Nearshore Thermal Gradients of the Colorado River near 

the Little Colorado River Confluence, Grand Canyon 

National Park, Arizona

Open-File Report 2013–1013

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia 2013

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Suggested citation:

Ross, R.P., and Grams, P.E., 2013, Nearshore thermal gradients of the Colorado River near the Little Colorado River 

confluence, Grand Canyon National Park, Arizona, 2010: U.S. Geological Survey Open-File Report 2013–1013, 65 p.

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iii

Contents

Abstract ......................................................................................................................................................................... 1

Introduction.................................................................................................................................................................... 1

Purpose and scope........................................................................................................................................................ 2

Physical setting and sites .............................................................................................................................................. 4

Site RM 63.33L .......................................................................................................................................................... 5

Site RM 63.56R.......................................................................................................................................................... 6

Site RM 63.65R.......................................................................................................................................................... 7

Site RM 63.81R.......................................................................................................................................................... 8

Site RM 64.29R.......................................................................................................................................................... 9

Site RM 65.10L .........................................................................................................................................................10

Site RM 65.70L .........................................................................................................................................................11

Site RM 65.78L .........................................................................................................................................................12

Methods........................................................................................................................................................................13

Results..........................................................................................................................................................................18

Discussion ....................................................................................................................................................................28

References cited...........................................................................................................................................................30

Appendix A. Results of Temperature-accuracy Tests on Thermistors ..........................................................................31

Appendix B. Raw Temperature Data from All Thermistors ...........................................................................................37

Appendix C. Filtered Temperature Data for All Study Sites ..........................................................................................38

Figures

1. Location map of study area between Little Colorado River (LCR) and Lava Canyon Rapid. ..................................... 3

2. Location map of site RM 63.33L.. .............................................................................................................................. 5

3. Location map of site RM 63.56R................................................................................................................................ 6

4. Location map of site RM 63.65R................................................................................................................................ 7

5. Location map of site RM 63.81R................................................................................................................................ 8

6. Location map of site RM 64.29R................................................................................................................................ 9

7. Location map of site RM 65.10L.. ............................................................................................................................ 10

8. Location map of site RM 65.70L. . ........................................................................................................................... 11

9. Location map of site RM 65.78L.. ............................................................................................................................ 12

10. Photograph showing array of thermistors at RM 65.81R.. ..................................................................................... 14

11. Generalized schematic of thermistor array. ........................................................................................................... 15

12. Thermistor response to changing flow conditions.. ................................................................................................ 16

13. Average temperature differences from mainstem to shoreline for all study intervals. ............................................ 20

14. Examples of nearshore temperature during fluctuating flows in July and August, 2010 (A-D) and September 

and October 2010 (E-H).. ....................................................................................................................................... 24

15. Comparison of consistently submerged thermistors during fluctuating flows (August) to similarly placed 

thermistors during steady flows (September).......................................................................................................... 25

16. Cumulative frequency distributions showing the fraction of time the thermal gradient from mainstem to 

shoreline is greater than or equal to value indicated............................................................................................... 26

17. Cumulative temperature frequency distribution for all study intervals.. .................................................................. 27

18. Average (A) and maximum (B) thermal gradients per meter (dT/dX) for all study sites/intervals........................... 29
iv

Tables

1. List of temperature monitoring sites showing number of thermistors deployed, shoreline types, duration of 

instrumentation, spacing between thermistors, and approximate slope of site ......................................................... 4

2. Operational data for Onset Technologies HOBO Pro v2 Water Temperature datalogging thermistors.................... 14

3. Discharge ranges used to filter temperature data during fluctuating flow intervals.. ................................................ 17

4. Summary of discharge characteristics, mainstem water temperatures, air temperatures, and temperature 

gradients for each study interval............................................................................................................................. 18
v

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft.) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square inch (in2) 6.452 square centimeter (cm2)

square mile (mi2) 2.590 square kilometer (km2) 

Volume

cubic foot (ft3) 0.02832 cubic meter (m3) 

Flow rate

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

cubic foot per day (ft3/d) 0.02832 cubic meter per day (m3/d)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C=(°F-32)/1.8

Horizontal coordinate information is referenced to North American Datum of 1983 (NAD 83). 
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Nearshore Thermal Gradients of the Colorado River near 

the Little Colorado River Confluence, Grand Canyon 

National Park, Arizona

By Robert P. Ross and Paul E. Grams

Abstract

Construction and operation of Glen Canyon Dam has dramatically impacted the flow of the 

Colorado River through Glen, Marble, and Grand Canyons. Extremes in both streamflow and water 

temperature have been suppressed by controlled releases from the dam. Trapping of sediment in Lake 

Powell, the reservoir formed by Glen Canyon Dam, has also dramatically reduced the supply of 

suspended sediment entering the system. These changes have altered the riverine ecosystem and the 

habitat of native species, including fish such as the endangered humpback chub (Gila cypha). Most 

native fish are adapted to seasonally warm water, and the continuous relatively cold water released by 

the dam is one of the factors that is believed to limit humpback chub growth and survival. While 

average mainstem temperatures in the Colorado River are well documented, there is limited 

understanding of temperatures in the nearshore environments that fish typically occupy. Four nearshore 

geomorphic unit types were studied between the confluence of the Colorado and Little Colorado Rivers 

and Lava Canyon in the summer and fall of 2010, for study periods of 10 to 27 days. Five to seven sites 

were studied during each interval. Persistent thermal gradients greater than the 0.2 °C accuracy of the 

instruments were not observed in any of the sampled shoreline environments. Temperature gradients 

between the shoreline and mainstem on the order of 4 °C, believed to be important to the habitatseeking behavior of native or nonnative fishes, were not detected. 

