STUDY PLAN TO EXAMINE THE EFFECTS OF USING 

LARVAL RAZORBACK SUCKER OCCURRENCE IN THE GREEN 

RIVER AS A TRIGGER FOR FLAMING GORGE DAM PEAK RELEASES

Prepared by the Larval Trigger Study Plan Ad Hoc Committee

Kirk LaGory, Argonne National Laboratory

Tom Chart, Upper Colorado River Endangered Fish Recovery Program

Kevin Bestgen, Colorado State University, Larval Fish Laboratory

Jerry Wilhite, Western Area Power Administration

Shane Capron, Western Area Power Administration

David Speas, Bureau of Reclamation

Heather Hermansen, Bureau of Reclamation

Kevin McAbee, U.S. Fish and Wildlife Service

Jana Mohrman, U.S. Fish and Wildlife Service

Melissa Trammell, National Park Service

Brandon Albrecht, Environmental Group Representative

Coordinated by 

The Upper Colorado River Endangered Fish Recovery Program 

Final Report March 2012
Larval Trigger Study Plan ii March 2012

TABLE OF CONTENTS

EXECUTIVE SUMMARY ...........................................................................................................iii

INTRODUCTION .......................................................................................................................... 1

PROPOSED MONITORING AND RESEARCH.......................................................................... 4

Topic 1: Entrainment and Retention of Razorback Sucker Larvae in Floodplain Wetlands...... 9

Topic 2: Survival and Escapement of Entrained Razorback Suckers....................................... 11

Topic 3: Availability of Colorado Pikeminnow Habitat........................................................... 13

Topic 4: Sediment Mobilization and Channel Maintenance..................................................... 14

Topic 5: Fish Community Response......................................................................................... 15

RESEARCH PRIORITIES AND IMPLEMENTATION............................................................. 17

REFERENCES ............................................................................................................................. 20
Larval Trigger Study Plan iii March 2012

EXECUTIVE SUMMARY

Flow recommendations were developed for the Green River below Flaming Gorge Dam 

by Muth et al. (2000) to assist with conservation and recovery of endangered fishes. These flow 

recommendations identified annual peak flow magnitudes and durations needed to connect the 

river to razorback sucker floodplain nursery habitats in the middle Green River (Table 1). In 

order to achieve these recommended peak flow magnitudes and durations, the Bureau of 

Reclamation (Reclamation) has timed the release of water from Flaming Gorge Reservoir to 

match the peak flow in the Yampa River. A primary purpose of those spring operations at 

Flaming Gorge Dam is to provide nursery habitat for endangered razorback sucker (Xyrauchen 

texanus) in the middle Green River so early life stages (larvae) can access productive floodplain 

wetlands via connections with the river. Despite successfully meeting or exceeding peak flow 

magnitudes and durations in the targeted reach, consistent and substantial razorback sucker 

recruitment has not been observed. In a recently completed synthesis report (Bestgen et al. 

2011), researchers concluded that in most years since 1993, releases from Flaming Gorge Dam 

occurred too early relative to presence of razorback sucker in the Green River. They 

recommended that the Upper Colorado River Endangered Fish Recovery Program (Recovery 

Program) and Reclamation implement a schedule of altered timing of flow releases from 

Flaming Gorge Dam to coincide more closely with presence of razorback sucker larvae, or 

perhaps, presence of abundant larvae, in the middle Green River. The Recovery Program has 

proposed that Reclamation use the occurrence of razorback sucker larvae in channel margin 

habitats (as determined by real-time monitoring) as the “trigger” to determine when peak 

releases should occur from Flaming Gorge Dam. Determining the effectiveness of this larval 

trigger in recruiting razorback suckers is the primary focus of this study plan, but other potential 

effects would also be evaluated.

Evaluating the effectiveness of operating Flaming Gorge Reservoir using a larval trigger 

requires a targeted hypothesis-based monitoring and research program that examines aspects of 

the life cycle and recruitment limitations of razorback sucker. The topics to be examined under 

the study plan, hypotheses to be tested within each, and the general methods to be employed are 

described here. Five topics are included in this plan: (1) entrainment and retention of larval 

razorback suckers in floodplain wetlands; (2) survival of larvae and escapement of juvenile and 

adult fish entrained as larvae into floodplain wetlands; (3) availability of young-of-the-year 

Colorado pikeminnow habitat at base flow; (4) sediment mobilization and channel maintenance; 

and (5) fish community response. Studies associated with Topics 1 and 2 are considered the 

highest priority because these studies address razorback sucker entrainment and recruitment, 

which are the intended benefits of using a larval trigger. Information from other species, 

particularly co-evolved native catostomids, as well as historical information, will be used to 

support patterns observed for razorback sucker particularly if their larvae are rare in some years. 

Topics 3, 4, and 5 address other potential consequences of using a larval trigger, and, although 

important, are considered lower priority for testing the efficacy of using a larval trigger. A total 

of nine hypotheses are identified under Topics 1 and 2; an additional eight hypotheses are 

identified under Topics 3, 4, and 5. 

Wetlands that hold the greatest promise for entraining and recruiting razorback suckers,

and that are representative of other wetlands in the system should be the focus of studies
Larval Trigger Study Plan iv March 2012

developed under this plan. Because study wetlands connect with the main channel at different 

flow levels, some can only be studied at higher peak flows. Under the study plan, up to eight

wetlands would be sampled in a given year with the three in the lowest flow category (Stewart 

Lake, Above Brennan, and Old Charley Wash) being sampled in all years. As practicable, 

proposed studies should address a range of flow magnitudes and durations, and we consider three 

years with flows < 18,600 cfs and three years with flows > 18,600 cfs, with connecting flows in 

each of these years of at least seven days duration, as minimally necessary to complete the study.

The specific objectives, tasks, and expected outcomes for individual studies developed 

under this plan will be identified in statements of work approved by the Recovery Program. 

These projects and the resulting project reports will go through the standard Recovery Program 

review protocols. It is anticipated that in addition to an annual review of the data collected, a 

synthesis report will be developed that summarizes results from individual projects, integrates 

results, summarizes conclusions, and makes recommendations for future implementation of a 

larval trigger. As for any study plan, additional knowledge will be gained during implementation, 

and it will be important to have enough flexibility to adjust studies and overall approaches in 

response to this new information. Toward this end, the results of studies will be evaluated each 

year to determine the need for modification.

This Study Plan was drafted by an ad hoc Committee, which included representatives 

from Reclamation, Western Area Power Administration, Colorado State University, U.S. Fish 

and Wildlife Service, National Park Service, Argonne National Laboratory, and environmental 

interests. Development of the Study Plan was coordinated by the Recovery Program and 

benefited greatly from input by members of the Biology Committee and principal investigators 

conducting studies in the Green River Subbasin.
1 INTRODUCTION

The razorback sucker (Xyrauchen texanus), an endemic species of the Colorado River 

Basin, is federally listed as endangered. A critically important population of this species inhabits

the middle Green River, Utah, between the confluence of the Yampa River downstream to the 

head of Desolation-Gray Canyon. Razorback suckers congregate in spring at two spawning areas 

in the upstream portion of the reach, at Razorback (river kilometer [RK] 500.9) and Escalante 

(RK 493.7) spawning bars, and have successfully reproduced from 1992-2011, as evidenced by 

annual collections of larval fish downstream of spawning areas (Bestgen et al. 2011; annual 

Recovery Program reports, Project 22f). However, very few naturally produced razorback 

suckers recruit from the larval stage to sexual maturity. Researchers believe that in order to 

successfully recruit, young-of-the-year (YOY) need to overwinter for one or more years in offchannel floodplain nursery habitats before returning to the main channel (Muth et al. 2000). 

Because the river must reach a specific height before each nursery habitat is connected to the 

main channel, recruitment of larval razorback suckers to adulthood is closely tied to high spring 

peak flows. 

