U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2012–5234

Prepared in cooperation with the City of Aurora, Colorado Springs Utilities, Lower Arkansas 

Valley Water Conservancy District, Pueblo Board of Water Works, Southeastern Colorado Water 

Conservancy District, and Upper Arkansas Water Conservancy District

Characterization of Streamflow, Water Quality, and 

Instantaneous Dissolved Solids, Selenium, and Uranium 

Loads in Selected Reaches of the Arkansas River, 

Southeastern Colorado, 2009–2010
COVER. (1) Arkansas River at Fourmile Road, Canon City, Colorado (photo taken by Roderick Ortiz, 

U.S. Geological Survey, 6/19/2009). (2) Arkansas River at Swink, Colorado (photo taken by Roderick 

Ortiz, U.S. Geological Survey, 2/5/2010).
Characterization of Streamflow, Water 

Quality, and Instantaneous Dissolved 

Solids, Selenium, and Uranium Loads in 

Selected Reaches of the Arkansas River, 

Southeastern Colorado, 2009–2010

By Tamara Ivahnenko, Roderick F. Ortiz, and Robert W. Stogner, Sr.

Prepared in cooperation with the City of Aurora, Colorado Springs Utilities, Lower 

Arkansas Valley Water Conservancy District, Pueblo Board of Water Works, 

Southeastern Colorado Water Conservancy District, and Upper Arkansas Water 

Conservancy District

Scientific Investigations Report 2012–5234 

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2013

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Suggested citation:

Ivahnenko, Tamara, Ortiz, R.F., and Stogner, R.W., Sr., 2013, Characterization of streamflow, water quality, and instantaneous dissolved solids, selenium, and uranium loads in selected reaches of the Arkansas River, southeastern 

Colorado, 2009–2010: U.S. Geological Survey Scientific Investigations Report 2012–5234, 60 p.
iii

Contents

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

Introduction.....................................................................................................................................................2

Purpose and Scope .......................................................................................................................................2

Description of the Study Area .....................................................................................................................5

Previous Investigations.................................................................................................................................5

Methods...........................................................................................................................................................9

Available Streamflow Data..................................................................................................................9

U.S. Geological Survey Streamflow Data ................................................................................9

Colorado Division of Water Resources Streamflow Data.....................................................9

Wastewater Treatment Plant Streamflow Data....................................................................12

Available Continuous Water-Quality Monitor Data.......................................................................12

Water-Quality Sampling Techniques and Analytical Methodology............................................12

Periodic Water-Quality Sampling............................................................................................12

Synoptic Water-Quality Sampling...........................................................................................12

Statistical Analyses............................................................................................................................16

Quality-Assurance Procedures........................................................................................................16

Characteristics of Streamflow, Water Quality, and Instantaneous Dissolved Solids, Selenium, 

and Uranium Loads in the Arkansas River Basin from Canon City to Near Portland .........23

Streamflow Characteristics ..............................................................................................................23

Water-Quality Characteristics..........................................................................................................23

Specific Conductance...............................................................................................................25

Major Ions...................................................................................................................................25

Dissolved Solids.........................................................................................................................26

Nutrients......................................................................................................................................28

Trace Elements...........................................................................................................................28

Hydrogen and Oxygen Isotopes ..............................................................................................34

Characteristics of Instantaneous Dissolved Solids, Selenium, and Uranium Loading ...........34

Characteristics of Streamflow, Water Quality, and Instantaneous Dissolved Solids, Selenium, 

and Uranium Loads in the Arkansas River Basin from Avondale to Las Animas ................39

Streamflow Characteristics ..............................................................................................................39

Water-Quality Characteristics..........................................................................................................42

Specific Conductance...............................................................................................................42

Major Ions...................................................................................................................................42

Dissolved Solids.........................................................................................................................45

Nutrients......................................................................................................................................45

Trace Elements...........................................................................................................................45

Hydrogen and Oxygen Isotopes ..............................................................................................51

Characteristics of Instantaneous Dissolved Solids, Selenium, and Uranium Loading ...........53

Summary........................................................................................................................................................56

Acknowledgments.......................................................................................................................................58

References Cited..........................................................................................................................................58
iv

Figures

1. Location of the Arkansas River Basin in Colorado..................................................................4

2. Delineation of the Upper Arkansas River Basin study area and locations of 

streamflow and water-quality sites, Canon City to Pueblo Reservoir, Colorado................6

3. Delineation of the Lower Arkansas River Basin study area and location of 

streamflow and water-quality sites, Avondale to Las Animas, Colorado............................7

4. Annual Arkansas River hydrographs (2010) (A) Arkansas at Portland, (B) Arkansas 

near Avondale, and (C) Arkansas at Las Animas ....................................................................8

5. Streamflow hydrographs for (A) Arkansas River at Canon City and (B) Arkansas 

River at Portland on the Arkansas River, 1990–2010.............................................................24

6. Seasonal streamflow hydrographs for Beaver Creek, Bear Creek, Brush Hollow, 

Fourmile Creek, and Hardscrabble Creek, Upper Arkansas River Basin, 2009–2010......24

7. Continuous specific conductance values at main-stem sites in the Upper Arkansas 

River Basin from Canon City to near Portland, 2009–2010 ...................................................25

8. Comparison of median concentrations of six major ions in the Arkansas River, 

from Canon City to near Portland, 2009–2010.........................................................................26

9. Generalized geology and 2010 irrigated cropland in the Upper Arkansas River 

Basin, from Canon City to near Portland.................................................................................27

10. Comparison of dissolved-solids in the Arkansas River and selected tributaries 

from Canon City to near Portland, 1990–2010.........................................................................29

11. Relation between dissolved-solids concentration and streamflow in the 

(A) Arkansas River and (B) selected tributaries from Canon City to near Portland, 

2009–2010 .....................................................................................................................................31

12. Comparison of concentrations of dissolved nitrite-plus-nitrate and total 

phosphorus in the Arkansas River from Canon City to near Portland, 1990–2010 ...........32

13. Comparison of concentrations of dissolved selenium and dissolved uranium in 

the Arkansas River from Canon City to near Portland, 2009–2010......................................33

14. Relation of concentrations of (A) dissolved selenium and (B) dissolved uranium 

with specific conductance and dissolved solids in the Arkansas River from Canon 

City to near Portland, 1990–2010 ..............................................................................................35

15. Relation of concentrations of dissolved selenium and dissolved uranium with 

streamflow in the Arkansas River from Canon City to near Portland, 2009–2010 ............35

16. Delta D and delta18O in samples from the Arkansas River and selected tributaries 

from Canon City to near Portland, 2009–2010.........................................................................36

17. Streamflow hydrographs in the Arkansas River: (A) Arkansas River near Avondale 

and Arkansas River near Nepsta, (B) Arkansas River at Catlin Dam and Arkansas 

River near Rocky Ford, (C) Arkansas River at La Junta and Arkansas River at 

Las Animas, 1990–2010 ..............................................................................................................41

18. Annual streamflow hydrographs (A) for Huerfano River, Timpas Creek, and 

Crooked Arroyo and seasonal hyrdrographs (B) for Sixmile Creek, Chicosa 

Creek, Apishipa River, Patterson Hollow, and Hungerford Hollow, 2009–2010 ................44

19. Continuous specific conductance values at main-stem sites in the Arkansas 

River Basin from Avondale to Las Animas, 2009–2010 .........................................................42

20. Generalized geology and 2010 irrigated cropland in the Lower Arkansas River 

Basin, from Avondale to Las Animas.......................................................................................43

21. Comparison of median concentrations of six major ions in the Arkansas River, 

from Avondale to Las Animas, 2009–2010...............................................................................44
v

22. Comparison of dissolved-solids concentrations in the Arkansas River and 

selected tributaries from Avondale to Las Animas, 1990–2010...........................................46

23. Concentrations of dissolved solids and streamflow in the Arkansas River (A) and 

selected tributaries (B) from Avondale to Las Animas, 2009–2010 ....................................48

24. Comparison of concentrations of (A) nitrite-plus-nitrate and (B) total phosphorus 

in the Arkansas River from Avondale to Las Animas, 1990–2010 .......................................49

25. Comparison of concentrations of (A) dissolved selenium and (B) dissolved uranium 

in the Arkansas River from Avondale to Las Animas, 2009–2010 .......................................49

26. Relation of concentrations of (A) dissolved selenium and (B) dissolved uranium 

with specific conductance and dissolved solids in the Arkansas River from 

Avondale to Las Animas, 1990–2010........................................................................................50

27. Relation of concentrations of dissolved selenium and dissolved uranium with 

streamflow in the Arkansas River from Avondale to Las Animas, 2009–2010 ..................51

28. Delta D and delta18O in samples from (A) main-stem sites on the Arkansas River 

from Avondale to Las Animas and (B) selected tributaries, 2009–2010.............................52

Tables

1. Site names, station names, information, and monitoring equipment information 

for sites in and near the Upper Arkansas River Basin and Lower Arkansas River 

Basin study areas, 2009–2010...................................................................................................10

2. Analytical methods for analyses of water-quality samples collected in the 

Arkansas River Basin study areas, 2009–2010.......................................................................13

3. Periodic water-quality sampling sites in the Upper Arkansas River Basin and 

Lower Arkansas River Basin study areas, 2009–2010 ..........................................................14

4. Synoptic water-quality sampling sites in the Upper Arkansas River Basin and 

Lower Arkansas River Basin study areas, 2009–2010 ..........................................................17

5. Results of field blank quality-assurance samples collected in the Arkansas River 

Basin study areas, 2009–2010...................................................................................................20

6. Relative percent difference between replicate quality-assurance samples 

collected in the Arkansas River Basin study areas, 2009–2010..........................................21

7. Relative percent differences in concentration of dissolved selenium and specific 

conductance between EWI and point samples collected in the Arkansas River 

Basin study areas, 2009–2010...................................................................................................22

8. Summary statistics for Arkansas River streamflow data 1990–2008 and 2009–2010 

for Arkansas River at Canon City and Arkansas River at Portland gage stations ...........25

9. Regression equations relating dissolved-solids concentration and specific 

conductance in the Upper Arkansas River and selected tributaries from Canon 

City to near Portland, 2009–2010 ..............................................................................................30

10. Measured and calculated ungaged streamflow, contributions to dissolved-solids 

loads and dissolved-solids concentration for three reaches in the Upper Arkansas 

River Basin, September 15, 2010 ..............................................................................................38

11. Change in streamflow and loads of dissolved solids, selenium and uranium 

between main-stem sites in the Upper Arkansas River Basin, September 15, 2010.......39

12. Summary statistics for Arkansas River streamflow data 1990–2008 and 2009–2010 

for Arkansas River at Avondale, Arkansas River near Nepesta, Arkansas River at 

Catlin Dam, Arkansas River near Rocky Ford, Arkansas River at La Junta, and 

Arkansas River at Las Animas gage stations.........................................................................41
vi

13. Regression equations relating dissolved solids and specific conductance in the 

Lower Arkansas River and selected tributaries from Avondale to Las Animas, 

2009–2010 .....................................................................................................................................47

14. Measured and calculated ungaged streamflow, contributions to dissolved-solids 

loads and dissolved-solids concentrations for six reaches in the Lower Arkansas 

River Basin, November 16-19, 2009..........................................................................................54

15. Change in streamflow and loads of dissolved solids, selenium, and uranium between 

main-stem sites in the Lower Arkansas River Basin, November 16-19, 2009...................55
vii

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)

yard (yd) 0.9144 meter (m)

Area

acre 4,047 square meter (m2

)

acre 0.004047 square kilometer (km2

)

square mile (mi2

) 259.0 hectare (ha)

square mile (mi2

) 2.590 square kilometer (km2) 

Volume

gallon (gal) 3.785 liter (L) 

gallon (gal) 0.003785 cubic meter (m3

) 

gallon (gal) 3.785 cubic decimeter (dm3

) 

million gallons (Mgal) 3,785 cubic meter (m3

)

cubic inch (in3

) 16.39 cubic centimeter (cm3) 

cubic inch (in3

) 0.01639 liter (L)

cubic foot (ft3

) 0.02832 cubic meter (m3) 

acre-foot (acre-ft) 1,233 cubic meter (m3

)

acre-foot (acre-ft) 0.001233 cubic hectometer (hm3

) 

Flow rate

acre-foot per day (acre-ft/d) 0.01427 cubic meter per second (m3

/s)

cubic foot per second (ft3

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

million gallons per day (Mgal/d) 0.04381 cubic meter per second (m3

/s)

Mass

ton per day (ton/d) 0.9072 metric ton per day

ton per day (ton/d) 0.9072 megagram per day (Mg/d)

pound per day (lb/d) 0.4536 kilogram per day (k/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

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25°C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L).

Conversion Factors
viii

Acronyms Used in this Report

DS Dissolved solids

DWR Division of Water Resources

USGS U.S. Geological Survey

RRPG Regional Resource Planning Group

NWIS National Water Information System

SC Specific conductance

USEPA U.S. Environmental Protection Agency

UARB Upper Arkansas River Basin

LARB Lower Arkansas River Basin

NWQL National Water Quality Laboratory
Abstract

As a result of continued water-quality concerns in the 

Arkansas River, including metal contamination from historical 

mining practices, potential effects associated with storage and 

movement of water, point- and nonpoint-source contamination, population growth, storm-water flows, and future changes 

in land and water use, the Arkansas River Basin Regional 

Resource Planning Group (RRPG) developed a strategy to 

address these issues. As such, a cooperative strategic approach 

to address the multiple water-quality concerns within selected 

reaches of the Arkansas River was developed to (1) identify 

stream reaches where stream-aquifer interactions have a 

pronounced effect on water quality and (or) where reactive 

transport, and physical and (or) chemical alteration of flow 

during conveyance, is occurring, (2) quantify loading from 

point sources, and (3) determine source areas and mass loading for selected constituents. 

To date in the Arkansas River Basin, most existing 

water-quality data have been collected for the purposes of 

characterizing current water-quality conditions for selected 

river reaches as they pertain to in-stream classifications and 

standards or had not been collected in a manner that allows for 

rigorous and accurate analysis of mass loading. Poor understanding of streamflow from ungaged tributary and return 

flows frequently result in poorly defined water budgets that are 

the basis for determining source contributions. The purpose 

of this report is to characterize streamflow and water-quality 

data collected along the selected reaches of the Arkansas River 

from Canon City to near Portland (Upper Arkansas River 

Basin (UARB)) and from Avondale to Las Animas (Lower 

Arkansas River Basin (LARB)) during 2009 and 2010, identify critical stream reaches where stream-aquifer interactions 

may potentially have a pronounced effect on water quality 

(or where point-source discharges are a significant load to the 

stream), and to identify potential load source areas for selected 

constituents within the UARB and LARB study reaches. 

Data-collection activities were conducted for 2 years in 

the UARB and LARB. Water-quality samples were collected 

from June through December 2009 and from May through 

October 2010. In each study area, samples were collected 

periodically to characterize the water quality throughout the 

various hydrologic conditions observed in the Arkansas River 

Basin. Additionally, a limited number of synoptic (time-oftravel) sampling events were conducted to provide a better 

understanding of the in-stream water-quality processes using 

mass-loading analyses. 

Main-stem dissolved solids (DS) (salinity) median concentrations increase downstream in the Arkansas River from 

Canon City (153 milligram per liter (mg/L)) to near Portland 

(264 mg/L). In addition to irrigation return flows, inflows from 

groundwater may contribute to the high DS concentrations 

in the tributaries, especially those tributaries below Florence, 

Colorado, and those that drain the Penrose, Colorado, area 

where hay, pasture, orchard, and cropland acres are irrigated. 

Additionally, the higher DS concentrations downstream from 

Florence can be attributed to differences in geology and variation in the chemical composition of the bedrock. 

Concentrations of dissolved selenium and dissolved uranium increase in water samples downstream from Canon City 

to near Portland. One concentration of selenium in a sample 

from Bear Creek and concentrations of uranium in samples 

from Bear, Hardscrabble, and Ranger Creeks, and Brush 

Hollow equaled or exceeded U.S. Environmental Protection 

Agency (USEPA) drinking-water standards (50 and 30 μg/L, 

respectively) for those elements.

There were statistical differences between the DS loads 

in the Arkansas River during low- streamflow conditions 

in 2009–2010. Using the Tukey’s test for multiple comparisons to determine which sites were different from each 

other resulted in showing statistically significant differences 

between instantaneous DS loads for sites Arkansas River at 

Canon City to Arkansas River at Portland. The same sites 

were significant for selenium loads. The river section from 

Arkansas River at Canon City to Arkansas River at Portland 

is a potential source area of dissolved solids and selenium to 

the UARB. No statistically significant difference between sites 

was determined for uranium loads. 

To estimate the amount of ungaged contribution to DS 

loads from ungaged flows and other sources, the water balance 

Characterization of Streamflow, Water Quality, and 

Instantaneous Dissolved Solids, Selenium, and Uranium 

Loads in Selected Reaches of the Arkansas River, 

Southeastern Colorado, 2009–2010

By Tamara Ivahnenko, Roderick F. Ortiz, and Robert W. Stogner, Sr.
2 Characterization Studies of the Arkansas River, Colorado, 2009–2010

for the synoptic sampling event on September 15, 2010, was 

used as the basis for calculating the concentration of DS from 

the ungaged sources. Calculated DS concentrations from 

ungaged sources for the UARB were Arkansas River at Canon 

City to Arkansas River near Canon City, 500.4 mg/L; Arkansas River near Canon City to Arkansas River at Portland, 

706.8 mg/L; and Arkansas River at Portland to Arkansas River 

near Portland, 191 mg/L. The calculated DS concentrations 

for each of the reaches in the UARB fall within the reported 

range of groundwater DS concentrations, and groundwater is a 

plausible source for the ungaged contribution to the DS load. 

In addition to groundwater, ungaged irrigation return flow and 

tributaries may also be sources of the DS load; however, proportions of contributions from these sources are unknown. The 

ungaged contributions to DS load, ungaged streamflow, and 

calculated DS concentration are only for the synoptic samples 

collected in September 2010, and may likely vary throughout 

the year for different streamflow and water-quality conditions.

The LARB main-stem DS median concentrations

increase downstream from Avondale (557 mg/L) to Las 

Animas (1,725 mg/L). Median DS concentrations in tributaries and the La Junta wastewater treatment plant (WWTP) 

range from 1,050 mg/L in Hungerford Hollow to 2,770 mg/L 

in water sampled from the Huerfano River. As in the UARB, 

irrigation return flows and inflows from groundwater likely 

contribute to the high DS concentrations in all the tributaries, 

whereas differences in DS concentration ranges between the 

selected tributaries can be attributed to differences in geology 

and the chemical composition of the bedrock.

Median dissolved selenium concentrations vary in the 

main-stem sites and ranged from 8.4 to 12.2 μg/L in Arkansas River near Avondale to Arkansas River at Las Animas, 

and selenium concentrations increase downstream at sites 

from Arkansas River near Nepesta to Arkansas River at La 

Junta. Median selenium concentrations measured in tributaries generally were higher than those in the main-stem sites; 

however, no concentrations of selenium in any of the sampled 

sites exceeded USEPA primary drinking-water standards. 

The median dissolved uranium concentration (39 μg/L) in 

Sixmile Creek exceeded the USEPA drinking-water standard. 

The tributaries, Sixmile Creek, IR-43, Huerfano River, and 

Chicosa Creek all drain the lower part of the Cretaceous-age 

Pierre Shale and upper part of the Niobrara Formation, which 

are suspected geologic sources of uranium. Patterson Hollow 

had two samples that exceeded the drinking-water standard, 

one collected in November 2009 and the other in June 2010. 

Timpas Creek and Crooked Arroyo, each had one sample that 

exceeded the drinking-water standard. Patterson Hollow, 

Timpas Creek, and Crooked Arroyo basins are underlain by 

the upper part of the Niobrara Formation. The samples with 

exceedences were collected in the fall, when flows in these 

tributaries were low. 

The tributary samples and samples from the La Junta 

WWTP plot below the GMWL and the Colorado LMWL, indicating that all samples are enriched in δD and d18O, probably 

as a result of evaporation. A line calculated for the tributary 

samples is similar to the main stem (slope of 6.16). Samples 

from Sixmile Creek, IR-43, Huerfano River, Crooked Arroyo, 

and La Junta WWTP group with little or no temporal variation 

(within 1 per mil unit) in the isotope concentrations, suggesting the source water for these tributaries does not undergo 

evaporation through irrigation return and reuse. The remaining 

sampled tributaries have greater temporal variation in isotopic 

ratios suggesting multiple evaporation cycles or, as for the 

main-stem sites, possibly increasing contribution of groundwater that has been affected by evaporation and irrigation 

recharge. However, as in the UARB, no LARB groundwater 

isotope samples are available for comparison to the surfacewater samples. 

A Kruskall-Wallis and Tukey’s test on periodic and synoptic DS loads in 2009–2010, indicated statistically significant 

differences between DS loads at the main-stem sites. The 

significant differences between instantaneous DS loads at the 

main-stem sites were from Arkansas River near Rocky Ford 

to Arkansas River at La Junta. Further investigation would 

be needed to evaluate potential dissolved-solids source areas 

within the Arkansas River from Arkansas River near Rocky 

Ford to Arkansas River at La Junta. A Kruskall-Wallis test 

on selenium loads from samples collected during low-flow 

periods for this study from 2009–2010 indicated statistically 

significant differences between loads at the main-stem sites. 

Using the Tukey’s test for multiple comparisons, the differences between the sites were statistically significant for Arkansas near Nepesta to Arkansas River at La Junta. The same 

statistical test for uranium loads in the LARB indicated the 

differences between the sites are significant, as with selenium, 

for Arkansas near Nepesta to Arkansas River at La Junta. 

Further investigation would be needed to describe the source 

or sources in these reaches in greater detail.

To estimate the amount of ungaged contribution to DS 

loads from ungaged flows and other sources in the LARB, 

the water balance for the synoptic on November 16–19, 2009 

was used as the basis for calculating the concentration of DS 

from the ungaged sources. Calculated DS concentrations from 

ungaged sources for the LARB were Arkansas River near 

Avondale to Arkansas River near Nepesta, 1,069 mg/L; Arkansas River near Nepesta to Arkansas River at Catlin Dam, 89.6 

mg/L; Arkansas River at Catlin Dam to Arkansas River near 

Rocky Ford, not calculated; Arkansas River near Rocky Ford 

to Arkansas River at Swink, 4,926 mg/L; Arkansas River at 

Swink to Arkansas River at La Junta, 843 mg/L; and Arkansas River at La Junta to Arkansas River at Las Animas, 1,725 

mg/L. The DS concentrations of ungaged sources for each 

of the reaches in the LARB fall within the reported range of 

groundwater DS concentrations, and groundwater is a plausible source for the estimated DS load. In addition to groundwater, ungaged irrigation return flow and tributaries may also 

be sources of the DS load; however, proportions of contributions, as in the UARB, from these sources are unknown. The 

ungaged streamflow, ungaged contributions to DS load, and 

calculated DS concentration for an assumed ungaged single 

source for all six reaches in the LARB are applicable only for 
Introduction 3

November 16–19, 2009, and may likely vary throughout the 

year for different streamflow and water-quality conditions.

Introduction

Water within the Arkansas River Basin in Colorado is 

used for a wide variety of purposes including agricultural, 

municipal, recreational, power production, industrial, livestock, domestic, and mining. In 2009, more than 85 percent 

of the water delivered in the Arkansas River Basin was for 

irrigation or storage (Colorado Division of Water Resources, 

2010). Although municipal and industrial uses accounted for 

only 12 percent of the total water deliveries in 2009 (Colorado Division of Water Resources, 2010), population growth 

in the basin will likely drive changes in water storage, water 

releases, and (or) transfer of water within or outside of the 

Arkansas River Basin. As such, numerous entities have 

expressed concerns about a wide range of water-quality issues 

in the Arkansas River Basin in Colorado. 

In the Arkansas River Basin upstream from Pueblo Reservoir (fig. 1), water-quality concerns generally are associated 

with metal contamination from historical mining practices, 

fish and biological integrity issues, increased salt loading, 

and concerns about potential effects associated with storage 

and movement of water. Downstream from Pueblo Reservoir, 

water-quality concerns are associated with a wide variety of 

environmental stressors including point- and nonpoint-source 

contamination, population growth, storm-water flows, and 

future changes in land and water use.

As a result of continued water-quality concerns, the 

Arkansas River Basin Regional Resource Planning Group 

(RRPG), developed a strategy to address multiple water-quality 

concerns in the Arkansas River Basin. Currently (2011), the 

Arkansas River Basin RRPG entities include the City of 

Aurora, Colorado Springs Utilities, Lower Arkansas Valley 

Water Conservancy District, Pueblo Board of Water Works, 

Southeastern Colorado Water Conservancy District, and the 

Upper Arkansas Water Conservancy District. The goals of the 

strategy include (1) synthesizing electronically available data 

and studies that could be integrated to help provide a basinwide understanding of water-quality conditions, (2) summarizing and characterizing available water-quality data to address 

concerns identified and prioritized by stakeholders, and (3) 

identifying gaps in existing data. The U.S. Geological Survey 

(USGS) is working in cooperation with the Arkansas River 

Basin RRPG to help address these goals.