Introduction

The construction and operation of Glen Canyon Dam (GCD) has greatly changed the character 

of the Colorado River through Glen, Marble, and Grand Canyons. Dam operation for water storage and 

production of hydropower has resulted in reduced seasonal variations in flow, but much larger daily 

fluctuations (Topping and others, 2003). The average predam annual peak flow was about 87,000 cubic 

feet per second (ft3

/s) and winter low flows were typically under 5,000 ft3/s (Topping and others, 2003). 

Current flows typically fluctuate by 5,000 to 8,000 ft3/s on a daily basis, with daily maximum flows less 

than about 22,000 ft3/s and daily minimum flows greater than about 6,000 ft3/s. Because the water 

intake structure is at a fixed elevation deep in the reservoir, the water released from Glen Canyon Dam 

is cool and typically varies between 8 and 12 °C in comparison with predam seasonal variations of 

between 0 and 30 °C (Wright and others, 2009). The changes in daily, seasonal, and annual temperature 

extremes have affected the aquatic habitat used by native species, including the humpback chub (Gila 

cypha). Temperature gradients along the shorelines are not well documented; previous work on 

mainstem shorelines (Korman and others, 2006; Vernieu and Andersen, unpublished data) has been 

limited to short durations of one to three days. Most temperature data collection has been in backwaters 

and shoreline cavities with some degree of isolation from the mainstem. A better understanding of both 
2

persistent and ephemeral temperature gradients relative to seasonal temperature variations and flow 

regime is needed to better predict the use of nearshore native fish habitats associated with various 

surficial geologic features, such as bedrock, talus slopes, cobble bars, and debris fans.

Purpose and scope

This report documents measurement and analysis of water temperatures in the mainstem 

Colorado River and nonisolated nearshore habitats between the Little Colorado River (LCR) and Lava 

Canyon Rapid (river miles[RMs] 61.10 to 65.78, respectively; fig. 1) from July 2010 to October 2010. 

We have used the term “mainstem” throughout the report to indicate main-channel flow with typical 

downstream velocity and a high degree of mixing. Sites were monitored during three intervals ranging 

from 10 to 26 days in July, August/September, and October. Although measurements are reported in SI 

system, locations and place names along the Colorado River are referred to by the convention of river 

miles (RM) downstream from Lees Ferry that was formalized in 2006 (U.S. Geological Survey, 2006). 

During the study period, the streamflow regime consisted of high-volume fluctuating flows in the 

summer and low-volume steady flows in the fall. We report both instantaneous and average 

temperatures and temperature gradients for seven study sites that encompass four different shoreline 

habitat types. 

A 
3

Figure 1. Location map of study area between Little Colorado River (LCR) and Lava Canyon Rapid, Grand 

Canyon National Park, Arizona. A, Study area (shown in green box) is from RM 63.33 to RM 65.78 in Grand 

Canyon. River miles show distance downstream from Lees Ferry, Arizona (U.S. Geological Survey, 2006). B, 

Detailed location map corresponding to inset study area in A. Red line is Carbon Creek, blue rectangle is 

Palisades weather station, and green squares, orange pentagons, and red triangles are locations of thermistor 

arrays.

B 
4

Physical setting and sites

Five sites were monitored during the first sampling interval (July 2010). An additional two sites 

were monitored in each of the subsequent sampling intervals to encompass the range of shoreline 

habitats where intensive fish sampling was being conducted in a concurrent study (Dodrill, 2012; table 

1). The four shoreline habitat types that are the focus of fish monitoring utilize the most common 

classification of geomorphic features that occur in the reach: (1) debris fans and associated sandbars, (2) 

talus, (3) cobble bars, and (4) bedrock. We selected a greater number of debris fan and talus monitoring 

sites, because a greater proportion of the shoreline is composed of those feature classes. Each study site 

is referred to by river mile and either the left (L) or right (R) side of river when facing downstream. 

Table 1. List of temperature monitoring sites showing number of thermistors deployed, shoreline types, duration of 

instrumentation, spacing between thermistors, and approximate slope of site. 

Site RM 63.33R RM 63.56R RM 63.65R RM 63.81R RM 64.29R RM 65.10L RM 65.70L RM 65.78L

Shoreline typei CB BR T T/DF DF T DF DF

Approximate Slopeii 3 2 3 1 1 2 1 1 

Deployment dates in July N/A N/A 7/12-

7/23/10 7/11-7/23/10 7/11-7/23/10 7/12-7/23/10 N/A 7/11-7/23/10

Thermistor spacing 0.5 m 1.0 m 0.5 m 0.5 m 0.5 m

Number of thermistors 8 6 10 5 10

Deployment dates in August 8/11-9/4/10 8/11-9/4/10 N/A 8/11-9/4/10 8/11-9/4/10 8/10-9/4/10 8/11-9/4/10 8/10-9/4/10

Thermistor spacing 0.75 m 0.75 m 1.0 m 0.75 m 0.5 m .75 m 0.75 m

Number of thermistors 6 7 6 7 5 6 5 

Deployment dates in 

September 9/4-9/15/10 9/4-9/15/10 N/A 9/4-9/15/10 9/4-9/15/10 9/4-9/15/10 9/4-9/15/10 9/4-9/15/10

Thermistor spacing 0.75 m 0.75 m 1.0 m 0.75 m 0.5 m .75 m 0.75 m

Number of thermistors 6 7 6 7 5 6 5 

Deployment dates in October 10/16-

10/27

10/16-

10/27/10 N/A 10/17-10/27/10

10/17-

10/27/10

10/17-

10/27/10

10/16-

10/27/10

10/16-

10/27/10

Thermistor spacing 0.5 m 1.0 m 1.0 m 1.0 m 0.5 m 1.0 m 1.0 m

Number of thermistors 4 5 6 5 5 5 5 

i Shoreline types are abbreviated from the following: CB=cobble bar, BR=bedrock, T=talus, and DF=debris fan

ii Approximate slope is defined by three classes: 1=low angle shoreline (~2 meters/~6+ meters), 2=intermediate shoreline (~2 meters/~4 meters), and 3=steep shoreline 

(~2 meters/2 meters or less)
5

Site RM 63.33L 

This site is located on the only cobble bar in the study reach (fig. 2). The bar is locally referred 

to as Hart Island and is a large cobble island in the center of the channel, with mainstem flow branching 

around either side. The island is dominated by medium to large cobbles (from 0.04 to 0.10 m in 

diameter) and has sparse vegetation. The instruments were located at the downstream end of the island 

in fast downstream-oriented current. The shoreline is relatively steep and reaches a water depth of about 

2 m within about 2 m from shore at the range of discharges that were monitored. This site was 

instrumented in three monitoring periods—August, September, and October 2010 (table 1).