Flow recommendations (Muth et al. 2000) were developed for the Green River 

downstream of Flaming Gorge Dam to provide the necessary flows to support recovery of the 

razorback sucker and three other endangered fishes in the Green River (Colorado pikeminnow, 

Ptychochelius lucius; humpback chub, Gila cypha; and bonytail, G. elegans). These flow 

recommendations identified annual peak flow magnitudes and durations, as measured at the 

Jensen, Utah gage (U.S. Geological Survey [USGS] gage 09261000), needed to connect the river 

to razorback sucker floodplain nursery habitats in the middle Green River (Table 1). Although 

connection of these habitats to the river appears to be critical to razorback sucker recovery, 

connection is only biologically meaningful if it occurs when razorback sucker larvae are drifting 

in the water column and available for transport into floodplain habitats. In addition, floodplain 

wetland habitats must consistently offer suitable habitat (i.e., sufficient size, depth, and water 

quality) to support fish until subsequent annual peak flows reconnect habitats to the river and 

allow for escapement of subadults.

Following the Record of Decision for the Flaming Gorge environmental impact statement 

published in 2006, the Bureau of Reclamation (Reclamation), as operator of Flaming Gorge 

Dam, and in collaboration with the interagency Flaming Gorge Technical Work Group, has 

provided annual peak flows that meet or exceed the annual peak flow recommendations 

presented in Muth et al. (2000). In order to achieve these recommended peak flow magnitudes 

and durations, Reclamation has timed the release of water from Flaming Gorge Reservoir to 

match the peak flow in the Yampa River, thus, minimizing the amount of released water needed 

to achieve the peak flow targets. Although this operational approach is consistent with the 

recommendations in Muth et al. (2000), a recent synthesis by Bestgen et al. (2011) suggests that 

it may not be accomplishing its intended biological purpose, i.e., to provide for successful 

recruitment of razorback suckers. Razorback sucker recruitment has not been observed since the

Record of Decision despite successfully meeting or exceeding target peak flow magnitudes and 

durations.
Larval Trigger Study Plan 2 March 2012

TABLE 1. Spring Peak Flow Recommendations for the Green River between the 

Confluences of the Yampa and White Rivers (Muth et al. 2000).(a)

Hydrologic Condition

Wet

(0 to 10%

Exceedance)

Moderately Wet

(10 to 30% 

Exceedance)

Average

(30 to 70% 

Exceedance)

Moderately Dry 

(70 to 90% 

Exceedance)

Dry

(90 to 100% 

Exceedance)

General 

recommendation

Peak flows should be of the magnitude, timing, and duration to provide floodplain inundation in 

the Ouray portion of the river for at least 2 weeks in 4 of 10 years and at least bankfull flows in 1 

of 2 years. In all years, peak flows should be of sufficient magnitude and duration to provide at 

least some in-channel habitat maintenance throughout the reach. No upper limits are placed on 

recommended peak flows in any hydrologic condition. The duration of peak flows less than 

527 m

3

/s (18,600 cfs) should be limited, because neither floodplain nor backwater habitats are 

available at these flows. 

Peak-flow 

magnitude

≥ 748 m3/s 

(26,400 cfs)

≥ 575 m3/s 

(20,300 cfs)

≥ 527 m3/s 

(18,600 cfs) in 1 

of 2 average 

years; ≥ 235 m

3

/s 

(8,300 cfs) in 

other average 

years

≥ 235 m3/s (8,300 cfs)

Peak-flow 

duration

Flows > 643 m

3

/s 

(22,700 cfs) 

should be 

maintained for 2 

weeks or more, 

and flows greater 

than 527 m

3

/s 

(18,600 cfs) for 4 

weeks or more.

Flows > 527 m

3

/s 

(18,600 cfs) 

should be 

maintained for 2 

weeks or more.

Flows > 527 m

3

/s 

(18,600 cfs) 

should be 

maintained for at 

least 2 weeks in 

at least 1 of 4 

average years.

Flows >235 m

3

/s 

(8,300 cfs) should 

be maintained for 

at least 1 week. 

Flows > 235 m

3

/s 

(8,300 cfs) should 

be maintained for 

2 days or more 

except in 

extremely dry 

years (≥ 98% 

exceedance).

Peak-flow timing Peak flows should coincide with peak and immediate post-peak spring flows in the Yampa 

River.

(a) All flow targets are as measured at the Jensen, Utah gage (USGS 09261000).

Bestgen et al. (2011) evaluated the effectiveness of matching Yampa River Peak flows 

with high releases from Flaming Gorge Dam, and found that after 1993 releases were premature

relative to larval razorback sucker drift. They found that by the time razorback sucker larvae 

were drifting, peaks flows were often receding, which limited the number of days of connection 

to floodplain nursery habitats and reduced the opportunity for entrainment of larvae. This led 

them to conclude that 

“Longer duration and especially, higher magnitude flows, timed to occur when 

razorback sucker larvae were present, may be minimally sufficient conditions to 

enhance recruitment of razorback suckers in the middle Green River, Utah.” 

Bestgen et al. (2011) provided a number of recommendations related to developing a 

better understanding of the relationships between the timing of drift, entrainment rates of larvae 
Larval Trigger Study Plan 3 March 2012

in floodplain wetland habitats, the ability of different floodplain wetland habitats to overwinter 

fish, and timing peak flows to coincide with larval drift periods. Recommendations, paraphrased

from the original text, included:

• Study early life history of razorback sucker in the Green River Basin to better understand 

the role of altered spring thermal ecology on timing of spawning, development of 

embryos, and emergence of razorback sucker larvae, as well as the potential effects on 

spawning of nonnative fishes.

• Determine timing of spawning, hatching, emergence, habitat use, and survival of 

razorback sucker larvae in the lower Green River [Reach 3 of Muth et al. 2000]. This 

may be especially important if timing of releases from Flaming Gorge Dam, or flow 

magnitude or duration, is altered.

• Evaluate utility of floodplain wetlands as recruitment habitat for early life stages of 

razorback sucker. Important aspects include colonization/entrainment rates of larvae into 

single-breach wetlands, utility of terrace wetlands as temporary habitat for razorback 

sucker larvae, and sedimentation of breaches.

• Evaluate utility of floodplain wetlands as overwinter habitat for young razorback sucker, 

and develop plans to enhance fish overwintering capability of key wetlands. 

• Consider utility and feasibility of scheduling filling of gated wetlands with Green River 

water only when high densities of razorback sucker larvae are present. 

• Develop a simple population dynamics tool to assist with modeling entrainment and 

survival rates of early life stages of razorback suckers in various floodplain wetlands. 

• Implement a schedule of altered timing of flow releases from Flaming Gorge Dam to 

coincide more closely with presence of razorback sucker larvae, or perhaps, presence of 

abundant larvae, in the middle Green River. Reliable real-time monitoring is already in 

place to guide timing of releases. In lieu of that, develop relationships based on physical 

attributes, mostly water temperature and time of year, which would predict timing of 

emergence. 

• Investigate the feasibility of increased magnitude and duration of spring flow releases 

from Flaming Gorge Dam, after razorback sucker larvae are present, to maintain 

connections with floodplain wetlands and increase entrainment rates. Subsequent effects 

on base flow levels, among other biotic and abiotic factors, will also need to be 

considered.

On the basis of the findings and recommendations in Bestgen et al. (2011), the U.S. Fish 

and Wildlife Service’s (USFWS) Upper Colorado River Endangered Fish Recovery Program

(Recovery Program) requested that releases from Flaming Gorge Reservoir in the spring of 2011 

be experimentally timed to coincide with the occurrence of razorback sucker larvae in the middle 

Green River. Unusually high Yampa River flows, inflows to Flaming Gorge Reservoir, and 

Flaming Gorge Dam releases resulted in extended periods of connection between river and 
Larval Trigger Study Plan 4 March 2012

floodplain habitats during the larval drift period of 2011. Flows were continuously > 18,600 cfs

for more than 40 days in 2011, and razorback sucker larvae were present for at least 19 of those 

days (Recovery Program annual report, project 22f; K. R. Bestgen, unpublished data).