To provide an electronically available data set, the USGS 

developed, and is maintaining, a water-quality data repository 

for selected study areas in Colorado including the Arkansas 

River Basin. This repository combines water-quality data from 

the USGS National Water Information System (NWIS) and 

the U.S. Environmental Protection Agency (USEPA) STORET 

databases and represents data from Federal, State, academia, 

and local sources. The interactive data repository for the 

Arkansas River Basin (U.S. Geological Survey, 2009) can be 

accessed at http://rmgsc.cr.usgs.gov/cwqdr/Arkansas/index.

shtml.

The occurrence of high concentrations of dissolved solids 

(DS), selenium, and uranium in groundwater and surface water 

affecting agricultural productivity, in-stream water quality, and 

(or) drinking-water quality in selected parts of the basin was 

identified as a priority water-quality concern by stakeholders 

(Miller and others, 2010). In 2007, the USGS, in cooperation 

with the Arkansas River Basin RRPG, began a retrospective 

evaluation of available data to characterize the occurrence 

and distribution of dissolved solids, selenium, and uranium 

in groundwater and surface water from the headwaters of the 

Arkansas River to Coolidge, Kansas (Miller and others, 2010).

Numerous data gaps were identified in the existing data 

for the Arkansas River Basin. In general, streamflow has been 

and is being monitored in the Arkansas River near the mouths 

of major tributaries and in the major canals. However, minor 

tributary streamflow and irrigation return flow typically are not 

monitored, although the data are needed for accurately determining water budgets and other hydrologic aspects. Existing 

water-quality data generally were collected to characterize 

water-quality conditions for selected river reaches as they 

pertain to in-stream classifications and standards. These data 

were primarily periodic samples only and are not necessarily 

applicable to rigorous and accurate analysis of mass loading 

and determining source contributions. Data needed to assess 

the dominant physical, chemical, or biological processes that 

created the existing water-quality conditions are generally 

unavailable. 

In order to address the gaps in data, a cooperative strategic approach was developed to (1) identify stream reaches 

where stream-aquifer interactions have a pronounced effect 

on water quality and (or) where reactive transport, and physical and (or) chemical alteration of flow during conveyance is 

occurring, (2) quantify loading from point sources, and (3) 

determine source areas and mass loading for selected constituents. In 2009, the USGS in cooperation with the entities of the 

Arkansas River Basin RRPG, began a study to help address 

some of the gaps in data and provide hydrologic information 

needed to characterize streamflow, water quality, and instantaneous dissolved-solids, selenium, and uranium loads for 

selected reaches of the Arkansas River.

Purpose and Scope

The purpose of this report is to describe the characteristics of streamflow, water quality, and instantaneous dissolvedsolids, selenium, and uranium loads in selected reaches of 

the Arkansas River Basin in southeastern Colorado, identify 

critical Arkansas River reaches where stream-aquifer interactions may potentially have a pronounced effect on water 

quality (or where point-source discharges are a significant 

load to the stream), and to identify potential load source areas 

for selected constituents within the study reaches. The first 
Crooked

Wild Horse 

Creek

Tallahassee Creek

NEW MEXICO 

BACA

LAS ANIMAS

PARK

PUEBLO

BENT

KIOWA

LINCOLN

SAGUACHE

EL PASO

ELBERT

OTERO

CHEYENNE

PROW

FREMONT

HUERFANO

CONEJOS

KIT CARSON

COSTILLA

GUNNISON CHAFFEE

CUSTER

CROWLEY

LAKE

ALAMOSA

TELLER

PITKIN

RIO GRANDE

DOUGLAS

EAGLE

MINERAL ARCHULETA

JEFFERSON SUMMIT

Lamar

Salida

Trinidad

La Junta

Leadville

Westcliffe

Walsenburg

Buena

Vista Colorado Springs

Penrose

Portland

Purgatoire River

Horse Creek

Huerfano River

Apishapa River

Fountain Creek

Adobe Creek

Timpas Creek

Saint Charles River

Currant Creek

Chicosa Creek

Chicosa Arroyo

Big Sandy Creek

Rush Creek

Chico Creek

Las Animas

106° 105° 104° 103° 

39°

38°37°

COLOR

PUEBLO

RESERVOIR

JOHN MARTIN 

RESERVOIR

Base from U.S. Geological Survey Data, 2011, various scalesUniversal Transverse Mercator projectionZone 13

EXPLANATION

Watershed boundary

Upper Arkansas River Basin study areaLower Arkansas River Basin study area

0 100 200 MILES

0 150 300 KILOMETERS

COLORADO

Monument

Pueblo

Fowler

Avondale

Rocky Ford

Sixmile Creek Nepesta

Lake 

Creek

Canon City

Arkansas River

Creek

 Arroyo

Figure 1. Location of the Arkansas River Basin in Colorado.
Introduction 5

main study-area reach includes the Arkansas River, tributaries, 

and diversions from Canon City to near the upstream end of 

Pueblo Reservoir and is designated in this report as the Upper 

Arkansas River Basin (UARB) reach (fig. 2). The second main 

study-area reach is downstream from Pueblo Reservoir and 

includes the Arkansas River, tributaries, and diversions from 

Avondale to Las Animas and is designated in this report as the 

Lower Arkansas River Basin (LARB) reach (fig. 3). 

Existing data and data collected specifically for this study 

were used. Numerous study-specific temporary streamflowgaging stations were installed and operated during 2009–2010 

along the Arkansas River and its tributaries to augment the 

existing streamflow-gaging network in the study area. Several 

continuous water-temperature and specific-conductance monitors were installed and operated seasonally along the Arkansas 

River. Periodic water-quality sampling was conducted from 

June through December 2009 and from May through October 

2010 at numerous main-stem, tributary, and diversion sites. 

Existing water-quality data also were used, although data for 

pre-1990 samples were not included because field-sampling 

protocols and laboratory analysis procedures for trace elements and radionuclides were not directly comparable to 

1990–2010 field protocols and laboratory procedures. Synoptic sampling using Lagrangian (time-of-travel) sampling 

designs were conducted during various streamflow conditions 

in both main reaches during 2009–2010 to provide a better 

understanding of the in-stream water-quality processes using 

mass-loading analyses. Statistical methods were used to determine significant differences between sites for various characteristics. Mass-balance methods were used to help determine 

ungaged streamflow and ungaged contributions to dissolved 

solids loads between sites in the two main reaches.

Description of the Study Area

The Arkansas River originates among some of the 

highest peaks in the continental United States, flowing from 

the high mountain basins onto the plains of southeastern 

Colorado. Upstream from Canon City, the Arkansas River is 

characterized by a steep gradient and high-velocity flows that 

are confined to a relatively narrow rock and cobble stream 

channel (Miller and others, 2010). Downstream from Canon 

City, the river gradient decreases as the river flows generally 

eastward out of the mountains onto the plains. Pueblo Reservoir is the first main-stem reservoir on the Arkansas River 

(fig. 1) and is used to control the release of stored water to 

the predominantly agricultural river valley downstream from 

Pueblo. Water is stored in Pueblo Reservoir for a variety of 

reasons, including flood control, irrigation, and public water 

supply. Fountain Creek is a major tributary in the basin and 

flows into the Arkansas River within the city limits of Pueblo 

(fig. 1). Downstream from Fountain Creek, the river channel is 

a shifting sand channel that meanders along the alluvial flood 

plain (Miller and others, 2010). For nearly 120 miles downstream from Pueblo Reservoir, water from the river is diverted 

through a network of irrigation canals and is applied and 

reapplied to grow crops in the valley. Near Las Animas, the 

river flows into John Martin Reservoir which is used to regulate the streamflow for downstream uses. Storage decreases 

substantially in both reservoirs by the end of the growing 

season because of decreased inflow and large releases to meet 

downstream demands for irrigation water (Lewis and Brendle, 

1998). 

Streamflow in the Arkansas River exhibits considerable 

seasonal variability (fig. 4). The majority of the total annual 

streamflow results from snowmelt runoff in the mountains at 

the western boundary of the basin, including water native to 

the watershed and water brought into the watershed by transmountain diversions (Bureau of Reclamation, 2012). Streamflow generally is at an annual minimum from October through 

March and is maintained by natural base flow and reservoir 

releases. Starting in April, streamflow begins to increase as 

snow begins to melt leading to maximum streamflow during 

May and June. During July through September, streamflow in 

the Arkansas River includes decreasing amounts of snowmelt 

and is augmented by releases of stored water. During this time, 

streamflow can increase substantially over short periods following intense rainstorms and subsequent runoff from tributaries. During July through September, most streamflow in the 

plains is diverted for irrigation uses. 

Previous Investigations

Streamflow and water quality in the Arkansas River 

Basin have been analyzed previously by numerous investigators. In 1993, a comprehensive 3-year water-quality datacollection program was completed in the Arkansas River 

Basin in Colorado (Dash and Ortiz, 1996). An assessment of 

the surface-water quality in the basin was done using these 

data and included analysis of dissolved solids (DS), major 

ions, trace elements, nutrients, radio-chemical constituents, 

pesticides, suspended sediment, and bacteria (Ortiz and others, 

1998). Gaydos (1980), Abbott (1985), Cain (1985; 1987), and 

Mueller and others (1991) described the quality of the Arkansas River including water quality of irrigation-return flows, 

relations to specific conductance (SC), and operation of water 

systems. Miles (1977) focused on the downstream increase 

in salinity in the Arkansas River. Cain and Edelmann (1980) 

and Cain and others (1980) investigated the effects of municipal and industrial wastewater discharges to the Arkansas 

River near Pueblo. Miller and others (2010) investigated the 

occurrence and distribution of dissolved solids, selenium, and 

uranium in the Arkansas River Basin, based on retrospective 

data. Summaries of previous investigations, including those 

listed here, are provided in Miller and others (2010). Various 

publications documenting ongoing research by Colorado State 

University in the Lower Arkansas River Basin are available

(Colorado State University, 2009). These studies provided historical water-quality information for and assisted in developing the scope of this study.
Minnequa Canal Fremont Ditch Oil Creek Ditch Oak Creek Beaver Creek un-named Bear Creek Creek Hardscrabble Creek Fourmile Creek Brush Hollow Sand Bear Cr Ranger Cr Beaver Cr Fourmile Cr Brush Hollow Hardscrabble Cr Ark n

Ark at Canon City Ark trib2 at CR119 Ark trib1 at CR119 Oil Cr Ditch return Ark trib1 at Hwy120 Oak Cr Sand Cr Chandler Cr Oak Cr trib Ark at Portland Ark trib1 at CR 112 Ark trib2 at CR 112 Canon City Florence Penrose 

Ark nr Canon City Rainbow Park WWTP Ark rtn at Hwy 115 BRUSH HOLLOW RESERVOIR 105°15' 105°10' 105°05' 105°00' 

38°25' 38°20'

67

0 1 2 3 4 5 MILES

0 2 4 8 KILOMETERS

Rainbow Park WWTP Ark rtn at Hwy 115Arkansas River

EXPLANATION

TownsUpper Arkansas River Basin study area

Periodic streamflow and periodic water-quality siteSeasonal continuous streamflow and periodic water-quality siteLong-term continuous streamflow and periodic water-quality site with longSeasonal continuous streamflow and periodic water-quality site with seas

Brush HollowRanger CreekArk near PortlandArk at Canon City

6

Arkansas River Creek

Chandler 

50

115

67

115

120

115

128

Figure 2. Delineation of the Upper Arkansas River Basin study area and locations of streamflow and water-quality siteReservoir, Colorado. 
La Junta

Apishapa River

Timpas Creek

Huerfano River

Sixmile Creek

Anderson Arroyo

Chicosa Creek

Crooked Arroyo

Chicosa Creek

Timpas Creek

Horse Cr

Adobe Cr

Colorado Canal

Fort Lyon Storage Canal

Holbrook Canal

Purgato

i

IR-43

La Junta WWTP

Fowler

Nepesta

Avondale

Rocky Ford

Arkansas River

Sixmile Cr

Huerfano R

Chicosa Cr

Ark at Swink

Oxford Canal

Ft Lyon Canal

Collier Ditch

Ark nr Nepesta

Colorado Canal

Highline Canal

Ark at La Junta

Anderson 

Arroyo

Ark nr Rocky Ford

Timpas Cr

Crooked 

Arroyo

Lake Meredith 

outlet

Ark nr Avondale

104°30' 104°00' 103°30'

38°00'

Base from U.S. Geological Survey Data, 2011, various scalesUniversal Transverse Mercator projectionZone 13

0 5 10 20 Miles0 5 10 20 30 Kilometers

15

15 25

Fort Lyon Canal

Fowler

Rocky Ford

Catlin Canal

Rocky Ford Highline Canal

Otero Canal

Patterson Hollow

Arkansas River

Rocky Ford Canal

Oxford Farmers Ditch

Apishapa R at Hwy 50 Otero Canal

Rocky Ford Ditch

Ark at Catlin Dam Hungerford Hollow

Ft Lyon Storage Canal

Apishapa R nr Fowler

Holbrook Ditch

Catlin Canal

Chico Creek

10

96

50

350 

Apishapa River

EXPLANATION

Lower Arkansas River Basin study area

Periodic streamflow and periodic water-quality siteLong-term continuous streamflow and periodic water-quality siteSeasonal continuous streamflow and periodic water-quality siteLong-term continuous streamflow and periodic water-quality site with 

Huerfano RiverIR-43

Ark at Las Animas

Patterson Hollow

Long-term continuous streamflow and periodic water-quality site with Seasonal continuous streamflow and periodic water-quality site with 

Ark at Las Animas

Patterson Hollow

Figure 3. Delineation of the Lower Arkansas River Basin study area and location of streamflow and water-quality sites, Avondale to Las Animas, Colorado. 
8 Characterization Studies of the Arkansas River, Colorado, 2009–2010

A

0

1,000

2,000

3,000

4,000

5,000

6,000

Streamflow, in cubic feet per second

B

C

Streamflow, in cubic feet per second

1/1/2010

2/1/2010

3/1/2010

4/1/2010

5/1/2010

6/1/2010

7/1/2010

8/1/2010

9/1/2010

10/1/2010

11/1/2010

12/1/2010

1/1/2010

2/1/2010

3/1/2010

4/1/2010

5/1/2010

6/1/2010

7/1/2010

8/1/2010

9/1/2010

10/1/2010

11/1/2010

12/1/2010

1/1/2010

2/1/2010

3/1/2010

4/1/2010

5/1/2010

6/1/2010

7/1/2010

8/1/2010

9/1/2010

10/1/2010

11/1/2010

12/1/2010

600

1,000

1,200

1,400

1,600

1,800

400

200

0

800

Streamflow, in cubic feet per second

0

1,000

2,000

3,000

4,000

5,000

6,000

Figure 4. Annual Arkansas River hydrographs (2010) (A) Arkansas at Portland, (B) Arkansas near Avondale, and (C) 

Arkansas at Las Animas.
Methods 9

Methods

General information for sites in and near the UARB and 

LARB is listed in table 1. In this report, sites generally will be 

identified by the site name. The following section characterizes the types of available streamflow and water-quality data 

evaluated in this report. Quality-assurance procedures also are 

discussed.

Available Streamflow Data

Streamflow data analyzed for this report are of two 

general types: instantaneous or daily mean values. Instantaneous streamflow data can be either measured directly by 

volumetric or streamflow methods at study-specific temporary 

gaging stations or derived from developed stage/streamflow 

relations at long-term gaging stations. Appropriate USGS 

streamflow-measuring techniques, as defined by Rantz and 

others (1982a), were used to determine streamflow (including 

estimated streamflow) at the sampling sites. Where available, 

instantaneous streamflows were determined from long-term or 

study-specific temporary streamflow stations maintained by 

the USGS or from long-term streamflow stations maintained 

by the Colorado Division of Water Resources (CODWR). 

Computed daily mean streamflow values for long-term or 

study-specific temporary stations operated by either agency or 

for wastewater treatment facilities also were compiled for the 

analysis of daily mass loading.

U.S. Geological Survey Streamflow Data

No long-term streamflow measuring stations are operated by the USGS in the UARB study area. However, two 

study-specific temporary gaging stations on the main stem 

were operational during parts of the study period (table 1 and 

fig. 2). Pressure transducer equipment was installed at sites 

Ark nr Canon City and Ark nr Portland. Generally, daily mean 

streamflow values were computed for July through December 

2009 and for May through November 2010. Suggested procedures for maintaining these records are described in Rantz and 

others (1982a,b). 

In addition, pressure transducers were installed and operated seasonally at five tributary stations in the UARB to aid 

in the quantification of inflows to the river during the study 

(table 1). Seasonal data were available for Fourmile Cr, Brush 

Hollow, Hardscrabble Cr, Bear Cr, and Beaver Cr. Generally, 

daily mean streamflow values were computed for a period of 

record similar to that of the two main-stem stations with the 

exceptions of Beaver Cr, which only was operational in 2009, 

and Brush Hollow, which only was operational in 2010.

Two long-term main-stem streamflow–gaging stations are 

operated by the USGS in the LARB study area (fig. 3). At least 

20 years of streamflow data are available for Ark nr Avondale 

and Ark at Las Animas (U.S. Geological Survey, 2012). Unitvalue (15-minute record) and daily mean streamflow data were 

available from each of these two stations for the study period 

2009–2010. One study-specific temporary gaging station on 

the main stem of the river—Ark at Swink—was operational 

during parts of the study period (table 1). Pressure transducer 

equipment was installed and operated at this site and, generally, daily mean streamflow values were computed for July 

through December 2009 and for May through November 

2010. Procedures for maintaining these records are described 

in Rantz and others (1982a,b). 

Two long-term streamflow stations on tributaries to the 

Arkansas River—the Huerfano R and Timpas Cr—are operated by the USGS (U.S. Geological Survey, 2012). Pressure 

transducers were installed and operated seasonally at six 

tributary stations in the LARB to aid in the quantification 

of inflows to the river during the study (table 1). Seasonal 

data were available for Sixmile Cr, Chicosa Cr, Hungerford 

Hollow, Apishapa R, Crooked Arroyo, and Anderson Arroyo. 

Generally, daily mean streamflow values were computed for 

a period of record similar to that of the main-stem station at 

Swink, with the exception of Anderson Arroyo, which was 

only operational in 2009. 

Colorado Division of Water Resources 

Streamflow Data

In the UARB, the CODWR operates two streamflow 

measuring stations on the Arkansas River (fig. 3, table 1). 

At least 20 years of streamflow data are available for Ark 

at Canon City and Ark at Portland. Generally, daily mean 

streamflow values were available for all dates during the study 

period. Streamflow data for most stations managed by the 

State of Colorado can be accessed through the Colorado’s Surface Water Conditions Web site at http://www.dwr.state.co.us/

SurfaceWater/default.aspx. One diversion site in the UARB, 

Minnequa Canal, was used to quantify diversion flows from 

the river during the study period (table 1). 

The CODWR also maintains three streamflow measuring 

stations on the Arkansas River in the LARB (fig. 3, table 1). 

At least 20 years of streamflow data are available for Ark nr 

Nepesta, Ark nr Rocky Ford, and Ark at La Junta. Generally, 

daily mean streamflow values were available for all dates during the study period. Seasonal streamflow data are collected by 

the State at several diversion sites in the LARB. Seasonal data 

were available for Collier Ditch, Colorado Canal, Highline 

Canal, Oxford Canal, Otero Canal, Catlin Canal, Holbrook 

Ditch, Fort Lyon Storage Canal, Rocky Ford Ditch, and Fort 

Lyon Canal. Data from these 10 sites were used to quantify 

diversion of flows from the river during the study period 

(table 1). Generally, daily mean streamflow values were available for a period of record when water was being diverted at 

the headgates. 
10 Characterization Studies of the Arkansas River, Colorado, 2009–2010

Site name USGS station identification number USGS station name Site type

Available 

monitoring 

equipment on site

Upper Arkansas River Basin study reach: Canon City to Pueblo Reservoir (in downstream order)

Lake Cr1 07084500 Lake Creek above Twin Lakes Reservoir TR eQgs

Ark at Granite1 07086000 Arkansas R at Granite MS eQgs

Ark at Canon City 07096000 Arkansas R at Canon City MS eQco, eQWgs

Sand Cr 382536105113901 Sand Creek near mouth at Canon City TR --

Fourmile Cr 07096500 Fourmile Creek near Canon City TR sQgs

Ark nr Canon City 07096515 Arkansas R at Fourmile Rd near Canon City MS sQgs, sQWgs

Oil Cr Ditch return 382520105104101 Oil Creek Ditch return flow at Fourmile Road near

 Canon City TR --

Minnequa Canal 07096530 Minnequa Canal at headgate near Florence DV --

Chandler Cr 382409105081701 Chandler Creek at Hwy 115 near Florence TR --

Oak Cr 382348105073901 Oak Creek at Hwy 115 near Florence TR --

Oak Cr trib 382334105071701 Oak Creek tributary at Hwy 115 at Florence TR --

Ark trib 2 at CR 119 382330105052301 Arkansas R trib 2 at CR 119 near Florence TR --

Ark trib 1 at CR 119 382323105045001 Arkansas R trib 1 at CR 119 near Florence TR --

Ark rtn at Hwy 115 382324105040601 Arkansas R return at Hwy 115 near Florence TR --

Rainbow Park WWTP 382322105040501 Rainbow Park Regional WWTP effluent outfall 

 near Florence WW user

Brush Hollow 382410105032301 Brush Hollow at Hwy 115 near Florence TR sQgs2

Hardscrabble Cr 382337105014600 Hardscrabble Creek at Hwy 120 at Portland TR sQgs

Ark at Portland 07097000 Arkansas R at Portland MS eQco, eQWgs

Bear Cr 382320105002901 Bear Creek at Hwy 120 at Portland TR sQgs

Ark trib 1 at CR 112 382240105010301 Arkansas R trib at CR 112 near Portland TR --

Ark trib 2 at CR 112 382226105001501 Arkansas R trib 2 at CR 112 near Portland TR --

Ark trib 1 at Hwy 120 382308104583201 Arkansas R trib 1 at Hwy 120 near Portland TR --

Ranger Cr 382236104580101 Ranger Creek at Old Portland Hwy near Portland TR --

Beaver Cr 07099100 Beaver Creek near Portland TR sQgs3

Ark nr Portland 07099200 Arkansas R near Portland MS sQgs, sQWgs

Lower Arkansas River Basin study reach: Avondale to Las Animas (in downstream order)

Ark nr Avondale5 07109500 Arkansas R near Avondale MS eQgs, eQWgs

Sixmile Cr 381440104234401 Sixmile Creek trib abv Hwy 50 near Avondale TR sQgs

Collier Ditch 381632104202001 Collier Ditch at Hwy 50 near Avondale DV eQco4-

IR-43 381412104202101 IR-43Avondale D at BR 50 near Avondale TR --

Table 1. Site names, station names, information, and monitoring equipment information for sites in and near the Upper Arkansas River 

Basin and Lower Arkansas River Basin study areas, 2009–2010.

[USGS, U.S. Geological Survey; MS, main stem; TR, tributary; R, River; DV, diversion canal or ditch; WWTP, wastewater treatment plant; eQgs, existing USGS 

streamflow station; eQco, existing Colorado Division of Water Resources streamflow station; sQgs, seasonal USGS streamflow station; user; user supplied streamflow data; eQWgs; existing USGS water-quality station; sQWgs, seasonal USGS water-quality station; WWTP, wastewater treatment plant; CODWR; Colorado 

Division of Water Resources; --, no equipment]
Methods 11

Site name USGS station identification number USGS station name Site type

Available 

monitoring 

equipment on site

Lower Arkansas River Basin study reach: Avondale to Las Animas (in downstream order)—Continued

Colorado Canal 381443104153301 Colorado Canal at Hwy 209 near Boone DV eQco

Huerfano R 07116500 Huerfano R near Boone TR eQgs

Highline Canal 381334104142301 Rocky Ford Highline Canal headgate near Boone DV eQco

Ark R blw Highline 

Canal6 381317104135400 Arkansas R below Rocky Ford Highline headgate MS sQgs3

Oxford Canal 381046104091501 Oxford Canal at Huckleberry Rd near Nepesta DV eQco

Ark nr Nepesta 07117500 Arkansas R near Nepesta MS eQco, sQWgs

Chicosa Cr 07117600 Chicosa Creek near Fowler TR sQgs

Otero Canal 380819104020601 Otero Canal at sluice near Fowler DV eQco4

Hungerford Hollow 380739103593701 Hungerford Hollow at Hwy 50 near Fowler TR sQgs

Apishapa R 380715103564701 Apishapa R at Hwy 50 near Fowler TR sQgs

Catlin Canal 380727103544201 Catlin Canal at CR 9 near Manzanola DV eQco

Ark at Catlin Dam5 07119700 Arkansas R at Catlin Dam near Fowler MS eQco, eQWgs

Holbrook Ditch 380636103444801 Holbrook Ditch at Hwy 71 near Rocky Ford DV eQco

Ft Lyon Storage Canal 380633103444901 Fort Lyon Storage Canal at Hwy 71 near Rocky 

 Ford DV eQco

Rocky Ford Ditch 380610103483501 Rocky Ford Ditch at Road 14.5 near Vroman DV eQco

Patterson Hollow 380456103473701 Patterson Hollow at Hwy 50 near Vroman TR --

Ark nr Rocky Ford 07120500 Arkansas R near Rocky Ford MS eQco, sQWgs

Timpis Circle 07121500 Timpas Creek at mouth near Swink TR eQgs

Ark at Swink 07121700 Arkansas R at Swink MS sQgs, sQWgs

Ft Lyon Canal 07122005 Fort Lyon Canal near La Junta DV eQco

Crooked Arroyo 375955103351201 Crooked Arroyo at Hwy 50 near La Junta TR sQgs

Anderson Arroyo 07122800 Anderson Arroyo at La Junta TR sQgs3

Ark at La Junta 07123000 Arkansas R at La Junta MS eQco, sQWgs

La Junta WWTP 07123080 La Junta sewage effluent at La Junta WW user

Ark at Las Animas5 07124000 Arkansas R at Las Animas MS eQgs, eQWgs

1

Site data only applicable for background isotope data collected in 2009.

