Using the Tracer-Dilution Discharge 

Method to Develop Streamflow Records 

for Ice-Affected Streams in Colorado

By Joseph P. Capesius, Joseph R. Sullivan, Gregory B. O’Neill, and Cory A. Williams

Prepared in cooperation with the Colorado Water Conservation Board

Scientific Investigations Report 2004–5164

U.S. Department of the Interior

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

Gale A. Norton, Secretary

U.S. Geological Survey

P. Patrick Leahy, Acting Director

U.S. Geological Survey, Reston, Virginia: 2005

For sale by U.S. Geological Survey, Information Services

Box 25286, Denver Federal Center

Denver, CO 80225

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

World Wide Web: http://www.usgs.gov/

Any use of trade, product, or firm names in this publication 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:

Capesius, J.P., Sullivan, J.R., O’Neill, G.B., and Williams, C.A., 2005, Using the tracer-dilution discharge method to 

develop streamflow records for ice-affected streams in Colorado: U.S. Geological Survey Scientific Investigations 

Report 2004–5164, 14 p.
Contents

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

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

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

Background............................................................................................................................................3

Methods of Streamflow Measurement.............................................................................................3

Current-Meter Discharge Measurements...............................................................................3

Tracer-Dilution Discharge Measurements..............................................................................3

Conventional Streamflow-Record Computation..............................................................................4

Methods of Tracer-Gage Operations.................................................................................................4

Methods of the Pump-Reliability Test................................................................................................5

Using the Tracer-Gage Method to Develop Streamflow Records.........................................................5

Discharge-Measurement Comparison..............................................................................................5

Streamflow-Record Computation ......................................................................................................5

Streamflow-Record Comparison........................................................................................................7

Tracer-Pump Evaluation.......................................................................................................................9

Equipment and Error Analysis ..........................................................................................................10

Instream Flow Water Rights and Tracer-Gage Streamflow Records.........................................13

Summary and Conclusions.........................................................................................................................13

References Cited..........................................................................................................................................14

Figures

1. Map showing location of study sites in Colorado...................................................................2

2–6. Graphs showing:

 2. Concentration of chloride ions during a tracer-injection study in the stream 

water at Brandon Ditch near Whitewater, Colorado.......................................................6

 3. Comparison of current-meter and tracer-dilution discharge measurements 

at Brandon Ditch near Whitewater, Colorado ..................................................................6

 4. Comparison of current-meter and tracer-dilution discharge measurements 

at Keystone Gulch near Dillon, Colorado...........................................................................8

 5. Streamflow-record comparison between estimated, current-meter, and 

tracer-dilution methods, Brandon Ditch near Whitewater, Colorado.........................11

 6. Streamflow-record comparison between estimated, current-meter, and 

tracer-dilution methods, Keystone Gulch near Dillon, Colorado.................................11

iii
Tables

1. Setup properties for the tracer-pump test................................................................................5

2. Tracer-gage and current-meter discharge measurements, Brandon Ditch 

near Whitewater, Colorado.........................................................................................................7

3. Tracer-gage and current-meter discharge measurements, Keystone Gulch 

near Dillon, Colorado....................................................................................................................8

4. Comparison of current-meter, tracer-gage, and estimated streamflow record, 

Brandon Ditch near Whitewater, Colorado..............................................................................9

5. Comparison of current-meter, tracer-gage, and estimated streamflow record, 

Keystone Gulch near Dillon, Colorado ....................................................................................10

6. Results from the tracer-pump evaluation...............................................................................12

7. Outside and inside air temperatures for the tracer-pump evaluation ...............................13

Conversion Factors

Multiply By To obtain

mile (mi) 1.609 kilometer (km)

foot (ft) 0.3048 meter (m)

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

milliliters per minute (mL/min) 0.03785 gallon per minute (gal/min)

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

Abbreviations

mg/L milligrams per liter

μS/cm microsiemens per centimeter at 25 degrees Celsius

iv
Abstract

Accurate ice-affected streamflow records are difficult 

to obtain for several reasons, which makes the management 

of instream-flow water rights in the wintertime a challenging endeavor. This report documents a method to improve 

ice-affected streamflow records for two gaging stations in 

Colorado. In January and February 2002, the U.S. Geological 

Survey, in cooperation with the Colorado Water Conservation 

Board, conducted an experiment using a sodium chloride 

tracer to measure streamflow under ice cover by the tracerdilution discharge method. The purpose of this study was to 

determine the feasibility of obtaining accurate ice-affected 

streamflow records by using a sodium chloride tracer that 

was injected into the stream. The tracer was injected at two 

gaging stations once per day for approximately 20 minutes 

for 25 days. Multiple-parameter water-quality sensors at the 

two gaging stations monitored background and peak chloride 

concentrations. These data were used to determine discharge 

at each site. A comparison of the current-meter streamflow 

record to the tracer-dilution streamflow record shows different levels of accuracy and precision of the tracer-dilution 

streamflow record at the two sites. At the lower elevation and warmer site, Brandon Ditch near Whitewater, the 

tracer-dilution method overestimated flow by an average 

of 14 percent, but this average is strongly biased by outliers. 

At the higher elevation and colder site, Keystone Gulch near 

Dillon, the tracer-dilution method experienced problems 

with the tracer solution partially freezing in the injection 

line. The partial freezing of the tracer contributed to the 

tracer-dilution method underestimating flow by 52 percent 

at Keystone Gulch. In addition, a tracer-pump-reliability test 

was conducted to test how accurately the tracer pumps can 

discharge the tracer solution in conditions similar to those 

used at the gaging stations. Although the pumps were reliable 

and consistent throughout the 25-day study period, the pumps 

underdischarged the tracer by 5.8–15.9 percent as compared to the initial pumping rate setting, which may explain 

some of the error in the tracer-dilution streamflow record 

as compared to current-meter streamflow record.

Introduction

Accurate wintertime streamflow records are difficult to 

obtain for several reasons. First, ice cover changes the stagedischarge relation by causing a variable backwater condition, 

which can introduce error to the stage-discharge record during 

the weeks between discharge measurements. Second, ice cover 

may interfere with accurate gage-height measurement, especially on small streams with thick ice and snow cover. Third, 

wintertime weather makes current-meter measurements more 

difficult to make and potentially less accurate. These reasons, 

along with travel hazards, cause hydrographers to estimate 

large portions of the wintertime streamflow records at some 

sites in Colorado.

The inaccuracy of ice-affected streamflow records is 

particularly troublesome for streams where Instream-Flow 

(ISF) water rights exist. ISF water rights are administered 

by the State of Colorado for protection of the natural environment in support of the Instream Flow and Natural Lake 

Level Program. As part of this program, the State of Colorado

acquired ISF water rights for more than 8,000 miles of streams 

and 475 natural lakes in the State. Water-use activities such as 

snowmaking at ski resorts can infringe on the amount of water 

needed to maintain the natural environment to a reasonable 

degree as stated in the Colorado Senate Bill 97, which was 

signed into law in 1973 (http://cwcb.state.co.us/isf/Programs/

Docs/ISFQuantPolicy.pdf). To balance the demands of water 

users with ISF water rights, water-resource managers need 

more accurate wintertime streamflow records.