Figure 2. Location map of site RM 63.33L. Site is marked with an orange pentagon and was instrumented in the 

August, September, and October intervals. Site is located on the downstream end of a large cobble island and 

is in the cobble bar class of fish habitat sampling units. Direction of streamflow is from top to bottom.
6

Site RM 63.56R 

This site is located along a bedrock shoreline in an indentation that allowed access and 

installation of monitoring equipment (fig. 3). In addition to bedrock, there is a small sand deposit and 

isolated talus along the water edge. The slope into the main channel is steep, and current is mostly 

downstream, with a very weak eddy current. This site was instrumented from August to October (table 

1).

Figure 3. Location map of site RM 63.56R. Site is marked with an orange pentagon and was instrumented in the 

August, September, and October intervals. Site is located in a shoreline cavity that contacts bedrock and is in 

the bedrock class of fish habitat sampling units. Direction of streamflow is from top to bottom.
7

Site RM 63.65R 

This talus-dominated site is on the upstream end of a large debris fan (fig. 4). It is a steep rocky 

shoreline, with talus consisting of locally derived clasts on the order of 0.2 to 3 m in diameter. The talus 

continues into the channel along the full length of the thermistor array. The initial 0.5 m of the shoreline 

is low-angle due to shallow rock surface, but it then drops off abruptly to 2 m or deeper over the next 

0.25 m. 

There is typically a small zone of upstream-directed eddy current in the nearest 1 m from the 

shoreline, and beyond that is downstream-oriented current. This site was only instrumented in July 

(table 1).

Figure 4. Location map of site RM 63.65R. Site is marked with a green square, and was only instrumented 

during the July monitoring interval. The site is located on the far upstream end of a debris fan and is in the 

talus class of fish habitat sampling units. Direction of streamflow is from top to bottom.
8

Site RM 63.81R 

This site is at the transition from a debris fan to talus-dominated shoreline, at the downstream 

end of the debris fan referenced for site RM 63.65R (fig. 5). The substrate along the thermistor string 

consists of large talus blocks. The site has a pronounced eddy current and is shallow and low-angle, not 

reaching 2 m in depth until about 8 m from shore. This site was instrumented for all intervals (table 1).

Figure 5. Location map of site RM 63.81R. Site is marked with a red triangle, and was instrumented during all 

study intervals. The site is located on the downstream end of large debris fan in a pronounced shoreline cavity 

and is situated at the transition point between debris fan and talus classes of fish habitat sampling units. 

Direction of streamflow is from top to bottom.
9

Site RM 64.29R

This site on river-right is an extensive sandy shoreline associated with a large debris fan (fig. 6). 

The slope is low-angle, dropping to 2 m water depth over about 8 m. The upper bank is heavily 

vegetated, while the lower bank is clear of vegetation due to the fluctuating flow regime over the lowangle shoreline. There is no noticeable eddy at this site; all current is downstream. This site was

instrumented during all intervals, with a duplicate line installed during the October interval (table 1).

Figure 6. Location map of site RM 64.29R. Site is marked with a red triangle and was instrumented during all 

study intervals. The site is grouped within the debris fan class of fish habitat sampling units. Direction of 

streamflow is from top to bottom.
10

Site RM 65.10L

This site is near Carbon Creek (fig. 1) along a talus shoreline with a moderately steep slope (fig. 

7). Along the shoreline, the talus blocks have interstitial sand, and the bank is heavily vegetated. Talus 

boulders continue into the channel, and the shoreline drops over the course of ~3 m to more than 2 m 

depth. A small, weak eddy current exists, but the site has mostly downstream flow. It was instrumented 

during all intervals (table 1). 

Figure 7. Location map of site RM 65.10L. Site is marked with a red triangle and was instrumented during all 

study intervals. Site is located on the toe of large talus cone and is within the talus class of fish habitat 

sampling units. Direction of streamflow is from top to bottom.
11

Site RM 65.70L

This site is located at the upstream end of the debris fan that is upstream from site RM 65.78L 

(fig. 8). It is at the mouth of a tributary drainage, and the substrate is sand intermixed with well-sorted 

gravel, cobbles, and boulders. The debris fan is heavily vegetated. The thermistor string was located 

within a small eddy created by bank irregularaties along the shoreline of the debris fan. The shoreline is 

low-angle over the first 2 m, with a rapid increase to 2 m depth over the next 2 m. This site was 

instrumented from August to October (table 1).

Figure 8. Location map of site RM 65.70L. Site is marked with an orange pentagon and was instrumented in the 

August, September, and October study intervals. The site is located at mouth of a debris fan and is within the 

debris fan class of fish habitat sampling units. Direction of streamflow is from top to bottom.
12

Site RM 65.78L

This site is located within a large eddy downstream from a prominent debris fan (fig. 9). The 

shoreline where the thermistor string was located is composed of sand and is sparsely covered with 

vegetation. The character of this shoreline varied among the sampling intervals, because sand deposition 

occurred during the transition from fluctuating flows during the summer to steady flow in September. 

Following that transition, a large sandbar projected into the channel and changed the slope of the site 

significantly. There is an eddy current at this site near shore. This site was instrumented during all 

intervals (table 1).