2

Seasonalstreamflow only in 2010.

3

Seasonal streamflow only in 2009.

4

Daily streamflow data provided by CODWR Division 2 Water Commissioner, 2010, in written communication.

5

Water-quality samples only collected during synoptic sampling events.

Table 1. Site names, station names, information, and monitoring equipment information for sites in and near the Upper Arkansas River 

Basin and Lower Arkansas River Basin study areas, 2009–2010.—Continued

[USGS, U.S. Geological Survey; MS, main stem; TR, tributary; R, River; DV, diversion canal or ditch; WWTP, wastewater treatment plant; eQgs, existing USGS 

streamflow station; eQco, existing Colorado Division of Water Resources streamflow station; sQgs, seasonal USGS streamflow station; user; user supplied streamflow data; eQWgs; existing USGS water-quality station; sQWgs, seasonal USGS water-quality station; WWTP, wastewater treatment plant; CODWR; Colorado 

Division of Water Resources; --, no equipment]
12 Characterization Studies of the Arkansas River, Colorado, 2009–2010

Wastewater Treatment Plant Streamflow Data

Data for streamflow discharged from wastewater treatment plants (WWTP) were obtained from the operators of 

the Rainbow Park Regional WWTP in the UARB and from 

the La Junta WWTP in the LARB. The Fremont Sanitation 

District processes wastewater from a service area that includes 

Canon City, Florence, and 13 State and Federal prisons at 

the Rainbow Park Regional WWTP. The plant discharges an 

average of 4.5 million gallons of treated wastewater per day to 

the Arkansas River (U.S. Environmental Protection Agency, 

2011). The La Junta WWTP in 2010 discharged an average 

of 1.81 million gallons per day to the Arkansas River (U.S. 

Environmental Protection Agency, 2011).

Available Continuous Water-Quality Monitor 

Data

The USGS operated eight study-specific continuous 

water-quality monitors in the Arkansas River Basin during 2009–2010. Procedures for maintenance, calibration, 

and development of records for monitor sites are described 

in Wagner and others (2006). Two long-term water-quality 

monitoring sites on the Arkansas River are operated by the 

USGS in the UARB study area (fig. 2). At least 20 years of 

water-temperature and specific-conductance data are available 

for Ark at Canon City and Ark at Portland. Additionally, two 

water-quality monitors were installed on the main stem of the 

river at Ark nr Canon City and Ark nr Portland (table 1; fig. 2). 

The monitors were operational from July through December 

2009 and from May through November 2010; daily mean 

water-temperature and specific-conductance values generally 

are available for these dates. Between December and April, 

icing prevented operation of the monitors. 

In the LARB, three established water-quality monitoring 

sites are operated by the USGS on the Arkansas River (fig. 3). 

At least 20 years of water-temperature and specific-conductance data are available for Ark nr Avondale, Ark at Catlin 

Dam, and Ark at Las Animas. Additionally, one water-quality 

monitor was installed on the main stem of the river at Ark at 

Swink (table 1; fig. 3). The monitor was operational from July 

through December 2009 and from May through November 

2010; daily mean water-temperature and specific-conductance 

values generally are available for these dates. Between 

December and April, ice prevented operation of the monitors.

Water-Quality Sampling Techniques and 

Analytical Methodology

Data-collection activities were conducted for two years 

in the two study areas. In general, water-quality samples were 

collected using established USGS techniques (U.S. Geological 

Survey, 2006) from June through December 2009 and from 

May through October 2010 to allow comparison of data from 

water-quality samples with data from specific-conductance 

monitors. In each study area, samples were collected periodically to characterize the water quality throughout the various 

hydrologic conditions observed in the Arkansas River Basin. 

Additionally, a limited number of synoptic (time-of-travel) 

sampling events were conducted to provide a better understanding of the in-stream water-quality processes using massloading analyses. Samples analyzed for dissolved constituents 

were filtered at the time of collection using a 0.45-micrometer 

capsule filter and preserved with acid to a pH of 2 (U.S. 

Geological Survey, 2006). Various types of quality-assurance 

samples also were collected in association with all the waterquality samples collected. Analytical procedures used for the 

analysis of water-quality samples are listed in table 2. Most 

water-quality samples were analyzed at the USGS National 

Water Quality Laboratory (NWQL) in Denver, Colo. In addition, a select number of samples were submitted to the USGS 

Reston Stable Isotope Laboratory in Reston, Va., for analysis 

of oxygen and hydrogen isotopes (Fishman and Friedman, 

1989). 

Periodic Water-Quality Sampling

Nine periodic water-quality sampling events were 

conducted in the UARB along a reach from Ark at Canon 

City downstream to Ark nr Portland; samples were collected 

between July 9, 2009, and October 7, 2010. Ten periodic sampling events were conducted in the LARB along a reach just 

downstream from Ark nr Avondale to Ark at La Junta; samples 

were collected between July 16, 2009, and October 5, 2010. 

Periodic samples were collected using established protocols as 

described in U.S. Geological Survey (2006). Field measurements of water temperature, pH, and specific conductance 

were collected at main-stem and tributary sites in each of the 

two study areas (Wilde, F.D., variously dated), but generally 

only water temperature and specific conductance were measured at diversion sites. Table 3 summarizes the general types 

of water-quality data obtained for the 22 sampled sites in the 

UARB and for the 24 sampled sites in the LARB. 

Synoptic Water-Quality Sampling

A limited number of synoptic (time-of-travel) sampling 

events were conducted to provide a better understanding of the 

in-stream water-quality processes using mass-loading analyses. Each synoptic sampling event used a Lagrangian sampling 

design, in order to follow the same parcel of water as it moves 

downstream (Zuellig and others, 2007). Analyses of available 

velocity profiles from established streamflow measuring stations were used to estimate time-of-travel between sample-collection sites (Patrick Edelmann, U.S. Geological Survey, oral 

commun., 2010). For this study, time-of-travel was defined 

as the estimated amount of elapsed time for a parcel of water 

to travel between two monitoring sites. Travel-time estimates 

were calculated several days prior to the date of sample collection using seasonally averaged velocities. Minor adjustments 
Methods 13

Field parameter or 

analyzed constituent Analysis method Method reporting limit

Reporting 

unit

Water temperature, field Digital thermistor 0.5 degrees Celsius

Specific conductance, field Digital meter variable μS/cm at 25 °C 

pH, field Digital meter 0.1 S.U.

Escherichia coli Quanti-Tray® 2000 variable MPN/ 100 ml

Total coliform Quanti-Tray® 2000 variable MPN/ 100 ml

Dissolved solids Residue on evaporation at 180 °C 10 mg/L

Calcium, dissolved ICP 0.044 mg/L

Magnesium, dissolved ICP 0.016 mg/L

Sulfate, dissolved ICP 0.18 mg/L

Potassium, dissolved ICP 0.064 mg/L

Sodium, dissolved ICP 0.10 mg/L

Acid neutralizing capacity, fixed endpoint, 

 lab determination Electrometric titration 8.0 mg/L

Chloride, dissolved ICP 0.12 mg/L

Fluoride, dissolved ICP 0.08 mg/L

Silica, dissolved Automated batch analyzer 0.11 mg/L

Ammonia plus organic nitrogen, dissolved Colorimetric 0.10 mg/L, as N

Ammonia, dissolved Colorimetric 0.020 mg/L, as N

Nitrate plus nitrite, dissolved Colorimetric 0.04 mg/L, as N

Nitrite, dissolved Colorimetric 0.002 mg/L, as N

Orthophosphate, dissolved Colorimetric 0.008 mg/L, as P

Phosphorus, whole water Colorimetric 0.008 mg/L

Aluminum, dissolved ICP-MS 3.4 μg/L

Zinc, dissolved ICP-MS 2.8 μg/L

Selenium, dissolved ICP-MS 0.04 μg/L

Iron, whole water ICP-AES 4.6 μg/L 

Boron, dissolved ICP-MS 3 μg/L

Uranium, dissolved ICP-MS 0.008 μg/L

Hydrogen isotope ratio1 DI-IRMS variable parts /1,000

Oxygen isotope ratio1 DI-IRMS variable parts /1,000

1

Samples submitted to U.S. Geological Survey Reston Stable Isotope Laboratory.

Table 2. Analytical methods for analyses of water-quality samples collected in the Arkansas River Basin study areas, 2009–2010.

[oC, degrees Celsius; MPN, most probable number; mL, milliliter; S.U., standard units; μS/cm, microsiemens per centimeter; μg/L, micrograms per liter; mg/L, 

milligrams per liter; ICP, inductively coupled plasma; N, nitrogen; MS, mass spectrometry; AES, atomic emission spectrometer; DI-IRMS, dual inlet isotoperatio mass spectrometer]
14 Characterization Studies of the Arkansas River, Colorado, 2009–2010

Site name Site 

type

Field 

measurements

Major 

ions

Dissolved 

solids Nutrients Trace metals Boron1 Silica2

Hydrogen/

Oxygen 

isotope

Upper Arkansas River Basin study reach: Canon City to Pueblo Reservoir (in downstream order)

Ark at Canon City MS x x x -- x -- -- x

Sand Cr TR x -- x -- -- -- -- --

Fourmile Cr TR x -- x -- x -- -- --

Ark nr Canon City MS x x x -- x -- -- x

Oil Cr Ditch return TR x -- x -- -- -- -- --

Minnequa Canal3 DV -- -- -- -- -- -- -- --

Chandler Cr TR x -- x -- --- -- -- --

Oak Cr TR x -- x -- -- -- -- --

Oak Cr trib TR x -- x -- -- -- -- --

Ark trib 2 at CR 119 TR x -- x -- x4 -- -- --

Ark trib 1 at CR 119 TR x -- x -- -- -- -- --

Ark rtn at Hwy 115 TR x -- x -- -- x -- --

Fremont WWTP WW x -- x -- x5 x -- x

Brush Hollow TR x -- x -- x -- -- --

Hardscrabble Cr TR x -- x -- x -- -- --

Ark at Portland MS x x x x x -- -- x

Bear Cr TR x -- x -- x -- -- --

Ark trib 1 at CR 112 TR x x x -- --6 -- x --

Ark trib 2 at CR 112 TR x -- x --6 --6 -- x --

Ark trib 1 at Hwy 120 TR x -- x -- -- -- -- --

Ranger Cr TR x -- x -- x4 -- -- --

Beaver Cr TR x -- x -- -- -- -- --

Ark nr Portland MS x x x x x -- -- x

Lower Arkansas River Basin study reach: Avondale to Las Animas (in downstream order)

Sixmile Cr TR x -- x x4 x4 -- x --

Collier Ditch DV x7 -- -- -- -- -- -- --

IR-43 TR x -- x -- x4 -- x4 --

Colorado Canal DV x7 -- -- -- -- -- -- --

Highline Canal DV x7 -- -- -- -- -- -- --

Oxford Canal DV x7 -- -- -- -- -- -- --

Ark nr Nepesta MS x x x x x x x x

Chicosa Cr TR x -- x -- x4 -- x4 --

Otero Canal DV x7 -- -- -- -- -- -- --

Table 3. Periodic water-quality sampling sites in the Upper Arkansas River Basin and Lower Arkansas River Basin study areas, 

2009–2010.

[Complete site name given in table 1; MS, main stem; TR, tributary; DV, diversion canal or ditch; WWTP, wastewater treatment plant; --, none available]
Methods 15

1 Dissolved boron data only collected in 2009.

2 Dissolved silica data only collected in 2010.

3 No samples were collected at this site; water-quality data from Ark nr Canon City were used as a surrogate. 

4

 Selected constituents sampled no more than twice at this site for regional characterization purposes.

5 Selenium was the only trace metal targeted at the Fremont County wastewater treatment plant.

6

 Requested analyses were not completed due to sample loss at the U.S. Geological Survey National Water Quality Laboratory. 

7

 Field measurements included water temperature and specific conductance but not pH. 

8

 Samples collected only in 2009 at the La Junta WWTP. 

Site name Site 

type

Field 

measurements

Major 

ions

Dissolved 

solids Nutrients Trace metals Boron1 Silica2

Hydrogen/

Oxygen 

isotope

Lower Arkansas River Basin study reach: Avondale to Las Animas (in downstream order)—Continued

Hungerford Hollow TR x -- x -- x4 -- x4 --

Apishapa R TR x -- x -- x4 -- x4 --

Catlin Canal DV x7 -- -- -- -- -- -- --

Holbrook Ditch DV x7 -- -- -- -- -- -- --

Ft Lyon Storage Canal DV x7 -- -- -- -- -- -- --

Rocky Ford Ditch DV x7 -- -- -- -- -- -- --

Patterson Hollow TR x -- x -- x4 -- x4 --

Ark nr Rocky Ford MS x x x x x x x x

Timpas Cr TR x -- x -- x4 -- x4 --

Ark at Swink MS x x x x x x x x

Ft Lyon Canal DV x7 -- -- -- -- -- -- --

Crooked Arroyo TR x -- x -- x4 -- x4 --

Anderson Arroyo TR x -- x -- x4 -- x4 --

Ark at La Junta MS x x x x x x x x

La Junta WWTP8 WW x -- x -- x x -- x

Table 3. Periodic water-quality sampling sites in the Upper Arkansas River Basin and Lower Arkansas River Basin study areas, 

2009–2010.—Continued

[Complete site name given in table 1; MS, main stem; TR, tributary; DV, diversion canal or ditch; WWTP, wastewater treatment plant; --, none available]
16 Characterization Studies of the Arkansas River, Colorado, 2009–2010

were made to the estimates just prior to the synoptic event 

using available real-time data. In general, sampling times for 

tributary and diversion sites were scheduled to coincide with 

the sampling time for the main-stem parcel of water in the 

Arkansas River. 

Synoptic sampling events in the UARB occurred on 

September 2, 2009, June 22, 2010, July 20, 2010, and September 15, 2010. During each sampling event, samples were 

collected during a single day along the main-stem reach of the 

Arkansas River (approximately 23 mi) as determined by the 

estimates of travel time between sites. Tributary sites were 

sampled accordingly, given the travel-time estimates. Equalwidth incremental sampling techniques were used to collect 

a representative sample when streamflow conditions allowed. 

During high-flow conditions, bank sampling or multiple vertical techniques were used to ensure the safety of the sampling 

crews. Sampling techniques used in this study are described 

in U.S. Geological Survey (2006). A list of sampling sites and 

general types of analyses can be found in table 4. The analytical list of constituents is similar to those for the periodic sampling in the UARB (table 3); however, analyses of Escherichia

coli and total coliform bacteria were included as part of the 

synoptic effort. Nutrient samples requiring refrigeration and 

time-dependent samples generally were not collected as part 

of the synoptic sampling events. 

Synoptic sampling events in the LARB occurred on 

October 20–23, 2009, November 16–19, 2009, June 8–10, 

2010, and August 17–20, 2010. Samples were collected over 

the course of several days as determined by estimates of 

travel time between sites; the overall length of this river reach 

was approximately 76 mi. Tributary and diversion sites were 

assigned sample times to coincide with the estimated sampling 

time for the Arkansas River main-stem parcel at the mouth of 

the tributary or diversion site. 

Synoptic sampling in the LARB was problematic for 

several reasons, and as such, the approach taken for sampling 

this study area was substantially different than that followed 

for the UARB. The study reach was too long to allow for 

typical synoptic sampling techniques because some of the 

scheduled sampling times were at night, creating a substantial safety hazard for the sampling crews. Additionally, water 

operations in the LARB are highly variable. Releases from 

Pueblo Reservoir and calls for diversions to canals and ditches 

can quickly and substantially change streamflow conditions 

in the Arkansas River. In order to circumvent these issues, 

automated samplers were used at main-stem sites to collect a 

single composited sample over a 24-hour period (one sample 

aliquot collected every 2 hours). This sampling strategy 

resulted in the collection of a sample that represented a daily 

mean concentration of analytes that would be less susceptible 

to short temporal changes in streamflow. 

Composited 24-hour samples were not collected at tributary sites. These sites were less susceptible to large changes in 

streamflow and generally were considered to be in a “quasi” 

steady-state condition over the course of the sampling period. 

As such, synoptic sampling of tributary sites in the LARB was 

scheduled to coincide as closely as practical with the estimated 

arrival time of the main-stem parcel being sampled at the 

mouth of a tributary, and the collected samples were assumed 

to represent a daily mean concentration. 

Statistical Analyses

 The nonparametric Kruskall-Wallis test was used to 

determine whether there were statistically significant differences in median concentrations of constituents (such as, 

dissolved solids, dissolved selenium, and dissolved uranium) 

and dissolved-solids loads between selected sites. Nonparametric analysis allows the user to analyze data without assuming an underlying distribution (Helsel and Hirsch, 2002). The 

probability value (p-value) is the probability of rejecting the 

null hypothesis if it is true. A p-value of less than 0.05 was 

used in this report to indicate if a statistical test was significant 

and reject the null hypothesis. The nonparametric MannWhitney test examines the equality of two population medians. A confidence coefficient of 0.95 and error level of test α = 

0.05 was selected for the test. As with the Kruskall-Wallis test, 

a p-value of less than 0.05 is significant and the null hypothesis is rejected. A nonparametric multiple-comparison Tukey’s 

test (Helsel and Hirsch, 2002) was used to determine significant differences between sampling sites on the main stem of 

the Arkansas River if the results of the Kruskall-Wallis test 

indicated significant differences in constituent concentrations 

or loads between selected sites. 

Simple linear regression was used to determine the best 

straight-line fit between two variables (one dependent and 

one independent variable) and obtain a predictive equation. 

The coefficient of determination, r-squared (r2

), is a statistical 

measure of how well the predictive equation (regression line) 

fits the real data points (Helsel and Hirsch, 2002). It varies 

between 0 and 1 and indicates the amount of variability in 

the dependent variable described by the independent variable 

(an r2

 value of 0 indicates that there is no relation between 

the dependent and independent variables; and an r2

 value of 1 

indicates that there is a perfect relation between the dependent 

and independent variables). The range and description of r2

 in 

this report is 0 to 0.50 is weak, 0.51 to 0.85 is moderate, and 

greater than 0.86 is a strong correlation.

Quality-Assurance Procedures

Quality-assurance procedures followed USGS protocols (U.S. Geological Survey, 2006), and quality assurance/

quality-control samples accounted for about 10 percent of the 

total number of samples collected in the study. Part per-billion 

collection protocols (U.S. Geological Survey, 2006) were 

used for equipment cleaning and handling, sample and blank 

collection, and field-processing procedures to minimize contamination and bias. Equal-width increment (EWI) samples 

collected by USGS personnel and point samples collected by 

automated samplers were compared to evaluate any potential 
Methods 17

Site name Site 

type

Field 

measurements

Major 

ions

Dissolved 

solids

NutrientsTrace 

metals Boron1 Silica2

Hydrogen/

oxygen 

isotope 

E. coli

Total 

coliform 

bacteria

Upper Arkansas River Basin study reach: Canon City to Pueblo Reservoir (in downstream order)

Ark at Canon City MS x x x -- x x x x x x

Sand Cr TR x -- x -- -- -- x x x x

Fourmile Cr TR x -- x -- -- -- x x x x

Ark nr Canon City MS x x x -- x x x x x x

Oil Cr Ditch return TR x -- x -- -- -- -- x x x

Minnequa Canal3 DV -- -- -- -- -- -- -- -- -- --

Chandler Cr TR x -- x -- -- -- x x x x

Oak Cr TR x -- x -- -- -- x x x x

Oak Cr trib TR x -- x -- -- -- -- x x x

Ark trib 2 at CR 119 TR x -- x -- -- -- -- x x x

Ark trib 1 at CR 119 TR x -- x -- -- -- x x x x

Ark rtn at Hwy 115 TR x x4 x -- x4 x4 x4 x x x

Fremont WWTP WW x -- x -- x5 -- -- x x x

Brush Hollow TR x -- x -- -- -- x x x x

Hardscrabble Cr TR x -- x -- -- -- x x x x

Ark at Portland MS x x x x x x x x x x

Bear Cr TR x -- x -- -- -- x x x x

Ark trib 1 at CR 112 TR x x4 x -- -- -- x x x x

Ark trib 2 at CR 112 TR x -- x -- -- -- x x x x

Ark trib 1 at Hwy 120 TR x -- x -- -- -- x x x x

Ranger Cr TR x -- x -- -- -- x x x x

Beaver Cr TR x -- x -- -- -- x x x x

Ark nr Portland MS x x x x x x x x x x

 Lower Arkansas River Basin study reach: Avondale to Las Animas (in downstream order)

Ark nr Avondale MS x x x -- x x x x x x

Sixmile Cr TR x x x x4 x x x x x x

Collier Ditch DV x6 -- x -- x -- -- x -- --

IR-43 TR x x x x4 x x x x x x

Colorado Canal DV x6 -- x -- x -- -- x -- --

Huerfano R TR x x x x4 x x x x x x

Highline Canal DV x6 -- x -- x -- -- x -- --

Table 4. Synoptic water-quality sampling sites in the Upper Arkansas River Basin and Lower Arkansas River Basin study areas, 

2009–2010.

[Complete site name given in table 1; E. coli, Escherichia coli; MS, main stem; TR, tributary; DV, diversion canal or ditch; WWTP, wastewater treatment plant; 

--, none available]
18 Characterization Studies of the Arkansas River, Colorado, 2009–2010

Site name Site 

type

Field 

measurements

Major 

ions

Dissolved 

solids

NutrientsTrace 

metals Boron1 Silica2

Hydrogen/

oxygen 

isotope 

E. coli

Total 

coliform 

bacteria

Lower Arkansas River Basin study reach: Avondale to Las Animas (in downstream order)—Continued

Ark R blw Highline 

Canal7 MS x x x -- x -- x x x x

Oxford Canal DV x6 -- x -- x -- -- x -- --

Ark nr Nepesta MS x x x x x x x x x x

Chicosa Cr TR x x x x4 x x x x x x

Otero Canal DV x6 -- x -- x -- -- x -- --

Hungerford Hollow TR x x x x4 x x x x x x

Apishapa R TR x x x x4 x x x x x x

Catlin Canal DV x6 -- x -- x -- -- x -- --

Ark at Catlin Dam MS x x x x x x x x x x

Holbrook Ditch DV x6 -- x -- x -- -- x -- --

Ft Lyon Storage Canal DV x6 -- x -- x -- -- x -- --

Rocky Ford Ditch DV x6 -- x -- x -- -- x -- --

Patterson Hollow TR x x x x4 x x x x x x

Ark nr Rocky Ford MS x x x x x x x x x x

Timpas Cr TR x x x x4 x x x x x x

Ark at Swink MS x x x x x x x x x x

Ft Lyon Canal DV x6 -- x -- x -- -- x -- --

Crooked Arroyo TR x x x x4 x x x x x x

Anderson Arroyo TR x x x x4 x x x x x x

Ark at La Junta MS x x x x x x x x x x

La Junta WWTP7 WW x -- x -- x x -- x x x

Ark at Las Animas8 MS x x x x x x x x x x

1 Dissolved boron data only collected in 2009.

2 Dissolved silica data only collected in 2010.

3 No samples were collected at this site; water-quality data from Ark nr Canon City were used as a surrogate. 

4

 Selected constituents sampled once at this site for regional characterization purposes.

5 Seleniumwas the only trace metal targeted at the Rainbow Park regional wastewater treatment plant.

6

 Field measurements included water temperature and specific conductance but not pH.

7

 Only sampled in 2009.

