In cooperation with the Illinois Department of Natural Resources, Offices of Water Resources, 

Realty and Environmental Planning–Conservation 2000 Program, and Resource Conservation; 

and with the Illinois Department of Transportation

Estimating Flood-Peak Discharge Magnitudes and 

Frequencies for Rural Streams in Illinois

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2004-5103

Estimating Flood-Peak Discharge 

Magnitudes and Frequencies for Rural 

Streams in Illinois

By David T. Soong, Audrey L. Ishii, Jennifer B. Sharpe, and Charles F. Avery

In cooperation with the Illinois Department of Natural Resources, Offices of Water Resources, 

Realty and Environmental Planning–Conservation 2000 Program, and Resource Conservation; 

and with the Illinois Department of Transportation

Scientific Investigations Report 2004-5103

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: 2004

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

Box 25286, Denver Federal Center

Denver, CO 80225

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

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

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

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

reproduce any copyrighted materials contained within this report.

Photograph in the upper left shows a bridge over the Fox River near Lotus Woods in Lake County, Illinois (photograph 

by Gary P. Johnson, U.S. Geological Survey, Illinois Water Science Center). Photograph in the lower right shows a 

bank-erosion site on Canteen Creek in St. Clair County, Illinois (photograph by Timothy D. Straub and Donald P. Roseboom, U.S. Geological Survey, Illinois Water Science Center).
iii

Contents

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

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

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

Report Organization..............................................................................................................................3

Previous Flood-Frequency Analyses for Illinois ..............................................................................3

Flood-Peak Discharge Magnitudes and Frequencies at Gaged Sites..................................................5

Data Availability ....................................................................................................................................5

Determination of Recurrence Intervals ............................................................................................6

Estimates of Flood-Frequency Relations Based on Annual Maximum Series .........................10

Estimates of Flood-Frequency Relations Based on Partial Duration Series.............................10

Effect of Updated Flood-Frequency Analysis on At-Site Flood Quantiles Based 

on Annual Maximum Series...........................................................................................................10

Example At-Site Frequency Curves for Annual Maximum Series and 

Partial Duration Series....................................................................................................................12

Regional Frequency Analysis.....................................................................................................................12

Hydrologic Regions for Illinois..........................................................................................................12

Basin Characteristics.........................................................................................................................12

Multiple Regression Analysis ...........................................................................................................12

Measurement of Explanatory Variables..........................................................................................15

Techniques for Estimating Flood-Peak Discharge Magnitudes and Frequencies............................16

Annual Maximum Series Regional Equations for Rural, Unregulated Streams .......................22

Application of Annual Maximum Series Regional Equations.............................................23

Ungaged Sites ...................................................................................................................24

Gaged Sites........................................................................................................................24

Near Gaged Sites .............................................................................................................26

Accuracy and Limitation of the Annual Maximum Series Regional Equations...............27

Regulated and Urban Streams ...............................................................................................27

Partial Duration Series Regional Equations for Rural, Unregulated Streams ..........................28

Summary and Conclusions.........................................................................................................................35

Acknowledgments.......................................................................................................................................39

Selected References...................................................................................................................................39

Glossary.........................................................................................................................................................44

Appendix 1. Data Preparation...................................................................................................................47

Appendix 2. At-Site Analysis of Flood-Peak Series ..............................................................................54

Appendix 3. Generalized Skew Coefficients for Illinois........................................................................61

Appendix 4. Effects of Additional Flood Records and Updated Generalized Skew 

Coefficients on At-Site Flood Quantiles Based on Annual Maximum Series.......................66

Appendix 5. Determination of Basin Characteristics............................................................................72

Appendix 6. Hydrologic Regions for Illinois............................................................................................78

Appendix 7. Regression Analysis.............................................................................................................82

Appendix 8. Digital Data and Plots (CD-ROM in pocket)......................................................................91
iv

Figures

 1. Diagram showing development of flood-frequency relations and regional 

 flood-frequency techniques for rural streams in Illinois........................................................4

 2A-5. Maps showing—

 2A. Location (as of 2002) of U.S. Geological Survey streamflow-gaging stations

 in Illinois and adjacent States for which annual maximum series (AMS)

 were retrieved and flood quantiles were estimated, and stations used in

 the regression analysis ........................................................................................................7

 2B. Location (as of 2002) of U.S. Geological Survey streamflow-gaging stations in 

 northern Illinois and adjacent States for which annual maximum series (AMS)

 were retrieved and flood quantiles were estimated, and stations used in the 

 regression analysis ...............................................................................................................8

 3. Location (as of 2002) of U.S. Geological Survey streamflow-gaging stations in 

 Illinois and adjacent States for which partial duration series (PDS) were 

 retrieved, and stations used in the regression analysis .................................................9

 4. Updated generalized skew-coefficient map for Illinois. ...............................................11

 5. Hydrologic regions for flood-frequency analysis of rural streams in Illinois. ...........13

 6-7. Graphs showing—

 6. Flood-frequency curve based on annual maximum series (AMS) analysis for 

 Bluegrass Creek at Potomac, Vermilion County Ill., (03336500)...................................14

 7. Flood-frequency curve based on partial duration series (PDS) analysis for 

 Bluegrass Creek at Potomac, Vermilion County Ill., (03336500)...................................14

 8A-8D. Maps showing—

 8A. Average soil permeability (from 0.2 to less than 1.0 inch per hour) for 

 Illinois. Average soil permeability is obtained by taking the arithmetic 

 average of the high and low soil-permeability values from the STATSGO 

 database ...............................................................................................................................17

 8B. Average soil permeability (from 1.0 to less than 2.0 inches per hour) for 

 Illinois. Average soil permeability is obtained by taking the arithmetic 

 average of the high and low soil-permeability values from the STATSGO 

 database ...............................................................................................................................18

 8C. Average soil permeability (from 2.0 to less than 3.0 inches per hour) for 

 Illinois. Average soil permeability is obtained by taking the arithmetic 

 average of the high and low soil-permeability values from the STATSGO 

 database ...............................................................................................................................19

 8D. Average soil permeability (from 3.0 to greater than 8.0 inches per hour) for 

 Illinois. Average soil permeability is obtained by taking the arithmetic 

 average of the high and low soil-permeability values from the STATSGO 

 database ...............................................................................................................................20

 9. Diagram showing procedures developed during the present study to estimate

 flood quantiles at a stream location. .......................................................................................21

 10. Map showing Blackberry Creek watershed, Kane and Kendall Counties, Illinois, 

 and site locations of three subbasins used in examples of application of the 

 annual maximum series (AMS) regional equations..............................................................25
v

 11-14. Graphs showing—

 11. Estimated magnitudes of flood-peak discharges for selected recurrence 

 intervals for the regulated reach of the Big Muddy River in southern Illinois. .........29

 12. Estimated magnitudes of flood-peak discharges for selected recurrence 

 intervals for the regulated reach of the Fox River in northern Illinois........................30

 13. Estimated magnitudes of flood-peak discharges for selected recurrence 

 intervals for the regulated reach of the Kaskaskia River in southern Illinois. ..........31

 14. Estimated magnitudes of flood-peak discharges for selected recurrence 

 intervals for the Illinois River. ............................................................................................32

Tables

 1. Flood-peak discharges for recurrence intervals, T, of 2, 5, 10, 25, 50, 100, and 

 500 years estimated from the annual maximum series at streamflow-gaging

 stations in Illinois and adjacent States ...................................................................................93

 2. Flood-peak discharges for recurrence intervals, T, of 0.8, 1.01, 1.5, 2, 3, and 

 5 years estimated from the partial duration series at streamflow-gaging

 stations in Illinois and adjacent States .................................................................................125

 3. Coefficients and exponents for equation 3 based on annual maximum series 

 for hydrologic regions 1, 3, and 5, Illinois, for specified recurrence intervals. ................23

 4. Coefficients and exponents for equation 4 based on annual maximum series 

 for hydrologic regions 2, 6, and 7, Illinois, for specified recurrence intervals. ................23

 5. Coefficients and exponents for equation 5 based on annual maximum series 

 for hydrologic region 4, Illinois, for specified recurrence intervals...................................24

 6. Selected basin characteristics and equivalent years of record for the 288

 streamflow-gaging stations in Illinois and adjacent States ..............................................133

 7. Selected flood-peak information for the 288 streamflow-gaging stations in

 Illinois and adjacent States.....................................................................................................141

 8. Parameter space for the annual maximum series regional equations in Illinois.............28

 9. Coefficients and exponents for equation 6 based on partial duration series 

 for hydrologic region 1, Illinois, for specified recurrence intervals...................................33

 10. Coefficients and exponents for equation 7 based on partial duration series 

 for hydrologic region 2, Illinois, for specified recurrence intervals...................................33

 11. Coefficients and exponents for equation 8 based on partial duration series 

 for hydrologic region 3, Illinois, for specified recurrence intervals...................................34

 12. Coefficients and exponents for equation 9 based on partial duration series 

 for hydrologic region 4, Illinois, for specified recurrence intervals...................................34

 13. Coefficients and exponents for equation 10 based on partial duration series 

 for hydrologic regions 5, 6, and 7 (stations in region 7 are combined with 

 region 6 in the analysis), Illinois, for specified recurrence intervals.................................34

 14. Parameter space for the partial duration series regional equations in Illinois................35
vi

Conversion Factors, Symbols, Datums, Abbreviations, and 

Acronyms

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) 2.590 square kilometer (km2) 

Volume

Flow rate

foot per mile (ft/mi) 0.1894 meter per kilometer (m/km)

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

inch per year (in/yr) 25.4 millimeter per year (mm/yr)

< less than

> greater than

= equal to

≤ less than or equal to

≥ greater than or equal to

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

(NAVD 88). However, gage datum presently (2004) still refers to National Geodetic Datum of 

1929 (NGVD 29).

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

Historical data collected and stored as North American Datum of 1927 (NAD 27).

Other abbreviations used in the report:

AEYR — average equivalent years of record

AMS — annual maximum series

DEM — digital elevation model

EGLS — estimated generalized least squares method

GIS — Geographic Information System 

MLR — multiple linear regression

APE — average prediction error of the regional equation, in percent

SEE — standard errors of estimators, in percent

NLCD — National Land Cover Data. The URL is http://landcover.usgs.gov/nationallandcover.html

NWIS — National Water Information System

OLS — Ordinary least squares

PDS — partial duration series
vii

STATSGO — State Soil Geographic database

USGS — U.S. Geological Survey
viii

ERRATA

Updated August 10, 2010: 

Equation 13, on page 26 is incorrect. The correct equation 13 is:

The purpose of this equation is to improve the Illinois rural regional-regression 

estimates of peak-flow quantiles at ungaged sites that are upstream or downstream 

from appropriate nearby streamgages by adjusting the regional estimate to reflect the 

peak-flow quantile estimates computed at the nearby streamgages. The final factor, the 

ratio Asite/Agage, is missing in the report. This ratio is an approximate scaling factor applied to adjust the peak-flow quantile estimate at the streamgage to the quantile 

estimate at the ungaged site. Equation 13 is not used if the regional estimate for the 

ungaged site is not adjusted with the peak-flow quantiles at a nearby gage. 

Equation 13 is to be used only where the drainage area of the ungaged site is 

between 50 and 150 percent of the nearby gaged drainage area. If the incorrect equation, as shown on page 26, is used to adjust the peak-flow quantiles estimates at an 

ungaged site, the size of the resulting difference in the estimates for QT|adjusted from 

the intended estimates (that include the scaling factor approximation), will depend 

on the proportion of the ungaged to gaged drainage areas. The total difference in the 

resulting QT|adjusted is expected to be smaller than the average prediction error of the 

regional regression equations in all cases. 

The National Streamflow Statistics Program, available at this URL: 

http://water.usgs.gov/osw/programs/nss/summary.html

can be used to compute the Illinois regional-regression equation estimates of peakflow quantiles at ungaged sites and to adjust the ungaged peak-flow quantile estimates 

for quantiles computed at a nearby gaged site. This program uses the correct form of 

equation 13, as given above.

Updated June 28, 2012:

Note: A correction was made to the figure reference at the top of page 26.

Note: The regional estimate computation in step 2 at the top of page 26 was corrected 

from Q100|

weighted to Q100|regional.

Note: The numeric values in the example at the top of page 26 were corrected to be 

consistent with table 7.

Note: The absolute value sign in equation 12 on page 26 was corrected to include the 

“-1” term.

Note: A note was added to equation 13 on page 26 to refer the reader to the Errata at 

the top of this page. The version of equation 13 on page 26 is not corrected.

Note: The example on page 27 was corrected to use the corrected version of equation 

13, and to be consistent with the numeric values listed on pages 26 and 27.
ix

Note: The citation for Harvey and Eash (1996) on page 40 was corrected to reference 

“U.S. Geological Survey Water-Resources Investigations Report 95-4287.” 

Note: Annotations on figure 2-3 and figure 2-4 in appendix 2 on page 52 were corrected from “GROUP” to “GP” to be consistent with the figure captions.

Note: The example on page 89 for computing model error at an ungaged site was corrected to reference equation (7-16) rather than (7-14). The notation and numeric values 

were made consistent.

Note: A station number for Salt Fork near Homer, Ill., “03338000” was added to table 

1 in appendix 8, on page 93. 

Note: The Hydrologic Region number for 05430500, Rock River at Afton, Wis. was 

corrected to region “1” in table 1 in appendix 8, on page 97.

Note: Duplicate stations were removed from table 1 in appendix 8 on pages 120, 122, 

and 123. Stations 05592000 Kaskaskia River at Shelbyville, Ill.; 05592500 Kaskaskia 

River at Vandalia, Ill.; 05593000 Kaskaskia River at Carlyle, Ill.; 05597000 Big 

Muddy River at Plumfield, Ill.; and 05599500 Big Muddy River at Murphysboro, Ill. 

were removed. 

Note: The Total Drainage Area (TDA) was corrected to 1,953 mi2 for station 5592500 

in table 6 in appendix 8, on page 139.
x
Abstract

Flood-peak discharge magnitudes and frequencies at streamflow-gaging sites were developed with the annual 

maximum series (AMS) and the partial duration series (PDS) in this study. Regional equations for both flood series 

were developed for estimating flood-peak discharge magnitudes at specified recurrence intervals of rural Illinois 

streams. The regional equations are techniques for estimating flood quantiles at ungaged sites or for improving 

estimated flood quantiles at gaged sites with short records or unrepresentative data. Besides updating at-site floodfrequency estimates using flood data up to water year 1999, this study updated the generalized skew coefficients 

for Illinois to be used with the Log-Pearson III probability distribution for analyzing the AMS, developed a program for analyzing the partial duration series with the Generalized Pareto probability distribution, and applied the 

BASINSOFT program with digital datasets in soil, topography, land cover, and precipitation to develop a set of basin 

characteristics. The multiple regression analysis was used to develop the regional equations with subsets of the basin 

characteristics and the updated at-site flood frequencies. Seven hydrologic regions were delineated using physiographic and hydrologic characteristics of drainage basins of Illinois. The seven hydrologic regions were used for 

both the AMS and PDS analyses. 