Figure 9. Location map of site RM 65.78L. Site is marked with a red triangle and was instrumented during all 

study intervals. Site is located downstream from a debris fan, above Lava Canyon Rapid, and is within the 

debris fan class of fish habitat sampling units. Direction of streamflow is from top to bottom.
13

Methods

Nearshore temperatures were monitored during unsteady flow conditions in July and August 

(July 11–23 and August 11-September 4, 2010) and relatively steady flow conditions in September and 

October (September 5–25 and October 17–27, 2010). Flow fluctuated from approximately 9,250 to 

22,850 ft3

/s in the fluctuating-flow regime of July and August, and from approximately 8,200 to 8,800 

ft3

/s in the steady flow regime of September and October. Note that steady flow still entails some 

fluctuation, due to inputs from the Little Colorado River (LCR) and other tributaries. Discerning local 

discharge at the study sites was problematic, as different flows, rates of change to flow, and variable 

conditions led to varying travel times of changes in flow. To remedy this, local discharge data was 

modeled using the program Colorado River Flow and Sediment (CRFS; after Wiele and Griffin, 1997; 

Ecometric Research, Inc., v.1.0.1.0, Canmore, Alberta, Canada, 2011), using discharge data from gages 

maintained by the Grand Canyon Monitoring and Research Station (GCMRC) located at RM 60.88 and 

above the mouth of the LCR at RM 61.7. Modeled output was considered at cross sections near each site 

to approximate instantaneous discharge for all monitoring periods. Air temperatures were compiled 

from the nearby Palisades weather station about 708 m downstream and about 138 m inland from site 

RM 65.78L. 

Thermal gradients and temperature differences for this study are considered latitudinal changes 

from the shoreline to the mainstem channel. Thermal gradients are measurements of average 

temperature change per meter (°C/m), and temperature differences are the absolute change in 

temperature between measurement points (°C). Arrays of HOBO Pro v2 Water Temperature Data

Logger thermistors (manufactured by Onset Computer Corp., Bourne, MA) were deployed at each site 

perpendicular to the shoreline (table 1; fig. 10). Each thermistor measured and recorded temperature at 

coincident 15-minute intervals, with published accuracy of ~0.2 °C and resolution of 0.02 °C (table 2). 

Accuracy tests conducted during the course of this study indicate a better-than-published accuracy for 

these units, between 0.1 and 0.14 °C (appendix A). The thermistors were programmed with a scheduled 

start time, and were downloaded after retrieval; the use of a computer in the field was unnecessary. The 

downloaded files were exported as comma-delimited text files, and were subsequently organized by site 

and distance from shoreline (appendix B). Instruments were attached to a floating polypropylene line 

(lengths varying from 3 m to 8 m) at spacing determined by site characteristics (between 0.5 and 1.0 m, 

with increased spacing over sites with low-angle shorelines). The thermistors were attached with cable 

ties in a downward orientation, to minimize chances of thermistors rotating to the top of the line. The 

floating lines were attached at the shoreline either to existing rocks and (or) vegetation, or rebar where 

existing attachment points were not available. A system of buoys made from sealed PVC pipe, scuba 

weights attached to nylon line, and rocks attached with cable ties was engineered to keep thermistors 

continuously submerged approximately 2 cm below the water surface (fig. 11). As the stage changes 

during fluctuating flows, the thermistors near the shoreline are often exposed subaerially or submerged 

in shallow water depths as the shoreline recedes or advances (fig. 12). 
14

Figure 10. Photograph showing array of thermistors at RM 65.81R. This 6-m instrument string has six thermistors 

deployed at approximately 1-m intervals. View is from the right bank of the river looking downstream.

Table 2. Operational data for Onset HOBO Pro v2 Water Temperature data logging thermistors. 

[Thermistor specifications available at 

http://www.onsetcomp.com/callback/show-specs?n=2331]

Operation Range Accuracy Resolution Response time Stability

40 °C to 70 °C (air); 

Max sustained 50 °C 

water

0.2 °C over 

0 °C to 50 

°C

0.02 °C @ 25 °C 5 minutes in water 0.1 °C/yr 

Clock Battery life Memory Buoyancy Shock

± 1 minute/month 6 years 64 Kb +13 grams 1.5 meter
15

Figure 11. Generalized schematic of thermistor array. System is composed of sealed PVC pipe floats, floating 

line, scuba weights, and stone ballasts, with Onset Technologies HOBO Pro v2 Water Temperature data 

logging thermistors. Floating lines were attached to the shoreline with either rebar or natural anchors, and 

floats and weights were arranged such that thermistors were positioned approximately 0.02 m below water 

surface. The floats and stone ballasts kept submerged thermistors in this orientation during fluctuating flows, 

and the scuba weights fixed the lines in a position perpendicular to the shoreline. 
16

Figure 12. Thermistor response to changing flow conditions. Generalized schematic shows how instrument line 

drops with water surface when flow decreases. Line is attached to fixed point on shore, and as flow 

decreases, nearshore thermistors become subaerial. At lowest discharge, the effective shoreline (the area 

where the first submerged thermistor is) moves towards the mainstem of the channel. Floats, ballast, and 

weights keep the instrument line perpendicular to shore, and instruments remain approximately 0.02 m below 

water surface. 

The data were filtered to remove values recorded when the thermistors were exposed above the 

water surface, which occurred during the low-flow periods during fluctuating flows in July and August, 

as well as during some periods of steady flow in September and October (note there is still natural 

variation from the LCR input during this time period). The filter was based on modeled local discharge 

values at each 15-minute interval created with CRFS. Periods of known exposure were compared to 

discharge values to approximate the point in upramp or downramp fluctuation at which each thermistor 

became exposed (table 3). Because of differences in duration of release between the two legs of the flow 

change, the thermistors respond differently, relative to the varying rates of stage increase/decrease. 

