U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2011–5106

Prepared in cooperation with the Bureau of Reclamation, the Colorado River

Basin Salinity Control Forum, and the Colorado River Water Conservation District

Characterization of Salinity and Selenium Loading and 

Land-Use Change in Montrose Arroyo, Western Colorado, 

from 1992 to 2010 
Front Cover:

Montrose Arroyo mancos shale outcropping. Photograph by Jennifer Moore, 

January 20, 2011. 
Characterization of Salinity and Selenium 

Loading and Land-Use Change in Montrose 

Arroyo, Western Colorado, from 1992 to 

2010

By Jennifer L. Moore

Prepared in cooperation with the Bureau of Reclamation, the Colorado River 

Basin Salinity Control Forum, and the Colorado River Water Conservation District

Scientific Investigations Report 2011–5106

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: 2011

For more information on the USGS—the Federal source for science about the Earth, its natural and living 

resources, natural hazards, and the environment, visit http://www.usgs.gov or call 1–888–ASK–USGS.

For an overview of USGS information products, including maps, imagery, and publications, 

visit http://www.usgs.gov/pubprod

To order this and other USGS information products, visit http://store.usgs.gov

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Moore, J.L., 2011, Characterization of salinity and selenium loading and land-use change in Montrose Arroyo, western 

Colorado, from 1992 to 2010: U.S. Geological Survey Scientific Investigations Report 2011-5106, 23 p.

Library of Congress Cataloging-in-Publication Data

Moore, Jennifer L.

 Characterization of salinity and selenium loading and land-use change in Montrose Arroyo, Western Colorado, 

from 1992 to 2010 / by Jennifer L. Moore ; prepared in cooperation with the Bureau of Reclamation, the Colorado 

River Basin Salinity Control Forum, and the Colorado River Water Conservation District.

 p. cm. -- (Scientific investigations report ; 2011-5106)

 Includes bibliographical references.

 ISBN 978-1-4113-3223-2 (pbk.)

 1. Water salinization--Environmental aspects--Colorado--Montrose. 2. Selenium--Environmental aspects-

-Colorado--Montrose. 3. Water--Pollution--Colorado--Montrose. 4. Land use--Environmental aspects--Colorado-

-Montrose. I. United States. Bureau of Reclamation. II. Geological Survey (U.S.) III. Colorado River Basin Salinity 

Control Forum. IV. Colorado River Water Conservation District (Colo.) V. Title. VI. Series: Scientific investigations report 

; 2011-5106. 

 TD427.S24M66 2011

 363.73809788’19--dc23

 2011036591
iii

Contents

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

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

Purpose and Scope ..............................................................................................................................3

Salinity and Selenium Control Efforts in the Colorado River Basin ..............................................3

Previous Studies ...................................................................................................................................3

Description of Study Area ...................................................................................................................4

Methods for Analysis of Salinity and Selenium Loading.........................................................................4

Data Collection......................................................................................................................................4

Load Calculations..................................................................................................................................5

Trend Calculations ................................................................................................................................6

Methods for Analysis of Land Use..............................................................................................................6

Salinity and Selenium Loading ....................................................................................................................6

Land-Use Change.........................................................................................................................................11

Summary........................................................................................................................................................21

Acknowledgments.......................................................................................................................................22

References Cited..........................................................................................................................................22

Figures

1. Map showing overview of the Lower Gunnison River Basin, Colorado..............................2

2. Map showing Montrose Arroyo study area, sampling sites, and 

surficial geology in Montrose, Colorado...................................................................................5

3. Photos showing land use in Montrose Arroyo, Montrose, Colorado, 

in 1993 and 2009 ............................................................................................................................7

4. Scatterplot of daily average salinity loads at MA2 and MA4 in Montrose 

Arroyo, Montrose, Colorado, from March 1992, to February 2010........................................8

5. Scatterplot of daily average selenium loads at MA2 and MA4 in Montrose 

Arroyo, Montrose, Colorado, from March 1992, to February 2010........................................9

6. Scatterplot of monthly salinity concentrations at MA2 in Montrose 

Arroyo, Montrose, Colorado, from March 1992, to February 2010......................................12

7. Scatterplot of monthly selenium concentrations at MA2 in Montrose 

Arroyo, Montrose, Colorado, from March 1992, to February 2010......................................12

8. Graph showing relation of selenium concentration and specific 

conductance at MA2 in Montrose Arroyo, Montrose, Colorado, from 

March 1992, to February 2010...................................................................................................13

9. Trilinear diagrams of major-ion concentrations at MA2 in Montrose 

Arroyo, Montrose, Colorado, from March 1992, to February 2010......................................13

10. Trilinear diagrams of major-ion concentrations at MA4 in Montrose 

Arroyo, Montrose, Colorado, from June 1998, to February 2010.........................................14

11-15. Maps showing:

 11. Land-use categorizations in Montrose Arroyo, Montrose, Colorado, 1993.............15

 12. Land-use categorizations in Montrose Arroyo, 

 Montrose, Colorado, 2002 ................................................................................................16
iv

 13. Land-use categorizations in Montrose Arroyo, Montrose, Colorado, 2009.............17

 14. Area identified in the current (2008) City of Montrose comprehensive 

 plan for potential land-use change in Montrose Arroyo, Colorado ..........................19

 15. Potential for land-use change beyond the current (2008) Montrose City 

 comprehensive plan in Montrose Arroyo, Colorado ...................................................20

Tables

1. Mann-Whitney and step-trend assessment at 95-percent confidence 

interval for salinity and selenium at MA4 in Montrose Arroyo, Colorado.........................10

2. Mann-Whitney and step-trend assessment at 95-percent confidence 

interval for salinity and selenium at MA2 in Montrose Arroyo, Colorado.........................10

3. Summary of land use in Montrose Arroyo, Colorado ...........................................................18

4. Summary of land-use change in Montrose Arroyo, Colorado ............................................18

5. Summary of projected potential land-use change in Montrose Arroyo, 

Montrose, Colorado....................................................................................................................18
v

Conversion Factors

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 0.4047 hectare (ha)

square foot (ft2) 0.09290 square meter (m2)

Volume

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

)

Flow rate

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

Mass

pound, avoirdupois (lb) 0.4536 kilogram (kg) 

ton 0.9072 megagram (Mg)

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

°C=(°F-32)/1.8

Horizontal coordinate information is referenced to 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).

Water year is defined in this report as the 12-month period October 1 through September 30, 

designated by the calendar year in which it ends.

Abbreviations

CDPHE Colorado Department of Public Health and Environment

CRBSCF Colorado River Basin Salinity Control Forum 

BLM U.S. Department of the Interior, Bureau of Land Management

FSA U.S. Department of Agriculture, Farm Service Agency

GIS Geographic information system

NAIP National Agriculture Imagery Program

NIWQP National Irrigation Water Quality Program

NWQL National Water Quality Laboratory

USDA U.S. Department of Agriculture

USFWS U.S. Fish and Wildlife Service

USGS U.S. Geological Survey 
Abstract

Salinity and selenium are naturally occurring and pervasive in the lower Gunnison River Basin of Colorado, including the watershed of Montrose Arroyo. Although some of the 

salinity and selenium loading in the Montrose Arroyo study 

area is from natural sources, additional loading has resulted 

from the introduction of intensive irrigation in the watershed. With increasing land-use change and the conversion 

from irrigated agricultural to urban land, land managers and 

stakeholders need information about the long-term effects of 

land-use change on salinity and selenium loading. In response 

to the need to advance salinity and selenium science, the 

U.S. Geological Survey, in cooperation with the Bureau of 

Reclamation, Colorado River Basin Salinity Control Forum, 

and Colorado River Water Conservation District, developed a 

study to characterize salinity and selenium loading and how 

salinity and selenium sources may relate to land-use change 

in Montrose Arroyo. This report characterizes changes in 

salinity and selenium loading to Montrose Arroyo from March 

1992 to February 2010 and the magnitude of land-use change 

between unirrigated desert, irrigated agricultural, and urban 

land-use/land-cover types, and discusses how the respective 

loads may relate to land-use change. Montrose Arroyo is an 

approximately 8-square-mile watershed in Montrose County 

in western Colorado. Salinity and selenium were studied in 

Montrose Arroyo in a 2001 study as part of a salinity- and 

selenium-control lateral project. The robust nature of the 

historical dataset indicated that Montrose Arroyo was a prime 

watershed for a follow-up study. 