8

 Synoptic sampling at Ark at Las Animas did not use automated pumping sampler.

Table 4. Synoptic water-quality sampling sites in the Upper Arkansas River Basin and Lower Arkansas River Basin study areas, 

2009–2010.—Continued

[Complete site name given in table 1; E. coli, Escherichia coli; MS, main stem; TR, tributary; DV, diversion canal or ditch; WWTP, wastewater treatment plant; 

--, none available]
Methods 19

sampling bias associated with the location of the automated 

sampler intakes and any variations in sample chemistry 

relative to samples collected from the entire channel cross 

section. Water-quality-assurance samples were analyzed by the 

USGS National Water-Quality Laboratory (NWQL), Denver, 

Colorado.

Field-blank and field-split samples were collected to evaluate potential sample contamination and variability associated 

with sampling processing (U.S Geological Survey, 2006). As 

part of this study, four field-blank quality-assurance samples 

were collected. Field blanks are collected and processed at the 

field site in the same manner and using the same equipment 

as the environmental samples except inorganic-free blank 

water was used instead of an environmental sample. Certified 

inorganic-free blank water was obtained from the NWQL. 

Field blanks are used to evaluate the potential for sample contamination as a consequence of equipment cleaning and handling, sample collection, and (or) laboratory procedures. Field 

blank analyses (table 5) indicated that equipment cleaning and 

handling protocols, field sampling, and laboratory processing 

procedures were generally effective in preventing or minimizing contamination of water-quality samples collected. Most 

constituents analyzed in field blanks were not quantified and 

were reported as a less than value. 

One field blank indicated potential contamination for one 

dissolved constituent: ammonia, as nitrogen (0.036 mg/L). This 

field-blank concentration represented about 65 to 80 percent of 

the dissolved ammonia present in the environmental sample 

(0.054 mg/L as nitrogen and 0.057 mg/L as ammonia). The 

source of ammonia contamination in this field blank is unclear. 

Concentrations of other nitrogen-based nutrient constituents 

analyzed were below laboratory reporting levels, and specific 

blanks that targeted different steps along the processing procedure were not collected. However, several potential sources of 

ammonia have been identified. Animal manure odor from local 

feedlots and (or) areas of manure application were present 

during numerous site inspections, and ammonia is contained in 

animal manure odor (McGinn and others, 2003). Whether or 

not airborne ammonia concentrations were noticeable 

during sampling on the date of concern was not noted; however, sample contamination by short-term exposure to airborne 

ammonia, if present, could possibly contribute to the concentrations observed in the field blank. Contamination during field 

and (or) laboratory procedures also is a possibility. Increasing 

the number of blank samples with nutrient analyses may be 

a benefit to future sampling efforts. The additional nutrient 

analyses would permit evaluation of ammonia contamination 

as an isolated occurrence or a systemic problem. Also, the 

addition of blank samples that target procedural steps could 

potentially provide insight into contamination sources.

Eighteen split samples were collected and processed in 

the field to evaluate variability in sample concentration as a 

function of sample processing. The relative-percent difference 

(RPD) between split samples was computed when the constituent concentration of each respective subsample was greater 

than the laboratory reporting level: 

 (1)

 

where

RPD is relative percent difference,

R1 is concentration of split sample 

1, and

R2 is concentration of split sample 2.

Ten field-split sample pairs were evaluated for differences in concentration of nutrients (table 6). RPDs of concentrations of most split pairs were very similar; 96.3 percent of 

split pair concentrations varied by less than 5 percent. RPDs 

of concentrations of ammonia, as N, varied by 10.00 percent, 

and represented the only occurrence of RPDs between nutrient 

concentrations in field-split sample pairs greater than 

5 percent. The large RPDs of ammonia concentrations, as 

N, were disregarded due to the possible sample contamination as previously described. Sixteen field-split samples were 

evaluated for differences in concentration of major, minor, and 

trace inorganics. RPDs of major inorganic concentration of all 

field-split sample pairs were less than 2.5 percent. RPDs of 

minor and trace inorganic concentration were less than 

5 percent in 24 of 26 sample pairs (92.3 percent). Eighteen 

field-split samples were evaluated for differences in physical 

properties. RPD in physical properties of all field-split sample 

pairs varied by less than 5 percent. Sixteen field-split samples 

were evaluated for differences in concentration of uranium. 

RPDs of uranium concentration of 15 of 16 (93.8 percent) 

field-split sample pairs were less than 5 percent. Based on the 

low occurrence of RPD greater than 5 percent in constituent 

concentrations in field-split sample pairs, sample-processing 

procedures were considered to be consistent and to have minimal effect on constituent concentrations, with the exception of 

ammonia. 

Differences in sampling methods can introduce bias into 

a data set. Two methods were used to collect water-quality 

samples—depth-integrated EWI and discrete-point sampling. 

The EWI method involves collection of a depth-integrated 

sample at discrete intervals across the entire width of the channel. The discrete-point sampling consists of water collected at 

one discrete point, generally close to the stream bank, by an 

automated pumping sampler. The primary source of potential 

bias between the two sampling methods was identified as 

unequal mixing of dissolved and suspended constituents with 

depth and across the width of the channel at the sampling site. 

To evaluate potential bias based on differences in sampling 

methods, 18 EWI/point-sample pairs were collected (table 7). 

Two or more sample pairs were collected at most sites. Computed RPDs were used to evaluate differences in dissolved 

selenium concentrations and SC in EWI and point samples.

An evaluation of the differences between the EWI- and 

point-sample concentrations indicates point-sample concentrations of dissolved selenium generally were larger than 

the dissolved selenium concentration in the associated EWI 

sample for each station. Point-sample concentrations were 

RPD R R

R R = −

+

























1 2

1 2

2

*100
20 Characterization Studies of the Arkansas River, Colorado, 2009–2010

Field measurement or analyzed 

constituents Station identification number

Date

07121700 07121700 07109500 381334104142301

12/16/2009 6/29/2010 8/17/2010 11/16/2009

Ammonia, mg/L as N NA 0.036 NA NA

Nitrite, mg/L as N NA <0.002 NA NA

Ammonia plus organic nitrogen, 

mg/L as N NA <0.1 NA NA

Nitrite plus nitrate, mg/L as N <0.04 <0.016 NA NA

Phosphorus, mg/L NA <0.008 NA NA

Orthophosphate, mg/L as P NA <0.008 NA NA

Calcium, mg/L <0.04 <0.04 <0.04 NA

Magnesium, mg/L <0.016 <0.016 <0.016 NA

Sodium, mg/L <0.1 <0.1 <0.1 NA

Potassium,mg/L <0.06 <0.06 <0.06 NA

Chloride, mg/L <0.12 <0.12 <0.12 NA

Sulfate, mg/L <0.18 <0.18 <0.18 NA

Fluoride, mg/L <0.08 <0.08 <0.08 NA

Silica, mg/L NA <0.1 <0.1 NA

ANC <8 <8 <8 NA

Boron, μg/L <3 NA NA NA

Zinc, μg/L NA <2.8 NA NA

Aluminum, μg/L NA <3.4 NA NA

Selenium, μg/L <0.04 <0.04 <0.04 <0.04

Residue on evaporation, mg/L <10 <10 <10 <10

Uranium (natural), μg/L <0.01 <0.01 <0.01 <0.01

Table 5. Results of field blank quality-assurance samples collected in the Arkansas River Basin study areas, 2009–2010.

[mg/L, milligrams per liter; N, nitrogen; P, phosphorus; ANC, acid neutralizing capacity; <, less than; μg/L, micrograms per liter; NA, not 

analyzed]
Methods 21

Table 6. Relative percent difference between replicate quality-assurance samples collected in the Arkansas River Basin study areas, 

2009–2010.

[ID, identification number; sites listed in table 1; mg/L, milligrams per liter; N, nitrogen; P, phosphorus; NH4, ammonia; ANC, acid neutralizing capacity; mg/L, 

micrograms per liter; red values indicate relative percent difference greater than 5 percent;--, not applicable]

Station ID Date Time

Ammonia plus

organic nitrogen, 

mg/L as N

Nitrite plus

nitrate,

mg/L as N

Ammonia,

mg/L

as N

Nitrite,

mg/L

as N

Orthophosphate,

mg/L as P

Phosphorus,

mg/L

07097000 9/15/2010 1500 –1.35 0.10 -- –3.85 0.00 0.42

07099200 12/1/2009 1430 –1.11 –0.65 -- 0.00 –2.63 4.43

07099200 7/20/2010 1720 –2.22 0.00 -- -- –0.85 –0.25

07117500 8/31/2009 1645 -- 0.36 -- -- -- --

07117600 6/8/2010 1600 -- 0.00 -- -- -- --

07120500 9/22/2009 1345 -- 0.00 -- -- -- --

07120500 7/27/2010 1320 1.25 0.68 -- 0.00 0.00 0.51

07121700 7/29/2009 0930 -- –0.44 -- -- -- --

07121700 11/9/2009 0830 -- 0.26 -- -- -- --

07121700 6/29/2010 1330 2.29 0.93 10.00 0.00 1.24 –2.18

Station ID Date Time ANC Calcium,

mg/L

Chloride,

mg/L

Fluoride,

mg/L

Magnesium,

mg/L

Potassium,

mg/L

Silica,

mg/L

Sodium

adsorption

ratio, water,

number

Sodium,

mg/L

Sodium, water, 

percent in

equivalents of

major cations

Sulfate, 

07097000 9/15/2010 1500 0.00 –0.65 0.00 0.00 0.00 0.08 –0.24 –0.31 –0.64 0.00 0.00

07099200 12/1/2009 1430 0.00 –0.58 0.07 0.00 –0.42 –1.17 -- –0.46 –0.64 0.00 0.12

07099200 7/20/2010 1720 0.54 –0.71 –0.41 –1.49 –1.72 –0.75 –0.76 0.49 0.18 1.43 –0.03

07109500 8/17/2010 1000 –0.20 0.48 –0.12 0.31 –0.11 –0.42 0.28 –0.45 –0.18 0.00 0.00

07117500 8/31/2009 1645 1.81 0.15 0.00 0.00 0.09 0.29 -- 0.00 0.22 0.00 0.00

07117600 6/8/2010 1600 -- -- -- -- -- -- –0.26 -- -- -- --

07120500 9/22/2009 1345 –0.14 –0.61 0.00 –0.27 –0.17 –0.53 -- –0.26 –0.48 0.00 –0.43

07120500 11/18/2009 0445 0.00 –0.59 1.16 –0.73 –0.48 –0.69 -- –0.53 –0.68 –0.94 0.08

07120500 7/27/2010 1320 –0.15 –0.23 –0.11 –0.29 –0.41 –0.31 0.77 0.09 –0.17 0.63 0.36

07121700 7/29/2009 0930 0.92 0.19 0.10 –0.31 0.13 0.42 -- 0.34 0.40 0.00 0.00

07121700 11/9/2009 0830 0.00 0.82 –0.08 –0.26 0.30 0.85 -- –0.61 –0.21 0.00 0.00

07121700 6/29/2010 1330 –0.35 0.00 0.44 –0.76 0.17 1.01 0.31 1.33 1.45 1.11 0.35

07123000 10/22/2009 2000 0.12 –0.30 –0.96 –0.27 –0.58 –0.36 -- 0.00 –0.22 0.91 –0.20

07123000 6/9/2010 1810 0.17 –0.09 0.31 –0.36 –0.10 –0.30 0.35 –0.22 –0.11 0.00 0.21

07124000 8/20/2010 0900 0.00 –0.13 0.05 –0.50 0.00 0.16 –0.36 0.11 0.00 0.00 0.03

382337105014600 9/15/2010 1405 -- -- -- -- -- -- -- 0.49 -- -- --

381334104142301 6/8/2010 1245 -- -- -- -- -- -- -- -- -- -- --

Station ID Date Time Aluminum,

μg/L

Boron,

μg/L

Cadmium,

μg/L

Selenium,

μg/L

Uranium

(natural),

μg/L

Hardness,

mg/L as

CaCO3

Residue on

evaporation,

dried at 180 degrees 

Celsius, water, fil

Residue,

sum of

constituents,

mg/L

Residue, 

tons per 

acre-ft

07097000 9/15/2010 1500 -- -- -- 0.00 –0.95 –0.45 –0.07 -- 0.00

07099200 12/1/2009 1430 -- -- -- -- 0.15 –0.64 0.00 –0.11 0.00

07099200 7/20/2010 1720 1.74 -- 0.00 0.53 –0.18 –1.02 –3.03 -- –2.31

07109500 8/17/2010 1000 -- -- -- 0.32 –0.07 0.28 –0.65 0.06 –0.79

07117500 8/31/2009 1645 -- –0.54 -- –0.88 –0.84 0.14 0.22 0.42 0.27

07117600 6/8/2010 1600 -- -- -- 0.43 –0.51 -- –0.03 -- 0.00

07120500 9/22/2009 1345 -- –2.29 -- –0.67 –0.23 –0.46 0.18 –0.36 0.19

07120500 11/18/2009 0445 -- –0.14 -- –0.50 –0.35 –0.57 0.00 –0.22 0.00

07120500 7/27/2010 1320 14.36 -- -- 0.00 0.24 –0.26 0.00 0.24 0.00

07121700 7/29/2009 0930 -- –7.86 -- –2.82 –3.33 0.20 –0.14 0.27 –0.20

07121700 11/9/2009 0830 -- 1.83 -- 1.21 0.38 0.58 0.68 0.12 0.84

07121700 6/29/2010 1330 –0.78 -- -- 0.00 0.00 0.00 0.66 0.30 0.60

07123000 10/22/2009 2000 -- –2.99 -- 0.69 –5.27 –0.35 0.00 –0.22 0.00

07123000 6/9/2010 1810 -- -- -- 0.40 0.65 –0.08 –0.70 0.10 –0.74

07124000 8/20/2010 0900 -- -- -- 0.21 0.10 0.00 –0.87 0.00 –0.89

381334104142301 6/8/2010 1245 -- -- -- 0.44 –0.08 -- –0.43 -- –0.53

382322105040501 6/22/2010 1230 -- -- -- 0.65 -- -- 0.17 -- 0.25

382337105014600 9/15/2010 1405 -- -- -- -- -- -- 0.00 -- 0.06
22 Characterization Studies of the Arkansas River, Colorado, 2009–2010

 Station 

identification 

 number

Date Sample 

type

ConcentrationSample 

type

Concentration Rmk RPD Average 

RPD

Point 

sample 

bias for station

Concentration 

difference

Selenium, dissolved, in μg/L

07109500 10/20/2009 EWI 13.1 PNT 13.5 -3.01

-5.69 (+)

-0.4

07109500 11/16/2009 EWI 13.1 PNT 14.0 -6.64 -0.9

07109500 8/17/2010 EWI 7.8 PNT 8.4 -7.41 -0.6

07117500 10/20/2009 EWI 10.7 PNT 8.5 22.9

6.93 (-)

2.2

07117500 11/16/2009 EWI 11.7 PNT 11.4 2.60 0.3

07117500 8/17/2010 EWI 6.2 PNT 6.5 -4.72 -0.3

07119700 10/20/2009 EWI 8.8 PNT 9.1 -3.35

-7.71 (+)

-0.3

07119700 11/16/2009 EWI 10.9 PNT 12.3 -12.1 -1.4

07120500 10/21/2009 EWI 9.8 PNT 10.9 -10.6

-5.89 (+)

-1.1

07120500 11/17/2009 EWI 13.7 PNT 14.1 -2.88 -0.4

07120500 8/19/2010 EWI 9.4 PNT 9.8 -4.17 -0.4

07121700 10/22/2009 EWI 10.3 PNT 12.2 -16.9

-13.6 (+)

-1.9

07121700 11/18/2009 EWI 16.6 PNT 19.3 -15.0 -2.7

07121700 8/19/2010 EWI 10.9 PNT 11.9 -8.77 -1.0

07123000 10/22/2009 EWI 11.5 PNT 11.4 0.87

1.39 (-)

0.1

07123000 11/19/2009 EWI 17.5 PNT 17.4 0.57 0.1

07123000 8/19/2010 EWI 11.2 PNT 10.9 2.71 0.3

381317104135400 8/17/2010 EWI 7.7 PNT 7.9 -2.56 -0.2

Specific conductance, in μS/

cm at 25oC

07109500 10/20/2009 EWI 855 PNT 876 -2.43

-2.65 (+)

-21

07109500 11/16/2009 EWI 1010 PNT 1030 -1.96 -20

07109500 8/17/2010 EWI 714 PNT 740 -3.58 -26

07117500 10/20/2009 EWI 919 PNT 932 -1.40

-0.95 (+)

-13

07117500 11/16/2009 EWI 1070 PNT 1090 -1.85 -20

07117500 8/17/2010 EWI 720 PNT 717 0.42 3

07119700 10/20/2009 EWI 985 PNT 1110 -11.9

-8.38 (+)

-125

07119700 11/16/2009 EWI 1010 PNT 1060 -4.83 -50

07120500 10/21/2009 EWI 1240 PNT 1310 -5.49

-3.18 (+)

-70

07120500 11/17/2009 EWI 1400 PNT 1410 -0.71 -10

07120500 8/19/2010 EWI 1180 PNT 1220 -3.33 -40

07121700 10/22/2009 EWI 1290 PNT 1530 -17.0

-10.3 (+)

-240

07121700 11/18/2009 EWI 1860 PNT 2140 -14.0 -280

07121700 8/19/2010 EWI 1310 PNT 1310 0.00 0

07123000 10/22/2009 EWI 1530 PNT 1550 -1.30

-1.06 (+)

-20

07123000 11/19/2009 EWI 2090 PNT 2100 -0.48 -10

07123000 8/19/2010 EWI 1410 PNT 1430 -1.41 -20

381317104135400 8/17/2010 EWI 753 PNT 767 -1.84 -14

Table 7. Relative percent differences in concentration of dissolved selenium and specific conductance between equal width increment 

and discrete-point samples collected in the Arkansas River Basin study areas, 2009–2010.

[ID, identification number; RPD, relative percent difference; μg/L, microgram per liter; μS/cm, microsiemens per centimeter; oC, degree Celsius; EWI, 

equal width increment; PNT, point; Rmk, remark; (+), positive bias, discrete-point-sample concentration generally greater than EWI sample concentration; 

(-), negative bias, discrete-point-sample concentration generally less than EWI sample concentration; sites listed in table 1]
Characteristics of Streamflow from Canon City to Near Portland 23

larger than the associated EWI sample concentration in 13 of 

18 sample pairs. However, the range in the absolute values of 

the differences was small, 0.1 to 2.7 μg/L. RPDs were greater 

than 5 percent in 8 of 18 samples. The perceived large RPDs 

(greater than 5 percent) between concentrations of eight EWI 

and point-sample pairs is an artifact of the very small concentrations measured in the EWI and point samples; large RPDs 

can be reported from small differences if the initial concentrations are very small. 

SC was used as a surrogate for dissolved solids and major 

ions. SC of point samples also tended to be larger than the SC 

in the associated EWI samples for each station; point-sample 

values were larger than the associated EWI sample values 

in 16 of 18 sample pairs. Similar to differences in dissolved 

selenium concentrations, the range in the absolute values of 

the differences between the SC of the EWI sample and the SC 

of the point sample generally were small. RPDs of specific 

conductance between EWI and point samples were greater 

than 5 percent in 4 of 18 sample pairs.

The Mann-Whitney test (Helsel and Hirsch, 2002) was 

used to evaluate the significance of perceived differences in 

the dissolved selenium concentration and specific conductance 

between the EWI and point samples. The p-value, for differences between median dissolved selenium concentrations, 

0.5690, and specific conductance, 0.5583, in EWI and point 

samples were greater than the α value of 0.05. Therefore, there 

are no statistically significant differences between the concentrations of dissolved selenium or specific conductance values 

of the EWI and point samples.

The general tendency for point-sample concentrations to 

be greater than EWI sample concentrations suggests a small 

positive bias in dissolved selenium concentration and SC in 

point samples. The small range of differences in dissolved 

selenium concentration and SC between EWI and point 

samples, however, indicates that the stream generally was well 

mixed relative to the collection locations of the point sample 

and the EWI sample on the days sampled. Therefore, the point 

samples collected by automated pumping samplers are considered to adequately represent the chemistry of the stream, and 

the use of automated pumping samplers is not considered to 

have introduced substantial bias into the data set.

Characteristics of Streamflow, Water 

Quality, and Instantaneous Dissolved 

Solids, Selenium, and Uranium Loads 

in the Arkansas River Basin from 

Canon City to Near Portland

Water quality in the reach from Canon City to near 

Portland (UARB) is influenced by natural and anthropogenic 

factors associated with land- and water-use practices. Streamflow was measured at both long-term and at study-specific 

temporary gages for use in water-quality analysis and computing instantaneous loads. Data for periodic as well as synoptic water-quality samples were used to assess water quality 

and instantaneous loads of dissolved solids, selenium, and 

uranium. 

Streamflow Characteristics

In the UARB, long-term (pre-2000) streamflow data 

are available for two stations—Ark at Canon City and Ark at 

Portland. These stations are operated and the data administered by the CODWR (Colorado Division of Water Resources, 

2011). Temporary streamflow gages were installed at the Ark 

nr Canon City and Ark nr Portland stations for this study, and 

streamflow records were collected for 2009–2010. 

Streamflow data for 2009–2010 from both long-term 

Arkansas River stations were compared to data for 1990–2008 

(fig. 5). To increase the number of observations and the power 

of statistical tests in this study, streamflow data beginning in 

1990 were included in selected data sets. Summary statistics 

for streamflow for stations and time periods are shown in 

table 8. The 2009–2010 hydrographs for the sites at Ark at 

Canon City and Ark at Portland are similar to hydrographs 

for those same stations for previous years, with the exception 

of years 1995 and 2002. The high flows in 1995 and the low 

flows in 2002 account for the differences in the maximum 

and minimum flows between the historical record and the 

2009–2010 study period (fig. 5). However, for each of the two 

stations, the median and mean flows for the two periods are 

similar.

Seasonal hydrographs for 2009–2010 (fig. 6) are available for five tributary stations—Fourmile Cr, Brush Hollow, 

Hardscrabble Cr, Bear Cr, and Beaver Cr (fig. 2)—whereas 

only instantaneous flow data are available for other tributary 

stations and diversion sites in the study reach. Median flows 

for these five tributaries ranged from a low of 0.76 ft3

/s in 

Beaver Cr to a high of 24 ft3

/s in Fourmile Cr. The smallest 

minimum, 0.01 ft3

/s, was measured in Hardscrabble Cr; the 

largest maximum, 150 ft3

/s, was measured in Fourmile Cr. Of 

the seasonally measured tributaries, Fourmile Cr had the largest flow. 

Water-Quality Characteristics

Water samples for main-stem and tributary sites were 

analyzed for selected constituents to characterize the water 

quality in the UARB and to identify potential source areas for 

dissolved solids, selenium, and uranium loads. Constituents 

sampled include specific conductance, major ions, dissolved 

solids, nutrients, select dissolved trace elements (selenium and 

uranium), and hydrogen and oxygen isotopes. Larger scale 

Arkansas River Basin water-quality studies (Dash and Ortiz, 

1996; Ortiz and others, 1998) began in the 1990s; previous 

to this, studies were generally limited in scope and scale. 

To increase the number of observations and the power of 
24 Characterization Studies of the Arkansas River, Colorado, 2009–2010

0

A

0

2,000

3,000

4,000

5,000

6,000

7,000

Streamflow, in cubic feet per second

1/1/1990

1/1/1991

1,000

1/1/1992

1/1/1993

1/1/1994

1/1/1995

1/1/1996

1/1/1997

1/1/1998

1/1/199

1/1/2000

1/1/2001

1/1/2002

1/1/2003

1/1/2004

1/1/2005

1/1/2006

1/1/2007

1/1/2008

1/1/20091/1/2010

8,000

2,000

3,000

4,000

5,000

6,000

7,000

Streamflow, in cubic feet per second

1/1/1990

1/1/1991

1/1/1992

1/1/1993

1/1/1994

1/1/1995

1/1/1996

1/1/1997

1/1/1998

1/1/1999

1/1/2000

1/1/2001

1/1/2002

1/1/2003

1/1/2004

1/1/2005

1/1/2006

1/1/2007

1/1/2008

1/1/2009

1/1/2010

1,000

B

Figure 5. Streamflow hydrographs for (A) Arkansas River at Canon City and (B) Arkansas River at Portland on the 

Arkansas River, 1990–2010.

Figure 6. Seasonal streamflow hydrographs for Beaver Creek, Bear Creek, Brush Hollow, Fourmile Creek, and Hardscrabble Creek, 

Upper Arkansas River Basin, 2009–2010.

