Glen Canyon Sloughs: 

Proposed Modifications 

Glen Canyon National Recreation Area, Arizona 

Upper Colorado Basin Region 

U.S. Department of the Interior August 2023 
 

Mission Statements 

The U.S. Department of the Interior protects and manages the 

Nation’s natural resources and cultural heritage; provides scientific 

and other information about those resources; honors its trust 

responsibilities or special commitments to American Indians, 

Alaska Natives, and affiliated Island Communities. 

The mission of the Bureau of Reclamation is to manage, develop, 

and protect water and related resources in an environmentally and 

economically sound manner in the interest of the American public. 

Cover Photo – Cover photo: Looking upstream at upper slough in Glen Canyon on a gravel bar near 

river mile -12. Under existing conditions, the isolated slough provides habitat for warm-water nonnative fish species (photograph by Michael Sixta, Reclamation, 10/18/2022). 
Glen Canyon Sloughs: 

Proposed Modifications 

Glen Canyon National Recreation Area, Arizona 

Upper Colorado Basin Region 

Prepared by: Michael Sixta and Timothy J. Randle 

Bureau of Reclamation 

Technical Service Center 

Denver, Colorado 

U.S. Department of the Interior August 2023 

 

Glen Canyon Sloughs: 

Proposed Modifications 

Glen Canyon National Recreation Area, Arizona 

Upper Colorado Basin Region 

Prepared by: Michael Sixta, MS, PE, Civil Engineer (Hydraulics) 

Sedimentation and River Hydraulics Group 

Prepared by: Timothy J. Randle, PhD, PE, D.WRE, Civil Engineer (Hydraulics) 

Sedimentation and River Hydraulics Group 

Peer Review Certification 

This section has been reviewed and is believed to be in accordance with the service agreement 

and standards of the profession. 

Peer reviewed by: Colin Byrne, PhD, Civil Engineer (Hydraulics) 

Sedimentation and River Hydraulics Group 

Peer reviewed by: Kerri Pedersen, Biologist 

UCRO Adaptive Management Group 

 

 

 

 

 

 

Symbols, Acronyms, and Abbreviations 

o

C degrees Celsius (temperature) 

> greater than 

< less than 

% percent 

1D one-dimensional 

2D two-dimensional 

AZ Arizona 

cfs cubic-feet-per-second (flow rate) 

DEM Digital Elevation Model 

ft feet (length) 

ft2 square feet (area) 

ft/s feet-per-second (flow velocity) 

gal/min gallons-per-minute 

GIS Geographic Information System 

HEC-RAS Hydrologic Engineering Center-River Analysis System (U.S. Army Corps 

of Engineers 1D hydraulic model) 

lidar light detection and ranging (topography data) 

NAD83 North American (horizontal) Datum established in 1983 

NAVD88 North American Vertical Datum established in 1988 

RM river mile 

SMS Surface-water Modeling System (Aquaveo software that interfaces with 

SRH-2D) 

SRH-2D Sedimentation and River Hydraulics (Reclamation’s 2D hydraulic model) 

TSC Technical Service Center 

U.S. United States 

UCRO Upper Colorado [Basin] Regional Office 

USACE U.S. Army Corps of Engineers 

USGS U.S. Geological Survey 

version 

yd3 cubic yards (volume) 

v 

Contents 

Page 

1.0 Introduction..........................................................................................................................1 

2.0 Existing Conditions..............................................................................................................4 

3.0 Previously Developed Alternatives....................................................................................13 

4.0 Proposed Actions ...............................................................................................................14 

4.1 Phase 1 Action .......................................................................................................15 

4.2 Phase 2 Action .......................................................................................................19 

4.3 Phase 3 Action .......................................................................................................20 

4.4 Implementation Summary......................................................................................21 

5.0 Hydraulic Analysis.............................................................................................................23 

5.2 Two-Dimensional Hydraulic Modeling Methodology ..........................................24 

5.2.1 Model Selection..........................................................................................24 

5.2.2 Hydraulic Model Simulations ....................................................................24 

5.2.3 Model Terrains ...........................................................................................25 

5.2.4 Model Meshes ............................................................................................26 

5.2.5 Model Material Types and Hydraulic Roughness......................................27 

5.2.6 Model Boundary Conditions ......................................................................29 

5.2.7 Model Runs and Monitoring ......................................................................30 

5.3 Model Results and Observations ...........................................................................30 

5.3.1 Existing Conditions Model Results............................................................30 

5.3.2 Phase 1 Conditions Model Results.............................................................32 

5.3.3 Phase 2 Conditions Model Results.............................................................33 

5.3.4 Phase 3 Conditions Model Results.............................................................35 

5.4 Model Limitations..................................................................................................36 

6.0 Temperature Analysis ........................................................................................................38 

6.1 Phase 1 Temperature Conditions ...........................................................................39 

6.2 Phase 2 Temperature and Velocity Conditions......................................................40 

6.3 Phase 3 Temperature and Velocity Conditions......................................................41 

7.0 Conclusions........................................................................................................................42 

Acknowledgments..........................................................................................................................45 

i 
Tables 

Page 

Table 1.—Summary of Lake Powell water surface elevations from 1980 to April 2023 .............. 9 

Table 2.—Summary of Colorado River water temperatures at Lees Ferry, AZ from 1980 to 

April 2023 ....................................................................................................................................... 9 

Table 3.—Summary of crew size, duration, and relative cost for each phase.............................. 22 

Table 4.—Values used in Darcy's law to estimate groundwater flow from Colorado River to 

upper slough.................................................................................................................................. 23 

Table 5.—SRH-2D modeled flows............................................................................................... 25 

Table 6.—Material types with corresponding Manning’s roughness values................................ 29 

Table 7.—Downstream boundary condition water surface elevations ......................................... 29 

Table 8.—Upper slough estimates of wetted surface area, volume, and water exchange time 

under existing and after Phase 1 conditions.................................................................................. 39 

Table 9.—Mixing rates in the lower slough under Phase 2 and Phase 3 conditions .................... 40 

Figures 

Page 

Figure 1.—Project location overview map ..................................................................................... 2 

Figure 2.—Colorado River through Glen Canyon where upper and lower sloughs are 

associated with a large gravel bar at RM -12...................................................................... 2 

Figure 3.—Aerial photographs of Glen Canyon near RM -12 on September 14, 1952 (A) and 

August 2, 2015 (B).............................................................................................................. 5 

Figure 4.—Mean-daily discharge of the Colorado River at Lees Ferry from 1962 to May 2023.. 6 

Figure 5.—The surface of the gravel bar has been armored with cobbles, boulders, and gravel... 7 

Figure 6.—Hydrographs of Lake Powell water surface elevations and Colorado River water 

temperature at Lees Ferry for the period 1986 through May 2023..................................... 8 

Figure 7.—Water temperature and discharge through Glen Canyon Dam into the Colorado 

River as measured at Lees Ferry during July and August 2022 ....................................... 10 

Figure 8.—Water temperature and discharges through Glen Canyon Dam into the Colorado 

River as measured at Lees Ferry during December 2022 and January 2023.................... 10 

Figure 9.—Discharges through Glen Canyon Dam into the Colorado River and water 

temperature at Lees Ferry and the lower slough during 2015 .......................................... 11 

Figure 10.—Discharges through Glen Canyon Dam into the Colorado River and water 

temperature at Lees Ferry and the lower slough during August 2022.............................. 12 

Figure 11.—Proposed actions on the Colorado River through Glen Canyon showing the 

location of the upper and lower sloughs. .......................................................................... 14 

Figure 12.—Looking upstream at the upper slough where most of the depths are shallow 

while the upstream end is deeper ...................................................................................... 15 

ii 
Figure 13.—Looking upstream from the lower slough along the existing channel connecting 

the upper and lower sloughs ............................................................................................. 16 

Figure 14.—Phase 1 and Phase 2 elevation profiles..................................................................... 17 

Figure 15.—Proposed Phase 1 conditions showing a partially drained upper slough.................. 18 

Figure 16.—Proposed excavation and fill areas for Phase 3 ........................................................ 21 

Figure 17.—DEM representation of all three phases in project area of interest........................... 26 

Figure 18.—Example area of coarse model mesh showing quadrilateral elements along the 

main channel surrounded by triangular elements on the overbanks ................................. 27 

Figure 19.—Project area of interest under Phase 2 conditions showing hydraulic model 

roughness zone delineation boundaries ............................................................................ 28 

Figure 20.—Project area of interest under Phase 2 conditions showing filled hydraulic model 

roughness zone delineations ............................................................................................. 28 

Figure 21.—Velocity magnitudes and inundation extent of existing conditions at 5,000 cfs...... 31 

Figure 22.—Velocity magnitudes and inundation extent of existing conditions at 20,000 cfs.... 31 

Figure 23.—Velocity magnitudes and inundation extent of existing conditions at 45,000 cfs.... 32 

Figure 24.—Depths and inundation extent of Phase 1 conditions at 15,000 cfs .......................... 33 

