In cooperation with the Illinois Environmental Protection Agency

Evaluation of Measurements Collected with 

Multi-Parameter Continuous Water-Quality Monitors in 

Selected Illinois Streams, 2001–03

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2005–5060
Evaluation of Measurements Collected 

with Multi-Parameter Continuous WaterQuality Monitors in Selected Illinois 

Streams, 2001–03

By George E. Groschen and Robin B. King

In cooperation with the Illinois Environmental Protection Agency

Scientific Investigations Report 2005–5060

U.S. Department of the Interior

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

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

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

Printed: 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:

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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.
iii

Contents

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

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

Description of Multi-Parameter Monitors ............................................................ 2

Temperature ....................................................................................... 2

Dissolved oxygen .................................................................................... 4

pH ................................................................................................ 4

Specific conductivity .............................................................................. 4

Turbidity ............................................................................................. 4

Description of Study Area and Monitor Installations ................................................ 5

Illinois River at Valley City ........................................................................ 5

Lusk Creek near Eddyville ........................................................................ 6

Mazon River near Coal City ........................................................................ 6

Middle Fork Vermilion River above Oakwood ................................................... 8

North Fork Vermilion River near Bismarck ...................................................... 8

Rayse Creek near Waltonville ..................................................................... 8

Salt Creek at Western Springs .................................................................. 8

Vermilion River near Danville ..................................................................... 9

Evaluation of Water-Quality Measurements ......................................................... 9

Rating continuous water-quality measurements ................................................ 10

Temperature ....................................................................................... 12

Dissolved oxygen .................................................................................... 12

pH ................................................................................................ 12

Specific conductivity .............................................................................. 12

Turbidity ............................................................................................. 12

Environmental conditions that affect the measurement quality .............................. 12

Major Reasons for Lost or Unusable Data ...................................................... 13

Sensor Failure ................................................................................. 13

Sensor Internal Drift ........................................................................ 13

Sonde Failure ................................................................................. 14

Fouling.......................................................................................... 14

Operator error ................................................................................. 15

Comparison of Continuous Data with Discrete Measurements .............................. 15

Analysis of Water-Quality Data ........................................................................ 23

Summary ................................................................................................... 34

Acknowledgments ....................................................................................... 37

References Cited .......................................................................................... 38

Appendix: Graphs of time series for water-quality parameters at eight 

streamflow-gaging stations in Illinois and discrete independent measurements 

for comparison, 2001–03. ........................................................................ 42
iv

Figures

1. Map showing ecoregions in Illinois and locations of study watersheds and 

monitoring stations ........................................................................... 3

2. Example correction applied to dissolved-oxygen concentrations for Middle Fork 

Vermilion River above Oakwood, Illinois, March 26 to April 11, 2002 .................. 10

3–7. Graphs showing the relation between discrete independent measurements 

and instantaneous sonde measurements of:

3. Temperature at eight monitoring stations, Illinois .......................................... 16

 4. Dissolved oxygen at eight monitoring stations, Illinois .................................... 17

 5. Specific conductivity at eight monitoring stations, Illinois ................................. 18

 6. pH at eight monitoring stations, Illinois ...................................................... 19

 7. Turbidity at eight monitoring stations, Illinois ................................................ 20

8. Graph showing relations among streamflow, daily precipitation, and turbidity in the 

Vermilion River near Danville, Illinois, May 5–14, 2002 .................................... 22

9–16. Graphs showing daily maximum, minimum, and range for dissolved-oxygen 

concentration from August 2001 through September 2003 at:

9. Illinois River at Valley City, Illinois ............................................................ 24

10. Lusk Creek near Eddyville, Illinois ............................................................ 25

11. Mazon River near Coal City, Illinois ............................................................ 26

12. Middle Fork Vermilion River above Oakwood, Illinois ....................................... 27

13. North Fork Vermilion River near Bismarck, Illinois .......................................... 28

14. Rayse Creek near Waltonville, Illinois ......................................................... 29

15. Salt Creek at Western Springs, Illinois ...................................................... 30

16. Vermilion River near Danville, Illinois ......................................................... 31

17. Graph showing water temperature and dissolved-oxygen concentration at Salt Creek

 at Western Springs, Illinois, from August 2001 through September 2003 ............ 33

18. Graph showing dissolved-oxygen concentration, daily precipitation, and streamflow 

at North Fork Vermilion River near Bismarck, Illinois, during August 2002 ............ 33

19. Graph showing the relation between dissolved-oxygen concentration and water 

temperature at Illinois River at Valley City, Illinois ....................................... 34

20. Graph showing relations among daily precipitation, streamflow, and specific 

conductivity in Mazon River near Coal City, Illinois, March 7–27, 2002 ............... 35

21. Graph showing specific conductivity and streamflow at Salt Creek at Western Springs, 

Illinois, February 7–March 21, 2002 ......................................................... 35

22. Graphs showing continuous and 7-day running mean dissolved-oxygen 

concentrations in three Illinois streams from June 1 through October 7, 2002, 

(A) at Salt Creek at Western Springs, (B) North Fork Vermilion River above 

Bismarck, and (C) Lusk Creek near Eddyville ............................................. 36

Tables

1. Station name, U.S. Geological Survey station number, basin area, watershed 

disturbance, and general location for water-quality monitoring stations 

in Illinois, 2001–03 .............................................................................. 6

2. Land cover, in percent of watershed, in the Illinois study watersheds ........................ 7

3. Criteria for water-quality data shifts for this study ................................................ 11
v

4. Maximum allowable limits of acceptability for water-quality sensor values used

 in this study .................................................................................... 11

5. Rating continuous water-quality data collected in this study .................................... 11

6. Summary of field visits and quality of data for five parameters at eight Illinois stations 

(40 sensors) during August 2001–June 2003 ................................................ 11

7. Coefficients of determination for five parameters at eight stations in Illinois .................. 21

8. Mean total nitrogen and phosphorus concentrations and mean dissolved-oxygen 

concentration maximums, minimums, and daily range at the eight 

stations in Illinois, July 2001 through June 2003 .......................................... 32
Multiply By To obtain

Length

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2) 259.0 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2) 

Flow rate

foot per second (ft/s) 0.3048 meter per second (m/s)

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

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

 °F=(1.8×°C)+32

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

 °C=(°F-32)/1.8

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

(NAVD 88). 

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

Altitude, as used in this report, refers to distance above the vertical datum.

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

25°C).

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

or micrograms per liter (μg/L).

Voltage is given in millivolts (mV).

Conversion Factors, Datums, and Abbreviations
Abstract

Eight streams, representing a wide range of environmental and water-quality conditions across Illinois, were monitored from July 2001 to October 2003 for five water-quality 

parameters as part of a pilot study by the U.S. Geological 

Survey (USGS) in cooperation with the Illinois Environmental 

Protection Agency (IEPA). Continuous recording multi-parameter water-quality monitors were installed to collect data on 

water temperature, dissolved-oxygen concentrations, specific 

conductivity, pH, and turbidity. The monitors were near USGS 

streamflow-gaging stations where stage and streamflow are 

continuously recorded. During the study period, the data collected for these five parameters generally met the data-quality 

objectives established by the USGS and IEPA at all eight stations. A similar pilot study during this period for measurement 

of chlorophyll concentrations failed to achieve the data-quality 

objectives. Of all the sensors used, the temperature sensors 

provided the most accurate and reliable measurements (generally within ±5 percent of a calibrated thermometer reading). 

Signal adjustments and calibration of all other sensors are 

dependent upon an accurate and precise temperature measurement. The dissolved-oxygen sensors were the next most 

reliable during the study and were responsive to changing 

conditions and accurate at all eight stations. Specific conductivity was the third most accurate and reliable measurement 

collected from the multi-parameter monitors. Specific conductivity at the eight stations varied widely—from less than 40 

microsiemens (μS) at Rayse Creek near Waltonville to greater 

than 3,500 μS at Salt Creek at Western Springs. In individual 

streams, specific conductivity often changed quickly (greater 

than 25th percent in less than 3 hours) and the sensors generally provided good to excellent record of these variations at all 

stations. The widest range of specific-conductivity measurements was in Salt Creek at Western Springs in the Greater Chicago metropolitan area. Unlike temperature, dissolved oxygen, 

and specific conductivity that have been typically measured 

over a wide range of historical streamflow conditions in many 

streams, there are few historical turbidity data and the full 

range of turbidity values is not well known for many streams. 

Because proposed regional criteria for turbidity in regional 

streams are based on upper 25th percentiles of concentration 

in reference streams, accurate determination of the distribution 

of turbidity in monitored streams is important. 

Digital data from all five sensors were recorded within 

each of the eight sondes deployed in the streams and in 

automated data recorders in the nearby streamflow–gaging 

houses at each station. The data recorded on each sonde were 

retrieved to a field laptop computer at each station visit. The 

feasibility of transmitting these data in near-real time to a 

central processing point for dissemination on the World-Wide 

Web was tested successfully.

Data collected at all eight stations indicate that a number of factors affect the dissolved-oxygen concentration in 

the streams and rivers monitored. These factors include: 

temperature, biological activity, nutrient runoff, and weather 

(storm runoff). During brief periods usually in late summer, 

dissolved-oxygen concentrations in half or more of the eight 

streams and rivers monitored were below the 5 milligrams per 

liter minimum established by the Illinois Pollution Control 

Board to protect aquatic life. Because the streams monitored 

represent a wide range in water-quality and environmental 

conditions, including diffuse (non-point) runoff and wastewater-effluent contributions, this result indicates that deleterious 

low dissolved-oxygen concentrations during late summer may 

be widespread in Illinois streams.

INTRODUCTION

The U.S. Environmental Protection Agency (USEPA) has 

determined the regional stream nutrient criteria as a maximum 

concentration of total phosphorus, total nitrogen, chlorophyll 

a (a specific form of chlorophyll), and turbidity allowable in 

streams and rivers for the Nation. These criteria are based on 

the concept of nutrient ecoregions (U.S. Environmental Protection Agency, 2000). The USEPA drafted guidance for States 

to set dissolved-oxygen standards in 1986 (U.S. Environmental Protection Agency, 1986). The USEPA recognized the 

close link between nutrients and dissolved oxygen in streams 

and the proposed regional stream nutrient criteria are designed, 

in part, to help prevent deleterious low dissolved-oxygen concentrations that could result from nutrient enrichment.

In Illinois, the current dissolved-oxygen water-quality 

standard promulgated in Illinois Pollution Control Board regulations dates from the early 1970s and may not be applicable 

to present-day (2004) water-quality conditions and reflective 

of modern water-quality monitoring capabilities. Natural diurnal and interregional variation in dissolved-oxygen concentraEvaluation of Measurements Collected with MultiParameter Continuous Water-Quality Monitors in 

Selected Illinois Streams, 2001–03
2 Evaluation of Measurements with Multi-Parameter Water-Quality Monitors in Selected Illinois Streams, 2001–03

tion might be greater than recognized with the present standards, particularly in streams and rivers in southern Illinois.

The U.S. Geological Survey (USGS), in cooperation 

with the Illinois Environmental Protection Agency (IEPA), 

conducted a study beginning in 2001 to investigate the 

applicability of the USEPA nutrient criteria to water-quality 

and environmental conditions in Illinois streams. The study 

involved the deployment of continuous water-quality monitoring monitors to eight stream or river locations across the 

State, representing a range of water-quality and environmental 

conditions (fig. 1). Data were collected on water temperature, 

dissolved-oxygen concentrations, specific conductivity, pH, 

and turbidity. Water-quality data were analyzed to provide 

some context for comparing water-quality standards for the 

protection of aquatic life.

This report presents the results of a study that monitored 

selected water-quality parameters continuously at eight stream 

and river sites in the State of Illinois from 2001 to 2003. The 

sites are in selected watersheds that represent a range in: 

land cover, watershed disturbance, climate, and hydrologic 

characteristics. The stations were operated from July 2001 

through September 2003. The report is organized into two 

parts. In the first part of the report, the accuracy and reliability 

of water-quality monitors are evaluated. Monitor performance 

is related, where possible, to environmental and hydrologic 

factors. In the second part of the report, selected water-quality 

data from the continuous monitors are analyzed for parameter-specific and temporal relations among temperature, dissolved oxygen, nutrients, diurnal variability, seasonality, and 

stormflow.

DESCRIPTION OF MULTI-PARAMETER 

MONITORS

The terms monitor, sensor, sonde, and telemetry are 

defined below as they are used in this report. 

Monitor—Complete hydrologic instrument system that 

measures multiple water-quality parameters of interest. A 

monitor generally comprises sensors and a sonde, and may 

have the capacity to operate remotely and in a user-determined 

manner for an extended period of weeks to months. A monitor 

may store data internally and/or transmit it to the user through 

telemetry.

Sensor—Device that measures one or more water-quality 

parameter(s) and is attached to a sonde. Can be synonymous 

with the term probe, although sensor is the preferred term in 

this report.

Sonde—Device that manages the operation of the waterquality sensor(s) in a user-determined manner. Generally, the 

sonde will be connected to multiple sensors, supply power to 

the sensors, store the water-quality data, and function as an 

interface to the telemetry. 

Telemetry—Transmission of data measured by the 

water-quality monitoring system to a remote location where it 

is recorded and inspected, or the hardware and physical installation associated with the system.

Wagner and others (2000) describe the major factors that 

should be considered in the operation of a continuous waterquality monitoring station. These factors include sensor and 

monitor type, monitor configuration, site selection, sensor 

location in the stream cross section, and the actual use, calibration, and operation of the continuous water-quality monitor. 

In this study, the water-quality monitors were deployed in situ 

(the sensors were immersed continuously in the water being 

measured) at the eight field stations. Each sonde was equipped 

with an internal data logging and power-supply system that 

was able to operate the water-quality sensors and record the 

data independently of the external power and telemetry. The 

water-quality monitors were installed during July or August 

2001 and remained operational through September 2003. The 

water-quality monitors usually were serviced about every 

2 to 6 weeks. Four of the monitors also were connected to 

a telemetry system that supplied power to the monitors and 

transmitted the water-quality data to USGS Illinois Water Science Center computer systems. Streamflow data for the eight 

stations are stored in the USGS National Water Information 

System (NWIS), are published in the Illinois Water Science 

Center Annual Data Report for 2003, and are available at 

URL: http://il.water.usgs.gov/annual_report/start.htm. The 

continuous water-quality data recorded by the sondes are also 

stored in NWIS.

Temperature

The effect of water temperature on the development and 

growth of aquatic systems is well understood (Laws, 2000, 

chap. 8; Allan, 1995, p. 69–78). Aquatic organisms grow most 

efficiently over a limited temperature range. Deviations from 

this limited range may cause reduced growth rates, although 

translating water-temperature changes directly into environmental stress indicators usually is difficult because other stress 

indicators generally are present in the aquatic system. 

In this study, water temperature was measured with a 

high-precision thermistor. The thermistor is constructed of a 

thin-walled titanium sleeve and operates with an algorithm 

that converts internal electrical resistance to water temperature. The accurate performance of the water-temperature sensor especially is critical because other water-quality sensors 

(including chlorophyll, dissolved oxygen, pH, specific conductivity, and turbidity) rely on concurrently measured temperature data to operate correctly. The water-temperature sensor is 

considered maintenance-free by the manufacturer and requires 

no field calibration. However, the performance of the watertemperature sensor at each station was compared to a certified 

thermometer about twice per year using methods described by 

Radtke, Kurklin, and Wilde (1998). 
Figure 1. Ecoregions in Illinois and locations of study watersheds and monitoring stations (modified from Omernik, 1995).

Description of Multi-Parameter Monitors 3
4 Evaluation of Measurements with Multi-Parameter Water-Quality Monitors in Selected Illinois Streams, 2001–03

Dissolved oxygen

Dissolved oxygen is the most important dissolved gas 

in the aquatic environment because it is essential to aerobic respiration, and many chemical and biological reactions 

depend on the presence of oxygen in water. The main factors affecting dissolved oxygen in aquatic ecosystems are 

atmospheric reaeration (source and sink of oxygen), the 

photosynthetic activities of green aquatic plants, and respiration by aquatic organisms (Radtke, White, and others, 1998; 

Boyd, 2000, chap. 4; among many others). Dissolved oxygen 

can be depleted indirectly because of excessive nutrients in 

water bodies. Excessive nutrients encourage the rapid growth 

of algal biomass that, in turn, increases the growth of oxygen consuming bacteria. In daytime, photosynthetic activity 

outstrips respiration and reaeration, and the dissolved-oxygen 

concentration in the water body increases. At night, photosynthetic activity stops, but respiration continues and decreases 

dissolved-oxygen concentrations. 

In this report, the dissolved oxygen is reported as a 

concentration in units of milligrams per liter (mg/L). A rapid 

pulse, temperature-compensated, dissolved-oxygen Clark-cell 

amperometric sensor with an instrument warm-up time of 48 

seconds per measurement was used in this study. The sensor 

was calibrated at each station visit according to monitor manufacturer methods (Yellow Springs Instruments, 2003).

pH

The pH of water is important because many reactions 

that affect water quality are pH-dependent. The pH of water is 

a measure of the effective hydrogen-ion concentration that is 

represented by the negative base-10 logarithm of hydrogen-ion 

activity, in moles per liter (Radtke, Busenberg, and Kurklin, 

1998). The pH is affected by interrelated chemical reactions 

that either produce or consume hydrogen ions (Hem, 1992, p. 