Two sites from the 2001 study were used to monitor 

salinity and selenium loads in Montrose Arroyo in the followup study. Over the period of 2 water years and respective 

irrigation seasons (2008–2010), 27 water-quality samples were 

collected and streamflow measurements were made at the 

historical sites MA2 and MA4. Salinity and selenium concentrations, loads, and streamflow were compared between the 

Characterization of Salinity and Selenium Loading and 

Land-Use Change in Montrose Arroyo, Western Colorado, 

from 1992 to 2010

By Jennifer L. Moore

pre-lateral-project and post-growth periods and between the 

post-lateral-project and post-growth periods.

No significant differences in streamflow, salinity (concentration and load), or selenium (concentration and load) were 

found at MA4 between the pre-lateral project and post-growth 

periods or between the post-lateral-project and post-growth 

periods. The statistical analysis indicated no significant differences in streamflow or salinity (both concentration and 

load) between the pre-lateral-project and post-growth periods 

or between the post-lateral-project and post-growth periods 

at MA2; however, selenium concentrations and loads were 

significantly greater between the pre-lateral-project and 

post-growth periods and between the post-lateral-project and 

post-growth periods at MA2. Land-use change between MA4 

and MA2 may have contributed to the determined differences 

in selenium values, but the specific mechanisms causing the 

increases between periods are unknown.

The size of the urbanized area in Montrose Arroyo was 

quantified for 1993, 2002, and 2009 by using a geographic 

information system (GIS) with imagery from the specified 

years. The greatest change in land use from 1993 to 2009 was 

the increase of urban land due to conversion from irrigated 

agricultural land. The conversion of previously unirrigated 

desert to urban land or irrigated agriculture could become 

more common if urbanization and development expands into 

the eastern part of the watershed because a majority of the 

un-urbanized land in eastern Montrose Arroyo is unirrigated 

desert. 

By applying GIS to the City of Montrose 2008 comprehensive growth plan, it was estimated that approximately 

786 acres of previously irrigated agricultural land will 

be converted to urban land and 689 acres of unirrigated desert 

will be converted to urban land under the plan scenario. New 

development on previously unirrigated land in shale areas 

would likely increase the potential for mobilization of selenium and salinity from new sources to Montrose Arroyo and 

the Lower Gunnison River Basin. 
2 Characterization of salinity and selenium loading in Montrose Arroyo, western Colorado, from 1992 to 2010

Delta!(

!(

!(

!

Cedar

Uncompahgre

North Fork Gunnison

Gunnison River

Gunnison River

Gunnison

Gunnison River

Ridgway

Reservoir

Sweitzer Lake

Creek

River

Montrose

Arroyo

Whitewater

Lower

Gunnison River

Basin 

107°30'

107°55'

108°20'

108°45'

39°10'

38°20'

37°55'

38°45'

")

COLORADO

DENVER Grand

Junction

Colorado R.

Base from U.S. Geological Survey digital data 1:1,000,000

Universal Transverse Mercator projection Zone 13

North American Datum of 1983 (NAD 83)

River

Montrose

Ridgway

River

0

0

10

10 20 KILOMETERS

20 MILES

Platte R.

Arkansas R.

South

UNCOMPAHGRE UPLIFT

Figure 1. Overview of the Lower Gunnison River Basin, Colorado.

Introduction

Salinity and selenium are known water-quality impairments to streams in the Lower Gunnison Basin (fig. 1) of 

western Colorado. Salinity (or the dissolved-solids concentration) is a concern because of its adverse effects on agricultural, 

municipal, and industrial users (Butler, 2001). Selenium, a 

trace element, can dissolve in oxygenated water and move 

through the aquatic environment, where it can bioaccumulate 

in organisms and potentially reach toxic levels (Lemly, 2002). 

Salinity and selenium in the lower Gunnison River Basin are 

naturally occurring and pervasive. Many water bodies in the 

lower Gunnison River Basin, including the lower Gunnison 

River and Uncompahgre River, exceed the State chronic 

aquatic-life selenium standard of 4.6 micrograms per liter 

(Thomas, 2009). The Gunnison River Basin annually contributes roughly 1.1 million tons of salinity to the Colorado River 

(Leib, 2008). Although some of the salinity and selenium 

loading in the study area is from natural sources, additional 

loading has resulted from the introduction of intensive irrigation in the watershed. Additionally, irrigation canals and laterals delivering water are commonly unlined and, consequently, 

some irrigation water seeps into the ground and becomes 

“deep percolation,” which has been defined as any water infiltrating below the top 12 inches of soil (Mayo, 2008). Salinity 

and selenium concentration can increase as the seepage from a 

given canal or lateral moves through the groundwater system 

and ultimately to a stream (Butler, 2001). 
Introduction 3

In the Montrose Arroyo watershed, Montrose County, 

Colo., surface water and groundwater discharge to ditches and 

natural drainages that, in turn, discharge to the Uncompahgre 

and Gunnison Rivers and eventually to the Colorado River. 

Montrose Arroyo watershed (referred to hereafter simply as 

“Montrose Arroyo”) and surrounding areas have experienced 

rapid population growth and thus a change in land use. Historically, land use in Montrose Arroyo was primarily agricultural. Mayo (2008) documented, on a site-specific basis, a 

decrease in water use and deep percolation associated with the 

conversion of agricultural lands to urban land use. (The term 

“urban” in this report refers to land use as residential development, industrial facilities, commercial development, and 

public parks.) The conversion of land from irrigated agricultural to urban results in substantially lower irrigation-water 

application, reduced deep percolation, and less salinity loading 

per developed acre, with the exception of unlined ponds 

(Mayo, 2008). The conversion of previously unirrigated land 

to irrigated agricultural land or urban land is associated with 

increases in the mobilization of salinity and selenium (Fahy 

and others, 2000). With increasing land-use change and the 

conversion from irrigated agricultural to urban land, land managers and stakeholders need information about the long-term 

effects of land-use change on salinity and selenium loading.

A robust historical dataset of regional water quality 

indicated that Montrose Arroyo was a prime watershed for a 

follow-up study. Salinity and selenium were studied in Montrose Arroyo in 2001 (Butler, 2001) as part of a salinity- and 

selenium-control project. In response to the need to advance 

salinity and selenium science, the U.S. Geological Survey 

(USGS)—in cooperation with the Bureau of Reclamation; 

Colorado River Basin Salinity Control Forum (CRBSCF); 

and Colorado River Water Conservation District—developed 

a study to characterize salinity and selenium loading and how 

salinity and selenium sources may relate to land-use change in 

Montrose Arroyo. The study was designed to collect waterquality and streamflow samples at two identical sites from the 

original eight sites used by Butler (2001) and to determine 

whether salinity and selenium loading has changed over time.

Purpose and Scope

This report characterizes changes in salinity and selenium loading to Montrose Arroyo from March 1992 to 

February 2010 and discusses how changes in loads may relate 

to land-use change. The water-quality and streamflow data 

were collected and evaluated by the USGS. The data were 

used to detect salinity and selenium trends throughout the 

study period. The report also includes a detailed analysis to 

determine the magnitude of land-use change between unirrigated desert, irrigated agricultural land, and urban land-use 

types. Throughout this report, the use of the term “selenium” 

refers to dissolved selenium, in micrograms per liter, and the 

use of the term “salinity” refers to dissolved-solids concentration, in milligrams per liter.

Salinity and Selenium Control Efforts in the 

Colorado River Basin 

The history of salinity control efforts in the Colorado 

River Basin dates back to the 1970s when the Salinity Control Act, Public Law 93–320, prompted the creation of the 

CRBSCF to investigate, plan, and construct projects to reduce 

salinity loading to the Colorado River (Bureau of Reclamation, 2009). The Bureau of Reclamation (2003) estimates that 

47 percent of the salinity load in the Colorado River Basin is 

derived from natural sources, including geological formations, 

saline springs, and surface runoff. Approximately 37 percent 

of the salinity load in the Colorado River Basin results from 

irrigation, and the remaining 16 percent results from reservoirstorage effects and municipal and industrial practices.