Examples are presented to illustrate the use of the AMS regional equations to estimate flood quantiles at an 

ungaged site and to improve flood-quantile estimates at and near a gaged site. Flood-quantile estimates in four 

regulated channel reaches of Illinois also are approximated by linear interpolation. Documentation of the flood data 

preparation and evaluation, procedures for determining the flood quantiles, basin characteristics, generalized skew 

coefficients, hydrologic region delineations, and the multiple regression analyses used to determine the regional 

equations are presented in the main text and appendixes. 

INTRODUCTION

Knowledge of the frequency and magnitude of flood-peak discharges is essential for water-resources planning, 

risk management, and project design. The magnitude of flood-peak discharge and associated exceedance probability 

can be estimated from various approaches (National Research Council, 1988) and flood-frequency analysis is one of 

them that is based on statistical inference. In flood-frequency analysis, characteristics of the observed instantaneous 

flood-peak discharge magnitudes from a stream location are analyzed and a probability distribution is selected to 

fit the observed peak data. The analysis (also termed as at-site analysis because only data at the study site are used) 

produces a best-fit line (a flood-frequency curve) between the observed flood-peak magnitudes and their estimated 

exceedance probabilities. From the flood-frequency curve, the magnitude of flood-peak discharge at a specific 

exceedance probably (a flood quantile) can be estimated for the site. Overall, the advantages of flood-frequency 

analysis over other approaches are that the flood quantiles are estimated based on observed flood-peak magnitudes 

and the exceedance probabilities are estimated from actual floods, not from rainfalls as in the design-storm method 

in the rainfall-runoff modeling approach (where the exceedance probability for a flood-peak discharge is assumed 

equal to that of the design rainfall). However, data availability and representativeness of the available data also are 

the limiting factors for the accuracy of quantiles estimated by the flood-frequency analysis. In the flood-frequency 

curve, the exceedance probability (P) indicates the chances that the magnitude of the corresponding flood peak have 

been equaled or exceeded by an actual flood peak in a specified time interval. For an easier engineering interpretation, the recurrence interval (T) that has a time unit is used instead of P. Therefore, T is to be used in a probabilistic 

sense. For a given T, the corresponding flood-peak magnitude is termed as flood quantile (QT) in this study. In 

Estimating Flood-Peak Discharge Magnitudes and 

Frequencies for Rural Streams in Illinois

By David T. Soong, Audrey L. Ishii, Jennifer B. Sharpe, and Charles F. Avery
2 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

1

 A water year is the period from October 1 to September 30 and is designated by the calendar year in which it ends.

addition to the “at-site” analysis, the flood-frequency estimates at a number of sites within a hydrologically homogeneous region may be combined and analyzed with explanatory variables taken from physiographic and basin characteristics to improve the at-site estimates by essentially “substituting space for time,” effectively extending the length 

of streamflow records (National Research Council, 1988), and providing a technique for estimating flood-frequency 

relations at ungaged sites.

Although systematic flow records for Illinois streams have been established since the late-1800’s or early-1900’s 

at some streamflow-gagging stations, however, the available flood data at most streamflow-gaging stations generally are insufficient for reliably estimating QT for extreme (such as the 100-year flood) events. At-site and regional 

flood-frequency curves for Illinois were last determined using streamflow data collected through water year1 (WY) 

1985 (Curtis, 1987). Additional flood data, advancements in analytical techniques and geographic information system (GIS) as well as digital databases have became available in the past decade. To provide updated flood-frequency 

estimates for the State of Illinois, the U.S. Geological Survey (USGS)—in cooperation with the Illinois Department 

of Natural Resources, Offices of Water Resources, Realty and Environmental Planning–Conservation 2000 Program, 

and Resource Conservation; and with the Illinois Department of Transportation—began a study in 2000. Flood 

records collected through WY 1999 were used in this study. Components of the study are briefly introduced as follows.

The annual maximum series (AMS) and partial duration series (PDS) are used in this study for estimating atsite flood frequencies. Up to the present (2004), the AMS has been used in most of the statewide flood-frequency 

analyses conducted by the USGS throughout the country; the analysis on PDS presented is thought to be the first 

statewide application of PDS in Illinois. The AMS and PDS represent different ways the instantaneous flood-peak 

discharge magnitudes are organized from the station records; therefore, these series have different definitions on 

common terms such as the T and might be fitted with different probability distributions. The AMS and PDS are analyzed and presented separately in this report. Users need to differentiate their definitions and applications. 

The AMS results have been used for flood prevention and protection. The AMS consists of the list of instantaneous maximum discharge values for each water year. In this report, no streamflow record of shorter duration 

than 10 years is utilized in determining the flood-frequency relations. The method used to determine flood-peak 

discharge magnitudes associated with the annual exceedance probabilities from 0.002 to 0.5 (corresponding to 500-

year flood to the 2-year recurrence intervals, respectively) at a gaged site is described in the guidelines published as 

Bulletin 17B of the Interagency Advisory Committee on Water Data, Hydrology Subcommittee (1982). Bulletin 17B 

recommends the AMS and the Log-Pearson Type III (LP3) probability distribution be used in estimating the floodfrequency relations. 

Because only one flood peak is chosen per year in AMS, it is not possible to evaluate the magnitudes of secondary flood-peak discharges if the effects of these discharges also are of concern, or to determine flood-peak discharge 

magnitudes that could occur more than once in a year (T ≤ 1 year). In recent years, interests on understanding floodpeak discharge of higher frequencies (for example, T equal to 1-3 years) increase because of its potential application 

to studies of habitat restoration and protection. These more frequent floods can be important in studying channel 

formation, stability, and migration; and floodplain vegetation and habitat. The PDS, organized from a streamflow 

record with all instantaneous flood-peak discharges above a threshold magnitude at the site, is suitable for analyzing 

flood-frequency relations where secondary flood-peak discharges are important (for example, Chow, 1964a). In this 

study, the Generalized Pareto (GP) distribution (for example, Rao and Hamed, 2000) was used to fit the PDS data 

and for determining the PDS at-site flood-frequency relations.

The regional flood-frequency technique used in this study was based on multiple-regression analysis, with subsets of newly determined explanatory variables from basin characteristics and updated at-site flood frequencies for 

selected recurrence intervals. Seven hydrologic regions have been delineated in the State of Illinois, and the optimal 

group of variables was determined using the technique of ordinary least squares (OLS). The technique of generalized least squares (GLS) (Stedinger and Tasker, 1986, 1985; Tasker and Stedinger, 1989) that accounts for unequal 

record length and concurrent flows in streamflow records was used in developing the final AMS regional equations. 

However, the OLS technique was used in the regional analysis for PDS.

Although the flood-frequency relations and techniques determined in this study are the latest available (2004), 

the limitations and assumptions made in deriving the techniques should be understood when using the results.
Purpose and Scope

This report presents the flood-frequency relations at rural gaged sites for selected recurrence intervals, and 

regional flood-frequency techniques for seven hydrologic regions in Illinois based on AMS and PDS. The scope of 

the study included:

1. Compiling the AMS and PDS at streamflow-gaging stations from the USGS peak-flow files available through 

WY 1999;

2. Deriving the at-site AMS flood quantiles based on Bulletin 17B procedures (Interagency Advisory Committee 

on Water Data, 1982);

3. Deriving the at-site PDS flood quantiles based on accepted methodologies;

4. Derive physiographic and drainage-basin characteristics using geographic information system (GIS) technology with the updated statewide databases;

5. Updating or developing new hydrologic regions;

6. Updating regional flood-frequency techniques for estimating AMS flood-peak discharge magnitudes using 

updated at-site flood quantiles and newly developed physiographic and hydrometeorological basin characteristics, and with the multiple regression analysis; and

7. Developing regional flood-frequency techniques for estimating PDS flood-peak discharge magnitudes using 

newly determined at-site flood quantiles and physiographic and hydrometeorological basin characteristics, 

and with the multiple regression analysis.

Databases for deriving these basin characteristics included the updated rainfall frequency for Illinois (Huff and 

Angel, 1992), digital topographic elevation model (DEM), State Soil Geographic (STATSGO) database, (Natural 

Resources Conservation Service, 1993), and the USGS National Land Cover Data (NLCD). The BASINSOFT program (Harvey and Eash, 1996) and ArcInfo procedures were used for determining basin geometric parameters from 

the DEM, and basin-weighted soil, land-use, and rainfall variables from the other GIS data layers.

Report Organization

This report contains a main text and eight appendixes. The main text presents the background of the study, definitions of the technical terms, and results of at-site as well as regional flood-frequency analyses. The appendixes are 

used to document work conducted at various key stages of AMS and PDS analyses of the study. This organization is 

intended to make the report an efficient reference for the reader, whereas keeping the data, techniques, and documentation available to future flood-frequency or other studies. The tasks and analytical steps of the study are shown in 

the flowchart (fig. 1). Digital data and images (with zooming capability) are included in the attached CD-ROM that 

is documented in appendix 8.

Previous Flood-Frequency Analyses for Illinois

The first statewide flood-frequency estimating techniques for Illinois were developed based on 108 stations with 

drainage areas greater than 10 mi2 (Mitchell, 1954). Mitchell graphically fitted the distribution curves to station data 

and applied a technique (index flood procedure, Dalrymple, 1960) to conduct the regional analysis. The hydrologic 

regions were modified slightly from the physiographic divisions outlined by Leighton and others (1948), and the 

index mean annual flood (Q2.33) for each region was estimated using (1) drainage area, (2) volume per area (related 

to the climatological factors), and (3) a lag index (measure of time lag because of storage that is related to physiographic factors). Mitchell then derived regional flood-frequency curves for three hydrologic divisions of Illinois; the 

regional frequency curves were used to estimate flood quantiles at ungaged sites for T between 1.1 and 50 years.

Using the procedures in Bulletin 15 (that is, fitted with the LP3 distribution) (U.S. Water Resource Council, 

1967), Carns (1973) presented regression equations for estimating flood quantiles at T of 1.25, 2, 5, 10, 25, 50, and 

100 years. The equations were derived using 172 stations with at least 10 years of record at the end of WY 1967. 

Carns (1973) analyzed residuals to delineate four hydrologic regions for Illinois. The multiple regression analysis evaluated nine basin-characteristic parameters and derived the regional equations using TDA (drainage area, in 

Introduction 3
4 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Figure 1. Development of flood-frequency relations and regional flood-frequency techniques for rural streams in Illinois (Text in italics indicates the part of the report where the listed components are fully described). 
square miles, mi2

), MCS (main-channel slope, in feet per mile, ft/mi), TTF (precipitation index, using the 2-year, 

24-hour, rainfall depth per 24 hours, in inches), and RF (a regional factor). Note that the abbreviations are cited differently from their original reports in order to maintain consistency with variables used in this report. The RF was 

determined in each region by averaging the residuals in that region and the RF was used in all QT equations. The RF

was adjusted when streams crossed regional boundaries, or when sites were close to the boundary between regions. 

The applicability of the regression equations for TDA ranged from 0.02 mi2 (Bear Creek tributary near Reeders, Ill.) 

to 28,600 mi2 (Wabash River at Mount Carmel, Ill.), and for MCS from 0.7 ft/mi (Wabash River at Mount Carmel, 

Ill.) to 269.81 ft/mi (Big Muddy River Tributary near Gorham, Ill.). The TTF, which ranged from 2.6 to 3.6 in, was 

obtained from the U.S. Weather Bureau (Technical Paper 40, Hershfield, 1961). Concerning the lack of data in representing the variable TDA in the group of 25 mi2

 or less, Carns (1973) developed a separate set of equations based 

on the index flood method to estimate the magnitudes of 50- and 100-year flood-peak discharges for drainage basins 

in this group. Carns also summarized PDS data for streamflow-gaging stations with 5 or more years of record.

Curtis (1977b) estimated the flood quantiles for T ranged from 2 to 500 years at 303 gaging stations based on 

using the procedures outlined in Bulletin 17 (U.S. Water Resources Council, 1976). All these stations had at least 10 

years of record by the end of WY 1975. The weighted skew method was applied to all stations except for stations 

with less than 25 years of record, where the generalized skew coefficient was used instead. Regional equations for 

rural streams were developed based on 241 sites not affected by urbanization or regulation. Among these 241 stations, 87 were from the USGS Small Stream Network (drainage area ranging from 0.02 to 10.2 mi2

). Curtis (1977b) 

also examined nine basin characteristics in developing the regional equations but selected the same explanatory 

variables used by Carns (1973). A constant 2.5 was subtracted from TTF to decrease the range and the magnitude 

of the regression coefficients for each T. The boundaries of four hydrologic regions were modified slightly with the 

consideration of physiographic characteristics, river-basin boundaries, and regression residuals. The RF values were 

different from those determined in 1973 but the differences were not appreciable. In his analysis, Curtis concluded 

that the RF reduced the standard error by about 3 percent, and stratifying the drainage area by size did not improve 

the standard errors significantly. The range of explanatory variables for TDA was from 0.02 (05586850, Bear Creek 

tributary near Reeders, Ill.) to 9,551 mi2 (05446500, Rock River near Joslin, Ill.), for MCS was from 0.69 (05526000, 

Iroquois River near Chebanse, Ill.) to 228.6 ft/mi (05558050, Coffee Creek tributary near Florid, Ill.), and the rainfall 

depth remained the same (Hershfield, 1961).

Curtis (1987) updated frequency analysis using station data ending in WY 1985 and procedures outlined in 

Bulletin 17B (Interagency Advisory Committee on Water Data, 1982). At-site flood quantiles for T = 2, 5, 10, 25, 

50, 100, and 500 years were evaluated at 394 streamflow-gaging stations with 10 or more years of data. Among 

the 394 gaging stations, 268 stations were on rural streams suitable for regression analysis. In developing regional 

equations, the same hydrologic regions and explanatory variables were retained but the GLS method (Stedinger and 

Tasker, 1986, 1985) was used. The RFs were defined for each T in each region. The range of explanatory variables 

remained the same as those defined in 1977 except TDA for station 05446500 (Rock River near Joslin, Ill.) was 

reported as 9,549 mi2.