Since the local modeled discharge is approximate, and the pattern of exposure is different for each site 

and change in flow, a range of discharge at which a thermistor is becoming subaerial or subaqueous is 

reported (table 3). Gross filtering was done based on discharge values where exposure was known, and 

then data were manually inspected for changes in discharge ranges where exposure was questionable 

(table 3). This procedure was applied to the fluctuating flow periods, and the data were manually filtered 

for the steady flow periods. 

The filtered data were inspected and compared to the unfiltered data, and any data that were 

erroneously filtered were replaced. The filtered temperature data were plotted as a time series to inspect 

18

Results

All filtered data are contained in appendix B. All intervals of filtered data (July, August, September, and 

October) were examined for magnitudes of temperature differences, patterns of change relative to 

shoreline characteristics, and change relative to insolation, air temperature, and flow regime (appendix 

C and fig. 13). Averages of these metrics for each interval give an overview of results (table 4). Mean 

temperature gradients range from 0.0 to 0.1 °C/m during all periods. Maximum and minimum 

temperature gradients range from -0.2 to 2.6 °C/m in July and August to -0.2 to 5.4 °C/m in October. 

September displays the smallest magnitude of thermal gradients, with a minimum of -0.3 °C/m and a 

maximum value of 0.5 °C/m. 

Table 4. Summary of discharge characteristics, dam release and mainstem water temperatures, air temperatures, 

and nearshore temperature gradients for each study interval. 

July August September October

Flow regime Fluctuating Fluctuating Steady Steady

Mainstem temperature (°C) 

Mean 12.8 12.8 13.4 12.2

Max 13.9 14 15 14.1

Min 11.7 11.8 12.2 10.8

Air temperature (°C)

Mean 34.7 30.8 28.7 18.4

Max 45.6 42.1 41.9 27.2

Min 22 17.1 16.8 9.7

Discharge (ft3/s) 

Mean 13767 14862 8819 8572

Max 18211 22847 15385 15658

Min 9253 9334 8198 8173

Discharge range (ft3/s) 8958 13513 7187 7485

Dam release temperature 

(°C) i 

Mean 10.3 10.6 11.1 10.9

Max 11.4 11.7 11.8 11.7

Min 9.3 9.4 10 9.6

Temperature gradient 

(°C /m ) 

Mean 0.1 0.1 0 0.1

Mean AM 0 0.5 0 0.1

Mean PM 0.1 0.7 0 0.2

Max 2.6 2.3 0.5 3.1

Max AM 0.8 2.3 0.4 2.8

Max PM 2.6 1.7 0.5 3.1

Min -0.2 -0.7 -0.3 -2

Min AM -0.1 -0.7 -0.2 -2

Min PM -0.2 -0.5 -0.3 -1.1

i Nancy Hornewer, written commun., 2011
19
20

Figure 13. Average temperature differences from mainstem to shoreline for all study intervals, shown by bar 

height and numerical value. Columns are color-coded by shoreline type, where yellow is cobble bar/island, 

green is bedrock, blue is talus, and red is debris fan. 
21

Temperature gradients from the mainstem to within 0.5 m of the first submerged thermistor were much 

less than 1 °C across all monitoring periods (fig. 14). However, significant gradients between the first 

submerged thermistor and the next thermistor towards the mainstem were observed during the discharge 

upramp and downramp phases of fluctuating flows. These differences are most likely related to the 

different placement of thermistors during fluctuating flows and steady flows rather than differences in 

thermal gradients between the flow regimes. During steady flows, the thermistor nearest the shoreline 

was always 0.5 m or more from the shoreline. During fluctuating flows, thermistors were sometimes 

nearer to the shoreline yet still submerged. Thus, during fluctuating flows, we sampled shallower water 

nearer the shoreline. The magnitude of the observed temperature difference between the two nearshore 

thermistors is greatest during the afternoon decrease in discharge, when air temperatures and insolation 

are the highest and are more pronounced (fig. 14). Air temperature and discharge are the main external 

controls on shoreline water temperature, and effects of insolation are reduced as the sun moves behind 

the canyon rim; this can be seen in July and August 2010 three-day subintervals for site RM 65.10L, 

which show a greater gradient during late afternoon hours when air temperature is higher (fig. 14).

Because the sampling environments between fluctuating and steady flows were different, six 

sites were evaluated for temperature differences and gradients, using only the thermistors that were 

constantly submerged (those still submerged during the lowest discharge) for all flows. This provides a 

comparison between flow regimes based on the same number of thermistors. The August fluctuating 

flow interval and the September steady flow interval were chosen for the comparison because air 

temperature and degree of insolation were similar, with the only marked variables being flow regime 

and release temperature from Glen Canyon Dam. The comparison shows that the temperature difference 

from the mainstem thermistors to the first consistently submerged thermistor is less than the accuracy of 

the instruments for both the fluctuating and steady-flow regimes (fig. 15).

Thermal gradients of significant values are more abundant in both frequency and magnitude 

during the July, August, and October intervals than in the September interval (figs. 16 and 17). 

Frequency of site-averaged thermal differences above the accuracy level of the thermistors (0.2 °C) 

ranges from 0 to 40 percent of the time in July, 10–20 percent of the time in August, 0–15 percent of the 

time in September, and and 5–20 percent of the time in October (fig. 17). Temperature differences of 

approximately 4 °C are deemed significant to fish behavior over short time periods of 4–6 hours (T.A. 

Kennedy, oral commun., 2011) Thermal differences of these magnitudes were not observed for any time 

periods more than 2 hours and were generally much more short-lived. 