0.01

0.1

1

10

100

1,000

Streamflow, in cubic feet per second

1/1/2009

2/1/2009

3/1/2009

4/1/2009

5/1/2009

6/1/2009

7/1/2009

8/1/2009

9/1/2009

10/1/2009

11/1/2009

12/1/2009

1/1/2010

2/1/2010

3/1/2010

4/1/2010

5/1/2010

6/1/2010

7/1/2010

8/1/2010

9/1/2010

10/1/2010

11/1/2010

12/1/2010

Brush Hollow

Beaver Creek

Fourmile Creek

EXPLANATION

Bear Creek

Hardscrabble Creek
Characteristics of Streamflow from Canon City to Near Portland 25

statistical tests in this study, water-quality data from Dash and 

Ortiz (1996) and Ortiz and others (1998) beginning in 1990 

were included in selected data sets.

Specific Conductance

Specific conductance (SC) values within the UARB varied spatially and temporally. SC values increased downstream 

from Ark at Canon City to Ark at Portland during 2009–2010, 

but the magnitude of the downstream increases between sites 

was variable (fig. 7). The lowest SC values (for all four sites) 

occurred in late May to early June during periods of high flow 

in both 2009 and 2010, whereas the highest values occurred 

in September during periods of low flow. When the flow in 

the river was dominated by snowmelt runoff during May and 

June, SC values varied little and the magnitude of the downstream increases between sites was small. During periods of 

low flow (base flow) in September, when local inflows, such 

as groundwater inflow and (or) irrigation return flow represent 

a greater portion of the total flow as compared to periods of 

high flows from snowmelt, SC values were higher and the 

downstream increases in SC values were larger between sites 

(fig. 7). Groundwater, recharged largely by irrigation water 

enriched in dissolved solids, discharging to the Arkansas River 

under low-flow conditions, may be providing a concentrated 

source of dissolved constituents, thereby increasing the SC in 

September. 

Major Ions

Water from main-stem sites beginning at Canon City are 

a calcium-magnesium-bicarbonate water type and transition 

downstream to a calcium-magnesium-sulfate water type at the 

Ark nr Portland site (fig. 8). A previous study by Ortiz and 

others (1998) reported calcium and bicarbonate as dominant 

ions in samples from main-stem Arkansas River site, but that 

study did not include the Ark nr Portland site. Changes in 

water type are likely due to changes in geology (fig. 9) from 

igneous and metamorphic to sedimentary bedrock and the 

changes in the chemical composition of the various bedrocks 

(Crouch and others, 1984). 

Table 8. Summary statistics for Arkansas River streamflow data 1990–2008 and 2009–2010 for Arkansas River at Canon City 

and Arkansas River at Portland gage stations.

Complete site name given in table 1; ft3/s, cubic feet per second]

Figure 7. Continuous specific conductance values at main-stem sites in the Upper Arkansas River Basin from Canon City to near 

Portland, 2009–2010.

0

100

200

300

400

500

600

700

800

9/9/2008

12/18/2008

3/28/2009

7/6/2009

10/14/2009

1/22/2010

5/2/2010

8/10/2010

11/18/2010

2/26/2011

Ark at Canon City

Ark nr Canon City

Ark at Portland

Ark nr Portland

EXPLANATION

Specific conductance, in microsiemens 

per centimeter at 25 degrees Celsius 

Site name Maximum flow, ft3

/s Minimum flow, ft3/s Median flow, ft3/s Mean flow, ft3/s

1990–2008 2009–2010 1990–2008 2009–2010 1990–2008 2009–2010 1990–2008 2009–2010

Ark at Canon City 6,580 4,940 98 144 451 457 686 728

Ark at Portland 6,830 5,010 91 184 465 477 728 749
26 Characterization Studies of the Arkansas River, Colorado, 2009–2010

Dissolved Solids

Main-stem DS (salinity) median concentrations, as 

previously reported by Dash and Ortiz (1996), Ortiz and others 

(1998), and Miller and others (2010), increase (fig. 10) 

downstream from the Ark at Canon City site (153 mg/L) to the 

Ark near Portland site (264 mg/L). Median DS concentrations 

in tributaries and the Rainbow Park WWTP outflow varied 

and ranged from a low of 213 mg/L in Oak Cr to a high of 

6,305 mg/L in water sampled from Ark trib 1 at CR 112. 

Miller and others (2010) reported higher DS concentrations 

in areas of the Arkansas River Basin is likely due to the use, 

return, and reuse of irrigation water and potential irrigation 

water recharge of local unconfined aquifers. Inflows from irrigation recharged groundwater may contribute to the high DS 

concentrations in the UARB tributaries, especially those tributaries below Ark trib 2 at CR 119 near Florence, Colo., and 

those that drain the Penrose, Colo., area where hay, pasture, 

orchard and cropland acres are irrigated (fig. 9). As with SC, 

the higher DS concentrations downstream from Florence can 

also be attributed to variations in geology (Miller and others, 

2010) and the chemical composition of the bedrock. 

In natural waters, SC is a good estimator of DS concentrations (Hem, 1985). To determine the relation, DS concentrations in the main-stem Arkansas River and selected tributaries 

were analyzed and statistically compared to SC. Table 9 lists 

results of the linear regression analysis of water samples 

from individual sites. Many of the sites, both main stem and 

tributary, even with a small number of samples (<10), indicate 

a statistically significant relation between DS and SC 

(r2

 >0.92). However, four tributaries downstream from Brush 

Hollow have DS and SC concentrations that are less correlated 

(r2

 values ranging from 0.039 to 0.740; table 9). Small data 

sets and outliers may be the cause of the less statistically significant relations between DS concentrations and SC in these 

tributaries. Ranger Cr, as an example, with r2

 = 0.039 (the 

lowest r2

 for the tributary samples), has two outlier samples 

in a small (n = 7) data set producing a regression with a weak 

correlation. Additional samples from a variety of streamflow 

regimes may improve the DS and SC relation for this Arkansas River tributary.

Based on DS concentrations and instantaneous streamflow values for samples collected during 2009–2010, the 

natural logarithm (log) of the DS concentration can be 

reasonably predicted from the natural log of streamflow 

(r2

 = 0.64; p-value <0.00001) for all three main-stem sites 

(fig. 11A). Tributary sites had fewer samples collected, and for 

the 2009–2010 data set, the correlation ( r2

 = 0.11) between 

the natural log of DS concentration and natural log of streamflow was weak (fig. 11B). Continued collection of paired DS 

and streamflow data, under a variety of flow regimes, might 

be used to build a statistical relation for each tributary, so that 

continuously monitored streamflow can be used to estimate 

DS for all seasonal streamflow conditions instead of collecting 

and analyzing several water-quality samples for DS.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Major ion concentration, 

in milliequivalents per liter 

Sodium

Magnesium

Calcium

Bicarbonate

Sulfate

Chloride

Arkansas River

at Canon City

Arkansas River

near Canon City

Arkansas River

at Portland

Arkansas River

near Portland

EXPLANATION

Figure 8. Comparison of median concentrations of six major ions in the Arkansas River, from Canon City to near 

Portland, 2009–2010.
0 1 2 

0 2 4 

Oak Creek

Beaver Creek Bear Creek

un-named

Chandler Creek

Hardscrabble Creek

Fourmile Creek

 

Sand

CANON CITY

FLORENCE

PENROSE

Base from U.S. Geological Survey Data, 2011, various scalesUniversal Tranverse Mercator projectionZone 13

Ark at Canon City

Fourmile Creek

Sand Creek

Oil Creek Ditch return

Ark near Canon City

Chandler Creek

Oak Creek

Oak Creek trib

Ark trib2 at CR119 Ark trib1 at CR119

Brush Hollow

Hardscrabble Cr

Bear Cr

Ark trib 1 at Hwy 120

Ranger Creek

Beaver Creek

Ark n

Rainbow Park WWTP

Ark at Portland

Ark trib 1 at CR 112

Ark trib 2 at CR 112

Brush Hollow

105°15' 105°10' 105°05' 105°00' 

38°25'

38°20'

Creek

Figure 9. Generalized geology and 2010 irrigated cropland in the Upper Arkansas River Basin, from Canon City to near Portland.
28 Characterization Studies of the Arkansas River, Colorado, 2009–2010

Nutrients

As part of this study, dissolved nutrient samples were 

only collected from the Ark at Portland and Ark nr Portland 

main-stem sites during 2009–2010. In addition to concentrations for these samples, nutrient concentrations of samples 

collected since 1990 from main-stem sites as part of previous 

investigations were included in this analysis. Nutrient concentrations for samples collected from three main-stem Arkansas 

River sites during 1990–2010 indicate a downstream increase 

in median total phosphorus concentrations from 0.03 mg/L at 

Ark at Portland to 0.08 mg/L nr Portland. Median concentration of dissolved nitrite-plus-nitrate increased from 0.05 mg/L 

at Ark at Canon City to 0.17 mg/L at Ark at Portland and 

decreased slightly to 0.16 mg/L at Ark nr Portland (fig. 12). 

Concentrations of nitrite-plus-nitrate do not exceed the USEPA 

drinking water standard (10 mg/L as N) (U.S. Environmental 

Protection Agency, 2011), and no drinking-water standards 

have been established for total phosphorus. 

Trace Elements

Concentrations of dissolved selenium and dissolved 

uranium were analyzed in samples collected from main-stem 

and selected tributary sites during 2009–2010. Main-stem 

sites were sampled more frequently for trace elements than 

the tributary sites (table 3). Boxplots (fig. 13) of selenium 

and uranium concentrations in water samples collected from 

the Arkansas River sites indicate that concentrations increase 

downstream from Canon City to near Portland. Median dissolved selenium concentrations increased downstream from 

0.17 micrograms per liter (μg/L) at Ark at Canon City to 

0.43 μg/L at Ark nr Canon City to 0.69 μg/L at Ark at 

Portland and 1.10 μg/L at Ark nr Portland. Median dissolved 

uranium concentrations increased downstream from 2.57 μg/L

Ark at Canon City to 3.20 μg/L at Ark nr Canon City to 

4.38 μg/L at Ark at Portland and 4.76 μg/L at Ark nr Portland. Concentrations of dissolved selenium and dissolved 

uranium in water samples collected from the Arkansas River 

in the UARB did not exceed USEPA drinking water standards 

of 50 μg/L and 30 μg/L, respectively (U.S. Environmental 

Protection Agency, 2011).

The concentrations of dissolved selenium in water 

samples collected from Arkansas River tributaries generally were less than the USEPA drinking-water standard of 

50 μg/L. However, the concentration of dissolved selenium 

in one sample collected in Bear Cr (66 μg/L) exceeded the 

USEPA drinking-water standard of 50 μg/L. Concentrations 

of dissolved uranium in water samples from the Arkansas 

River tributary sites of Brush Hollow (34 μg/L), Hardscrabble 

Cr (30 μg/L), Bear Cr (36 μg/L and 37 μg/L), and Ranger 

EXPLANATION

Quaternary alluvial deposits

Sedimentary rocks of Cretaceous age

Sedimentary rocks of Jurassic age

Sedimentary rocks of Triassic age

Sedimentary rocks of Permian age

Crystalline rocks of Precambrian age 

Towns

Ranger Creek

Periodic streamflow and periodic water-quality site and site name

Brush Hollow

Seasonal continuous streamflow and periodic water-quality site and site name

Ark at Canon City

Long-term continuous streamflow and periodic water-quality site with long-term 

water-quality monitoring and site name

Ark near Portland

Seasonal continuous streamflow and periodic water-quality site with seasonal 

water-quality monitoring and site name

 

Geology modified from Green (1992)

Geology

2010 irrigated cropland

Irrigated cropland modified from 

U.S. Department of Agriculture (2011)
x x 0 1,000 2,000 3,000

4,0005,0006,0007,0008,000

105

Dissolved solids, in milligrams per liter

38 7 20 12 7 5 77 7 7 10 8 6 19 7 6 5 7 7 21 13

Ark at Canon City Sand Cr Fourmile Cr Ark nr Canon City Oil Cr Ditch Chandler Cr Oak Cr Oak Cr Trib Ark trib 2 at CR119 Ark trib 1 at CR119 Ark Return flow Fremont WWTP Brush Hollow Hardscrabble Cr Ark at Portland Bear Cr Ark trib 1 at CR112 Ark trib 2 at CR112 Ark trib 1 at Hwy120 Ranger Cr Beaver Cr Ark nr Portland

10 Num

EXPLANATI

Ma

 IQR

Interquartile range

x

Out(greOuts(greUpp(les75th 50th (Me25th Low(les

Figure 10. Comparison of dissolved-solids in the Arkansas River and selected tributaries from Canon City to near Portland, 1990–2010.
30 Characterization Studies of the Arkansas River, Colorado, 2009–2010

Cr (47 μg/L), equaled or exceeded USEPA drinking-water 

standard (30 μg/L) (U.S. Environmental Protection Agency, 

2011). All samples with exceedences were collected in the 

summer and fall, when flows in these tributaries were low. 

Regression analysis of dissolved selenium and uranium 

concentrations in samples from the three main-stem sites 

indicate that concentrations of these trace elements can be 

reasonably predicted from DS, SC, and streamflow. Based on 

data for 1990–2010, selenium in the main-stem samples from 

Canon City to near Portland is correlated with SC (r2

 = 0.79) 

and DS (r2 = 0.81) (fig. 14A); however, uranium concentrations in the main-stem samples are significantly correlated 

with SC (r2 = .94) and DS (r2 =.93) (fig. 14B). Whittmore and 

others (2010) and Snow and Spalding (1994) reported similar 

uranium-SC and uranium-DS correlations in the Arkansas 

and Platte Rivers, respectively. The significant correlation of 

dissolved uranium with both SC and DS in samples from the 

Arkansas River Basin is a result of sulfate mineral dissolution and clay mineral ion-exchange reactions in the weathered 

shale bedrock and shaley soils (Zielinski and others, 1995).

The statistically significant correlation of uranium (and less 

so for selenium) with SC could allow for good estimates of 

uranium (and possibly selenium) concentrations and loads in 

the Arkansas River at gaging stations with continuous specificconductance monitors (Whittmore and others, 2010).

Dissolved selenium and uranium concentrations in 

samples from main-stem Arkansas River sites during 2009–

2010 decreased with increasing streamflow (fig. 15); however, 

uranium concentrations have a stronger correlation (r2 = 0.75) 

with streamflow than does selenium (r2 = 0.28). The seleniumstreamflow and uranium-streamflow relations may indicate 

that groundwater contributes more of these constituents to the 

Arkansas River than surface-water runoff or tributaries at low 

streamflows. At higher streamflows (greater than 500 ft3/s; 

fig. 15), surface-water flow may dilute dissolved selenium 

and uranium concentrations in groundwater discharge and 

(or) impede groundwater containing dissolved selenium and 

uranium from discharging to the Arkansas River. 

Table 9. Regression equations relating dissolved-solids concentration and specific conductance in the Upper 

Arkansas River and selected tributaries from Canon City to near Portland, 2009–2010.

 Site name Equation Number of 

comparisons r2

Ark at Canon City DS = 0.607SC + 1.371 13 .970

Sand Cr DS = 0.67SC - 33.3 7 .923

Fourmile Cr DS = 0.751SC - 51.7 7 .956

Ark nr Canon City DS = 0.636SC - 0.765 13 .981

Oil Cr Ditch return DS = 0.687SC - 11.4 7 .976

Chandler Cr DS = 0.575SC + 11.5 5 .975

Oak Cr DS = 0.618SC + 4.39 7 .988

Oak Cr trib DS = 0.592SC + 9.61 7 .991

Ark trib 2 at CR 119 DS = 0.637SC - 2.83 7 .977

Ark trib 1 at CR 119 DS = 0.934SC - 223.9 7 .997

Ark rtn at Hwy 115 DS = 0.778SC - 75.4 9 .999

Brush Hollow DS = 1.02SC - 377.7 7 .897

Hardscrabble Cr DS = 1.11SC - 479.4 6 .995

Ark at Portland DS = 0.637SC + 1.06 21 .983

Bear Cr DS = 0.807SC + 485.7 7 .465

Ark trib 1 at CR 112 DS = 1.50SC - 2758.7 7 .586

Ark trib 2 at CR 112 DS = 0.540SC + 1470 6 .740

Ark trib1 at Hwy 120 DS = 1.37SC - 1129.4 7 .964

Ranger Cr DS = 0.665SC + 913.1 7 .039

Beaver Cr DS = 1.02SC - 230.4 7 .993

Ark nr Portland DS = 0.667SC - 10.4 13 .988

[Complete site names given in table 1; DS, dissolved solids, in milligrams per liter; SC, specific conductance, in microsiemens per 

centimeter at 25 degrees Celsius; r2, coefficient of determination]
Characteristics of Streamflow from Canon City to Near Portland 31

y = -0.4847x + 8.3925

r2 = 0.6439 

0

1

2

3

4

5

6

7

4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5

Natural log dissolved solids

concentration, in milligrams per liter 

Natural log streamflow, in cubic feet per second

y = -0.27x + 6.8075

r2 = 0.1067 

0

1

2

3

4

5

6

7

8

9

10

-6 -4 -2 0 2 4 6

Natural log streamflow, in cubic feet per second

Natural log dissolved solids

concentration, in milligrams per liter 

A

B

Figure 11. Relation between dissolved-solids concentration and streamflow in the (A) Arkansas River and (B) selected 

tributaries from Canon City to near Portland, 2009–2010.
Figure 12. Comparison of concentrations of dissolved nitrite-plus-nitrate and total phosphorus in the Arkansas River from Canon City to near Portla

xxxx

x

xx

42 63 70 62

Ark at Canon City Ark at Portland Ark nr Portland

Nitrite + nitrate Nitrite + nitrate Total Nitrite + nitrate

phosphorus

Total

phosphorus

4

Too few

samples

Total

phosphorus

Concentrations of dissolved nitrite-plus-nitrate and total

phosphorus, in milligrams per liter

0.00.20.40.60.81.01.21.4

10 Nu

EXPLANAT

 IQR

Interquartile range

x

Ou(grOu(grUp(les75t50t(M25tLow(les
Figure 13. Comparison of concentrations of dissolved selenium and dissolved uranium in the Arkansas River from Canon City to near Por

Selenium Uranium Selenium Uranium Selenium Uranium Selenium Uranium Ark at Canon City Ark nr Canon City Ark at Portland Ark nr Portland

6 6 6 6 7 7 7 7

Concentrations of dissolved selenium and dissolved uranium,

in micrograms per liter 

02468

10

10 Num

EXPLANATI

 IQR

Interquartile range

Upp(les75th 50th (Me25th Low(les
34 Characterization Studies of the Arkansas River, Colorado, 2009–2010

Hydrogen and Oxygen Isotopes

The isotopic compositions of hydrogen and oxygen in the 

water molecules of surface water from main-stem Arkansas 

River and tributary sites were analyzed to determine potential sources of water and the effects of evaporation. Isotopes 

are atoms of the same element that differ in mass because of 

a difference in the number of neutrons in the nucleus. As an 

example, 2

H (deuterium, commonly shown as D) is hydrogen 

with one proton and one neutron in the atom nucleus, whereas 

1

H has only one proton and no neutron in the nucleus. Isotopic 

fractionation is the partitioning of isotopes by either physical or chemical processes and is proportional to their masses, 

and because of the intimate association with water, isotopic 

fractionation of hydrogen and oxygen are usually covariant, 

and any change will be in the same direction and magnitude 

(Coplen, 1993).

Isotopic compositions of the stable isotopes D and 18O are 

expressed as a ratio of 2

H to 1H and 18O to 16O and are reported 

as the delta (δ) value of the heavier isotope (for example, δ18O 

because it is heavier than 16O) in units of per mil (1/1,000) 

with respect to a standard. For example, a water sample with 

a δ18O value of +25.0 per mil is enriched in 18O by 25.0 per 

mil relative to the standard and is isotopically “heavy” relative to the standard. A negative delta value indicates that the 

sample is depleted in the heavy isotope relative to the standard; therefore, the sample is isotopically “light” in relation to 

the standard (Coplen, 1993). Oxygen and hydrogen isotopes 

in water samples are reported relative to the Vienna Standard 

Mean Ocean Water (VSMOW).

The relation between δD and δ18O in precipitation varies 

along a global meteoric water line (GMWL) with the equation

 δD = 8 δ18O + d (2)

where d is the deuterium excess parameter. Craig (1961) found 

the global mean value of d for freshwater sources to be +10 

per mil. However, factors such as temperature and humidity can affect the value of d, and factors such as humidity, 

temperature, and salt concentration can affect the slope (here 

represented by the value of 8 in equation 2). Evaporation from 

surface-water bodies can enrich δD and δ18O in water causing a reduction in slope to values less than 8, usually ranging 

between 3 and 6 (Coplen, 1993). 

The δD and δ18O for main-stem and tributary samples 

(fig. 16) for the Arkansas River plot along a linear trend; 

however, most samples plot below the GMWL and are similar 

to the local meteoric water line (LMWL) for Colorado (Kendall and Coplen, 2001). The LMWL is derived from isotope 

samples collected for a specific location, and the slope of 

the line (6.3 for Colorado) for those samples will vary with 

humidity, which imparts a distinctive evaporative signature 

to precipitation (Kendall and Coplen, 2001). Arkansas River 

main-stem and some tributary samples plot into three groups 

(fig. 16) showing temporal variability. The first group (A) 

consists of main-stem samples and tributary samples for 

Chandler Cr, Oak Cr, and Lake Cr collected in May 2010 and 

are the most isotopically light, suggesting that the water has 

not been affected by evaporation. Lake Cr near the headwaters 

of the Arkansas River was sampled to represent a background 

site for snowmelt and the isotope ratios plot within this group, 

indicating that precipitation is the source for this water. The 

second grouping of samples (B) includes those main-stem 

samples collected in June, July, and August and the tributary samples from Fourmile Cr, Sand Cr, Hardscrabble Cr, 

Ark trib 1 at CR119, Ark trib 2 at CR119, and Ark trib 1 at 

CR112. Evaporation, decreasing snowmelt runoff, and possible increasing contribution of groundwater that may contain 

isotopically heavier (higher values of δD and δ18O) irrigation 

recharge may explain the progression of isotope ratios in 

water samples collected from the main-stem Arkansas River 

from being isotopically light in May to heavier in August. 

No groundwater-isotope samples have been collected in this 

area of the Arkansas River Basin, however, for comparison to 

surface-water isotope samples. 

The third and isotopically heaviest group of tributary 

samples (C) includes those collected from Beaver Cr, Ranger 

Cr, Bear Cr, and Brush Hollow indicating that water from 

these tributaries either has been substantially affected by 

evaporation or contains greater amounts of irrigation return 

flows than Fourmile Cr, Sand Cr, Hardscrabble Cr, Ark trib 1 

at CR119, Ark trib 2 at CR119, Ark trib 1 at CR112, and Ark 

trib 2 at CR112 (fig. 16). Beaver Cr, Ranger Cr, Bear Cr and 

Brush Hollow drain from the north of the Arkansas River 

through the irrigated agricultural areas in and around the community of Penrose, Colo. (fig. 9) increasing the potential for 

greater amounts of irrigation return flows in those tributaries. 

Brush Hollow Reservoir, northwest of Penrose, provides water 

storage for irrigation and recreational use and increases the 

residence time and potential for evaporation of water in Brush 

Hollow before it flows into the Arkansas River or is diverted 

for agricultural uses. Conversely, the tributaries, Hardscrabble 

Cr, Ark trib 1 at CR112, and Ark trib 2 at CR112, drain from 

the south of the Arkansas River, primarily through range land 

where agricultural irrigation is limited (fig. 9). The lighter 

isotopic ratios of Hardscrabble Cr, Ark trib 1 at CR112, and 

Ark trib 2 at CR112 than Beaver Cr, Ranger Cr, Bear Cr, and 

Brush Hollow suggest less effects of evaporation to surface 

waters, groundwater recharge, and possibly less irrigation 

return flows.

Characteristics of Instantaneous Dissolved 

Solids, Selenium, and Uranium Loading 

The synoptic sampling events using the Lagrangian sampling design provide an understanding of the in-stream waterquality processes and an opportunity to see water-quality 

changes for the same parcel of water as it moves downstream. 

Water quality of the tributaries provides additional information 

about mass-loading input to the river. Analysis is focused on 

flows measured during selected synoptic events and the DS 
Characteristics of Streamflow from Canon City to Near Portland 35

concentrations of samples collected during these events. As an 

example, DS, dissolved selenium, and dissolved uranium loads 

in the UARB were characterized for the September 15, 2010, 

synoptic event (low streamflow). Loads for DS were calculated by multiplying the daily mean streamflow, in cubic feet 

per second, by the constituent concentration, in milligrams per 

liter, and a unit conversion factor (0.0027) to obtain tons per 

day. Selenium and uranium loads were calculated similar to 

DS loads, except concentrations were in micrograms per liter, 

which required a unit conversion factor of 0.0054 to obtain 

pounds per day.