Selenium concerns are more recent than salinity concerns 

in the Colorado River Basin. The existence of a selenium 

toxicity issue in the United States was first brought to light in 

1983 at Kesterson Reservoir in the western San Joaquin Valley 

in California (Ohlendorf and others, 1990). The U.S. Fish and 

Wildlife Service (USFWS) discovered incidences of mortality, 

deformities, and decreased reproduction in fish and aquatic 

birds at the reservoir attributed to selenium toxicity induced 

by irrigation. In order to quantify the biological effects of selenium, the National Irrigation Water Quality Program (NIWQP) 

was formed in 1985. NIWQP is a multiagency program within 

the Department of the Interior; it has done investigations at 

various irrigation projects in the Western United States to 

determine whether irrigation drainage was having adverse 

effects on water quality and on fish and wildlife (Butler and 

Leib, 2002). In 1988, NIWQP studies were initiated in the 

Uncompahgre River Basin. A robust historical dataset exists as 

a result of the NIWQP studies. More attention was given to the 

selenium issue in Colorado in 1997, when the Colorado State 

Water Quality Control Commission revised chronic aquaticlife criterion for dissolved selenium from 17 μg/L to 4.6 μg/L. 

In cooperation with NIWQP, the Gunnison Basin Selenium 

Task Force (Task Force) was established in 1998 as a group of 

private, local, State, and Federal interests to develop ideas and 

projects for reducing selenium levels in the Gunnison River 

Basin. 

Previous Studies

In 1999, the USGS, in cooperation with the Bureau of 

Reclamation and the Salinity Control Program, began a study 

(hereafter, 2001 study) to assess the effect of piping irrigation 

canals and laterals to reduce salinity and selenium loading 

in Montrose Arroyo (Butler, 2001). Five open-ditch laterals 

in the Montrose Arroyo watershed were piped as part of the 

project (hereafter, lateral project). Approximately 8.5 miles 

of open laterals were replaced with about 7.5 miles of polyvinyl chloride (PVC) pipe. The lateral project significantly 

decreased salinity loads (by 16 percent) and selenium loads 

(by 28 percent) in Montrose Arroyo. 
4 Characterization of salinity and selenium loading in Montrose Arroyo, western Colorado, from 1992 to 2010

In 2007, the USGS published a report analyzing selenium loads from 1978 to 2005 in the Lower Gunnison Basin 

(Thomas and others, 2008) in support of the development of 

Total Maximum Daily Loads (TMDLs) for selenium. Results 

for the Uncompahgre River indicated that a reduction of 69 

percent of the mean annual load of selenium for water years 

2001–2005 would be needed to meet the water-quality standard for the Uncompahgre River at Delta, Colo., streamflowgaging station.

In 2008, the USGS published a report of salinity trends 

(1986–2003) in the upper Colorado River Basin upstream 

from the Grand Valley Salinity Control Unit (Leib and Bauch, 

2008). The report details results for trends in salinity concentrations, loads, and selected major ions at selected USGS 

streamflow-gaging stations near the Grand Valley Salinity 

Control Unit. Downward trends in salinity load (average trend 

slope of 11,200 tons per year) were observed at the Gunnison 

River near Grand Junction, Colo., streamflow-gaging station.

Also in 2008, the USGS studied the effects of land-use 

change on deep percolation in the Grand Valley (Mayo, 2008). 

The report from the study presents estimates of the effects of 

conversion of agricultural land to urban land on deep percolation of irrigation water in the Grand Valley. Effects on the 

amounts of applied irrigation water and salt loading in the 

Colorado River also are discussed. The report concludes that 

the conversion of land from agricultural to residential subdivisions results in reduced irrigation-water application, reduced 

deep percolation, and reduced salt loading per developed acre, 

with the exception of unlined ponds.

A third USGS study in 2008 analyzed salinity and 

selenium trends (concentrations and loads) in the Upper 

Colorado River Basin, specifically, in three tributaries to the 

Colorado River in the Grand Valley (Leib, 2008). The report 

states that a reduction in annual salinity load in Lewis Wash 

of approximately 2,450 tons may have occurred as a result of 

either salinity-control work or land-use change, particularly 

the conversion of agricultural land to residential development. 

In 2009, the USGS analyzed selenium loading from 

contributing sources (surface water and groundwater) to 

Sweitzer Lake, which is listed on the Colorado Department 

of Public Health and Environment (CDPHE) 303d list for 

selenium impairment (Thomas, 2009). The report concluded 

that the selenium load from Garnet Canal and the Diversion 

Drain would need to be reduced (73 and 40 percent, 

respectively) in an effort to meet the chronic aquatic-life 

standard. 

Description of Study Area

Montrose Arroyo is an approximately 8-square-mile 

watershed located in Montrose County in western Colorado 

(fig. 2). Montrose Arroyo is a tributary to Cedar Creek, which 

is a tributary to the Uncompahgre River. The primary land use 

in the study area is agriculture and unirrigated desert; however, residential and commercial development is increasing 

with population growth. Public lands managed by the U.S. 

Department of the Interior, Bureau of Land Management 

(BLM) account for approximately 30 percent of Montrose 

Arroyo (City of Montrose, 2010). 

The major population center is the City of Montrose 

(population 17,834) (City of Montrose, 2010). The population 

in Montrose increased 21 percent from 2000 to 2008 (U.S. 

Census Bureau, 2010). Changes in land use have resulted in a 

shift from unirrigated desert and irrigated agricultural land to 

residential and urban land use, as well as the use of independent septic drainage systems (Thomas and others, 2008). In 

1997–98, golf course construction began in the area between 

the two sample sites MA2, Montrose Arroyo at East Niagara 

Street (USGS site identification number 382802107513301), 

and MA4, Montrose Arroyo at 6750 and Ogden Roads (USGS 

site identification number 382702107493701) (fig. 2). Another 

change in the watershed was the Montrose Arroyo demonstration project in 1998–2000, by which the Salinity Control 

Forum funded a project to replace about 8.5 miles of open 

laterals with PVC piping.

Climate in the study area is semiarid, with an average 

annual precipitation in Montrose of approximately 9.5 inches 

(Western Regional Climate Center, 2010). Average annual 

high and low temperatures range from 63 to 34 degrees Fahrenheit in Montrose. 

The geology of Montrose County is dominated in the 

west by the Uncompahgre Uplift. The Uncompahgre Uplift 

forms the Uncompahgre Plateau and is an asymmetrical anticline that plunges northwest to southeast (Thomas, 2009). The 

Uncompahgre Plateau is bordered on the east by the Uncompahgre River. On the east side of the Uncompahgre River are 

gray and yellow slopes of Mancos Shale, a marine deposit 

of the Late Cretaceous age. The Mancos Shale layer, which 

contains high levels of salinity and selenium, underlies and 

is interspersed with alluvium and gravels in the fertile floodplain of the Uncompahgre River. Most of the eastern part of 

Montrose Arroyo is underlain by Mancos Shale (8,896 acres) 

(fig. 2).

Methods for Analysis of Salinity and 

Selenium Loading

Data Collection

Two sites were used to monitor salinity and selenium 

loads in Montrose Arroyo. Over the period of 2 water years 

and respective irrigation seasons (2008–2010), 27 waterquality samples and streamflow measurements were collected 

at the historical sites MA2 and MA4 (fig. 2). The control site 

(MA4) and the outflow site (MA2) were the same sites as 

used in the Butler study that assessed the effects of the lateral 

project (2001). The current study collected data at the same 

locations to determine whether concentrations and loads had 
Methods for Analysis of Salinity and Selenium Loading 5

Figure 2. Montrose Arroyo study area, sampling sites, and surficial geology in Montrose, Colorado (from Tweto, 

1979; location within Colorado is shown in fig. 1).

changed. Butler’s study (2001) analyzed data from water years 

1992–2000. 