FLOOD-PEAK DISCHARGE MAGNITUDES AND FREQUENCIES AT GAGED SITES

Data Availability

Both AMS and PDS were retrieved from the peak-flow files in the National Water Information System (NWIS) 

of the USGS. The instantaneous peak discharges also are part of streamflow record published annually by USGS 

Science Center offices, for example, LaTour and others (1996) or at URL: http://il.water.usgs.gov/nwis-w/IL/ for Illinois. Secondary instantaneous peak discharges above a selected base discharge and associated stages are reported if 

the flow above the gaging station is not appreciably regulated. The base discharge generally is selected such that, on 

average, three independent flood-peak discharges, including the annual maximum peak discharge, exceed the base 

discharge each water year. Criteria for selecting peak discharges, concerning the selection of independent events, 

secondary peaks, base discharge at the gaging station, and others are given in Novak (1985). 

The AMS series were compiled for streamflow-gaging stations with a minimum of 10 years of records. There 

were 419 streamflow-gaging stations – including both rural and urban watersheds as well as active and inactive stations – used in at-site AMS analysis. Out of the 419 records, 288 were identified as rural streamflow records that 

are suitable for developing the regional equations. Descriptions of data preparation for AMS data are described in 

Flood-Peak Discharge Magnitudes and Frequencies at Gaged Sites 5
6 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

appendix 1 (Data Preparation). The locations of the 419 and 288 streamflow-gaging stations are shown in figures 

2A, b. 

The PDS series were first retrieved from all stations with available records and examined. Gage operations 

could create breaks in a station’s continuous record for both AMS and PDS, and also could create records having 

combined continuous and one record per year (when the gage was operated as a crest-stage gage (CSG)) for a period 

of time. The CSG data were included as part of the PDS data if they did not constitute a major portion of the record. 

The missing data (secondary peaks) in the CSG operation period may not be a major concern in this study because 

the emphasis of PDS analysis is on estimating flood quantiles at smaller T’s. The effect of missing data on estimating flood quantiles at larger T’s should be investigated, however. A general rule used for selecting systematic PDS 

data in this study was that the record had adequate coverage of large and small flood events and, generally, had 15 

or more data points (an average of 5 years or more of record). With 15 or more data points and 5 years in time span, 

the magnitudes of smaller flood peak discharges generally are sufficiently represented in the PDS dataset. As the 

result of record examination, there were 222 streamflow-gaging station records in Illinois suitable for the PDS analysis of which 142 stations were in rural watersheds suitable for developing the PDS regional equations. The PDS 

station locations are shown in figure 3. Descriptions of data preparation for the PDS and other potential sources for 

organizing PDS data also are described in appendix 1 (Data Preparation).

Determination of Recurrence Intervals

For AMS, the recurrence interval, T, has a time unit, year, and is defined as the inverse of exceedance probability P. The T for AMS is easy to understand because these data are taken at yearly intervals. For a 100-year flood, it 

means that there is a 0.01 probability or 1-percent chance, on average, that the flood-peak discharge magnitude will 

be exceeded (an exceedance) in any and each year, including successive years. For PDS, because multiple flood 

peak-discharges are counted in a year, the definition of T for PDS is the expected time between exceedances, on 

average, in which a flood-peak discharge (no implication of annual maximum) exceeds the specified flood magnitude. The methods for approximating T’s for the AMS and the PDS are discussed below.

Because each observed flood-peak discharge in a flood series is necessarily a limited sample of the full range of 

possible events, the exceedance probability P for each flood sample in the series is approximated by using the plotting position formula (Chow, 1964b). Chow (1964b) or others have discussed various formulas for this purpose. In 

this study, the Weibull plotting position formula is used for approximating the T for the AMS (Interagency Advisory 

Committee on Water, 1982). The Weibull formula is specified in equation 1 below as 

 T = 1 = N + 1 ,

 P m (1)

where T is the recurrence interval, P is the exceedance probability, N is the total number of observed events (samples), and m is the rank of the event in ascending order. Note that here the number of observed data is equal to the 

number of years in the flood series. For a flood series with 20 samples, for example, the highest ranked flood event 

has T equal to 21 years (P = 0.0476), and the lowest event has T equal to 1.05 (P = 0.952, P cannot exceed 1.0).

Equation 1 also is used to estimate the T for each sample in the PDS. Dalrymple (1960) considered that equation 1 would work better than other plotting position formulas for the PDS because a better estimate of smaller T’s 

can be obtained – as there are multiple flood-peak discharges in a year, the N is greater than the number of years 

of the flood series. In the PDS analysis, the average number of flood peaks in a year, r, must be considered in the 

definition of T’s. Therefore, the T computed with equation 1 represents an average of rT events for the PDS. In this 

study, the T for PDS is approximated by

 T = ——— = — (in years . ) (2) r rP

 1

—P 1 annual

event ( )

8 Estimating Flood-Peak Disch

Figure 2B. Location (as of 2002) of U.

annual maximum series (AMS) were figure 2a for area location.)

10 Estimating Flood-Peak Disc

The value of r increases or decreasof r could affect the structure of the 

average of 1.6 events per year is usto 0.66 year. Therefore, the differe

and is dependent on the value of r. model but are recognized in the PDsuitable value of r for at-site analys

Estimates of Flood-Frequen

At-site AMS flood frequencie

program PEAKFQ; version 4.1; Thbecause of random sample errors osory Committee on Water Data, 19the sample skew of the systematic rof generalized skew map for Illinoiaround –0.4. During this study, the dix 3, Generalized Skew Coefficienshown in figure 4. Estimated AMS presented in table 1. The results of table 1 (at back of report). 

Estimates of Flood-Frequen

At-site PDS flood-frequencies 

tion. A computer program was devnential, GP, and LP3), and generat(Rao and Hamed, 2000) were used for these distributions. The GP disDischarges). The formulas of the G1.01, 1.5, 2, 3, and 5 years for the 2

Effect of Updated Flood-FreSeries

A review of the AMS data bet

and updated generalized skew coefFlood Records and Updated Gener

Additional streamflow records 

ing stations used by Curtis (1987) (inactive streamflow-gaging stationregional flood-frequency analysis. ing the number of stations into thremedium record-length group (15 ye

in the long record-length group incthe other two groups, with the 10 n

In addition to increases in reco

Mississippi River Basin, occurred iadditional flood records, new statiothat were not new station records b

discharge magnitude matched the 1

newly recorded floods, the hydrolobe given later. These floods were r
Figure 4. Updated generalized skew-coefficient map for Illinois.

Flood-Peak Discharge Magnitudes and Frequencies at Gaged Sites 11
12 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

The additional flood records had more apparent effects on at-site flood-frequency estimates than the updated 

skew coefficients. The analysis showed that the with the additional flood records, the widths of confidence intervals 

for both Q2 and Q100 (only statistics associated with these two T’s were evaluated) at the 116 stations were reduced, 

and estimated magnitudes of Q2 and Q100 were increased in regions 2, 3, 4, 5, and 6 but decreased in region 7. The 

updated generalized skew coefficients showed a general pattern of increasing the magnitudes and width of the confidence interval for Q100, but decreasing the magnitudes and width of the confidence interval for Q2

.

The analysis also identified the lack of updated data for small watersheds in Illinois. There are only 83 stations (of the total current 288 stations) having drainage area less than or equal to 5 mi2

. No new data were available 

for these small watersheds after 1980. Also, 64 of the 83 small watersheds have less than 25 years of record and 

18 stations have 25 years of record. The lack of data for the 1990’s (a period with high flood peaks) may bias the 

frequency estimates for small watersheds.

Example At-Site Frequency Curves for Annual Maximum Series and Partial Duration Series

Flood-frequency curves derived for the AMS and PDS models are shown in figures 6 and 7, respectively. 

Streamflow records at Bluegrass Creek at Potomac, Illinois, are used in the example. The AMS results are plotted 

with P as the x-axis, the way they are obtained from the PEAKFQ output; the PDS results are plotted with T as the 

x-axis for the purpose of illustrating flood estimates at lower recurrence intervals. All at-site flood-frequency curves 

for both flood series are presented in the CD-ROM as documented in appendix 8.

REGIONAL FREQUENCY ANALYSIS

Hydrologic Regions for Illinois

Seven hydrologic regions for Illinois (fig. 5) are delineated for the regional flood-frequency analysis. The 

revised region delineations are developed on the basis of physiographic features (Leighton and others, 1948) and 

hydrologic characteristics (Mitchell, 1954; Singh, 1981) (appendix 6, Hydrologic Regions for Illinois), and the 

results are different from those developed by Curtis (1987a, b; 1977) that were developed based on analysis of 

residuals of the regional regression analysis. By using physiographic features and hydrologic characteristics as the 

bases for hydrologic region delineation, it is expected that the delineations won’t be altered appreciably in the future 

analyses and can reasonably be used in both AMS and PDS analyses. River basins of Illinois in these seven hydrologic regions are given in appendix 6. 

Basin Characteristics

Thirty-eight basin characteristics describing the morphometric, soil, precipitation, and land use were defined 

(Eash, 2001). BASINSOFT (version 1.0, 2001) and various Arc/Info AML (Arc Macro Language) programs in 

conjunction with 100,000-scale DEM, STATSGO, and NLCD datasets were applied to determine the values of these 

basin characteristics (appendix 5, Determination of Basin Characteristics). Note that some values were averaged 

for the basin and presented at the basin centroid. The TDA is assumed to have the same value as CDA (contributing 

area) in this study. Although CDA is more relevant to the flow analysis than TDA, a recognized means for determining the CDA has not been yet reached, however. The values of these basin characteristics for the 288 stations are 

given in the CD-ROM (appendix 8).

Multiple Regression Analysis

Regional equations were developed by using the updated flood quantiles for rural watersheds (as response variables) and subgroups of the newly derived basin characteristics (as explanatory variables) with the multiple regression analysis. A preliminary analysis (appendix 7, Regression Analysis) identified the following basin characteristics 

that are suitable for developing the regional equations for Illinois. They are: TDA (total drainage areas, in square 

miles), MCS (main-channel slope, in feet per mile), PermAvg (average permeability, in inches per hour), %Water
FIgure 5. Hydrologic regions for flood-frequency analysis of rural streams in Illinois.

Regional Frequency Analysis 13
14 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Figure 6. Flood-frequency curve based on annual maximum series (AMS) analysis for Bluegrass Creek at Potomac, Vermilion County Ill., 

(03336500).

Figure 7. Flood-frequency curve based on partial duration series (PDS) analysis for Bluegrass Creek at Potomac, Vermilion County Ill., 

(03336500).
(area of open water and herbaceous wetland, in percent of basin area), BL (basin length, in miles), BW (basin width, 

in miles), MCL (main-channel length, in miles), and TTF (2-day, 24-hour rainfall depth, in inches). In addition, 

dummy variables were used as the surrogate for factors affecting QT’s that could not be properly expressed as variables (for example, Helsel and Hirsch, 1992). With the use of dummy variables, all rural watersheds in the State 

were included in each regression analysis, not just those stations in the specified hydrologic region (appendix 7). 

When the analysis was concluded, the numerical values of the dummy variable were converted to regional factors 

for those regions using the same group of explanatory variables. At the screen stage, the OLS technique was used 

for identifying the optimal grouping of explanatory variables. 

Using the multiple regression analysis, the regional equations were developed for each recurrence interval separately. Such procedures could result in different groupings of explanatory variables for different QT’s in the same 

hydrologic region. Although conceptually correct, such a formulation could result in discontinuity in the flood-frequency curves. That is, the estimated QT at a lower T might have large magnitude than the estimated QT at the next 

higher T. Results like such were observed at a few small basins especially those located near the regional boundaries. In this study, the same group of explanatory variables was used for all the QT’s in the same region. Selection 

of the optimal group of explanatory variables for all the QT’s in the same region was determined by evaluating the 

sum of square of errors (SSE) for each of the potential regression equations in the screen stage. The final regression 

model was identified if the SSE for one explanatory variable group was within a specified tolerance level of the SSE

in each of the QT model (appendix 7). For the AMS regional equations, a relative low tolerance level, 10 percent, 

could be reached. For the PDS regional equations, the tolerance level had to be relaxed to 25 percent for hydrologic 

region 2, 17 percent for region 3, and 10 percent for all other regions.

The screening analysis indicated that TDA and MCS were the main explanatory variables in the AMS regional 

equations. These results were the same as obtained by Curtis (1987, 1977b). The TTF was not selected in this 

analysis probably because the updated rainfall depths (Huff and Angel, 1992) among different regions became less 

distinguishable to the regression analysis. Instead, the PermAvg, %Water, or BL was selected as the auxiliary (third) 

explanatory variable. The final AMS regional equations were determined by GLS analysis. The BL and BW variables could be alternatives to TDA and appeared more frequently in the PDS regional equations in this study. After 

converting BW to TDA and BL, TDA is the main explanatory variable for the PDS regional equations. The PDS 

regional equations were determined by OLS analysis.

Measurement of Explanatory Variables

When the values of explanatory variables for the sites in question need to be measured, the same procedures and 

datasets (as described in the Basin Characteristics section) should be used whenever possible. Suggestions for using 

other means to determine the values of these variables are given below. However, users are first reminded of these 

three points.

1. Use the same definitions and units for these variables (see appendix 5).

2. Refer to published watershed boundaries or be familiar with the delineation of watersheds. Watershed boundaries determined with automated procedures need to be verified.

3. Pay attention to the map scales if multiple maps are used. Errors can result from using maps at different 

scales.

When measuring the variables from a map, a grid method can be used to minimize measurement errors. The 

method involves dividing the study area into small, uniform grids; users then can measure the variable value in each 

grid, and sum up the individual values. The following is a brief summary for determining each of the five explanatory variables.

•	 TDA, basin drainage areas, in square miles. TDA can be measured by planimetering the USGS 7.5-minute 

topographic, county, or other maps; obtained from DEM with other computerized programs; from published 

reports or other reliable means. Field reconnaissance to verify the delineated watershed always is helpful.

•	 MCS, main-channel slope, in feet per mile. MCS is computed from the difference in streambed elevations 

at points 10 percent and 85 percent of the distance along the main channel from the basin outlet to the basin 

divide, MCS=(E85-E10)/(0.75MCL). The MCL (measured with a displacement gage) and elevations can be 

obtained using the USGS 7.5-minute topographic, county, or other maps; from DEM with other computerized 

programs, field survey, published reports, or other reliable means.

Regional Frequency Analysis 15
16 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

•	 PermAvg, average soil permeability, in inches per hour. This term is the area-weighted (represented at the 

basin centroid) arithmetic mean of PermH(igh) and PermL(ow), which are determined from the STATSGO 

database. Other computer programs with GIS capabilities (the BASINSOFT program was used in this study) 

can be used with STATSGO to determine the PermH and PermL. The average soil permeability for the State 

is illustrated in figures 8A-D, in which different ranges of the PermAvg values are prepared. Users also can 

use the digital version of these figures in the attached CD-ROM to zoom in the areas under study.