Figure 25.—Velocity magnitudes and inundation extent of Phase 2 conditions at 8,000 cfs ...... 34 

Figure 26.—Velocity magnitudes and inundation extent of Phase 2 conditions at 30,000 cfs .... 34 

Figure 27.—Velocity magnitudes and inundation extent of existing conditions at 15,000 cfs.... 35 

Figure 28.—Velocity magnitudes and inundation extent of Phase 3 conditions at 15,000 cfs .... 36 

Figure 29.—Factors influencing the thermal regime of rivers ..................................................... 38 

iii 

 

 

 

 

 

Executive Summary 

The elevation of Lake Powell outside of Page, Arizona, reached historically low levels in spring 

2023 (3520 feet) due to sustained drought conditions. When the reservoir was nearly full in the 

early 1980s (water surface elevation > 3650 feet), the temperature of water released from Lake 

Powell through Glen Canyon Dam (dam) averaged 9 degrees Celsius (o

C). With the current 

reservoir levels, the release temperature of the water is now higher (12 oC) because the warmer 

surface water is closer to the penstocks where the water is released, and the river water 

downstream from the dam is more susceptible to warming. Fish reside in the warmer waters 

(epilimnion) of Lake Powell, and when the elevation is low, the warmer water aligns with the 

penstocks of the dam meaning that the fish are more likely to becoming entrained as the water 

passes through the penstocks. 

After completion of Glen Canyon Dam in 1963, Lake Powell began trapping the sediment load 

of the Colorado River. In response, the clearwater releases from Glen Canyon Dam resulted in 

the channel downstream to incise, armor, and narrow. Approximately 3.5 miles downstream 

from Glen Canyon Dam (river mile (RM) -12), a large portion of the riverbed became exposed 

above the water surface as a large gravel bar. The finer sediments (sand and gravel) were eroded 

from the riverbed leaving a layer of cobbles and boulders to armor the riverbed. 

The temperature of the Colorado River water does not change much after it is released through 

the dam and before it travels 3.5 miles to the upper and lower sloughs at RM -12. Water in the 

sloughs exchanges slowly with the river and backs up into the lower slough when the river 

discharge is greater than 6,000 cubic-feet-per-second (cfs). River water does not connect with the 

upper slough until the discharge increases to 20,000 cfs. Except for a slow groundwater 

connection, the upper slough is isolated from the main river channel most of the year. 

Consequently, the water temperatures in the sloughs vary daily and seasonally with air 

temperature. Water in the lower slough is mostly exchanged with Colorado River water 

throughout the year during daily hydro-peaking operations, but water temperatures in the lower 

slough are still 2 to 5 o

C higher than the river during summer months. On summer days when 

Colorado River discharge is relatively steady because of typical dam operations on weekends 

and holidays, water temperatures in the lower slough can warm as much as 5 to 15 oC above the 

river water temperatures. 

Seven alternatives were previously developed to alter the sloughs to make the habitat less 

favorable to non-native, warm-water fish (TSC Report No. SRH 2018-17). These alternatives 

included draining the upper slough, filling in the upper and lower sloughs, creating flow paths 

(through open channels or culverts) from the river to the upper sloughs, pumping river water into 

the upper slough, or pumping warm water out of the sloughs. 

ES-1 
 

Glen Canyon Sloughs: Proposed Modifications 

In this report, a new alternative is presented that could be implemented in three phases. In Phase 

1, a narrow, but deeper channel (using manual labor and hand tools) would be excavated 

between the upper and lower sloughs to partially drain the upper slough. In Phase 2 a longer and 

narrow side channel would be excavated (using small, mechanized equipment) between the 

Colorado River and the upper slough to allow Colorado River water to continually enter the 

upper slough and continue flowing through the lower slough. Some of the gravel, cobble, and 

sand excavated from the channel would be used to fill in the deepest portions of the upper 

slough. In Phase 3 the width of the lower slough (using small, mechanized equipment) would be 

narrowed by placing gravel, cobble, and sand from the bar along the right, and shallow bank of 

the lower slough. 

Implementation of the three phases is expected to result in water temperatures in the upper and 

lower sloughs that approximate water temperatures in the adjacent Colorado River. Only natural 

materials (gravel, cobble, and sand) would be used to modify the habitat. Due to the length of 

time that the large and armored gravel bar has existed, the habitat modifications are expected to 

be self-sustaining and require little, if any, maintenance. 

ES-2 
 

 

1.0 Introduction 

Glen Canyon Dam and Lake Powell near Page, Arizona (AZ), have significantly changed river 

flow, sediment, and temperature regimes of the Colorado River through Glen, Marble, and Grand 

Canyons, and consequently the geomorphology and ecology of the river corridor downstream 

from the dam (United States (U.S.) Department of Interior, 1995). Given the low elevation of the 

dam’s outlet through the penstocks to the powerplant, water released from the dam has 

historically been clear and cold, with an average temperature of 9 degrees Celsius (oC) (Webb et 

al., 1999). However, sustained drought conditions in the western part of the U.S. over the past 

two decades, has decreased the reservoir elevation significantly resulting in warmer water 

releases and warmer water temperatures in the Colorado River. 

Two sloughs associated with a gravel bar that is 3.5 river miles downstream from Glen Canyon 

Dam or 12 river miles upstream from Lees Ferry, Arizona (AZ) (RM -12; Figure 1) are warmer 

than the Colorado River for much of the year creating habitat for warmwater non-native fishes. 

The lower (downstream) slough has a surface water connection with the Colorado River. 

However, the upper (upstream) slough is normally only connected with the Colorado River via 

groundwater through the sediments of the gravel bar (Figure 2). Darcy’s Law suggests this 

groundwater connection takes weeks to exchange water with the upper slough. At a river 

discharge of at least 20,000 cubic-feet-per-second (cfs; high end of daily fluctuating flow) water 

flows over the gravel bar and establishes a temporary surface water connection between the 

Colorado River and the upper slough. 

1 
Glen Canyon Sloughs: Proposed Modifications 

Figure 1.—Project location overview map. 

Lower Slough Upper Slough 

Figure 2.—Colorado River through Glen Canyon where upper and lower sloughs are associated with a 

large gravel bar at RM -12 (3.5 miles downstream from Glen Canyon Dam). 

2 
Glen Canyon Sloughs: Proposed Modifications 

Alterations of the slough habitats are expected to help prevent non-native fish species from 

spawning in the slough. Due to the difference in temperature of the sloughs, as well as the low 

velocity habitat, the current conditions provide habitat for these warmwater non-native fishes. 

In 2018, the Bureau of Reclamation’s (Reclamation) Technical Service Center (TSC) completed 

an analysis of various options to modify the slough (TSC Report No. SRH 2018-17). Due to 

decreasing elevations of Lake Powell, the environmental conditions have changed considerably 

since the initial report, thus prompting a re-evaluation of options to modify the sloughs. 

3 
 

 

Glen Canyon Sloughs: Proposed Modifications 

2.0 Existing Conditions 

Following the closure of Glen Canyon Dam in 1963, Lake Powell began trapping sediment that 

resulted in erosion of the Colorado River channel through Glen Canyon creating a narrower and 

deeper channel (Pemberton, 1976, and Grams et al, 2007). Near RM -12, the channel thalweg has 

lowered 10 ft and the water surface elevation has lowered 5 ft subsequent to 1959 (Grams et al, 

2007 see figure 9 of that report). Notations on a 1952 aerial photograph indicate that the left side 

of the river channel was shallow with a submerged “gravel and rock bar” at the upstream end and 

a sandbar at the downstream end (Figure 3A). In other words, the upper and lower slough as they 

exist today did not exist prior to the dam. After many years of streambed erosion, the channel 

became deeper along the right side of the canyon while the left side of the former riverbed 

became exposed as a high gravel bar. Depressions on the left side of the exposed gravel bar now 

function as the upper and lower sloughs (Figure 3B). 

4 
A 

B 

 

Glen Canyon Sloughs: Proposed Modifications 

Figure 3.—Aerial photographs of Glen Canyon near RM -12 on September 14, 1952 (A) and August 2, 

2015 (B) show that the Colorado River channel has become narrower with areas of former riverbed now 

exposed along the left bank. Mean daily discharge on September 14, 1952, was 7,050 cfs and 9,160 cfs 

on August 2, 2015, at 7:30 AM. 

5 
 

Glen Canyon Sloughs: Proposed Modifications 

With the trapping of Colorado River sediments in Lake Powell and occasional high discharges 

from Glen Canyon Dam (Figure 4), fine sediments from the riverbed and gravel bar surface have 

eroded leaving an armored layer of large cobbles, small boulders, and gravel (Figure 5). Without 

frequent floods, vegetation has grown on the high bar surfaces. 

Figure 4.—Mean-daily discharge of the Colorado River at Lees Ferry, AZ from 1962 to May 2023. 