The Recovery Program has proposed that Reclamation use the occurrence of razorback 

sucker larvae in channel margin habitats (an indication that larval drift is occurring in the river) 

as the “trigger” to determine when peak releases should occur from Flaming Gorge Dam. This 

“larval trigger” would initially be implemented during an experimental period of about six years, 

depending on flows conditions realized, and is consistent with the Muth et al. (2000) flow 

recommendations in which initial appearance of larval suckers was identified as one of several 

examples of real-time information to be considered when determining the onset of spring peak 

flows (see Table 5.3 of Muth et al. 2000). Determining the effectiveness of this larval trigger in

recruiting razorback suckers is the primary focus of this study plan, but other potential effects are 

also evaluated. Based on information in Bestgen et al. (2011), using the larval trigger would shift 

the timing of Flaming Gorge peak releases to later in the runoff period. For the 1993 to 2008 

period examined in Bestgen et al. (2011), the shift in timing of releases relative to peak Yampa 

River flows could be earlier, about the same, or as much as 17 days later if the first detection of 

larvae was used as the trigger, based on comparison of timing of flow releases in that period 

relative to first occurrence of larvae for those 16 years. 

2 PROPOSED MONITORING AND RESEARCH

The Green River Study Plan (Green River Study Plan ad hoc Committee 2007), identified 

studies to evaluate the effectiveness of the flow recommendations of Muth et al. (2000). One of 

these recommended studies was the floodplain synthesis performed by Bestgen et al. (2011). 

This larval trigger study plan is a consequence of the findings of studies identified in the original 

Green River Study Plan, and is considered an important next step by the Recovery Program 

Biology Committee towards refining the implementation of the flow recommendations.1

Evaluating the effectiveness of operating Flaming Gorge Reservoir under a “larval 

trigger” scenario requires a targeted hypothesis-based monitoring and research program. The

topics to be examined under the study plan, hypotheses to be tested within each, and the general 

methods to be employed are described here. Five topics are included in this plan: (1) entrainment 

and retention of larval razorback suckers in floodplain wetlands; (2) survival and eventual 

escapement of larvae entrained in floodplain wetlands; (3) availability of YOY Colorado 

 

1 This next step in refining the implementation of the flow recommendations is consistent with the expression of 

Reclamation’s intent in the 2006 Record of Decision (ROD) “to work through the Upper Colorado Endangered Fish 

Recovery Program, along with the cooperating agencies on the EIS and the interested public, to assess the possibility 

of improving connectivity of floodplain habitats, identifying ways to improve entrainment of larval razorback 

suckers into floodplain habitats, maintain the river channel, restore natural variability of the river system, and meet 

other goals of the Flow and Temperature Recommendations at lower peak flow levels where feasible.” The 2006 

ROD also recognizes that “such additional knowledge gained through the adaptive management process may result 

in future refinement of the 2000 Flow and Temperature Recommendations that would maintain or improve 

conditions for the four endangered fish species while minimizing negative effects to the authorized purposes of 

Flaming Gorge Dam.”
Larval Trigger Study Plan 5 March 2012

pikeminnow habitat at base flow; (4) sediment mobilization and channel maintenance; and 

(5) fish community response. Studies associated with Topics 1 and 2 are considered the highest 

priority because these studies address razorback sucker entrainment and recruitment, which are

the intended benefits of using a larval trigger. Information from other species, particularly coevolved native catostomids, will be used to support patterns observed for razorback sucker 

particularly if razorback larvae are rare in some years. Topics 3, 4, and 5 address potential other 

consequences of using a larval trigger, and, although important, are considered lower priority for 

testing the efficacy of using a larval trigger. It is important to note that the priorities assigned to 

topics in this study plan are relative to their importance to testing the effectiveness of 

implementing the larval trigger and not to overall priorities of the Recovery Program. Wherever 

possible, the study plan identifies existing projects that could be modified or expanded to test 

hypotheses, in order to capitalize on well-established protocols.

Floodplain wetlands in the middle Green River consist of terrace and depression wetlands

(Irving and Burdick 1995; Valdez and Nelson 2004). Floodplain depressions hold water for an 

extended period of time because they are separated from the river by higher ground (natural or 

manmade levees), but terrace wetlands do not hold water, and fill and drain as the river rises and 

falls. Some depression wetlands may provide important nursery habitat for the entire period 

between sequential annual peak flows, thus augmenting recruitment of juveniles and sub adults 

into riverine habitats. Because of this ability to hold water for extended periods, the study plan 

focuses on depression wetlands only.

Depression wetlands are single-breach or multiple-breach floodplain wetlands (also 

called flow-through) based on the number of inlets and/or outlets that exist at elevations above

the initial connecting flow. Hedrick et al. (2009) and Bestgen et al. (2011) suggested that there 

were important differences between these two depression wetland types in terms of entrainment 

rates, in that flow-through wetlands entrain far greater volumes of water than single-breach 

types. This study plan proposes examinations of differences between these two wetland types. 

Table 2 presents a proposed study matrix to be used as a guide in testing hypotheses

(Table 3) associated with the larval trigger. Table 2 identifies key single-breach and flowthrough wetlands that should be evaluated. As indicated in Table 2, studies should be 

implemented over a range of peak flow magnitudes and durations to test the effectiveness of 

using a larval trigger under a variety of conditions. Flow magnitudes less than 18,600 cfs should 

be evaluated as suggested in Table 2 because some levees have been breached to allow 

connection at lower flows and several wetlands (e.g., Stewart Lake) have manually operated inlet 

gates that allow connection at lower flows.
Larval Trigger Study Plan 6 March 2012

TABLE 2. Matrix to Be Used in Studying the Effectiveness of a Larval Trigger

Peak Flow (x) as 

Measured at Jensen, 

Utah Proposed Study Wetlands(a, b)

Number of Days (x) Flow to Be Exceeded and 

Corresponding Hydrologic Conditions(c)

1 < x < 7 7 < x < 14 x >14

8,300 < x < 14,000 cfs Stewart Lake (f), Above Brennan (f), 

Old Charley Wash (s)

Dry Moderately 

dry

Moderately 

dry and

average 

(below 

median)

14,000 < x < 18,600 cfs Same as previous plus Thunder Ranch 

(f), Bonanza Bridge (f), Johnson 

Bottom (s), Stirrup (s), Leota 7 (s)

Average 

(below 

median)

Average 

(below 

median)

Average 

(below 

median)

18,600 < x < 20,300 cfs Same as previous Average 

(above 

median)

Average 

(above 

median)

Average 

(above 

median)

20,300 < x < 26,400 cfs Same as previous plus Baeser Bend 

(s), Wyasket (s), additional Leota 

units (7a and 4), Sheppard Bottom (s)

Moderately 

wet

Moderately 

wet

Moderately 

wet

x > 26,400 cfs Same as previous Wet Wet Wet 

(a) f = flow-through wetland, s = single-breach wetland

(b) Up to eight wetlands would be sampled in a given year with the three in the lowest flow category being sampled 

in all years. 

(c) Refer to Table 1 for exceedance percentages and peak flow recommendations for each hydrologic condition.

Note that the hydrologic conditions presented are the driest that could support a particular combination of peak flow 

magnitude and duration. For any combination, wetter hydrology could also support an experiment.

Wetlands that hold the greatest promise for entraining and recruiting razorback suckers,

and that are representative of other wetlands in the system should be the focus of studies

developed under this plan. Based on discussions with researchers and information presented in 

Valdez and Nelson (2004), Tetra Tech (2005), Hedrick et al. (2009), and Bestgen et al. (2011),

the authors identified candidate study wetlands (Table 2). Because study wetlands connect with 

the main channel at different flow levels, some can only be studied at higher peak flows (Table 

2). Under the study plan, up to eight wetlands would be sampled in a given year with the three in 

the lowest flow category (Stewart Lake, Above Brennan, and Old Charley Wash) being sampled 

in all years.