One alternative to an estimated streamflow record is to 

increase the frequency of streamflow measurement, either 

with a current meter or by using the tracer-dilution discharge 

method. Increased current-meter measurement frequency is 

costly and generally not used except for specific short-term 

studies. Tracer-dilution discharge methods are a potentially cost-effective alternative. This method has been used 

to determine discharge in studies conducted in Wyoming 

(Kilpatrick and Cobb, 1985) and Iowa (Soenksen, 1990; 

Melcher and Walker, 1990). These studies used a fluorescent 

dye, rhodamine-WT, to determine discharge for streams. None 

of these studies, however, attempted to develop a streamflow 

record determined exclusively by the tracer-dilution discharge 

measurements.

Using the Tracer-Dilution Discharge Method to Develop 

Streamflow Records for Ice-Affected Streams in Colorado

By Joseph P. Capesius, Joseph R. Sullivan, Gregory B. O’Neill, and Cory A. Williams
In 2002, the U.S. Geological Survey (USGS), in cooperation with the Colorado Water Conservation Board, began 

a study to assess the feasibility and accuracy of the tracerdilution discharge method to develop streamflow records at 

two streamflow-gaging stations: Keystone Gulch near Dillon, 

Colorado, and Brandon Ditch near Whitewater, Colorado 

(fig. 1). In addition, the study also examined a key piece of 

equipment for the study, the tracer-injection pumps, to determine their reliability and accuracy in discharging the tracer 

into the stream over a period of weeks.

Purpose and Scope

The purpose of this report is to describe the results 

of a study to determine the feasibility and accuracy of iceaffected streamflow records using the tracer-dilution discharge method. Data were collected from January 9 through 

February 12, 2002. This study expanded upon the methods 

used in previous tracer-dilution discharge-measurement 

studies (Kilpatrick and Cobb, 1985; Soenksen, 1990; Melcher 

and Walker, 1990; J.E. Vaill, U.S. Geological Survey, written 

commun., 1999) by incorporating automated tracer injection 

and near-continuous (3-minute interval) tracer-concentration 

measurements to determine discharge in conjunction with 

conventional current-meter measurements to produce a 

streamflow record. The tracer-dilution streamflow and 

estimated streamflow determined by straight-line interpolation methods were compared to streamflow determined from 

current-meter measurements. For simplicity of terminology, 

“tracer gage” refers to a system of equipment set up to automatically inject a tracer into a stream for which streamflow 

can be measured using the tracer-dilution discharge method. 

In addition, a separate test was conducted to measure the 

reliability and accuracy of the tracer pumps over a period 

of 25 days.

Figure 1. Location of study sites in Colorado.

0

0 25 50 KILOMETERS

25 50 MILES

Base from U.S. Geological Survey digital line graphs

1:100,000, Lambert Conformal Conic projection

Standard parallels 33o and 45o, central meridian –105o30' 

37o 

38o 

39o 

40o 

41o 108o 109o 107o 106o 105o 104o 103o

Sterling

Fort Collins

Craig

Glenwood

Springs

Gunnison

Durango

Salida

Alamosa

Grand

Junction

Whitewater

Brandon Ditch near Whitewater

390053108181700

Keystone

 Gulch near Dillon

09047700

Dillon Denver

Colorado Springs

Pueblo

Lamar

Trinidad

 Using the Tracer-Dilution Discharge Method to Develop Streamflow Records for Ice-Affected Streams in Colorado
Background

The equipment for the tracer gages was installed at two 

sites with conventional streamflow-gaging stations (fig. 1): 

Brandon Ditch near Whitewater, Colorado (gaging station 

number 390053108181700), and Keystone Gulch near Dillon, 

Colorado (gaging station number 09047700). The Brandon 

Ditch gage was chosen because of a Parshall flume at the site 

that, when covered with insulating materials, was reported to 

remain free from ice during the winter. After the project had 

begun, however, it was noted that there was enough ice formation along the sides of the flume to prevent accurate discharge 

measurement using the Parshall flume. Therefore, currentmeter measurements were used to determine discharge at this 

site.

A tracer gage also was installed at Keystone Gulch. 

This site was chosen because it had an ISF water right (Case 

Number 5–86CW092) and a wintertime water user in close 

proximity (Keystone Ski Resort). Also, the site is located at 

an elevation of 9,350 ft on the northwest side of Keystone 

Mountain (11,935 ft). The Keystone Gulch site is subject to 

extremely cold wintertime temperatures, with overnight lows 

commonly reaching below 10° Fahrenheit. This site represented a field application of the tracer-gage methodology on 

a natural stream at high elevation, as it is noted that many 

of the streams with both an ISF water right and a substantial 

wintertime water user are located at high elevations (more 

than 7,500-ft elevation) in Colorado.

The tracer pumps were tested after the tracer gage was 

tested. It was noted from a previous tracer-gage experiment 

conducted at the Western Slope (Grand Junction, Colorado) 

office of the USGS (J.E. Vaill, U.S. Geological Survey, written 

commun., 1999) that the pumping rate was the only variable 

that could not be verified because the tracer-injection lines 

were buried and frozen in place. Therefore, a tracer-pump 

evaluation was conducted in an attempt to replicate onsite 

field conditions while accurately measuring the tracer volumes. This test was run for approximately 25 days.

Methods of Streamflow Measurement

The following is a brief summary of the two streamflowmeasurement methods used in this study: the current-meter 

discharge-measurement method and the tracer-dilution 

discharge-measurement method. Those interested in further 

explanation of these or other streamflow-measurement methods are referred to Rantz and others (1982).

Current-Meter Discharge Measurements

Current-meter discharge measurements are the 

most commonly used method to determine streamflow by 

the USGS (Rantz and others, 1982). Streamflow is measured by subdividing a stream cross section and measuring 

water velocity by using a current meter. The product of the 

width and depth of the subsections and the velocity in the 

middle of the subsection is the subsection discharge. The 

subsection discharges are added to determine total discharge 

or streamflow.

Most discharge measurements are given a measurement rating or an evaluation of the accuracy of the measurement. Current-meter discharge measurements are rated from 

Excellent to Poor, depending on percentage of total flow in 

any one subsection. Ideally, no one subsection should have 

more than 5 percent of the total streamflow. Measurements 

rated Poor have over 8 percent of the total discharge in a 

single subsection, although the measurement rating may be 

downgraded based on site conditions such as changes in flow 

or possible meter malfunction. There are instances when a 

Poor current-meter discharge measurement is the best that can 

be obtained. For example, the stream cross section may be too 

narrow to allow for an adequate number of velocity measurements. Current-meter discharge measurements at sites with 

ice cover are commonly rated Poor. Current-meter discharge 

measurements were made daily at the two tracer gages from 

January 9 through February 12, 2002.