•	 %Water, the percentage of area classified as open water and herbaceous wetland in a watershed. This value 

can be determined using computer programs with GIS capabilities on the NLCD database or other equivalent 

land-cover dataset (for example, the Land Cover of Illinois by the Illinois Department of Natural Resources, 

1996), or measured by planimetering the U.S. Fish and Wildlife Service (USFWS) National Wetland Inventory maps, other suitable maps, by field survey, published reports, or other reliable means. The USFWS maps 

are available in both hard-copy and digital formats for all of Illinois and are printed at the same scale as the 

USGS topographic maps. There also is an interactive mapper available through the USFWS Web site (http://

mapper.tat.fws.gov/nwi/viewer.htm, accessed December 2003), where specific study areas can be printed. 

Note that users need to add a constant of 5 (percent) to this explanatory variable before applying it to the 

regional equations. A constant 5 (percent) was added to this explanatory variable during the regression analysis to avoid zeros when transforming to logarithmic values.

•	 BL, basin length, in miles. BL can be measured by using a displacement gage on the USGS 7.5-minute topographic, county, or other maps; obtained from DEM with other computerized programs; or from field survey, 

published reports, or other reliable means. When using a map, measure along a line from the basin outlet to 

the intersection of the main channel (extended, if necessary) with the upper basin boundary. The line should 

not cross outside the drainage-basin boundaries. 

TECHNIQUES FOR ESTIMATING FLOOD-PEAK DISCHARGE MAGNITUDES AND 

FREQUENCIES

The regional equations, for both AMS and PDS, are presented in a general form in the next two sections with 

tables containing the values of coefficients and exponents of the explanatory variables for each selected T. Also 

presented in the tables are parameters for estimating the accuracy of the regional equations. As mentioned previously, the AMS flood estimates presented here are suitable for flood protection and prevention analyses, similar to 

those presented in previous statewide flood-frequency analysis. The PDS flood estimates, on the other hand, could 

be more suitable than AMS estimates in environmental studies where damages caused by more frequent or repeated 

flood magnitudes in a year are of concern. However, the PDS estimates are presented only for flood-peak discharge 

magnitudes smaller than 5 years in this study (see appendix 1). If the Q5 is required, the AMS result should be used 

unless the purpose of the study requires use of the PDS. The AMS and PDS results are presented separately and the 

meaning of their results should be distinguished clearly. Users also are reminded that the resulting regional equation 

estimates the mean (logarithmic) value of QT of different basins in the region with the same set of explanatory variables (William Kirby, U.S. Geological Survey, written commun., 2003). Local features that can affect flow magnitudes are not accounted for in the regional equations.

The AMS regional equations are developed for T’s of 2, 5, 10, 25, 50, 100, and 500 years. The estimates of Q500

are included for applications in floodplain delineation and flood-insurance studies. Understandably, higher uncertainties are associated with estimates at larger T’s no matter which flood series is used. The PDS regional equations 

are developed for T’s equal to 0.8, 1.01, 1.5, 2, 3, and 5 years. An overview of the procedures for computing the 

flood-frequency estimates at an ungaged or gaged site is illustrated in the flowchart illustrated in figure 9, including 

techniques described in the following sections. After the applicability of the regional equations is determined (fig. 

9), the user can proceed through the procedures as follows:

1. Identify the region where the site is located using figure 5; then identify the explanatory variables from the 

corresponding general regional equation. The general equation for the AMS regional equations is found in the 

Annual Maximum Series Regional Equations section. The general equation for the PDS regional equations is 

found in the Partial Duration Series Regional Equations section. Note that if the site is at a streamflow-gaging 

station, where at-site QT’s have been estimated in this report, the at-site estimates may be obtained from table 

1 (AMS) or table 2 (PDS). See item 4 below.
Figure 8A. Average soil permeability (from 0.2 to less than 1.0 inch per hour) for Illinois. Average soil permeability is obtained by taking the arithmetic average of the high and low soil-permeability values from the STATSGO database (Natural Resources Conservation 

Service, 1993).

Techniques for Estimating Flood-Peak Discharge Magnitudes and Frequencies 17
18 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Figure 8B. Average soil permeability (from 1.0 to less than 2.0 inches per hour) for Illinois. Average soil permeability is obtained by taking the arithmetic average of the high and low soil-permeability values from the STATSGO database (Natural Resources Conservation 

Service, 1993).
Figure 8C. Average soil permeability (from 2.0 to less than 3.0 inches per hour) for Illinois. Average soil permeability is obtained by taking the arithmetic average of the high and low soil-permeability values from the STATSGO database (Natural Resources Conservation 

Service, 1993).

Techniques for Estimating Flood-Peak Discharge Magnitudes and Frequencies 19
20 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Figure 8D. Average soil permeability (from 3.0 to greater than 8.0 inches per hour) for Illinois. Average soil permeability is obtained by 

taking the arithmetic average of the high and low soil-permeability values from the STATSGO database (Natural Resources Conservation 

Service, 1993).
Figure 9. Procedures developed during the present study to estimate flood quantiles at a stream location.

Techniques for Estimating Flood-Peak Discharge Magnitudes and Frequencies 21
22 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

2. Determine representative values of the explanatory variables by using tables and figures provided in the 

report, or measure these values from other maps (see Measurement of Explanatory Variables section above).

3. Compute QT’s by substituting in values of the explanatory variables and selecting coefficients and exponents 

corresponding to the selected T’s from the tables in the appropriate section below where the regional equations are described. The computed QT’s are designated for ungaged sites.

4. For the AMS flood quantiles, if the site is on the same stream and nearby a streamflow-gaging station (use figures 2A and 2B or the digital versions in the CD-ROM) where at-site QT’s have been estimated in this report 

(table 1), a weighting procedure can be applied to improve the regional equation estimates. See provided 

examples in the Gaged Sites and Near Gaged Sites sections for adjusting the computed QT’s. No weighting 

procedure has been developed for the PDS series.

5. For QT’s at recurrence intervals other than those given in the equations, users can develop the frequency curve 

on log-probability plotting paper and interpolate for the QT’s in question. 

The following sections (1) present the AMS regional equations; (2) illustrate the use of the AMS regional 

equations for an ungaged site with an example application; (3) illustrate the use of the AMS regional equations to 

improve flood quantiles at a gaged site; (4) provide a technique for improving flood quantiles near a gaged site; (5) 

describe the limitations and accuracy of the AMS regional equations techniques; and (6) present a technique to transfer flood quantiles in four regulated channel reaches of Illinois. 

Although no example for PDS regional equation is prepared, the procedures for applying the AMS and PDS 

regional equations are similar, however. The PDS regional equations and their limitations and accuracy are presented after the AMS regional equation techniques. No techniques for improving the PDS regional equations 

estimates through regional weighting or areal adjustment, or transferring the estimates in regulated channels are 

provided, as these items are beyond the scope of this study. 

Annual Maximum Series Regional Equations for Rural, Unregulated Streams

A general form of the regression equation for hydrologic regions 1, 3, and 5 is given in equation 3 below, and 

the coefficient for each term is presented in table 3. 

QT = a(TDA)b (MCS)

c (PermAvg)d

 RF (N) [for hydrologic regions 1, 3, and 5] (3)

The parameters that can be used to measure the uncertainty and accuracy of these regression equations also are 

shown in table 3. These parameters were obtained from the GLSNET program (Tasker and Stedinger, 1989) output and their values were the averaged values for all the 288 rural-watershed stations. The method for using APE

to evaluate model accuracy and uncertainty is given in appendix 7 (Regression Analysis). The AEYR describes the 

accuracy of the regression equation. It is an estimate of the number of years of streamflow record that must be collected at a streamflow-gaging station to estimate the magnitude of flood-peak discharge for a selected frequency with 

an accuracy equivalent to that of the regression equation (Hardison, 1971, p. C232). 

A general form of the regression equation for hydrologic regions 2, 6, and 7 is given in equation 4. A constant, 

5, has been added to %Water to avoid zero values when transformed to logarithmic values. The regression equations were developed with this constant added to the variable; therefore, the user also must add the constant (5) to the 

determined value of %Water, as shown in the equation. The corresponding coefficient for each term is presented in 

table 4.

QT = a(TDA)b (MCS)c (%Water + 5)d RF (N) [for hydrologic regions 2, 6, and 7] (4)

The regression equation for hydrologic region 4 is given as

 QT = a(TDA)b (MCS)c (BL)d [for hydrologic region 4]. (5)

The corresponding coefficient for each term is given in table 5. 

The explanatory variables of the AMS regional equations are similar to those used by Curtis (1987), except the 

third variable. The exponents of PermAvg in equation 3, as well as those of %Water in equation 4, depict the reverse 

relation of infiltration and storage to the magnitude of flood-peak discharges. Originally, explanatory variables 
selected for region 4 were TDA, BL, and BW. Watersheds in region 4 (described by equation 5), in general, contain 

bedrock topography, dissected upland, and deep incised channels, where varying hydraulic factors may have considerable effects on channel flows. BL and BW, variables describing the flow time, were selected instead of the contributing area. The BW is a function of TDA and BL (appendix 5), thus, equation 5 has a similar form as equations 3 and 

4.

Numerical values of selected basin characteristics used in the regression analysis and equivalent years of record 

for each recurrence interval for the 288 stations are given in table 6, and record length, historical events, and floodpeak information for these stations are presented in table 7. Tables 6 and 7 are located at the back of the report.

Application of Annual Maximum Series Regional Equations

Examples on how to use the AMS regional equations to compute flood quantiles at ungaged streams are given 

here. Note that the regional equations estimate the mean (logarithmic) value of QT of streams in the region with one 

set of explanatory variables. When estimated at-site flood quantiles are available or the site is near a gaging station 

Table 3. Coefficients and exponents for equation 3 based on annual maximum series for hydrologic regions 1, 3, and 5, Illinois, for specified recurrence intervals.

[QT, flood quantile, in cubic feet per second, ft3/s; a, coefficient; b, exponent for drainage area TDA, in square miles, mi2; c, exponent for main-channel slope 

MCS, in feet per mile, ft/mi; d, exponent for averaged permeability PermAvg, in inches per hour, in/hr; RF(N) regional factor for region N; APE%, average 

prediction error of the regional equation, in percent, %; γ2, model error variance, in log value; γ%, standard error of the model, in percent, %; AEYR, average 

equivalent years of record, in years, yr]

QT

(ft 3/s) a b c d RF(1) RF(3) RF(5) APE%

(%) γ2 γ%

(%)

AEYR

(yr)

Q2 22.2 0.749 0.401 -0.224 1.467 1.620 2.128 39.5 0.0257 38.2 2.7

Q5 34.1 .743 .437 -.223 1.563 1.811 2.360 40.0 .0263 38.7 3.2

Q10 41.8 .740 .457 -.224 1.618 1.913 2.476 41.6 .0282 40.2 3.9

Q25 50.8 .738 .478 -.224 1.686 2.030 2.612 44.2 .0315 42.6 4.7

Q50 57.0 .737 .491 -.223 1.738 2.113 2.711 46.6 .0345 44.8 5.2

Q100 62.7 .736 .503 -.222 1.790 2.192 2.809 49.0 .0378 47.1 5.6

Q500 74.5 .735 .527 -.219 1.917 2.371 3.037 54.9 .0462 52.7 6.2

Table 4. Coefficients and exponents for equation 4 based on annual maximum series for hydrologic regions 2, 6, and 7, Illinois, for specified recurrence intervals.

[QT, flood quantile, in cubic feet per second, ft3/s; a, coefficient; b, exponent for drainage area TDA, in square miles, mi2; c, exponent for main-channel slope 

MCS, in feet per mile, ft/mi; d, exponent of the (%Water +5) term (%Water+5 is the percentage of open water and herbaceous wetland, where 5 is added to 

avoid zero values), in percent, %; RF(N) regional factor for region N; APE%, average prediction error of the regional equation, in percent, %; γ2, model error 

variance, in log value; γ%, standard error of the model, in percent, %; AEYR, average equivalent years of record, in years, yr]

QT

(ft 3/s) a b c d RF(2) RF(6) RF(7) APE%

(%) γ2 γ%

(%)

AEYR

(yr)

Q2 54.7 0.728 0.341 -0.470 1 2.963 3.515 40.4 0.0268 39.1 2.6

Q5 94 .721 .374 -.527 1 3.119 3.281 40.7 .0271 39.3 3.1

Q10 120 .718 .393 -.550 1 3.241 3.226 42.0 .0288 40.6 3.8

Q25 151 .716 .413 -.573 1 3.409 3.217 44.7 .0321 43.1 4.6

Q50 174 .715 .426 -.586 1 3.540 3.236 46.9 .0350 45.2 5.2

Q100 195 .714 .437 -.598 1 3.672 3.269 49.2 .0381 47.3 5.6

Q500 241 .714 .461 -.619 1 3.980 3.377 55.0 .0464 52.8 6.2

Techniques for Estimating Flood-Peak Discharge Magnitudes and Frequencies 23
24 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

on the same stream, the regional estimates could be weighted with the at-site flood quantiles to improve the estimates at that location. These three cases (ungaged, gaged, and a nearby streamflow-gaging station) are illustrated 

with three sites in the Blackberry Creek watershed (a tributary to the Fox River) in Kane and Kendall Counties, 

northern Illinois (fig. 10). The examples are illustrated with estimation of the Q100.

Ungaged Sites

The first site is at the outlet of a Lake Run tributary (fig. 9), an ungaged site. The estimated magnitude of 100-

year flood discharge is calculated as follows.

1. Identify the hydrologic region and explanatory variables. From figure 5, this site is located in hydrologic 

region 2; equation 4 and table 4 will be needed for calculating the Q100. The explanatory variables are TDA, 

MCS, and %Water, and a regional factor.

2. Determine the values of drainage area TDA, in square miles; main-channel slope, MCS, in feet per mile; and 

the percent area classified as open water and herbaceous wetland, %Water, where the constant 5 is added to 

make the variable (%Water+5), using the procedures discussed in the Measurement of Explanatory Variables 

section. The values for Lake Run tributary are 14.0 mi2, 11.4 ft/mi, and 6.34 percent for TDA, MCS, and 

(%Water+5), respectively.

3. From table 4, the coefficient and exponents corresponding to the 100-year flood quantile are 195, 0.714, 

0.437, and –0.598 for the constant, TDA, MCS, and (%Water+5), respectively. The regional factor RF(2) is 1.