Table 3 summarizes hypotheses, variables to be measured, related studies, and priorities 

for each. Hypotheses, variables, and related studies are described for each topic in the remainder 

of this section; priorities are discussed further in Section 3. Additional details on related studies 

are presented in the Appendix.
Larval Trigger Study Plan 7 March 2012

TABLE 3. Larval Trigger Study Plan Topics, Hypotheses, Variables, Related Studies, and 

Priorities

Hypotheses Variables Related Studies and Data (a)

Topic 1: Entrainment and Retention of Razorback Sucker Larvae in Floodplain Wetlands (Priority: High)

H1: Entrainment and retention of 

larvae in floodplain wetlands are not 

related to the timing of connecting 

flows relative to the timing of larval 

drift (Priority: High)

Timing, duration, and abundance of 

larvae in the main channel (Priority: 

High)

Ongoing and expanded project 22f

and new floodplain studies (projects 

FR-164 and FR-165). New modeling 

effort to predict the timing of larval 

drift.

Timing of connecting flows 

(Priority: High)

New field study needed. Related to 

ongoing project C6-hydro.

Volume of water entrained into 

wetlands during the period of larval 

drift (Priority: High)

New field study needed. Related to 

completed project FR-FP synthesis, 

and ongoing project C6-hydro and 

flow gage data.

Larval presence and relative 

abundance in wetlands after flows 

recede and connection with the main 

channel has ended (Priority: High)

Ongoing and expanded project 22f, 

and new floodplain studies (projects 

FR-164 and FR-165)

H2: Entrainment and retention of 

larvae in floodplain wetlands are not 

related to the magnitude of 

connecting flows when larvae are 

present (Priority: High)

Same as H1 plus:

Magnitude of connecting flows 

(Priority: High)

Ongoing and expanded project 22f, 

new floodplain studies (projects FR164 and FR-165), completed project

Cap-6 rz/entr, and ongoing C6-

hydro.

H3: Entrainment and retention of 

larvae in floodplain wetlands are not 

related to the duration of connecting 

flows when larvae are present 

(Priority: High)

Same as H1 plus:

Duration of connecting flows 

(Priority: High)

Ongoing and expanded project 22f, 

new floodplain studies (projects FR164 and FR-165), ongoing C6-

hydro, and completed project Cap-6 

rz/entr.

H4: Entrainment and retention of 

larvae in floodplain wetlands are not 

related to floodplain wetland 

characteristics (e.g., single-breach 

and flow-through, location of 

wetland, breach/connection 

elevation) (Priority: High)

Same as H1 plus:

Physical characteristics of study 

wetlands (Priority: High)

New field study needed. Related to 

ongoing project C6-hydro.

Topic 2: Survival and Escapement of Entrained Razorback Suckers (Priority: High)

H1: Relative abundance and 

condition of YOY razorback suckers 

in autumn are not related to 

floodplain wetland characteristics 

(e.g., single-breach and flowthrough, breach/connection 

elevation, surface area, and depth)

(Priority: High)

Relative abundance and condition of 

YOY suckers in floodplain wetlands 

in autumn (Priority: High)

Ongoing and expanded project 22f, 

new floodplain studies (projects FR164 and FR-165), ongoing efforts at 

the Stirrup floodplain (projects Cap6 RZ/recr), and past studies (Cap-6 

rz/bt, Cap-6 bt/rz, and data collected 

in 2011).
Larval Trigger Study Plan 8 March 2012

Table 3 (Cont.)

Hypotheses Variables Related Studies and Data(a)

H2: Relative abundance and 

condition of age 1 and other

razorback suckers at the end of the 

winter period are not related to 

floodplain wetland characteristics 

(Priority: High)

Relative abundance and condition of 

age 1 and other razorback suckers in 

floodplain wetlands at the end of 

winter prior to peak runoff (Priority: 

High)

New floodplain study (FR-164).

Related to the recently completed 

portion of projects Cap-6 RZ/recr, 

and past studies (Cap-6 rz/bt, and 

Cap-6 bt/rz).

H3: Number of razorback suckers 

that are able to escape floodplain 

wetland habitats to the main channel 

river is not related to floodplain 

wetland characteristics (Priority: 

High)

Number of razorback suckers 

escaping from floodplain wetlands 

during peak flows (Priority: High)

PIT tag arrays will be deployed at 

the Stirrup floodplain (Cap-6 

RZ/recr) and at Stewart Lake (new 

study FR-165). Ongoing projects

123a, 123b, 128, 138, and 158

(currently funded through 2012)

could detect escaped fish.

Degree of connection in subsequent 

years that would provide an 

opportunity for escapement 

(Priority: High)

New modeling study needed similar 

to FR-FP synthesis. Related to 

recently completed portions of 

project Cap-6 RZ/recr as well as 

ongoing deployment of PIT tag 

array, and ongoing project C6-

hydro.

H4: Floodplain wetlands are not

different in terms of surface area,

depth, and cover at peak, post-peak, 

autumn, and end of winter (Priority: 

High)

Surface area, depth, and cover of 

floodplain wetlands post-peak, in 

autumn, and at end of winter 

(Priority: High)

New field study needed and/or 

supplement new floodplain studies 

(FR-164 and FR-165). Related to 

completed project Cap-6 bt/rz and 

ongoing project C6-hydro.

H5: Floodplain wetlands are not 

different in terms of water quality 

through the summer and winter

(Priority: High)

Water quality in floodplain wetlands 

through the summer and winter 

period (Priority: High)

New field study needed, and/or 

supplement new floodplain studies 

(FR-164 and FR-165). Related to 

completed project Cap-6 bt/rz.

Topic 3: Availability of Colorado Pikeminnow Habitat (Priority: Medium)

H1: Base flow magnitude is not 

affected by the use of a larval trigger 

(Priority: Medium)

Base flow magnitude (Priority: 

Medium)

Ongoing USGS gage data collection. 

H2: The amount of backwater habitat 

available for Colorado pikeminnow 

during the base flow period is not 

affected by the use of a larval trigger 

(Priority: Medium)

Surface area, volume, and depth of 

backwaters at base flow (Priority: 

Medium)

Ongoing Argonne/Western 

backwater study. 

H3: The number of Colorado 

pikeminnow found in backwater 

nursery habitats in late summer is 

not affected by the use of a larval 

trigger (Priority: Medium)

Number of Colorado pikeminnow 

captured in backwater habitats in 

late summer/early autumn (Priority: 

Medium)

Ongoing projects 138 and 158.
Larval Trigger Study Plan 9 March 2012

Table 3 (Cont.)

Hypotheses Variables Related Studies and Data(a)

Topic 4: Sediment Mobilization and Channel Maintenance (Priority: Medium)

H1: The amount of suspended 

sediment transport is not affected by 

the use of a larval trigger (Priority: 

Medium)

Suspended sediment transport rates 

(Priority: Medium)

New field study may be needed. 

Related to completed project 85f.

H2: Bedload transport is not affected

by the use of a larval trigger 

(Priority: Medium)

Bedload transport rates (Priority: 

Medium)

New field study may be needed.

Related to completed project 85f.

H3: Channel width and complexity 

are not affected by the use of a larval 

trigger (Priority: Medium)

Channel width (Priority: Medium) New field study needed (aerial 

photography).

Channel complexity including the 

size and number of sandbars that 

provide backwater habitats 

(Priority: Medium)

New field study needed (aerial 

photography).

Topic 5: Fish Community Response (Priority: Low)

H1: The diversity and abundance of 

native and nonnative fish 

established in floodplain wetlands is 

not affected by the use of a larval 

trigger (Priority: Low)

Native and nonnative fish diversity 

and abundance in floodplain 

wetlands (Priority: Low)

New field study needed. Related to 

completed project Cap-6 RZ/recr, 

Cap-6 rz/bt, and Cap-6 bt/rz.