Tracer-Dilution Discharge Measurements

Streamflow also may be measured using tracers. Tracerdilution discharge measurements rely on the conservation of 

mass law. A tracer of known concentration is injected into a 

stream at a constant rate for a predetermined period of time 

upstream from the site of interest, which usually is a gaging 

station. Concentrations of the tracer are measured before, 

during, and after the tracer passes the site. Typically, the 

tracer concentrations rapidly increase, remain steady, and 

then decline rapidly at the downstream section. This form of 

tracer-dilution discharge measurement is called the constantrate method (Rantz and others, 1982). Discharge is determined 

by using the following equation, modified from Soenksen 

(1990):

Qs5.89 –7 u10 

c

i Qi  

c

g

c

b   – = ---------------------- (1)

where

Qs is the discharge of the stream, in cubic feet per 

second (ft3/s);

ci is the concentration of the tracer injected, in 

milligrams per liter (mg/L);

Qi is the rate of tracer injection, in milliliters per 

minute (mL/min);

cg is the concentration of tracer at the gaging station, 

in milligrams per liter (mg/L);

and

cb is the background concentration of the tracer at 

the gaging station, in milligrams per liter (mg/L).

Introduction 
Discharge determined by the tracer-dilution method is 

total discharge or streamflow. Use of the constant-rate tracerdilution discharge-measurement method also assumes that the 

tracer is uniformly mixed throughout the cross section and 

that there is no loss of the tracer due to sorption or a chemical 

reaction (Rantz and others, 1982). There is no assigned rating 

of the quality of the tracer-dilution discharge measurement. 

Tracer-dilution discharge measurements were made concurrently with the current-meter measurements from January 9 

through February 12, 2002.

Conventional Streamflow-Record Computation

Normally, stage is measured continuously at a stream 

site and discharge is measured periodically to develop a 

stage-discharge relation. In an ideal situation, this relation is 

constant and the amount of streamflow can be determined for 

all ranges in stage and discharge throughout the year. A dailymean discharge is determined by averaging the individual 

values of discharge measured throughout the day to develop 

a streamflow record.

At many sites subject to subfreezing temperatures, ice can 

affect the stage-discharge relation. The formation of ice causes 

a variable backwater condition that changes the stage-discharge 

condition. The variable nature of the backwater condition 

introduces error to the stage-discharge relation, but the amount 

of change is unknown during the periods between discharge 

measurements. During periods of ice, hydrographers try to 

minimize the uncertainty within the streamflow record by using 

climatologic information such as overnight low temperature 

and periods of precipitation or streamflow at hydrologically 

similar sites that are not affected by ice to estimate the streamflow record. Unfortunately, it is difficult to estimate streamflow 

record on a real-time basis at sites that become ice covered.

Another, although less preferred, method to estimate 

streamflow under ice cover is to assume a constant change 

in base flow between discharge measurements (Rantz and 

others, 1982). This method is only used at sites with complete 

ice cover in very cold conditions where temperature remains 

below freezing during long periods of time and when there are 

no comparable sites on which to base streamflow estimates.

Methods of Tracer-Gage Operations

The tracer-dilution discharge method was implemented at two tracer gages: Brandon Ditch near Whitewater, 

Colorado, and Keystone Gulch near Dillon, Colorado. The 

approach at the two sites was the same. A sodium chloride 

solution (greater than 100,000 mg/L) was injected with a 

piston-driven pump once per day, and a multiple-parameter 

water-quality sensor downstream from the gaging station 

measured chloride ion concentrations, water temperature, 

specific conductance, and pH.

One assumption of using the tracer-dilution discharge 

method is that the tracer is uniformly mixed in the streamflow. 

To ensure uniform mixing, the water-quality sensor must be 

placed at a sufficiently long distance downstream from the 

injection point. The mixing reach can be shortened by using 

multiple tracer-injection ports to help disperse the tracer more 

quickly (Rantz and others, 1982). At Brandon Ditch, the mixing reach between tracer injection and tracer measurement 

was about 300 ft along a relatively uniform channel where 

flow was expected to remain below 1 ft3/s. At Keystone Gulch, 

which consists of many pools and riffles, only a 100-ft reach 

immediately downstream from the gage was suitable for placement of the water-quality sensor. To ensure complete mixing 

of the tracer, several injection ports were used to inject the 

tracer and, before the site was covered with ice, small cobbles 

were moved to further help mix the tracer.

To test for the assumption of complete mixing, specific conductance of the stream water was measured in the 

stream at the same location as the chloride probe, both before 

and during the study period at Keystone Gulch. Specific 

conductance was measured at several points across the 

stream to ensure the chloride tracer was uniformly mixed. 

Specific conductance ranged from 131 to 137 μS/cm across 

the stream where the probe was located during an initial 

test run in November 2001 and also during the initial setup 

on January 2002. Given the consistency of the two tests, 

the tracer was assumed uniformly mixed and further measurements were not made. At Brandon Ditch, no specificconductance measurements were made due to an oversight; 

however, the chloride probe was located at the mouth of the 

Parshall flume, which was about 300 ft downstream from 

the injection point for a channel that was less than 3 ft wide. 

Therefore, the tracer was assumed to have been uniformly 

mixed, which is plausible given the long mixing reach and 

small amounts of water at Brandon Ditch.

One tracer injection per day was used at each site so as 

to limit the time between refills of the tracer reservoir and to 

reduce tracer loading into the streams. Both sites were visited 

daily to make current-meter discharge measurements through 

an opening in the ice cover and to ensure that the equipment was working properly. The tracer pump was activated 

automatically by the data-collection platform at Brandon 

Ditch. At Keystone Gulch, electrical problems in the datacollection platform prevented automatic injection, so the 

pump was activated manually. In addition, a 200-hour candle 

was placed inside the shelter containing the tracer pump to 

add a small heat source to prevent the tracer from freezing and causing damage to the pump. The two tracer gages 

were operated from January 9 through February 12, 2002, 

in an effort to record 25 days of data. At the end of the datacollection period, the tracer-gage equipment was removed 

from both sites.

 Using the Tracer-Dilution Discharge Method to Develop Streamflow Records for Ice-Affected Streams in Colorado
Methods of the Pump-Reliability Test

Upon completion of the tracer-gage portion of the 

study, the equipment was used in the pump-reliability test. 

The objective of this test was to determine the reliability and 

accuracy of the pumps to inject the sodium chloride tracer 

solution for the specified period. The tracer-pump-reliability 

test was conducted from February 23 through March 19, 2002, 

at the Western Slope office of the USGS in Grand Junction, 

Colorado. Table 1 lists the properties of each pump setup:

Setup #1 was intended to determine the consistency of the 

pumping rates for field conditions without a heat source. 