61-65). The pH of natural water is an indicator of the status of 

aqueous chemical reactions. The pH of pure water at 25°C is 

7.00, although the pH of river water is generally in the range 

6.5 to 8.5. The pH can be affected by the presence of dissolved 

gases, particularly carbon dioxide. Photosynthesis by aquatic 

organisms will cause pH to increase because photosynthesis 

consumes carbon dioxide which is a weak acid in water. Algal 

respiration, which occurs primarily at night in water bodies, 

will cause pH to decrease because carbon dioxide is the major 

product of respiration.

A temperature-compensated ion-selective electrode (ISE) 

sensor that includes an integral reference electrode was used to 

measure pH. The ISE is calibrated in the field with solutions 

of known pH, to establish a relation between ISE electrical 

output and pH. Internal sonde software then is used to automatically correct the pH values for the water temperature at 

the time of the sensor measurement.

Specific conductivity

Specific conductivity is an operationally defined measurement of electrical conductivity in water, normalized to 

a unit length and unit cross section at a specified temperature (Radtke, Davis, and Wilde, 1998). Although there is no 

absolute theoretical relation between concentration of dissolved solids and specific conductivity, the capacity of water 

to conduct an electrical current generally is a function of the 

electrical charge and the concentrations of dissolved ions that 

make up the majority of dissolved solids in the water. Usually, 

an empirical relationship between specific conductivity and 

dissolved ions can be developed for a specific water type or at 

a specific sampling site. In this report, specific conductivity is 

reported in microsiemens per centimeter (μS/cm) at 25°C. 

Turbidity

Turbidity is an operationally defined measure of water 

clarity and is used as an indicator of the condition and productivity of aquatic systems (Wilde and Gibbs, 1998). The 

primary causes of turbidity include clay, silt, organic and inorganic matter, soluble colored compounds, phytoplankton, and 

microscopic organisms (American Public Health Association 

and others, 1992). Turbidity can be determined by measuring the scattering effect that suspended solids have on a light 

beam; the greater the light scatters in water, the greater the turbidity. The measurement of turbidity is quantitative between 

0 and 1,000 formazin nephelometric units (FNU), however, 

it often cannot be directly correlated to the concentration of 

suspended solids in the water column. A station-specific or 

matrix-specific relation can usually be developed between 

turbidity measurements and suspended solids after collection 

of sufficient data. During the course of this study, turbidity in most streams exceeded the 1,400 FNUs upper limit of 

quantitation; therefore, it may not be possible using this sensor 

to develop a sufficiently accurate relation between turbidity 

and suspended solids. 

In this study, turbidity was measured in terms of FNUs 

according to method 7027 of the International Organization 

for Standardization (ISO) (Anderson, 2004). The scatter from 

a near-infrared light beam at a wavelength of 860 nanometers 

(nm), ±30 nm is measured with this method. The optical face 

of the sensor was cleaned before each measurement with an 

automated wiper mechanism and the turbidity over a 12-second time interval was averaged and recorded. A data-filtering 

algorithm was used to suppress occasional data spikes that can 

be caused by large particles or air bubbles passing in front of 

the turbidity sensor at measurement time. The turbidity sensor was calibrated during each field inspection, as necessary, 

to zero using a zero turbidity solution. Occasionally, a twopoint calibration was performed, using both the zero solution 

and a diluted formazin suspension, which can be prepared to 

simulate turbidity at the desired FNU. The turbidity sensor 

was designed and manufactured to provide reliable monitor 
performance to an upper limit of about 1,000 FNUs. During this study, the sensor appeared to reliably perform to an 

upper limit of about 1,400 FNUs; therefore, data values below 

1,400 FNUs only are included in this report. The data above 

1,400 FNU are stored in the USGS NWIS database but are of 

unknown quality.

DESCRIPTION OF STUDY AREA AND 

MONITOR INSTALLATIONS

The State of Illinois has a total area of 55,600 mi2 and 

has a population of about 11.8 million people (U.S. Department of Commerce, 2003a). The study area included in the 

water-quality network is 28,700 mi2, although 3,850 mi2 (13.4 

percent) is outside of the State of Illinois. Some of the study 

area watersheds include area in adjoining States—primarily 

Indiana and Wisconsin. The population in the parts of Illinois 

in the study area is about 10.4 million people (modified from 

the U.S. Department of Commerce, 2003b) and includes the 

metropolitan areas of Chicago, Decatur, Kankakee, Peoria, and 

Springfield. The study area includes approximately 45 percent 

of the land area in Illinois (fig. 1). Most of the land is used for 

agriculture, particularly the production of corn and soybeans. 

The climate in Illinois is classified as continental; typically, 

summers are warm and humid and winters are cold and dry. In 

northern Illinois, the average temperature is –4°C in January 

and 24°C in July. In southern Illinois, the average temperature 

is 2°C in January and 26°C in July (National Oceanic and 

Atmospheric Administration, 2003). The average annual rainfall ranges from about 35 in. in the north to about 48 in. in the 

south, whereas average annual snowfall ranges from about 40 

in. in the north to about 12 in. in the south (Illinois State Water 

Survey, 2000).

The installation of a water-quality monitoring system 

requires the consideration of multiple and sometimes conflicting station-selection factors such as, but not limited to, 

stream-channel morphology, water depth, and flow regime. 

Ideally, the water-quality sensors should be placed in a 

location that best represents the section of the stream being 

monitored, while considering the likely range of flow conditions. However, other factors, such as station accessibility, bank stability, avoidance of high-water debris damage, 

monitor fouling potential, and protection from vandalism also 

must be considered (Wagner and others, 2000). For this study, 

the following specific factors were considered important: (1) 

the sensors were placed at sufficient depth to stay submerged 

during low-flow conditions; (2) the sonde housing structures 

were designed to be accessible during most flow conditions; 

(3) the sonde housing structures were designed to withstand 

the forces of high-flow conditions (in particular, the collisions 

with moving instream debris, such as large trees, that occur 

during floods); and (4) the streams generally were well mixed 

and cross-sectional variability was small.

Eight sites on rivers and streams with a range of environmental and hydrologic conditions across the State were 

selected for monitoring. The primary criteria in station selection were watershed area, location type, and station type (table 

1). The eight stations are on rivers or streams ranging in drainage area from 42.9 mi2

 to 26,700 mi2. The State was divided 

into three general regions—northern, central, and southern—

and at least two field stations were selected from within each 

region (table 1). The availability of stream chemistry data was 

the final criterion in station selection. The watershed areas of 

the eight stations selected further were classified into one of 

three types: least disturbed, slightly disturbed, and disturbed. 

Watershed disturbance is an IEPA qualitative measure that 

considers biological conditions, historical nutrient concentrations, and State regulatory agency concerns (R. Smogor, 

Illinois Environmental Protection Agency, written commun. 

2005). The watersheds of seven of the eight stations were classified as either disturbed or slightly disturbed (table 1). 

The watershed areas monitored with the water-quality 

network collectively make up approximately 40 percent of 

the land area of Illinois. However, most of this area is within 

a single watershed, the drainage area above Illinois River at 

Valley City (fig. 1). Land cover in each watershed is shown by 

category in table 2.

Illinois River at Valley City

 The Illinois River at Valley City watershed drains 26,700 

mi2. However, diversion of the Chicago River that began 

January 17, 1900, added to the flow of the Illinois River 

streamflow that originally went to Lake Michigan (Lanyon, 

2000). Where the USGS streamflow-gaging station and 

water-quality monitor (station 5586100) were installed, the 

river channel was relatively straight for 3-5 mi both upstream 

and downstream and the nearby banks generally were covered with a heavy growth of trees and forbs. The streambed 

comprised silt, sand, and gravel. The Illinois River is an 

important navigable waterway with substantial barge traffic. 

During the course of the study, barges usually passed within 

about 50 to 100 feet (ft) of the water-quality monitor, which 

was on the streamward side of a concrete bridge-support pier 

near the shelter for the USGS streamflow-gaging station. The 

water-quality sensors were positioned at a water depth of 1.2 

ft of stage, which is about 0.8 ft below typical low-flow stage 

and about 15 ft above the streambed. At low-flow conditions 

(below 2.0 ft of stage), the stream velocities at the sensors 

always were measurable and the stream was well mixed at all 

stages. 

Land cover during 1999–2000 in the watershed upstream 

of the Illinois River at Valley City (table 2) was 77.5 percent 

agricultural, 9.1 percent forest, 9.3 percent urban and built-up, 

2.6 percent wetland, 1.4 percent surface water, and 0.1 percent 

barren and exposed (modified from the Illinois Department of 

Agriculture, 2002). 

Description of Study Area 5
6 Evaluation of Measurements with Multi-Parameter Water-Quality Monitors in Selected Illinois Streams, 2001–03

At Illinois River at Valley City, the long-term (Oct. 

1980–Sept. 1996) average concentration of total nitrogen (total 

Kjeldahl nitrogen plus nitrite and nitrate nitrogen) as nitrogen (N) was 5.7 mg/L and the average concentration of total 

phosphorus (total-P) was 0.31 mg/L (Short, 1999, appendix, 

p. B-15). During August 2001 through September 2003, the 

average concentration of total-N was 6.6 mg/L, which is 116 

percent of the long-term average and total-P was 0.40 mg/L, 

which is 129 percent of the long-term average.

Lusk Creek near Eddyville

 The Lusk Creek near Eddyville watershed drains 42.9 

mi2. The water-quality monitor was on a gentle bend near 

the left bank (while looking downstream) and the sonde was 

submerged in a shallow pool. The channel was fairly straight 

for 150 ft upstream and then turned gently to the right (while 

looking upstream). Looking downstream from the water-quality monitor, the channel bent gently to the left and then was 

fairly straight for about 500 ft. The USGS streamflow-gaging 

station number 03384450 was about 300 ft downstream from 

the water-quality monitor. The local banks were steep and covered with heavy growth of trees and forbs and the streambed 

comprised sand, pebbles, cobbles, and boulders. At low flow, a 

sand-and-gravel riffle 30 ft downstream controlled the depth of 

water at the sonde. The water-quality sensors were positioned 

at a water depth of 1.8 ft of stage, which was about 0.2 ft 

below the point of zero flow and about 0.3 ft above the streambed. At low-flow conditions (below about 2.5 ft of stage), the 

stream velocities at the sensors sometimes approached the 

point of being immeasurable. Under very low-flow conditions 

(below about 1.5 ft of stage), the stream mixing was poor. The 

stream mixing usually was good at all other flow conditions. 

The streambed immediately beneath the water-quality sensors consisted mostly of sand and small amounts of scattered 

pebbles. In the summer months, the streambed was covered 

with heavy vegetation both upstream and downstream from the 

water-quality monitor.

Land cover during 1999-2000 in the Lusk Creek near 

Eddyville watershed (table 2) was 76.4 percent forest, 17.8 

percent agricultural, 4.0 percent wetland, 1.6 percent urban 

and built-up, and 0.2 percent surface water (Illinois Department of Agriculture, 2002). 

Mazon River near Coal City

 The Mazon River near Coal City watershed drains 

455 mi2. The water-quality monitor was on a straight section of channel near the right bank, about 20 ft upstream of 

a bridge. The channel was fairly straight for more than 500 

ft upstream. Looking downstream from the water-quality 

monitor, the channel bent gently to the left and then was fairly 

straight for about 500 ft. The USGS streamflow-gaging station 

(05542000) was about 75 ft downstream of the water-quality 

monitor. The local banks were steep and covered with forbs 

and a moderate density of trees. The sonde was positioned 

with the sensors submerged in a shallow pool (less than 3 ft 

deep) about 30 ft downstream of a rock-and-gravel riffle. At 

low flow (below 1.0 ft of stage), the stream velocities at the 

sensors ranged down to immeasurable, and the streamflow was 

confined to a small outlet on the upstream riffle, about 25 ft 

instream. The water-quality sensors were positioned at a water 

depth of 0.2 ft of stage, which is approximately the point of 

zero flow and about 0.2 ft above the streambed. The streambed immediately beneath the water-quality sensors comprised 

silt, although the streambed in the general vicinity comprised 

bedrock, gravel, and scattered rocks.

Table 1. Station name, U.S. Geological Survey (USGS) station number, basin area, watershed disturbance, and general location for 

water-quality monitoring stations in Illinois, 2001-03. 

[Watershed disturbance is an Illinois Environmental Protection Agency qualitative measure that considered biological conditions, historical nutrient concentrations, and State regulatory agency concerns (R. Smogor, Illinois Environmental Protection Agency, written commun. 2005)]

Station name USGS station 

number

Basin area, in 

square miles

Watershed Disturbance General location within 

Illinois

Illinois River at Valley City 5586100 26,700 Disturbed Central

Lusk Creek near Eddyville 3384450 43 Least disturbed Southern

Mazon River near Coal City 5542000 455 Slightly disturbed Northern

Middle Fork Vermilion River above 

Oakwood

3336645 432 Slightly disturbed Central

North Fork Vermilion River near Bismarck3338780 262 Slightly disturbed Central

Rayse Creek near Waltonville 5595730 88 Disturbed Southern

Salt Creek at Western Springs 5531500 115 Disturbed Northern

Vermilion River near Danville 3339000 1,290 Slightly disturbed Central
Table 2. Land cover, in percent of watershed, in Illinois study watersheds. [NA, not applicable]

[Codes in parentheses in column headings are Illinois Environmental Protection Agency station identifiers]

Anderson level 2 land-cover 

category

Area in land-cover category, by watershed (percent)

Illinois River 

at Valley City

(D-32)

Lusk Creek 

near 

Eddyville

(AK-02)

Mazon River 

near Coal 

City

(DV-04)

Middle Fork 

Vermilion 

River above 

Oakwood

(BPK-07)

North Fork 

Vermilion 

River near 

Bismarck

(BPG-09)

Rayse Creek 

near 

Waltonville

(NK-01)

Salt Creek 

at Western 

Springs

(GL-09)

Vermilion 

River near 

Danville

(BP-08)

11 Corn NA 0.64 45.4 44.5 NA 14.6 0.19 NA

12 Soybeans NA 2.36 42.4 38.6 NA 29.5 .16 NA

13 Winter wheat NA .01 .25 .15 NA 5 0 NA

14 Other small grains 

and hay

NA .18 .22 0 NA 1.73 0 NA

15 Winter wheat/soybeansNA 0 .13 .09 NA 6.05 0 NA

16 Other agriculture NA .2 .02 .01 NA 1.28 0 NA

17 Rural grassland NA 14.4 5.91 9.99 NA 11.8 1.78 NA

TOTAL AGRICULTURE 77.5 17.8 94.3 93.4 95.5 158 2.13 90.8

21 Upland NA 55.8 0.56 2.23 NA 18.4 10.1 NA

25 Partial canopy/savannah uplandNA 2.38 .36 .47 NA 1.26 4.52 NA

26 Coniferous NA 18.1 0 0 NA 0 0 NA

TOTAL FOREST 9.07 76.4 0.92 2.69 1.43 19.6 14.7 2.67

31 High density urban 

and built-up land 

NA 0.05 0.69 1.07 NA 0.47 13.9 NA

32 Low/medium density 

urban and built-up land

NA 1.16 1.25 .54 NA .75 43.4 NA

35 Urban open space NA .41 .62 .27 NA .51 22.2 NA

TOTAL URBAN and 

BUILT-UP

9.25 1.62 2.55 1.89 2.18 1.73 79.6 4.15

41 Shallow marsh/wet 

meadow

NA 0.08 0.04 0.02 NA 1.03 0.9 NA

42 Deep marsh NA 0 0 .01 NA .27 .22 NA

43 Seasonally/temporally 

flooded

NA 0 .16 .02 NA .34 0 NA

44 Flood-plain forest NA 3.79 .52 1.49 NA 6.42 .14 NA

48 Swamp NA 0 0 0 NA 0 0 NA

49 Shallow water NA .14 .03 .04 NA .19 0 NA

TOTAL WETLAND 2.6 4.01 0.75 1.57 0.71 8.25 1.26 1.54

51 Surface water 1.43 0.25 1.43 0.48 0.17 0.28 2.34 0.73

TOTAL SURFACE 

WATER

1.45 0.25 1.43 0.48 0.17 0.28 2.34 0.73

52 Barren and exposed 

land

0.13 0 0.03 0.01 0 0.09 0.03 0.09

TOTAL BARREN and 

EXPOSED

0.13 0 0.03 0.01 0 0.09 0.03 0.09

Description of Study Area 7
8 Evaluation of Measurements with Multi-Parameter Water-Quality Monitors in Selected Illinois Streams, 2001–03

Land cover during 1999-2000 in the watershed upstream 

of the Mazon River near Coal City (table 2) was 94.3 agricultural, 2.6 percent urban or built-up, 1.4 percent surface water, 

0.9 percent forest, and 0.8 percent wetland (Illinois Department of Agriculture, 2002). At Mazon River near Coal City, 

the long-term average concentration of total-N was 9.6 mg/L 

and the average concentration of total-P was 0.09 mg/L (Short, 

1999, appendix, p. B-27). During August 2001 through June 

2003, the average concentration of total-N was 11.4 mg/L, 

which is 119 percent of the long-term average and total-P was 

0.09 mg/L, which is 100 percent of the long-term average. 

Middle Fork Vermilion River above Oakwood

 The Middle Fork Vermilion River above Oakwood 

watershed drains 432 mi2. The water-quality monitor was on a 

fairly straight section of channel near the right bank, at USGS 

streamflow-gaging station (03336645). A bridge is about 250 

ft downstream. The banks, especially the banks near the waterquality monitor, were steep to vertical and were actively eroding and slumping into the channel during the study period. The 

banks were covered with a dense growth of forbs and trees. 