For the September 15, 2010, synoptic event, from Ark 

at Canon City to Ark nr Canon City, streamflow increased 

downstream; however, the contribution (21.9 ft3

/s) (table 10) 

from the measured tributaries, although large in comparison 

to tributaries in other parts of the UARB, does not account 

for the increase in streamflow at Ark nr Canon City. This suggests the presence of ungaged inflow(s) in this reach, possibly 

groundwater; however, groundwater discharge data have not 

been collected for this reach. In the reach from Ark nr Canon 

City to Ark at Portland, the Minnequa Canal (fig. 2) can divert 

large amounts of water. On September 15, 2010, diversions 

to this canal were 92.3 ft3

/s (table 10). The effluent from the 

Fremont WWTP represented approximately 3 percent of the 

measured streamflow at Ark at Portland; streamflow from the 

nine measured tributaries accounted for another 4 percent. As 

for the river reach from Ark at Canon City to Ark nr Canon 

City, the groundwater contribution to the Arkansas River 

flow between Ark nr Canon City to Ark at Portland is at 

present uncharacterized. Measured streamflow decreased in 

the main-stem sites from Ark at Portland to Ark nr Portland 

(table 10); however, this decrease (6 percent from Ark at 

Portland to Ark nr Portland) is within instrument measurement 

error. Tributaries contributed 4.12 ft3

/s of the measured flow 

at Ark nr Portland, and as for the other reaches of the UARB, 

any groundwater contribution is uncharacterized.

Instantaneous DS loads on September 15, 2010, at the 

main-stem sites in the UARB, increased from Ark at Canon City 

(102.5 tons per day (tons/d)) to Ark nr Portland (241.7 tons/d) 

Figure 14. Relation of concentrations of (A) dissolved selenium and (B) dissolved uranium with specific conductance and 

dissolved solids in the Arkansas River from Canon City to near Portland, 1990–2010.

Figure 15. Relation of concentrations of dissolved selenium and 

dissolved uranium with streamflow in the Arkansas River from Canon 

City to near Portland, 2009–2010.

A B

0

0.5

1

1.5

2

2.5

0 200 400 600 800

Concentration of dissolved

selenium, in micrograms per liter 

0

1

2

3

4

5

6

7

8

9

0 200 400 600 800

Specific conductance and concentrations of dissolved solids,

in milligrams per liter

Specific conductance

y = 0.0031x - 0.3941

r2 = 0.79 

Dissolved solids

y = 0.0049x - 0.3814

r2 = 0.81 

Specific conductance

y = 0.0118x - 0.225 

r2 = 0.94

Dissolved solids

y = 0.0182x - 0.0903

r2 = 0.93 

Concentration of dissolved

uranium, in micrograms per liter 

Specific conductance and concentrations of dissolved solids,

in milligrams per liter

0

1

2

3

4

5

6

7

8

9

0 500 1,000 1,500 2,000 2,500

Streamflow, in cubic feet per second

Uranium

y = - 2.035ln(x) + 16.755

r2 = 0.75

Selenium

y = - 0.361ln(x) + 2.9703

r2 = 0.28 

Concentration of dissolved 

selenium and dissolveduranium, 

in micrograms per liter
Figure 16. Delta D and delta18O in samples from the Arkansas River and selected tributaries from Canon City to near Portland, 

-140-130-120-110-100

-90-80-70-60

Delta D, in millimeters 

Delta18O, in millimeters 

Global meteoric water line

Local meteoric water line

A

B

C

-18 -17 -16 -15 -14 -13 -12 -11 -10 -9 
Characteristics of Streamflow from Canon City to Near Portland 37

(table 10). Tributary loads generally were small compared to 

main-stem loads. Fourmile Cr, which had the largest tributary load to the river, contributed 25.7 tons/d of the total DS 

load at Ark nr Canon City (205.3 tons/d) followed by Bear Cr 

contributing 23.2 tons/d of the total DS load at Ark nr Portland 

(241.7 tons/d). The Minnequa Canal diverted approximately 

32 percent of the total DS load at Ark nr Canon City. The 

greatest increase in DS load for main-stem sites (table 10) was 

between Ark at Canon City and Ark nr Canon City. 

A Kruskall-Wallis test on DS loads from samples (periodic and synoptic) collected during 2009–2010, excluding 

high-flow samples collected in May, June, and July, indicated 

significant differences (p-value = 0.004) in instantaneous loads 

between the main-stem sites. The groundwater DS load contribution to the water quality of the Arkansas River may be small 

in relation to the surface-water contribution during high-flow 

periods, and removing the high-flow samples from the data set 

allows for the groundwater contribution to be accounted for 

in the statistical analysis. Using the Tukey’s test for multiple 

comparisons to determine which sites were different from each 

other indicated significant differences between instantaneous 

DS loads for the main-stem sites from Ark at Canon City to 

Ark at Portland. Further investigation would be needed to 

evaluate potential DS source areas within the Arkansas River 

from Ark at Canon City to Ark at Portland.

To estimate the DS load and to calculate concentration originating from ungaged contributions (+) or losses (–) 

between main-stem sites, mass balance equations were developed for use with the data from the synoptic events:

 Quc=Qmdr–Qmur–Qmctr+Qmcdr (3)

 DSLuc=DSLmdr–DSLmur–DSLmctr+DSLmctr (4)

 DSCuc= DSLuc÷0.0027 (5)

where the main variables are

 Q is streamflow, in cubic feet per second;

DSL is dissolved-solids load, in tons per day; and

DSC is dissolved solid concentration, in milligrams

per liter; 

and the subscripts associated with the main variables are

uc is ungaged contributions (+) or losses (–) 

within the reach;

mdr is measured Q or calculated DSL at down-

 stream end of main-stem reach;

mur is measured Q or calculated DSL at upstream

 end of main-stem reach;

 mctr is measured Q or calculated DSL 

contributions from tributaries within the 

reach; and

 mcdr is measured Q or calculated DSL losses (–) 

from diversions within the reach.

The factor 0.0027 is used to convert units in equation 5 to 

milligrams per liter.

For the reach from Ark at Canon City to Ark nr Canon 

City for the synoptic event of September 15, 2010, streamflow 

in the main-stem increased from 211 ft3

/s at Ark at Canon City 

to 287 ft3

/s at Ark nr Canon City (table 10). The measured 

contribution to streamflow from tributaries within the reach 

was 21.9 ft3

/s. No diversions were present within the reach. 

Therefore, the ungaged streamflow from contributions within 

the reach was calculated to be 54.1 ft3

/s. The DS load in the 

same reach increased from 102.5 tons/d at Ark at Canon City 

to 205.3 tons/d at Ark nr Canon City. The contribution to the 

DS load from tributaries was calculated to be 29.7 tons/d. 

Therefore, the DS load from ungaged contributions within 

the reach was estimated to be 73.1 tons/d. Assuming a single 

source, such as groundwater discharge, for all the ungaged 

streamflow and DS load from ungaged contributions, the 

DS concentration of the single source is calculated to be 

500.4 mg/L.

For the reach from Ark nr Canon City to Ark at Portland 

for the same synoptic event, the streamflow from ungaged 

contributions within the reach was calculated to be 22.9 ft3

/s 

and the DS load was calculated to be 43.7 tons/d. Assuming 

a single source, such as groundwater discharge, for all the 

ungaged streamflow and ungaged contributions to DS load, the 

DS concentration of the ungaged single source is calculated to 

be 706.8 mg/L (table 10). 

For the reach from Ark at Portland to Ark nr Portland, 

for September 15, 2010, the streamflow from ungaged losses 

within the reach was calculated to be –15.1 ft3

/s and the 

ungaged contribution to DS load was –7.8 tons/d. Assuming a 

single source for these ungaged losses, such as leakage from 

the river to groundwater, for all the ungaged streamflow and 

ungaged contributions to DS load, the DS concentration of the 

ungaged single source is calculated to be 191 mg/L (table 10). 

The negative value for streamflow in the reach between Ark 

at Portland and Ark nr Portland indicates that for the period 

evaluated, the Arkansas River within this reach is losing water 

to the aquifer, or inflows and loads from ungaged sources are 

smaller than discharge from the Arkansas River to the aquifer. 

The ungaged streamflow, ungaged contributions to DS load, 

and DS concentration for an assumed ungaged single source 

for all three reaches in the UARB are applicable only for 

September 2010, and may likely vary throughout the year for 

different streamflow and water-quality conditions. Substantial 

differences between DS concentration in the main stem and 

for the ungaged contributions could indicate unaccounted for 

inputs or losses in the reach or streamflow measurement error.

Groundwater in Quaternary alluvial deposits hydraulically connected to the Arkansas River and tributaries in the 

UARB is reported to have a median DS concentration of 

220 mg/L, with concentrations ranging from 100 to more than 

1,000 mg/L (Miller and others, 2010). Calculated DS concentrations of ungaged sources in each of the reaches in the 

UARB are within the reported range of groundwater DS concentrations; therefore, groundwater is a plausible source for 
38 Characterization Studies of the Arkansas River, Colorado, 2009–2010

the ungaged contribution to DS loads. In addition to groundwater, however, ungaged irrigation return flow and ungaged 

tributaries may also be sources of DS loads, but the contributions from these sources are unknown. 

Selenium loads in the Arkansas River on September 15, 

2010 (table 11), increased from 0.19 pounds per day (lbs/d) 

at Ark at Canon City to 1.05 lbs/day at Ark nr Canon City, 

an increase of 0.86 lbs/d, to 1.38 lbs/d at Ark at Portland, an 

increase of 0.33 lbs/d, and to 2.51 lbs/d at Ark nr Portland, an 

increase of 1.13 lbs/d. Tributary loads or load removal through 

the Minnequa Canal were not calculated for September 15, 

2010, as no tributary or Minnequa Canal selenium samples 

were collected. The Rainbow Park WWTP, however, contributes approximately 0.12 lbs/d to the selenium load at Ark at 

Portland. 

A Kruskall-Wallis test was done using selenium loads 

for all samples (periodic and synoptic) collected during 

2009–2010. High-flow samples were not removed from the 

data set, because too few samples would have been available 

for analysis. Statistically significant (p-value ≤ 0.0005) differences were found between dissolved selenium loads at the 

main-stem sites. Using the Tukey’s test for multiple comparisons, the differences between selenium loads are significant 

for the reach from Ark at Canon City to Ark at Portland, 

suggesting this section of the Arkansas River is the likely 

source for the increase in selenium loads. With the available 

selenium load data, the reach from Ark at Portland to Ark nr 

Portland was not statistically significant as a possible source 

for selenium loads, even though selenium loads in this reach 

increased by 1.13 lbs/d on September 15, 2010.

Uranium loads on September 15, 2010 (table 11), for 

main-stem sites generally increased downstream from 4.71 

lbs/d at Ark at Canon City, to 8.15 lbs/d Ark nr Canon City, 

to 7.79 lbs/d at Ark at Portland, and to 9.25 lbs/d at Ark nr 

Portland. Tributary or Rainbow Park WWTP loads and load 

removal through the Minnequa Canal were not calculated for 

September 15, 2010, as no tributary, WWTP, or Minnequa 

Canal uranium samples were collected. A Kruskall-Wallis test 

on uranium loads from main-stem samples (periodic and synoptic) collected during 2009–2010 indicated (p-value 0.224) 

no statistically significant differences between instantaneous 

loads at the main-stem sites. 

 Reach Site name or component 

of flow

Streamflow, in 

cubic feet per 

second

Dissolved solids 

concentration, 

in milligrams per 

liter

Dissolved solids load, 

in tons per day

Reach 1

 Ark at Canon City 211 180 102.5

 Tributaries3 21.9 864 29.7

 UC 54.1 500.41 73.1

 Ark nr Canon City 287 265 205.3

Reach 2

 Ark nr Canon City 287 265 205.3

 Tributaries3 14.4 10,179 33.7

 Diversions -92.3 265 -66

 UC 22.9 706.81 43.7

 Ark at Portland 232 346 216.7

Reach 3

 Ark at Portland 232 346 216.7

 Tributaries3 4.12 21,180 32.8

 UC2 -15.1 1911 -7.8

 Ark nr Portland 221 405 241.7

1

 Value is based on the assumption that the ungaged contribution (UC) is from a single source.

2

 No ungaged contributions quantified within this reach.

3

 Dissolved solids concentration summed for all tributaries in the reach.

Table 10. Measured and calculated ungaged streamflow, contributions to dissolved-solids load, and 

dissolved-solids concentration for three reaches in the Upper Arkansas River Basin, September 15, 2010.

[Complete site names given in table 1; UC, ungaged contribution; negative numbers indicate a loss of water from the river]
Characteristics of Streamflow from Avondale to Las Animas 39

Characteristics of Streamflow, Water 

Quality, and Instantaneous Dissolved 

Solids, Selenium, and Uranium Loads 

in the Arkansas River Basin from 

Avondale to Las Animas

Streamflow and water quality in the LARB from Avondale to Las Animas are influenced by natural and anthropogenic factors associated with land- and water-use practices. 

Irrigation diversions have been expanded since the first 

diversion rights were granted in the early 1830s (The Ditch 

Project, 2011) to move water from the Arkansas River to fields 

in the valley. Streamflow was measured at both long-term 

and at study-specific temporary gages for use in water-quality 

analysis and computing instantaneous loads. Data for periodic 

as well as synoptic water-quality samples were used to assess 

water quality and instantaneous loads of dissolved solids, 

selenium, and uranium. 

Streamflow Characteristics

In the LARB, long-term (pre-2000) data are available for 

six main-stem stations—Ark nr Avondale, Ark at Nepesta, Ark 

at Catlin Dam, Ark nr Rocky Ford, Ark at La Junta, and Ark 

at Las Animas (fig. 17). The streamflow-gaging stations for 

Ark nr Avondale and Ark at Las Animas are maintained and 

administered by the USGS (U.S. Geological Survey, 2012), 

whereas gaging stations at Ark nr Nepesta, Ark at Catlin Dam, 

Ark nr Rocky Ford, and Ark at La Junta are administered by 

Colorado Division of Water Resources (2011). A temporary 

gage was installed and operated by the USGS at the Ark at 

Swink station for this study, and streamflow records were 

collected for 2009 through 2010. 

The hydrographic record from 1990 through 2010 for 

all six long-term gages on the main-stem Arkansas River in 

the LARB are shown in figure 17; data collection for Ark nr 

Rocky Ford did not begin until 1999. Summary statistics 

for 1990–2008 and 2009–2010 are shown for each station in 

table 12. The 2009–2010 hydrographs for the six main-stem 

stations were similar to hydrographs for those same sites for 

previous years, with the exception of the years 1999 and 2002. 

The high flows in April 1999 resulted from a large storm and 

caused flooding of Fountain Creek (National Center for Atmospheric Research, 2011) and the low flows in 2002 account for 

the differences in the maximum and minimum flows between 

the historical record and the 2009–2010 study period (fig. 17). 

However, for most of the six sites, the median and mean flows 

for the two periods are similar.

Annual hydrographs for 2009–2010 are available for 

three tributary stations (fig. 18A), Huerfano R, Timpas Cr, and 

Crooked Arroyo. Median flows for the three tributaries having 

annual records ranged from 3.2 ft3

/s in Crooked Arroyo to 

67.5 ft3

/s in Timpas Cr. During the study period, Timpas Cr 

had the highest maximum flow, 684 ft3

/s, while the Huerfano 

R was dry during parts of this study. 

Table 11. Change in streamflow and loads of dissolved solids, selenium, and uranium between 

main-stem sites in the Upper Arkansas River Basin, September 15, 2010.

Site name

Streamflow, 

in cubic feet 

per second

Percent change Dissolved 

solids load, 

in tons per 

day

Percent change

Within 

reach Cumulative Within reach Cumulative

Ark at Canon 211 102.5

Ark nr Canon 287 36 36 205.3 100 100

Ark at Portland 232 -19 10 216.7 6 111

Ark nr Portland 221 -5 5 241.7 12 136

[Complete site names given in table 1]

Site name

Selenium 

load, in 

pounds per 

day

Percent change Uranium 

load, in 

pounds 

per day

Percent change

Within 

reach Cumulative Within reach Cumulative

Ark at Canon .19 4.71

Ark nr Canon 1.05 453 453 8.15 73 73

Ark at Portland 1.38 31 626 7.79 -4 65

Ark nr Portland 2.51 82 1,221 9.25 19 96
40 Characterization Studies of the Arkansas River, Colorado, 2009–2010

Figure 17. Streamflow hydrographs in the Arkansas River: (A) Arkansas River near Avondale and 

Arkansas River near Nepsta, (B) Arkansas River at Catlin Dam and Arkansas River near Rocky Ford, 

(C) Arkansas River at La Junta and Arkansas River at Las Animas, 1990–2010.

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

18,000

Streamflow, in cubic feet per second

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

18,000

Streamflow, in cubic feet per second

0

5,000

10,000

15,000

20,000

25,000

Streamflow, in cubic feet per second

A

B

C

1/1/1990

1/1/1991

1/1/1992

1/1/1993

1/1/1994

1/1/1995

1/1/1996

1/1/1997

1/1/1998

1/1/199

1/1/2000

1/1/2001

1/1/2002

1/1/2003

1/1/2004

1/1/2005

1/1/2006

1/1/2007

1/1/2008

1/1/2009

1/1/2010

1/1/1990

1/1/1991

1/1/1992

1/1/1993

1/1/1994

1/1/1995

1/1/1996

1/1/1997

1/1/1998

1/1/199

1/1/2000

1/1/2001

1/1/2002

1/1/2003

1/1/2004

1/1/2005

1/1/2006

1/1/2007

1/1/2008

1/1/2009

1/1/2010

1/1/1990

1/1/1991

1/1/1992

1/1/1993

1/1/1994

1/1/19951/1/1996

1/1/1997

1/1/1998

1/1/199

1/1/2000

1/1/2001

1/1/2002

1/1/2003

1/1/2004

1/1/2005

1/1/2006

1/1/2007

1/1/2008

1/1/2009

1/1/2010

Ark at Catlin Dam

Ark near Rocky Ford

EXPLANATION

Ark near Avondale

Ark near Nepesta

EXPLANATION

Ark at La Junta

Ark at Las Animas

EXPLANATION
Characteristics of Streamflow from Avondale to Las Animas 41

Figure 18. Annual streamflow hydrographs (A) for Huerfano River, Timpas Creek, and Crooked Arroyo and seasonal 

hyrdrographs (B) for Sixmile Creek, Chicosa Creek, Apishipa River, Patterson Hollow, and Hungerford Hollow, 2009–2010.

0

100

200

300

400

500

600

700

800

Streamflow, in cubic feet per second

0

50

100

150

200

250

300

Streamflow, in cubic feet per second

A

B

1/1/2009

2/1/2009

3/1/2009

4/1/2009

5/1/2009

6/1/2009

7/1/2009

8/1/2009

9/1/2009

10/1/2009

11/1/2009

12/1/2009

1/1/2010

2/1/2010

3/1/2010

4/1/2010

5/1/2010

6/1/2010

7/1/2010

8/1/2010

9/1/2010

10/1/2010

11/1/2010

12/1/2010

1/1/2009

2/1/2009

3/1/2009

4/1/2009

5/1/2009

6/1/2009

7/1/2009

8/1/2009

9/1/2009

10/1/2009

11/1/2009

12/1/2009

1/1/2010

2/1/2010

3/1/2010

4/1/2010

5/1/2010

6/1/2010

7/1/2010

8/1/2010

9/1/2010

10/1/2010

11/1/2010

12/1/2010

Huerfano River

Timpas Cr

Crooked Arroyo

EXPLANATION

Sixmile Cr

Chicosa Cr

Apishipa R

Patterson Hollow

Hungerford Hollow

EXPLANATION

Site name

 Maximum flow, ft3

/s Minimum flow, ft3/s Median flow, ft3/s Mean flow, ft3/s

1990-2008 2009-2010 1990-2008 2009-2010 1990-2008 2009-2010 1990-2009 2009-2010

Ark nr Avondale 12,300 5,660 87 194 565 517 884 823

Ark nr Nepesta 16,700 3,610 1.6 80 431 402 681 622

Ark at Catlin Dam 16,200 5,660 .53 194 352 517 571 823

Ark nr Rocky Ford1

 2,440 2,800 .41 23 172 215 340 446

Ark at La Junta 19,000 1,220 3.4 26 93 115 269 253

Ark at Las Animas 22,600 1,560 7.5 26 123 133 278 211

1

 Data are available only for 1990–2010.

Table 12. Summary statistics for Arkansas River streamflow data 1990–2008 and 2009–2010 for Ark at Avondale, Ark nr Nepesta, Ark at 

Catlin Dam, Ark nr Rocky Ford, Ark at La Junta, and Ark at Las Animas gage stations.

[Complete site names given in table 1; ft3/s, cubic feet per second]
42 Characterization Studies of the Arkansas River, Colorado, 2009–2010

Seasonal hydrographs for 2009–2010 are available for 

five stations (fig. 18B); Sixmile Cr, Chicosa Cr, Apishipa R, 

and Patterson and Hungerford Hollows. Of the seasonally 

measured tributaries, the lowest flow, 0.25 ft3

/s, was 

measured in Patterson Hollow, the highest maximum flow, 

251 ft3

/s, was measured in the Apishipa R. Median flows 

in these tributaries ranged from 1.7 ft3

/s in Hungerford and 

Patterson Hollows to 21 ft3

/s in the Apishipa R. In general, 

streamflow in the tributaries is less than in the Arkansas River, 

and larger instantaneous flows are associated with storm 

runoff. Seasonal increases in streamflows in many of these 

tributaries can be attributed to irrigation return flows during 

the growing season and, in turn, seasonal decreases in streamflows during the fall and winter months can be attributed to 

reduced irrigation return flow (fig. 18). 

Water-Quality Characteristics

Water samples from the main stem and tributaries were 

analyzed for selected constituents to characterize the Avondale 

to Las Animas reach of the Arkansas River and to identify 

potential source areas for dissolved solids, selenium, and 

uranium. Constituents sampled include specific conductance, 

major ions, dissolved solids, nutrients, select dissolved trace 

elements (selenium and uranium), and hydrogen and oxygen 

isotopes. Larger scale Arkansas River Basin water-quality 

studies (Dash and Ortiz, 1996; Ortiz and others, 1998) began 

in the 1990s; previous to this, studies were generally limited 

in scope and scale. To increase the number of observations 

and the power of statistical tests in this study, water-quality 

data from Dash and Ortiz (1996) and Ortiz and others (1998) 

beginning in 1990 were included in selected data sets.

Specific Conductance

Specific conductance values within the LARB generally 

increased downstream from Ark nr Avondale to Ark at Las 

Animas during 2009–2010 (fig. 19). Temporally, the lowest 

SC values occurred in June in both 2009 and 2010, whereas 

the highest values measured occurred in March and April. 

Spring runoff (predominantly snowmelt low in dissolved 

solids) and releases from Pueblo Reservoir in May and June 

produce high flows in the river with a low SC. In March and 

April, most of the streamflow in the river is presumably irrigation recharged groundwater, which could have a high SC. 

Major Ions

Water in the main stem of the LARB is a calcium-sulfate 

type, as reported previously by Ortiz and others (1998), and 

is likely a result of groundwater and surface-water interaction with marine shales and sandstones of Cretaceous age 

containing calcareous cements and marine limestones (fig. 20). 

The calcium-sulfate water type dominates from Avondale to 

Las Animas; however, the concentrations of the major ions 

increase downstream from Avondale to a maximum at Las 

Animas (fig. 21). Use and reuse of irrigation return flows 

(Miller and others, 2010) and inflows from groundwater are 

the likely causes for the increase in major ions.

Figure 19. Continuous specific conductance values at main-stem sites in the Arkansas River Basin from Avondale to Las Animas,

2009–2010.

 0

 500

1,000

1,500

2,000

2,500

3,000

3,500

Specific conductance, 

in microsiemens per centimeter

12/3/2008

3/13/2009

6/21/2009

9/29/2009

1/7/2010

4/17/2010

7/26/2010

11/3/2010

2/11/2011

Ark nr Avondale

Ark nr Nepesta

Ark at Catlin Dam

Ark nr Rocky Ford

Ark at Swink

Ark at La Junta

Ark at Las Animas

EXPLANATION
Base from U.S. Geological Survey Data, 2011, various scales Universal Transverse Mercator projection Zone 13 0 10 

0 10 

104°30' 104°00' 103°30'

38°00'

Pueblo

Rocky Ford

La Junta

Ark nr Avondale

Sixmile Cr

Apishapa R

Huerfano R

Chicosa Cr

Ark at Swink

Ark nr Nepesta

Ark at La Junta

Anderson Arroyo

Ark nr Rocky Ford

Ark at Catlin Dam

Hungerford Hollow

IR-43

La Junta WWTP

Patterson Hollow 

Sixmile Creek

Huerfano River

Chicosa Creek

Arkansas River

Apishapa Rive

r

Timpas Creek Crooked Arroyo

Anderson

Ark R blw Highline Canal

Arroyo

Figure 20. Generalized geology and 2010 irrigated cropland in the Lower Arkansas River Basin, from Avondale to Las Animas.
44 Characterization Studies of the Arkansas River, Colorado, 2009–2010

0

5

10

15

20

25

Arkansas 

River near 

Avondale

Arkansas 

River near 

Nepesta

Arkansas 

River at 

Catlin Dam

Arkansas 

River near 

Rocky Ford

Arkansas 

River at 

Swink

Arkansas 

River at 

La Junta

Arkansas 

River at 

Las Animas

Sodium

Magnesium

Calcium

Bicarbonate

Sulfate

Chloride

EXPLANATION

Major ion concentration, in

milliequivalents per liter 

Figure 21. Comparison of median concentrations of six major ions in the Arkansas River, from Avondale to Las Animas, 

2009–2010.