Setup #1 represents the most basic tracer-gage setup. In 

setup #2, a heat source was added to determine if the heat 

source improved the consistency of the pumping rate, noting 

that a heat source (200-hour candle) was used at the Keystone 

Gulch site. Setup #3 was used to determine what effect a 

200-ft electrical line and relay switch had on the pumping rate. This option was chosen because it was observed 

that battery voltage varied considerably during pumping, 

as recorded on the data-collection platforms; but there was 

less variation between the two streamflow-measurement 

methods at Brandon Ditch, where a relay switch and 200-ft 

electrical line were used to activate the pump. A heat source 

also was added to setup #3. Each pump setup was placed 

inside a small shelter, similar to those used at the two field 

sites.

Using the Tracer-Gage Method 

to Develop Streamflow Records

The months of October 2001 through January 2002 

were unusually dry and warm for much of Colorado. Therefore, 

it was decided to implement the tracer-gage portion of the study 

during the coldest months of the year to ensure complete ice 

cover at the two gaging stations and the tracer-pump evaluation 

would be conducted afterward. The site locations for the tracer 

gages were determined in October 2001, and the tracer-gage 

equipment was installed in early January 2002. During the 

tracer-gage study, the background chloride concentrations in the 

streams were generally low (less than 5 mg/L) and rose rapidly 

due to injection of the tracer solution, and then fell rapidly 

after the tracer pumping stopped (fig. 2). The tracer-dilution 

discharge measurements were compared with current-meter 

measurements. For the comparison, 24 tracer-dilution discharge 

measurements were made at Brandon Ditch near Whitewater 

and another 23 measurements were made at Keystone Gulch 

near Keystone. One measurement at Brandon Ditch and two 

at Keystone Gulch were not used due to equipment failures 

or the depletion of the tracer reservoir part way through the 

pumping cycle. The tracer-pump-reliability test was done from 

February 23 through March 19, 2002, at the Western Slope 

office of the USGS in Grand Junction, Colorado.

Discharge-Measurement Comparison

The tracer-dilution discharge measurements had varying levels of comparability with the current-meter discharge 

measurements at the two sites. At the Brandon Ditch gage, the 

tracer-dilution discharge measurements overestimated streamflow by an average of 14 percent (fig. 3 and table 2). However, 

the standard deviation between the two types of measurement 

was 49 percent and a 25-percent trimmed mean (mean of the 

central 50 percent of data) of –2 percent, which indicates the 

differences between the two methods was strongly biased by 

a few outliers.

At Keystone Gulch, the tracer-dilution discharge 

method was compared to the current-meter discharge by an 

average of 52 percent with a standard deviation of 13 percent 

(fig. 4 and table 3). Unlike the Brandon Ditch site, the average and standard deviation at the Keystone Gulch site were 

not greatly influenced by outliers as the 25-percent trimmed 

mean of percent differences between the two methods is 

–55 percent.

Streamflow-Record Computation

The ultimate goal of the tracer-dilution discharge 

method is to improve the accuracy of wintertime 

streamflow records. Normally, much of the iceaffected streamflow record for high-elevation sites in 

Table 1. Setup properties for the tracer-pump test.

[mL/min, milliliters per minute; <, less than; ft, feet]

Setup #1 Setup #2 Setup #3

Pumping rate 500 mL/min 450 mL/min 490 mL/min

Heat source No Yes Yes

Length of electrical wire <5 ft <5 ft 200 ft

Solar panel Yes Yes Yes

Relay switch No No Yes

Each pump was set to a pumping rate similar to that 

used at the gaging stations (450–500 mL/min). In an attempt 

to mimic how the tracer pumps were installed at the tracer 

gages, the pumps were set for a consistent rate within 

this range, rather than setting the pump rates to a specific 

value. The final pumping rate for each setup was determined at the beginning of the tracer-pump evaluation by 

filling a graduated cylinder with the sodium chloride solution for 1 minute. Once the pump rate was between 450 

and 500 mL/min, three additional rate tests were done to 

ensure consistency in the pump rates. This initial pump rate 

was used for comparison with the actual volume of solution 

pumped for 10 minutes during the 25 days of the tracer-pump 

evaluation.

Using the Tracer-Gage Method to Develop Streamflow Records 
Colorado is estimated. The estimated streamflow record 

at a site often involves using a combination of the streamflow record of a nearby site with a more reliable streamflow 

record and measurements of climatic variables such as air 

temperature. Unfortunately, the Keystone Gulch site is subject to such cold temperatures (similar to arctic conditions) 

that air-temperature variations have little effect upon 

streamflow because the stream is covered with ice and a 

heavy layer of snow. Because midwinter air temperatures 

have little effect on the amount of streamflow at the 

Keystone Gulch gage, a slow and uniform recession in 

streamflow was assumed, and a straight-line interpolation between the first (January 9) and last (February 4) 

current-meter discharge measurements was used to determine the estimated streamflow record.

At Brandon Ditch, the amount of streamflow was 

controlled by a ditch headgate upstream from the site, and 

a relatively constant discharge also was assumed. Because 

wintertime record is not computed at Brandon Ditch 

and a straight-line interpolation between current-meter 

measurements was used at Keystone Gulch to determine 

the estimated streamflow record, the estimated streamflow record for this study was interpolated between the 

first and last current-meter discharge measurements made 

on January 19 and February 12, respectively. Although 

this interpolation may not be the most desired method to 

determine wintertime streamflow record, it does provide a 

point of reference that can be used to compare the currentmeter and tracer-gage methods of record computation. 

Each discharge measurement during the study is treated as 

the daily-mean discharge, and the three methods of record 

determination—tracer dilution, current meter, and straightline interpolation (estimated)—were examined. Although 

multiple tracer-dilution measurements per day would be 

more useful for monitoring the ISF water right, this study 

used daily measurements due to the developmental nature 

of the methodology.

Figure 3. Comparison of current-meter and tracer-dilution 

discharge measurements at Brandon Ditch near Whitewater, 

Colorado.

0

0.2

0.4

0.6

0.8

0

0.2 0.6 0.8 1 1.2 1.4 1.6

CURRENT-METER DISCHARGE,

IN CUBIC FEET PER SECOND

TRA

CER-DILUTION

DISCHARGE,

IN

C

UBIC FEET PER SEC

O

N

D

1:1 Line

0.4

01/17/2002

01/18/2002

01/19/2002

01/20/2002

01/21/2002

0

20

40

60

80

100

120

140

10:43 10:58 11:12 11:26 11:41 11:55 12:10

TIME

C

HLO

RIDE C

O

N

CEN

T

R

A

TIO

N,

IN

MILLIG

R

A

M

S

PER LITER

Figure 2. Concentration of chloride ions during a tracer-injection study in the stream water at Brandon Ditch 

near Whitewater, Colorado.

 Using the Tracer-Dilution Discharge Method to Develop Streamflow Records for Ice-Affected Streams in Colorado
Streamflow-Record Comparison

Results of the three methods for determining streamflow records for Brandon Ditch and Keystone Gulch were 

compared. Current-meter measurements are typically the 

standard by which other streamflow-measurement methods are 

judged; therefore, the tracer-dilution and estimated records were 

compared to the current-meter record (tables 4 and 5). The 

daily-mean streamflow record based on current-meter discharge 

measurements, tracer-dilution discharge measurements, and 

estimated record, percent difference from the current-meter 

record, and averages of both the streamflow record and the 

percent differences are listed in tables 4 and 5.