The monitor sensors were submerged in a shallow pool about 

350 ft upstream of a sand, gravel, boulder, and bedrock riffle 

at a water depth of 0.2 ft of stage, which is approximately 

the point of zero flow and about 0.3 ft above the streambed. 

On some occasions after high flow, large uprooted trees were 

deposited near or on the monitor housing structure—interfering with the natural flow near the sonde—and these trees were 

moved as soon as practical. At low flow (below about 1.0 ft of 

stage) the stream velocities in the pool generally were measurable (greater than 0.2 feet per second) and sufficient to maintain adequate mixing. The streambed immediately beneath 

the water-quality sonde comprised silt and sand, although 

generally, the streambed near the sonde comprised bedrock, 

boulders, and gravel. 

Land cover during 1999-2000 in the watershed upstream 

of the Middle Fork Vermilion River above Oakwood (table 2) 

was 93.4 percent agricultural, 2.7 percent forest, 1.9 percent 

urban or built-up, 1.5 percent wetland, and 0.5 percent surface 

water (Illinois Department of Agriculture, 2002). 

North Fork Vermilion River near Bismarck

 The North Fork Vermilion River near Bismarck watershed drains 262 mi2

. The water-quality monitor was beneath a 

bridge on the left bank and was attached to a concrete revetment at the USGS streamflow-gaging station (03338780). The 

channel was straight and there was a rock-and-gravel riffle 

about 300 ft downstream of the water-quality monitor and a 

similar riffle about 200 ft upstream. The banks comprised sand 

and mud, and were densely covered with trees and forbs. The 

streambed comprised pebbles, cobbles, and some scattered 

small boulders. The water-quality sensors were positioned 

at a water depth of 4.9 ft of stage, which is the point of zero 

flow and about 2.5 ft above the streambed. At low flow (below 

about 5.5 ft of stage), the stream velocities always were sufficient to maintain adequate mixing at the sensors. The streambed immediately beneath the water-quality sensors comprised 

silt and cobbles. 

Land cover during 1999-2000 in the watershed upstream 

of the North Fork Vermilion River near Bismarck (table 2) was 

95.5 percent agricultural, 1.4 percent forest, 2.2 percent urban 

or built-up, 0.7 percent wetland, and 0.2 percent surface water 

(Illinois Department of Agriculture, 2002). 

Rayse Creek near Waltonville

The Rayse Creek near Waltonville watershed drains 

88.0 mi2. The water-quality monitor was beneath a bridge on 

the right bank and was attached to a concrete block at USGS 

streamflow-gaging station (05595730). The channel was 

straight for about 50 ft upstream and 100 ft downstream. There 

was a mud-and-sand riffle about 150-200 ft downstream and 

the streambed comprised mud, pebbles, and cobbles, along 

with various types of debris. The banks were high and steep 

with dense growth of trees and forbs. The water-quality monitor and surrounding area occasionally were buried beneath 

trees and other natural debris, that were removed as soon as 

practical. The water-quality sensors were positioned at 0.5 ft 

of stage, which is about 0.2 ft below the point of zero flow 

and about 1.5 ft above the streambed. At low-flow conditions 

(below about 1.0 ft of stage), the stream velocities at the sonde 

ranged down to immeasurable. The streambed immediately 

beneath the water-quality sensors comprised mostly silt, 

although some pebbles and cobbles were present. 

Land cover during 1999-2000 in the watershed upstream 

of Rayse Creek near Waltonville (table 2) was 70.0 percent 

agricultural, 19.6 percent forest, 8.3 percent wetland, 1.7 

percent urban or built-up, 0.3 percent surface water, and 0.1 

percent barren and exposed (Illinois Department of Agriculture, 2002). 

Salt Creek at Western Springs

The Salt Creek at Western Springs watershed drains 115 

mi2. The water-quality monitor was attached to the upstream 

side of the left bridge pier, at USGS streamflow-gaging station (05531500). The channel was straight for over 1,500 ft 

upstream and downstream. The streambed comprised mud, 

pebbles, cobbles, and some scattered small boulders. The 

banks were low with gentle slopes and were densely covered 

with trees and forbs. The water-quality monitor occasionally 

was buried beneath floating tree branches and other natural 

debris, although these burials almost always occurred above 

the submerged sensors. The sensors were positioned at a water 

depth of 0.8 ft of stage, which is approximately the point of 

zero flow and about 0.4 ft above the streambed. At low-flow 

conditions (below 1.5 ft of stage), the stream velocities at 

the sonde always were measurable, although sometimes the 
Evaluation of Water-Quality Measurements 9

stream mixing at the sonde was incomplete. The streambed 

immediately beneath the water-quality sensors comprised 

pebbles and cobbles. 

Land cover during 1999-2000 in the watershed upstream 

of the Salt Creek at Western Springs (table 2) was 79.6 percent 

urban or built-up, 14.7 percent forest, 2.3 percent surface 

water, 2.1 percent agricultural, and 1.3 percent wetland (Illinois Department of Agriculture, 2002). 

Vermilion River near Danville

 The Vermilion River near Danville watershed drains 

1,290 mi2. The water-quality monitor was attached to the 

right bank about 100 ft downstream of the water-treatment 

plant bridge, at USGS streamflow-gaging station (03339000). 

The channel was straight for about 300 ft upstream and for 

over 1,500 ft downstream. The streambed comprised sand, 

pebbles, and cobbles, with some scattered small- to mediumsized boulders lying on top of bedrock. The banks were high 

with steep to vertical slopes and were densely covered with 

trees and forbs. The water-quality sensors were positioned at 

1.4 ft of stage, which is approximately the point of zero flow 

and about 0.7 ft above the streambed. At low-flow conditions (below 2.0 ft of stage), the stream velocities at the sonde 

always were measurable, although they were usually much 

slower (less than 0.5 ft/s) near the sonde than in the middle 

of the stream. At low flow, stream mixing usually was poor 

around the sensors but was good at all higher flow conditions 

(above 2.0 ft of stage). The streambed immediately beneath 

the water-quality sensors comprised silt and sand. 

Land cover during 1999-2000 in the watershed upstream 

of Vermilion River near Danville (table 2) was 90.8 percent 

agricultural, 4.2 percent urban or built-up, 2.7 percent forest, 

1.5 percent wetland, and 0.7 percent surface water, and 0.1 

percent barren or exposed (Illinois Department of Agriculture, 

2002).

EVALUATION OF WATER-QUALITY 

MEASUREMENTS

Data-quality objectives for the study were to obtain and 

record fair to excellent data (see table 5, p. 11) from the multiparameter monitors at least 75 percent of the deployment for 

temperature, dissolved-oxygen concentration, specific conductivity, pH, and turbidity at the eight stations. 

The major sources of uncertainty (error) in continuous 

water-quality monitoring data include monitor fouling, instrument drift, and cross-sectional location. When the error caused 

by monitor fouling and instrument drift exceeds a specified 

threshold, then a correction needs to be applied to the data 

(table 3). The correction from monitor fouling and drift is 

applied by prorating the observed error between monitor 

service visits over the associated time interval, as described 

in Wagner and others (2000). An example correction applied 

to the dissolved-oxygen data record at Middle Fork Vermilion 

River above Oakwood for the time period from March 26, 

2002, to April 11, 2002, is shown figure 2. The dissolved-oxygen sensor, which had been cleaned and calibrated and was 

operating accurately after the March 26, 2002, service visit, 

had a measurement error of 1.4 mg/L of dissolved oxygen on 

April 11, 2002, service visit. This 1.4 mg/L error was prorated backward in time, to the point of the previous monitor 

service visit, 16 days prior. The records for all the continuously monitored water-quality data at the eight field stations 

were evaluated similarly, by parameter, and corrections were 

applied as necessary. Because the cross-sectional variability 

at the eight stations generally was small, cross-sectional error 

was considered negligible during this study.

The maximum allowable limits for reporting data from 

continuous water-quality monitoring sensors vary by parameter (table 4). Data that differ by more than the maximum 

allowable limit were not used in computations or published. 

Wagner and others (2000) established the maximum allowable 

limits as the data value that is 10 times the accuracy resolution 

of the water-quality sensor, although stricter criteria are used 

for dissolved oxygen and turbidity. A maximum allowable 

limit is not specified for chlorophyll, because the chlorophyll 

sensor manufacturer does not claim any specific accuracy for 

this monitor. Occasionally, one or more individual sensors 

provided acceptable data for a short period, but completely 

failed between the service visits. For example, after a dissolved-oxygen sensor was cleaned and calibrated, it operated 

within acceptable accuracy after a routine service visit, but 

then suddenly and completely failed without warning before 

the next service visit, perhaps because of a ruptured membrane. In such cases, the dissolved-oxygen sensor provided 

acceptable data up to the time of the failure, but no dissolvedoxygen data would be retained between the failure and the 

next service visit. Another complication that usually is more 

common than sensor failure is the error introduced by gradual 

monitor fouling or instrument drift. Reasonable judgment 

was used to identify the point in the water-quality data record 

when the errors exceeded the maximum allowable accuracy 

limits. To assist in making this judgment, the water-quality 

monitor provides diagnostic data on sensor performance that 

are recorded with the sensor-data values. The diagnostic data 

sometimes provided additional insight into sensor performance 

and data quality, and aided decision-making regarding when 

the maximum allowable limits were exceeded.

After the water-quality data had been evaluated and 

screened with respect to the maximum allowable limits, the 

resultant verified data from each sensor were classified into 

one of four possible accuracy grades: excellent, good, fair, and 

poor (table 5). These accuracy grades were assigned based on 

the sensor values that were recorded before any corrections 

were made to the data. The accuracy grades for dissolved 

oxygen, pH, and water temperature are considered in terms 

of their associated reporting units (mg/L, pH standard units, 

and °C, respectively), whereas the accuracy grades for specific 
10 Evaluation of Measurements with Multi-Parameter Water-Quality Monitors in Selected Illinois Streams, 2001–03

Figure 2. Example correction applied to dissolved-oxygen concentrations for Middle Fork Vermilion River above Oakwood, Illinois, 

March 26 –April 11, 2002.

Shifted data

Original continuousmonitor data

DISSOLVED-OXYGEN CONCENTRATION,

IN MILLIGRAMS PER LITER

15

14

13

12

11

10

9 26 27 28 29 30 31 1 2 3 4 5 6 7 9 8 10 11 12 13 14

MARCH APRIL

2002

conductivity and turbidity are considered in terms of the percentage deviation from their associated true value. No attempt 

was made to assess the accuracy of the chlorophyll data in this 

data-quality classification scheme.

Rating continuous water-quality measurements

The water-quality measurements, including the daily 

values for the minimum, maximum, and mean, were evaluated. 

A final set of criteria for publication or other data distribution 

(table 6) are listed below and were considered when collating 

these daily values (Wagner and others, 2000): 

• If less than 100 percent of the data values are available 

on a specific day, the expected daily minimum and 

expected daily maximum values must be present to 

compute the mean value. 

• If less than 100 percent of the data values are available 

on a specific day, the daily minimum, daily maximum, 

and daily mean data values will be computed if flow 

conditions are steady or gradually changing; the minimum and maximum data values occur at a reasonable 

time; and sufficient data values are present according 

to the following parameter-specific criteria: 

• Dissolved oxygen—At least 25 percent of the data 

values are present.

• pH—At least 25 percent of the data values are present. 

• Specific conductivity—At least 50 percent of the 

data values are present. 

• Turbidity—At least 50 percent of the data values are 

present. 

• Water temperature—At least 25 percent of the data 

values are present.

Except for dissolved-oxygen concentration daily maximums and minimums, discussed later, daily values are not 

used or discussed in this report.
Evaluation of Water-Quality Measurements 11

Table 3. Criteria for water-quality data shifts for this study (modified from Wagner and others, 2002). 

[°C, degree Celsius; μS/cm, microsiemens per centimeter; mg/L, milligram per liter; FNU, formazin nephelometric units]

Parameter Shift criteria (apply shift when deviation exceeds this value)

Temperature (may affect the other properties) ± 0.2°C

Specific conductivity The greater of ± 5 μS/cm or ± 3 percent of the measured value

Dissolved oxygen ± 0.3 mg/L

pH ± 0.2 pH standard unit

Turbidity The greater of ± 2 FNU or ± 5 percent of the measured value

Table 4. Maximum allowable limits of acceptability for water-quality sensor values used in this study (modified from Wagner and 

others, 2000).

[°C, degrees Celsius; mg/L, milligrams per liter]

Parameter Maximum allowable limits for water-quality sensor values

Temperature ±2.0°C

Specific conductivity ±30 percent

Dissolved oxygen Greater of ±2.0 mg/L or 20 percent

pH ±2 pH standard units

Turbidity ±30 percent

Table 5. Rating continuous water-quality data collected in this study (modified from Wagner and others, 2000).

[≤, less than or equal to; >, greater than; °C, degrees Celsius; mg/L, milligram per liter]

Parameter Excellent Good Fair Poor

Dissolved oxygen ≤±0.3 mg/L >±0.3 to 0.5 mg/L >±0.5 to 0.8 mg/L >±0.8 mg/L

pH ≤±0.2 standard unit >±0.2 to 0.5 standard 

unit

>±0.5 to 0.8 standard 

unit

>±0.8 standard unit

Specific 

conductivity

≤±3 percent >±3 to 10 percent >±10 to 15 percent >±15 percent

Temperature ≤±0.2°C >±0.2 to 0.5°C >±0.5 to 0.8°C >±0.8°C

Turbidity ≤±5 percent >±5 to 10 percent >±10 to 15 percent >±15 percent

Table 6. Summary of field visits and quality of data for five parameters at eight Illinois stations (40 sensors) during August 2001–

June 2003. 

[°C degree Celsius; mg/L, milligram per liter]

Parameter Number 

of service 

visits

Time in 

calibration, 

percent

Time out of 

calibration, 

percent

Time 

excellent data, 

percent

Time good 

data, 

percent

Time fair 

data, 

percent

Time poor 

data, 

percent

Mean error Median 

error

Temperature 268 88.1 11.9 50.4 26.8 5.9 5 0.2°C 0.1°C

Specific 

conductivity

265 86 14 46.9 24.7 6.1 8.3 5.1 percent 2.6 percent

Dissolved oxygen 268 77.9 22.1 21.9 16.3 19.3 20.4 0.6 mg/L 0.6 mg/L

pH 265 84.9 15.1 51.1 22.3 6.4 5.1 0.4 standard unit0.2 standard unit

Turbidity 227 72 28 13.1 6.4 19 33.5 13 percent 11 percent
12 Evaluation of Measurements with Multi-Parameter Water-Quality Monitors in Selected Illinois Streams, 2001–03

Temperature

The performance of the water-temperature sensor at each 

field station was compared to a certified thermometer about 

twice per year using methods described by Radtke, Kurklin, 

and Wilde (1998). The water-temperature sensors were serviced in the field 268 times during August 2001 through September 2003. The water-temperature sensors either exceeded 

the maximum allowable limit (±2°C) or otherwise failed to 

provide usable water-temperature values at 11.9 percent of the 

service visits. During the remaining 88.1 percent of the visits, 

the water-temperature data values were within the maximum 

allowable limit and were rated as follows: 50.4 percent were 

excellent; 26.8 percent were good; 5.9 percent were fair; and 

5.0 percent were poor. The mean error was 0.2°C and the 

median error was 0.1°C for the 88.1 percent of visits when the 

water-temperature sensors were reading within the maximum 

allowable limit. Performance characteristics for temperature 

and the other four parameters monitored during July 2001 

through June 2003 are summarized in table 6.

Dissolved oxygen

The dissolved-oxygen sensors were serviced in the field 

268 times during August 2001 through September 2003. The 

dissolved-oxygen sensors exceeded the maximum allowable 

limit (the greater of ±2.0 mg/L or ±20 percent of the correct 

value) or otherwise failed to provide usable dissolved-oxygen values at 22.1 percent of the visits. During the remaining 

77.9 percent of the visits, the dissolved-oxygen data values 

were within the maximum allowable limit and were rated as 

follows: 21.9 percent were excellent; 16.3 percent were good; 

19.3 percent were fair; and 20.4 percent were poor. The mean 

error was 0.9 mg/L and the median error was 0.7 mg/L at the 

77.9 percent of visits when the dissolved-oxygen sensors were 

reading within the maximum allowable limit.

pH

The pH sensors were serviced in the field 265 times during August 2001 through September 2003. The pH sensors 

exceeded the maximum allowable limit (±2 pH standard units) 

or otherwise provided unusable pH values at 15.1 percent of 

the visits. During the remaining 84.9 percent of the visits, 

the pH data values were within the maximum allowable limit 

and were rated as follows: 51.1 percent were excellent; 22.3 

percent were good; 6.4 percent were fair; and 5.1 percent were 

poor. The mean error was 0.4 pH standard unit and the median 

error was 0.2 pH standard units for the 84.2 percent of visits 

when the pH sensors were reading within the maximum allowable limit.

Specific conductivity

The specific-conductivity sensors were serviced in the 

field 265 times during August 2001 through September 2003. 

The specific-conductivity sensors either exceeded the maximum allowable limit (±30 percent of the correct value) or 

otherwise failed to provide usable specific conductivity values 

at 14.0 percent of the service visits. During the remaining 86.0 

percent of the service visits, the specific-conductivity data 

values were within the maximum allowable limit and were 

rated as follows: 46.9 percent were excellent; 24.7 percent 

were good; 6.1 percent were fair; and 8.3 percent were poor. 

The mean error was 5.1 percent and the median error was 2.6 

percent for the 86.0 percent of visits when the specific-conductivity sensors were reading within the maximum allowable 

limit.