We did not observe consistent differences in mainstem-to-shoreline temperature gradients 

among the four shoreline types that were monitored (fig. 13). One of the debris-fan shoreline sites (RM 

65.78L) tended to have the lowest temperature gradients, but other debris fan sites had gradients similar 

to the other shoreline types. The debris fan at RM 65.78L is located near a persistent backwater and is 

on the downstream end of an eddy that is prone to rapid deposition, as was evidenced by the formation 

of an extensive offshore reattachment bar in August 2010. 
22
23
24

Figure 14. Examples of nearshore temperatures during fluctuating flows in July and August 2010 (A-D), and 

September and October 2010 (E-H). Temperatures are shown at site RM 65.10L for July 19, 2010, during the 

early morning increase in discharge (A) and late afternoon decrease in discharge (B), and for August 19, 

2010, during early morning increase in discharge (C) and late afternoon decrease in discharge (D). 

Temperatures are shown at site RM 65.10L for September 10, 2010, during early morning (E) and late 

afternoon (F), and for October 19, 2010, during early morning (G) and late afternoon (H). These time periods 

are similar to the periods of increases and decreases in discharge examined for July and August during 

fluctuating flow. Vertical scales are water temperature in degrees Celsius, and are set at a common scale for 

all plots. Horizontal scale is distance from effective shoreline in meters, defined as the point where the first 

submerged thermistor is located. This point does not change in the intervals presented for the two periods of 

steady flow. In fluctuating flows, significant increases in temperature relative to the rest of the line exist for the 

thermistors nearest to shore. Note that periods of significant increase in temperature are limited to very short 

time periods, often in the range of 15 minutes to 1 hour. In both instances examined during steady flow, there 

is no observed thermal gradient near the shore, although there is an increase in overall water temperature 

during the afternoon interval for September, and a possible thermal gradient in the afternoon interval for 

October. This temperature difference from mainstem to nearshore in October is short-lived (less than 0.7 °C 

over 2 hours). Each series is a simultaneous reading of all thermistors on the line; the legend in the upper right 

corner indicates the time of measurement for each series.
25

Figure 15. Comparison of constantly submerged thermistors during fluctuating flows (August) to similarly placed 

thermistors during steady flows (September). The comparison is inconclusive; all pairs are within 0.2 °C of 

each other and therefore are insignificant relative to the accuracy of the data loggers (0.2 °C). 
26

Figure 16. Cumulative frequency distributions showing the fraction of time the thermal gradient from mainstem to 

shoreline is greater than or equal to value indicated. Plotted data are average values of all sites for each 

measurement interval. For example, in July the observed thermal gradient is greater than or equal to -0.1 

°C/m 100 percent of the time interval and greater than or equal to 1.1 °C/m 0 percent of the time. All intervals 

display thermal gradients in excess of the accuracy of the HOBO thermistors (~0.2 °C) less than 15 percent of 

the time. Maximum thermal gradients approach 1–1.5 °C/m during July and August fluctuating flows, and 

0.2– 0.6 °C/m in September and October steady flows.
27

Figure 17. Cumulative temperature frequency distribution for all study intervals. A, Cumulative frequency 

distribution for all study sites in July 2010. Plotted data are magnitudes of thermal gradient from mainstem to 

shoreline in degrees Celsius per meter against percentage of time during interval that gradient is greater than 

or equal to value shown; for example, at site RM 63.65R (purple series), observed thermal gradient is greater 

than or equal to 0.0 °C/m 100 percent of the time interval and greater than or equal to 2.6 °C/m 0 percent of 

the time. In general, thermal gradients greater than the tolerances of the Onset HOBO Pro v2 (~0.2 °C) are 

observed in the frequency interval of 0–15 percent, with RM 63.65R reaching significant gradient (more than

0.2  ̊C/m) up to 40 percent of the study interval period. Dashed orange line represents the averaged value of 

data for all sites, with temperature differences greater than 0.2 oC occurring 0 to 10 percent of the time.

Thermal differences for all sites in excess of 1 °C occur 5 percent of the time or less. 
28

B, Cumulative frequency distribution of absolute temperature differences for all study sites in August 2010. 

Plotted data are magnitudes of thermal gradient from mainstem to shoreline, in degrees Celsius per meter 

against percent of time during interval that gradient is greater than or equal to value shown; for example, at 

site RM 63.33R (purple series), observed thermal gradient is greater than or equal to -0.4 °C/m 100 percent of 

the time interval, and greater than or equal to 1.0 °C/m 0 percent of the time. In general, thermal gradients 

greater than the tolerances of the Onset HOBO Pro v2 (~0.2 °C) are observed in the frequency interval of 0–

10 percent, with site RM 63.33R reaching significant gradient up to 20 percent of the study interval period. 

C, Cumulative frequency distribution for all study sites in September 2010. Plotted data are magnitudes of 

thermal gradient from mainstem to shoreline in degrees Celsius per meter against percent of time during 

interval that gradient is greater than or equal to value shown; for example, for RM 63.81R (light green series), 

observed thermal gradient is greater than or equal to 0 °C/m 100 percent of the time interval, and greater than 

or equal to 0.4 °C/m 0 percent of the time. For five of the seven sites, thermal gradients greater than the 

accuracy of the Onset HOBO Pro v2 (~ 0.2 °C) are not observed, with sites RM 63.81R and RM 63.33R 

reaching significant gradient up to 15 percent of the study interval period. Dashed aquamarine line represents 

the averaged value of all data at all sites, with significant gradient not observed. No thermal gradients in 

excess of 1 °C/m were observed. Site RM 65.78L displayed cooling from mainstem to near shore during a 

significant part of the study interval, likely due to the formation of an offshore sandbar in early September. 