At Brandon Ditch (fig. 5 and table 4), the average 

current-meter streamflow for the period of daily measurements 

was 0.48 ft3/s. By comparison, the average for the same period 

using the tracer-dilution method was 0.49 ft3/s, overestimating 

streamflow by an average difference of 14 percent. Most of the 

overestimation of streamflow occurred during three particular 

measurements for which the streamflows were different by 

50 percent or more. The estimated (straight-line interpolation) 

record at the site averaged 0.59 ft3/s, overestimating streamflow by an average of 64 percent.

The Keystone Gulch site had vastly different results compared to the Brandon Ditch site. The average current-meter 

discharge was 2.8 ft3/s (fig. 6 and table 5). The average tracerdilution discharge was 1.3 ft3

/s with an average of 52 percent 

underestimation of streamflow. The estimated record (straightline interpolation) was closer to the current-meter record, 

averaging 3.2 ft3/s, overestimating streamflow by an average 

of 20 percent.

Table 2. Tracer-gage and current-meter discharge measurements, Brandon Ditch near Whitewater, Colorado.

[conc., concentrations; mg/L, milligrams per liter; mL/min, milliliters per minute; ft3/s, cubic feet per second. ci, Qi, cg, and Qs are variables in equation 1]

Date

Tracer properties Chloride concentration Discharge

Tracer conc.,

mg/L,

ci

Injection rate,

mL/min,

Qi

Peak conc.,

mg/L,

cg

Background,

mg/L,

cb

Stream 

discharge,

ft3/s,

Qs

Measured,

ft3/s

Percent

difference

01/09/02 146,000 450 94 2 0.42 0.49 –14

01/11/02 146,000 450 117 2 0.34 0.36 –6

01/12/02 146,000 450 83 2 0.48 0.56 –14

01/13/02 146,000 450 70 2 0.57 0.74 –23

01/14/02 146,000 450 450 2 0.09 0.06 50

01/15/02 146,000 540 100 1 0.47 0.43 9

01/16/02 146,000 540 106 1 0.44 0.50 –12

01/17/02 146,000 540 115 1 0.41 0.47 –13

01/18/02 146,000 540 98 1 0.48 0.41 17

01/19/02 146,000 540 93 1 0.50 0.18 178

01/20/02 146,000 540 92 1 0.51 0.67 –24

01/21/02 146,000 540 90 1 0.52 0.55 –5

01/22/02 146,000 540 93 1 0.50 0.60 –17

01/23/02 146,000 540 101 1 0.46 0.47 –2

01/24/02 146,000 540 104 1 0.45 0.45 0

01/25/02 146,000 540 91 1 0.52 0.22 136

01/26/02 146,000 540 86 1 0.55 0.65 –15

01/27/02 146,000 540 78 1 0.60 0.38 58

01/28/02 146,000 540 92 1 0.51 0.55 –7

01/29/02 146,000 540 89 1 0.53 0.47 13

02/01/02 146,000 540 80 1 0.59 0.47 26

02/02/02 146,000 540 80 1 0.59 0.48 23

02/03/02 146,000 540 78 1 0.60 0.66 –9

02/04/02 146,000 505 74 1 0.59 0.70 –16

Average 0.49 0.48 14

Chloride concentration accuracy limit is ±2 mg/L Standard deviation 0.11 0.16 49

Using the Tracer-Gage Method to Develop Streamflow Records 
The results of the streamflow-record comparison 

using the available data are somewhat inconclusive; however, the inaccuracy of the Keystone Gulch tracer-dilution 

record is apparent. The inaccuracy at this site could be due 

more to equipment problems related to partial freezing of 

the tracer rather than to shortcomings of the methods. The 

results from the Brandon Ditch site show how streamflow 

records may be improved using the tracer-dilution method 

compared to using an estimated (straight-line interpolation) 

record. At most ice-affected sites, the additional data collection by the current-meter method cannot be justified due 

to the cost of visiting a site once per day. At sites where ISF 

water rights and wintertime water users come in conflict, 

however, the tracer-dilution method may provide improved 

streamflow record to help water-resource managers equitably 

distribute water.

Table 3. Tracer-gage and current-meter discharge measurements, Keystone Gulch near Dillon, Colorado.

[conc., concentrations; mg/L, milligrams per liter; mL/min, milliliters per minute; ft3/s, cubic feet per second. ci, Qi, cg, and Qs are variables in equation 1]

Date

Tracer properties Chloride concentration Discharge

Tracer conc.,

mg/L,

ci

Injection rate,

mL/min,

Qi

Peak conc.,

mg/L,

cg

Background,

mg/L,

cb

Stream 

discharge,

ft3/s,

Qs

Measured,

ft3/s

Percent

difference

01/17/02 108,000 560 30 2 1.3 3.9 –67

01/18/02 108,000 560 20 2 2.0 3.4 –41

01/19/02 108,000 560 60 2 0.61 1.9 –68

01/20/02 108,000 560 33 1 1.1 2.8 –61

01/21/02 108,000 560 30 1 1.2 3.2 –63

01/22/02 108,000 560 20 1 1.9 3.6 –47

01/23/02 108,000 560 32 2 1.2 2.8 –57

01/24/02 108,000 560 49 1 0.74 2 –63

01/25/02 108,000 560 38 1 0.96 3.1 –69

01/26/02 108,000 560 24 1 1.6 3.4 –53

01/27/02 108,000 560 24 2 1.6 3.1 –48

01/28/02 108,000 560 25 2 1.6 3.1 –48

01/29/02 108,000 560 25 2 1.6 2.9 –45

02/01/02 108,000 560 45 2 0.83 1.8 –54

02/02/02 178,000 670 62 2 1.2 2.3 –48

02/04/02 178,000 640 65 3 1.1 2.9 –62

02/05/02 178,000 510 30 2 1.9 2.5 –24

02/06/02 178,000 480 30 2 1.8 2.6 –31

02/07/02 178,000 620 66 3 1.0 3.1 –68

02/09/02 178,000 280 28 2 1.1 2.4 –54

02/10/02 178,000 250 20 2 1.5 2.3 –35

02/11/02 178,000 260 22 2 1.4 2.3 –39

02/12/02 178,000 210 24 2 1.0 2.5 –60

Average 1.3 2.8 –52

Chloride concentration accuracy limit is ±2 mg/L Standard deviation 0.38 0.54 13

Figure 4. Comparison of current-meter and tracer-dilution 

discharge measurements at Keystone Gulch near Dillon, Colorado.