Turbidity

The turbidity sensors were serviced in the field 227 times 

during August 2001 through September 2003. The turbidity sensors exceeded the maximum allowable limit (±30 

percent of the correct value) or otherwise failed to provide 

usable turbidity values at 28.0 percent of the visits. During the 

remaining 72.0 percent of the visits, the turbidity data values 

were within the maximum allowable limit and were rated as 

follows: 13.1 percent were excellent; 6.4 percent were good; 

19.0 percent were fair; and 33.5 percent were poor. The mean 

error was 13.3 percent and the median error was 10.6 percent 

at the 72.0 percent of visits where the turbidity sensors were 

reading within the maximum allowable limit.

Environmental conditions that affect the 

measurement quality

Low ionic strength affects the responsiveness and reliability of pH measurements—especially for the rugged gel-filled 

pH sensors used in this study. For most Illinois streams, low 

ionic strength may not be an issue; however, one station (Lusk 

Creek near Eddyville) generally had low ionic strength. This 

low ionic strength may have affected the responsiveness of the 

pH sensor in Lusk Creek (see p. 21). Potentially, pH measurements in other streams in watersheds similar to Lusk Creek 

near Eddyville may be affected. Large suspended-sediment 

concentrations also can affect pH measurements (Radtke, 

Busenberg, and Kurklin, 1998). 

Ideally, sondes would be placed in streams where they 

would remain completely submerged for the entire period of 

deployment without being moved. Furthermore, the sonde 

would be in the well-mixed and representative part of the 

stream channel. In many smaller or shallow streams considered here, such as Mazon River near Coal City, there are riffle 

zones too shallow for sonde placement where there is greater 

oxygen exchange between the atmosphere and streamwater. 

These riffle zones may be more representative of the overall 
Evaluation of Water-Quality Measurements 13

stream dissolved-oxygen concentration. To assure that the 

sonde is underwater even under the lowest flow conditions, 

they are generally placed in “pools’ or deeper zones within 

shallow streams with slower moving water that may not be 

as well-oxygenated as the shallow riffle zones. Therefore, 

the placement of some sondes in these deeper and sometimes 

stagnant flow zones tends to bias the dissolved oxygen toward 

lower concentrations than other placements. This bias likely 

is negligible during conditions of moderate to high flow for 

a particular stream; however, low dissolved oxygen usually 

is more frequent during low-flow conditions, in part, because 

low flows tend to occur in late summer to early fall when 

water temperatures are generally highest.

Because placement of the sondes is limited to a single 

point within the stream channel, they are of limited utility 

in poorly mixed streams or stream segments. Weather and 

streamflow have substantial affects on the capability of the 

sondes to record accurate data. 

Major Reasons for Lost or Unusable Data

 Lost or unusable (quality worse than poor and rejected) 

water-quality values were caused by various factors, including 

sensor failure, sensor internal drift, excessive sensor fouling, 

total sonde failure, and other directly related factors. On some 

occasions, failure was simultaneously caused by more than 

one factor, and when this happened, the most important factor 

has been considered the cause of the failure. For example, 

total sonde failure also caused sensor failure; however, in 

this circumstance, the cause of the data loss was classified as 

total sonde failure rather than sensor failure. Each factor is 

discussed below (along with selected related factors) and the 

estimated percentage of missing or unusable data relative to 

all data loss is reported. The distinction between sensor failure 

and total sonde failure is important; sensor failure results in 

lost data for a single parameter, whereas total sonde failure 

results in loss of all parameters for the station. If the monitor recorded no data, the data were considered missing. If the 

monitor recorded data but the data exceeded the maximum 

allowable limit (table 4), the data were considered unusable 

and were rejected.

Sensor Failure

About 15 percent of oxygen-sensor failures were caused 

either by membrane rupture or membrane leakage. The 

membrane, fastened to the sensor with a rubber O-ring, is a 

thin (0.002 inch) hydrophobic oxygen-permeable material that 

separates the electrodes from the environmental water, and 

is easily damaged. Membrane ruptures were sudden with no 

warning, typically caused by objects such as woody debris or 

aquatic organisms, and resulted in immediate sensor failure. 

By contrast, membrane leakage generally was slow, usually 

caused by a loose or failing O-ring or a slightly abraded membrane, and the sensor gradually failed over a period of hours 

or sometimes days. The majority of membrane-related failures 

were caused by rupture, and identifying unusable oxygen 

values (defined as errors greater than maximum allowable 

limit) in this circumstance was obvious upon inspection of the 

dissolved-oxygen concentration data values. When unusable 

oxygen values resulted from membrane leakage, and if the 

distinction between usable and unusable data was not obvious 

upon inspection, then the distinction was made with a diagnostic parameter that measured oxygen-sensor performance. 

On some occasions (about 3 percent of deployment time), the 

oxygen sensor failed without warning because of an internal 

electronic malfunction. When internal failure of the sensor 

occurred, collection of oxygen data immediately ceased and 

the defective sensor required replacement.

Most failures (about 13 percent) of the pH sensors were 

caused by small fractures in the thin glass membrane that 

separates the pH electrode from the environmental water. 

The glass membrane has a small protective shield. However, 

objects occasionally breached the membrane; as a result, reference solution was lost and the pH sensor failed immediately. 

Rarely (about 3 percent), the pH sensors failed because of 

internal failure, as indicated by the inability to restore operability to nonresponsive sensors that were clean and structurally intact.

Among the water-quality sensors used in this study, the 

turbidity sensors were the most prone to failure. The turbidity sensors were self-cleaning and operated by measurement 

of optical backscatter. The turbidity sensors usually failed for 

one of two main reasons. Most turbidity sensor failures (38 

percent) were caused by malfunction of the automated-cleaning mechanism, and about 19 percent were caused by internal 

sensor failure.

Water temperature and specific conductivity were measured with a single, combination sensor. Only about 7 percent 

of water-temperature and specific-conductivity data losses 

were caused by internal failure of the combination sensor, 

and all such data losses were caused by failure of the watertemperature sensor. Failure of the water-temperature sensor, 

though rare, resulted in failure of the dissolved-oxygen, pH, 

and specific-conductivity sensors, all of which required watertemperature data to operate. The combination temperature 

and specific conductivity sensor rarely failed independently 

of other failures, and there was never an internal failure of the 

specific-conductivity sensor that occurred independently of 

some other failure.

Sensor Internal Drift

Unusable sensor values caused by internal sensor drift 

were relatively rare, accounting for about 3.9 percent of data 

loss for the specific-conductivity sensor and no data loss for 

the water-temperature sensor. Data losses from sensor internal 

drift were more common for dissolved oxygen (5.9 percent) 

and pH (13 percent). The data losses from internal drift in 

the oxygen and pH sensors were quantified using diagnostic electrical measurements, collected concurrently with the 
14 Evaluation of Measurements with Multi-Parameter Water-Quality Monitors in Selected Illinois Streams, 2001–03

water-quality data. For example, the pH sensor had an associated diagnostic parameter that recorded the difference, in millivolts (mV), of the electrical voltage across the pH reference 

electrode at a user-specified pH standard. If the user-specified 

standard had a pH value of 7, then the associated pH reference 

electrode voltage should have been in the range ±30 mV. Voltage in excess of this range indicated the pH sensor was operating beyond the maximum allowable error limit and needed 

replacement. Internal drift was of little or no importance for 

the two self-cleaning optical sensors that measured turbidity 

and chlorophyll.

Sonde Failure

Sonde failure was a major cause (66 percent) of lost data. 

Sonde failure was caused by any of three possible events, 

including internal sonde flooding, software failure, and operator error. Sonde failure caused all sensors to cease operation 

and data recording to stop. The sensor values stored internally 

prior to the failure usually were erased or corrupted, and if the 

sonde was not connected to telemetry prior to the sonde failure, all measurements made after the previous field visit and 

subsequent to the field visit in which the failure was identified 

usually were irretrievably lost. The relative percentage that 

each type of failure contributed to all the sonde failures and 

resultant data losses are described below.

About 15 percent of sonde failures were the result of 

internal sonde flooding. When one or more of the internal 

compartments were breached, streamwater entered the sonde. 

The battery compartment was most often flooded, though in 

some instances, the circuitry compartment, adjacent to the 

battery compartment, was breached. Breaches of the sonde 

resulted from small fractures in the battery compartment 

lid—in close proximity to the locking screws—or fracture of 

the main casing, usually near the end of the sonde where the 

sensors were attached.

During the winter of 2001-02, a software upgrade failed, 

affecting multiple stations during a period when field visits were relatively infrequent (usually every 3 weeks). The 

manufacturer’s software upgrade was intended to improve the 

function of the optical-port wiping mechanisms. However, an 

error in the software caused rapid internal-memory saturation and battery-power depletion, resulting in partial to total 

shutdown of the affected sondes subsequent to the upgrade and 

shortly after they were redeployed. This software error caused 

about 13 percent of the sonde failures.

If the water at the sensors was not frozen, generally the 

sondes performed best in cold weather. This is because fouling 

was negligible in the near-freezing water. The most important 

negative effect of low temperature (defined as air temperature 

below 0°C) was field servicing became much more difficult 

and sometimes impossible, because some sondes occasionally 

were stuck in, or under, ice. This lack of sonde access, in turn, 

resulted in data-quality problems and sometimes loss of all 

sensor data for the period that the sonde could not be accessed. 

Subfreezing temperatures resulted in some sondes being outof-service for substantial periods of time, particularly when 

the sonde was operating only on internal battery power. The 

stations where icing prevented timely access were, in order 

of more to less frequent problems, Mazon River at Coal City, 

North Fork Vermilion River near Oakwood, and Illinois River 

at Valley City. The problems at Coal City were most severe; 

there were periods in the winter of 2001–02 when that sonde 

was stuck under ice for weeks at a time. 

Fouling

Excessive sensor fouling caused about 10 percent of all 

unusable values. Fouling generally did not cause complete 

data loss. Observation of the performance of the sensors after 

field cleaning and during calibration allowed identification of 

unusable values that resulted from sensor fouling. Usually, the 

errors caused by sensor fouling decreased greatly or disappeared after field cleaning and, if so, the sensor usually then 

calibrated without difficulty. Fouling was most severe during 

periods of low streamflow and during peak water temperature, 

often in late summer. These conditions sometimes caused 

growth of biofilm on the sensors that interfered with the proper 

operation. The biofilm sometimes rapidly grew to considerable thickness, and as biofilm thickness increased, the quality 

of the measurements could decrease to inutility. Generally, 

the oxygen and pH sensors were most vulnerable to biofilm 

fouling—about 29 percent of dissolved-oxygen data losses and 

11 percent of pH data losses were the result of fouling. The 

water-temperature sensor never failed because of excessive 

fouling. Assuming the automated cleaning mechanisms were 

operating properly, sensor fouling caused loss of turbidity data 

less than 3 percent of the time with the exception of Middle 

Fork Vermilion River above Oakwood, Rayse Creek near 

Waltonville, and Vermilion River near Danville. At these three 

stations, the fouling was more severe and caused about 10 to 

15 percent of turbidity data to be unusable, despite automated 

cleaning mechanisms that operated properly. During periods 

of increased streamflow and/or periods of cooler (but above 

freezing) water temperatures, sensor fouling usually was much 

less severe and seldom the sole cause of unusable values. 

Sometimes, the increased streamwater velocities associated 

with high streamflow or floods effectively cleaned the sensors, 

by removing biofilm and sediment build-up on and around 

the sensors. Cooler water temperatures reduced algal-related 

activity and resulted in reduced biofilm growth on the sensors, 

and, consequently, reduced the frequency and extent of sensor 

fouling. Some unusable values were caused by accumulation 

of sand- and silt-sized sediment lodged on and around the sensors. The specific-conductivity sensor was most vulnerable to 

sediment fouling, and fouling by sediment build-up caused 12 

percent of its data losses.
Evaluation of Water-Quality Measurements 15

Operator error

About 72 percent of data losses were caused by operator 

error, which was the most frequent cause of missing or unusable data values. Two types of operator errors caused nearly 

all these failures; either the sonde was deployed in a location 

that was not representative of the stream, or the operator erred 

during data retrieval or routine field servicing. Data losses 

caused by poor sonde deployment became apparent during 

low flow at two stations—the Mazon River near Coal City 

and the Vermilion River near Danville. In the Mazon River 

near Coal City during periods of extreme low flow (usually lasting not more than 5–7 days), the water level dropped 

sufficiently to leave the sonde exposed to the air. For this 

report, extreme low flow in the Mazon River near Coal City is 

defined as streamflow less than about 0.5 ft3/s (the 90-percent 

exceedance streamflow is 1.6 ft3/s). During low flow, Mazon 

River streamflow was mostly confined to an exposed rock 

riffle in the center of the stream channel that was from about 

0.1 to 0.2 ft deep. Deployment of the Mazon River sonde at or 

near the riffle was not feasible. The sonde was deployed in a 

shallow mud-bottomed pool near the right bank, downstream 

of the riffle, where low-flow stream velocity was slow (less 

than 0.1 ft/s). The water-quality measurements in the shallow 

pool likely were not representative of water quality in the riffle 

zone. The deployment location of the sonde in the Vermilion 

River near Danville periodically also was not representative of 

the stream during low flow—defined as less than the 90-percent exceedance flow of 52 ft3

/s. 

The optimum deployment of a sonde is confounded 

by the need to satisfy conflicting requirements. The sensors 

must be at an appropriate stream depth so they are continuously submerged and the location should be representative of 

the stream overall. Furthermore, the sonde and sensors need 

to be readily accessible during most streamflow conditions 

for routine field servicing. These competing requirements 

are sometimes further complicated by the need to place the 

sonde within a reasonable distance (generally less than 500 ft) 

of the streamflow-gaging station, if real-time data-transmission capabilities are required. At the Mazon River station, 

the sonde was lowered incrementally, as needed, to keep the 

sensors continuously submerged during the extreme low-flow 

periods. Lowering the monitor elevation in large increments 

was not feasible, because the water depth at the deployment 

location was less than 0.5 ft during the extreme low-flow periods. About 16 percent of all total data loss was caused by the 

deployment of the sonde in a location that did not adequately 

represent the stream during low streamflow. 

About 58 percent of all data loss was caused by operator 

errors during field visits and data retrieval, and nearly all these 

errors occurred in 2003, the final year of data collection. Data 

losses through operator error were most frequent at the following stations (relative percentage of data loss by indicated station shown in parentheses): Rayse Creek near Waltonville (80 

percent), Lusk Creek near Eddyville (68 percent), Middle Fork 

Vermilion River above Oakwood (51 percent), and Vermilion 

River near Danville (41 percent).

Comparison of Continuous Data with Discrete 

Measurements

Another means of assessing the performance of the monitors is by comparison with independent measurements of the 

parameters by USGS personnel prior to servicing and by other 

observers; in this case, IEPA employees during their periodic 

sampling visits to the stations. The IEPA maintains an ambient 

water-quality-monitoring program in Illinois (Illinois Environmental Protection Agency, 1989). This program includes 214 

(as of 2002) stations across the State in selected representative watersheds; the eight stations where the water-quality 

monitors were deployed were included in the program. The 

stations were visited every 6 weeks through the year and water 

temperature, pH, dissolved oxygen, specific conductivity, and 

turbidity were measured with calibrated monitors, similar to 

those deployed for this study but for discrete measurements 

to accompany the sample analytical data. Field methods for 

sample collection and measurement of pH, specific conductivity, temperature, dissolved oxygen were comparable to USGS 

methods (Illinois Environmental Protection Agency, 1989) and 

the methods for field measurement of turbidity were given in a 

written procedure (Illinois Environmental Protection Agency, 

2002). For the remainder of this report, these discrete measurements will be referred to as discrete independent measurements (DIMs). 

The correspondence between DIMS and the sonderecorded values are shown in figures 3–7. The sonde-recorded 

values were those nearest in time to the sampling time. To 

avoid using values recorded during or after a service visit 

when the sensors were recalibrated, preference was given to 

the sonde value recorded immediately prior to the sample 

time. The linear-regression lines, along with the equations and 

coefficients of determination (R2), also are shown in figures 

3–7. Generally, a strong relation (generally R2 greater than 

0.75) resulted between the DIMs and instantaneous sonde 

measurements for water temperature. Most values indicate 

a one-to-one correspondence (fig. 3). The lowest R2 is 0.93 

for the sonde deployed at Vermilion River near Danville. The 

highest R2 is 1.0 for the sonde deployed at Illinois River at 

Valley City. 

As previously discussed, each station was selected as representative of a subset of environmental conditions. Because 

each of the sondes was deployed continuously at a given station for the study duration, except for problems, it is not possible to distinguish specific sonde-related issues from those of 

the environmental conditions at each station. In other words, 

the high correspondence between DIMs and sonde values at 

Illinois River at Valley City (see figures 3–7) could result from 

the relatively stable and slowly changing nature of the Illinois 

River at this location, or equally, to the robust and accurate 

response of the temperature sensor and sonde deployed there. 
16 Evaluation of Measurements with Multi-Parameter Water-Quality Monitors in Selected Illinois Streams, 2001–03

Figure 3. The relation between discrete independent measurements and instantaneous sonde measurements of temperature at eight monitoring stations, Illinois. R2 is the coefficient of determination of the regression. 1:1 or line of perfect correspondence (black) only shown where the data and/or regression line will not be obscured.