H2: The diversity and abundance of 

native and nonnative fish in main 

channel habitats is not affected by 

the use of a larval trigger (Priority: 

Low)

Native and nonnative fish diversity 

and abundance in main channel 

habitats (Priority: Low)

Related to completed project 144, 

and ongoing projects 123a, 123b, 

138, and 158.

Main channel water temperatures

(Priority: Low)

Entrainment of burbot through 

power turbines, bypass or spillway

(Priority: High)

Ongoing water temperature gage 

data collection.

Risk Assessment review conducted 

by NPS, NNF coordinator and Utah.

(a) Ongoing and completed projects are described in the Appendix.

Topic 1: Entrainment and Retention of Razorback Sucker Larvae in Floodplain 

Wetlands

Topic 1 addresses factors that may affect entrainment and retention of larval razorback 

suckers in floodplain wetlands. Included under this topic is an examination of the role of peak 

flow characteristics (e.g., timing, magnitude, and duration of connecting flows relative to the 

timing of larval drift) and floodplain wetland characteristics (e.g., single-breach and flowthrough, location of wetland, breach/connection elevation) in relation to the entrainment and 
Larval Trigger Study Plan 10 March 2012

retention of razorback sucker larvae in floodplain wetlands. To evaluate this topic, peak releases 

and connecting flows would be timed to coincide with the presence of larvae, but there could be 

significant variation in abundance during the peak release period. Data collected for this portion 

of the study would be compared to historical data (i.e., Bestgen et al. 2011) collected when the 

Yampa River trigger was used.

Hypotheses2to be tested under Topic 1 include:

H1: Entrainment and retention of larvae in floodplain wetlands are not related to the timing of 

connecting flows relative to the timing of larval drift. 

H2: Entrainment and retention of larvae in floodplain wetlands are not related to the magnitude 

of connecting flows when larvae are present.

H3: Entrainment and retention of larvae in floodplain wetlands are not related to the duration of 

connecting flows when larvae are present.

H4: Entrainment and retention of larvae in floodplain wetlands are not related to floodplain 

wetland characteristics (e.g., single-breach and flow-through, location of wetland, 

breach/connection elevation).

To test hypotheses for Topic 1 (Table 3), a variety of data should be collected, and some 

data will be useful for testing more than one of the hypotheses listed above. Data needs, related 

existing studies, and, where applicable, the need for new studies are presented next. Although 

separate hypotheses are considered for the potential effects of timing, magnitude, and duration of 

flows, it may be difficult to separate the effects of these variables since they can effect 

entrainment both collectively and individually.

 Timing, duration, and abundance of larvae in the main channel. Ongoing project 22f

would be used to gather these data. Bestgen et al. (2011) also suggested that it may be 

possible to develop relationships based on physical attributes (e.g., water temperature and 

time of year) to predict the timing of larval drift. Such modeling would be useful for 

operational planning and should be developed and used to predict the first occurrence of 

larvae, but should not replace direct measurements of drift to test this hypothesis.

 Timing, magnitude, and duration of connecting flows. A new field study would be needed 

to collect these data, but could tier from ongoing project C6-hydro to assess actual 

connection flow (i.e., when river flow begins to enter wetlands) at each study wetland at 

the beginning of the study, and perhaps every year thereafter until study completion. It 

may be necessary to develop new river flow and entrainment relationships at the 

beginning of the study, and periodically during the study, if breach elevations are altered 

by annual high flows. Green River researchers have noted the poor concordance between 

published (i.e., Valdez and Nelson 2004; Bestgen et al. 2011) connecting flows and 

actual connecting flows following high-flow years. These differences between actual and 

 

2 All hypotheses are written as null hypotheses, i.e., that there is no effect or difference between the elements 

compared.
Larval Trigger Study Plan 11 March 2012

published connecting flows may be especially noticeable following the very high flows in 

2011. Thus, it would be important to assess breach condition and elevations prior to 

spring peak flow in 2012, if possible.

 Volume of water entrained into wetlands during the period of larval drift. Data collected 

to determine connection flows, flow gage data, and the relationships developed by 

Bestgen et al. (2011) should be used to determine annual water volume entrained into 

wetlands. The relationships used may need to be modified annually or occasionally if 

breach elevations are altered by annual high flows.

 Larval presence and abundance in wetlands after flows recede and connection with the 

main channel has ended. Modifications to existing project 22f and new studies by 

UDWR and USFWS in floodplain wetlands will provide sampling needed to inform this 

information need. Those studies follow aspects of sampling protocols used in 2011 to 

evaluate larval presence in floodplain wetlands. Based on experience in 2011, it may be 

difficult to accurately assess the presence and abundance of larvae in wetlands after flows 

recede. This is at least partly a result of the large size of some of the study wetlands, 

sampling effort, and the number of larvae entrained. For this reason, it is recommended 

that even if larvae were not detected initially in study wetlands, these wetlands be 

sampled again before the subsequent spring peak to determine if razorback suckers had 

been entrained. Abundance estimates should be quantified to the extent possible, but may 

need to rely on effort-based estimates or estimates of relative abundance. In addition, a 

research project using marked individuals (e.g., Hedrick et al. 2009) could be used to 

better quantify abundance of larvae in light trap samples and calibrate sampling effort 

and results accordingly. Physical characteristics of study wetlands. Important physical 

characteristics of study wetlands include (1) number of inlets/outlets, (2) 

breach/connection elevations, and (3) distance from spawning areas. Some of the 

physical characteristics of potential study area wetlands are well known (e.g., number of 

inlets/outlets and distance from spawning areas), but, as mentioned above,

breach/connection elevations should be assessed initially and annually if possible 

following protocols in project C6-hydro.

Topic 2: Survival and Escapement of Entrained Razorback Suckers

Topic 2 addresses factors that may affect the survival of razorback suckers entrained as 

larvae into floodplain wetlands and their eventual escapement from those wetlands into the main

channel of the river. Survival of larvae and eventual escapement of subadults are essential 

elements of the razorback sucker life cycle (e.g., Muth et al 2000.). Entrainment into wetlands 

that cannot support razorback suckers through at least one and potentially several years provides 

no benefit to the species, and could have a negative effect if these wetlands functioned as sinks

from which suckers could not re-enter the main channel. Studies would focus on evaluating the 

abundance and condition of YOY and subadult fish as related to floodplain wetland 

characteristics that could affect their suitability to serve as nursery and overwinter habitats, while 

also allowing escapement to the main channel. Wetland characteristics to evaluate under Topic 2 

include floodplain wetland type (e.g., the number of connections (single-breach vs. flowthrough), breach/connection elevation, surface area, depth, cover, and water quality, particularly 
Larval Trigger Study Plan 12 March 2012

temporal patterns of dissolved oxygen. When evaluating each study wetland, there should be

consideration of whether or not the wetland had been reset in previous years (i.e., drained or

dried sufficiently to eradicate nonnative resident fish). Note that the relative abundance of YOY 

will also depend on entrainment rates, and, therefore, testing the hypotheses of Topic 2 will 

require controlling for previous entrainment rates.

Hypotheses to be tested under Topic 2 include:

H1: Relative abundance and condition of YOY razorback suckers in autumn are not related to

floodplain wetland characteristics (e.g., single-breach and flow-through, breach/connection 

elevation, surface area, depth, and cover).

H2: Relative abundance and condition of age 1 and other razorback suckers at the end of the 

winter period are not related to floodplain wetland characteristics.

H3: Number of razorback suckers that are able to escape floodplain wetland habitats to the main 

channel is not related to floodplain wetland characteristics.

H4: Floodplain wetlands are not different in terms of surface area, depth, and cover at peak, postpeak, autumn, and end of winter.

H5: Floodplain wetlands are not different in terms of water quality through the summer and

winter.

To test hypotheses for Topic 2 (Table 3), a variety of data should be collected. Data 

needs, related existing studies, and, where applicable, the need for new studies are presented 

next.