CURRENT-METER DISCHARGE,

IN CUBIC FEET PER SECOND

TRA

CER-DILU

TIO

N

DISCHARGE,

IN

C

UBIC FEET PER SEC

O

N

D

1:1 Line

0

1

2

3

4

0 1 2 3 4 5 6 7 8

 Using the Tracer-Dilution Discharge Method to Develop Streamflow Records for Ice-Affected Streams in Colorado
Tracer-Pump Evaluation

The reliability and accuracy of the piston-driven tracer 

pumps were evaluated during a test done from February 23 

through March 19, 2002 (table 6). The pumps were evaluated 

because there was no practical method of independently measuring the output from the tracer pump, such as using a flow 

meter or volumetric measurements, because the injection lines 

were frozen in place. Also, this pump evaluation examined the 

consistency of three identical pumps to determine the pump 

reliability in a simulated field environment.

The three pumps were set up to automatically discharge 

a sodium chloride tracer solution for 10 minutes once per day. 

The setups were numbered in sequential order as setup #1, 

setup #2, and setup #3, and the specifications of each pump 

setup are given in table 1. Table 7 shows the air temperatures 

outside and inside the shelters while the pump test was in 

progress as an indication of how well the heat source helped 

to regulate the air temperature inside the structure. For the 

pump test, a propane pilot light was used as a heat source.

Each pump setup underdischarged the tracer as compared 

to the initial setting (see table 6). Setup #1 underdischarged 

the tracer by an average of –10.4 percent, setup #2 by 

–15.9 percent, and setup #3 by –5.8 percent. However, each 

pump setup was consistent in the volume of tracer discharged. 

Setup #1 had a standard deviation of the percent differences of 

5.0 percent, setup #2 a standard deviation of 2.8 percent, and 

setup #3 a standard deviation of 3.6 percent.

These results from the tracer-pump evaluation support the 

assumption that the tracer pumps can consistently discharge 

the tracer solution for short time periods. However, to properly 

gage the tracer-injection rate, the results of this study also 

show that care must be taken when determining the pump rate 

before the pump is allowed to function automatically.

Table 4. Comparison of current-meter, tracer-gage, and estimated streamflow record, Brandon Ditch near Whitewater, Colorado.

[ft3/s, cubic feet per second; --, no data]

Date Measured,

ft3/s

Tracer,

ft3/s

Percent

difference

Estimated,

ft3/s

Percent

difference

01/09/02 0.49 0.42 –14 0.49 0

01/10/02 -- -- -- 0.50 --

01/11/02 0.36 0.34 –6 0.51 41

01/12/02 0.56 0.48 –14 0.51 –8

01/13/02 0.74 0.57 –23 0.52 –29

01/14/02 0.06 0.09 50 0.53 783

01/15/02 0.43 0.47 9 0.54 25

01/16/02 0.50 0.44 –12 0.55 9

01/17/02 0.47 0.41 –13 0.55 18

01/18/02 0.41 0.48 17 0.56 37

01/19/02 0.18 0.50 178 0.57 217

01/20/02 0.67 0.51 –24 0.58 –14

01/21/02 0.55 0.52 –5 0.59 7

01/22/02 0.60 0.50 –17 0.59 –1

01/23/02 0.47 0.46 –2 0.60 28

01/24/02 0.45 0.45 0 0.61 36

01/25/02 0.22 0.52 136 0.62 181

01/26/02 0.65 0.55 –15 0.63 –4

01/27/02 0.38 0.60 58 0.63 67

01/28/02 0.55 0.51 –7 0.64 17

01/29/02 0.47 0.53 13 0.65 38

01/30/02 -- -- -- 0.66 --

01/31/02 -- -- -- 0.67 --

02/01/02 0.47 0.59 26 0.67 43

02/02/02 0.48 0.59 23 0.68 42

02/03/02 0.66 0.60 –9 0.69 5

02/04/02 0.70 0.59 –16 0.70 –0

Average 0.48 0.49 14 0.59 64

Using the Tracer-Gage Method to Develop Streamflow Records 
Equipment and Error Analysis

Probably the largest source of unquantifiable error in 

the tracer-dilution method is how the equipment works under 

extremely cold conditions. Although the pump-evaluation 

test examined the pump reliability to automatically inject the 

tracer during cold temperatures, the temperatures were not 

as cold as those at the Keystone Gulch gaging station. The 

tracer-gage results at Keystone Gulch showed that the tracer 

gage underestimated discharge at this site. It was noted during 

the study that the tracer pump had difficulty discharging the 

tracer solution, and partial freezing of the tracer solution in the 

injection lines was observed. Any reduction in the discharge 

of the tracer would cause an underestimation of discharge 

using the tracer-dilution method. In addition, a subsequent 

tracer-gage study (David Clow, U.S. Geological Survey, oral 

commun., 2003) experienced difficulty in accurately measuring the instream tracer concentration at a nearby site while the 

same probe performed well at a lower elevation, warmer site. 

Future work in the tracer-dilution method will need to focus 

on improving the reliability of the equipment at very cold 

temperatures.

There are additional sources of error that are quantifiable 

that also may have introduced error in the accuracy analysis of 

the tracer-dilution method. First, there are accuracy limitations 

to the equipment that was used during the study. The chloride 

ion probes had a 2-mg/L accuracy limit. This accuracy limit 

may have introduced an error ranging from less than 1 percent 

to 3 percent for the flows at Brandon Ditch and an error ranging 

from 3 to 10 percent at Keystone Gulch, based on peak-chloride 

concentrations. A lower peak-chloride concentration was used 

at Keystone Gulch compared to Brandon Ditch because high 

sodium chloride concentration could be potentially harmful to 

the ecosystem of the stream. A higher concentration, however, 

would have helped reduce any probe-accuracy errors.

The use of chloride as the tracer has limited application 

in Colorado. One requirement for a tracer is that the background concentrations of the tracer are low. Because sodium 

Table 5. Comparison of current-meter, tracer-gage, and estimated streamflow record, Keystone Gulch near Dillon, Colorado.

[ft3/s, cubic feet per second; --, no data]

Date Measured,

ft3/s

Tracer,

ft3/s

Percent

difference

Estimated

record

Percent

difference

01/17/02 3.9 1.3 –67 3.9 0

01/18/02 3.4 2.0 –41 3.8 12

01/19/02 1.9 0.61 –68 3.8 100

01/20/02 2.8 1.1 –61 3.7 32

01/21/02 3.2 1.2 –63 3.7 16

01/22/02 3.6 1.9 –47 3.6 0

01/23/02 2.8 1.2 –57 3.6 29

01/24/02 2.0 0.74 –63 3.5 75

01/25/02 3.1 0.96 –69 3.5 13

01/26/02 3.4 1.6 –53 3.4 0

01/27/02 3.1 1.6 –48 3.4 10

01/28/02 3.1 1.6 –48 3.3 6

01/29/02 2.9 1.6 –45 3.2 10

01/30/02 -- -- -- 3.2 --

01/31/02 -- -- -- 3.1 --

02/01/02 1.8 0.83 –54 3.1 72

02/02/02 2.3 1.2 –48 3.0 30

02/03/02 -- -- -- 3.0 --

02/04/02 2.9 1.1 –62 2.9 0

02/05/02 2.5 1.9 –24 2.9 16

02/06/02 2.6 1.8 –31 2.8 8

02/07/02 3.1 1.0 –68 2.8 –10

02/08/02 -- -- -- 2.7 --

02/09/02 2.4 1.1 –54 2.6 8

02/10/02 2.3 1.5 –35 2.6 13

02/11/02 2.3 1.4 –39 2.5 9

02/12/02 2.5 1.0 –60 2.5 0

Average 2.8 1.3 –52 3.2 20

10 Using the Tracer-Dilution Discharge Method to Develop Streamflow Records for Ice-Affected Streams in Colorado
0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