50

353025201510

0 5 10 15 20 25 30 35

50

3025201510

0 5 10 15 20 25 30

50

3025201510

0 5 10 15 20 25 30 5

3025201510

5 10 15 20 25 30

50

353025201510

0 5 10 15 20 25 30 35 -5

50

353025201510

-5 0 5 10 15 20 25 30 35

50

3025201510

0 5 10 15 20 25 30 -5

50

353025201510

-5 0 5 10 15 20 25 30 35

TURE (SONDE), IN TEMPERA TER WA

DEGREES CELSIUS

ILLINOIS RIVER AT

VALLEY CITY

WATER TEMPERATURE (DISCRETE), IN

DEGREES CELSIUS

LUSK CREEK NEAR

EDDYVILLE

MAZON RIVER NEAR

COAL CITY

MIDDLE FORK VERMILION

RIVER ABOVE OAKWOOD

VERMILION RIVER

NEAR DANVILLE

SALT CREEK AT WESTERN

SPRINGS

RAYSE CREEK NEAR

WALTONVILLE

NORTH FORK VERMILION

RIVER NEAR BISMARCK

Y = -0.15 + 1.01X R = 1.0 2

WATER TEMPERATURE (DISCRETE), IN

DEGREES CELSIUS

WATER TEMPERATURE (DISCRETE), IN

DEGREES CELSIUS

WATER TEMPERATURE (DISCRETE), IN

DEGREES CELSIUS

WATER TEMPERATURE (DISCRETE), IN

DEGREES CELSIUS

WATER TEMPERATURE (DISCRETE), IN

DEGREES CELSIUS

WATER TEMPERATURE (DISCRETE), IN

DEGREES CELSIUS

WATER TEMPERATURE (DISCRETE), IN

DEGREES CELSIUS

Y= -0.44 + 1.00X R = 0.99 2 Y=-0.66 + 1.02X R = 0.96 2 Y= -1.83 + 1.09X R = 0.94 2

Y = -0.76 + 1.06X R = 0.93 2 Y = -1.11 + 1.07X R = 0.98 2 Y= 0.30 + 0.95X R = 0.98 2 Y= -0.18 + 1.0X R =0.99 2

TURE (SONDE), IN TEMPERA TER WA

DEGREES CELSIUS

1:1, OR LINE OF PERFECT

CORRESPONDENCE
Evaluation of Water-Quality Measurements 17 Figure 4. The relation between discrete independent measurements and instantaneous sonde measurements of dissolved oxygen at eight monitoring stations, Illinois. R2 is the coefficient of determination of the regression. 1:1 or line of perfect correspondence (black) only shown where the data and/or regression line will not be obscured.

987

1413121110

7 8 9 10 11 12 13 14

DISSOLVED-OXYGEN CONCENTRATION

NOR

(DISCRETE), IN MILLIGRAMS PER LITER

TH FORK VERMILION

RIVER NEAR BISMARCK

86420

141210

0 2 4 6 8 10 12 14

RAYSE CREEK NEAR

WALTONVILLE

Y = -2.82 + 1.38X R = 0.96

864

16141210

4 6 8 10 12 14 16

SALT CREEK AT WESTERN

SPRINGS

TION XYGEN CONCENTRA VED-O DISSOL

(SONDE), IN MILLIGRAMS PER LITER

86

1816141210

6 8 10 12 14 16 18

MAZON RIVER NEAR

COAL CITY

864

141210

4 6 8 10 12 14

LUSK CREEK NEAR

EDDYVILLE

864

16141210

4 6 8 10 12 14 16

ILLINOIS RIVER AT

VALLEY CITY

987

151413121110

7 8 9 10 11 12 13 14 15

MIDDLE FORK VERMILION

RIVER ABOVE OAKWOOD

864

1816141210

4 6 8 10 12 14 16 18

VERMILION RIVER

NEAR DANVILLE

2

DISSOLVED-OXYGEN CONCENTRATION(DISCRETE), IN MILLIGRAMS PER LITER

DISSOLVED-OXYGEN CONCENTRATION

(DISCRETE), IN MILLIGRAMS PER LITER

DISSOLVED-OXYGEN CONCENTRATION

(DISCRETE), IN MILLIGRAMS PER LITER

DISSOLVED-OXYGEN CONCENTRATION(DISCRETE), IN MILLIGRAMS PER LITER

DISSOLVED-OXYGEN CONCENTRATION(DISCRETE), IN MILLIGRAMS PER LITER

DISSOLVED-OXYGEN CONCENTRATION

(DISCRETE), IN MILLIGRAMS PER LITER

DISSOLVED-OXYGEN CONCENTRATION(DISCRETE), IN MILLIGRAMS PER LITER

TION XYGEN CONCENTRA VED-O DISSOL

(SONDE), IN MILLIGRAMS PER LITER

Y = -1.31 + 1.18X R = 0.92 2 Y = -0.72 + 1.1X R = 0.91 2 Y = 5.74 + 0.56X R =0.36 2

Y = 1.27 + 0.93X R = 0.70 2 Y = 1.52 + 0.88X R = 0.72 2 Y = 0.12 + 0.98X R =0.94 2 Y = -0.99 + 1.13X R = 0.96 2

1:1, OR LINE OF PERFECTCORRESPONDENCE 1:1, OR LINE OF PERFECT CORRESPONDENCE 1:1, OR LINE OF PERFECT CORRESPONDENCE

1:1, OR LINE OF PERFECTCORRESPONDENCE 1:1, OR LINE OF PERFECT

CORRESPONDENCE 1:1, OR LINE OF PERFECT CORRESPONDENCE 1:1, OR LINE OF PERFECT CORRESPONDENCE
18 Evaluation of Measurements with Multi-Parameter Water-Quality Monitors in Selected Illinois Streams, 2001–03

Figure 5. Relation between discrete independent measurements and instantaneous sonde measurements of specific conductivity at eight monitoring stations, Illinois. R2 is the coefficient of determination of the regression. 

900800700600500400300200

200 300 400 500 600 700 800 900

750700650600550500450

450 500 550 600 650 700 750

900800700600500400300

300 400 500 600 700 800 900

80

180160140120100

80 100 120 140 160 180

1,1001,000

900800700600500400300

1,100 1,000 900 800 700 600 500 400 300

SPECIFIC CONDUCTIVITY (DISCRETE), IN

MICROSIEMENS PER CENTIMETER

SONDE SPECIFIC CONDUCTIVITY (SONDE),

OSIEMENS PER CENTIMETER IN MICR

1,1001,000

900800700600500400300

1,100 1,000 900 800 700 600 500 400 300 1,800 1,600 1,400 1,200 1,000 800 600 400

 1,800

1,6001,4001,2001,000

800600400

LUSK CREEK NEAR

EDDYVILLE

ILLINOIS RIVER AT VALLEY CITY MAZON RIVER NEAR COAL CITY MIDDLE FORK VERMILION RIVER ABOVE OAKWOOD

SALT CREEK AT WESTERN

SPRINGS

VERMILION RIVER

NEAR DANVILLE

NORTH FORK VERMILION

RIVER NEAR BISMARCK

RAYSE CREEK NEAR

WALTONVILLE

SPECIFIC CONDUCTIVITY (DISCRETE), IN

MICROSIEMENS PER CENTIMETER

SPECIFIC CONDUCTIVITY (DISCRETE), IN

MICROSIEMENS PER CENTIMETER

SPECIFIC CONDUCTIVITY (DISCRETE), IN

MICROSIEMENS PER CENTIMETER

SPECIFIC CONDUCTIVITY (DISCRETE), IN

MICROSIEMENS PER CENTIMETER

SPECIFIC CONDUCTIVITY (DISCRETE), IN

MICROSIEMENS PER CENTIMETER

SPECIFIC CONDUCTIVITY (DISCRETE), IN

MICROSIEMENS PER CENTIMETER

SPECIFIC CONDUCTIVITY (DISCRETE), IN

MICROSIEMENS PER CENTIMETER

SONDE SPECIFIC CONDUCTIVITY (SONDE),

OSIEMENS PER CENTIMETER IN MICR

Y = 236 + 0.64X R = 0.70 2 2 Y = 77.1 + 0.92X R = 0.92 2 Y = 61.3 + 0.94X R = 0.56 2 Y = -44.4 + 1.06X R = 0.98

2 Y = 68.0 + 0.93X R = 0.88 2 Y = 16.0 + 0.81X R = 0.82 2 Y = 311 + 0.66X R = 0.41

200 400 600 800 1,200 1,000

800600400200

1,2001,000 2 Y = 99.8 + 0.81X R = 0.59

1:1, OR LINE OF PERFECTCORRESPONDENCE

1:1, OR LINE OF PERFECTCORRESPONDENCE 1:1, OR LINE OF PERFECT

CORRESPONDENCE

1:1, OR LINE OF PERFECTCORRESPONDENCE 1:1, OR LINE OF PERFECT CORRESPONDENCE 1:1, OR LINE OF PERFECT CORRESPONDENCE 1:1, OR LINE OF PERFECT CORRESPONDENCE

1:1, OR LINE OF PERFECTCORRESPONDENCE
Evaluation of Water-Quality Measurements 19 Figure 6. Relation between discrete independent measurements and instantaneous sonde measurements of pH at eight monitoring stations, Illinois. R2 is the coefficient of determination of the regression. 7.8 7.68

8.48.2

7.6 7.8 8 8.2 8.4

ARD UNITS AND pH (SONDE), IN ST

pH (DISCRETE), IN STANDARD UNITS

8.48.2

8.0

7.87.67.47.2

7.0

7.2 7.4 7.6 7.8 8.2 8.4 7.0 8.0

9.0

8.5

8.0

7.5

7.0

6.5

6.0

6.5 7.5 8.5 8.0 9.0 6.0 7.0

9.5

9.0

8.5

8.0

7.5

7.0

7.5 8.5 9.5 7.0 8.0 9.0

8.48.2

8.0

7.87.67.47.2

7.2 7.4 7.6 7.8 8.2 8.4 8.0

9.0

8.5

9.0

7.5

7.0

6.5

6.5 7.5 8.5 8.0 9.0 7.0 7.2 7.4 7.6 7.8 8.2 8.0 7.0

8.2

8.0

7.87.67.47.2

7.0

6.5 7.5 7.0 8.0 6.0

8.0

7.5

7.0

6.5

6.0

LUSK CREEK NEAR

EDDYVILLE

ILLINOIS RIVER AT

VALLEY CITY

MAZON RIVER NEAR

COAL CITY

MIDDLE FORK VERMILION

RIVER ABOVE OAKWOOD

NORTH FORK VERMILION

RIVER NEAR BISMARCK

RAYSE CREEK NEAR

WALTONVILLE

SALT CREEK AT WESTERN

SPRINGS

VERMILION RIVER

NEAR DANVILLE

ARD UNITS AND pH (SONDE), IN ST

pH (DISCRETE), IN STANDARD UNITS pH (DISCRETE), IN STANDARD UNITS pH (DISCRETE), IN STANDARD UNITS

pH (DISCRETE), IN STANDARD UNITS pH (DISCRETE), IN STANDARD UNITS pH (DISCRETE), IN STANDARD UNITS pH (DISCRETE), IN STANDARD UNITS

2 Y = 2.65 + 0.68X R = 0.71 2 Y = 3.21 + 0.56X R = 0.37

2 Y = 4.61 + 0.38X R = 0.30 2 Y = 3.92 + 0.53X R = 0.65

2 Y = 1.93 + 0.76X R = 0.51 2 Y = 1.07 + 0.89X R = 0.78 2 Y = 6.11 + 0.13X R = 0.35

2 Y = 4.43 + 0.45X R = 0.29

1:1, OR LINE OF PERFECT

CORRESPONDENCE 1:1, OR LINE OF PERFECT CORRESPONDENCE 1:1, OR LINE OF PERFECT CORRESPONDENCE

1:1, OR LINE OF PERFECTCORRESPONDENCE

1:1, OR LINE OF PERFECT

CORRESPONDENCE 1:1, OR LINE OF PERFECT CORRESPONDENCE 1:1, OR LINE OF PERFECT CORRESPONDENCE

1:1, OR LINE OF PERFECTCORRESPONDENCE
20 Evaluation of Measurements with Multi-Parameter Water-Quality Monitors in Selected Illinois Streams, 2001–03

0

50

150100

0 50 100 150

0

500400300200100

0 100 200 300 400 500 0

50

150100

0 50 100 150

0

80604020

120100

0 20 40 60 80 100 120

0

600500400300200100

0 100 200 300 400 500 600

0

700600500400300200100

0 100 200 300 400 500 600 700

50

25201510

0 5 10 15 20 25

LUSK CREEK NEAR

EDDYVILLE

ILLINOIS RIVER AT

VALLEY CITY

MAZON RIVER NEAR

COAL CITY MIDDLE FORK VERMILION RIVER ABOVE OAKWOOD

NORTH FORK VERMILION

RIVER NEAR BISMARCK

RAYSE CREEK NEAR

WALTONVILLE

SALT CREEK AT WESTERN

SPRINGS

VERMILION RIVER

NEAR DANVILLE

TURBIDITY (SONDE), IN FORMAZIN

NEPHELOMETRIC UNITS

TURBIDITY (DISCRETE), IN NEPHELOMETRIC

TURBIDITY UNITS

0 200 400 600 800 1,000 1,200 0

800600400200

1,2001,000

2 Y = 8.15 + 0.090X R = 0.023 2 Y = -35.2 + 3.90X R = 0.90 2 Y = 38.9 + 1.01X R = 0.47

TURBIDITY (DISCRETE), IN NEPHELOMETRIC

TURBIDITY UNITS

TURBIDITY (DISCRETE), IN NEPHELOMETRIC

TURBIDITY UNITS

TURBIDITY (DISCRETE), IN NEPHELOMETRIC

TURBIDITY UNITS

TURBIDITY (DISCRETE), IN NEPHELOMETRIC

TURBIDITY UNITS

TURBIDITY (DISCRETE), IN NEPHELOMETRIC

TURBIDITY UNITS

TURBIDITY (DISCRETE), IN NEPHELOMETRIC

TURBIDITY UNITS

TURBIDITY (DISCRETE), IN NEPHELOMETRIC

TURBIDITY UNITS

2 Y = 27.1 + 1.09X R = 0.82 2 Y = -0.49 + 1.00X R = 0.96

2 Y = 14.1 + 0.335X R = 0.05

2 Y = -1.12 + 1.14X R = 0.83

Y = 36.3 + 0.67X R =0.96 2

1:1, OR LINE OF PERFECTCORRESPONDENCE

1:1, OR LINE OF PERFECTCORRESPONDENCE 1:1, OR LINE OF PERFECT

CORRESPONDENCE

1:1, OR LINE OF PERFECTCORRESPONDENCE

1:1, OR LINE OF PERFECT

CORRESPONDENCE

1:1, OR LINE OF PERFECT

CORRESPONDENCE

1:1, OR LINE OF PERFECTCORRESPONDENCE

TURBIDITY (SONDE), IN FORMAZIN

NEPHELOMETRIC UNITS

Figure 7. Relation between discrete independent measurements and instantaneous sonde measurements of turbidity at eight monitoring stations, Illinois. R2 is the coefficient of determination of the regression. 1:1 or line of perfect correspondence (black) only shown where the data and/or regression line will not be obscured.
Evaluation of Water-Quality Measurements 21

ity of 122 μS/cm; mean alkalinity of 32 mg/L as CaCO3). pH 

electrodes have also been demonstrated to be less accurate in 

water with specific conductivity less than about 100 μS/cm 

(Busenberg and Plummer, 1987). Specific conductivity in 

Lusk Creek near Eddyville often was less than 100 μS/cm. 

Gel-filled pH electrodes, such as used in the deployed sondes, 

may be particularly susceptible to inaccuracy in water of low 

ionic strength. Lusk Creek near Eddyville has the second 

lowest correspondence between DIM pH and instantaneous 

sonde pH. Further, Lusk Creek is the only station where the 

pHs were consistently near neutral (7.0 standard units). All 

other monitors recorded pH values generally centered above 

neutral (pH of 7.5-8.0 or higher), indicating greater alkalinity 

and buffering capacity than Lusk Creek. Salt Creek at Western Springs has the lowest correspondence between DIM pH 

and instantaneous sonde pH; however, Salt Creek at Western 

Springs had the highest specific conductivity. Salt Creek at 

Western Springs is downstream of about 12 wastewater-treatment plants (WWTP) and during the study period in late summer, fall, and winter, a majority of the streamflow was WWTP 

effluent. Most WWTPs adjust pH during the treatment process 

and this could affect the stream pH independent of any natural 

buffering capacity or alkalinity in the streamwater.

Correspondence between DIM turbidity and instantaneous sonde turbidity is partly a consequence of the timing 

of sample collection. Generally, samples are collected only 

during periods of low turbidity—not during the early part 

of storm runoff. Turbidity data recorded at Vermilion River 

at Danville from May 5 through May 14, 2002, illustrate 

stormflow-related turbidity (fig. 8). The maximum turbidity 

generally occurs during the rising limb of a stormflow peak. 

Runoff from land surfaces during the early part of a rainstorm 

has the highest concentration of suspended sediment. Usually, suspended sediment is a major component of turbidity in 

Illinois streams and rivers. In the short period (hours) following the initial rise of a stormflow peak and later during stable 

or slowly changing streamflow periods, the turbidity tends 

to decrease and stabilize and then decrease further to levels 

before the storm. Rarely are samples taken, or the sondes 

serviced, during these short increases in turbidity and, thereLikewise, the relatively low correspondence between DIMS 

and sonde values at Vermilion River near Danville could be 

due to the sensor or sonde itself, or to the environmental conditions of the Vermilion River at that location.