D, Cumulative frequency distribution for all study sites in October 2010. Plotted data are magnitudes of 

thermal gradients from mainstem to shoreline in degrees Celsius per meter against percent of time during 

interval that gradient is greater than or equal to value shown; for example, at site RM 64.29R (fuchsia series), 

observed thermal gradient is greater than or equal to -0.2 °C/m 100 percent of the time interval and greater 

than or equal to 0.8 °C/m 0 percent of the time. In general, thermal gradients greater than the tolerances of 

the Onset HOBO Pro v2 (~0.2 °C) are observed in the frequency interval 0–15 percent of the time. Dashed 

aquamarine line represents the averaged value of all data for all sites, with significant gradient observed 

overall in the 0–12 percent frequency interval. Thermal gradients in excess of 1 °C/m are only observed for 

sites RM 64.33R and 65.78L and are negligible (less than 2 percent frequency). Note that site RM 65.78L still 

displays some cooling from mainstem to near shore, but to a much lesser extent than observed in September, 

likely due to erosion of the offshore sandbar noted after the transition from fluctuating to steady flow. 

The rapid decrease in channel depth led to erratic temperature measurements, as the offshore sandbar 

created a partial barrier to the mainstem system. A significant temperature change was observed during 

fluctuating flows, with maximum rates of change between 0.2 and 2.6 °C/m (fig. 18).This is caused by 

flow fluctuation moving the actual shoreline to the initial submerged thermistor, and is ephemeral in 

nature. There was no pattern between shoreline types within the accuracy of the thermistors, and 

temperature differences were not significant within the bounds of instrument tolerance for any of the 

sites. 

Discussion

None of the study locations detected a persistent thermal difference greater than the accuracy of 

the instruments used for monitoring (within about 0.2 °C). More importantly, temperature differences of 

4 °C, believed to be important to the habitat-seeking behavior of native or nonnative fishes, were not 

detected. However, we did observe an ephemeral temperature gradient during times of transition 

between high and low fluctuating flows, with rates of change possibly approaching 2.5 °C over a 

distance of 1 m (fig. 18). These gradients are likely an artifact of the sampling method during 

fluctuating flow, where extremely shallow depths at distances less than 0.5 m from the shoreline were 
29

Figure 18. Average (A) and maximum (B) thermal gradients per meter (dT/dX) for all study sites and 

measurement intervals. There is no clear pattern in average thermal gradients between study intervals, 

although maximum thermal gradient values tend to be highest in July and August. The average values for all 

sites and intervals are less than the accuracy of the instruments (~0.2 °C). Examining thermistors coincident 

between transient and steady flow periods (the consistently submerged final thermistors in fluctuating flow and 

the initial thermistors in steady flow) showed no significant thermal gradient between study intervals.

sampled. This type of gradient may be present in steady flows as well, but was not measured because 

thermistors were never closer than 0.5 m from the shoreline. 

The temperature gradients in this zone within 0.5 m of the shoreline only occur for short 

durations, typically less than 2 hours at a time. The frequency and magnitude of these periods of 

nearshore warming are dependent in part on differences in sampling methods between steady flows and 

fluctuating flows. At the point where the thermistors nearest to shore are almost subaerial, they are 

sampling temperature in a much smaller volume of water, where air temperature and insolation have a 

greater effect on water temperature. 

We believe that the temperature data represent changes in flow regime where the thermistor is 

just submerged in a small volume of water as flow increases or decreases, capturing the highest 

variability of temperature at the shoreline. However, further work should be undertaken to confirm that 

these measurements are accurate. This would require that a technician be physically present at each site 

for the transition periods, noting the exact times that thermistors become subaerial. When results of 

study intervals during fluctuating flows from August 2010 are compared to results during September 

steady flows, the temperature gradients from consistently submerged thermistors during the fluctuating 
30

flows are not substantially different from those during steady flows (fig. 15). Detailed survey of each 

shoreline would create a local stage-discharge relationship and better constrain the modeled discharge 

results from CRFS. 

References cited

Dodrill, M.J., 2012, Habitat relationships of small-bodied fish in the Grand Canyon reach of the 

Colorado River, Arizona—Emphasis on native fish and evaluation of backwater habitats: M.S. 

thesis, Gainesville, University of Florida. 

Korman, J., Kaplinski, M., and Buszowski, J., 2006, Effects of air and mainstem water temperatures, 

hydraulic isolation, and fluctuating flows from Glen Canyon Dam on water temperatures in 

shoreline environments of the Colorado River in Grand Canyon: Ecometric Research Inc., 

Northern Arizona University, and Namtek Inc., submitted to U.S. Geological Survey, Grand 

Canyon Monitoring and Research Center, Flagstaff, AZ, cooperative agreement no. 

04WRAG00006, Modification 1, 52 p. 

Topping, D.J., Schmidt, J.C., and Vierra, L.E., 2003, Computation and analysis of the instantaneousdischarge for the Colorado River at Lees Ferry, Arizona—May 8, 1921, through September 30, 

2000: U.S. Geological Survey Professional Paper 1677, 118 p. 

U.S. Geological Survey, 2006, Colorado mileage system [Spatial database, 

GIS.BASE_GCMRC_TenthMile] 1st rev. ed.: Grand Canyon Monitoring and Research Center 

[http://www.gcmrc.gov/dasa/default.aspx, last accessed January 18, 2013.]. 

Wiele, S.M., and Griffin, E.R., 1997, Modifications to a one-dimensional model of unsteady flow in the 

Colorado River through the Grand Canyon, Arizona: U.S. Geological Survey Water-Resources 

Investigations Report 97–4046, 17 p. 