01/09/2002 01/14/2002 01/19/2002 01/24/2002 01/29/2002 02/03/2002

DATE

DIS

C

H

ARGE, IN CUBIC FEET PER SECOND

Current meter

Tracer dilution

Estimated

No measurement

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

01/17/2002 01/24/2002 01/31/2002 02/07/2002

DATE

DISCHARGE, IN CUBIC FEET PER SECOND

Current meter

Tracer dilution

Estimated

No measurement

No measurement

No measurement

Figure 5. Streamflow-record comparison between estimated, current-meter, and tracer-dilution methods, 

Brandon Ditch near Whitewater, Colorado.

Figure 6. Streamflow-record comparison between estimated, current-meter, and tracer-dilution methods, 

Keystone Gulch near Dillon, Colorado.

Using the Tracer-Gage Method to Develop Streamflow Records 11
chloride and magnesium chloride are used as road deicers 

throughout Colorado, the sites where chloride can be used 

as a tracer is limited to sites where few roads exist. In the 

mountains of Colorado, most roads follow at least portions of 

the stream valleys, thus limiting the use of chloride as a tracer 

to a few sites without roads that are used in the winter, such as 

Keystone Gulch.

Another limiting factor in the use of chloride as a tracer 

is the accuracy limitations of the in-situ water-quality probes. 

Most standard in-situ water-quality probes at present (2003) 

have an accuracy limit of 2 mg/L for the detection of chloride. 

Therefore, the peak concentration of chloride in the stream during the tracer-injection period should be at least 40–100 mg/L. 

Use of a tracer with a lower detection limit, such as rhodamineWT dye, would allow for less tracer loading into the stream 

and a greater contrast between the background and peak tracer 

concentrations, which would reduce the error introduced by the 

accuracy limitations of the tracer probe. Another benefit of a 

lower detection limit of another tracer is the less frequent need 

to refill the tracer reservoir.

A second source of error in the equipment is the tracer 

pump. The pump-reliability test showed that the tracer pumps 

discharged less tracer solution as compared to the initial setting of the pumps. The pump-reliability test showed the pumps 

discharged 5.8–15.9 percent less tracer solution than was originally set. This decrease in the tracer discharge into the stream 

would cause an underestimation of discharge at the gaging 

stations by the same magnitude, but much of this error could 

be reduced with additional preparation of the tracer gage. 

One solution to this problem is to develop a better method 

to measure how much tracer is being discharged during a 

tracer-dilution study and to repeatedly check the pumping rate 

throughout the study without interfering with the operation of 

the tracer-injection equipment.

Table 6. Results from the tracer-pump evaluation.

[%, percent; mL, milliliters; --, no data]

Date

Pump setup #1: 5,000 mL Pump setup #2: 4,500 mL Pump setup #3: 4,900 mL

Volume % difference 

from 4,800 mL Volume % difference from 4,500 mL Volume % difference from 4,900 mL

02/23/2002 4,930 –1.4 -- -- 4,738 –3.3

02/24/2002 4,820 –3.6 -- -- 4,659 –4.9

02/25/2002 4,210 –15.8 -- -- 4,040 –17.6

02/26/2002 4,029 –19.4 -- -- 4,536 –7.4

02/27/2002 -- -- -- -- -- --

02/28/2002 4,376 –12.5 -- -- 4,396 –10.3

03/01/2002 -- -- -- -- -- --

03/02/2002 4,622 –7.6 4,000 –11.1 4,500 –8.2

03/03/2002 4,148 –17.0 3,876 –13.9 4,548 –7.2

03/04/2002 4,216 –15.7 3,746 –16.8 4,425 –9.7

03/05/2002 4,288 –14.2 3,844 –14.6 4,503 –8.1

03/06/2002 4,396 –12.1 3,902 –13.3 4,652 –5.1

03/07/2002 4,559 –8.8 3,648 –18.9 4,712 –3.8

03/08/2002 4,720 –5.6 3,734 –17.0 4,693 –4.2

03/09/2002 4,594 –8.1 3,897 –13.4 4,805 –1.9

03/10/2002 4,364 –12.7 3,560 –20.9 4,550 –7.1

03/11/2002 4,590 –8.2 3,728 –17.2 4,724 –3.6

03/12/2002 4,498 –10.0 3,860 –14.2 4,744 –3.2

03/13/2002 4,210 –15.8 3,784 –15.9 4,722 –3.6

03/14/2002 4,104 –17.9 3,904 –13.2 4,810 –1.8

03/15/2002 4,574 –8.5 3,734 –17.0 4,714 –3.8

03/16/2002 4,656 –6.9 3,876 –13.9 4,784 –2.4

03/17/2002 4,704 –5.9 3,794 –15.7 4,670 –4.7

03/18/2002 4,688 –6.2 3,726 –17.2 -- --

03/19/2002 4,716 –5.7 3,512 –22.0 4,670 –4.7

Mean 4,479 –10.4 3,785 –15.9 4,618 –5.8

Standard deviation 248 5.0 126 2.8 176 3.6

Minus first 2 days

Mean 4,441 –11.2 3,766 –16.3 4,610 –5.9

Standard deviation 224 4.5 118 2.6 182 3.7

12  Using the Tracer-Dilution Discharge Method to Develop Streamflow Records for Ice-Affected Streams in Colorado
Table 7. Outside and inside air temperatures for the tracer-pump 

evaluation.

[°C, degrees Celsius; --, no data]

Date Temperatures for pump test

Inside, °C Outside, °C

02/23/2002 14 14

02/24/2002 20 5

02/25/2002 13.5 1.5

02/26/2002 6 –4

02/27/2002 –3 –4

02/28/2002 12 1

03/01/2002 -- --

03/02/2002 10 –4

03/03/2002 10 –6

03/04/2002 11.5 –4

03/05/2002 13 5

03/06/2002 17 8

03/07/2002 9 9

03/08/2002 14.5 3.5

03/09/2002 4 –4

03/10/2002 18 8.5

03/11/2002 23.5 11

03/12/2002 20 10.5

03/13/2002 27 15

03/14/2002 15.5 1

03/15/2002 22 11.5

03/16/2002 14 0.5

03/17/2002 17 7

03/18/2002 6.5 3.5

A third source of quantifiable error is the accuracy of 

the current-meter discharge measurements. At Brandon Ditch, 

most of the measurements made for this study are rated Poor 

or Fair, which indicates a greater than 8-percent accuracy 

limit and an 8-percent accuracy limit, respectively (Rantz and 

others, 1982). The measurement of any volume of water with 

a current meter has small amounts of potential error that, at 

small discharges, may become a larger portion of the total 

flow. At the Brandon Ditch gage, depth of flow did not exceed 

0.9 ft during the study period, and 9 of the 25 current-meter 

measurements had more than 75 percent of the depth measurements less than 0.3 ft, which is the minimum depth of water 

needed for proper operation of a pygmy flow meter. Similarly 

at Keystone Gulch, all but one of the discharge measurements 

were rated Poor. While the error in the current-meter measurements was probably the least of the three error sources 

and likely the most difficult to remove, a combination of the 

unquantifiable error from equipment difficulties at very low 

temperatures and all three quantifiable error sources may 

help to explain some of the disagreement between the tracerdilution and current-meter streamflow records.