Instantaneous streamflow was measured by using methods described by Rantz and others (1982). Streamflow was 

measured with every sample collected to enable computation 

of loads. Salinity concentrations were determined by using the 

dissolved solids sum of constituents method, which computes 

the salinity concentration by summing the major-ion concentrations. Water-quality samples were analyzed by using 

standard USGS techniques and procedures (U.S. Geological 

Survey, variously dated). 

Approximately 10 percent of the samples collected were 

quality-control samples, which included equipment blanks and 

replicate samples. No detections were found in blank samples. 

All concentrations in replicate pairs were within 5 percent of 

each other. The percent difference is calculated using the following equation:

Percentage difference = (R1-R2) / ((R1+R2)/2) x 100 (1)

where

R1 = replicate 1

R2 = replicate 2

All samples were analyzed by the National Water 

Quality Laboratory (NWQL) in Lakewood, Colo. Major ions 

were analyzed by using methods described in Fishman and 

Friedman (1989). General quality control for the NWQL is 

described in Pritt and Raese (1992).

Load Calculations

Salinity and selenium loads were calculated by using 

the streamflow and concentration data. Instantaneous load is 

calculated by multiplying the streamflow by the concentration 

#

#

#

Base from U.S. Geological Survey digital data 1:1,000,000 

Universal Transverse Mercator projection Zone 13 North American 

Datum of 1983 (NAD 83)

Km Mancos Shale

Qgo Older gravels and alluviums

River

Watershed boundary

Sampling site

Qa Modern alluvium

Canal or lateral

EXPLANATION

Road

#

107°52'30" 107°50' 107°47'30" 107°45'

38°30'

38°27'30"

38°25'

Uncompahgre

US HIGHWAY 550

Cedar Creek

South Canal

Montrose Arroyo

River

US HIGHWAY 50

MMAA22

MA4

0 0.5 1

0 0.5 1

1.5 2 MILES

1.5 2 KILOMETERS

Dry Cedar Creek

Loutzenhizer Canal
6 Characterization of salinity and selenium loading in Montrose Arroyo, western Colorado, from 1992 to 2010

and then by a conversion factor. Salinity loads are reported 

in tons per day, and selenium loads are reported in pounds 

per day. For the loading analysis, samples for each site were 

separated into a pre-lateral-project period (all data before 

April 1999), a post-lateral-project period (April 1999–

October 2002), and a post-growth period (March 2008–

February 2010). 

Trend Calculations

To determine whether the concentrations and loads were 

significantly different between time periods, a step-trend twosample t-test (Helsel and Hirsch, 2002) was used on the data. 

Because some of the data were nonparametric (did not have a 

normal distribution), a two-sample Mann-Whitney test (Helsel 

and Hirsch, 2002) also was used. Both tests were set up so that 

the calculated probability is the likelihood that the true difference between the mean values of each time-period comparison 

is equal to zero. Salinity and selenium concentrations, loads, 

and the streamflow were compared between the pre-lateralproject period and the post-growth period and between the 

post-lateral-project period and the post-growth period. Trends 

were evaluated on the basis of statistical significance and are 

considered “highly significant” when the trend test reports 

a confidence level greater than 99 percent (p-value less than 

0.01) and “significant” when the trend test reports a confidence 

level greater than 95 percent (p-value of 0.05). Statistically 

significant trend directions were determined on the basis of 

increasing or decreasing mean and median values.

Methods for Analysis of Land Use

The size of the urbanized area in Montrose Arroyo was 

quantified for the years 1993, 2002, and 2009 by using GIS 

with imagery from the specified years. Land-use orthoquads 

for 1993 were obtained from the USGS Seamless Data Warehouse (U.S. Geological Survey, 2010). Land-use imagery for 

2002 was acquired from the City of Montrose GIS department 

(City of Montrose, 2010). Land-use imagery for 2009 was 

obtained from the U.S. Department of Agriculture (USDA) 

Farm Service Agency (FSA) under the National Agriculture 

Imagery Program (NAIP), which is aerial imagery taken 

during the agricultural growing season (U.S. Department of 

Agriculture, 2010). 

Categories of land use were consolidated into three major 

groups (irrigated agricultural, unirrigated desert, and urban 

land). All detailed land-use data were manually classified 

in ArcGIS (ESRI, 2006) to fit into the three major land-use 

classes. The image was scaled to 1:6,000, and individual plots 

of land were identified and classified by their apparent land 

use. Interpretive errors can be associated with this method of 

land-use classification. Errors with comparison between the 

specified years also exist because resolution and color conventions were not consistent between the three sets of imagery.

 Land use in riparian or wetland areas surrounded by 

unirrigated desert was classified as unirrigated desert. Land 

use in riparian or wetland areas surrounded by irrigated 

agricultural land was classified as irrigated agricultural land 

because the adjacent irrigated land is assumed to be leaching irrigation water through the adjacent riparian and wetland 

areas. Small-acreage farms, including lawns and gardens, 

surrounded by irrigated agricultural fields were classified as 

irrigated agricultural land. Urban parks and golf courses were 

classified as urban land. Land that was irrigated agricultural in 

1993, dormant in 2002, and urban land in 2009 was classified 

as irrigated agricultural land in 1993 and 2002 and classified 

as urban land in 2009. This classification scenario for dormant 

land was used because previously irrigated land, which typically sits dormant for only a short time before being developed 

as residential property, has previously been irrigated and 

leached and does not represent a true desert scrub landscape 

(previously unirrigated land) during the intermediate time 

period.

Salinity and Selenium Loading

Levels of salinity and selenium loading in Montrose 

Arroyo are an ongoing concern for local, State, and Federal agencies, local water providers, and landowners. As 

mentioned previously, Montrose Arroyo is a tributary to the 

Uncompahgre River and was studied to better understand 

salinity and selenium loading in the lower Gunnison Basin. 

Within Montrose Arroyo, several land-use changes had the 

potential to affect salinity and selenium loading. Throughout 

the land-use study period (1993–2009), urban areas expanded 

consistently within the watershed (fig. 3). This urban growth 

generally occurred to the northwest of site MA4. Again, within 

the statistical analysis, salinity and selenium data for each site 

were separated into a pre-lateral-project period (all data before 

April 1999), a post-lateral-project period (April 1999–October 2002), and a post-growth period (March 2008–February 

2010) (figs. 4–5). The lateral piping project in 1998–2000 was 

implemented between sites MA4 and MA2 as a salinity- and 

selenium-control effort. Individual-sample data for MA2 and 

MA4 can be found in the National Water Information System 

database at http://waterdata.usgs.gov/usa/nwis/rt . 

No significant trends in streamflow, salinity (concentration and load), or selenium (concentration and load) were 

found at MA4 between the pre-lateral-project and post-growth 

periods or the post-lateral-project and post-growth periods 

(table 1). Site MA4 was a control site with minimal land-use 

change and management change upstream. At MA2, the trend 

analysis indicated no significant differences in streamflow 

or salinity (both concentration and load) between the prelateral-project and post-growth periods and between the postlateral-project and post-growth periods; however, selenium 

concentrations and loads were significantly greater between 

the pre-lateral-project and post-growth periods and between 

the post-lateral-project and post-growth periods (table 2). 
Salinity and Selenium Loading 7

Figure 3. Land use in Montrose Arroyo, Montrose, Colorado, in (A) 1993 and 

(B) 2009.

107°51' 107°50'

38°28'30"

38°27'30"

107°51'30" 107°50'30" 107°49'30"

38°28'30"

38°27'30"

EXPLANATION

Watershed boundary

Base from U.S. Geological Survey digital data 

1:1,000,000 Universal Transverse Mercator projection 

Zone 13 North American Datum of 1983 (NAD 83)

#

#

#

# Sampling site

 (A) 1993

 (B) 2009

MA2

MA4

Urban land-use boundary

MA2

MA4

#
8 Characterization of salinity and selenium loading in Montrose Arroyo, western Colorado, from 1992 to 2010

Figure 4. Scatterplot of daily average salinity loads at MA2 and MA4 in Montrose Arroyo, 

Montrose, Colorado, from March 1992, to February 2010.