6 
 

 

Glen Canyon Sloughs: Proposed Modifications 

Figure 5.—The surface of the gravel bar has been armored with cobbles, boulders, and gravel (A and 

C). In the absence of frequent floods, grasses and shrubs have grown on the high surface of the bar 

(B). 

During periods when Lake Powell was relatively full (3650 feet), cold water was released from 

deep within the reservoir (200 ft deep) to the Colorado River downstream. Because warm-water, 

non-native fish reside in the warmer surface waters of the lake, the probability of them becoming 

entrained or passing through the penstocks of the dam was very low. In addition, the water 

released into the Colorado River below the dam was colder than ideal for warm water fishes and 

the gravel bar and sloughs at RM -12 were commonly inundated by daily peak discharges. 

Due to sustained drought conditions, the elevation of Lake Powell is much lower, meaning the 

penstocks of the dam are now closer to the warmer surface waters where warmwater fishes 

reside thus increasing the likelihood of entrainment. With lower elevations in Lake Powell, daily 

peak discharge is almost always less than (<) 20,000 cfs making the upper and lower sloughs 

more isolated from river flows. In fact, the U.S. Geological Survey (USGS) gage at Lees Ferry, 

AZ, shows that 20,000 cfs has only been exceeded five times during the past decade (April 2013 

through April 2023). 

7 
 

Glen Canyon Sloughs: Proposed Modifications 

Lake Powell filled for the first time in June 1980 when the reservoir water surface reached an 

elevation of 3700 feet. Since then, the reservoir has varied over time but mostly decreased since 

2001 during drought conditions (Figure 6). Reservoir levels reached a low elevation of 

3519.9 feet on April 13, 2023, the lowest elevation since July 1965 when the reservoir was 

initially filling and two years after the dam was completed in 1963. 

Figure 6.—Hydrographs of Lake Powell water surface elevations and Colorado River water temperature 

at Lees Ferry, AZ for the period 1986 through May 2023. 

Because the water is drawn through the penstocks from deep within the lake, the water released 

is cold (averaging 9 °C). Prior to October 1, 1991, hourly discharge from Glen Canyon Dam 

fluctuated throughout the day following the demands for electricity in the western U.S. However, 

hourly discharge fluctuations were constrained beginning in October 1991. Prior to constraints 

on the daily fluctuations, releases regularly caused the river water surface to flow over the gravel 

bar at RM -12 during the peak power months of January and December and July and August. 

The river water was cold and regularly exchanged with water in the sloughs. 

8 
 

Glen Canyon Sloughs: Proposed Modifications 

From April 2003 through March 2021, mean Lake Powell water surface elevations were much 

lower than the prior period (Table 1) and warmer water was released from the reservoir 

(Figure 6, Table 2). From 2003 to March 2021, mean-daily river water temperatures increased by 

1.3 oC, maximum temperatures increased by 3.4 oC, and the standard deviation doubled relative 

to the prior period (Table 2). From April 2021 to April 2023, the same pattern ensued with 

reservoir water surface elevations decreasing, and river water temperatures increasing again 

(Table 2). 

Table 1.—Summary of Lake Powell water surface elevations from 1980 to April 2023 

Time Period 

Lake Powell Elevation (ft) Depth to 

Penstock 

Mean Minimum Maximum Intake (ft) 

6/1/1980 3/31/2003 3670 3607 3708 230 

4/1/2003 3/31/2021 3606 3555 3661 85 

4/1/2021 4/17/2023 3537 3520 3567 50 

Table 2.—Summary of Colorado River water temperatures at Lees Ferry, AZ from 

1980 to April 2023 

Time Period 

Colorado River mean-daily water temperature at Lees 

Ferry, AZ (oC) 

Mean Minimum Maximum 

Standard 

Deviation 

6/1/1980 3/31/2003 9.3 6.6 12.3 0.9 

4/1/2003 3/31/2021 10.6 7.3 15.7 1.8 

4/1/2021 4/17/2023 12.0 6.9 20.8 4.0 

Beginning October 1, 1991, constraints on hourly discharge fluctuations prevented Colorado 

River flows from regularly flowing over the gravel bar at RM -12. Hydrographs of river 

discharge and water temperature for July and August 2022 show that daily peak releases were 

between 9,000 and 16,000 cfs and water temperatures ranged from roughly 17 to 21 o

C 

(Figure 7). During the winter months, the cold air temperatures, in addition with the turning over 

of Lake Powell, result in colder reservoir water temperatures and colder Colorado River water 

temperatures (Figure 8). 

9 
 

Glen Canyon Sloughs: Proposed Modifications 

Figure 7.—Water temperature and discharge through Glen Canyon Dam into the Colorado 

River as measured at Lees Ferry, AZ, during July and August 2022. 

Figure 8.—Water temperature and discharges through Glen Canyon Dam into the Colorado 

River as measured at Lees Ferry, AZ, during December 2022 and January 2023. 

10 
 

Glen Canyon Sloughs: Proposed Modifications 

Upper slough water temperatures are more susceptible to seasonal fluctuations than the river. 

Water temperatures in the upper slough typically become even colder than the river water during 

the winter because there is not normally a surface water connection between the river and upper 

slough during this time of the year. The reverse pattern happens during the summer when the 

slough water temperatures warm with the air temperature. Discharge and temperature data from 

2015 show that water temperatures in the lower slough vary with season and with river discharge 

(Figure 9). Water temperatures in the slough were colder than the river water during that year in 

January, February, much of March, November, and December. However, water temperatures in 

the lower slough were significantly warmer than the river channel during May, June, July, 

August, and September. Similar trends were observed in other abbreviated data sets provided by 

the National Park Service for 2013 through 2017, and 2022. Additionally, discharge and 

temperature data from August 2022 show that water temperatures in the lower slough were 

significantly warmer during weekends when the discharges are relatively steady, indicating that 

daily discharge fluctuations during the week exchange water with the lower slough and limit 

warming (Figure 10). 

Figure 9.—Discharges through Glen Canyon Dam into the Colorado River and water temperature at 

Lees Ferry, AZ, and the lower slough during 2015. 

11 
Glen Canyon Sloughs: Proposed Modifications 

Figure 10.—Discharges through Glen Canyon Dam into the Colorado River and water temperature at 

Lees Ferry, AZ, and the lower slough during August 2022. 

12 
 

 

 

Glen Canyon Sloughs: Proposed Modifications 

3.0 Previously Developed Alternatives 

In 2016, Reclamation’s TSC looked at options for modifying the sloughs to make them less 

desirable for non-native fishes. Seven conceptual alternatives were looked at including draining 

the upper slough, filling-in the upper and lower sloughs or meeting a water temperature 

reduction goal. Each alternative was evaluated and compared using several criteria such as the 

necessary cut and fill volumes, level of operations and maintenance, and planimetric area of 

disturbance. Required maintenance was foreseen to include such activities as sediment removal 

to maintain excavations as designed and pipe or culvert connections, promotion of vegetation 

establishment, and equipment maintenance. Additional details can be found in the report 

(TSC Report No. SRH 2018-17). 

13 
Glen Canyon Sloughs: Proposed Modifications 

4.0 Proposed Actions 

Environmental conditions have changed substantially since the original alternatives for modifying 

the slough were developed in 2018. To evaluate the options with the current environmental 

conditions, a field trip to the Colorado River in Glen Canyon, AZ, was conducted on October 18, 

2022, to investigate the upper and lower sloughs and their relationship to the main river channel. 

Authors, Mike Sixta, and Tim Randle (TSC) joined Kerri Pedersen from Reclamation’s 

Upper Colorado Basin Region out of Salt Lake City, UT, and several local scientists from the 

National Park Service on this field trip (Sixta and Randle, 2023). Some of the reasons that the 

previous alternatives were not implemented were discussed including the need to bring in heavy 

equipment, concern with where to dispose dredge material, and disturbing habitat for other 

species. These concerns were considered in developing a new proposed action. A new alternative 

to connect the sloughs to the Colorado River was developed during this field trip with the idea 

that it could be implemented all at once or in three phases (Figure 11). This new alternative is 

described below. 

Phase 1 

Excavation 

Phase 2 

Excavation 

Phase 3 

Channel Fill 

Upper Slough 

 

 

Figure 11.—Proposed actions on the Colorado River through Glen Canyon showing the location of the 

upper and lower sloughs. Background photograph from Google Earth on August 2, 2015, at 7:30 AM 

when the discharge was 9,160 cfs. 

14 
 

 

Glen Canyon Sloughs: Proposed Modifications 

4.1 Phase 1 Action 

During Phase 1, a channel would be excavated between the upper slough and the lower slough to 

convert much of the upper slough to floodplain that would lower the outlet elevation and 

partially drain the slough. This initial phase could be implemented at low cost and low impact 

while compliance for the other two phases could be completed. If preferred, Phase 1 could be 

omitted, and the other two phases could be implemented at the same time. If Phase 1 were not 

implemented first, Phase 2 would supersede the need for Phase 1 by converting the upper slough 

into a continuously flowing side channel through providing a surface water connection to both 

sloughs. 