The correspondence between DIMs and sonde values for 

dissolved oxygen, pH, specific conductivity, and turbidity at 

all eight stations is shown in figures 4–7. In general, the correspondence between DIMs and sonde values is poorer for dissolved oxygen, specific conductivity, pH, and turbidity, than 

for temperature. The coefficients of determination for these 

four parameters ranged as low as 0.02 for turbidity at Lusk 

Creek near Eddyville (the least disturbed watershed) to as high 

as 0.98 for specific conductivity at North Fork Vermilion River 

near Bismarck (table 7). 

The general sequence (from highest to lowest) of coefficients of determination is

Temperature > Dissolved Oxygen > Specific Conductivity > 

Turbidity > pH.

This sequence may be misleading. The coefficients of determination for turbidity are higher than pH, in part, because there 

is a wide range in turbidity values, when compared to temperature or dissolved-oxygen concentration. In all of the datasets, 

except for North Fork Vermilion River near Bismarck, only 

one or two values are in the highest range of turbidity and 

these single values substantially affect the coefficient of determination for the linear regressions on turbidity. For example, 

turbidity at Mazon River near Coal City (fig. 7 and table 7) has 

a high coefficient of determination (R2 of 0.90) even though 

the regression line deviates substantially from a one-to-one 

correspondence because of a single value. Further, that single 

value (data pair) appears to be an outlier (the sonde value or 

the DIM value) based on visual inspection of the graph. 

The relatively poor correspondence between DIM pH and 

sonde pH may be related to the buffer capacity and, indirectly, 

on the dissolved-solids concentration of the stream or river. 

The higher the dissolved solids (indicated by the specific 

conductivity) and the greater amount of alkalinity, the greater 

the buffer capacity of the streamwater and, generally, the 

more stable the pH. Lusk Creek near Eddyville had the lowest 

specific conductivity and alkalinity (mean specific conductivTable 7. Coefficients of determination for five parameters at each of eight stations in Illinois.

Station Temperature Specific 

conductivity

Dissolvedoxygen 

concentration

pH Turbidity

Illinois River at Valley City 1.0 0.88 0.96 0.29 0.96

Lusk Creek near Eddyville .99 .82 .93 .35 .02

Mazon River near Coal City .96 .41 .72 .78 .90

Middle Fork Vermilion River above Oakwood .93 .59 .70 .71 .47

North Fork Vermilion River near Bismarck .99 .98 .92 .51 .83

Rayse Creek near Waltonville .95 .56 .96 .37 .05

Salt Creek at Western Springs .98 .92 .91 .30 .96

Vermilion River near Danville .93 .70 .36 .65 .82
22 Evaluation of Measurements with Multi-Parameter Water-Quality Monitors in Selected Illinois Streams, 2001–03

fore, the DIMs rarely include the highest stream turbidities. 

Unfortunately, the high values occur during relatively high 

flow during storms when a substantial proportion of sediment 

and possible contaminants attached to sediments flow past the 

station. Therefore, the lack of data from samples collected during high-turbidity periods represents a substantial uncertainty 

in measuring the transport of sediments and potential contaminants. The lowest turbidity generally occurs during low-flow 

conditions when ground-water inflow is the largest component 

of flow in the stream or river.

The available data on turbidity of natural streams in 

Illinois are limited. Unlike temperature, dissolved oxygen, 

and specific conductivity, that have been typically measured 

over a wide range of historical streamflow conditions at many 

stations, there are few historical turbidity data and the full 

range of turbidity values is not well known for many streams. 

Turbidity has not been convenient to measure until a type of 

sensor (such as used in this study) was developed in the mid1990s. Prior to the development of deployable, in-situ sensors, turbidity was chiefly measured in the laboratory with a 

bench-top instrument. Typically, the requirements of drinkingwater clarity were the objectives for these measurements. In 

fact, the standard USEPA method, 180.1 (U.S. Environmental 

Protection Agency, 1993), is useful only for the range of 0 to 

40 nephelometric turbidity units (NTUs: NTUs are equivalent 

to FNUs, but are measured by a different method). Turbidity 

in most Illinois streams commonly exceeds 40 NTUs for substantial periods of time, particularly during storm runoff. The 

sensors used for this study were not certified for greater than 

1,400 FNUs. Considerable uncertainty still remains concerning the calibration and reliability of in-situ measurements of 

turbidity exceeding 1,000 FNUs. Because the turbidity at most 

of the eight stations in this study exceeded 1,400 FNUs for 

at least brief periods during high streamflow, it is not known 

how these transient high-turbidity periods affect the average 

turbidity, either on a time-weighted or flow-weighted basis. 

Development of sensors capable of reliably and reproducibly 

measuring turbidities much greater than 1,000 FNUs (possibly 

up to 2,000 to 5,000 FNUs) and reliable, stable standards for 

calibration at the upper range may allow the evaluation of the 

full range of turbidities. Reliable and accurate measurement 

of stream turbidity greater than 1,000 FNUs would potentially 

allow for development of much more accurate empirical relations between turbidity and suspended solids or suspended 

sediment (Ziegler, 2002).

Turbidity and suspended sediment in the Illinois River 

is unusual compared to the other seven streams in the study 

because of the nature of large high-order rivers, and because 

the river is a navigable waterway. Barges regularly passed by 

the Illinois River at Valley City monitor and because of the 

amount of propulsion required for moving barges, substantial 

amounts of sediment are re-entrained in the water unrelated to 

rainfall or runoff and, thus, increased the turbidity in at least 

part of the river at regular intervals.

Figure 8. Relations among streamflow, daily precipitation, and turbidity in the Vermilion River near Danville, Illinois, May 5–14, 2002. STREAMFLOW, IN CUBIC FEET PER SECOND

20,000

15,000

10,000

5,000

0

2.0

1.5

1.0

0.5

0

DAILY PRECIPITATION, IN INCHES

MAY 2002

5 6 7 8 9 10 11 12 13 14

TURBIDITY, IN FORMAZIN NEPHELOMETRIC UNITS

1,000

800

600

400

200

0

Turbidity Streamflow

Daily

precipitation
Analysis of Water-Quality Data 23

ANALYSIS OF WATER-QUALITY DATA

Comparison of laboratory-measured chlorophyll concentrations with the fluorescence (sensor)-measured chlorophyll 

concentrations indicated that there was no relation between 

the two datasets. Other characteristics of the sensor-derived 

chlorophyll concentrations, such as uncharacteristically large 

hourly or daily variations (sometimes an order of magnitude), 

indicated that the sensors were not reliably detecting chlorophyll. Based on the data collected for this study, it cannot 

be determined if the problems with the sensor-derived chlorophyll concentrations data were chiefly with the sensor or 

monitor, the deployment and calibration protocols, or with 

the particular environmental conditions at the stations, or a 

combination of these reasons. These chlorophyll data are not 

analyzed or discussed further in this report.

The daily maximums, the daily minimums, and the daily 

range of dissolved oxygen measured with the monitors for 

the eight stations from July 2001 through September 2003 are 

shown in figures 9 through 16. Dissolved oxygen is critical 

to ecosystem health and especially to native fish and other 

respiring organisms (Allen, 1995). The State of Illinois has 

set a regulatory minimum of 5 mg/L of dissolved oxygen 

(this regulatory minimum is shown in the figures 9 through 

16) to maintain health of fish and of respiring aquatic organisms. During periods, generally in late summer, even the least 

developed (least disturbed) watershed—Lusk Creek near 

Eddyville—recorded dissolved-oxygen concentrations below 

the regulatory minimum (5 mg/L). Nutrient data collected at 

this station indicate, that of the eight watersheds monitored 

in this study, it is the least affected by nutrient runoff (table 

8) and should be least likely of the eight watersheds monitored to have oxygen depletion caused by algal bloom die-off. 

Other watersheds, such as that of the Illinois River at Valley 

City, had daily oxygen maximum concentrations during short 

intervals in late summer (hours to days) that did not rise above 

the 5-mg/L dissolved-oxygen minimum. Rayse Creek near 

Waltonville has the most variable daily maximums, minimums, and range of dissolved-oxygen concentration.

The patterns of dissolved oxygen and temperature during August 2001 through September 2003 at Salt Creek at 

Western Springs are shown in figure 17. In general, and for all 

stations, not just Salt Creek, dissolved-oxygen concentration 

and temperature through the year are nearly completely out 

of phase—one reaches its annual minimum when the other 

reaches its annual maximum. This phase disparity probably 

results from the following or other factors. First, the solubility of gasses declines with increasing temperature; therefore, 

at equilibrium and in well-mixed conditions, oxygen in water 

will decline with increasing temperature. Second, respiration 

by plants, animals, and bacteria increases with temperature, so, 

barring other limitations, dissolved oxygen will be used more 

quickly as water temperature increases. Finally, production 

of organic matter (particularly algal biomass) increases with 

temperature. In late summer, algal blooms are most frequent 

and the resultant decay of a large mass of algal material can 

quickly deplete dissolved oxygen in nutrient-enriched streams. 

Oxygen is constantly replenished in flowing water in contact 

with the atmosphere; however, biota-driven processes, such 

as photosynthesis and respiration, can be much faster than 

physical replenishment. High photosynthetic release of oxygen 

during sunny periods usually is faster than the rate at which 

oxygen can diffuse to the surface of the stream and exsolve 

to the atmosphere. Uptake of oxygen by bacteria and other 

organisms at night generally is faster than the rate at which 

oxygen dissolves in water and diffuses to the zone of lower 

concentration (Hem, 1992).

Daily fluctuations of dissolved-oxygen concentration in 

response to generation of oxygen by macrophytes and algae 

during sunny days, and decomposition and respiration by 

oxygen-using organisms at night are important in all streams 

and rivers. These daily fluctuations and the overriding effect 

of stormflow on dissolved oxygen are shown in figure 18 for 

North Fork Vermilion River near Bismarck during August 

2002. The range of daily dissolved oxygen at this station is 

close to the average for the eight stations monitored during 

this study. The two storms in the second half of the month 

(as indicated by the two streamflow peaks on figure 26) show 

that the storms dampen or eliminate the daily fluctuations and 

depress the dissolved-oxygen concentrations for days. The 

lowered dissolved-oxygen concentration at this station does 

not go below the 5-mg/L regulatory minimum. 

The relation between discrete measurements of temperature and discrete dissolved-oxygen measurements in the 

Illinois River at Valley City is shown in figure 19. There is a 

strong linear relation between river water temperature and dissolved-oxygen concentration. This station is the only station 

of the eight monitored with a strong correlation between water 

temperature and dissolved-oxygen concentration. The river is 

wide and relatively shallow with a turbidity-limited euphotic 

zone (probably less than 2-3 ft). Small streams (less than 

5,000 ft3/s) have lower turbidity and usually have substantial 

biomass of periphyton attached to the streambed. The generation of oxygen by the periphyton greatly outstrips the depletion by respiration during sunny days in small streams. The 

narrow euphotic zone in the Illinois River limits photosynthesis by periphyton and to some extent even the free-floating 

phytoplankton. Production of oxygen by algae in the Illinois 

River probably does not greatly outstrip the capacity of the 

river to equilibrate with atmospheric oxygen; therefore, water 

temperature could be the main determinant of the amount of 

dissolved oxygen in the Illinois River at Valley City. Illinois 

River at Valley City has the smallest range in dissolved oxygen 

of the eight monitored stations (fig. 9). However, there are 

periods where the dissolved-oxygen concentration fell below 

the regulatory 5-mg/L minimum, likely because of decomposition of algal biomass.

The coefficient of determination for the linear correlation between temperature and dissolved oxygen at Valley City 

is 0.90. The highest coefficient of determination at any other 

station is 0.72 at Lusk Creek near Eddyville. Lusk Creek is 
24 Evaluation of Measurements with Multi-Parameter Water-Quality Monitors in Selected Illinois Streams, 2001–03

Figure 9. Daily maximum, minimum, and range for dissolved-oxygen concentration at Illinois River at Valley City, Illinois, from August 2001 through September 2003.

86420

1816141210

Daily maximum

Dailyminimum

Range

ILLINOIS RIVER AT VALLEY CITY

TION, XYGEN CONCENTRA VED-O DISSOL

IN MILLIGRAMS PER LITER Illinois General Use 

Water

Minimum Concentration

J A S O N D J F M A M J J A S O N D J F M A M J J A

S

2001 2002 2003

Gaps indicate missing data
Analysis of Water-Quality Data 25Figure 10. Daily maximum, minimum, and range for dissolved-oxygen concentration at Lusk Creek near Eddyville, Illinois, from August 2001 through September 2003.

86420

16141210

Dailymaximum

Dailyminimum

Range

TION, XYGEN CONCENTRA VED-O DISSOL

IN MILLIGRAMS PER LITER

J A S O N D J F M A M J J A S O N D J F M A M J J A

S

2001 2002 2003

LUSK CREEK NEAR EDDYVILLE

Illinois General Use 

Water

Minimum Concentration

Gaps indicate missing data
26 Evaluation of Measurements with Multi-Parameter Water-Quality Monitors in Selected Illinois Streams, 2001–03

Figure 11. Daily maximum, minimum, and range for dissolved-oxygen concentration at Mazon River near Coal City, Illinois, from August 2001 through September 2003.

50

3025201510

Dailymaximum

Dailyminimum

Range

TION, XYGEN CONCENTRA VED-O DISSOL

IN MILLIGRAMS PER LITER

J A S O N D J F M A M J J A S O N D J F M A M J J A

S

2001 2002 2003

Illinois General Use 

Water

Minimum Concentration

MAZON RIVER NEAR COAL CITY

Gaps indicate missing data
Analysis of Water-Quality Data 27 Figure 12. Daily maximum, minimum, and range for dissolved-oxygen concentration at Middle Fork Vermilion River above Oakwood, Illinois, from August 2001 through September 2003.

50

201510

Dailymaximum

Dailyminimum

Range

TION, XYGEN CONCENTRA VED-O DISSOL

IN MILLIGRAMS PER LITER

J A S O N D J F M A M J J A S O N D J F M A M J J A

S

2001 2002 2003

Illinois General Use 

Water

Minimum Concentration

MIDDLE FORK VERMILION RIVER ABOVE OAKWOOD

Gaps indicate missing data
28 Evaluation of Measurements with Multi-Parameter Water-Quality Monitors in Selected Illinois Streams, 2001–03

Figure 13. Daily maximum, minimum, and range for dissolved-oxygen concentration at North Fork Vermilion River near Bismarck, Illinois, from August 2001 through September 2003.

50

201510

Dailymaximum

Dailyminimum

Range

TION, XYGEN CONCENTRA VED-O DISSOL

IN MILLIGRAMS PER LITER

J A S O N D J F M A M J J A S O N D J F M A M J J A

S

2001 2002 2003

Illinois General Use 

Water

Minimum Concentration

NORTH FORK VE

RMILION RIVER NEAR BISMARC

K
Analysis of Water-Quality Data 29Figure 14. Daily maximum, minimum, and range for dissolved-oxygen concentration at Rayse Creek near Waltonville, Illinois, from August 2001 through September 2003.

86420

16141210

Daily maximum

Dailyminimum

Range

TION, XYGEN CONCENTRA VED-O DISSOL

IN MILLIGRAMS PER LITER

J A S O N D J F M A M J J A S O N D J F M A M J J A

S

2001 2002 2003

Illinois General Use 

Water

Minimum Concentration

RAYSE CREEK NEAR WALTONVILLE

Gaps indicate missing data
30 Evaluation of Measurements with Multi-Parameter Water-Quality Monitors in Selected Illinois Streams, 2001–03

Figure 15. Daily maximum, minimum, and range for dissolved-oxygen concentration at Salt Creek at Western Springs, Illinois, from August 2001 through September 2003.

86420

201816141210

SALT CREEK AT 

WESTERN SPRINGS Daily maximum

Dailyminimum

Range

TION, XYGEN CONCENTRA VED-O DISSOL

IN MILLIGRAMS PER LITER

J A S O N D J F M A M J J A S O N D J F M A M J J A

S

2001 2002 2003

Illinois General Use 

Water

Minimum Concentration

Gaps indicate missing data
Analysis of Water-Quality Data 31Figure 16. Daily maximum, minimum, and range for dissolved-oxygen concentration at Vermilion River near Danville, Illinois, from August 2001 through September 2003.

50

25201510

Daily maximum

Daily minimum

Range

TION, XYGEN CONCENTRA VED-O DISSOL

IN MILLIGRAMS PER LITER

J A S O N D J F M A M J J A S O N D J F M A M J J A

S

2001 2002 2003

Illinois General Use 

Water

Minimum Concentration

VERMILION RIVER NEAR DANVILLE

Gaps indicate missing data
32 Evaluation of Measurements with Multi-Parameter Water-Quality Monitors in Selected Illinois Streams, 2001–03

Table 8. Mean total nitrogen and phosphorus concentrations and mean dissolved-oxygen concentration maximums, minimums, 

and daily range, in milligrams per liter, at the eight stations in Illinois, July 2001 through June 2003.

