships 

with tribal neighbors such as the Southern Paiute to the north, the Hopi to the east, and many 

other tribes in the region which also call the Grand Canyon home. 

Hualapai look at the entire ecosystem in its totality, including the rock elements, the plants, the 

birds, insects, fish, mammals, the river itself, and their role among them as caretakers of this land 

that they call home. The TMF is an effort to restore an aspect of this ecosystem back to its 

former realm by working to find ways to promote the native fish that once abounded. Hualapai 

support decisions that attempt to restore balance and health to how the ecological system once 

functioned pre-dam. We are presented with many challenges in our modern era, including how 

the dam has dramatically changed the original landscape. Although Hualapai are working within 

a different set of circumstances than their ancestors encountered, they believe in adapting to seek 

the highest good for the land and its many inhabitants. 

Recommendations/Next Steps

There are many unknowns related to whether implementation of a TMF experiment would be 

successful or not. The hydrograph that would be most effective needs to be determined. The 

first step would be to re-examine the existing management and experimental actions available for 
22

managing trout (Tables 2, 6), and to determine whether there are any that could be explored that 

would have fewer negative effects on other resources. Since the focus of this evaluation was on 

TMFs, these tools were not thoroughly evaluated in this report. It would also be valuable to 

determine if there are additional tools that could be evaluated or implemented. 

It is unclear whether the costs to other resources of implementing a TMF outweigh the benefits. 

Implementation of a TMF conflicts with tribal concerns with taking of life (Appendix A) and 

anglers’ concerns about the impact on the recreational rainbow trout fishery. However, if the 

recruitment model predicts that rainbow trout recruitment will be very high or the TMF is 

expected to positively impact humpback chub, implementation might be considered. Regardless, 

if a TMF is to be further considered as an option, there are several questions that could be 

addressed by designing a study to make a more informed decision related to implementation

(Table 7). 

 

If a TMF is determined to be a viable option or there is a desire to implement it as an 

experimental action, the bioeconomic model could be used to identify the conditions under 

which it should be implemented, similar to the sediment conditions required for implementing a 

high flow experiment. A multi-agency group of experts could then be assembled to evaluate the 

conditions and determine whether implementation of a TMF was warranted. As with the 

implementation of any experiment, the current conditions should be considered and a TMF 

should only be implemented if it is determined that no unacceptable adverse impacts are likely to 

occur. Per the LTEMP ROD, an unacceptable adverse impact will include significant negative 

impacts on resources as a result of experimental treatments that were not analyzed for 

Alternative D in the LTEMP EIS.

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