The upper slough currently has an area of roughly 14,000 ft2 (200-foot-length and 70-foot-width) 

as measured from Google Earth. Most of the upper slough is less than 3 ft deep while the 

upstream portion of the upper slough is as deep as 6 ft (Figure 12). The distance between the 

upper and lower slough is only about 20 ft (Figure 13). 

Figure 12.—Looking upstream at the upper slough where most of the depths are shallow (2-3 ft) while 

the upstream end is deeper (6 ft); the proposed excavated Phase 1 channel is outlined. 

15 
 

 

Glen Canyon Sloughs: Proposed Modifications 

Figure 13.—Looking upstream from the lower slough along the existing channel connecting the upper 

and lower sloughs; the proposed excavated Phase 1 channel is outlined. 

In Phase 1, a narrow channel would be excavated (about 2 ft wide at the bottom) to hydraulically 

connect the two sloughs and partially drain the upper slough. The channel could be excavated 

with manual labor using picks, shovels, and pry bars. The excavation should proceed in the 

upstream direction from the lower slough toward the upper slough. Initially, the depth of 

excavation where the channel connects to the lower slough only needs to be 0.1 ft deep. 

However, as the channel cuts through the existing high surface that separates the sloughs, the 

excavation depth would increase to a maximum of roughly 3 ft (Figure 14). The longitudinal 

slope of the excavated channel would be basically flat (< 0.1 percent (%)). The side slopes of the 

excavation should be about 50% (the channel widens by 2 ft for every 1 ft of elevation gain) to 

maintain channel bank stability. The excavation volume is expected to be 65 cubic yards (yd3). 

Channel excavation would likely take a crew of six people 5 days to complete. Because the 

Phase 1 channel would be dug using manual labor, the excavated cobbles, gravel, and sand 

would be cast aside along the channel. Additional labor time would be needed to place the 

excavated material in the deeper portions of the upper slough. 

16 
 

Glen Canyon Sloughs: Proposed Modifications 

3120 

3125 

3130 

3135 

3140 

0 200 400 600 800 1,000 1,200 

Ground Elevation (feet) 

Channel Length (feet) 

5,000 cfs Water Level 8,000 cfs Water Level 20,000 cfs Water Level 

Phase 1 Excavation Phase 2 Excavation Existing Ground 

Upper 

Slough 

Lower 

Slough 

CO 

River 

Figure 14.—Phase 1 and Phase 2 elevation profiles. 

As the Phase 1 channel excavation progresses toward the upper slough, seepage flow rates into 

the channel are expected to increase. As the excavation reaches the ponded water of the upper 

slough, the rates of flow entering the new channel will temporarily increase and laborers may 

have to step out of the channel until flow rates decrease. Once flow rates decrease, channel 

excavation should continue through the upper slough until most of the ground surface 

surrounding the channel excavation is dry or barely wet; there will still likely be several feet of 

ponding upstream of the excavation in the deepest part of the slough. If large boulders or 

bedrock are encountered during the excavation, the channel would have to realigned around the 

immovable rock or the depth of excavation limited to the top of rock. 

Once the new channel has been excavated, native floodplain vegetation could be planted (if 

desired) to cover the bare substrate of the upper slough and along the new channel banks and 

bottom to help stabilize the new channel. Vegetation planting is expected to take a crew of six 

people 2 days to complete. The new channel may widen over time if flows are greater than 

20,000 cfs and flow over the gravel bar and spill into the upper slough. However, channel 

widening is not expected be a prominent concern because the gravel bar has already armored. 

17 
 

 

Glen Canyon Sloughs: Proposed Modifications 

After excavation of the Phase 1 channel, the upstream end of the upper slough (upstream from 

the excavation) may still be 2- to 3 ft deep. The Phase 1 wetted surface area is anticipated to be 

roughly one-third the size of the current upper slough (0.1 acres compared to 0.3 acres, 

respectively; Figure 15) until it becomes connected to the main channel upstream at 20,000 cfs. 

This assumes that material excavated from the channel is not placed in the remaining deeper 

portion of the upper slough, which could be done. Any fish in this remaining ponded area would 

have difficulty swimming downstream to the lower slough during periods of normal river flow 

because water depths along the connecting channel are expected to be less than 0.1 ft. However, 

a net could be installed across the exit of the upper slough to further reduce the possibility of any 

fish leaving the ponded area. 

Figure 15.—Proposed Phase 1 conditions showing a partially drained upper slough; flow is from rightto-left. The light blue line shows the channel that would be excavated to connect the upper slough to 

the lower slough. 

18 
 

 

 

 

Glen Canyon Sloughs: Proposed Modifications 

4.2 Phase 2 Action 

After most of the upper slough has been drained, there would still be a small area with water 

(2- to 3 ft deep) that would warm during the summer months. In addition, slow-moving water 

through the lower slough would warm from solar radiation. During Phase 2, a lower-elevation 

side channel would be excavated to continually connect the upper slough to the Colorado River 

channel. This lower-elevation side channel would connect a small portion of Colorado River 

streamflow at most flows (1.3% at 20,000 cfs) from the main river channel, through the upper 

slough, and to the lower slough. This deeper side channel would be 730 ft long to provide a 

hydraulic connection between the river and the sloughs. Part of this excavation would follow the 

same alignment as the Phase 1 channel, but at a lower elevation (Figure 11). 

The side channel to connect the river to the upper slough could be excavated using small, 

mechanized equipment with a backhoe and bucket (e.g., mini excavator) to remove cobbles, 

gravel, and sand from the gravel bar along with a Bobcat for material transport and placement. 

The alignment of the side channel would generally follow along the existing side channel path 

near the left edge of the gravel bar that has formed from overtopping flows greater than (>) 

20,000 cfs. However, the alignment may have to be offset to the north so that the left bank is not 

closer than 20 ft to the valley hillslopes adjacent to the south canyon wall. This offset is proposed 

so that any sediment or debris eroding over the canyon walls or valley hillslopes would have a 

chance to deposit on the gravel bar surface and not enter the new channel. If large boulders or 

bedrock are encountered during the excavation, the channel would have to be realigned. If the 

channel excavation cannot be realigned, then the excavation depth will have to be limited to the 

top of bedrock or large boulders. 

The lower-elevation side channel would be excavated along a flat longitudinal slope from the 

bottom of the upper slough to the Colorado River channel where river water from the main 

channel would begin to enter the side channel at a river discharge of 5,000 cfs (Figure 14). The 

side channel would have a flat bottom width of 2 ft and 50% side slopes. Side channel 

excavation would proceed upstream from the upstream most end of the lower slough toward the 

Colorado River channel. The bottom elevation of the side channel, at the downstream end, would 

be determined by a field survey to ensure the upstream end of the side channel would intersect 

the Colorado River channel just below the water surface elevation corresponding to a river 

discharge of 5,000 cfs. At a river discharge of 5,000 cfs, a small amount of Colorado River water 

would flow (0.2 cfs) through the new side channel. As river discharges increase, the amount of 

water flowing through the side channel would also increase. At a river discharge of 20,000 cfs, 

flow rates through the side channel would increase to 300 cfs. 

Side-channel excavation depths may range from 3 to 6.5 ft (Figure 14). Channel top widths 

would range from 15 to 30 ft depending on the excavation depth. Cobble, gravel, and sand 

excavated from the side channel would comprise a cut volume of approximately 1,300 yd3

. This 

excavated material would be used to fill in deep areas of the upper slough on either side of the 

excavated channel. All the excavated material should fit within the low areas of the upper 

slough, that presently has a volume of 2,000 yd3

. Excavation of the new side channel using 

small, mechanized equipment and a crew of four people, is expected to take 7 days. 

19 
 

 

Glen Canyon Sloughs: Proposed Modifications 

As excavation of the side channel progresses toward the Colorado River channel, (groundwater) 

seepage flow rates into the side channel are expected to increase. The final excavation to connect 

the channel to the Colorado River should be timed so that discharge is at or near the daily low 

(typically during morning hours), preferably during a month with a low release volume from 

Glen Canyon Dam. The rates of flow entering the new channel significantly increase with 

increases in river discharge and laborers and equipment may have to temporarily exit the new 

side channel as the upstream connection becomes established and water starts flowing into the 

channel. Laborers would be able to reenter the new side channel once river discharge has 

decreased. 

The mechanized equipment could be transported to RM -12 via barge from Lees Ferry, AZ, or 

via helicopter. To the extent possible, disruption of native vegetation should be avoided. Once 

the new side channel has been excavated, riparian vegetation could be planted along the channel 

banks. The vegetation would help to stabilize the channel and minimize establishment of nonnative vegetation. Vegetation planting at the upstream end of the upper slough and along the 

banks of the new side channel is expected to take a crew of six people 5 days to complete. 

The new side channel may widen over time when river discharges are greater than 20,000 cfs. 

However, the gravel bar is already armored and there should not be need for additional 

maintenance considering the lack of an upstream sediment supply. 