Station Mean total 

nitrogen 

concentration

Mean total 

phosphorus

concentration

Mean of daily 

dissolved-oxygen 

maximums

Mean of daily 

dissolved-oxygen 

minimums

Mean of daily range 

dissolved-oxygen 

concentration

Illinois River at Valley City 6.6 0.4 9.4 8.6 0.76

Lusk Creek near Eddyville .45 .02 8.9 7.8 1.2

Mazon River near Coal City 11.4 .09 13.2 8.1 5

Middle Fork Vermilion River 

above Oakwood

9.2 .11 12.3 9.5 2.9

North Fork Vermilion River near 

Bismarck

8.2 .16 11.8 9.9 1.8

Rayse Creek near Waltonville 1.9 .24 6.7 4.7 2

Salt Creek at Western Springs 7.4 1.5 10.5 8.1 2.4

Vermilion River near Danville 7.2 .32 12 9.1 2.9

the watershed least affected by nutrients in runoff (table 8) 

of the eight watersheds monitored. Lusk Creek (fig. 10) has 

the second smallest average daily range in dissolved oxygen 

of the eight stations monitored. The effect of algal blooms on 

dissolved oxygen in Lusk Creek should be small relative to 

other small streams; thus, temperature likely is a major factor 

affecting dissolved-oxygen concentration in the stream.

The relation between streamflow and specific conductivity is nearly the opposite of that between turbidity and 

streamflow (fig. 20). Early in the rising limb of a storm 

hydrograph, specific conductivity drops because of the precipitation directly on the stream and early overland flow from 

riparian areas of the watershed. The specific conductivity and 

streamflow for Mazon River near Coal City for the period 

March 7–27, 2002, shows examples of this phenomenon. In 

most small streams, (mean annual flow about 500 ft3/s or less), 

specific conductivity is an indicator of ground-water inflow to 

the stream because ground water generally has much higher 

dissolved-solids concentrations than storm runoff in streams. 

Specific conductivity increases as ground water becomes a 

larger portion of the streamflow as the flow recedes during late 

summer, fall, and winter.

Most streams and rivers in Illinois follow the general 

relation between specific conductivity and streamflow, but 

the effect often is subdued in some streams because of the 

relatively high dissolved-solids concentration in streams and 

rivers relative to precipitation and shallow ground water. 

Further, streams that have relatively large contributions from 

WWTPs, such as Salt Creek at Western Springs, have high dissolved-solids concentrations because of the effluent (fig. 21) 

or from road-salt runoff. The high dissolved-solids concentration of WWTP effluent and consistent discharge to the stream 

throughout the year and road salt from snow melt create specific-conductivity peaks in the stream that tend to mimic and 

often overwhelm the increase in specific conductivity caused 

by ground-water inflow, especially during the winter months 

(fig. 21).

Since 1972, dissolved-oxygen concentration in Illinois 

streams has been regulated. The Illinois Pollution Control 

Board (IPCB) requires that dissolved-oxygen concentration 

in Illinois General Use (those without special regulations) 

streams will not be less than 6.0 mg/L for at least 16 hours of 

any 24-hour period or less than 5.0 mg/L at any time (Illinois Pollution Control Board, 2004). Compliance with this 

regulation can be difficult to monitor, in part, because there 

is no definition or restriction regarding the 16-hour period to 

which the regulation applies. Furthermore, dissolved-oxygen 

concentration generally reaches its minimum around dawn, 

when water-quality measurements are rarely collected. This 

discussion refers primarily to the instantaneous standard (5 

mg/L) rather than the 16-hour minimum because most periods 

of noncompliance in the eight streams of this study involved 

this standard rather than the 16-hour minimum. Dissolvedoxygen concentration typically fluctuates daily as a result of 

photosynthesis and algal and other microbial respiration, and 

relatively low-velocity streams often have larger daily fluctuations compared to faster flowing streams. Some Illinois 

streams, especially in the southern part of the State, are 

naturally warmer and, thus, may have lower dissolved-oxygen 

concentration than cooler streams. The higher stream temperatures often increase the rates of photosynthesis and respiration 

and, therefore, the range of the daily dissolved-oxygen fluctuations. Most noncompliance with respect to the dissolvedoxygen standard occurs from June through October each year 

with near-normal weather and rainfall. The dissolved-oxygen 

concentration from the sondes and the 7-day mean dissolved 

oxygen for Salt Creek at Western Springs, North Fork Vermilion River near Bismarck, and Lusk Creek near Eddyville 

are shown in figure 22. The current (2004) dissolved-oxygen 

standards and example proposed standards also are shown in 

the figure to compare how these three streams comply when 

assessed with these standards. These three streams have a wide 

variety of impairment and environmental conditions and, thus, 

represent most Illinois streams.
Analysis of Water-Quality Data 33

Figure 17. Water temperature and dissolved-oxygen concentration at Salt Creek at Western Springs, Illinois, August 2001 through 

September 2003.

Figure 18. Dissolved-oxygen concentration, daily precipitation, and streamflow at North Fork Vermilion River near Bismarck, Illinois, 

during August 2002. DISSOLVED-OXYGEN CONCENTRA- TION, IN MILLIGRAMS PER LITER

WATER TEMPERATURE,IN DEGREES CELSIUS

20

15

10

5

0

35

30

25

20

15

10

5

0

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

Dissolved

oxygen

Temperature

Gaps indicate

missing data

5

6

7

8

9

10

DISSOLVED-OXYGEN CONCENTRATION,

IN MILLIGRAMS PER LITER

STREAMFLOW, IN CUBIC FEET PER SECOND

1 2 3 4 5 7 6 8 9 10 11 1213 1415 16 17 1819 20 21 22 23 24 25 26 27 28 29 30

AUGUST 2002

10

100

1,000

10,000

100,000

Streamflow

Dissolved

oxygen

0

0.7

1.4

2.1

2.8

3.5

DAILY PRECIPITATION, IN INCHES

Daily

precipitation
34 Evaluation of Measurements with Multi-Parameter Water-Quality Monitors in Selected Illinois Streams, 2001–03

Figure 19. The relation between dissolved-oxygen concentration 

and water temperature at Illinois River at Valley City, Illinois. R2 is 

the coefficient of determination of the regression.

obtain and record fair to excellent data from the multi-parameter monitors for at least 75 percent of the deployment at the 

eight stations—were achieved. The pilot deployment of eight 

multi-parameter water-quality continuous monitors generally 

yielded good to excellent data for five parameters—water 

temperature, dissolved-oxygen concentration, specific conductivity, pH, and turbidity. Data from sensors for chlorophyll 

generally did not meet data-quality objectives for this study. 

The results indicate that the station-visit frequency (from 2 to 

6 weeks) was adequate to maintain the accuracy and precision 

of four of five of the sensors (temperature, specific conductivity, dissolved oxygen, and turbidity). The temperature sensors 

provided the most accurate and reliable measurements (generally within ±5 percent of a calibrated thermometer reading). 

The dissolved-oxygen sensors were the next most reliable 

during the study and were responsive to changing conditions 

and accurate at all eight stations. Specific conductivity was the 

third most accurate and reliable measurement collected from 

the multi-parameter monitors. Measurements of pH generally 

met data-quality objectives; however, the pH sensors apparently were the least responsive to changes of the five sensors 

tested and yielded the least-reliable data. Turbidity sensors 

collected measurements that met the data-quality objectives 

below about 1,400 formazin nephelometric units (FNUs).

Operator error was the most frequent cause of missing or 

unusable data values, accounting for 72 percent of overall data 

losses. Excessive sensor fouling caused about 10 percent of all 

unusable values. Fouling was most severe during periods of 

low streamflow and during peak water temperature, often in 

late summer. Generally, the oxygen and pH sensors were most 

vulnerable to fouling—about 29 percent of dissolved-oxygen 

data losses and 11 percent of pH data losses were the result of 

fouling. Sensor fouling caused loss of turbidity data less than 3 

percent of the time with the exception of Middle Fork Vermilion River above Oakwood, Rayse Creek near Waltonville, 

and Vermilion River near Danville. At these three stations, the 

fouling was more severe and caused about 10 to 15 percent 

of turbidity data to be unusable. About 15 percent of oxygensensor data losses were caused either by membrane rupture or 

membrane leakage.

The performance of the monitors also was evaluated by 

comparison with discrete independent measurements of the 

parameters by IEPA and USGS sampling personnel during 

their periodic sampling visits to the stations. Coefficients of 

determination of linear regressions of the discrete measurements on the instantaneous sonde value are indicators of 

the correspondence between the two sets of measurements. 

The correspondence between the discrete temperature measurements and sonde temperature yielded the highest coefficients—0.93 at Lusk Creek near Eddyville to 1.0 at Illinois 

River at Valley City. The coefficients of determination for dissolved oxygen, specific conductivity, pH, and turbidity ranged 

as low as 0.02 for turbidity at Lusk Creek near Eddyville 

(the least disturbed watershed) to as high as 0.98 for specific 

conductivity at North Fork Vermilion River near Bismarck. 

The relatively poor correspondence between discrete indepenDISSOLVED OXYGEN CONCENTRATION, IN MILLIGRAMS PER LITER

2 Y = 13.7 - 0.28X R = 0.90

WATER TEMPERATURE, IN DEGREES CELSIUS

4

6

8

10

12

14

16

0 5 10 15 20 25 30 35

SUMMARY

The U.S. Geological Survey (USGS), in cooperation with 

the Illinois Environmental Protection Agency (IEPA), conducted a study from July 2001 to October 2003 to investigate 

the applicability of the USEPA nutrient criteria to water quality and environmental conditions in streams in Illinois. The 

study involved the deployment of multi-parameter continuous 

water-quality monitors at eight stream or river locations across 

the State, representing a range of water quality and environmental conditions. The stations are (in alphabetical order):

• Illinois River at Valley City, Illinois;

• Lusk Creek near Eddyville, Illinois;

• Mazon River near Coal City, Illinois;

• Middle Fork Vermilion River above Oakwood, Illinois;

• North Fork Vermilion River near Bismarck, Illinois;

• Rayse Creek near Waltonville, Illinois;

• Salt Creek at Western Springs, Illinois, and;

• Vermilion River near Danville, Illinois

Data on water temperature, dissolved-oxygen concentrations, specific conductivity, pH, and turbidity were collected 

with these monitors. Data-quality objectives for the study—to 
Summary 35

Figure 20. Relations among daily precipitation, streamflow, and specific conductivity in Mazon River near Coal City, Illinois, March 

7–27, 2002. Gray lines indicate beginning of stormflow rise.

Figure 21. Specific conductivity and streamflow at Salt Creek at Western Springs, Illinois, February 7–March 21, 2002. SPECIFIC CONDUCTIVITY, IN MICROSIEMENS PER CENTIMETER STREAMFLOW, IN CUBIC FEET PER SECOND

MARCH 2002

7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

Specific

Conductivity

Streamflow

1,000

800

600

400

200

0

3,600

3,200

2,800

2,400

2,000

1,600

1,200

800

400

0 0

0.07

0.14

0.21

0.28

0.35

DAILY PRECIPITATION, IN INCHES

Daily

precipitation

Streamflow

Specific

Conductivity

SPECIFIC CONDUCTIVITY, IN MICROSIEMENS

PER CENTIMETER

STREAMFLOW, IN CUBIC FEET PER SECOND

7 9 11 13 15 17 19 21 23 25 27 1 3 5 7 9 11 13 15 17 19

FEBRUARY MARCH

2002

3,400

3,000

2,600

2,200

1,800

1,400

1,000

1,000

800

600

400

200

0
36 Evaluation of Measurements with Multi-Parameter Water-Quality Monitors in Selected Illinois Streams, 2001–03

Figure 22. Continuous and 7-day running mean dissolved-oxygen concentrations in three Illinois streams from June 1 through October 7, 2002, (A) at Salt Creek at Western Springs, (B) North Fork Vermilion River above Bismarck, and (C) Lusk Creek near Eddyville, 

Illinois. Current (2004) dissolved-oxygen standards and example proposed standards are shown for comparison.

DISSOLVED-OXYGEN STANDARDS

Current (2004) 16-hour minimum

Current (2004) instantaneous minimum

Proposed 7-day mean mimimum

Proposed instantaneous minimum

EXPLANATION

2

4

6

8

10

12

14

JUNE JULY AUG SEPT OCT

2002

Continuous dissolved oxygen

7-day running mean

dissolved oxygen

DISSOLVED-OXYGEN CONCENTRATION, IN MILLIGRAMS PER

LITER

5

6

7

8

9

10

11

JUNE JULY AUG SEPT OCT

2002

Continuous dissolved oxygen

7-day running mean

dissolved oxygen

DISSOLVED-OXYGEN CONCENTRATION, IN MILLIGRAMS PER

LITER

3

4

5

6

7

8

9

JUNE JULY AUG SEPT OCT

2002

Continuous dissolved oxygen

7-day running mean

dissolved oxygen

DISSOLVED-OXYGEN CONCENTRATION, IN MILLIGRAMS PER

LITER

A

B

C
Acknowledgments 37

dent measurements of pH and sonde pH, relative to the other 

four parameters, may be related to the buffer capacity and, 

indirectly, on the dissolved-solids concentration of the stream 

or river.

Specific conductivity at the eight stations varied widely—

from less than 40 microsiemens (μS) at Rayse Creek near 

Waltonville to greater than 3,500 μS at Salt Creek at Western 

Springs. In individual streams, specific conductivity often 

changed quickly (greater than 25 percent in less than 3 hours) 

and the sensors generally provided good to excellent record 

of these variations at all stations. The widest range of specific 

conductivity measurements was in Salt Creek at Western 

Springs in the Greater Chicago metropolitan area. The pH 

sensors apparently were the least responsive to changes of 

the five sensors tested and yielded the least-reliable data. It 

is not well understood why the pH sensors, in general, were 

less responsive to in-stream pH variations than the other four 

sensors. Depending on the data-quality objectives, monitoring 

of pH may require more intensive field maintenance than was 

undertaken during this study.

The three factors that limit determination of the accuracy and precision of the turbidity data above 1,400 FNUs 

are; the upper measurement limit of the sensor (1,400 FNUs), 

the lack of historical turbidity data covering a wide range of 

streamflow conditions, and the lack of independent turbidity 

measurements during times when turbidity is expected to be 

at the maximum. The upper limit on the turbidity sensor is 

a cause for concern because many peaks in turbidity at most 

stations exceeded 1,400 FNUs. In future deployments, an 

added effort to collect independent turbidity measurements 

during periods of maximum turbidity, such as the rising limb 

of stormflow peaks, would help determine the correspondence 

between sonde turbidity and the actual turbidity conditions in 

the stream. More effective calibration of the turbidity sensors 

during field visits using either high-turbidity (2,000 to 5,000 

FNUs) standards with sensors that accurately can sense high 

turbidity also would help ensure the reliability and precision of 

continuous turbidity monitoring. Continuous collection of turbidity data could be a valuable surrogate for suspended sediment given sufficient data and an empirical relation between 

turbidity and suspended sediment. Accurately determining 

the average range of turbidity would be important to waterresources managers in setting effective regulatory limits on 

turbidity in streams and rivers.

Data collected at all eight stations indicate that a number 

of factors affect the dissolved-oxygen concentration in the 

streams and rivers monitored. These factors include: temperature, biological activity, nutrient runoff, and weather (storm 

runoff). Dissolved-oxygen concentration at the station on the 

largest stream, Illinois River at Valley City, appeared to be 

dependent primarily on water temperature. This result may 

also reflect the limits on algal productivity at this site because 

of a turbidity-limited euphotic zone or other conditions. 

During brief periods usually in late summer, dissolvedoxygen concentrations in half or more of the eight streams and 

rivers monitored were below the 5-milligrams per liter (mg/L) 

minimum established by the Illinois Pollution Control Board 

to protect aquatic life. These changes result from a combination of factors, such as water temperature, algal production 

and die off, nutrient runoff, and rainstorms. The importance of 

each of these factors varies among the streams and rivers. The 

data collected on dissolved-oxygen concentration in the eight 

study streams and rivers also indicate that many streams in 

Illinois, even in relatively undisturbed watersheds, may have 

periods during which the 5-mg/L dissolved-oxygen concentration minimum criterion is not met. During July 2001 through 

September 2003, and generally during short late summer 

intervals, even the daily high dissolved-oxygen concentration 

did not rise above the minimum at Illinois River at Valley City, 

Lusk Creek near Eddyville, and Rayse Creek near Waltonville. 

ACKNOWLEDGMENTS

Gregg Good, Bob Mosher, and Roy Smogor, all of the 

Illinois Environmental Protection Agency, Bureau of Water, 

provided technical guidance in both the design of the waterquality monitoring network and formulation of the study 

objectives. Stephanie J. Fenimore of the USGS assisted with 

the various tasks related to water-quality data evaluation and 

analysis. Elizabeth A. Weldon of the USGS assisted with the 

archiving of the data. David H. Dupré, William S. Morrow, 

Jennifer L. Hogan, Elizabeth A. Murphy, and Paul J. Terrio, all 

of the USGS, assisted with the fieldwork, data-quality assurance, and related operational duties.
38 Evaluation of Measurements with Multi-Parameter Water-Quality Monitors in Selected Illinois Streams, 2001–03

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Anderson, Chauncey, W., August 2004, Turbidity (2d ed.): 

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proposed amendments to dissolved oxygen standard 35 

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Laws, E. A., 2000, Aquatic Pollution: (3rd edition), New York, 

New York, J. Wiley, 639 p.

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usgs.gov/osw/techniques/TSS/ZieglerT.pdf
40 Evaluation of Measurements with Multi-Parameter Water-Quality Monitors in Selected Illinois Streams, 2001–03
Appendix 41

APPENDIX
42 Evaluation of Measurements with Multi-Parameter Water-Quality Monitors in Selected Illinois Streams, 2001,–03

APPENDIX: Graphs of time series for water-quality parameters at eight 

streamflow-gaging stations in Illinois and discrete independent measurements 

for comparison, 2001–03.