 Relative abundance and condition of YOY suckers in floodplain wetlands in autumn.

Two new studies conducted by USFWS (project FR-164) and UDWR (project FR-165),

which will sample floodplain wetlands in the post-connection period, and an expanded

project 22f have been funded to accommodate these data needs. Other related projects 

that have been completed, but that could be tiered from include Cap-6 RZ/recr, Cap-6 

rz/bt, and Cap-6 bt/rz. Abundance estimates should be quantified to the extent possible, 

but may need to rely on effort-based estimates. Condition of individual fish should be 

based on calculations of relative weight or length-weight relationships; otherwise,

qualitative assessments of condition should be recorded.

 Relative abundance and condition of age 1 and other razorback suckers in floodplain 

wetlands at the end of winter prior to peak runoff. Two new studies conducted by 

USFWS (project FR-164) and UDWR (project FR-165), which will sample floodplain 

wetlands in the post-connection period, and expanded project 22f have been funded to 

accommodate these data needs. Sampling will be similar to that conducted in autumn. .

 Number of razorback suckers escaping from floodplain wetlands during peak flows. To 

gather these data, a new study would be needed that tags fish captured in autumn and prepeak spring samples using Passive Integrated Transponder (PIT) tags and uses PIT tag 
Larval Trigger Study Plan 13 March 2012

antenna arrays in breaches and points of connection to determine escapement. These 

studies will complement other tag-recapture studies including projects 123a, 123b, and 

128, and the new floodplain studies (projects FR-164 and FR-165). This study could tier 

from completed project Cap-6 RZ/recr. In addition, an evaluation of recaptures in 

subsequent years in ongoing main-channel sampling (e.g., projects 123b, 128, and 138) 

would provide information on the ultimate fate of fish escaping from floodplain wetlands.

 Degree of connection in subsequent years that would provide an opportunity for 

escapement. A post-hoc evaluation of escapement opportunity would be conducted using 

gage-based estimates of river elevation, previously derived estimates of 

breach/connection elevation, and previously reported fish passage criterion (Burdick 

1997) to determine the duration of escapement opportunity in any given year.

 Surface area, depth, and cover of floodplain wetlands post-peak, in autumn, and at end of 

winter. To gather these data, a new study would be needed, possibly as an expansion of 

project C6-hydro or Cap-6 bt/rz. The purpose of this study would be to gather 

information on the physical characteristics of floodplain wetlands that are most important 

in determining the ability of floodplain wetlands to provide for survival and escapement 

of razorback suckers. Although detailed survey-grade quantification of surface area and 

depth would be of greatest value, less detailed information, if representative and 

unbiased, could be gathered and used instead.

 Water quality in floodplain wetlands through the summer and winter period.

Eutrophication during the summer and a reduction in free water in the winter could result 

in a reduction in dissolved oxygen levels in floodplain wetlands that affect fish health and 

survivorship. A new study would be needed to monitor water quality through summer 

and winter and should focus on critical periods when water quality is considered 

potentially limiting. The study could tier from completed project Cap-6 bt/rz.

Topic 3: Availability of Colorado Pikeminnow Habitat

It is possible that using a larval trigger could have consequences on other components of 

the Green River ecosystem. Topic 3 addresses the effect of using a larval trigger on base flows 

and Colorado pikeminnow nursery habitats. As mentioned in Bestgen et al. (2011), using a

greater release volume to meet peak-flow targets could result in less water available for base 

flows, and, consequently, less Colorado pikeminnow nursery habitat through the summer and 

autumn. The analysis of this topic would include a comparison of new data and historical data.

Hypotheses to be tested under Topic 3 include:

H1: Base flow magnitude is not affected by the use of a larval trigger.

H2: The amount of backwater habitat available for Colorado pikeminnow during the base flow 

period is not affected by the use of a larval trigger.
Larval Trigger Study Plan 14 March 2012

H3: The number of Colorado pikeminnow found in backwater nursery habitats in late summer is 

not affected by the use of a larval trigger.

Data needs, related existing studies, and, where applicable, the need for new studies (e.g., 

Table 3) to address these hypotheses are presented next. 

 Base flow magnitude. Flows during the base flow period as measured at the Jensen gage 

would be used in this analysis. Comparisons would be made to historical data collected in 

years with comparable hydrology when a larval trigger was not used.

 Surface area, volume, and depth of backwaters at base flow. These data would be 

collected as part of the ongoing backwater topography and modeling project conducted 

annually by Argonne National Laboratory and Western Area Power Administration.

Comparisons would be made to historical data collected in years with comparable 

hydrology when a larval trigger was not used.

 Number of Colorado pikeminnow in backwater habitats in late summer. Ongoing project 

138 and perhaps project 158 (ongoing through 2012 and perhaps beyond) would be used 

to determine effort-based catch rates of YOY Colorado pikeminnow. Comparisons would 

be made to historical data collected in years when a larval trigger was not used.

Topic 4: Sediment Mobilization and Channel Maintenance

It is possible that using a larval trigger could have consequences on other components of 

the Green River ecosystem. Topic 4 addresses the effect of using a larval trigger on sediment 

mobilization and channel maintenance. Using a larval trigger could result in an overall reduction 

in annual peak flow magnitude in the middle Green River, because Flaming Gorge releases 

would not coincide with and add to Yampa River flows. This reduction in peak flow magnitude

in this reach could result in less sediment transport and channel maintenance. It is also possible 

that using a larval trigger could result in longer peak-flow duration in this reach, but lower 

magnitude peaks that affect erosion and deposition patterns.

Hypotheses to be tested under Topic 4 include:

H1: The amount of suspended sediment transport is not affected by the use of a larval trigger.

H2: Bedload transport is not affected by the use of a larval trigger.

H3: Channel width and complexity are not affected by the use of a larval trigger.

Data needs, related existing studies, and, where applicable, the need for new studies (see 

Table 3) to address these hypotheses are presented next. 

 Suspended sediment transport rates. Collection of these data would require a new study 

or renewal of elements of the recently completed project 85f, but it may be possible to 
Larval Trigger Study Plan 15 March 2012

use existing sediment transport equations from project 85f to estimate suspended 

sediment transport under different flow conditions.

 Bedload transport rates. Similar to the previous variable, collection of these data would 

require a new study or renewal of elements of the recently completed project 85f, but it 

may be possible to use existing sediment transport equations from project 85f to estimate 

bed load sediment transport under different flow conditions.

 Channel width. Collection of these data would require a new study that builds on existing 

aerial photography

 Channel complexity, including the size and number of sandbars that provide backwater 

habitats. Similar to the previous variable, collection of these data would require a new 

study that builds on existing aerial photography.

Topic 5: Fish Community Response

It is possible that using a larval trigger could have consequences on other components of 

the Green River ecosystem. Topic 5 addresses the effect of using a larval trigger on native nonendangered fishes, particularly co-evolved catostomids, and nonnative fish populations in 

floodplain wetlands and in the main channel. Using a larval trigger could result in a positive 

response by other native fishes as well as nonnative fishes, at least in part because annual peak 

flows that are thought to suppress nonnative fish populations may be lower if a larval trigger is 

used. The risk of entraining nonnative burbot (Lota lota) through power turbines, bypass tubes, 

or spillway has not been assessed and may increase with increased use of bypass during peak 

releases. These analyses would include a comparison of new data and historical data.

Hypotheses to be tested under Topic 5 include:

H1: The diversity and abundance of nonnative fish established in floodplain wetlands is not 

affected by the use of a larval trigger.

H2: The diversity and abundance of nonnative fish in main channel habitats is not affected by the 

use of a larval trigger.

Data needs, related existing studies, and, where applicable, the need for new studies 

(Table 3) to address these hypotheses are presented next. 