EXPLANATION

Quaternary alluvial deposits

Sedimentary rocks of Cretaceous age

 Pierre Shale

 Niobara Formation

 

Towns

IR-43

Periodic streamflow and periodic water-quality site and site name

Anderson Arroyo

Seasonal continuous streamflow and periodic water-quality site and site name

Ark at Catlin Dam 

Long-term continuous discharge and periodic water-quality site with long-term water-quality 

monitoring and site name

Ark at Swink

Seasonal continuous discharge and periodic water-quality site with seasonal water-quality 

monitoring and site name

 

Geology modified from Green (1992)

Geology

2010 irrigated cropland; modified

from U.S. Department of Agriculture (2011) 
Characteristics of Streamflow from Avondale to Las Animas 45

Dissolved Solids

Similar to the findings of Ortiz and others (1998) and 

Miller and others (2010), the LARB main-stem median DS 

concentrations generally increase downstream (fig. 22) from 

Avondale (557 mg/L) to Las Animas (1,725 mg/L). Median 

DS concentrations in samples from tributaries and the La Junta 

WWTP varied, ranging from a low of 1,050 mg/L in Hungerford Hollow to a high of 2,770 mg/L in water sampled from 

the Huerfano R. The Huerfano R., in addition to having the 

highest measured median DS concentration of the sampled 

tributaries, has the largest range of DS concentrations, from 

774 mg/L to 5,640 mg/L. Irrigation return flows (Miller and 

others, 2010) and inflows from groundwater likely contribute 

to the high DS concentrations in all the tributaries, whereas 

differences in the ranges of DS concentration among the 

selected tributaries can be attributed to differences in geology 

and the chemical composition of the bedrock among the tributary basins (fig. 20).

Dissolved-solids concentrations at the main-stem LARB 

Arkansas River sites were analyzed and compared to specific 

conductance and streamflow. Table 13 lists results of the linear 

regression analysis of DS and SC from water samples collected in 2009–2010 from individual sites. All of the sites, 

even with a small number of samples (less than 10), indicate 

a significant relation between DS and SC (r2

 >0.95). As in the 

UARB, the likely sources of DS in the LARB are geologic 

units and the continuous recharge and reuse of irrigation water 

that results in increased concentrations of DS (Miller and others, 2010).

A relation was developed between main-stem Arkansas 

River flow and DS concentrations. The natural log of DS is 

moderately correlated to the natural log of flow (r2

 = 0.59; 

p-value <0.00001) for the 2009–2010 main-stem site data set 

(fig. 23A). Regression analysis of the combined data set of DS 

for all tributary sites for the same period showed minimal correlation (r2

 = 0.16) to the natural log of streamflow (fig. 23B), 

indicating that streamflow would not be a reasonable variable 

to estimate DS concentrations in the tributaries. However, 

additional paired DS samples and streamflow measurements, 

in a variety of flow regimes, could be used to build a statistical 

relation for each tributary.

Nutrients

Multiple nutrient samples were collected during 

2009–2010 from selected main-stem sites, which include 

Ark at Nepesta, Ark nr Rocky Ford, Ark at Swink, and Ark 

at La Junta. One set of nutrient samples was collected during 

2009–2010 at each of 10 selected tributary sites—Sixmile Cr, 

IR-43, Huerfano R, Chicosa Cr, Hungerford Hollow, Apishipa 

R, Patterson Hollow, Timpas Cr, Crooked Arroyo, and Anderson Arroyo. For samples collected from 1990–2010, concentrations of dissolved nitrite-plus-nitrate concentrations from 

samples collected from Arkansas River main-stem and tributary sites ranged from a low of 1.43 mg/L, at Ark nr Nepesta, 

to a high of 2.73 mg/L, at Ark at La Junta (fig. 24A). The 

median nitrite-plus-nitrate concentration was constant from 

Ark nr Avondale to Ark at Swink, increased at Ark at La Junta 

and decreased downstream at Ark at Las Animas. Median 

concentrations of nitrite-plus-nitrate in samples collected from 

tributaries ranged from a low of 0.35 mg/L at Huerfano R, to a 

high of 3.07 mg/L at Crooked Arroyo (fig. 24A). The concentrations of nitrite-plus-nitrate in Crooked Arroyo and Apishipa 

River may be contributing to the concentrations at Ark at La 

Junta and Ark at Catlin Dam, respectively. As in the UARB, 

concentrations of dissolved nitrite-plus-nitrate for the LARB 

did not exceed the USEPA drinking-water standard (10 mg/L 

as nitrogen) (U.S. Environmental Protection Agency, 2011).

Multiple total phosphorus samples were collected during 

2009–2010 for Ark nr Nepesta, Ark nr Rocky Ford, Ark at 

Swink, and Ark at La Junta; however, to increase the amount 

of data for comparison, the boxplots shown in figure 24B are 

based on samples collected from 1990 through 2010 (Ortiz 

and others, 1998; and Miller and others, 2010). The median 

total phosphorus concentrations for the five main-stem sites 

decreased downstream from 0.3 mg/L at Ark nr Avondale to 

0.1 mg/L at Ark at Las Animas. Total phosphorous currently 

does not have a USEPA primary or secondary drinking-water 

standard (U.S. Environmental Protection Agency, 2011).

Trace Elements

Concentrations of dissolved selenium and dissolved 

uranium were analyzed in samples collected from main-stem 

and selected tributary sites during 2009–2010. Samples for 

trace-element analyses were collected more frequently at the 

main-stem sites than at sites on the selected tributaries 

(table 3). Median dissolved selenium concentrations at the 

main-stem sites ranged from 8.4 to 12.2 μg/L, increasing 

downstream from Ark nr Nepesta to Ark at La Junta 

(fig. 25A). However, the median selenium concentration 

decreased between Ark at La Junta and Ark at Las Animas, 

the most downstream site in the study area. Miller and others 

(2010) reported that historical selenium concentrations were 

variable between Ark nr Avondale and Ark at Las Animas. 

Median dissolved selenium concentrations for tributaries 

generally were higher than those for main-stem sites; however, 

the median concentration at tributary site IR-43 (7.5 μg/L) 

was lower than other tributary or main-stem sites. The highest 

median selenium concentration (18.9 μg/L) was measured in 

the La Junta WWTP effluent (fig. 25A). Patterson Hollow had 

the largest interquartile range of dissolved selenium concentrations, which indicates high variability in sample concentrations. No concentrations of dissolved selenium for any of the 

sampled sites exceeded the USEPA primary drinking-water 

standard of 50 μg/L (U.S. Environmental Protection Agency, 

2011). The origin of the selenium is likely geologic, as summarized by Miller and others (2010), and associated with 

groundwater or irrigation recharge coming in contact with the 

Cretaceous-age marine Pierre Shale and Niobrara Formation. 
0

1,0002,0003,0004,0005,0006,0007,000

Dissolved solids, in milligrams per liter

x

62 7 7 16 4 15 7 7 20 81 7 71 7 16 7 7 53 6 44

Ark nr Avondale Sixmile CrIR-43 Huerfano R Ark below Highline canal Ark near Nepesta Chicosa Cr Hungerford Hollow Apishipa R Ark at Catlin Dam Patterson Hollow Ark nr Rocky Ford Timpas Cr Ark at Swink Crooked Arroyo Anderson Arroyo Ark at LaJunta LaJunta WWTP Ark at LasAnimas

 IQR

Interquartile range

x

10 Num

EXPLANATIO

Main

Outs(greaUppe(less 75th 50th (med25th Lowe(less 

x

x

Figure 22. Comparison of dissolved-solids concentrations in the Arkansas River and selected tributaries from Avondale to Las Animas, 1990–2010.
Characteristics of Streamflow from Avondale to Las Animas 47

Concentrations of dissolved uranium in water samples 

from the LARB increase in the Arkansas River with distance 

downstream from Ark nr Avondale to Ark at Las Animas 

(fig. 3), with median concentrations ranging from 6.1 to 27.4 

μg/L (fig. 25B). The median uranium concentration (39 μg/L) 

in Sixmile Cr exceeded the USEPA drinking-water standard 

(30 μg/L) (U.S. Environmental Protection Agency, 2011). Sixmile Cr also had the highest median uranium concentration. 

The tributaries Sixmile Cr, IR-43, Huerfano R, and Chicosa Cr 

all drain the lower part of the Cretaceous-age Pierre Shale and 

upper part of the Niobrara Formation, which are thought to be 

geologic sources of uranium (Landis, 1959; Miller and others, 

2010). Two samples from Patterson Hollow exceeded the 

USEPA drinking-water standard for uranium—one collected in 

November 2009 and the other in June 2010. One sample from 

Timpas Cr and one from Crooked Arroyo, both collected in the 

fall when streamflow was low, exceeded the USEPA drinkingwater standard. Patterson Hollow, Timpas Cr, and Crooked 

Arroyo basins are underlain by the upper part of the Niobrara 

Formation. 

Regression analysis of dissolved selenium and dissolved 

uranium concentrations, as in the UARB, showed some 

correlation with SC, DS, and streamflow (figs. 26A and B). 

Selenium in the main-stem samples from Ark nr Avondale to 

Ark at Las Animas is weakly correlated with SC (r2

 = 0.42) 

and DS (r2

 = 0.50) (fig. 26A). However, uranium shows a 

strong correlation to SC and DS, with a linear regression r2

 of 

0.86 and 0.92 respectively (fig. 26B), suggesting that uranium 

concentrations can be reasonably estimated from either SC or 

DS. 

Both dissolved selenium and uranium concentrations in 

samples from main-stem LARB sites (2009 to 2010) decrease 

with increasing streamflow (fig. 27); however, uranium 

concentrations (r2

 = 0.53) show a moderate correlation with 

streamflow. As in the UARB, it can be suggested from the 

selenium-flow and uranium-flow relations that groundwater 

may be the main source of these constituents to the Arkansas 

River rather than main-stem or tributary surface-water runoff 

at lower streamflows. The increased surface-water flow in the 

Arkansas River may dilute selenium and uranium concentrations of groundwater inflows and (or) impede groundwater 

inflow containing dissolved selenium and uranium. However, 

data pertaining to groundwater and groundwater surface-water 

interaction in the LARB are insufficient to confirm this. 

Site name Equation Number of comparisons r2

Ark nr Avondale DS = 0.71SC – 35 13 0.979

Sixmile Cr DS = 0.99SC – 251 7 0.968

IR-43 DS = 0.88SC – 135 7 0.998

Huerfano R DS = 1.08SC – 577 5 0.995

Ark blw Highline Canal DS = 0.77SC – 72 4 0.957

Ark nr Nepesta DS = 0.76SC – 63 15 0.988

Chicosa Cr DS = 0.95SC – 240 7 0.997

Hungerford Hollow DS = 0.98SC – 357 7 0.983

Apishipa R DS = 1.04SC – 400 7 0.939

Ark at Catlin Dam DS = 0.86SC – 138 14 0.983

Patterson Hollow DS = 0.90SC – 151 7 0.961

Ark nr Rocky Ford DS = 0.86SC – 133 28 0.989

Timpas Cr DS = 1.07SC – 436 7 0.999

Ark at Swink DS = 0.88SC – 142 16 0.986

Crooked Arroyo DS = 1.08SC – 435 7 0.999

Anderson Arroyo DS = 1.04SC – 415 7 0.996

Ark at La Junta DS = 0.88SC – 149 16 0.992

Table 13. Regression equations relating concentrations of dissolved solids and specific conductance in the 

Lower Arkansas River and selected tributaries from Avondale to Las Animas, 2009–2010.

[Complete site names given in table 1; DS, dissolved solids, in milligrams per liter; SC, specific conductance, in microsiemens 

per centimeter at 25 degrees Celsius; r2, coefficient of determination]
48 Characterization Studies of the Arkansas River, Colorado, 2009–2010

Figure 23. Concentrations of dissolved solids and streamflow in the Arkansas River (A) and selected tributaries (B) from Avondale 

to Las Animas, 2009–2010.

5

5.5

6

6.5

7

7.5

8

3 4 5 6 7 8 9

5

5.5

6

6.5

7

7.5

8

8.5

-1 0 1 2 3 4 5 6

Natural log streamflow, in cubic feet per second

Natural log streamflow, in cubic feet per second

A

B

Natural log dissolved solids, 

in milligrams per liter 

y = -0.3583x + 8.7707

r2 = 0.59

y = -0.1284x + 7.5696

r2 = 0.16

Natural log dissolved solids, 

in milligrams per liter 
Figure 24. Comparison of concentrations of (

A) nitrite-plus-nitrate and (

B) total phosphorus in the Arkansas River from Avondale to L

Animas, 1990–2010.

A

B IQR

Interquartile range

x

10 Num

EXPLANATI

MaiOut(greOuts(greUpp(les75th 50th (Me25th Low(lesPer

A 

x

x

x

x

x

x

x

Ark nr Avondale

1993-2002

Huerfano R

1993-2010

Ark nr Nepesta 1993-2010

Chicosa Cr

1998-2010

Apishipa R

1998-2010

Ark at Catlin Dam

1993-2002

Ark nr Rocky Ford

2002-2010

Ark at Swink

2009-2010

Crooked Arroyo

1998-2010

Ark at LaJunta

1993-2010

Ark at LasAnimas

1993-2002

Ark nr Avondale

1990-2002

Ark at Catlin Dam

1990-2002

Ark nr Rocky Ford

2002-2010

Ark at LaJunta

2002-2010

Ark at LasAnimas

1990-2002

Nitrite-plus-nitrate, in milligrams per liter Total phosphorus, in milligrams per liter

02468

1012

0.00.51.01.52.0

18 16 12 16 16 19 12 10 16 29 19

26 26 5 5 26
Figure 25. Comparison of concentrations of (A) dissolved selenium and (B) dissolved uranium in the Arkansas River from Avondale to Las Animas

31 5 5 4 15 5 5 5 57 17 5 14 5 5 15 6 4

A 

4 5 5 4 14 5 5 5 4 5 14 145 5 145 6 4

B

Concentrations of dissolved selenium, 

in micrograms per liter 

Concentrations of dissolved uranium, 

in micrograms per liter 

 IQR

Interquartile range

x

10 Nu

EXPLANAT

Ma

Out(greOut(greUpp(les75t50t(Me25tLow(les

x

x

x

x

x

0

10203040

20406080

100

0

Ark nr Avondale Sixmile CrIR-43 Huerfano R Ark nr Nepesta Chicosa Cr Hungerford Hollow Apishipa R Ark at Catlin Dam Patterson Hollow Ark nr Rocky Ford Timpas Cr Ark at Swink Crooked Arroyo Anderson Arroyo Ark at LaJunta LaJunta WWTP Ark at LasAnimas
Characteristics of Streamflow from Avondale to Las Animas 51

Hydrogen and Oxygen Isotopes

Values of δD and δ18O for samples of water collected 

from main-stem sites in the LARB show a linear trend that 

plots below the GMWL (line slope of 8) and the Colorado 

LMWL (line slope of 6.3) (fig. 28A). These results indicate 

isotopic enrichment of δD and δ18O for these samples through 

evaporation and possibly evapotranspiration. The main-stem 

samples collected in early June 2010 are the most isotopically 

light samples in comparison to other samples collected at the 

same sites in other months; however, the main-stem samples 

for June are not isotopically similar to the Lake Cr snowmelt 

sample (δD= –126 per mil and d18O= –17 per mil). The water 

collected in June 2010 likely has been affected by evaporation and may be representative of water released from Pueblo 

Reservoir. The LARB main-stem samples from October and 

November 2009 and August 2010 plot (fig. 28A) with no clear 

temporal groupings such as those in the UARB. Samples from 

Ark at Las Animas, the most downstream site in the LARB, 

are the most isotopically heavy, suggesting possible increased 

surface-water evaporation and (or) increasing contribution of 

groundwater that has been affected by evaporation and irrigation recharge. 

Most of the tributary samples and samples from the La 

Junta WWTP plot (fig. 28B) below the GMWL and the Colorado LMWL, indicating enrichment in δD and δ18O, probably 

as a result of evaporation. A regression line calculated for 

Figure 26. Relation of concentrations of (A) dissolved selenium and (B) dissolved uranium with specific conductance and 

dissolved solids in the Arkansas River from Avondale to Las Animas, 1990–2010.

0

5

10

15

20

25

30

35

40

0 1,000 2,000 3,000 4,000 5,000 6,000

0

10

20

30

40

50

60

70

0 1,000 2,000 3,000 4,000 5,000 6,000

A B

Concentration of dissolved selenium, 

in micrograms per liter 

Specific conductance, in microseimens per centimeter 

and dissolved solids in milligrams per liter

Concentration of dissolved uranium, 

in micrograms per liter 

Specific conductance, in microseimens per centimeter 

and dissolved solids in milligrams per liter

Dissolved solids

y = 0.2544x 0.545

r

2 = 0.50

Specific conductance

y = 0.2301x 0.5356

r

2 = 0.42

Dissolved solids

y = 0.017x -1.7581

r2 = 0.92

Specific conductance

y = 0.0119x

r

2 = 0.86

0

5

10

15

20

25

30

35

40

0 1,000 2,000 3,000 4,000 5,000

Concentration of dissolved selenium and

dissolved uranium, in micrograms per liter 

Streamflow, in cubic feet per second

Uranium

y = -6.856ln(x) + 55.489

r

2 = 0.53

Selenium

y = -3.082ln(x) + 28.991

r2 = 0.49

Figure 27. Relation of concentrations of dissolved selenium and dissolved uranium with streamflow in the Arkansas River from 

Avondale to Las Animas, 2009–2010.
52 Characterization Studies of the Arkansas River, Colorado, 2009–2010

Figure 28. Delta D and delta18O in samples from (A) main-stem sites on the Arkansas River from Avondale to 

Las Animas and (B) selected tributaries, 2009–2010.

Delta 18O, inches per millimeter 

Delta 18O, inches per millimeter

A

B

-120

-115

-110

-105

-100

-95

-90

-85

-17 -16 -15 -14 -13 -12 -11

Delta D, inches per millimeter

Sixmile C

IR-43

Huerfano R

Chicosa Cr

Hungerford Hollow

Apishipa R

Patterson Hollow

Timpas Cr

Crooked Arroyo

Anderson Arroyo

LaJunta WWTP

Global meteoric water line 

Local meteoric water line 

y=6.16x -21.166

 

-130

-125

-120

-115

-110

-105

-100

-95

-90

-85

-18 -17 -16 -15 -14 -13 -12 -11

Delta D, inches per millimeter 

Ark near Avondale

Ark below Highline

Ark near Nepesta

Ark at Catlin Dam

Ark near Rocky Ford

Ark at Swink

Ark at La Junta

Ark at Las Animas

Lake Cr

Global meteoric water line

Local meteoric water line 

y=6.13x -22.073 

Samples collected June 2010 

EXPLANATION

EXPLANATION
Characteristics of Streamflow from Avondale to Las Animas 53

the tributary samples is similar to the main-stem line (slope 

of 6.16). Isotope ratios from Sixmile Cr, IR-43, Huerfano R, 

Crooked Arroyo, and La Junta WWTP group with little or no 

temporal variation (within 1 per mil unit), which suggests the 

source water for these tributaries does not undergo numerous 

evaporation cycles through irrigation return and reuse. The 

remaining sampled tributaries have greater temporal variation in isotopic ratios suggesting multiple evaporation cycles 

or, as for the main-stem sites, possibly increasing contribution of groundwater that has been affected by evaporation 

and irrigation recharge. However, as in the UARB, no LARB 

groundwater isotope samples are available for comparison to 

the surface-water samples. Finally, samples collected from 

Anderson Arroyo in October 2009 and Hungerford Hollow 

in November 2009 plot above the Colorado LMWL. A single 

sample from Patterson Hollow collected in October 2009 plots 

above the GMWL. In these samples, the δD and δ18O may 

have been affected by isotopically light water in runoff from 

localized storms.

Characteristics of Instantaneous Dissolved 

Solids, Selenium, and Uranium Loading 

Characterization of loading in the LARB focused on 

the synoptic samples collected during low streamflow 

(November 16–19, 2009) when the influence from groundwater inflow to the river would likely be large (table 14). In 

the Arkansas River from Ark nr Avondale to Ark nr Nepesta, 

streamflow increased downstream; however, the contribution 

(18.2 ft3

/s) from the measured tributaries (Huerfano R, IR-43, 

and Sixmile Cr), does not account for all of the increase 

in streamflow at Ark nr Nepesta. This suggests a possible 

groundwater contribution in this reach that has not been measured. From Ark nr Nepesta to Ark at Catlin Dam, diversion of 

water through the Otero Canal (table 14) diverts 40 ft3

/s of the 

measured streamflow at Ark at Catlin Dam, whereas tributaries in this reach contribute 26.6 ft3

/s of the streamflow. The net 

decrease in streamflow (table 14) from Ark nr Nepesta to Ark 

at Catlin Dam may be attributed in part to the diversions to the 

Otero Canal. From Ark at Catlin Dam to Ark nr Rocky Ford, 

water removed from the Arkansas River through the Fort Lyon 

Storage Canal had the dominant effect on the total flow in the 

river. The canal (active in the months November to February) 

diverted 382 ft3

/s of the flow within the reach between Ark at 

Catlin Dam and Ark nr Rocky Ford, whereas Patterson Hollow contributed 0.62 ft3

/s to the flow at Ark nr Rocky Ford. 

Downstream from the Ft Lyon Storage Canal diversion, flows 

generally increased with distance downstream to Ark at Las 

Animas. Timpas Creek contributed approximately 29 ft3

/s of 

the flow at Ark at Swink. No diversions remove water in the 

reaches Ark at Swink to Ark at La Junta and Ark at La Junta to 

Ark at Las Animas (table 14); however, Crooked Arroyo and 

Anderson Arroyo contribute minor amounts of inflow 

(10.6 ft3

/s) to the river at Ark at La Junta (table 14). For the 

synoptic event November 16–19, 2009, the La Junta WWTP 

contributed 1.97 ft3

/s to the streamflow at Ark at Las Animas, 

the only measured contribution in that reach. As in the other 

reaches of the LARB, the groundwater contribution was not 

measured but may be a source of streamflow in the reach from 

Ark at Swink to Ark at Las Animas. 

 Based on data collected November 16–19, 2009, in the 

LARB, DS loads followed a pattern similar to that of measured streamflow and increased in the Arkansas River from 

Ark nr Avondale to Ark at Catlin Dam (table 14). In the reach 

Ark nr Avondale to Ark nr Nepesta, the measured tributaries 

contributed 91.4 tons/d of DS load to the Arkansas River. In 

the reach Ark nr Nepesta to Ark at Catlin Dam, tributaries 

contributed 133.4 tons/d to the DS load, but the Otero Canal 

removed 93.9 tons/d. In the reach Ark at Catlin Dam to Ark 

nr Rocky Ford, DS loads decreased from 1,199.2 tons/d to 

408.4 tons/d. The Fort Lyon Storage Canal diverts 841.6 

tons/d, while tributaries contributed 5 tons/d of the DS load to 

Ark nr Rocky Ford (table 14). In the reach from Ark nr Rocky 

Ford to Ark at Swink, the DS load increased from 408.4 tons/d 

to 730.9 tons/d, the greatest increase in DS load (322.5 tons/d) 

for any of the study reaches (table 14). Timpas Cr, the only 

measured tributary in the Ark nr Rocky Ford to Ark at Swink 

reach, contributed 189.5 tons/d of the DS load at Ark at Swink. 

In the reaches Ark at Swink to Ark at La Junta, and Ark at 

La Junta to Ark at Las Animas, DS loads increased from 

730.9 tons/d at Ark at Swink to 1,044.9 tons/d at Ark at Las 

Animas. Tributaries in the reach Ark at Swink to Ark at La 

Junta contributed 76.8 tons/d to the main-stem Arkansas River.

A Kruskall-Wallis test on periodic and synoptic DS loads 

in 2009–2010, excluding high streamflow samples collected in 

May, June, and July, indicated statistically significant (p-value 

0.038) differences between DS loads at the main-stem sites. 

Using the Tukey’s test for multiple comparisons, the significant differences between instantaneous DS loads at the mainstem sites were from Ark nr Rocky Ford to Ark at La Junta. 

Further investigation would be needed to evaluate potential 

dissolved-solids source areas within the Arkansas River from 

Ark nr Rocky Ford to Ark at La Junta. 