4.3 Phase 3 Action 

Streamflow through the new side channel will increase flow velocities through the upper and 

lower sloughs and allow water temperatures in the sloughs to closely match temperatures of the 

Colorado River. However, the lower slough may still be too wide for higher flow velocities to 

persist throughout the entire slough. The current width of the lower slough ranges from 100- to 

150 ft. Water depths in the lower slough vary with the stage of the Colorado River, but current 

flow velocities are near stagnant (less than 0.1 feet-per-second (ft/s)) except when river 

discharges are higher than 20,000 cfs and water flows over the gravel bar. The slow velocities 

and shallow depths allow the lower slough to warm during summer, relative to water in the main 

river channel. It is possible that warmer temperatures in the lower slough may persist in the 

shallower margins even with excavation of a side channel. 

During Phase 3, the lower slough would be narrowed to an average width of 60 ft. The narrower 

width would fill in shallow areas, increase flow velocities, reduce water exchange time, and 

allow water temperatures to match more closely those of the Colorado River. Channel narrowing 

would be accomplished by relocating about 8,500 yd3 of cobbles, gravel, and sand from high 

elevations of the larger gravel bar to the north of the lower slough (Figure 16). The small, 

mechanized equipment from Phase 2 could be used to relocate the material. Deposited material 

would sink as it compresses the underlying soft sediments of the lower slough. The top surface 

of the deposited material would be graded to just above the water surface elevation 

20 
 

 

Glen Canyon Sloughs: Proposed Modifications 

corresponding to a river discharge of 8,000 cfs; discharges above 8,000 cfs would overtop the 

deposited material that is placed to narrow the lower slough. Native floodplain vegetation would 

be planted to cover this new substrate. Excavated areas of the high gravel bar surface would be 

graded and planted with native upland vegetation. 

Figure 16.—Proposed excavation and fill areas for Phase 3. 

The mechanized equipment could be transported to RM -12 via barge from Lees Ferry, AZ, or 

via a helicopter. Phase 3 could also be completed at the same time as Phase 2 to minimize the 

number of times mechanized equipment needed to be transported. To the extent possible, 

disruption of native vegetation should be avoided. Relocating material from the gravel bar 

surface to the lower slough using small, mechanized equipment and a crew of four people, is 

expected to take 30 days assuming a production rate of roughly 300 yd3 per day using the 

proposed two pieces of mechanized equipment working in tandem (mini excavator and Bobcat). 

Vegetation planting along the lower slough and disturbed areas of the gravel bar is expected to 

take a crew of six people 15 days to complete. 

4.4 Implementation Summary 

The crew size, duration, and relative cost for each phase are summarized in Table 3. 

Mobilization of mechanized equipment will depend on the logistics of transporting a barge 

(perhaps in pieces) by truck from the source to Lees Ferry, assembling and launching the barge, 

21 
 

Glen Canyon Sloughs: Proposed Modifications 

loading the equipment onto the barge, upstream river transport to RM -12, and unloading the 

equipment onto the gravel bar. Demobilization of equipment involves similar logistics, but in 

reverse order. Detailed cost estimates have not yet been prepared, but comparatively, Phase 1 is 

estimated to cost less than Phase 2, and Phase 2 would likely cost less than Phase 3. 

Table 3.—Summary of crew size, duration, and relative cost for each phase 

Phase Activity 

Crew 

Size 

Duration 

(days) 

Relative 

Cost 

1 Channel excavation, using hand tools 6 5 $ 

Optional planting of floodplain vegetation 6 2 $ 

Mobilization of equipment 4 $$ 

2 Excavation using small mechanical equipment 4 7 $$ 

Optional planting of floodplain vegetation 6 5 $$ 

3 

Placement of sand, gravel, & cobbles to narrow the 

slower slough using small mechanical equipment 4 30 $$ 

Optional planting of floodplain vegetation 6 15 $$ 

Demobilization of equipment 4 $$ 

22 
 

 

 

 

Glen Canyon Sloughs: Proposed Modifications 

5.0 Hydraulic Analysis 

5.1 Groundwater Flow Rate 

Under existing conditions, there is normally only a groundwater connection between the river 

and the upper slough. Based on visual observations on October 18, 2022, the rate of water 

exiting the upper slough appeared to be only a few gallons-per-minute (gal/min). Darcy’s Law 

can be used to estimate the groundwater flow rate between two locations. 

Where: 

Q = groundwater flow rate 

A = cross-sectional area through which the flow passes 

q = specific flow rate 

K = hydraulic conductivity 

Δh = difference in hydraulic head between two locations 

l = horizontal flow length between two locations 

Using equation 1 and various assumptions, the groundwater flow rate from the river to the upper 

slough was estimated to be 12 gal/min (Table 4). 

Table 4.—Values used in Darcy's law to estimate groundwater flow from 

Colorado River to upper slough 

Darcy’s law parameter Assumed Value 

Head difference (Δh) -3 ft 

Flow length (l) 330 ft 

Hydraulic conductivity (K) 9.84E-03 ft/s 

Horizontal flow rate (q) 8.95E-05 ft/s 

Width of groundwater flow 75 ft 

Depth of groundwater flow 4 ft 

Area of groundwater flow (A) 300 ft2 

Horizontal flow rate (Q) 0.027 cfs 

Horizontal flow rate (Q) 12 gal/min 

23 
 

 

Glen Canyon Sloughs: Proposed Modifications 

5.2 Two-Dimensional Hydraulic Modeling Methodology 

5.2.1 Model Selection 

The choice of model is often governed by time and budget constraints, project needs, access to 

and knowledge of existing models, and the availability of appropriate data to apply the model. It 

is important to understand the formulation of the selected model, recognize the model limitations 

(see 8.0), and apply the model in a manner that takes advantage of its strengths. 

TSC used the SRH-2D (v 3.2.3) numerical model to evaluate the conceptual design alternatives 

for this study. SRH-2D is a two-dimensional (2D) fixed or mobile-bed hydraulics and sediment 

transport model for river systems developed by Reclamation at the TSC (Lai, 2008). This study 

only used the fixed bed hydraulics module of the model. SRH-2D solves the depth-averaged 

dynamic wave equations with a depth-averaged parabolic turbulence model using a finitevolume numerical scheme. The model adopts a zonal approach for coupled modeling of channels 

and floodplains; a river system is broken down into modeling zones (delineated based on natural 

features such as topography, vegetation, and bed roughness), each with unique parameters such 

as flow resistance (Lai, 2010). One of the major features of SRH-2D is the adoption of an 

unstructured hybrid mixed element mesh, based on the arbitrarily shaped element method of 

Lai (2000) for geometric representation. This meshing strategy is flexible enough to facilitate the 

implementation of the zonal modeling concept, allowing for greater modeling detail in areas of 

interest that ultimately leads to increased modeling efficiency through a compromise between 

solution accuracy and computing demand. 

5.2.2 Hydraulic Model Simulations 

Various discharges were used for modeling the existing conditions and proposed (three) design 

phases based on the upstream operations of Glen Canyon Dam (Table 5). All discharges were 

used to simulate existing and Phase 2 conditions while select flows were used to simulate Phase 

1 and Phase 3 conditions. Modeled discharges for Phase 1 and Phase 3 were selectively chosen 

based on anticipated changes according to existing conditions model results (e.g., 5,000 and 

8,000 cfs doesn’t inundate the lower slough enough to interact with the proposed Phase 1 

channel excavation). 

24 
 

Glen Canyon Sloughs: Proposed Modifications 

Table 5.—SRH-2D modeled flows 

Flow 

(cfs) Description 

Existing 

Conditions 

Phase 1 

Conditions 

Phase 2 

Conditions 

Phase 3 

Conditions 

5,000 low night-time flow  X  

8,000 low day-time flow  X  

11,000 mean annual flow for minimum release year    

15,000 moderate power plant release    

20,000 limit of upper hydropeaking flow  X  

25,000 high flow overtopping bar  X  X 

30,000 near powerplant capacity  X  X 

35,000 max powerplant & low outlet works release  X  X 

40,000 max powerplant & moderate outlet works release  X  X 

45,000 max powerplant & max outlet works release  X  X 

(green check marks were modeled scenarios whereas red x’s were not modeled) 

5.2.3 Model Terrains 

The model domain spanned roughly 3 river miles, with the gravel bar and sloughs located near 

the lower one-third of the model domain. To represent the existing conditions topography within 

this domain, a model terrain was generated using a Geographic Information System (GIS) 

software that output a 3 ft posting Digital Elevation Model (DEM). The DEM used 

photogrammetric data acquired in 2021 stitched in with bathymetric (underwater) data that was 

collected between May 2013 to February 2016 (Kaplinski et al., 2022) that included both 

sloughs. To evaluate the three design phases (Figure 17), TSC modified the existing DEM 

surface using terrain modification tools in HEC-RAS (Hydrologic Engineering Center-River 

Analysis System, 2022). All model topography used the North American Datum (NAD83) State 

Plane Arizona Central 0202 (International ft) coordinate system. All elevations were shifted from 

the provided ellipsoid heights (in meters) to reference orthometric ground elevations under the 

North American Vertical Datum (NAVD88) datum in International ft. The geoidal height 

separation was calculated to be -77.1 ft, meaning that ground elevations are 77.1 ft lower than 

the elevations given in the provided DEM. 