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

100

90

80

70

60

50

40

30

20

10

Pre-lateral project Post-lateral project Post growth

Year

Post growth

Salinity load, in tons per day Salinity load, in tons per day

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

Year

20.0

17.5

15.0

12.5

10.0

7.5

5.0

MA4

Pre-lateral project Post-lateral project

MA2
0.7

0.6

0.5

0.4

0.3

0

0

.2

0.1

Post growth

5

4

3

2

1

0

Post growth

MA4

MA2

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

Selenium load, in pounds per day Selenium load, in pounds per day

Year

Year

Pre-lateral project

Pre-lateral project Post-lateral project

Post-lateral project

Figure 5. Scatterplot of daily average selenium loads at MA2 and MA4 in Montrose Arroyo, 

Montrose, Colorado, from March 1992, to February 2010.

Salinity and Selenium Loading 9
10 Characterization of salinity and selenium loading in Montrose Arroyo, western Colorado, from 1992 to 2010

Table 1. Mann-Whitney and step-trend assessment at 95-percent confidence interval for salinity and selenium at MA4 in 

Montrose Arroyo, Colorado. 

[p, probability that the true difference between the mean values of each time-period comparison are equal to zero; ft3/s, cubic feet per second; mg/L, milligrams per liter; lb, pounds; μg/L, micrograms per liter]

Constituent Units Time 

period1

Paired t-test

(two-sided) p Mean Significance

MannWhitney

(two-sided) p

Median Significance

Streamflow

Streamflow ft3

/s 1–3 0.149 0.66; 0.95 Not significant 0.7637 0.64; 0.53 Not significant

Streamflow ft3

/s 2–3 0.808 0.90; 0.95 Not significant 0.8339 0.77; 0.53 Not significant

Salinity

Load tons 1–3 0.849 7.99; 7.74 Not significant 1.000 6.41; 6.47 Not significant

Load tons 2–3 0.609 8.25; 7.74 Not significant 0.3074 7.79; 6.47 Not significant

Concentration mg/L 1–3 0.596 5,061; 4,652 Not significant 0.6809 6,305; 5,015 Not significant

Concentration mg/L 2–3 0.820 4,498; 4,652 Not significant 0.9803 3,710; 5,015 Not significant

Selenium

Load lb 1–3 0.927 0.16; 0.16 Not significant 0.6127 0.12; 0.14 Not significant

Load lb 2–3 0.123 0.11; 0.16 Not significant 0.1278 0.09; 0.14 Not significant

Concentration μg/L 1–3 0.600 52.2; 61.2 Not significant 0.7637 64.5; 48.4 Not significant

Concentration μg/L 2–3 0.104 35.7; 61.2 Not significant 0.1741 18.0; 48.4 Not significant

1

Time period 1 refers to pre-lateral-project period (before April 1999), period 2 refers to post-lateral-project period (April 1999–October 2000), and period 3 

refers to post-growth period (March 2008–February 2010).

Table 2. Mann-Whitney and step-trend assessment at 95-percent confidence interval for salinity and selenium at MA2 in Montrose 

Arroyo, Colorado.

[p, probability that the true difference between the mean values of each time-period comparison are equal to zero; ft3/s, cubic feet per second; mg/L, 

milligrams per liter; lb, pounds; μg/L, micrograms per liter] 

Constituent Units Time 

period1

Paired 

t-test

(two-sided) 

p

Mean Significance

MannWhitney

(two-sided) 

p

Median Significance

Streamflow

Streamflow ft3

/s 1–3 0.226 11.9; 9.16 Not significant 0.1632 8.0; 8.1 Not significant

Streamflow ft3

/s 2–3 0.881 9.46; 9.16 Not significant 0.7458 9.8; 8.1 Not significant

Salinity

Load tons 1–3 0.170 49.6; 42.1 Not significant 0.0910 56.3; 38.6 Not significant

Load tons 2–3 0.683 44.2; 42.1 Not significant 0.6315 46.6; 38.6 Not significant

Concentration mg/L 1–3 0.866 2,882; 2,806 Not significant 0.9335 1,820; 2,305 Not significant

Concentration mg/L 2–3 0.680 2,634; 2,806 Not significant 0.9175 1,780; 2,305 Not significant

Selenium

Load lb 1–3 0.044 1.89; 2.49 Significant 0.0477 1.90; 2.28 Significant

Load lb 2–3 0.000 1.33; 2.49 Highly significant 0.0000 1.36; 2.28 Highly significant

Concentration μg/L 1–3 0.038 58.3; 90.9 Significant 0.0093 36.0; 73.0 Highly significant

Concentration μg/L 2–3 0.002 40.9; 90.9 Highly significant 0.0001 27.0; 73.0 Highly significant

1

Time period 1 refers to pre-lateral-project period (before April 1999), period 2 refers to post-lateral-project period (April 1999– October 2002), and period 

3 refers to post-growth period (March 2008–February 2010).
Land-Use Change 11

Land-use change between MA4 and MA2 may have contributed to the trend in selenium values, but the specific mechanisms causing the increases between periods are unknown. 

Because no trend in streamflow was found at MA4 and MA2 

but a trend of increasing load was found, much of the trend 

apparently can be attributed to increasing selenium concentrations. It is unclear why the trend analysis shows that selenium 

concentrations and loads at MA2 increased between study 

periods while salinity concentrations and loads remained 

unchanged.

Salinity and selenium concentrations at MA2 have a 

consistent seasonal pattern. Low-streamflow conditions (base 

flow) occur in November–March, when the irrigation system 

is shut off during the nonirrigation season (figs. 6–7). As 

irrigation begins in April, the concentrations decrease with the 

increase in irrigation-water delivery. 

The monthly concentration plots for salinity at MA2 (fig. 

6) do not show the same pattern as that in the selenium plot 

(fig. 7). The monthly concentration plots for selenium at MA2 

show that selenium concentrations in most months during the 

post-growth period are elevated in comparison to historical 

data. It is apparent that selenium is being mobilized between 

sites MA4 and MA2 at a higher rate since sampling began in 

1992. The increase in selenium concentrations could be attributed to increased groundwater load or changes in groundwater 

chemistry; however, such analysis was beyond the scope of 

this study. 

Selenium concentrations correlated positively with 

specific conductance (fig. 8). The relation shows a shift in the 

intercept between the post-lateral-project and post-growth 

periods. This shift indicates that selenium concentrations 

increased relative to specific conductance during the period of 

growth in Montrose Arroyo. Specific conductance can be used 

as a surrogate for salinity concentration (Salt LOAD; Liebermann and others, 1987).

Trilinear diagrams or “Piper diagrams” are useful representations of the relative percentages of major ions within 

a water-quality sample and allow for the characterization of 

ionic chemistry between study periods. Trilinear diagrams are 

presented for MA2 and MA4 in figures 9 and 10. Separation 

of the data into three study periods allows for an examination 

of temporal variations in water chemistry at each site. 

Temporal variations of major ions over the three study 

periods are not apparent. Relative concentrations of calcium, 

magnesium, and sodium plus potassium at both sites were 

consistent throughout the three study periods. Comparisons 

between sites showed similar distribution in relative percentages of the major ions. MA2 appeared to have greater variation in calcium plus magnesium and sulfate plus chloride in 

comparison to MA4. 

Land-Use Change

Correlations between land-use change and water quality often are complex. As described in the previous section, 

selenium concentrations and loads increased in Montrose 

Arroyo (site MA2). An analysis of land use in Montrose 

Arroyo is presented here to parallel these results. No direct 

correlation between land use and selenium concentrations is 

given; however, previous studies have indicated that applying 

irrigation water to previously unirrigated land can potentially 

mobilize selenium (Fahy and others, 2000). 

A common land-use change in the Montrose Arroyo 

watershed is from irrigated agricultural to urban land. Mayo’s 

study in the Grand Valley, western Colorado, concluded 

that the conversion of agricultural land to residential urban 

subdivisions results in substantially lower irrigation-water 

application, reduced deep percolation, and less salt loading per 

developed acre (Mayo, 2008). It was assumed that the acreage 

of irrigated land would decrease because of the construction 

of impermeable surfaces such as roads and housing structures. 