4. Substitute these coefficients and exponents in equation 4, and the Q100 is computed as

 Q100 = 195(14.0)0.714 (11.4)0.437 (6.34)-0.598 (1), or

 Q100 = 1,232 ft3/s.

Gaged Sites

Flood quantiles at a gaged site are weighted using a procedure adopted from the equivalent years of record concept (Hardison, 1971). The procedure can be described in equation form as

 LogQT | weighted= (years of record)(logQT | at-site) + (EYR)(logQT

 | regional) ———————————————————————— , (11)

 (years of record + EYR)

where EYR is the equivalent years or record at this site. 

Table 5. Coefficients and exponents for equation 5 based on annual maximum series for hydrologic region 4, Illinois, for specified recurrence intervals.

[QT, flood quantile, in cubic feet per second, ft3/s; a, coefficient; b, exponent for drainage area TDA, in square miles, mi2; c, exponent for main-channel slope 

MCS, in feet per mile, ft/mi; d, exponent for basin length, BL, in miles, mi; APE%, average prediction error of the regional equation, in percent, %; γ2, model 

error variance, in log value; γ%, standard error of the model, in percent, %; AEYR, average equivalent years of record, in years, yr]

QT

(ft 3/s) a b c d APE%

(%) γ2 γ%

(%)

AEYR

(yr)

Q2 49.3 0.734 0.370 -0.006 41.1 0.0277 39.8 2.5

Q5 85.1 .772 .406 -.095 41.5 .0282 40.2 3.0

Q10 111 .792 .425 -.140 43.0 .0300 41.5 3.7

Q25 144 .812 .446 -.183 45.5 .0332 43.9 4.5

Q50 168 .823 .460 -.207 47.7 .0361 45.9 5.0

Q100 193 .833 .472 -.228 50.0 .0393 48.1 5.4

Q500 250 .852 .496 -.266 55.7 .0475 53.5 6.1
Figure 10. Blackberry Creek watershed, Kane and Kendall Counties, Illinois, and site locations of three subbasins used in examples of 

application of the annual maximum series (AMS) regional equations.

Techniques for Estimating Flood-Peak Discharge Magnitudes and Frequencies 25
26 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

For the Blackberry Creek watershed, a USGS streamflow-gaging station is present near Yorkville (05551700) 

near the outlet (fig. 10). Streamflow records are available since WY 1961 and there were 39 years of record up to 

WY 1999 that were used for this study (table 7). The estimated at-site Q100 is 2,850 ft3

/s, the equivalent years of 

record for Q100 at this site is 4.1 years (see table 6). The weighted Q100 is calculated as follows. 

1. Determine the values of TDA, MCS, and (%Water+5), that are 70.2 mi2, 6.43 ft/mi, and 6.06 percent, respectively. 

2. Compute the regional estimate of Q100 by substituting these values and the coefficients in equation 4. Because 

the regional factor is 1, it is not shown in the calculation resulting in

 Q100 | regional= 195(70.2)0.714 (6.43)0.437 (6.06)-0.598,

 = 3,120 ft3/s.

3. Determine the weighted Q100 using equation 11 as

 LogQT | weighted= (39)(log 2,850 | at-site) + (4.1)(log 3,120 | regional) ————————————————————— .

 (39 + 4.1)

The weighted Log Q100 is 3.458. Note that flood quantiles are calculated in log (base 10) values. Therefore, 

the corresponding arithmetic value is 2,870 ft3

/s (103.458 = 2,870). 

The weighted result is considered the best estimate for QT at a gaged site.

The regional estimates for QT’s at the 288 rural stations using regional equations 3–5 and the weighted results 

are listed in table 1. The weighted flood frequencies are considered the best flood estimates for these 288 ruralwatershed stations.

Near Gaged Sites 

Estimated flood quantiles can be adjusted at sites upstream or downstream from a gaging station on the same 

stream, depending on the proximity of the site to the gaging station. If the drainage area of the site in question is 

within ±50 percent of the drainage area of the gaging station, the estimated flood quantiles can be improved by using 

the ratio of the areas to compute an adjustment ratio between the regional estimate at the site and the estimate at the 

gaging station. Steps for estimating the flood quantiles are listed below.

1. Compute the QT using the appropriate regional equation.

2. Determine an adjustment ratio, ar, according to the drainage areas as

 (12)

where Asite is the TDA of the study site and Agage is the TDA of the gaging station. Note that the absolute value 

is used.

3. Calculate the adjusted flood quantile as

 QT | adjusted= QT | regional X ar + QT | weighted X (1 – ar) (

Asite )

* . (13)

 equation at gage 

______

* Use equation as listed in errata at front of this report.

 A

gage

If the TDA of the study site is not within ±50 percent of the TDA at the gaging station, the adjustment ratio ar is 1 

and the adjustment by weighted QT at the gaging station is zero.

 Asite – 1 X 2, if 0.5 < Asite ar = —– < 1.5 , —–

 Agage Agage

 = 1, otherwise,
For the Blackberry Creek watershed (fig. 10), the sub-watershed above Montgomery streamflow-gaging station 

(05551675 Blackberry Creek near Montgomery, fig. 10) is selected as an example for illustrating adjustment for sites 

near a gaged location. This station has been operating since WY 1998. Only 2 years of data were available by WY 

1999; therefore, this station is treated as an ungaged site. The Q100 at this site is estimated as follows.

1. The TDA, MCS, and (%Water+5) at the Montgomery station are 58.6 mi2, 7.9 ft/mi, and 6.12 percent, respectively. The estimated Q100 with the regional equation is 2,970 ft3

/s.

2. Because the drainage area is within 50 percent of that of the Blackberry Creek near Yorkville station and is 

on the main stem of the Blackberry Creek, the near gage adjustment will apply. The adjustment ratio, ar, is 

computed as

3. From the previous example, the weighted regional estimate of Q100 is 2,870 ft3/s. The adjusted Q100 using 

equation 13 is

 Q100 adjusted= 2,970 X 0.33 + 2,870 X 0.67(58.6 ) = 2,585 ft3/s .

 

70.2

Accuracy and Limitation of the Annual Maximum Series Regional Equations

The average prediction error, APE%, in percent, ranges approximately from 41.6 to 43 percent for Q10 computed by various regional equations, and ranges from 49 to 50 percent for Q100. The APE% and standard error of the 

model, γ% , listed in tables 3–5 are similar for all the recurrence intervals, indicating that the average standard error 

of sampling accounts for little additional unexplained variance of the regional equations (Wiley and others, 2000). 

The averaged equivalent years of record listed in tables 1–3 vary from 3.7 to 3.9 years for the 10-year flood and from 

5.4 to 5.6 for the 100-year flood, for example. This information also is included in table 12 for each selected recurrence interval at the 288 stations.

The developed regional equations are applicable to rural streams of Illinois. The equations are not applicable 

to locations where streamflows are altered appreciably by regulation, diversion, channelization, or urbanization in 

the watersheds. Unusual natural morphologic, hydrologic, or geologic conditions, such as karst terrain, minimal soil 

cover, large off-channel storage, stream sites downstream from bluffs, and others, may cause deviations from the 

expected flood frequencies. In certain situations, additional adjustments to fit local hydraulics may be needed; for 

example, Baldwin and Potter (1986) described a case concerning the timing of tributary floods.

In developing the regional equations, a joint-parameter space, defined by the ranges of selected explanatory 

variables in the analysis, is defined to indicate the limitation in which the regional equations remain valid. When 

flood quantiles at a site in question are estimated with these regional equations, the values of explanatory variables 

for the watershed should be within the parameter space. Because dummy variables are used in developing the 

regional equations in this study, the parameter spaces are defined by explanatory variables associated with all the 

streamflow-gaging stations used for analysis. The ranges of selected explanatory variables for the AMS models are 

listed in table 8. 

Record length, values of explanatory variables, and equivalent years of record are needed for applying the 

weighted adjustment procedures. Record length and other streamflow-record characteristics for the 288 stations are 

given in table 7. The values of explanatory variables and equivalent years of record for each recurrence interval for 

the 288 stations are given in table 12 (at back of report).

Regulated and Urban Streams 

Regional equations are not applicable to regulated and urban streams. However, in the case of channel regulation, when the concern is that regulations could have created non-homogeneous peak-flow records from preconstruction datasets, at-site flood-frequencies representing a relatively constant channel condition can be used to 

develop a linear interpretation between these streamflow-gaging stations if the flood series are examined and nonar = 58.6 mi2

 – 1 X 2 = 0.31 . ——–—

 69.4 mi2

Techniques for Estimating Flood-Peak Discharge Magnitudes and Frequencies 27
28 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

homogeneous data are removed. Regulated streams in Illinois include: Illinois River, Fox River below the Chain-ofLakes, Kaskaskia River below Lake Shelbyville, Saline River below mouth of Cypress Ditch, and Big Muddy River 

below Rend Lake. Note that the Ohio, Wabash, and Mississippi Rivers are not considered in the analysis because 

large portions of their watersheds are outside of Illinois.

Flood frequencies for specific reaches of the Illinois River, Kaskaskia River from downstream of Cowden to 

upstream of Carlyle Lake, Big Muddy River below Plumfield, and Fox River below Algonquin (see figs. 2A and 

2B) could be estimated by linearly interpolating between estimated flood frequencies at streamflow-gaging stations because channel conditions between the selected stations are reasonably consistent. To facilitate interpolation of flood frequencies between the gaging stations on these rivers on the basis of river miles, figures 11–14 were 

developed. However, these graphical interpretations only provide an approximation of the flood frequencies. The 

distances between these streamflow-gaging stations are large (miles and tens of miles). Other estimating methods 

than these graphs, such as hydraulic modeling, should be used for more detailed studies. For example, the U.S. 

Army Corps of Engineers has conducted a flood-frequency study on the Upper Mississippi River System (U.S. Army 

Corps of Engineers, 2004).

Reservoir operations, flow diversions, and/or inter-basin flow transfers modify the random nature of flood-peak 

discharge magnitudes; therefore, frequency analysis is not applicable. Discharge records for Saline River near Junction include inter-basin flow from the Wabash River through Cypress Ditch just upstream of the gaging station. The 

magnitude of the inter-basin flow depends on Wabash River stage, that, in turn, depends on Ohio River stage. The 

complexity of flood conditions precludes the use of regional equations on the Saline River at the confluence with the 

Wabash River.

Flood frequencies on urban streams in northeastern Illinois have been studied by Allen and Bejeck (1979). They 

have developed a set of curves (figure 6 in their report) for computing flood-peak discharge magnitudes at various 

levels of imperviousness or urbanization. The flood quantiles for urban streams were computed by multiplying their 

ratios to flood quantile for rural watersheds or less than 1-percent imperviousness. By assuming that the effects of 

urbanization on flood-peak discharges in northeastern Illinois are similar to urban effects in other parts of Illinois, 

the at-site flood quantiles and those regional equations can be used in conjunction with the correction factors developed by Allen and Bejeck (1979) to estimate flood frequencies on urban streams for other parts of Illinois. These 

estimates should be checked with estimates made with other methods such as rainfall-runoff models.

Partial Duration Series Regional Equations for Rural, Unregulated Streams

The PDS regional equations for rural, unregulated streams are presented in equations 6–12, and corresponding coefficients and exponents for T’s equal to 0.8, 1.01, 1.5, 2, 3, and 5 years are presented in tables 9–13. Procedures for applying the PDS regional equations are similar to those for the AMS regional equations, although the 

PDS regional analysis yielded different explanatory variables from those AMS equations in some of the hydrologic 

regions.

The regression equation for hydrologic region 1 is given in equation 6 and the corresponding coefficient for 

each term is presented in table 9. 

 QT = a(TDA)b (MCS)c (%Water + 5)d [for hydrologic region 1] (6)

Table 8. Parameter space for the annual maximum series regional equations in Illinois.

[TDA, total drainage area, in square miles, mi2; MCS, main-channel slope, in feet per mile, ft/mi; BL, basin length, in miles, mi; PermAvg, averaged permeability, in inches per hour, in/hr; (%Water+5), open water and herbaceous wetland plus a constant (5) in percent, %; -----, no data]

Explanatory Variables Minimum value Maximum value

TDA 0.03 9,554

MCS .81 317

BL .3 190

PermAvg .3 8.0

(%Water+5) 5 13

----- ----- -----
Figure 11. Estimated magnitudes of flood-peak discharges for selected recurrence intervals for the regulated reach of the Big Muddy River in southern Illinois.

Techniques for Estimating Flood-Peak Discharge Magnitudes and Frequencies 29
30 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Figure 12. Estimated magnitudes of flood-peak discharges for selected recurrence intervals for the regulated reach of the Fox River in northern Illinois.
Figure 13. Estimated magnitudes of flood-peak discharges for selected recurrence intervals for the regulated reach of the Kaskaskia River in southern Illinois.

Techniques for Estimating Flood-Peak Discharge Magnitudes and Frequencies 31
32 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Figure 14. Estimated magnitudes of flood-peak discharges for selected recurrence intervals for the Illinois River.
The regression equation for hydrologic region 2 is given in equation 7. Coefficients for each term are given in 

table 10.

 QT = a(TDA)b (BL)c (PermAvg)d [for hydrologic region 2] (7)

The regression equation for hydrologic region 3 is shown in equation 8. Coefficients for each term are given in 

table 11.

 QT = a(TDA)b (%Water + 5)c [for hydrologic region 3] (8)

The regression equation for hydrologic region 4 is shown in equation 9, and the coefficient for each term given 

in table 12.

 QT = a(TDA)b (MCS)

c (BL)d

 [for hydrologic region 4] (9)

The regression equation for hydrologic regions 5, 6 and 7 is shown in equation 10 with the coefficient for each 

term given in table 13. Note that stations in region 7 are combined into region 6 because there are 11 stations in 

region 6 but only 4 stations in region 7. The regional factors resulting from the use of dummy variables have been 

combined into the corresponding coefficients and exponents.

 QT = aN(TDA)bN (MCS)c (%Water + 5)d [for hydrologic regions 5, 6, and 7] (10)

Table 9. Coefficients and exponents for equation 6 based on partial duration series for hydrologic region 1, Illinois, for specified recurrence intervals.