25 
 

Glen Canyon Sloughs: Proposed Modifications 

Figure 17.—DEM representation of all three phases in project area of interest. 

5.2.4 Model Meshes 

A 2D mesh defines the SRH-2D model topography and solution spacing. Nodes that store 

ground elevation information from the model terrain comprise the mesh. The mesh consists of 

both quadrilateral and triangular shaped elements. To evaluate the existing and proposed phases, 

TSC constructed a mixed element mesh using Aquaveo’s Surface-water Modeling System 

(SMS) software. The design elevations were interpolated directly to the mesh from the DEM. 

Therefore, the mesh resolution was increased at the primary areas of interest (the gravel bar, 

excavation channels, and both sloughs) by using finer mesh cells. The mesh contours were later 

evaluated to ensure these features were being properly represented. TSC used coarser mesh cell 

sizes in other areas of the model. In doing so, meshes were built to provide both relevant detail as 

well as reasonable simulation run times. To maintain model stability, a separate, coarser mesh, 

was used for all simulations or river discharges greater than 30,000 cfs. 

The final meshes for this study consisted of structured (quadrilateral) elements within the main 

channel of the Colorado River, and unstructured (triangular) elements in the overbank areas. The 

element size varied based on location. For the fine meshes, the size of the quadrilateral elements 

started at 10 ft by 15 ft for the lateral (cross stream) and longitudinal (downstream) direction in 

the main channel, respectively, and gradually decreased down to 2 ft by 5 ft around the design 

areas of interest. The triangular elements in the floodplain areas increased in coarseness as the 

model approached the domain boundary up to a maximum size of 20 ft. For the coarse meshes, 

26 
 

Glen Canyon Sloughs: Proposed Modifications 

the size of the quadrilateral and triangular elements remained the same as what the fine meshes 

started at throughout the entire model domain (Figure 18). In total, an average of 

358,600 elements made up the fine meshes used to simulate both existing and proposed 

conditions while an average of 157,550 elements made up the coarse meshes. 

Figure 18.—Example area of coarse model mesh showing quadrilateral elements along the main 

channel surrounded by triangular elements on the overbanks. Flow is from right to left. 

5.2.5 Model Material Types and Hydraulic Roughness 

TSC also used SMS to delineate distinct roughness areas in the model domain. Hydraulic 

roughness is the resistance of the channel and overbank topography (vegetation) to the flow. 

SRH-2D quantifies hydraulic roughness using the Manning’s roughness coefficient (n), which 

influences the calculated depths, but more so the calculated velocity magnitudes. Assuming the 

channel and floodplain topography is well represented, roughness is the primary calibration 

parameter for 2D hydraulic modeling. 

Through SMS, the roughness coefficients were distributed spatially across the model domain 

according to material type/land cover. Topography and aerial imagery informed the roughness 

zone delineations (Figure 19). Across the floodplain, generalized patterns were observed in 

vegetation density, leading to two distinct levels of roughness classification. TSC assigned five 

distinct roughness zones for the existing and proposed conditions modeling (Figure 20). 

Manning’s n values were based on previous model efforts, published values (Bureau of 

Reclamation, 1957), and relative comparisons (Table 6). 

27 
Glen Canyon Sloughs: Proposed Modifications 

Figure 19.—Project area of interest under Phase 2 conditions showing hydraulic model roughness zone 

delineation boundaries. 

Figure 20.—Project area of interest under Phase 2 conditions showing filled hydraulic model roughness 

zone delineations. 

28 
 

 

 

 

 

Glen Canyon Sloughs: Proposed Modifications 

Table 6.—Material types with corresponding Manning’s roughness values 

Material Roughness 

Channel 0.03 

Bare ground 0.035 

Light vegetation 0.045 

Medium vegetation 0.05 

Canyon wall 0.03 

It was not possible to calibrate the main channel roughness value as flow data or water surface 

elevations for reference was not available. However, calibration was not deemed necessary for 

this effort as the primary interest was in relative comparisons between existing and proposed 

design conditions. Existing and proposed simulations all used the same roughness values. 

5.2.6 Model Boundary Conditions 

There were two boundary conditions in the model domain specified: one upstream representing 

the inflow via a discharge value (Table 5) and one downstream representing a subcritical outflow 

via a corresponding water surface elevation. A previous model (TSC Report No. SRH 2018-17) 

that extended further downstream was used to assign this water surface elevation (Table 7). All 

simulations assumed a steady flow that did not change over time. 

Table 7.—Downstream boundary condition water surface elevations 

Flow (cfs) Water Surface Elevation (ft) 

5,000 3124.0 

8,000 3125.6 

11,000 3127.0 

15,000 3128.6 

20,000 3130.2 

25,000 3131.9 

30,000 3133.5 

35,000 3134.8 

40,000 3136.1 

45,000 3137.1 

29 
 

 

 

Glen Canyon Sloughs: Proposed Modifications 

5.2.7 Model Runs and Monitoring 

A total of 27 model simulations (5.2.2) were completed, running various flow scenarios with 

existing terrain conditions and the conditions for the three phases of the proposed design. Each 

model run included monitoring lines that measure discharge and water surface elevation and 

monitoring points that measure a variety of scalar quantities including velocity and water depth 

throughout the simulation. A monitoring line and point at the downstream boundary were placed 

to check for model stability, to ensure the model ran long enough to achieve a state of 

equilibrium, and to check that mass balance was being met. Monitoring lines were also 

positioned across the proposed channels feeding the sloughs and the sloughs themselves helping 

to determine the quantity of main channel flow being routed through the sloughs under the 

proposed design phases. 

To achieve model stability, all simulations up to and including 30,000 cfs used a 0.5-second time 

step while all simulations above 30,000 cfs used a 0.2-second time step. A total simulation time 

of 24 hours was deemed adequate to achieve equilibrium, and all runs were started without water 

in the model; setting the initial condition to “dry”. 

5.3 Model Results and Observations 

5.3.1 Existing Conditions Model Results 

Existing conditions model results showed that the lower slough was directly affected by the 

amount of water flowing in the main channel. The water was stagnant at all flows below 20,000 

cfs and was not directly connected to the main channel at all during the lowest simulated flow of 

5,000 cfs (Figure 21). Model results further showed a localized increase in main channel 

velocities, especially at lower flows (5,000 cfs), where the lower slough meets the main channel 

because of an inset (deeper) channel constriction in that area. Water began to flow over the 

gravel bar at 20,000 cfs, sending a minimal amount of water (< 0.5 cfs) into the upper slough via 

an existing channelized flow path (Figure 22). At the highest simulated flow of 45,000 cfs, the 

gravel bar was completely inundated with water against both canyon walls and a pocket of lower 

velocity on the south side of the canyon identifies the general location of the sloughs (Figure 23). 

30 
Glen Canyon Sloughs: Proposed Modifications 

Figure 21.—Velocity magnitudes and inundation extent of existing conditions at 5,000 cfs. 

Figure 22.—Velocity magnitudes and inundation extent of existing conditions at 20,000 cfs. 

31 

Upper 

Slough 

Lower 

Slough 

Gravel Bar 

Gravel Bar 

Upper 

Slough 

Lower 

Slough 
Glen Canyon Sloughs: Proposed Modifications 

Figure 23.—Velocity magnitudes and inundation extent of existing conditions at 45,000 cfs. 

5.3.2 Phase 1 Conditions Model Results 

Phase 1 model results showed that water from the Colorado River would start to flow upstream 

from the lower slough into the upper slough via the excavation channel when the release 

discharge was approximately 15,000 cfs (Figure 24); the upper and lower sloughs were not 

connected at 15,000 cfs under existing conditions. Water was not expected to overtop the 

excavation channel when the water flowed from the lower slough to the upper slough, and the 

reduced ponded area in the upper slough (Figure 15) should remain once the flows were reduced. 

Water could flow over the excavation channel when flows were above 20,000 cfs and the 

Colorado River flows over the gravel bar would increase flow into the upper slough and 

temporarily increase the inundated area in the upper slough, but the reduced ponded area should 

return once the river discharge recedes. 

32 

Upper 

Slough 

Lower 

Slough 

Gravel Bar 
Glen Canyon Sloughs: Proposed Modifications 

 

Figure 24.—Depths and inundation extent of Phase 1 conditions at 15,000 cfs. 