Results from Mayo’s study (2008) showed that mean seasontotal deep percolation decreased about 91 percent for all vegetation types when irrigated agricultural land was converted to 

urban land. 

Previously unirrigated desert can be converted to irrigated 

residential or irrigated agricultural land. This land-use change 

could become more common if urbanization and development expands into the eastern part of the watershed because a 

majority of the undeveloped land in eastern Montrose Arroyo 

is unirrigated desert. The introduction of treated domestic 

water and the use of septic systems on previously unirrigated 

land have the potential to mobilize salinity and selenium in the 

soils and potentially result in increased selenium and salinity 

loads (Fahy and others, 2000; Mayo, 2008). New residential 

developments in Montrose Arroyo are connected to the city 

sewer system (Eric Svensen, City of Montrose, oral commun., 

2010).

Land-use categorizations for 1993, 2002, and 2009 

in Montrose Arroyo are shown in figures 11–13. Irrigated 

agricultural land in 2002 and 2009 was 3,074 and 2,032 acres, 

respectively (table 3). Unirrigated desert in 2002 and 2009 

was 7,450 and 7,320 acres, respectively. Urban land in 2002 

and 2009 was 2,040 and 3,212 acres, respectively. The greatest change in land use from 1993 to 2009 was the increase of 

urban land due to conversion from irrigated agricultural land 

(tables 3–4). The land-use change from unirrigated desert to 

urban land was 367 acres from 1993 to 2009. Those trends 

could intensify as development expands in the eastward 

direction and there is less and less irrigated agricultural land 

for development. There was also a small land-use change 

(34 acres) from 1993 to 2009 of irrigated agricultural to unirrigated desert. Some of this acreage could be attributed to 

irrigated agricultural land that has been taken out of agricultural production and has gone to scrub, even though housingdevelopment plans have been proposed and some new roads 

and driveways have already been constructed. 

Another land-use change of importance has been the construction of ponds. Ponds that are unlined and perched above 

the water table have the potential to seep. In Montrose Arroyo, 

ponds have been added in developed areas for recreational or 
12 Characterization of salinity and selenium loading in Montrose Arroyo, western Colorado, from 1992 to 2010

50

100

150

200

0

Chronic water-quality standard

Selenium concentration, in micrograms per liter

January

February

March

April

May

June

July

August

September

October

November

December

Data from before March 1, 2008

Data from March 2008 - February 2010 (post-growth period)

EXPLANATION

Month

Figure 7. Scatterplot of monthly selenium concentrations at MA2 in Montrose Arroyo, 

Montrose, Colorado, from March 1992, to February 2010.

Data from before March 1, 2008

Data from March 2008 - February 2010 (post-growth period)

January

February

March

April

May

June

July

August

September

October

November

December

Salinity concentration, in milligrams per liter

EXPLANATION

Month

1,000

2,000

3,000

4,000

5,000

6,000

7,000

Figure 6. Scatterplot of monthly salinity concentrations at MA2 in Montrose Arroyo, 

Montrose, Colorado, from March 1992, to February 2010.
Land-Use Change 13

Figure 8. Relation of selenium concentration and specific conductance at MA2 in Montrose Arroyo, Montrose, 

Colorado, from March 1992, to February 2010.

EXPLANATION

Data from before March 2008

 Data from March 2008 to February 2010 (post-growth period) 

0

50

100

150

200

250

0 1,000 2,000 3,000 4,000 5,000 6,000 7,000

Selenium concentration, in micrograms 

per liter

Specific conductance, in microsiemens per centimeter

Pre-lateral project

EXPLANATION

Post-growth period

Post-lateral project

PERCENT

MA2

Calcium

 100

 80

 60

 40

 20

 0

 0

 20

 40

 60

 80

 100

Magnesium

 0

 20

 40

 60

 80

 100

Sodium 

plus Potassium

Chloride, Fluoride

 0

 20

 40

 60

 80

 100

 100

 80

 60

 40

 20

 0

Bicarbonate

 100

 80

 60

 40

 20

 0

Sulfate

 0

 20

 40

 60

 80

 100

Sulfate plus Chloride 0

 20

 40

 60

 80

 100

Calcium 

plus Magnesium

 100

 80

 60

 40

 20

 0

 100

 80

 60

 40

 20

 0

PERCENT

PERCENT

Figure 9. Trilinear diagrams of 

major-ion concentrations at MA2 

in Montrose Arroyo, Montrose, 

Colorado, from March 1992, to 

February 2010.
14 Characterization of salinity and selenium loading in Montrose Arroyo, western Colorado, from 1992 to 2010

Figure 10. Trilinear diagrams of major-ion concentrations at MA4 in 

Montrose Arroyo, Montrose, Colorado, from June 1998, to February 2010.

PERCENT

MA4

Calcium

 100

 80

 60

 40

 20

 0

 0

 20

 40

 60

 80

 100

Magnesium

 0

 20

 40

 60

 80

 100

Sodium plus Potassium

Chloride

 0

 20

 40

 60

 80

 100

 100

 80

 60

 40

 20

 0

Bicarbonate

 100

 80

 60

 40

 20

 0

Sulfate

 0

 20

 40

 60

 80

 100

Sulfate plus Chloride

 0

 20

 40

 60

 80

 100

Calcium plus Magnesium

 100

 80

 60

 40

 20

 0

 100

 80

 60

 40

 20

 0

PERCENT

PERCENT

Pre-lateral project

EXPLANATION

Post-growth period

Post-lateral project
Figure 11. Land-use categorizations in Montrose Arroyo, Montrose, Colorado, 1993. Base from U.S. Geological Survey digital data 1:1,000,000 Universal Transverse Mercator projection Zone 13 North American Datum of 1983 (NAD 83)

#

#

107°52'30" 107°50' 107°47'30" 

38°30’

38°27'30"

0 0.5 10 0.5 1

1.5 2 MILES

1.5 2 KILOMETERS

Urban land use

Irrigated agricultureUnirrigated desert

EXPLANATION

Watershed boundary

# Sampling site

38°25'
Figure 12. Land-use categorizations in Montrose Arroyo, Montrose, Colorado, 2002.

#

#

Base from U.S. Geological Survey digital data 1:1,000,000 Universal Transverse Mercator projection Zone 13 North American Datum of 1983 (NAD 83)

EXPLANATION

Irrigated agriculture

Unirrigated desertUrban land use

Watershed boundary

# Sampling site

38°27'30"

107°52'30" 107°50' 107°47'30" 10

38°30'

38°25'

0 0.5 10 0.5 1

1.5 2 MILES

2 KILOMETERS
Figure 13. Land-use categorizations in Montrose Arroyo, Montrose, Colorado, 2009. Base from U.S. Geological Survey digital data 1:1,000,000 Universal Transverse Mercator projection Zone 13 North American Datum of 1983 (NAD 83)

#

#

38°30'

38°27'30"

107°47'30" 107°45'

EXPLANATION

Irrigated agriculture

Unirrigated desertUrban land use

Watershed boundary

# Sampling site

107°52'30" 107°50'

38°25'

0 0.5 10 0.5 1

1.5 2 MILES

1.5 2 KILOMETERS

Location of ponds
18 Characterization of salinity and selenium loading in Montrose Arroyo, western Colorado, from 1992 to 2010

Table 3. Summary of land use in Montrose Arroyo, 

Colorado.

Land use

Area (acres)

1993 2002 2009

Irrigated agricultural 3,604 3,074 2,032

Unirrigated desert 7,653 7,450 7,320

Urban land 1,306 2,040 3,212

Table 4. Summary of land-use change in Montrose 

Arroyo, Colorado.