 Native and nonnative fish diversity and abundance in floodplain wetlands. Two new 

studies conducted by USFWS (project FR-164) and UDWR (project FR-165), which will 

sample in floodplain wetlands in the post-connection period, and expanded project 22f 

have been funded to partially accommodate these data needs. Data collection will 

include fishes captured and measures of relative abundance (catch per unit effort 

[CPUE]) in floodplain wetland habitats. The study could tier from completed projects 

Cap-6 RZ/recr, Cap-6 rz/bt, and Cap-6 bt/rz.
Larval Trigger Study Plan 16 March 2012

 Native and nonnative fish diversity and abundance in main channel habitats. These data 

are being collected under ongoing projects 123a, 123b, 138, and 158 (at least through 

2012).

 Entrainment of burbot through power turbines, bypass tubes or spillway. A literature 

review, and risk assessment will be completed by the NPS, Utah, and the Nonnative Fish 

coordinator in 2012. 

 Main channel water temperatures. Existing water temperature gages would be used to 

monitor main channel temperature through the year. Comparisons would be made to 

historical data collected in years with comparable hydrology when a larval trigger was 

not used.
Larval Trigger Study Plan 17 March 2012

3 RESEARCH PRIORITIES AND IMPLEMENTATION

As described in Section 2, five topics are included in this study plan: (1) entrainment and 

retention of larval razorback suckers in floodplain wetlands; (2) survival and eventual 

escapement of entrained larvae in floodplain wetlands; (3) availability of Colorado pikeminnow 

habitat; (4) sediment mobilization and channel maintenance; and (5) fish community response. 

Studies associated with Topics 1 and 2 are considered the highest priority because these studies 

address the objectives of using a larval trigger (i.e., razorback sucker entrainment and 

recruitment). Topics 3, 4, and 5 address potential other consequences of using a larval trigger, 

and are considered lower priority. It is important to note that the priorities assigned to topics in 

this study plan are relative to their importance to testing the effectiveness of implementing a 

larval trigger and not to overall priorities of the Recovery Program. Studies identified as low 

priority here could be high priority for other Program elements.

Table 3 summarizes hypotheses, variables to be measured, related studies, and priorities 

for each topic addressed in the study plan. Overall priorities are categorized as high, medium, or

low based on the perceived relationship between the topic and the larval trigger, and the 

importance of the information in understanding that relationship and testing specific hypotheses.

Within topics, certain hypotheses and variables are considered higher priority than others

(Table 3). For Topic 1, all hypotheses and variables are considered high priority and essential for 

interpretation of the effectiveness of the larval trigger in achieving recovery of razorback 

suckers. For Topic 2, obtaining useful information on abundance and condition of fish in both 

the autumn and after the winter period prior to peak runoff is considered high priority. Numbers 

and condition in autumn would be useful for determining survival and growth during the 

summer, and could be used to interpret pre-peak numbers, but only winter data would enable a 

determination of the usefulness of wetlands for completing the cycle from entrainment to 

escapement. It is considered a high priority to measure the opportunity for escapement, but direct 

measurements of escapement using a PIT tag antenna array may be considered on an as-needed 

basis. Escapement should also be inferred from captures of wild-spawned subadults or adults in 

the main channel during ongoing survey projects (e.g., 123b and 138). Under Topic 2, 

measurements of wetland surface area and depth are considered most important at times when 

these variables would be at their minimum (i.e., limiting) values (e.g., at the end of the summer). 

Water quality data are considered high priority, because this information is relatively easily 

obtained and could be used to identify issues that could limit the value of floodplain wetland 

habitats.

The remaining topics are considered to be medium (Topics 3 and 4) or low (Topic 5) 

priority for purposes of testing the effectiveness of the larval trigger. These topics were identified 

in the Green River study plan (Green River Study Plan ad hoc Committee 2007) and will be 

evaluated as part of that process. Topic 3 would be evaluated using field data collected annually 

under existing ongoing projects. Thus, even though the topic is considered to be medium priority

for this study plan, it could be evaluated with minimal new or additional work, and it is 

recommended that this topic be evaluated. Topic 4 would require potentially expensive new field 

studies to measure sediment transport (unless transport relationships developed in project 85f 

could be used to estimate transport under different flow regimes), and collect aerial photography.
Larval Trigger Study Plan 18 March 2012

Topic 5 is considered low priority for this study plan, but ongoing studies would provide much 

of the data needed to test relevant hypotheses. To further evaluate the effects of using a larval 

trigger, Reclamation will conduct a retrospective analysis of dam release hydrographs that may 

have occurred since 2006 if the larval trigger had been implemented along with existing peakand base flow objectives specified in the 2006 Record of Decision. This modeling will be used 

primarily by the Flaming Gorge Technical Work Group to inform their flow planning process in 

future years.

As practicable, proposed studies should address the flow magnitude and duration bins 

shown in Table 2, but three years with flows < 18,600 cfs and three years with flows 

> 18,600 cfs and with connecting flows in each of these years of at least seven days duration are 

considered necessary to complete the study. Although it could be possible to complete the study 

in six years, ultimately the length of the study will be dependent on annual hydrologic 

conditions. Figure 1 shows a decision tree to be used when determining the need for monitoring 

actions in any given year.

Under the study plan, up to eight wetlands would be sampled in a given year. Only 

wetlands that are thought to hold the greatest promise for recruiting razorback suckers and that 

are representative of other wetlands in the system should be chosen for study (see Table 2 for 

wetlands identified as candidates for study). Some additional evaluation may be needed prior to 

selecting study wetlands to verify the flows at which wetlands connect to the main channel, and 

to determine their suitability as nursery habitat. In order to make meaningful statistical inferences 

from the data, it will be important to study the same wetlands each year to the extent possible.

The specific objectives, tasks, and expected outcomes for individual studies developed 

under this plan will be identified in statements of work approved by the Recovery Program. 

These projects and the resulting project reports will go through the standard Recovery Program 

review protocols. It is anticipated that a synthesis report will be developed that summarizes 

results from individual projects, integrates results, summarizes conclusions, and makes 

recommendations for future implementation of a larval trigger.

As for any study plan, additional knowledge will be gained during implementation, and it 

will be important to have enough flexibility to adjust studies and overall approaches in response 

to this new information. Emerging data gaps regarding the relative effects of flow magnitude, 

duration and timing on larval entrainment should also help guide the direction of research. The 

topics, hypotheses, variables, and priorities presented here are a starting point, rather than a fixed 

path forward.
Larval Trigger Study Plan 19 March 2012

FIGURE 1. Monitoring Decision Tree to Be Used in Evaluating the Larval Trigger
Larval Trigger Study Plan 20 March 2012

4 REFERENCES

Bestgen, K.R., G.B. Haines, and A.A. Hill. 2011. Synthesis of Flood Plain Wetland Information: 

Timing of Razorback Sucker Reproduction in the Green River, Utah, Related to Stream Flow, 

Water Temperature, and Flood Plain Wetland Availability. Final Report, Upper Colorado River 

Endangered Fish Recovery Program, Denver, Colo.

Burdick, B. D. (1997). Minimum Flow Recommendation for Passage of Colorado Squawfish and 

Razorback Sucker in the 2.3-Mile Reach of the Lower Gunnison River: Redlands Diversion Dam 

to the Colorado River Confluence, Colorado River Recovery Program.

Green River Study Plan ad hoc Committee. 2007. Study Plan for the Implementation and 

Evaluation of Flow and Temperature Recommendations for Endangered Fishes in the Green 

River Downstream of Flaming Gorge Dam. Final Report.

Hedrick, T.N., K.R. Bestgen, and K.D. Christopherson. 2009. Entrainment of Semi-Buoyant 

Beads and Razorback Sucker (Xyrauchen texanus) Larvae into Flood Plain Wetlands of the 

Middle Green River, Utah. Final Report, Upper Colorado River Endangered Fish Recovery 

Program, Denver, Colo.