5.3.3 Phase 2 Conditions Model Results 

The Phase 2 excavation channel was designed to activate when the Colorado River was around 

5,000 cfs. Phase 2 model results showed a hydraulic connection between the main channel and 

the upper and lower sloughs via the proposed side channel that would carry a very small 

percentage of the total flow (10 cfs) when the Colorado River had a discharge of 8,000 cfs 

(Figure 25). The flow in the side channel would continue to increase as the amount of river 

discharge increase above 8,000 cfs. At flows of 20,000 cfs, 265 cfs would enter the side channel 

and at flows of 30,000 cfs, 1,085 cfs would enter the side channel (Figure 26). Also, velocity in 

the lower slough increased further into the slough as discharge increased. 

33 

Upper 

Slough 

Lower 

Slough 

Gravel Bar 
Glen Canyon Sloughs: Proposed Modifications 

Figure 25.—Velocity magnitudes and inundation extent of Phase 2 conditions at 8,000 cfs; note 

stagnant water in a majority of the lower slough. 

Figure 26.—Velocity magnitudes and inundation extent of Phase 2 conditions at 30,000 cfs; note 

velocity plume through a majority of the lower slough. 

34 

Upper 

Slough 

Lower 

Slough 

Gravel Bar 

10 cfs 

Upper 

Slough 

Lower 

Slough 

Gravel Bar 

1,085 cfs 
 

Glen Canyon Sloughs: Proposed Modifications 

5.3.4 Phase 3 Conditions Model Results 

Phase 3 model results showed that the velocity increased considerably through the lower slough 

compared to existing conditions (Figure 27 and Figure 28; scaling was changed from figures 

above to highlight velocity differences), confirming that the water should move through the 

slough more quickly, allowing slough water temperatures to more quickly approximate river 

water temperatures. Phase 2 and Phase 3 model results were also used to calculate and compare 

the water exchange rates between the lower slough and mainstem under each condition by 

comparing the volume of water in the lower slough to the flow moving through. Compared to 

Phase 2, Phase 3 (when the lower slough is narrowed) shortens the water exchange time with the 

Colorado River because there was a reduced cross-sectional area and corresponding volume of 

water (see section 6.0 Temperature Analysis). 

Figure 27.—Velocity magnitudes and inundation extent of existing conditions at 15,000 cfs. 

35 
Glen Canyon Sloughs: Proposed Modifications 

Figure 28.—Velocity magnitudes and inundation extent of Phase 3 conditions at 15,000 cfs. 

5.4 Model Limitations 

The engineering science of hydraulic modeling is mature and numerical models do a very good 

job of predicting water surface elevations for a given discharge provided that the following 

conditions are met: (1) the digital surface representing the riverbed bathymetry and floodplain 

topography is accurate, and (2) the surface roughness is reasonably estimated. All numerical 

models require simplifying assumptions and thus have limitations that manifests in uncertainty. 

Numerical model predictions will always include some level of uncertainty because the physical 

processes being modeled are not completely represented within the governing equations applied 

in a numerical model. These must be considered while viewing and evaluating model results. For 

example, no flow loss was assumed for this modeling, the model terrain had a fixed-surface 

elevations, and no upstream sediment supply was considered. There are many factors that can 

affect the uncertainty of model results with some being easier to quantify than others. In general, 

the accuracy of model results depends on the quality of input data that in this case includes 

topographic data and model input parameters. 

More than any other factor, topography directly affects model results. The representative 

surfaces used in the model simulations comes from a combination of bathymetric and light 

detection and ranging (lidar) data that represents one instance in time. There are likely areas that 

are not accurately represented in the surfaces that were modeled. Ultimately, this is not expected 

to significantly affect these conceptual study results as these topographic features were not part 

of the focus area of interest. 

36 
Glen Canyon Sloughs: Proposed Modifications 

Model input parameters include Manning’s roughness, time step, and boundary conditions, and 

can all affect the resulting hydraulics. Roughness values were chosen based on literature and 

previous modeling efforts in the same area and were held constant for all simulations. However, 

roughness generally has a much smaller influence on hydraulics than the topography. Model 

stability and achievement of mass balance for the water flow was examined through evaluation 

of model residuals along with water surface elevation and flow rate fluctuations near the model 

boundary. The project area of interest was well within the model domain and outside of the 

boundary condition zone of influence, which should further reduce model uncertainty. 

37 
 

Glen Canyon Sloughs: Proposed Modifications 

6.0 Temperature Analysis 

Caissie (2006) describes four different factors influencing the thermal regime of rivers: (i) 

atmospheric conditions; (ii) topography; (iii) stream discharge; and (iv) streambed (Figure 29). 

Atmospheric conditions are the most important factor responsible for the heat exchange 

processes that take place at the water surface. Topography influences atmospheric conditions. 

Stream discharge influences the heating and cooling capacity through mixing of upstream 

surface water and groundwater, including hyporheic heat exchanges through the streambed. 

Figure 29.—Factors influencing the thermal regime of rivers (Caissie, 2006). 

The temperature of water released from Glen Canyon Dam is a function of season, reservoir 

water surface elevation, and whether the water is released through the penstocks of the dam or 

through the outlet works (Dibble et al., 2021). Once water is released from Glen Canyon Dam, 

the temperature changes very little as it is conveyed downstream along the 3.5 river miles to the 

sloughs at RM -12. In comparison, water temperature in the sloughs tends to vary over time both 

daily and seasonally due to isolation from the main channel and the low flow velocity. Daily 

temperatures are at a minimum during the early morning (at sunrise) and a maximum in late 

afternoon to early evening (Caissie, 2006). Hot air temperatures during summer warm water in 

the sloughs relative to the temperatures of the river. Cold air temperatures during winter cool 

water in the sloughs relative to the river. 

38 
 

 

 

Glen Canyon Sloughs: Proposed Modifications 

Under existing conditions, measurements indicate that water temperatures in the sloughs change 

in response to the changing air temperature. When water is exchanged directly between the 

slough and river, temperature change occurs within minutes and water temperature in the sloughs 

are expected to approximate that of the river. Currently, this only occurs when discharges are 

high enough for water to flow over the gravel bar and enter the sloughs (> 20,000 cfs). 

6.1 Phase 1 Temperature Conditions 

Phase 1 of the proposed action would partially drain the upper slough, reducing its wetted 

surface area, depth, and volume and subsequently allow groundwater to exchange more quickly 

with water in the upper slough. Estimates of surface area, volume, and flow exchange rate are 

presented in Table 8. Phase 1 actions would not change the frequency of river water flowing over 

the gravel bar and entering the upper slough and Phase 1 actions would have little to no effect on 

water temperatures in the lower slough. 

Table 8.—Upper slough estimates of wetted surface area, volume, and water exchange time under exist 

after Phase 1 conditions 

Scenario Wetted 

area (ft2) 

Average 

depth (ft) 

Water 

volume (ft3) 

Slough flow 

rate (cfs) 

Water exchange 

time 

0.027 20 days 

Existing conditions 13,700 3.8 52,430 10 90 minutes 

100 9 minutes 

0.027 8 days 

Phase 1 conditions 4,040 4.5 18,180 10 30 minutes 

100 3 minutes 

The ground water exchange time (or residence time) of the upper slough is currently estimated to 

be about 20 days, which provides enough time for the water in the upper slough to approximate 

the air temperature. After the upper slough is partially drained during Phase 1, the surface area of 

the slough may be reduced by two-thirds and the volume of stored water would also be reduced. 

Under these conditions, the ground water exchange time would be reduced to 8 days, which is 

still enough time for the water in the upper slough to approximate the air temperature. 

If river discharges are high enough (> 20,000 cfs) to flow over the gravel bar at a rate of 10 cfs, 

the water exchange times in the upper slough would be reduced to about 90 minutes under 

current conditions and 30 minutes under the Phase 1 conditions. With the reduced water 

exchange time under Phase 1 conditions, the water temperature in the upper slough would begin 

to approximate the river water temperature, but only when river flows are high enough to overtop 

39 
 

 

 

Glen Canyon Sloughs: Proposed Modifications 

the gravel bar. When river discharges are 25,000 cfs, water is predicted to flow through the 

sloughs at a rate of 100 cfs and the water exchange times in the upper slough would be reduced 

to 9 minutes under current conditions and 3 minutes under Phase 1 conditions. Under these 

conditions, water temperatures in the upper slough would equal the river water temperature. 

6.2 Phase 2 Temperature and Velocity Conditions 

Water in the lower slough is currently exchanged with the river water during daily flow 

fluctuations, which typically occur Monday through Friday. Under current conditions, summer 

water temperatures in the lower slough can fluctuate from 2.5 to 5 oC due to solar heating. When 

river discharge is relatively steady (typically weekends and holidays), lower slough water 

temperatures can increase by 5 to 15 o

C over the river temperatures in the summer months 

(Figure 10). During winter, lower slough water temperatures would tend to be colder than the 

river water, especially during steady flow periods. 