[QT, flood quantile, in cubic feet per second, ft3/s; a, coefficient; b, exponent for drainage area TDA, in square miles, mi2; c, exponent for main-channel slope 

MCS, in feet per mile, ft/mi; d, exponent of the (%Water+5) term (%Water is the percentage of open water and herbaceous wetland, where the constant (5) is 

added to avoid zero values), in percent, %; SEE, standard error of estimate of the regional equation, in percent, %; R2, coefficient of determination (multiple 

correlation coefficient)]

QT (ft 3/s) a b c d SEE (%) R 2

Q0.8 33.3 0.771 0.438 -0.400 50.2 0.83

Q1.01 52.6 .755 .458 -.515 44.1 .86

Q1.5 83.9 .745 .478 -.621 41.4 .86

Q2 107.0 .740 .488 -.673 40.9 .86

Q3 140.8 .736 .498 -.733 41.2 .86

Q5 185.9 .732 .508 -.793 41.7 .85

Techniques for Estimating Flood-Peak Discharge Magnitudes and Frequencies 33

Table 10. Coefficients and exponents for equation 7 based on partial duration series for hydrologic region 2, Illinois, for specified recurrence intervals.

[QT, flood quantile, in cubic feet per second, ft3/s; a, coefficient; b, exponent for drainage area TDA, in square miles, mi2; c, exponent for basin length, BL, in 

miles, mi; d, exponent for averaged permeability, PermAvg, in inches per hours, in/hr; SEE, standard error of estimate of the regional equation, in percent, %; 

R2, coefficient of determination (multiple correlation coefficient)]

QT (ft 3/s) a b c d SEE (%) R 2

Q0.8 17.9 0.775 0.223 -0.499 45.9 0.86

Q1.01 23.7 .772 .185 -.470 41.4 .88

Q1.5 32.8 .769 .152 -.448 39.9 .86

Q2 39.5 .766 .138 -.438 40.2 .87

Q3 49.1 .762 .125 -.428 40.9 .86

Q5 61.8 .755 .114 -.417 42.5 .85
34 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Table 12. Coefficients and exponents for equation 9 based on partial duration series for hydrologic region 4, Illinois, for specified recurrence intervals.

[QT, flood quantile, in cubic feet per second, ft3/s; a, coefficient; b, exponent for drainage area TDA, in square miles, mi2; c, exponent for main-channel slope, 

MCS, in feet per mile, ft/mi; d, exponent for basin length, BL, in miles, mi; SEE, standard error of estimate of the regional equation, in percent, %; R2, coefficient of determination (multiple correlation coefficient)]

QT (ft 3/s) a b c d SEE (%) R 2

Q0.8 60.3 0.907 0.386 -0.463 44.8 0.87

Q1.01 78.7 .910 .403 -.503 39.6 .87

Q1.5 104.9 .916 .431 -.540 37.1 .89

Q2 121.6 .919 .447 -.553 36.6 .89

Q3 142.3 .920 .465 -.562 37.1 .89

Q5 164.1 .920 .484 -.561 38.1 .88

Table 13. Coefficients and exponents for equation 10 based on partial duration series for hydrologic regions 5, 6, and 7 (stations in 

region 7 are combined with region 6 in the analysis), Illinois, for specified recurrence intervals.

[QT, flood quantile, in cubic feet per second, ft3/s; a5, coefficient for region 5; a6,7, coefficient for regions 6 and 7; b, exponent for drainage area TDA, in square 

miles, mi2; with b5 for region 5, and b6,7 for regions 6 and 7; c, exponent for main-channel slope, MCS, in feet per mile, ft/mi; d, exponent of the (%Water +5)

term (%Water is the percentage of open water and herbaceous wetland, where the constant (5) is added to avoid zero values), in percent, %; SEE, standard error 

of estimate of the regional equation, in percent, %; R2, coefficient of determination (multiple correlation coefficient)]

QT (ft 3/s) a5 a6,7 b5 b6,7 c d SEE (%) R 2

Q0.8 69.7 72.0 0.776 0.802 0.383 -0.397 44.6 0.87

Q1.01 101.7 87.4 .759 .822 .405 -.472 39.6 .89

Q1.5 151.4 105.0 .747 .854 .436 -.549 37.3 .89

Q2 186.6 115.2 .742 .874 .453 -.588 37.1 .89

Q3 236.6 128.2 .738 .898 .474 -.633 37.3 .89

Q5 300.9 142.9 .736 .926 .494 -.682 38.4 .88

QT (ft 3/s) a b c SEE (%) R 2

Q0.8 131.0 0.672 -0.456 50.7 0.83

Q1.01 207.1 .645 -.524 45.9 .84

Q1.5 336.9 .624 -.593 44.3 .84

Q2 434.5 .615 -.629 44.6 .84

Q3 580.4 .607 -.671 45.1 .83

Q5 777.7 .600 -.716 46.4 .82

Table 11. Coefficients and exponents for equation 8 based on partial duration series for hydrologic region 3, Illinois, for specified recurrence intervals.

[QT, flood quantile, in cubic feet per second, ft3/s; a, coefficient; b, exponent for drainage area TDA, in square miles, mi2; c, exponent of the (%Water +5) term 

(%Water+5 is the percentage of open water and herbaceous wetland, where 5 is added to avoid zero values), in percent, %; SEE, standard errors of estimate of 

the regional equation, in percent, %; R2, coefficient of determination (multiple correlation coefficient)]
The PDS flood quantiles estimated with regional equations 6-10 are listed in row 2 of table 2; those at-site flood 

quantiles estimated with the GP distribution are listed in row 1 of the same table. For the PDS estimates, weighted 

procedures were not developed in this study.

Similar to the AMS regional equations, the derived equations are applicable to rural streams in Illinois. The 

equations are not applicable to locations where streamflows are altered appreciably by regulation, diversion, channelization, or urbanization in the watersheds. Unusual natural morphologic, hydrologic, or geologic conditions, 

such as karst terrain, minimal soil cover, large off-channel storage, downstream of a bluff, and others, may cause 

deviations from the expected flood frequencies. The ranges of selected explanatory variables for the PDS regional 

equations are listed in table 14. 

The regression equations based on the PDS model are derived using the OLS technique. The uncertainty of prediction is expressed using the standard error of estimate, SEE, and the model accuracy is presented using the correlation coefficient, R2

. Values of these measures for the equations are given in tables 9–13. For example, the 1.5-year 

flood quantile has SEE varying from 37.1 to 44.3 (percent) and R2 varying from 0.84 to 0.89 in the seven hydrologic 

regions. Although the R2’s and data plots (not shown) are reasonable, large residuals are observed at some stations 

in various regions. After the usefulness of PDS analysis is validated in the field, improvements in the PDS data and 

analysis should be investigated in future studies.

SUMMARY AND CONCLUSIONS

Knowledge of the frequency and magnitude of flood-peak discharges is essential for water-resources planning, 

risk management, and project design. To provide up-to-date flood-frequency estimates for the State of Illinois, 

the U.S. Geological Survey—in cooperation with the Illinois Department of Natural Resources, Offices of Water 

Resources, Realty and Environmental Planning–Conservation 2000 Program, and Resource Conservation; and with 

the Illinois Department of Transportation—began a study in 2000 to analyze flood-frequency estimates for rural 

Illinois streams. At-site flood frequencies have been estimated with peak-flow discharge data through September 

1999 (the end of water year 1999), and regional equations have been developed with the at-site flood quantiles and 

geographic information system (GIS) derived basin characteristics. 

At-site flood-frequency relations were estimated using two types of flood series: the annual maximum series 

(AMS) and partial duration series (PDS). However, the two flood series are different in their data structure and 

definitions of commonly used terms, such as the recurrence intervals, are different. Applications of flood quantiles 

estimated from each flood series should not be mixed. The flood-frequency analysis based on the AMS series is 

used for estimating flood quantiles with recurrence intervals from 2 to 500 years, for applications in cases when 

the chances of the estimated flood quantiles having been exceeded by the highest annual flood-peak discharge are 

of concern. The AMS results are used in a manner similar to that in previous studies concerning flood prevention 

and protection. The flood-frequency relations based on the PDS are suitable for practices concerned with not only 

the annual maximum events but also include the magnitudes of secondary flood-peak discharges. In this study, the 

PDS analysis estimated flood quantiles with recurrence intervals from 0.8 to 5 years. Potential applications of these 

PDS results are investigations of channel morphology, floodplain habitat protection, and other restoration issues. 

Regional flood-frequency prediction equations, tables, and graphs for estimating flood quantiles of rural streams of 

Illinois are presented for both cases.

Table 14. Parameter space for the partial duration series regional equations in Illinois.

[Total drainage area TDA, in square miles, (mi2); main-channel slope MCS, in feet per mile, (ft/mi); BL, basin length, in miles, (mi); averaged permeability 

PermAvg, in inches per hour, (in/hr); (%Water+5), open water and herbaceous wetland plus a constant (5) in percent, (%); -----, no data]

Explanatory Variables Minimum value Maximum value

TDA 1.08 5,149

MCS .95 165

BL 1.22 123

PermAvg .4 6.0

(%Water+5) 5 11

----- ----- -----

Summary and Conclusions 35
36 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Both AMS and PDS flood series were compiled from the peak-flow files of NWIS. A set of basin characteristics 

has been derived using the BASINSOFT program and Arc/Info procedures in conjunction with DEM and the digital 

databases STATSGO and NLCD for 288 rural streamflow-gaging stations in Illinois. These newly derived basin 

characteristics were used in the regression analysis for developing regional flood-frequency equations. For developing regional equations, seven hydrologic regions were determined based on physiographic and hydrologic characteristics of the State and refined using the residual analysis. 

At-site AMS flood frequencies were estimated with the Log-Pearson type III distribution using the PEAKFQ 

program. During the study, the generalized skew coefficients were updated using kriging techniques, and seven 

hydrologic regions were delineated on the basis of physiographic and hydrologic characteristics of drainage basins 

of Illinois. The updated at-site AMS flood frequencies have changed noticeably at many stations as a result of 

major floods in the 1990’s, extended record lengths, and the updated generalized skew coefficients from the previous statewide results completed in 1987. For the 116 stations with additional flood records, new station flood-peak 

discharges were recorded at 40 stations; flood-peak discharges that were not new station records but exceeded the 

Q100 estimated previously were recorded at 16 stations, and flood-peak discharge magnitude matched the 1987 Q100

value at 1 station. These floods were recorded at stations primarily in regions 2, 3, and 5 but none were recorded in 

region 7.

The AMS regional equations and a listing of their corresponding coefficients for the seven hydrologic regions 

are presented below. Variables in these equations are defined as follows.

QT is the estimated flood quantile, in cubic feet per second, for the designated recurrence interval T, in years. For 

AMS, the T’s equal to 2, 5, 10, 25, 50, 100, and 500 years.

a is the coefficient of the equation, b, c, d, e, and f are exponents for variables TDA, MCS, PermAvg, BL, and 

(%Water+5), respectively.

TDA is the total drainage area, in square miles.

MCS is the main-channel slope, in feet per mile.

PermAvg is the averaged permeability of the watershed, in inches per hour.

BL is the basin length, in miles

(%Water +5) is the calculated percentage of open water and herbaceous wetland in the watershed plus a constant 

5 percent (to avoid zero values). The unit of the (%Water +5) term is percent.

RF(N) is the regional factor for hydrologic region N.

 QT = a(TDA)b (MCS)

c (PermAvg)e RF (N) [for hydrologic regions 1, 3, and 5]

 QT = a(TDA)b (MCS)

c (%Water + 5)f RF (N) [for hydrologic regions 2, 6, and 7]

 QT = a(TDA)b (MCS)c (BL)d [for hydrologic region 4]

Examples on how to use these equations at ungaged, gaged, and near gaged sites have been given. The accuracy 

of these equations, as measured with APE% (average prediction error of the regional equation), varies between 49 

percent and 50 percent for Q100 among the seven regions. The average equivalent years of record for the same T varies from 5.4 to 5.6 years.
The flood-frequency relations based on the PDS were developed using the Generalized Pareto distribution with 

an averaged 1.6 flood peaks per year. The same hydrologic regions and basin characteristics were used in developing the PDS regional equations. In the PDS regional equations, the TDA was the basic explanatory variable for all 

the regions. The regional equations and a listing of their corresponding coefficients for estimating flood quantiles 

for T’s equal to 0.8, 1.01, 1.5, 2, 3, and 5 years have been developed as follows. 

 QT = a(TDA)b (MCS)c (%Water + 5)f [for hydrologic region 1].

 QT = a(TDA)b (BL)c (PermAvg)e [for hydrologic region 2].

 QT = a(TDA)b (%Water + 5)f [for hydrologic region 3].

 QT = a(TDA)b (MCS)c (BL)d [for hydrologic region 4].

 QT = a(TDA)b (MCS)c (%Water + 5)f [for hydrologic regions 5, 6, and 7].

The accuracy of these equations, as measured with standard error of prediction, varies from 37.1 to 44.3 (percent) for Q1.5 among the seven regions, for example. The corresponding correlation between predicted and observed 

values, expressed as R2

, varies from 0.84 to 0.89.