Land-use change

Area (acres)

1993–2009 2002–2009

Irrigated agricultural to urban 

land 1,539 1,042

Unirrigated desert to urban 

land 367 130

Irrigated agricultural to 

unirrigated desert 34 0

storage purposes, typically near or around subdivisions, parks, 

or golf courses. The GIS analysis of the aerial photos found 

only one new pond as part of a new subdivision between 1993 

and 2010; however, nine ponds were developed as a result 

of the construction of a golf course in Montrose Arroyo. It is 

unknown whether these ponds are perched or lined with a synthetic liner. Irrigation on the golf course began in the summer 

of 2000. It is also unknown what the irrigation practices are 

at the golf course and whether they are comparable to previous practices when the land was irrigated agriculture. Results 

from Mayo’s study (2008) indicate that the estimated amount 

of seepage from a 1.086-acre unlined pond in Mancos Shale 

derived soils is 11.70 acre-feet per season. Mayo stated that 

these ponds were perched above the water table; thus, groundwater inflow could be ignored. Mayo concluded from the pond 

dataset that unlined ponds contribute more seepage per unit 

area than either irrigated urban sites with bluegrass as cover 

or irrigated agricultural fields. Mayo suggested that control 

of seepage from unlined ponds could be an important factor 

to consider for minimizing irrigation-induced salinity loading 

to the river. Estimates of seepage and load from ponds in the 

Montrose Arroyo watershed are undetermined, and making 

such estimates was beyond the scope of this study. 

Table 5. Summary of projected potential land-use change in 

Montrose Arroyo, Montrose, Colorado.

Potential 

land-use 

change

Area (acres)

Area identified in 

City of Montrose 

comprehensive 

plan

Additional potential 

growth outside 

comprehensive 

plan area

Irrigated agricultural 

to urban land 786 1,139

Unirrigated desert to 

urban land 689 2,815

A final exercise in the analysis of land-use change was a 

GIS comparison of growth projections in the City of Montrose 

2008 Comprehensive Plan with acreage of non-Federal land 

outside the plan area (City of Montrose, 2008). The GIS estimated projections for Montrose Arroyo are that approximately 

786 acres of previously irrigated agricultural land will be converted to urban land and 689 acres of unirrigated desert will be 

converted to urban land (table 5 and fig. 14). Outside of and 

primarily to the east of the area identified as potential land-use 

change (fig. 15) (according to the 2008 Comprehensive Plan) 

is approximately 3,954 acres of non-Federal land, of which 

2,815 acres is unirrigated desert (fig. 15). This property could 

be developed in the future with approval from the City of 

Montrose. New development on previously unirrigated land in 

shale areas (fig. 2) would likely increase the potential to mobilize new selenium and salinity to Montrose Arroyo and the 

Lower Gunnison Basin. The remainder of property in eastern 

Montrose Arroyo (3,954 acres) is managed by the BLM. 

Butler’s study in 2001 and Mayo’s study in 2008 indicated that salinity and selenium loads would decrease as a 

result of the lateral piping project and land-use change from 

irrigated agricultural to urban land in Montrose Arroyo. 

Results from the current study of the surface water indicate 

that, for unknown reasons, selenium loads have increased but 

salinity loads are unchanged. Studying the mechanisms of 

groundwater in Montrose Arroyo could possibly result in a 

better understanding of salinity and selenium interactions in 

the watershed.
Figure 14. Area identified in the current (2008) City of Montrose comprehensive plan for potential land-use change in Montrose Arroyo, Colorado.

#

#

Base from U.S. Geological Survey digital data 1:1,000,000 Universal Transverse Mercator projection Zone 13 North American Datum of 1983 (NAD 83)

107°52' 107°50' 107°47'30" 107

38°30'

38°27'30"

38°25'

EXPLANATION

Sampling site

Zoned for conversion fromirrigated agriculture tourban land use

Zoned for conversion fromunirrigated desert tourban land use

Streams

Watershed boundary

#

0 0.5 10 0.5 1

1.5 2 MILES

1.5 2 KILOMETERS
Figure 15. Potential for land-use change beyond the current (2008) Montrose City comprehensive plan in Montrose Arroyo, Colorado.

#0

##0

#

EXPLANATION

Base from U.S. Geological Survey digital data 1:1,000,000 Universal Transverse Mercator projection Zone 13 North American Datum of 1983 (NAD 83)

107°52'30" 107°50' 107°47'30" 

38°30'

38°27'30"

Potential conversion from Streamsirrigated agriculture tourban land usePotential conversion fromunirrigated desert tourban land use

Sampling site

Watershed boundary

38°25'

#

0 0.5 10 0.5 1

1.5 2 MILES

1.5 2 KILOMETERS
Summary 21

Summary

Salinity and selenium are known water-quality impairments to streams in the lower Gunnison River Basin of 

western Colorado. Many water bodies in the lower Gunnison 

River Basin, including the lower Gunnison River and Uncompahgre River, exceed the State chronic aquatic-life standard 

for selenium. Although some of the salinity and selenium 

loading in the study area is from natural sources, additional 

loading has resulted from the introduction of intensive irrigation in the watershed. In Montrose Arroyo, surface water and 

groundwater discharges to ditches and natural drainages that, 

in turn, discharge to the Uncompahgre and Gunnison Rivers 

and eventually to the Colorado River. 

Salinity and selenium were studied in Montrose Arroyo in 

2001 as part of salinity- and selenium-control lateral project. 

With increasing land-use change and the conversion from irrigated agricultural to urban land, land managers and stakeholders need information about the long-term effects of land-use 

change on salinity and selenium loading. The availability of 

a robust historical dataset of regional water quality makes 

Montrose Arroyo an ideal watershed for characterizing salinity 

and selenium loading with respect to land use over time. In 

response to the need to advance salinity and selenium science, 

the U.S. Geological Survey, in cooperation with the Bureau of 

Reclamation, Colorado River Basin Salinity Control Forum, 

and Colorado River Water Conservation District, developed a 

study to characterize salinity and selenium loading and how 

salinity and selenium sources may relate to land-use change in 

Montrose Arroyo. This report characterizes changes in salinity 

and selenium loading to Montrose Arroyo from March 1992 to 

February 2010 and discusses how changes in loads may relate 

to land-use change. An analysis of the magnitude of landuse change between unirrigated desert, irrigated agricultural 

land, and urban land-use types, as well as changes in salinity 

and selenium loading in Montrose Arroyo and ways in which 

changes in loads may relate to land-use change are provided. 

Montrose Arroyo is an approximately 8-square-mile 

watershed in Montrose County, Colorado. Montrose Arroyo is 

a tributary to Cedar Creek, which is a tributary to the Uncompahgre River. The primary land use in the study area is agriculture and unirrigated desert; however, residential and commercial development is increasing with population growth. 

Public lands managed by the U.S. Department of the Interior, 

Bureau of Land Management account for approximately 30 

percent of Montrose Arroyo. 

Over the period of 2 water years and respective irrigation seasons (2008-2010), 27 water-quality samples were 

collected and streamflow measurements were made at sites 

MA2 (control site) and MA4 (outflow site) from a 2001 study. 

Salinity and selenium loads were calculated from the streamflow and concentration data. For the loading analysis, samples 

for each site were separated into three study periods relative to 

an irrigation-lateral piping project and to a subsequent period 

of expanded urban land use: the pre-lateral-project period 

encompassed all data before April 1999, the post-lateral-project period was April 1999–October 2002, and the post-growth 

period was March 2008–February 2010. Salinity and selenium 

(concentrations and loads) and streamflow were compared by 

using parametric and nonparametric statistics between the prelateral-project period and the post-growth period and between 

the post-lateral-project period and the post-growth period.

No significant trends in streamflow, salinity (concentration and load), or selenium (concentration and load) were 

found at MA4 between all study periods. The trend analysis 

also indicated no significant trends in streamflow or salinity 

(both concentration and load) at MA2. Selenium concentration and load were significantly greater at MA2 between the 

pre-lateral-project and post-growth periods and between the 

post-lateral-project and post-growth periods. Land-use change 

between MA4 and MA2 may have contributed to the observed 

selenium trend, but the specific mechanisms causing selenium 

levels to increase are unknown.

By using a geographic information system (GIS) with 

imagery from 1993, 2002, and 2009, the change in land use 

was determined. Categories of land use from various data 

sources were consolidated into three major groups (irrigated 

agricultural, unirrigated desert, and urban land). All detailed 

land-use data were manually classified to fit into the three 

major land-use classes.