Instream Flow Water Rights and Tracer-Gage 

Streamflow Records

Initially during the study, it was assumed that Keystone 

Gulch near Dillon, given the high elevation and cold temperatures at the site, maintained a slowly receding base flow 

throughout the winter. The current-meter measurements made 

at the site show that this slow recession does not occur. Part 

of the reason for the variable discharge at Keystone Gulch is 

an apparent return flow of water from snowmaking activities. 

The return flows enter Keystone Gulch about 500 ft upstream 

from the gaging station. It is acknowledged that the ISF water 

right for Keystone Gulch is 1.8 ft3/s (ISF Program Database), 

and the current-meter measurements show that this minimal 

amount of streamflow was present in the stream throughout 

the study period (January 9–February 12).

Although the tracer-dilution method needs considerable 

improvement and refinement before it can help water-resource 

managers administer ISF water rights, future developments 

and technological improvements may provide unforeseen 

advancements. Development of in-situ water-quality probes to 

measure environmentally friendly tracers such as rhodamineWT dye and the implementation of satellite or other telemetry 

could allow a water-resource manager to cost-effectively and 

accurately administer ISF water rights.

As the tracer-dilution method is improved and applied, 

consideration needs to be given to the compatibility of 

current-meter and tracer-dilution streamflow records. Water 

rights have been historically based upon streamflow records 

determined at gaging stations where current meters are used 

to measure flow. Studies using tracers have shown that tracerdilution discharge measurements may overestimate flow in 

cobble-bed streams (Zellweger and others, 1989; Kimball, 

1996) because the tracer can be diluted by water flowing 

through the cobble alluvium that cannot be measured with 

a current meter. The discrepancies between the two methods 

may lead to conflicting streamflow values.

Summary and Conclusions

The U.S. Geological Survey, in cooperation with the 

Colorado Water Conservation Board, conducted a study 

examining the feasibility and accuracy of ice-affected streamflow records using an automatically injected tracer and the 

constant-rate tracer-dilution discharge method to determine 

streamflow. This tracer-dilution study had two components. 

The first was to compare the accuracy of a current-meter 

ice-affected streamflow record with both tracer-dilution and 

estimated (straight-line interpolation) streamflow records. The 

second was to examine the reliability of the tracer pumps to 

consistently and accurately discharge a specified volume of 

the tracer solution.

The first component of the study had varying levels of 

comparability between the current-meter, the tracer-dilution, 

and the straight-line interpolation streamflow records. At 

Summary and Conclusions 13
the lower elevation and warmer site, Brandon Ditch near 

Whitewater, the tracer-dilution method overestimated the 

amount of streamflow by an average of 14 percent. This average overestimation was strongly biased by 3 days for which 

the streamflows were 50 percent, or more, different. For the 

24 days where a successful tracer-dilution discharge measurement was made, the daily-mean flow was 0.49 ft3

/s, whereas 

the daily-mean flow using a current meter was 0.48 ft3/s and 

the daily-mean flow using straight-line interpolation was 

0.59 ft3/s. At the higher elevation and colder site, Keystone 

Gulch near Dillon, the tracer-dilution method underestimated 

flow by an average of 52 percent. For the 23 days where a 

successful tracer-dilution discharge measurement was made, 

the daily-mean flow was 1.3 ft3/s, whereas the daily-mean flow 

using a current meter was 2.8 ft3/s and the estimated straightline interpolation record was 3.2 ft3

/s.

Several factors contributed to the difficulties of developing streamflow records using the tracer-dilution discharge 

method. The first is the reliability of the equipment to work 

at a cold, high-elevation site as the tracer pump was less reliable at Keystone Gulch. Other factors that may have caused 

inaccuracies between the current-meter and tracer-dilution 

streamflow records are the tracer-probe accuracy limits and 

the accuracy of wintertime current-meter flow measurements 

that were mostly rated Poor. It appears that the performance 

of the equipment at cold, high-elevation sites is where most 

of the error exists between the current-meter and tracerdilution discharge records.

The second component of the study showed that the 

piston-driven pumps can consistently discharge the tracer 

solution in a field environment with no adjustments after 

installation. Although the pump setups performed consistently 

throughout the study period, care must be exercised when 

setting the discharge rate of the pumps, as each pump setup 

underdischarged the tracer as compared to its original setting.

The use of the tracer-dilution method has great potential 

and utility where streams are subject to variable backwater 

conditions from ice or a constantly shifting streambed. It is 

expected that advances in technology will allow water-resource 

managers to have timely and accurate near-real-time streamflow 

records. Improvements in the ability of the tracer pumps to 

perform in extreme cold, development of in-situ water-quality 

probes with lower (less than 2 mg/L) tracer-detection limits, and 

the use of satellite telemetry are examples of where improved 

technology will enhance the tracer-dilution method.

References Cited

Kilpatrick, F.A., and Cobb, E.D., 1985, Measurement of discharge using tracers: U.S. Geological Survey Techniques of 

Water-Resources Investigations Report, book 3, chap. A16, 

52 p.

Kimball, B.A., 1996, Use of tracer injections and synoptic 

sampling to measure metal loading from acid mine drainage: U.S. Geological Survey Fact Sheet 245–96, 4 p.

Melcher, N.B., and Walker, J.F., 1990, Evaluation of selected 

methods for determining streamflow during periods of ice 

effect: U.S. Geological Survey Open-File Report 90–554, 

51 p.

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

of streamflow, volume 1, Measurement of stage and discharge: U.S. Geological Survey Water-Supply Paper 2175, 

281 p.

Soenksen, P.J., 1990, Automatic tracer-dilution method 

used for stage-discharge ratings and streamflow hydrographs on small Iowa streams: U.S. Geological Survey 

Water-Resources Investigations Report 89–4187, 45 p.

Zellweger, G.W., Avanzino R.J., and Bencala K.E., 1989, 

Comparison of tracer-dilution and current-meter discharge 

measurements in a small gravel streams, Little Lost Man 

Creek, California: U.S. Geological Survey Water-Resources 

Investigations Report 89–4150, 20 p.

14  Using the Tracer-Dilution Discharge Method to Develop Streamflow Records for Ice-Affected Streams in Colorado