Wright, S.A., Anderson, C.R., and Voichick, N., 2009, A simplified water temperature model for the 

Colorado River below Glen Canyon Dam: River Research and Applications, v. 25, p. 675–686. 
31

Appendix A. Results of Temperature-accuracy Tests on Thermistors 
32

Serial number 

of Onset 

Technologies 

HOBO Pro v2 

Water 

Temperature 

Data Logger 

thermistors

BUCKET

TEST 

difference 

from 

average 

(July 

2010) OR 

other 

results 

Source of Results (if not from July 

2010 bucket test)

SEPTEMBER 

2010 

BUCKET 

TEST 

RESULTS 

(positive is 

higher than 

average, 

negative is 

lower than 

average)

SEPTEMBER 

29, 2010

BUCKET 

TEST 

RESULTS 

(positive is 

higher than 

average, 

negative is 

lower than 

average)

FEBRUARY 

2011

BUCKET 

TEST 

RESULTS 

(positive is 

higher than 

average, 

negative is 

lower than 

average)

OTHER 

TEST 

RESULTS Source of Result

1073436 -0.064 -0.058

1098590 0.040

1098591

LOW BY 

0.12 -0.140

1098592 -0.066

1098593

low by 

0.036 good KA100217H

1098594

high by 

0.03 river tests August 2010

1101692 -0.037

1101693 0.004

1101696

low by 

0.012 -0.018

1101697

low by 

0.005 river tests August 2010 good R0651002H

1161792

GOOD 

(<0.01) -0.043

1161793

high by 

0.035 river tests August 2010 good R0651002H2

1161795

high by 

0.04 river tests August 2010

1161797

low by 

0.046 -0.019

1161798

high by 

0.046

1161799

high by 

0.04 -0.006

1177166

high by 

0.029 river tests August 2010

1177158 0.050

1177159

high by 

0.04 river tests August 2010

1177160 0.044

1177164 0.036
33

1177167

high by 

0.042 0.015 0.006

1177169 0.007 0.024

1177170

low by 

0.031 river tests August 2010

1177171 0.032

1177172 low by 0.3 river tests August 2010

1177173 0.012

1177233 -0.093 -0.240

1177234

high by 

0.025 river tests August 2010 good R0871002H

1177235

low by 

0.031 -0.032 0.025

1177236

low by 

0.01 river tests August 2010

1177237

LOW BY 

0.007 river tests August 2010 good R0631002H

1177239

low by 

0.019 river tests August 2010 good HA100217H2

1193202

low by 

0.085 -0.071

1193203 -0.006 -0.010

1193204

high by 

0.031 -0.004 0.029

POWER 

WAS 

RESET' 

MESSAGE 

(ROB, 

2/11)

WORKED OK FOR FEB 2011 

BUCKET TEST

1193205

GOOD 

(<0.01) 0.012 -0.065

DID NOT 

READ 

OUT FOR 

ROB (2/11)

WORKED OK FOR FEB 2011 

BUCKET TEST

1193206

GOOD 

(<0.01) low by 0.01 river tests August 2010

1193207

high by 

0.025 -0.015 -0.091

VERY 

HIGH 

READINGS 

AND THEN 

DIED (ROB, 

2/11)

WORKED OK FOR FEB 2011 

BUCKET TEST

1193208

low by 

0.033 -0.017

1193209 -0.042

1193210

low by 

0.021

1193211

low by 

0.026 -0.010

1193212

GOOD 

(<0.01) -0.035

1193213

high by 

0.02 0.032
34

1193214

low by 

0.003 river tests August 2010

1193215

high by 

0.014 river tests August 2010 good R0651002_abv_lc_camp_h2

1258706 0.022

1258708

high by 

0.01 river tests August 2010

1258708

LOW BY 

.008

based on bucket test in August 2010 

with 2 other 'accurate' Hobos (see 

HOBO_BUCKET_TEST_AUG10.XLS

)

1258709

HIGH BY 

0.129 0.086

1258710

high by 

0.092 0.090

1258715

high by 

0.051 0.045

1258718

low by 

0.014

river tests August 2010 (165 Mile 

test)

1258721

GOOD 

(<0.01) 0.017 0.041

1258724

high by 

0.044 0.065

1258725 0.083

1258727

high by 

0.071 0.065

1258729

high by 

0.078

1258730

high by 

0.095 0.087

1258732

low by 

0.07 river tests August 2010

1258736

HIGH BY 

0.123 0.078 0.137

1258738 0.003

1258742 -0.043

1258744

GOOD 

(<0.01) -0.035 -0.015

1258746 0.043

1258747 0.094 0.088

1258748 -0.051

1258749 0.061

1258760

high by 

0.052 river tests August 2010 good HA100217H4

1258761 0.052
35

1258762

high by 

0.089 river tests August 2010 good R0631002_across_from_hi_h2

1258763 0.039

1258770

high by 

0.048 river tests August 2010 0.010

1258771 0.105

SKIPPED 

~ 40 

MINUTE 

READINGS 

IN FEB 

2011 

BUCKET 

TEST 

PROBABLY 

DO NOT 

TRUST 

1258772

high by 

0.113 river tests August 2010

1258773

high by 

0.100 river tests August 2010 good LU0217H2

1258774

high by 

0.1

river tests August 2010 (165 Mile 

test) 0.095

1258777

high by 

0.038 river tests August 2010

1258778 -0.026

1258779

high by 

0.1 to 0.2 river tests August 2010 0.093

high by 

~0.1 R0651002_abv_lc_camp_h

2342428

low by 

0.029 -0.004

2424205

low by 

0.072

2424206

low by 

0.058 -0.032

2424207

low by 

0.034

low by 

0.024 river tests August 2010

2424208

GOOD 

(<0.01) -0.019 -0.026

2424209

low by 

0.076 -0.075

2424210

low by 

0.02 -0.034 -0.013

2424211

low by 

0.06 -0.055

2424212

low by 

0.011

low by 

0.003 river tests August 2010

2424213

low by 

0.053 -0.070 -0.022

2424214

low by 

0.015 0.010 0.021

9790775 0.024

9790776 -0.029

9790777 -0.082 -0.134
36

9790778 0.008

9790779 -0.034 -0.013

9790780 0.026 0.038

9790781 

9790782 0.046

9790783 -0.052

9790784 0.029

9816300 -0.013

9816301 -0.025
37

Appendix B. Raw Temperature Data from All Thermistors

Spreadsheet available for download at http://pubs.usgs.gov/of/2013/1013/
38

Appendix C. Filtered Temperature Data for All Study Sites
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65