The most common land-use change from 1993 to 2009 

in the Montrose Arroyo watershed was from irrigated agricultural land to urban land. The introduction of treated domestic 

water and septic systems to previously unirrigated land has 

the potential to mobilize salinity and selenium in the soils and 

potentially result in increased selenium and salinity loads. This 

land-use change type could become more common as development expands eastward and less and less irrigated agricultural 

land is available for development. Another land-use change 

of importance is the construction of ponds, most of which 

were developed during the construction of the golf course in 

Montrose Arroyo. Ponds that are unlined and perched above 

the water table have the potential to seep. 

A final exercise in the analysis of land-use change was a 

GIS comparison of growth projections in the City of Montrose 

2008 comprehensive plan with acreage of non-Federal land 

outside the plan area. The GIS estimated projections for Montrose Arroyo are that approximately 786 acres of previously 

irrigated agricultural land will be converted to urban land and 

689 acres of unirrigated desert will be converted to urban land. 

Outside of and primarily to the east of the area identified as 

potential land-use change in the 2008 Comprehensive Plan 

is approximately 3,954 acres of non-Federal land, of which 

2,815 acres is unirrigated desert. New development on previously unirrigated land in shale areas would likely increase the 

potential to mobilize new selenium and salinity to Montrose 

Arroyo and the Lower Gunnison River Basin. 
22 Characterization of salinity and selenium loading in Montrose Arroyo, western Colorado, from 1992 to 2010

Acknowledgments

The author of this report acknowledges, with gratitude 

and thanks, Dave Butler (retired USGS) and Dr. Russ Walker 

(Mesa State College), for providing technical review comments; and Ken Leib (USGS), for providing patience and 

technical insight throughout the interpretive and report-writing 

process.

References Cited

Bureau of Reclamation, 2003, Quality of water—Colorado 

River Basin: U.S. Department of the Interior Progress 

Report 21, 89 p.

Bureau of Reclamation, 2009, Colorado River Basin salinity control program federal accomplishments report for 

fiscal year 2009: accessed October 13, 2010, available 

online at http://www.usbr.gov/uc/progact/salinity/pdfs/

FedAccompRep-2009.pdf.

Butler, D.L., 2001, Effects of piping irrigation laterals on 

selenium and salt loads, Montrose Arroyo Basin, western Colorado: U.S. Geological Survey Water-Resources 

Investigations Report 01–4204, 14 p.

Butler, D.L., and Leib, K.J., 2002, Characterization of 

selenium in the lower Gunnison River Basin, Colorado, 

1988–2000: U.S. Geological Survey Water-Resources 

Investigations Report 02–4151, 26 p.

City of Montrose, Colorado, 2008, Comprehensive Plan: 

accessed May 3, 2011, at http://cityofmontrose.org/DocumentView.aspx?DID=195.

City of Montrose, Colorado, 2010, GIS zoning coverages: GIS 

Department, accessed February 10, 2010, at http://www.

cityofmontrose.org/index.aspx?NID=276.

ESRI, 2006, ArcGIS Desktop: Release 9.2. Redlands, CA: 

Environmental Systems Research Institute.

Fahy, J., Brummer, J., Fisher, F., Dearstyne, D., Brown, K., 

and Loftis, J., 2000, Statistical evaluation of sample data 

from soil profiles: Denver, Colo., U.S. Bureau of Reclamation, 52 p.

Fishman, M.J., and Friedman, L.C., 1989, Methods for the 

determination of inorganic substances in water and fluvial 

sediments: U.S. Geological Survey Techniques of WaterResources Investigations, book 5, chap. A1, 545 p. 

Helsel, D.R., and Hirsch, R.M., 2002, Statistical methods in 

water resources: U.S. Geological Survey Techniques of 

Water-Resource Investigation, book 4, chap. A3, 510 p.

Leib, K.J., 2008, Concentrations and loads of selenium in 

selected tributaries to the Colorado River in the Grand Valley, western Colorado, 2004–2006: U.S. Geological Survey 

Scientific Investigations Report 2008–5036, 36 p.

Leib, K.J., and Bauch, N.J., 2008, Salinity trends in the upper 

Colorado River Basin upstream from the Grand Valley 

Salinity Control Unit, Colorado, 1986–2003: U.S. Geological Survey Scientific Investigations Report 2007–5288, 

21 p.

Liebermann, T.D., Middelburg, R.F., and Irvine, S.A., 1987, 

User’s manual for estimation of dissolved-solids concentrations and loads in surface water: U.S. Geological Survey 

Water-Resources Investigations Report 86–4124, 51 p.

Lemly, A.D., 2002, Selenium assessment in aquatic ecosystems—A guide for hazard evaluation and water quality 

criteria: New York, Springer-Verlag, 161 p.

Mayo, J.W., 2008, Estimating the effects of conversion of agricultural land to urban land on deep percolation of irrigation 

water in the Grand Valley, western Colorado: U.S. Geological Survey Scientific Investigations Report 2008–5086, 

58 p.

Montrose, Colorado, 2010, Montrose population: Accessed 

March 1, 2010, at http://www.montrose.org/index.

aspx?nid=220.

Ohlendorf, H.M, Hothem, R.L., Bunck, C.M., and Marois, 

K.C., 1990, Bioaccumulation of selenium in birds at Kesterson Reservoir, California: Archives of Environmental 

Contamination and Toxicology, v. 19, no. 4, p. 495–507.

Pritt, J.W., and Raese, J.W., eds., 1992, Quality assurance/

quality control manual: U.S. Geological Survey Open-File 

Report 92–495, 33 p.

Rantz, S.E., and others, 1982, Measurement and computation 

of streamflow: U.S. Geological Survey Water-Supply Paper 

2175, 631 p.

Thomas, J.C., Leib, K.J., and Mayo, J.W., 2008, Analysis of 

dissolved selenium loading for selected sites in the Lower 

Gunnison River Basin, Colorado, 1978–2005: U.S. Geological Survey Scientific Investigations Report 2007–5287, 

25 p.

Thomas, J.C., 2009, Analysis of dissolved selenium loading from surface water and groundwater to Sweitzer Lake, 

Colorado, 2006–07: U.S. Geological Survey Scientific 

Investigations Report 2009–5048, 17 p.

Tweto, Ogden, comp., 1979, Geologic map of Colorado: U.S. 

Geologic Survey State Geologic Map, scale 1:500,000 

(reprinted).
References Cited 23

U.S. Census Bureau, 2010, State & County Quick Facts, Montrose County, Colorado: Accessed March 1, 2010, at 

http://quickfacts.census.gov/qfd/states/08/08085.html.

U.S. Department of Agriculture, Farm Service Agency, 2010, 

National Agriculture Imagery Program: Accessed March 5, 

2010, at http://datagateway.nrcs.usda.gov/GDGOrder.aspx. 

U.S. Geological Survey, variously dated, National field manual for the collection of water quality data: U.S. Geological 

Survey Techniques of Water-Resources Investigations, book 

9, chaps. A1–A9, accessed February 24, 2010, at 

http://pubs.water.usgs.gov/twri9A.

U.S. Geological Survey, 2007, Seamless Data Warehouse 

Accessed February 10, 2010, at http://seamless.usgs.gov/

website/seamless/viewer.htm.

Western Regional Climate Center, 2010, Period of record 

monthly climate summary, Grand Junction 6 ESE, Colorado 

(053489): Accessed March 1, 2010, at http://www.wrcc.dri.

edu/cgi-bin/cliMAIN.pl?co3489.

Publishing support provided by: 

Denver Publishing Service Center

For more information concerning this publication, contact:

Director, USGS Colorado Water Science Center

Box 25046, Mail Stop 415

Denver, CO 80225

(303) 236-4882

Or visit the Colorado Water Science Center Web site at:

http://co.water.usgs.gov/
Jennifer L. Moore— Characterization of Salinity and Selenium Loading and Land-Use Change, Western Colorado, from 1992 to 2010—SIR 2011–5106