The series of 48 graphs on the following pages show the data collected from all sensors of the sondes deployed at the eight 

selected Illinois streamflow-gaging stations during 2001–03. The multi-parameter continuous water-quality monitors recorded 

dissolved oxygen, pH, specific conductivity, water temperature, and turbidity. Also shown in the graphs are the precipitation and 

streamflow at each of the stations. For comparison, the discrete independent measurements are shown in the graphs with associated error bars. The errors bars represent ±0.3 mg/L for dissolved-oxygen concentration, ±0.2 standard unit for pH, ±3 percent 

of the value for specific conductivity, ±0.2°C for temperature, and ±5 percent of the value for turbidity. The stations where these 

data were collected are (in alphabetical order):

Illinois River at Valley City, Illinois

Lusk Creek near Eddyville, Illinois

Mazon River near Coal City, Illinois

Middle Fork Vermilion River above Oakwood, Illinois

North Fork Vermilion River near Bismarck, Illinois

Rayse Creek near Waltonville, Illinois

Salt Creek at Western Springs, Illinois

Vermilion River near Danville. Illinois
Dissolved-oxygen concentration, pH, and specific conductivity in the Illinois River at Valley City from July 1, 2001, to September 

30, 2003. Blue solid lines are data from the sensors on the continuous in-situ monitor and the black dots with red error bars are the 

discrete measurements made by U.S. Geological Survey and Illinois Environmental Protection Agency personnel. Gaps in the blue 

line indicate periods of missing data.

Appendix: Graphs of time series for water-quality parameters at eight Illinois stations 43

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

2

4

6

8

10

12

14

16

DISSOLVED-OXYGEN CONCENTRATION, IN MILLIGRAMS PER LITER

Discrete with error bars

Continuous

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

8.8

8.6

8.2

7.8

7.4

7.0

8.4

8.0

7.6

7.2

pH, IN STANDARD UNITS

Discrete with error bars

Continuous

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

1,200

1,000

800

600

400

200

SPECIFIC CONDUCTIVITY, IN

MICROSIEMENS PER CENTIMETER

Discrete with error bars

Continuous
44 Evaluation of Measurements with Multi-Parameter Water-Quality Monitors in Selected Illinois Streams, 2001,–03

Water temperature, turbidity, and streamflow in the Illinois River at Valley City and precipitation at Beardstown from July 1, 2001, to September 30, 2003. Blue solid lines are data from the sensors on the continuous in-situ instrument and the black dots with red error bars are 

the discrete measurements made by U.S. Geological Survey and Illinois Environmental Protection Agency personnel. Gaps in the blue line 

indicate periods of missing data.

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

-5

0

5

10

15

20

25

30

35

WATER TEMPERATURE, IN

DEGREES CELSIUS

Discrete with error bars

Continuous

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

0

200

400

600

800

1,000

1,200

1,400

TURBDITY, IN FORMAZINNEPHELOMETRIC UNITS

Discrete with error bars

Continuous

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

120,000

100,000

80,000

60,000

40,000

20,000

0

STREAMFLOW, IN CUBIC FEET

PER SECOND

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0 DAILY PRECIPITATION, IN INCHES

Streamflow

Precipitation
Appendix: Graphs of time series for water-quality parameters at eight Illinois stations 45

Dissolved-oxygen concentration, pH, and specific conductivity in Lusk Creek near Eddyville from July 1, 2001, to September 30, 

2003. Blue solid lines are data from the sensors on the continuous in-situ instrument and the black dots with red error bars are the 

discrete measurements made by U.S. Geological Survey and Illinois Environmental Protection Agency personnel. Gaps in the blue 

line indicate periods of missing data.

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

2

4

6

8

10

12

14

16

DISSOLVED-OXYGEN CONCENTRATION, IN MILLIGRAMS PER LITER

Discrete with

error bars

Continuous

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

9.0

8.5

8.0

7.5

7.0

6.5

6.0

pH, IN STANDARD UNITS

Discrete with error bars

Continuous

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

Discrete with error bars

Continuous

20

40

60

80

100

120

140

160

180

SPECIFIC CONDUCTIVITY, IN

MICROSIEMENS PER CENTIMETER
46 Evaluation of Measurements with Multi-Parameter Water-Quality Monitors in Selected Illinois Streams, 2001,–03

Water temperature, turbidity, and streamflow in Lusk Creek near Eddyville and precipitation at Harrisburg from July 1, 2001, to September 

30, 2003. Blue solid lines are data from the sensors on the continuous in-situ instrument and the black dots with red error bars are the 

discrete measurements made by U.S. Geological Survey and Illinois Environmental Protection Agency personnel. Gaps in the blue line 

indicate periods of missing data.

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

Discrete with error bars

Continuous

0

5

10

15

20

25

30

35

WATER TEMPERATURE, IN

DEGREES CELSIUS

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

Discrete

with error

bars

Continuous

TURBDITY, IN FORMAZINNEPHELOMETRIC UNITS

0

100

200

300

400

500

600

700

800

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

STREAMFLOW, IN CUBIC FEET

PER SECOND

DAILY PRECIPITATION, IN INCHES

8,000

7,000

6,000

5,000

4,000

3,000

2,000

1,000

0

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

Streamflow

Precipitation
Appendix: Graphs of time series for water-quality parameters at eight Illinois stations 47

Dissolved-oxygen concentration, pH, and specific conductivity in Mazon River near Coal City from July 1, 2001, to September 30, 

2003. Blue solid lines are data from the sensors on the continuous in-situ instrument and the black dots with red error bars are the 

discrete measurements made by U.S. Geological Survey and Illinois Environmental Protection Agency personnel. Gaps in the blue 

line indicate periods of missing data.

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

0

5

10

15

20

25

30

Discrete with

error bars

Continuous

DISSOLVED-OXYGEN CONCENTRATION, IN MILLIGRAMS PER LITER

Discrete with error bars

pH, IN ST

Continuous

ANDARD UNITS

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

6.8

7.2

7.6

8.4

8.8

9.2

9.6

8.0

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

Discrete with error bars

Continuous

SPECIFIC CONDUCTIVITY, IN

MICROSIEMENS PER CENTIMETER4,000

3,000

2,000

1,000

0
48 Evaluation of Measurements with Multi-Parameter Water-Quality Monitors in Selected Illinois Streams, 2001,–03

Water temperature, turbidity, and streamflow in Mazon River near Coal City and precipitation at Morris from July 1, 2001, to September 30, 2003. Blue solid lines are data from the sensors on the continuous in-situ instrument and the black dots with red error bars 

are the discrete measurements made by U.S. Geological Survey and Illinois Environmental Protection Agency personnel. Gaps in the 

blue line indicate periods of missing data.

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

Discrete with

error bars

Continuous

WATER TEMPERATURE, IN

DEGREES CELSIUS

0

10

20

30

40

-5

Discrete with error bars

Continuous

TURBDITY, IN FORMAZINNEPHELOMETRIC UNITS

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

0

200

400

600

800

1,200

1,000

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

STREAMFLOW, IN CUBIC FEET

PER SECOND

DAILY PRECIPITATION, IN INCHES

15,000

12,500

10,000

7,500

5,000

2,500

0

3.0

2.5

2.0

1.5

1.0

0.5

0.0

Streamflow

Precipitation
Appendix: Graphs of time series for water-quality parameters at eight Illinois stations 49

Dissolved-oxygen concentration, pH, and specific conductivity in Middle Fork Vermilion River above Oakwood from July 1, 2001, to 

September 30, 2003. Blue solid lines are data from the sensors on the continuous in-situ instrument and the black dots with red error 

bars are the discrete measurements made by U.S. Geological Survey and Illinois Environmental Protection Agency personnel. Gaps 

in the blue line indicate periods of missing data.

4

6

8

10

12

14

16

18

20

Discrete with error bars

Continuous

DISSOLVED-OXYGEN CONCENTRATION, IN MILLIGRAMS PER LITER

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

Discrete with error bars

pH, IN ST

Continuous

ANDARD UNITS

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

7.2

7.5

7.8

8.1

8.4

8.7

9.0

Discrete with error bars

Continuous

pH, IN STANDARD UNITS

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

9.0

8.5

8.0

7.5

7.0

6.5
50 Evaluation of Measurements with Multi-Parameter Water-Quality Monitors in Selected Illinois Streams, 2001,–03

Water temperature, turbidity, and streamflow in Middle Fork Vermilion River above Oakwood and precipitation at Danville from July 

1, 2001, to September 30, 2003. Blue solid lines are data from the sensors on the continuous in-situ instrument and the black dots 

with red error bars are the discrete measurements made by U.S. Geological Survey and Illinois Environmental Protection Agency 

personnel. Gaps in the blue line indicate periods of missing data.

-5

0

5

10

15

20

25

30

35

Discrete with error bars

Continuous

WATER TEMPERATURE, IN

DEGREES CELSIUS

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

Discrete with error bars

Continuous

TURBDITY, IN FORMAZINNEPHELOMETRIC UNITS

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

0

200

400

600

800

1,000

STREAMFLOW, IN CUBIC FEET

PER SECOND

DAILY PRECIPITATION, IN INCHES

Streamflow

Precipitation

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

8,000

7,000

6,000

5,000

4,000

3,000

2,000

1,000

0

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0
Appendix: Graphs of time series for water-quality parameters at eight Illinois stations 51

Dissolved-oxygen concentration, pH, and specific conductivity in North Fork Vermilion River near Bismarck from July 1, 2001, to 

September 30, 2003. Blue solid lines are data from the sensors on the continuous in-situ instrument and the black dots with red error 

bars are the discrete measurements made by U.S. Geological Survey and Illinois Environmental Protection Agency personnel. Gaps 

in the blue line indicate periods of missing data.

0

5

10

15

20

Discrete with

error bars

Continuous

DISSOLVED-OXYGEN CONCENTRATION, IN MILLIGRAMS PER LITER

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

Discrete with error bars

Continuous

pH, IN STANDARD UNITS

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

9.0

8.5

8.0

7.5

7.0

6.5

0

100

200

300

400

500

600

700

800

900

Discrete with error bars

Continuous

SPECIFIC CONDUCTIVITY, IN

MICROSIEMENS PER CENTIMETER

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003
52 Evaluation of Measurements with Multi-Parameter Water-Quality Monitors in Selected Illinois Streams, 2001,–03

Water temperature, turbidity, and streamflow in North Fork Vermilion River near Bismarck and precipitation at Danville from July 1, 

2001, to September 30, 2003. Blue solid lines are data from the sensors on the continuous in-situ instrument and the black dots with 

red error bars are the discrete measurements made by U.S. Geological Survey and Illinois Environmental Protection Agency personnel. Gaps in the blue line indicate periods of missing data.

-5

0

5

10

15

20

25

30

35

Discrete with error bars

Continuous

WATER TEMPERATURE, IN

DEGREES CELSIUS

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

Discrete with

error bars

Continuous

TURBDITY, IN FORMAZINNEPHELOMETRIC UNITS

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

0

200

400

600

800

900

700

500

300

100

STREAMFLOW, IN CUBIC FEET

PER SECOND

DAILY PRECIPITATION, IN INCHES

Streamflow

Precipitation

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

14,000

12,000

10,000

8,000

6,000

4,000

2,000

0

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0
Appendix: Graphs of time series for water-quality parameters at eight Illinois stations 53

Dissolved-oxygen concentration, pH, and specific conductivity in Rayse Creek near Waltonville from July 1, 2001, to September 30, 

2003. Blue solid lines are data from the sensors on the continuous in-situ instrument and the black dots with red error bars are the 

discrete measurements made by U.S. Geological Survey and Illinois Environmental Protection Agency personnel. Gaps in the blue 

line indicate periods of missing data.

0

2

4

6

8

10

12

14

16

Discrete with error bars

Continuous

DISSOLVED-OXYGEN CONCENTRATION, IN MILLIGRAMS PER LITER

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

Discrete with error bars

Continuous

pH, IN STANDARD UNITS

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

5.5

6.5

7.5

7.0

8.0

6.0

Discrete with error bars

Continuous

SPECIFIC CONDUCTIVITY, IN

MICROSIEMENS PER CENTIMETER

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

0

500

1,500

1,000
54 Evaluation of Measurements with Multi-Parameter Water-Quality Monitors in Selected Illinois Streams, 2001,–03

Water temperature, turbidity, and streamflow in Rayse Creek near Waltonville and precipitation at Waltonville from July 1, 2001, to 

September 30, 2003. Blue solid lines are data from the sensors on the continuous in-situ instrument and the black dots with red error 

bars are the discrete measurements made by U.S. Geological Survey and Illinois Environmental Protection Agency personnel. Gaps 

in the blue line indicate periods of missing data.

0

5

10

15

20

25

30

Discrete with

error bars

Continuous

WATER TEMPERATURE, IN

DEGREES CELSIUS

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

Discrete with error bars

Continuous

TURBDITY, IN FORMAZINNEPHELOMETRIC UNITS

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

0

200

400

600

800

1,000

STREAMFLOW, IN CUBIC FEET

PER SECOND

DAILY PRECIPITATION, IN INCHES

Streamflow

Precipitation

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

7,000

6,000

5,000

4,000

3,000

2,000

1,000

0

2.5

2.0

1.5

1.0

0.5

0
Appendix: Graphs of time series for water-quality parameters at eight Illinois stations 55

Dissolved-oxygen concentration, pH, and specific conductivity in Salt Creek at Western Springs from July 1, 2001, to September 30, 

2003. Blue solid lines are data from the sensors on the continuous in-situ instrument and the black dots with red error bars are the 

discrete measurements made by U.S. Geological Survey and Illinois Environmental Protection Agency personnel. Gaps in the blue 

line indicate periods of missing data.

0

5

10

15

20

Discrete with

error bars

Continuous

DISSOLVED-OXYGEN CONCENTRATION, IN MILLIGRAMS PER LITER

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

Discrete with

error bars Continuous

pH, IN STANDARD UNITS

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

6.6

7.4

7.8

8.2

8.6

7.0

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

Discrete with error bars

Continuous

SPECIFIC CONDUCTIVITY, IN

MICROSIEMENS PER CENTIMETER4,000

3,000

2,000

1,000

0
56 Evaluation of Measurements with Multi-Parameter Water-Quality Monitors in Selected Illinois Streams, 2001,–03

Water temperature, turbidity, and streamflow in Salt Creek at Western Springs and precipitation at Midway Airport from July 1, 

2001, to September 30, 2003. Blue solid lines are data from the sensors on the continuous in-situ instrument and the black dots with 

red error bars are the discrete measurements made by U.S. Geological Survey and Illinois Environmental Protection Agency personnel. Gaps in the blue line indicate periods of missing data.

0

5

10

15

20

25

30

35

Discrete with error bars

Continuous

WATER TEMPERATURE, IN

DEGREES CELSIUS

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

0

100

200

300

400

500

600

700

800

Discrete with

error bars

Continuous

TURBDITY, IN FORMAZINNEPHELOMETRIC UNITS

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

STREAMFLOW, IN CUBIC FEET

PER SECOND

DAILY PRECIPITATION, IN INCHES

Streamflow

Precipitation

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

2,500

2,000

1,500

1,000

500

0

4.0

3.5

3.0

2.5

1.0

1.5

1.0

0.5

0.0
Appendix: Graphs of time series for water-quality parameters at eight Illinois stations 57

Dissolved-oxygen concentration, pH, and specific conductivity in Vermilion River near Danville from July 1, 2001, to September 30, 

2003. Blue solid lines are data from the sensors on the continuous in-situ instrument and the black dots with red error bars are the 

discrete measurements made by U.S. Geological Survey and Illinois Environmental Protection Agency personnel. Gaps in the blue 

line indicate periods of missing data.

0

5

10

15

20

25

Discrete with error bars

Continuous

DISSOLVED-OXYGEN CONCENTRATION, IN MILLIGRAMS PER LITER

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

Discrete with

error bars Continuous

pH, IN STANDARD UNITS

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

9.5

9.0

8.5

8.0

7.5

7.0

6.5

Discrete with

error bars

Continuous

SPECIFIC CONDUCTIVITY, IN

MICROSIEMENS PER CENTIMETER

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

200

300

400

500

600

700

800

900

1,000
58 Evaluation of Measurements with Multi-Parameter Water-Quality Monitors in Selected Illinois Streams, 2001,–03

Water temperature, turbidity, and streamflow in Vermilion River near Danville and precipitation at Danville from July 1, 2001, to 

September 30, 2003. Blue solid lines are data from the sensors on the continuous in-situ instrument and the black dots with red error 

bars are the discrete measurements made by U.S. Geological Survey and Illinois Environmental Protection Agency personnel. Gaps 

in the blue line indicate periods of missing data.

-5

0

5

10

15

20

25

30

35

Discrete with

error bars

Continuous

WATER TEMPERATURE, IN

DEGREES CELSIUS

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

Discrete with

error bars

Continuous

TURBDITY, IN FORMAZINNEPHELOMETRIC UNITS

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

0

200

400

600

800

1,000

STREAMFLOW, IN CUBIC FEET

PER SECOND

DAILY PRECIPITATION, IN INCHES

Streamflow Precipitation

J A S O N D J F M A M J J A S O N D J F M A M J J A S

2001 2002 2003

5,000

10,000

15,000

20,000

0

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0
George E. Groschen and Robin B. King—Evaluation of Measurements Collected with Multi-Parameter Continuous Water-Quality Monitors

 in Selected Illinois Streams—Scientific Investigations Report 2005–5060

Director

U.S. Geological Survey

Illinois Water Science Center

1201 W. University Avenue

Urbana, IL 61801

http:://il.water.usgs.gov