The flood-frequency results represent the most up-to-date (2004) information available. However, the analysis 

identified the lack of information for small watersheds since the 1980’s. Although the flood-frequency analyses 

based on the AMS have been used for many years and have been widely used in Illinois and throughout the United 

States, flood-frequency analyses based on PDS will require additional study before they are as commonly utilized 

as the AMS. The developed regional equations are applicable to rural, natural streams in Illinois. The equations are 

not applicable where streamflows are altered appreciably. The regional equations should not be extrapolated beyond 

QT a b c d e f RF(1) RF(3) RF(5)

Regions 1, 3, 5

Q2 22.2 0.749 0.401 0 -0.224 0 1.467 1.62 2.128

Q5 34.1 .743 .437 0 -.223 0 1.563 1.811 2.360

Q10 41.8 .740 .457 0 -.224 0 1.618 1.913 2.476

Q25 50.8 .738 .478 0 -.224 0 1.686 2.03 2.612

Q50 57.0 .737 .491 0 -.223 0 1.738 2.113 2.711

Q100 62.7 .736 .503 0 -.222 0 1.79 2.192 2.809

Q500 74.5 .735 .527 0 -.219 0 1.917 2.371 3.037

Regions 2, 6, 7 RF(2) RF(6) RF(7)

Q2 54.7 0.728 0.341 0 0 -0.47 1 2.963 3.515

Q5 94 .721 .374 0 0 -.527 1 3.119 3.281

Q10 120 .718 .393 0 0 -.55 1 3.241 3.226

Q25 151 .716 .413 0 0 -.573 1 3.409 3.217

Q50 174 .715 .426 0 0 -.586 1 3.54 3.236

Q100 195 .714 .437 0 0 -.598 1 3.672 3.269

Q500 241 .714 .461 0 0 -.619 1 3.98 3.377

Region 4

Q2 49.3 0.734 0.37 -0.006 0 0 0 0 0

Q5 85.1 .772 .406 -.095 0 0 0 0 0

Q10 111 .792 .425 -.14 0 0 0 0 0

Q25 144 .812 .446 -.183 0 0 0 0 0

Q50 168 .823 .46 -.207 0 0 0 0 0

Q100 193 .833 .472 -.228 0 0 0 0 0

Q500 250 .852 .496 -.266 0 0 0 0 0

Summary and Conclusions 37
38 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

QT a b c d e f

Region 1

Q0.8 33.3 0.771 0.438 0 0 -0.4

Q1.01 52.6 .755 .458 0 0 -.515

Q1.5 83.9 .745 .478 0 0 -.621

Q2 107 .74 .488 0 0 -.673

Q3 140.8 .736 .498 0 0 -.733

Q5 185.9 .732 .508 0 0 -.793

Region 2

Q0.8 17.9 0.775 0 0.223 -0.499 0

Q1.01 23.7 .772 0 .185 -.47 0

Q1.5 32.8 .769 0 .152 -.448 0

Q2 39.5 .766 0 .138 -.438 0

Q3 49.1 .762 0 .125 -.428 0

Q5 61.8 .755 0 .114 -.417 0

Region 3

Q0.8 131 0.672 0 0 0 -0.456

Q1.01 207.1 .645 0 0 0 -.524

Q1.5 336.9 .624 0 0 0 -.593

Q2 434.5 .615 0 0 0 -.629

Q3 580.4 .607 0 0 0 -.671

Q5 777.7 .6 0 0 0 -.716

Region 4

Q0.8 60.3 0.907 0.386 -0.463 0 0

Q1.01 78.7 .91 .403 -.503 0 0

Q1.5 104.9 .916 .431 -.54 0 0

Q2 121.6 .919 .447 -.553 0 0

Q3 142.3 .92 .465 -.562 0 0

Q5 164.1 .92 .484 -.561 0 0

Region 5

Q0.8 69.7 0.776 0.383 0 0 -0.397

Q1.01 101.7 .759 .405 0 0 -.472

Q1.5 151.4 .747 .436 0 0 -.549

Q2 186.6 .742 .453 0 0 -.588

Q3 236.6 .738 .474 0 0 -.633

Q5 300.9 .736 .494 0 0 -.682

Regions 6 and 7

Q0.8 72 0.802 0.383 0 0 -0.397

Q1.01 87.4 .822 .405 0 0 -.472

Q1.5 105 .854 .436 0 0 -.549

Q2 115.2 .874 .453 0 0 -.588

Q3 128.2 .898 .474 0 0 -.633

Q5 142.9 .926 .494 0 0 -.682
the range of selected explanatory variables in each equation. The conditions used in developing these results and 

equations and their limitations are explained, and the techniques should be used with caution. The equations are not 

designed for evaluating the effects of land-use changes within a watershed. Also, the equations estimate the mean 

values of flood quantiles of different basins in the hydrologic region with the same set of explanatory variables. 

Engineering judgment should be used in determining the applicability of these results to a given situation.

ACKNOWLEDGMENTS

Many USGS personnel assisted in this investigation. William Kirby and Gary Tasker (Reston, Va.) provided 

technical assistance on the AMS and PDS analysis, Katherine Flynn (Reston, Va.) on USGS program applications, 

and David Eash (Iowa City, Iowa) on determining basin characteristics. The AMS data from adjacent States were 

provided by William Krug (Madison, Wis.), David Eash (Iowa City, Iowa), Loyd Waite (Rolla, Mo.), Glenn Hodgkins (Augusta, Maine; provided data for Kentucky), Michael Griffin (Louisville, Ky.), and Donald Arvin (Indianapolis, Ind.). Thomas Over (Urbana, Ill.) was helpful in providing technical discussions throughout the study. Michael 

Friedel and Terri Arnold (Urbana, Ill.) conducted the kriging analysis. Angel Martin (Urbana, Ill.) completed the 

editorial review of the report.

The study of PDS in Illinois initially was proposed by Donald Roseboom (USGS, Urbana, Ill.), and formerly of 

the Illinois Department of Natural Resources (IDNR) State Water Survey (ISWS). James Angel and Vernon Knapp 

(ISWS) discussed the rainfall analysis and hydrologic regions in Illinois, respectively. Matthew Hoffman, Lisa Pun, 

Katarina Jegdic, Luke Lim, Nils Oberg, Bharath Krishnan and Megan Jupin, students from the University of Illinois 

at Urbana-Champaign, and Michael Barnes of Parkland College (part-time USGS employees) worked on various 

aspects of the study. 

Arlan Juhl (IDNR), Vernon Knapp, and John Atkins (USGS, Charleston, W. Va.) performed technical reviews, 

and William Kirby (USGS, Reston, Va.) and Timothy Cohn (USGS, Reston, Va.) reviewed specific technical aspects 

of the report. Their generosity in conducting detailed reviews and providing suggestions is appreciated greatly. The 

first author also wishes to express his sincere appreciation to the late Professor Ben Chie Yen of the University of 

Illinois for his guidance and advice.

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44 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

GLOSSARY

Annual maximum flood – The highest instantaneous peak discharge in a water year.

Annual maximum series – A list of annual maximum floods.

Pearson’s Correlation coefficient – A measure of the strength of the linear association between two continuous 

variables.

Equivalent years of record – A measure of the accuracy with which the regression model can estimate the QT at a 

site, expressed in years of at-site streamflow record. It is an estimate the years of record required at a site

in order to achieve an accuracy equivalent to the standard error of estimate (or prediction) of the regional 

equation.

Exceedance probability – Probability that a random event will exceed a specified magnitude in a given time period,

such as a water year or specified otherwise.

Frequency analysis – The estimation of how often a specified event will occur.

Flood Quantile – The flood-peak discharge magnitude corresponding to a specified exceedance probability (percent

quantile). Often it also is used with the corresponding recurrence interval, T; a T-year quantile. The symbol 

used in this report is QT .

Outlier – Data points that depart significantly from the trend of the remaining data. 

Partial duration series – A list of all instantaneous flood-peak discharges that exceed a specified threshold

discharge.

Population – In statistics, the entire collection of objects under consideration is called a population, or universe. 

The entire population often is not available, so a sample often is studied.

Recurrence Interval – The average time interval between actual occurrences of a hydrologic event of a given or 

greater magnitude. Also known as the return period. 

Water year – The 12-month period October 1 through September 30, during which streamflow data are collected,

compiled, and reported. The water year is designated by the calendar year in which it ends. For example, 

water year 1999 or the 1999 water year covers the period from October 1, 1998, to September 30, 1999.
 45

APPENDIXES AND TABLES
46 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois
Appendix 1. Data Preparation

Procedures for preparing peak-flow data in Illinois streams for at-site flood-frequency analysis are described in 

this appendix. The general guidelines for these preparation procedures can be found in Bulletin 17B (Interagency 

Advisory Committee on Water Data, 1982). Two types of flood series are used in the study: the annual maximum 

series (AMS) and partial duration series (PDS). The peak-flow data are instantaneous flood-peak discharge magnitudes retrieved from peak-flow files stored in the USGS National Water Information System (NWIS) that contain 

records of both continuous streamflow-gaging and crest-stage gages (CSG) stations. Data also can be obtained from 

the USGS Illinois Water Science Center annual water-data report, for example, Latour and others (1996), or from the 

Web page at URL http://il.water.usgs.gov/usgs. It is assumed that peak-flow data retrieved from NWIS are independent. When preparing flood-peak data for NWIS, three criteria in temporal space are specified for checking data 

independency (WRD Data Reports Preparation Guide: Novak, 1985, p. 93).

Preparation of Annual Maximum Series

Selection of stations

From streamflow-gaging stations that represent watersheds in Illinois or their flows drain into Illinois, 419 

stations, including active and inactive, rural and urban stations, that have 10 or more years of flood-peak discharge 

records are selected for at-site AMS analysis. Streamflow records that have been affected by substantial urbanization or other types of watershed changes, or channelization, diversion, or regulation, are excluded from the regional 

regression analysis. Those alterations modify flow characteristics and separate analysis is required. Whether the 

watershed changes or channel modifications have induced changes in streamflow characteristics is determined by 

field observation and noted in the NWIS with qualification codes. “Indices of Urbanization” (Sauer and others, 

1983, p. 7) also can be used to determine the degree of urbanization by users. Northeastern Illinois contains many 

urban streams. Regulated rivers include the Wabash River, Big Muddy River below Rend Lake, Kaskaskia River 

below Lake Shelbyville, Saline River below the mouth of Cypress Ditch, Fox River below the Chain-of-Lakes, Illinois River, Mississippi River, and Ohio River. 

Error checking

Possible copying, decimal, or code errors in the retrieved flood-peak discharge magnitudes are checked against 

the published USGS data reports for stations in Illinois starting from WY 1984 to WY 1999.

Missing or discontinuous records

Missing records (no reported values for a given year) are not filled. Discontinuous records (record consists 

of different time periods) are treated as one entity providing no major watershed changes affect the records (Interagency Advisory Committee on Water Data , 1982).

Historical events

Bulletin 17B (Interagency Advisory Committee on Water Data, 1982) recommended the inclusion of historical events in analysis providing that the reliability of the data, causes of the events (natural causes excluding ice 

jams, downstream constriction, backwater, and snow melt), and changes in watershed conditions are examined and 

the effects on the estimated frequency curve are evaluated. Bulletin 17B also provided procedures for computing a 

historically adjusted LP3 frequency curve. Historical adjustments can be bypassed (William Kirby, U.S. Geological 

Survey, written commun., 2003) if the following apply.

1. The systematic records are long.

2. There are no high outliers in the systematic record.

Appendix 1. Data Preparation 47
48 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

3. The magnitude and plotting position (1/(length of the historical period)) of the historical flood-peak discharge 

is consistent with the systematic-record frequency curve.

Historical events are reported with peak stages, date, and, in most cases, the estimated discharges in the 

retrieved peak-flow data. With the criteria in mind, historical events at streamflow-gaging stations with estimated 

discharge are included in at-site flood-frequency analysis in this study. Historical events without discharge values 

have been examined to determine whether the subsequent rating curve could be used for a reasonable estimate of 

discharge. Judgment is required to determine if adjustment at the high end of the rating curve can be verified with 

measured discharge(s) and if the rating can be extended to the stage of the historical flood. If a similar event is 

recorded at nearby stations, for example, such information also validates the use of the historical event at the station. 

Estimated flood-peak discharge magnitudes that were used in the frequency analysis at the stations are shown in 

table 1-1.

Additional stations excluded because of urbanization

In addition to those urban stations identified by Curtis (1987), the following stations (table 1-2) are considered 

to be affected by urbanization during most of the period of record. This analysis is based on evaluation of USGS 

quadrangle topographic maps of different time periods and general knowledge of development in the Chicago metropolitan area.

Table 1-1. Stations with discharge estimated for their historical events in Illinois.

[Stage, stage above the gage datum, in feet; ft; Date of flood peak: in month/year format, m/yr, except for station 05592050, where only the year is presented; 

Est Q, estimated discharge, in cubic feet per second, ft3/s; Station numbers are referred to in figure 2A]

Station 

(fig. 2A)

Stage 

(ft)

Date

(m/yr)

Est Q

(ft 3/s)

Station 

(fig. 2A)

Stage 

(ft)

Date

(m/yr)

Est Q

(ft 3/s)

03337500 13.50 5/33 4,000 05467500 23.20 6/24 11,000

03338000 18.60 3/39 20,500 05468000 15.70 5/35 2,600

03338500 24.40 3/39 41,000 05468500 18.10 6/24 12,000

03378900 35.50 1/50 42,800 05469000 27.80 6/24 20,000

05439500 11.90 3/37 12,500 05469500 19.00 6/24 7,000

05441000 15.90 2/38 11,000 05502040 17.93 8/39 24,000

05441500 18.70 2/37 47,000 05512500 18.40 9/26 35,000

05444000 19.60 6/38 6,800 05513000 19.50 8/16 24,000

05445500 17.74 6/37 6,500 05580500 17.40 7/29 14,000

05448000 7.40 6/36 4,500 05592050 16.80 1957 26,400

Table 1-2. Additional stations compared to Curtis (1987) determined to be affected by urbanization in Illinois.

Station number

(fig. 2B) Station name Recordlength Explanation

05529000 Des Plaines River near Des Plaines, Ill. 1938-99 Most of the watershed has been urbanized for more than 

half the period of record.

05536190 Hart Ditch at Munster, Ind.. 1943-99 Has been urbanized

05540110 Ferry Creek at Warrenville, Ill. 1961-79 Eastern Du Page County urbanized between early 1960’s and mid-1970’s as evidenced by revisions to 

topographic maps of the area

05540140 East Branch Du Page River nr Bloomingdale, Ill. 1961-79 Eastern Du Page County urbanized between early 1960’s and mid-1970’s as evidenced by revisions to 

topographic maps of the area.

05550450 Poplar Creek near Ontarioville, Ill. 1961-77 Cook County urbanized in early 1960’s and mid-1970’s as evidenced by revisions to topographic maps of the 

area
Streamflow records from urbanized watersheds

Some urban stations may have sufficient length of record before urbanization or regulation that are suitable for 

regression analysis. Stations that were not used by Curtis (1987) because of anomalous characteristics were reevaluated for possible use for this study. The 25 stations with records used in the regression analysis are given in table 

1-3.

Appendix 1. Data Preparation 49

Table 1-3. Additional stations compared to Curtis (1987) included in the regression analysis for Illinois.

Station number

(figs. 2A and B) Station name RecordUsed Explanation

05447200 Normandy Ditch at Normandy, Ill. 1956-71 Rural drainage, only possible problem is the effects 

from channelization.

05528150 Indian Creek at Diamond Lake, Ill. 1960-76 Lake County, not urbanized until mid- to late-1970’s 

well into the period of record as evidenced by 

revisions to topographic maps of the area.

05528170 Diamond Lake Drain at Mundelein, Ill. 1961-76 Lake County, not urbanized until mid- to late-1970’s 

well into the period of record as evidenced by 

revisions to topographic maps of the area.

05528200 Hawthorn Drainage Ditch near Mundelein, Ill. 1961-76 Lake County, not urbanized until mid- to late-1970’s 

well into the period of record as evidenced by 

revisions to topographic maps of the area.

05528440 Buffalo Creek near Lake Zurich, Ill. 1961-76 Lake County not urbanized until mid- to late-1970’s 

well into the period of record as evidenced by 

revisions to topographic maps of the area. Trend 

analysis indicated significant trend but site kept in 

analysis because the beginning of record occurred 

during drought.

05528470 Buffalo Creek at Long Grove, Ill. 1961-76 Lake County not urbanized until mid- to late-1970’s 

well into the period of record as evidenced by 

revisions to topographic maps of the area. Trend 

analysis indicated significant trend but site kept 

in the analysis because the beginning of record 

occurred during drought.