Irving, D.B. and B.D. Burdick. 1995. Reconnaissance Inventory and Prioritization of Existing 

and Potential Bottomlands in the Upper Colorado River Basin, 1993–1994. Final Report, Upper 

Colorado River Endangered Fish Recovery Program, Denver, Colo.

Muth, R.T., L.W. Crist, K.E. LaGory, J.W. Hayse, K.R. Bestgen, J.K. Lyons, T.P. Ryan, and 

R.A. Valdez. 2000. Flow Recommendations for Endangered Fishes in the Green River 

Downstream of Flaming Gorge Dam, Final Report, Upper Colorado River Endangered Fish 

Recovery Program Project FG-53, Denver, Colo.

Tetra Tech, Inc. 2005. Floodplain Habitat Restoration, 2005 Monitoring Final Report, Green 

River, Utah. Upper Colorado River Endangered Fish Recovery Program, Project Number C6-

Hydro, Denver, Colo.

Valdez, R.A. and P. Nelson. 2004. Green River Subbasin Floodplain Management Plan. Upper

Colorado River Endangered Fish Recovery Program, Project Number C-6, Denver, Colo.
Larval Trigger Study Plan 21 March 2012

APPENDIX

RECOVERY PROGRAM PROJECTS RELATED TO THIS STUDY PLAN
Larval Trigger Study Plan 22 March 2012

TABLE A-1. Ongoing and Completed Recovery Program Studies Identified in Table 3 and 

in the Text that Will Contribute Information to Hypotheses Testing.

Project No. Project Title(a) Comments

22f Interagency Standardized Monitoring 

Program (ISMP) assessment of endangered 

fish reproduction in relation to Flaming 

Gorge Dam operations in the middle Green 

and lower Yampa Rivers. 

Long-term (since 1992) standardized main channel 

light trapping for larval razorback suckers, which 

will provide real time information to trigger 

Reclamation’s experimental operations. Study was 

expanded to incorporate larval sampling in 

floodplain habitats. Addresses Topic 1 hypotheses.

FR-164 Middle Green River floodplain sampling. New study in 2012 (complements larval sampling 

covered in project 22f). Sample wetlands in spring 

to determine overwinter survival of razorback 

sucker; qualitatively describe fish community in 

wetlands; document entrainment and recruitment of 

razorback sucker in fall; collect water quality 

information at wetlands. Addresses Topic 1 and 2 

hypotheses.

FR-165 Use of the Stewart Lake floodplain by larval 

and adult endangered fishes

New study in 2012. Monitor entrainment of larval 

endangered fishes during high flows; examine fish 

community composition and habitat characteristics 

post floodplain connection; monitor escapement of 

native and nonnative fishes from Stewart Lake. 

Addresses Topic 1 and 2 hypotheses. 

C-6 hydro Physical evaluation of floodplain habitats 

restored/enhanced to benefit endangered 

fishes of the Upper Colorado River basin.

This ongoing study will need to be revised to 

address study plan information needs at floodplain 

habitats in Green River subbasin. Addresses Topics 

1 and 2 hypotheses.

FR-FP 

synthesis

Synthesis of flood plain wetland 

information. 

Completed study, which serves as a basis for the 

Larval Trigger Study Plan. Addresses Topic 1 

hypotheses. Results summarized in Bestgen et al. 

(2011).

Cap-6 rz/entr Entrainment of larval razorback sucker. Completed study, which serves as a basis for the 

Larval Trigger Study Plan. Addresses Topic 1 

hypotheses. Results summarized in Hedrick et al. 

(2009).

C-6 Green River Subbasin Floodplain 

Management Plan. 

Completed study, which provides background 

information related primarily to Topic 2 hypotheses.

Results summarized in Valdez and Nelson (2004).

Cap-6 rz/bt Larval razorback and bonytail survival in 

Baeser.

Completed study, which provides background 

information related primarily to Topic 2 hypotheses.

Results summarized in Brunson and Christopherson 

(2005). Larval razorback sucker and bonytail 

survival and growth in the presence of nonnative 

fish in the Baeser floodplain wetland of the middle 

Green River.
Larval Trigger Study Plan 23 March 2012

Table A-1 (Continued)

Project No. Project Title(a) Comments

Cap-6 bt/rz Larval bonytail and razorback sucker 

survival in floodplain habitats.

Completed study, which provides background 

information related primarily to Topic 2 hypotheses.

Results summarized in Modde and Haines (2005). 

Survival and growth of stocked razorback sucker 

and bonytail in multiple floodplain wetlands of the 

middle Green River under reset conditions.

Cap-6 

RZ/recr

Razorback sucker survival and emigration 

from the Stirrup floodplain

Research aspects of this study have been completed: 

Hedrick et al. (2012). Razorback sucker survival and 

emigration from the Stirrup floodplain, Middle 

Green River, Utah 2007-2010. UDWR will continue 

to deploy a PIT tag array during floodplain 

connection and monitor/augment water quality as 

needed at the Stirrup floodplain in 2012 and beyond. 

Primarily addresses Topic 2 hypotheses.

128 Abundance estimates for Colorado 

pikeminnow in the Green River Basin, Utah 

and Colorado These ongoing efforts comprise many hours of main 

channel electrofishing, which can detect razorback 

suckers escaping from floodplains, i.e. Topic 2 

hypotheses. Projects 123(a) and (b) can also 

evaluate changes in main channel fish community, 

i.e. address Topic 5 hypotheses.

123b Nonnative fish control in the middle Green 

River

123a Nonnative Fish Control in the Echo Park to 

Split Mountain Reach of the Green River, 

Utah

138 Annual fall monitoring of YOY Colorado 

pikeminnow and small-bodied native fishes.

This ongoing study provides a long term assessment 

of the small bodied fish community in backwater 

habitats throughout the middle Green River. 

Addresses Topic 3 hypotheses

144 Green River native fish response to 

nonnative control

Completed study, which provides background 

information related primarily to Topic 5 hypotheses.

Draft report in review.

158 Assessment of larval Colorado pikeminnow 

presence and survival in low velocity 

habitats in the middle Green River

Ongoing study with final year of field work 

scheduled in 2012 (may be extended). Verify that 

larval pikeminnow are arriving in nursery habitat; 

document abundance of larval Colorado 

pikeminnow in backwaters as season progresses; 

determine success of removing and excluding 

nonnative fish from backwaters using various 

blocking techniques and depletion treatments; assess 

small-bodied fish community effects from removing 

nonnative fishes from backwaters. Addresses Topic 

3 and 5 hypotheses.

FR-BW 

synthesis

Historical assessment of factors affecting 

young Colorado pikeminnow abundance 

and physical habitat availability in the 

Green River, Utah.

This synthesis (in preparation) incorporates longterm age-0 pikeminnow collection data (e.g., project

138) and sandbar topography (Argonne National 

Laboratory) to describe physical and biological 

habitat responses to middle Green River flows. 

Provides a baseline for Topics 3 and 4 hypotheses. 
Larval Trigger Study Plan 24 March 2012

Table A-1 (Continued)

Project No. Project Title(a) Comments

85f Gunnison and Green River sediment 

monitoring

Completed study, report in final revision; sediment 

transport equations could be used to address Topic 4 

hypotheses. Results presented in Williams et al.

(2011). Application of Sediment Characteristics and 

Transport Conditions to Resource Management in 

Selected Main-Stem Reaches of the Upper Colorado 

River, Colorado and Utah, 1967–2007.

FR-115 Monitoring effects of Flaming Gorge Dam 

releases on the Lodore/Whirlpool fish 

community

This ongoing fish community monitoring study will 

provide some evaluation of effects of Reclamation’s 

releases to meet the larval trigger in upstream 

reaches. Addresses Topic 5 hypotheses.

(a) Full scopes of work are available at: http://www.coloradoriverrecovery.org/documents-publications/work-plandocuments/project-scopes-of-work.html#I. Completed studies are available at: 

http://www.coloradoriverrecovery.org/documents-publications/technical-reports/habitat-restoration.html.