Under Phase 2 conditions, there would be near continual flow through the sloughs, so water 

temperatures in the lower slough would more closely match the river water temperature. At a 

flow rate of 10 cfs through the sloughs (river discharge of 8,000 cfs), the water exchange time in 

the lower slough is expected to be 2.9 hours (Table 9), which is short enough time for lower 

slough water temperatures to start matching river water temperatures. As discharges increase 

throughout the day with increasing hydropower demands, slough water temperatures would more 

closely approximate the river water temperature. For example, at a flow rate of 114 cfs through 

the sloughs (river discharge of 15,000 cfs), the water exchange rate in the lower slough is 

expected to be reduced to 1 hour and water temperature is expected to approximate that of the 

river. However, some shallow and more stagnant areas of the lower slough may still warm 

significantly during the summer months. 

Table 9.—Mixing rates in the lower slough under Phase 2 and Phase 3 conditions 

Scenario River 

discharge (cfs) 

Water volume 

(ft3) 

Slough flow rate 

(cfs) 

Water 

exchange time 

8,000 105,700 10 2.9 hours 

Phase 2 Conditions 11,000 244,000 46 1.5 hours 

15,000 460,100 114 1.1 hours 

20,000 712,200 265 0.7 hours (45 min) 

8,000 61,100 10 1.7 hours (102 min) 

Phase 3 Conditions 

11,000 132,000 46 0.8 hours (48 min) 

15,000 348,400 114 0.8 hours (51 min) 

20,000 599,500 265 0.6 

hours (38 min) 

40 
 

 

Glen Canyon Sloughs: Proposed Modifications 

6.3 Phase 3 Temperature and Velocity Conditions 

Phase 3 of the proposed action is meant to narrow the slough width and increase the velocity 

through the channel. The faster flow velocity would reduce the water exchange time and 

additionally help water temperatures of the lower slough approximate the river water 

temperature. By narrowing the width of the lower slough (Phase 3 actions), the water exchange 

time would be reduced compared to Phase 2 conditions (Table 9). Any shallow areas along the 

right bank of the slough would be eliminated and no longer able to provide warmwater habitat. 

There would be no change in water temperature in the upper slough relative to Phase 2 of the 

proposed action. 

41 
 

 

Glen Canyon Sloughs: Proposed Modifications 

7.0 Conclusions 

Due to sustained drought conditions, the water surface elevation of Lake Powell has been low 

enough that water released through the dam’s penstocks is aligned with the warm surface water 

where fish reside. This increased the chance of fish entrainment and passage through the dam. 

Following the closure of the dam in 1963, channel erosion exposed a high portion of the 

Colorado riverbed approximately 3.5 miles below the dam which is now a large gravel bar that 

contains the upper and lower slough. The upper and lower sloughs are warmer than the river 

which creates habitat for warmwater non-native fishes. Currently, water in the sloughs exchange 

slowly with the river, allowing sufficient time for the water to warm. During summer, water 

temperatures in the sloughs increase significantly more than water in the Colorado River 

channel. During winter, water temperatures in the sloughs are colder than the river. 

A new alternative was developed to modify the sloughs to make the habitat less desirable to 

warm water fishes that could be implemented in phases or all at once. 

• Phase 1 would involve excavation of a narrow, but deeper channel between the upper and 

lower sloughs to partially drain the upper slough. 

• Phase 2 would include excavation of a narrow and lower-elevation side channel to allow 

Colorado River water to enter the upper slough and continue flowing through the lower 

slough. 

• Phase 3 would include narrowing the width of the lower slough by placing gravel, cobble, 

sand from the gravel bar along the right and shallow bank of the slough to increase the 

velocity through the channel. Native flood plain vegetation would be planted in exposed 

areas to help create terrestrial habitat and help prevent the establishment of non-native 

species. 

The proposed new alternative would allow water temperatures in the upper and lower sloughs to 

approximate water temperatures in the adjacent Colorado River and would increase the velocity 

through the sloughs. There are very few upstream sediment sources, so filling of the excavated 

channels with sediment would be unlikely, except from rare rock falls from the left (looking 

downstream) canyon wall. A few rocks falling into the excavated side channel are not expected 

to prevent Colorado River water from entering the sloughs. High river discharge may cause some 

widening of the excavated channels. However, the large gravel bar at RM -12 existed before 

construction of Glen Canyon Dam and has been armored by periodic high and clearwater 

discharges for more than one-half of a century even though the sloughs formed after construction 

of the dam. Therefore, the habitat improvements are expected to be self-sustaining in the future. 

There is a risk that large boulders or bedrock, if present, would prevent the excavation to the full 

design depths of the Phase 1 or Phase 2 channels. 

42 
 

 

 

 

Glen Canyon Slough Proposed Habitat Improvements 

References 

Bureau of Reclamation, 1957. Tail water and degradation studies, Colorado River below Glen 

Canyon Dam: Denver, Colorado, U.S. Department of Interior, Bureau of Reclamation, 

10 p. 

Bureau of Reclamation, 2023. May 2023 24-Month Study Projections, Lake Powell and Lake 

Mead: End of Month Elevation Charts. https://www.usbr.gov/uc/water/crsp/studies/ 

images/PowellElevations.pdf Accessed on 5/19, 2023. 

Caissie, D., 2006. “The thermal regime of rivers: a review,” Freshwater Biology, Volume 51, 

Issue 8, pp 1389-1406, https://doi.org/10.1111/j.1365-2427.2006.01597.x 

Dibble, K.L., Yackulic, C.B., Kennedy, T.A., Bestgen, K.R., and Schmidt, J.C., 2021. “Water 

storage decisions will determine the distribution and persistence of imperiled river 

fishes,” Ecological Applications, 31(2), 2021, e02279, 

https://esajournals.onlinelibrary.wiley.com/doi/pdfdirect/10.1002/eap.2279 

Grams, P.E., Schmidt, J.C., and Topping, D.J. 2007. The rate and pattern of bed incision and 

bank adjustment on the Colorado River in Glen Canyon downstream from Glen Canyon 

Dam, 1956-2000: Geological Society of America Bulletin, v. 119, nos. 5-6, p. 556-575. 

Greimann, B. and Sixta, M., 2018. Temperature Reduction Options for Glen Canyon Slough; 

RM -12, Upper Colorado Regional Office, Technical Report No. SRH-2018-17, Bureau 

of Reclamation, Technical Service Center, Sedimentation and River Hydraulics Group, 

Denver, Colorado. 

Hydrologic Engineering Center. 2022. HEC-RAS River Analysis System [Computer software]. 

Davis, CA: U.S. Army Corps of Engineers. 

Kaplinski, M., Hazel, J.E., Grams, P.E., Gushue, T., Buscombe, D.D., and Kohl, K. 2022. 

Channel Mapping of the Colorado River from Glen Canyon Dam to Lees Ferry in Glen 

Canyon National Recreation Area, Arizona: U.S. Geological Survey Open-File Report 

2022-1057, 20p., https://doi.org/10.3133//ofr20221057. 

Lai, Y.G. 2000. Unstructured grid arbitrarily shaped element method for fluid flow simulation. 

AIAA Journal. 38(12), 2246-2252. 

Lai, Y.G. 2008. SRH-2D version 2: Theory and User’s Manual. Sedimentation and River 

Hydraulics – Two-Dimensional River Flow Modeling. U.S. Department of the Interior, 

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Lai, Y.G. 2010. Two-dimensional depth-averaged flow modeling with an unstructured hybrid 

mesh. Journal of Hydraulic Engineering. 136(1), pp. 12-23. 

43 
Glen Canyon Sloughs: Proposed Modifications 

Pemberton, E.L., 1976. Channel change in the Colorado River below Glen Canyon Dam, in 

Symposium 5- Channel adjustments of Proceedings of the Third Federal Interagency 

Sedimentations Conference, Denver, CO, March 22-25, 1976: Washington D.C., 

U.S. Subcommittee on Sedimentation, p. 61-73. 

Sixta, M. and Randle, T., 2023. Travel to Glen Canyon Sloughs (Page, AZ). Memorandum to 

Kerri Pedersen (UC-411), Biologist, UCRO Adaptive Management Group, Bureau of 

Reclamation, January 27, 2023. 

U.S. Department of Interior, 1995. Operation of Glen Canyon Dam final environmental impact 

statement. Colorado River Storage Project. Coconino Country, Arizona: Salt Lake City, 

Utah. U.S. Bureau of Reclamation. Upper Colorado Regional Office, 337 p. 

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44 
 

Acknowledgments 

The authors would like to acknowledge Reclamation’s Upper Colorado Basin Regional Office 

(UCRO) in Salt Lake City, UT, for funding this work. Additionally, we would like to thank Kerri 

Pedersen from the UCRO Adaptive Management Group along with Taryn Preston, Jeff Arnold, 

and Buddy Fazio from the National Park Service in Page, AZ, for helping conduct the incredibly 

valuable field visit in October 2022, and sharing background data along with their thoughts on 

the new proposed alternative. Joel Sankey, Charles Yackulic, Drew Eppehimer, and Paul Grams 

from the USGS Grand Canyon Monitoring and Research Center were also instrumental in 

sharing their data resources, including all the topographic (bathymetry) data that were used in the 

modeling and the historic pre-dam aerial imagery (Figure 3A). 

45