For the reach from Ark nr Avondale to Ark nr Nepesta 

for the synoptic event of November 16–19, 2009, streamflow 

in the main-stem increased from 453 ft3

/s at Ark nr Avondale to 539 ft3

/s at Ark nr Nepesta (table 14). The measured 

contribution to streamflow from tributaries within the reach 

was 18.2 ft3

/s. No diversions were present within the reach. 

Therefore, the ungaged streamflow within the reach was calculated to be 67.8 ft3

/s. The DS load in the same reach increased 

from 876.9 tons/d at Ark nr Avondale to 1,164.2 tons/d at Ark 

nr Nepesta. The contribution to the DS load from tributaries was 91.4 tons/d. Therefore, the ungaged contribution to 

DS load within the reach was calculated to be 195.9 tons/d. 

Using the water balance method, as discussed in the section 

“Characteristics of Instantaneous Dissolved Solids, Selenium, 

and Uranium Loading” for the UARB, and substituting values 

for Ark nr Avondale to Ark nr Nepesta, and assuming a single 

ungaged source, the DS concentration of the single source 

(such as groundwater) is calculated to be 1,069 mg/L.
54 Characterization Studies of the Arkansas River, Colorado, 2009–2010

For the reach from Ark nr Nepesta to Ark at Catlin Dam 

for the same synoptic event, the ungaged streamflow within 

the reach was calculated to be –18.6 ft3

/s and the DS load was 

calculated to be 4.5 tons/d. A negative streamflow in this reach 

indicates that for the period evaluated, the Arkansas River 

within this reach is losing water to the aquifer, or inflows and 

loads from ungaged sources are smaller than discharge from 

the Arkansas River to the aquifer. For the ungaged streamflow 

and ungaged contribution to DS load, the DS concentration is 

calculated to be 89.6 mg/L (table 14). 

For the reach from Ark at Catlin Dam to Ark nr Rocky 

Ford for the same synoptic event, the ungaged streamflow 

from contributions within the reach was calculated to be 

–0.62 ft3

/s and the DS load was calculated to be 45.8 tons/d 

(table 14). As in the reach from Ark nr Nepesta to Ark at 

Catlin Dam, a negative streamflow in this reach suggests the 

Arkansas is losing water to the local aquifer. The DS concentration for ungaged sources in the reach Ark at Catlin Dam to 

Ark nr Rocky Ford was not calculated because the of the small 

ungaged streamflow (–0.62 ft3

/s). 

For the reach from Ark nr Rocky Ford to Ark at Swink, 

the ungaged streamflow from contributions within the reach 

was calculated to be –10 ft3

/s, and the estimated DS load 

was calculated to be 133 tons/d. As in the reach from Ark 

nr Nepesta to Ark at Catlin Dam, a negative streamflow in 

this reach suggests the Arkansas is losing water to the local 

Table 14. Measured and calculated ungaged streamflow, contributions to dissolved-solids load, and dissolved-solids concentration 

for six reaches in the Lower Arkansas River Basin, November 16–19, 2009.

Reach Site name or component 

of flow

Streamflow, in cubic 

feet per second

Dissolved solids concentration, in milligrams per literDissolved solids load, in 

tons per day

Reach 1

 Ark nr Avondale 453 717 876.9

 Tributaries2 18.2 5,300 91.4

 UC 67.8 1,0691 195.9

 Ark nr Nepesta 539 800 1,164.2

Reach 2

 Ark nr Nepesta 539 800 1,164.2

 Tributaries2 26.6 5,640 133.4

 Diversions -40 869 -93.9

 UC -18.6 89.61 -4.5

 Ark at Catlin Dam 507 876 1,199.2

Reach 3

 Ark at Catlin Dam 507 876 1,199.2

 Tributaries2 0.62 3,000 5.0

 Diversions -382 816 -841.6

 UC -0.62 NA 45.8

 Ark nr Rocky Ford 125 1,210 408.4

Reach 4

 Ark nr Rocky Ford 125 1,210 408.4

 Tributaries2 29 2,420 189.5

 UC -10.0 4,9261 133.0

 Ark at Swink 144 1,880 730.9

Reach 5

 Ark at Swink 144 1,880 730.9

 Tributaries2 10.6 5,770 76.8

 UC 42.4 8431 96.5

 Ark at La Junta 197 1,700 904.2

Reach 6

 Ark at La Junta 197 1,700 904.2

 WWTP 1.97 NA NA

 UC 30.03 1,7251 140.7

 Ark at Las Animas 229 1,690 1,044.9

1

 Value is based on the assumption that the ungaged contribution (UC) is from a single source.

2

 Dissolved solids concentration summed for all tributaries in the reach.

[Complete site names given in table 1; UC, ungaged contribution; negative numbers indicate a loss of water from the river; WWTP, wastewater treatment 

plant]
Characteristics of Streamflow from Avondale to Las Animas 55

aquifer. For the ungaged streamflow and ungaged contributions to DS load, the DS concentration is calculated to be 

4,926 mg/L (table 14). Substantial differences between DS 

concentration in the main stem and for the ungaged contributions could indicate unaccounted for inputs or losses in the 

reach or streamflow measurement error.

For the reach from Ark at Swink to Ark at La Junta, for 

November 16–19, 2009, the ungaged streamflow from losses 

within the reach were calculated to be 42.4 ft3

/s and ungaged 

contributions to DS load was 96.5 tons/d. Assuming a single 

source, such as groundwater discharge, for the ungaged 

streamflow and ungaged contributions to DS load, the DS 

concentration of the ungaged single source is calculated to be 

843 mg/L (table 14). 

For the reach from Ark at La Junta to Ark at Las Animas, 

for the same synoptic event, the ungaged streamflow within 

the reach was calculated to be 30 ft3

/s and DS load was 

140.7 tons/d. Assuming an ungaged single source for the DS 

load, for the ungaged streamflow and ungaged contributions 

to DS load, the DS concentration of the single ungaged source 

is calculated to be 1,725 mg/L. The ungaged streamflow, 

ungaged contributions to DS load, and calculated DS concentration for an assumed ungaged single source for all six 

reaches in the LARB are applicable only for November 16–19, 

2009, and may likely vary throughout the year for different 

streamflow and water-quality conditions. 

The Quaternary alluvial deposits in the LARB are 

reported to have a median DS concentration of 1,700 mg/L, 

and concentrations range from approximately 150 to 7,000 

mg/L (Miller and others, 2010). The DS concentrations of 

ungaged sources for each of the reaches in the LARB fall 

within the reported range of groundwater DS concentrations, 

and groundwater is a plausible source for the estimated DS 

load. In addition to groundwater, ungaged irrigation return 

flow and tributaries may also be sources of the DS load; however, proportions of contributions, as in the UARB, from these 

sources are unknown. 

Selenium loads on November 16–19, 2009, in the LARB 

for main-stem sites increased from Ark at Avondale to Ark at 

Catlin (table 15), but loads decreased substantially from Ark at 

Catlin Dam to Ark nr Rocky Ford. Selenium loads increased 

again from Ark nr Rocky Ford to Ark at Las Animas. The Fort 

Lyon Storage Canal diverts 25.6 lbs/d of the selenium load 

from downstream of Ark at Catlin Dam removing the selenium 

from the Arkansas River. No selenium load contribution was 

calculated for the La Junta WWTP as no selenium samples 

were collected for this synoptic event. 

Site name

Streamflow, 

in cubic feet 

per second

Percent change

Dissolved 

solids load, 

in tons per 

day

Percent change

Within 

reach Cumulative Within reach Cumulative

Ark nr Avondale 453 876.9

Ark nr Nepesta 539 19 19 1164.2 33 33

Ark at Catlin Dam 507 -6 12 1199.2 3 37

Ark nr Rocky Ford 125 -75 -72 408.4 -66 -53

Ark at Swink 144 15 -68 730.9 79 -17

Ark at La Junta 197 37 -57 904.2 24 3

Ark at Las Animas 229 16 -49 1044.9 16 19

Table 15. Change in streamflow and loads of dissolved solids, selenium, and uranium between main-stem 

sites in the Lower Arkansas River Basin, November 16–19, 2009.

[Complete site names given in table 1]

Site name

Selenium load, 

in pounds per 

day

Percent change

Uranium 

load, in 

pounds per 

day

Percent change

Within 

reach Cumulative Within reach Cumulative

Ark nr Avondale 35.5 20.4

Ark nr Nepesta 35.2 -1 -1 30.3 49 49

Ark at Catlin Dam 38.8 10 9 34.5 14 69

Ark nr Rocky Ford 10.1 -74 -72 14.4 -58 -29

Ark at Swink 15.1 50 -57 26.3 83 29

Ark at La Junta 17.2 14 -52 32.7 24 60

Ark at Las Animas 18.9 10 -47 37.7 15 85
56 Characterization Studies of the Arkansas River, Colorado, 2009–2010

A Kruskall-Wallis test on selenium loads from samples 

collected during low-flow periods for this study from 2009–

2010 indicated statistically significant (p-value 0.0036) differences between loads at the main-stem sites. Using the Tukey’s 

test for multiple comparisons, the differences between the sites 

were statistically significant for Ark nr Nepesta to Ark at La 

Junta.

Uranium loads during November 16–19, 2009, increased 

in main-stem sites from Ark at Avondale to Ark at Catlin Dam 

(table 15), but loads decreased substantially from Ark at Catlin 

Dam to Ark nr Rocky Ford. Uranium loads increased again 

from Ark nr Rocky Ford to Ark at Las Animas. The Fort Lyon 

Storage Canal diverts 24.3 lbs/d of the uranium load from 

downstream of Ark at Catlin Dam removing the uranium from 

the Arkansas River. No uranium load contribution was calculated for the La Junta WWTP as no uranium samples were 

collected for this synoptic event. 

A Kruskall-Wallis test on uranium loads from low-flow 

samples collected for this study from 2009–2010, indicated 

statistically significant (p-value 0.048) differences between 

loads at the main-stem sites. Using the Tukey’s test for multiple comparisons, the differences between the sites are significant, as with selenium, for Ark nr Nepesta to Ark at La Junta. 

The river reaches from Ark nr Nepesta to Ark at La Junta are 

potential source areas of selenium and uranium. Further investigation would be needed to describe the source or sources in 

greater detail.

Summary

As a result of continued water-quality concerns in the 

Arkansas River, including metal contamination from historical 

mining practices, potential effects associated with storage and 

movement of water, point- and nonpoint-source contamination, population growth, storm-water flows, and future changes 

in land and water use, the Arkansas River Basin Regional 

Resource Planning Group (RRPG) developed a strategy to 

address these issues. As such, a cooperative strategic approach 

to address the multiple water-quality concerns within selected 

reaches of the Arkansas River was developed to (1) identify 

stream reaches where stream-aquifer interactions have a 

pronounced effect on water quality and (or) where reactive 

transport, and physical and (or) chemical alteration of flow 

during conveyance is occurring, (2) quantify loading from 

point sources, and (3) determine source areas and mass loading for selected constituents. 

To date in the Arkansas River Basin, most existing 

water-quality data have been collected for the purposes of 

characterizing current water-quality conditions for selected 

river reaches as they pertain to in-stream classifications and 

standards or had not been collected in a manner that allows for 

rigorous and accurate analysis of mass loading. Poor understanding of streamflow from ungaged tributary and return 

flows frequently result in poorly defined water budgets that are 

the basis for determining source contributions. The purpose 

of this report is to characterize streamflow and water-quality 

data collected along the selected reaches of the Arkansas River 

from Canon City to near Portland (Upper Arkansas River 

Basin (UARB)) and from Avondale to Las Animas (Lower 

Arkansas River Basin (LARB)) during 2009 and 2010, identify critical stream reaches where stream-aquifer interactions 

may potentially have a pronounced effect on water quality 

(or where point-source discharges are a significant load to the 

stream), and to identify potential load source areas for selected 

constituents within the UARB and LARB study reaches. 

Data-collection activities were conducted for 2 years in 

the UARB and LARB. Water-quality samples were collected 

from June through December 2009 and from May through 

October 2010. In each study area, samples were collected 

periodically to characterize the water quality throughout the 

various hydrologic conditions observed in the Arkansas River 

Basin. Additionally, a limited number of synoptic (time-oftravel) sampling events were conducted to provide a better 

understanding of the in-stream water-quality processes using 

mass-loading analyses. 

In the UARB, major-ion water-quality analyses indicate 

water from main-stem sites beginning at Canon City are a 

calcium-magnesium-bicarbonate water type and transition 

downstream to a calcium-magnesium-sulfate water type at the 

Arkansas River near Portland site. Changes in water type are 

likely due to changes in geology from igneous and metamorphic to sedimentary bedrock and the chemical composition of the various bedrocks within the basin with distance 

downstream. 

Main-stem dissolved solids (DS) (salinity) median concentrations increase downstream in the Arkansas River from 

Canon City (153 milligram per liter (mg/L)) to near Portland 

(264 mg/L). Median DS concentrations in tributaries range 

from a low of 213 mg/L in the Oak Creek tributary to a high 

of 6,305 mg/L in water sampled from Arkansas River tributary 

1 at CR 112 (Ark trib 1 at CR 112). In addition to irrigation 

return flows, inflows from groundwater may contribute to 

the high DS concentrations in the tributaries, especially those 

tributaries below Arkansas River tributary 2 at CR 119 (Ark 

trib 2 at CR 119) in Florence, Colorado, and those that drain 

the Penrose, Colorado, area where hay, pasture, orchard, and 

cropland acres are irrigated. The higher DS concentrations 

downstream from Florence can be attributed to differences 

in geology and variation in the chemical composition of the 

bedrock. 

Concentrations of dissolved selenium and dissolved uranium increase in water samples downstream from Canon City 

to near Portland. One concentration of selenium in a sample 

from Bear Creek and concentrations of uranium in samples 

from Bear, Hardscrabble, and Ranger Creeks and Brush 

Hollow equaled or exceeded U.S. Environmental Protection 

Agency (USEPA) drinking-water standards (50 and 30 μg/L, 

respectively) for those elements.

Hydrogen (δD) and oxygen (δ18O) isotope ratios in 

samples from main-stem and tributary waters for the Arkansas 
Summary 57

River have a linear trend; however, most samples plot below 

the global meteoric water line (GMWL). The samples have 

a slope similar to the Colorado local meteoric water line 

(LMWL) of 6.3. Arkansas River main-stem and some tributary 

samples can be grouped into three sets of samples showing 

temporal variability. The first set are main-stem and tributary 

samples were collected in May 2010; Chandler, Oak, and Lake 

Creeks are the most isotopically light samples, suggesting that 

the water has not been affected by evaporation. An isotope 

sample was collected at Lake Creek near the headwaters of the 

Arkansas River as a background site for snowmelt and plots 

within this group, indicating that precipitation is the source for 

this water. The next grouping of samples includes main-stem 

samples collected in June, July and August and the tributary 

sample from Oak Creek. The isotopically heaviest group of 

samples includes those collected from Beaver, Ranger, and 

Bear Creeks and Brush Hollow, indicating water from these 

tributaries has been substantially affected by or contains 

greater amounts of irrigation return flows than Fourmile 

Creek, Hardscrabble Creek, Arkansas River tributary 1 at 

CR119, Ark trib 2 at CR119, and Ark trib1 at CR112. Evaporation, decreasing snowmelt runoff, and possible increasing 

contribution of groundwater that may contain isotopically 

heavier (more δD and δ18O) water originating from irrigation recharge may explain the progression of Arkansas River 

main-stem isotope-light samples in May to heavier samples in 

August. No groundwater isotope samples have been collected 

in this area of the Arkansas River Basin, however, to compare 

to surface-water isotope samples. 

There were statistical differences between the DS loads 

in the Arkansas River during low-streamflow conditions in 

2009–2010. The groundwater DS load contribution to the 

water quality of the Arkansas River may be small in relation 

to the surface-water contribution during high-flow periods, 

and removing the high-flow samples from the data set allows 

for the groundwater contribution to be accounted for in the 

statistical analysis. Using the Tukey’s test for multiple comparisons to determine which sites were different from each 

other resulted in showing statistically significant differences 

between instantaneous DS loads for sites Arkansas River at 

Canon City to Arkansas River at Portland. The same sites 

were significant for selenium loads. The river section from 

Arkansas River at Canon City to Arkansas River at Portland 

is a potential source area of dissolved solids and selenium to 

the UARB. No statistically significant difference between sites 

was determined for uranium loads. 

To estimate the amount of ungaged contribution to DS 

loads from ungaged flows and other sources, the water balance 

for the synoptic on September 15, 2010, was used as the basis 

for calculating the concentration of DS from the ungaged 

sources. Calculated DS concentrations from ungaged sources 

for the UARB were Arkansas River at Canon City to Arkansas River near Canon City, 500.4 mg/L; Arkansas River near 

Canon City to Arkansas River at Portland, 706.8 mg/L; and 

Arkansas River at Portland to Arkansas River near Portland, 

191 mg/L. The calculated DS concentrations for each of the 

reaches in the UARB fall within the reported range of groundwater DS concentrations, and groundwater is a plausible 

source for the ungaged contribution to the DS load. In addition 

to groundwater, ungaged irrigation return flow and tributaries 

may also be sources of the DS load; however, proportions of 

contributions from these sources is unknown. The ungaged 

contributions to DS load, ungaged streamflow, and calculated 

DS concentration are only for the synoptic samples collected 

in September, 2010, and may likely vary throughout the year 

for different streamflow and water-quality conditions.

Major-ion water-quality analyses indicate water from 

main-stem sites in the LARB beginning at Avondale are a 

calcium-magnesium-sulfate type, and sulfate becomes the 

more dominant cation downstream. The LARB main-stem DS 

median concentrations increase downstream from Avondale 

(557 mg/L) to Las Animas (1,725 mg/L). Median DS concentrations in tributaries and the La Junta wastewater treatment 

plant (WWTP) range from 1,050 mg/L in Hungerford Hollow 

to 2,770 mg/L in water sampled from the Huerfano River. As 

in the UARB, irrigation return flows and inflows from groundwater likely contribute to the high DS concentrations in all the 

tributaries, whereas differences in DS concentration ranges 

between the selected tributaries can be attributed to differences 

in geology and the chemical composition of the bedrock.

Median dissolved selenium concentrations vary in the 

main-stem sites and ranged from 8.4 to 12.2 μg/L in Arkansas River near Avondale to Arkansas River at Las Animas, 

and selenium concentrations increase downstream at sites 

from Arkansas River near Nepesta to Arkansas River at La 

Junta. Median selenium concentrations measured in tributaries generally were higher than those in the main-stem sites; 

however, no concentrations of selenium in any of the sampled 

sites exceeded USEPA primary drinking-water standards. 

The median dissolved uranium concentration (39 μg/L) in 

Sixmile Creek exceeded the USEPA drinking-water standard. 

The tributaries, Sixmile Creek, IR-43, Huerfano River, and 

Chicosa Creek all drain the lower part of the Cretaceous-age 

Pierre Shale and upper part of the Niobrara Formation, which 

are suspected geologic sources of uranium. Patterson Hollow 

had two samples that exceeded the drinking-water standard, 

one collected in November 2009 and the other in June 2010. 

Timpas Creek and Crooked Arroyo each had one sample that 

exceeded the drinking-water standard. Patterson Hollow, 

Timpas Creek, and Crooked Arroyo basins are underlain by 

the upper part of the Niobrara Formation. The samples with 

exceedences were collected in the fall, when flows in these 

tributaries were low. 

Values of δD and δ18O for samples of water collected 

from main-stem sites in the LARB show a linear trend that 

plots below the GMWL (line slope of 8) and the Colorado 

LMWL (line slope of 6.3), indicating isotopic enrichment 

of δD and δ18O, representing water that has been affected by 

evaporation and possibly evapotranspiration. The main-stem 

samples collected in early June 2010 are the most isotopically 

light samples in comparison to other samples but have been 

affected by evaporation and may represent water released from 
58 Characterization Studies of the Arkansas River, Colorado, 2009–2010

the Pueblo Reservoir. The LARB main-stem samples from 

October and November 2009 and August 2010 have no clear 

temporal groupings such as those in the UARB. Samples from 

Arkansas River at Las Animas, the most downstream site in 

the LARB, are the most isotopically heavy, suggesting possible increased surface-water evaporation and (or) increasing 

contribution of groundwater that has been affected by evaporation and irrigation recharge. 

The tributary samples and samples from the La Junta 

WWTP plot below the GMWL and the Colorado LMWL, indicating that all samples are enriched in δD and δ18O, probably 

as a result of evaporation. A line calculated for the tributary 

samples is similar to the main stem (slope of 6.16). Samples 

from Sixmile Creek, IR-43, Huerfano River, Crooked Arroyo, 

and La Junta WWTP group with little or no temporal variation 

(within 1 per mil unit) in the isotope concentrations, suggesting the source water for these tributaries does not undergo 

evaporation through irrigation return and reuse. The remaining 

sampled tributaries have greater temporal variation in isotopic 

ratios suggesting multiple evaporation cycles or, as for the 

main-stem sites, possibly increasing contribution of groundwater that has been affected by evaporation and irrigation 

recharge. However, as in the UARB, no LARB groundwater 

isotope samples are available for comparison to the surfacewater samples. 

A Kruskall-Wallis test on periodic and synoptic DS loads 

in 2009–2010, excluding high streamflow samples collected in 

May, June, and July, indicated statistically significant (p-value 

0.038) differences between DS loads at the main-stem sites. 

Using the Tukey’s test for multiple comparisons, the significant differences between instantaneous DS loads at the 

main-stem sites were from Arkansas River near Rocky Ford 

to Arkansas River at La Junta. Further investigation would 

be needed to evaluate potential dissolved-solids source areas 

within the Arkansas River from Arkansas River near Rocky 

Ford to Arkansas River at La Junta. A Kruskall-Wallis test 

on selenium loads from samples collected during low-flow 

periods for this study from 2009–2010 indicated statistically 

significant (p-value 0.0036) differences between loads at the 

main-stem sites. Using the Tukey’s test for multiple comparisons, the differences between the sites were statistically significant for Arkansas River near Nepesta to Arkansas River at La 

Junta. The same statistical test for uranium loads in the LARB 

indicated the differences between the sites are significant, as 

with selenium, for Arkansas River near Nepesta to Arkansas 

River at La Junta. The river reaches from Arkansas River near 

Nepesta to Arkansas River at La Junta are potential source 

areas of selenium and uranium. Further investigation would be 

needed to describe the source or sources in greater detail.

To estimate the amount of ungaged contribution to DS 

loads from ungaged flows and other sources in the LARB, 

the water balance for the synoptic on November16–19, 2009, 

was used as the basis for calculating the concentration of 

DS from the ungaged sources. Calculated DS concentrations 

from ungaged sources for the LARB were Arkansas River 

near Avondale to Arkansas River near Nepesta, 1,069 mg/L; 

Arkansas River near Nepesta to Arkansas River at Catlin Dam, 

89.6 mg/L; Arkansas River at Catlin Dam to Arkansas River 

near Rocky Ford, not calculated; Arkansas River near Rocky 

Ford to Arkansas River at Swink, 4,926 mg/L; Arkansas 

River at Swink to Arkansas River at La Junta, 843 mg/L; and 

Arkansas River at La Junta to Arkansas River at Las Animas, 

1,725 mg/L. The Quaternary alluvial deposits in the LARB are 

reported to have a median DS concentration of 1,700 mg/L, 

and concentrations range from approximately 150 to 7,000 

mg/L. The DS concentrations of ungaged sources for each 

of the reaches in the LARB fall within the reported range of 

groundwater DS concentrations, and groundwater is a plausible source for the estimated DS load. In addition to groundwater, ungaged irrigation return flow and tributaries may also 

be sources of the DS load; however, proportions of contributions, as in the UARB, from these sources are unknown. The 

ungaged streamflow, ungaged contributions to DS load, and 

calculated DS concentration for an assumed ungaged single 

source for all six reaches in the LARB are applicable only for 

November 16–19, 2009, and may likely vary throughout the 

year for different streamflow and water-quality conditions.

Acknowledgments

The authors would like to thank the many individuals 

who assisted in the collection and interpretation of data in 

support of this report. Thanks to Colorado Division of Water 

Resources Water Commissioners (Division 2) John VanOort, 

Charlie Judge, and Don Taylor (retired) for their efforts collecting periodic specific-conductance samples. Thanks also to 

USGS personnel Sheryl Ferguson, Shaylynn Mincic, Pamela 

Tello, Crystal Brown, Ryan Kopp, Russ Lewins, and Brenden 

Ortiz (volunteer) for their tireless efforts collecting data at all 

hours of the day.

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For more information concerning this publication, contact:

Director, USGS Colorado Water Science Center

Box 25046, Mail Stop 415

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(303) 236-4882

Or visit the Colorado Water Science Center Web site at:

http://co.water.usgs.gov/
Ivahnenko and others— Characterization Studies of the Arkansas River, Colorado, 2009–2010—Scientific Investigations Report 2012–5234