05533200 Sawmill Creek Tributary near Tiedtville, Ill. 1961-79 Only one subdivision in the drainage basin shown 

on topographic map that dates back to around the 

beginning of the period of record. Trend analysis 

indicated significant trend but site kept in the 

analysis because the beginning of record occurred 

during drought.

05533300 Wards Creek near Woodridge, Ill. 1962-76 Only one housing area around a lake in the upper 

part of basin, as shown by topographic map that 

dates back to the beginning of the period of record. 

Trend analysis indicated significant trend but 

site kept in the analysis because the beginning of 

record occurred during drought.

05534400 North Branch Chicago River at Bannockburn, Ill. 1960-76 Lake County, not urbanized until mid- to late-1970’s 

well into the period of record as evidenced by 

revisions to topographic maps of the area.

05536325 Little Calumet River at Harvey, Ill. 1917-33 Assumed this area of southern Cook County not 

urbanized appreciably before 1933.

05539870 West Branch Du Page River at Ontarioville, Ill. 1961-79 Western Du Page County, not urbanized until midto late-1970’s well into the period of record as 

evidenced by revisions to topographic maps of the 

area.

05539890 West Branch Du Page River near Wayne, Ill. 1961-79 Western Du Page County, not urbanized until midto late-1970’s well into the period of record as 

evidenced by revisions to topographic maps of the 

area.

05539950 Klein Creek at Carol Stream, Ill. 1961-79 From topographic map that dates to the beginning of 

the period of record, only one housing subdivision 

present just upstream of station.
50 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Stations where basin characteristics could not be derived

When applying the BASINSOFT program to determine the specified basin characteristics, one or more basic 

basin characteristics at nine stations listed below (table 1-4) could not be determined because the drainage areas were 

too small (ranging from 0.08 to 1.10 mi2) to contain enough hypsographic and hydrographic features for computations. Thus, these stations are not included in the regression analysis.

Therefore, 291 stations initially were used in the regression analysis. During the analysis (appendix 7), three 

stations were excluded because of anomalous results. The final regional equations were developed using records 

from 288 streamflow-gaging stations.

Table 1-3. Additional stations compared to Curtis (1987) included in the regression analysis for Illinois--Continued.

Station number

(figs. 2A and B) Station name RecordUsed Explanation

05540030 West Br Du Page River at West Chicago, Ill. 1961-79 Western Du Page County, not urbanized until midto late-1970’s well into the period of record as 

evidenced by revisions to topographic maps of the 

area.

05540240 Prentiss Creek near Lisle, Ill. 1961-80 From topographic map that dates to the beginning 

of the period of record, only about 20 percent of 

drainage urbanized..

05549000 Boone Creek near Mc Henry, Ill. 1949-92 McHenry County, not urbanized until late-1980’s 

near end of period of record, no trend in peak 

discharge observed.

05549850 Flint Creek near Fox River Grove, Ill. 1962-96 McHenry County, not urbanized until late-1980’s 

near the end of the period of record, no trend in 

peak discharge observed.

05550470 Poplar Creek Tributary near Bartlett, Ill. 1961-79 From topographic map that dates to the beginning 

of the period of record, only about 25 percent of 

drainage urbanized.

05580700 Salt Creek Tributary at Middletown, Ill. 1961-76 Rural drainage, no perceived regulation or diversion.

05589780 Little Canteen Creek Tributary near Collinsville, Ill. 1959-72 From topographic map, one reservoir regulates only 

about 10 percent of drainage with other small 

ponds on various side channels.

05592000 Kaskaskia River at Shelbyville, Ill. 1908-99 The pre-dam period of record (1908-68) can be used. 

Regulated since 1969 by Lake Shelbyville. 

05592500 Kaskaskia River at Vandalia, Ill. 1908-99 The pre-dam period of record (1908-68) can be used. 

Regulated since 1969 by Lake Shelbyville.

05593000 Kaskaskia River at Carlyle, Ill. 1908-99 The pre-dam period of record (1908-67) can be used. 

Regulated since 1968 by Carlyle Lake.

05597450 Crab Orchard Creek Tributary near Pittsburg, Ill. 1960-72 Rural drainage, only problem may be the effects of 

earthen dam upstream of gage.

05599580 Big Muddy River Tributary near Gorham, Ill. 1961-76 Rural drainage, only problem may be the effects of 

steep channel slope and heavily forested drainage.

Table 1-4. Stations where basin characteristics could not be derived in Illinois.

Station number

(fig. 2A) Station Name Station number (fig. 2A) Station Name

03344250 Embarras River Tributary near Greenup, Ill. 05584450 Wigwam Hollow Creek nr Macomb, Ill.

03380300 Dums Creek Tributary near Iuka, Ill. 05586850 Bear Creek Tributary near Reeders, Ill.

03382520 Black Branch Tributary near Junction, Ill. 05587850 Cahokia Creek Tributary Number 2 nr Carpenter, Ill.

03612200 Q Ditch Tributary near Choat, Ill. 05592700 Hurricane Creek Tributary near Witt, Ill.

05572100 Wildcat Creek Tributary near Monticello, Ill.
Stationarity in annual maximum series of selected rural watershed – trend analysis

In order to use the past flood-peak data to estimate future flood-peak magnitudes, the past flood-peak data are 

assumed to be random homogeneous events. Trends in streamflow data result if there are alterations in the watershed or apparent changes in climatic patterns. If a trend in an AMS is detected, it indicates that results from this 

AMS analysis would be derived from nonrandom samples, and uncertainty in the derived flood-frequency relation is 

increased. However, if randomness is the only deviation from other data assumptions, nonrandom data may define 

unbiased estimates of future flood activity (Interagency Advisory Committee on Water Data, 1982). McCabe and 

Wolock (2002) analyzed streamflow statistics for 400 sites in the conterminous United States with data measured 

during 1941-99. Their results indicate a noticeable increase in annual minimum and median daily streamflow around 

1970, and a less noticeable mixed pattern of increase or decrease in annual maximum daily streamflow. These 

changes in annual streamflow statistics primarily occurred in the eastern U.S. Knapp and Markus (2003) analyzed 

records at 59 streamflow-gaging stations in Illinois with more than 50 years of data and found that more than half of 

the 48 selected stations showed significant positive trends in average flow, and roughly 25 percent showed positive 

trends in instantaneous flood-peak discharges. 

Trends in AMS series were analyzed by using Kendall’s tau (τ) analysis in the SWSTAT program (Lumb and 

others, 1990). A τ value (correlation coefficient) of zero indicates no trend; positive or negative τ values indicate 

positive or negative trends. Whether a trend is significant depends not only on the τ value but also on the sample 

size. Among the rural watersheds used in the AMS analysis, 62 streamflow-gaging station records are found to have 

significant trends within the 95-percent confidence level (p ≤ 0.05); 12 station records have negative trends and 50 

station records have positive trends. The range of absolute τ values is from 0.18 to 0.82. Among the stations with 

high τ values (either negative or positive), the majority have record lengths between 10 and 25 years, contain records 

collected from the 1950’s to 1980’s, and are inactive stations. The mid-1950’s was one of the driest periods in the 

conterminous U.S. and years after 1970 generally were wetter than average (McCabe and Wolock, 2002). Drought 

periods at either the beginning or end of the records result in trends for these stations. Therefore, these streamflowgaging station records are retained in the regression analysis.

Preparation of Partial Duration Series

Source of data

The PDS data also are retrieved from the peak-flow files. Secondary instantaneous flood-peak discharges and 

the associated stages above a selected base discharge are available if the flow above the station is not appreciably 

regulated. The base discharge generally is selected such that, on average, three independent flood-peak discharges, 

including the annual maximum peak discharge, will exceed the base discharge each water year. Criteria for deciding 

which streamflow-gaging stations that the secondary peaks are determined, for selecting the base discharge at the 

selected streamflow-gaging station, and for selecting the independent peaks greater than the base discharge are given 

in Novak (1985). With the availability of secondary peaks, PDS data are organized for 241 stations.

The Illinois Water Science Center maintains unit-value (or gage-value) files stored in the NWIS database. The 

unit-values are those recorded, transmitted, and/or computed from a streamflow-gaging station. Typical record intervals are 5, 15, or 30 minutes. These continuous hydrographs can be used to develop the PDS. However, at the time 

of this study, the unit-value files had been developed for 64 stations only with record starting in 1985. Samples of 

flood-peak discharge magnitudes are used to check the quality of the PDS data only.

Threshold value

Selecting an appropriate threshold value is a challenging task in practical use of the PDS analysis. Cunnane 

(1989) illustrated how selected threshold values affected the structure of flood series and, therefore, the fitted distributions. If too small a threshold is used, clusters of peak discharges of similar magnitudes are included in the PDS 

model. The threshold value can be the mean annual flood-peak discharge of a station, or generally it can be determined that, on average, three flood-peak discharges, including the annual maximum peak discharge a year will be 

reported (Novak, 1985).

Appendix 1. Data Preparation 51
52 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Error checking

The PDS data retrieved from peak-flow files are checked for recording errors and compared with published 

reports by Carns (1973), USGS water-data reports, and the unit-value files. Because of time constraints, the PDS 

data only are checked at randomly selected sites. Some differences in magnitudes were found in comparison to the 

unit-value files. The differences are minor and may be explained by the time interval used in the unit-value files. 

Duplicated events were checked for the entire PDS dataset. 

Available stations 

During at-site analysis (appendix 2), some stations were discarded because their flood-peak discharge magnitudes were clustered in a narrow range. Statistical parameters estimated from such datasets could not be used to fit 

the selected probability distributions. In all, a total of 222 stations are fitted with the Generalized Pareto distribution 

(appendix 2) and at-site flood quantiles are computed. Of the 222 stations, 142 rural-watershed stations are used in 

regression analysis. 

Possible future work

The present PDS analysis focuses on estimation at the lower recurrence intervals, those intervals from less than 

1 year to 5 years. Knowledge about the more frequent floods is needed in environmental studies for the protection 

and restoration of channels, aquatic habitat, and floodplains, as well as fishery management. If proven applicable, 

the PDS analysis could enhance the applications of flood-frequency analysis. The PDS model also can be used for 

estimating flood magnitudes at larger recurrence intervals (as those estimated with the AMS model), which could be 

beneficial for flood predictions at stations with short periods of record (for example, from 5 to 10 years) but with a 

well-represented range of peak discharges. 

The statistical significance of missing data and/or mixed data (both AMS and PDS) in the PDS dataset are not 

examined in detail in this study. Representative data for smaller flood-peak discharges are retained for analysis if a 

proper average number of peaks per year (r-value) is selected. On the other hand, the r-value will not affect samples 

from large flood magnitudes. Missing high flood-peak discharge magnitudes from these samples affect the ranking 

of all samples and the data structure. 

In developing possible future PDS analysis, the unit value could be a better data source than peak-flow files. 

Appreciably more streamflow-gaging stations than used in this study would be required for developing future PDS 

regional equations.

References

Carns, J.M., 1973, Magnitude and frequency of floods in Illinois: Illinois Department of Transportation, Division of Water 

Resources Management, 599 p.

Cunnane, C., 1989, Statistical distributions for flood-frequency analysis: World Meteorological Organization, Operational 

Hydrology Report No. 33. Secretariat of the World Meteorological Organization–No. 718, Geneva, Switzerland, 61 p. plus 

appendixes. 

Curtis, G.W., 1987, Technique for estimating flood-peak discharges and frequencies on rural streams in Illinois: U.S. Geological 

Survey Water-Resources Investigations Report 87-4207, 79 p.

Interagency Advisory Committee on Water Data, 1982, Guidelines for determining flood flow frequency, Bulletin 17-B of the 

Hydrology Subcommittee: Reston, Virginia, U.S. Geological Survey, Office of Water Data Coordination, 183 p. [Available 

from National Technical Information Service, Springfield, Va. 22161, as report no. PB 86 157 278. Also available in PDF 

format (28 Mbyte file size) from the Federal Emergency Management Agency Web site http://www.fema.gov/mit/tsd/dl_flow.

htm]

LaTour, J.K., Maurer, J.C., and Wicker, T.L., 1996, Water resources data, Illinois water year 1996—Volume 1, Illinois except 

Illinois River Basin: U.S. Geological Survey Water-Data Report IL-96-1, 247 p.
Lumb, A.M., Kittle, J.L., Jr., and Flynn, K.M., 1990, Users manual for ANNIE, a computer program for interactive hydrologic 

analyses and data management: U.S. Geological Survey Water-Resources Investigations Report 89-4080, 236 p.

Knapp, H.V., and Markus, M., 2003, Evaluation of the Illinois streamflow gaging network: Illinois State Water Survey, Watershed Science Section, Contract Report 2005-05, 97 p.

McCabe, G.J., and Wolock, D.M., 2002, A step increase in streamflow in the conterminous United States: Geophysical Research 

Letter, Vol. 29, No. 24, 2185, p. 38-1 to 38-4.

Novak, C.E., 1985, Water Resources Division data report preparation guide: U.S. Geological Survey Water-Resources Division,199 p. plus appendixes.

Sauer, V.B., Thomas, W.O., Stricker, V.A., and Wilson, K.V., 1983, Flood characteristics of urban watersheds in the United 

States: U.S. Geological Survey Water-Supply Paper 2207, 63 p. 

Appendix 1. Data Preparation 53
54 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Appendix 2. At-Site Analysis of Flood-Peak Series

Annual Maximum Series

Flood quantiles for the AMS model are estimated with the PEAKFQ program (Thomas and others, 1998) in 

which the LP3 distribution is used. In addition, the generalized skew coefficients for Illinois are analyzed and 

updated (appendix 3). The input and output files for the PEAKFQ program and probability plots of the fit at each 

streamflow-gaging station are included in the attached CD-ROM (appendix 8). 

Partial Duration Series

PDS analyses are conducted with a Fortran program developed for this study. The PEAKFQ program can be 

used to analyze the PDS data file with minor modifications on data code in the input file. However, the analysis of 

PDS data is then limited to the LP3 distribution if the PEAKFQ program is selected. Also, evaluation of parameter 

r, the average number of peaks per year, and presentation of the results at selected recurrence intervals have to be 

done outside of the PEAKFQ program.

A literature search indicated that three probability distributions are suitable for analyzing the PDS data: the 

Gumbel, exponential, and Generalized Pareto (GP) distributions. The GP distribution is the logical choice for modeling flood magnitudes that exceed a fixed threshold (Hosking and Wallis, 1987) where it can be assumed reasonably 

that successive flood flows follow a Poisson process with independent magnitudes (Rao and Hamed, 2000). These 

three distributions, and later the LP3 distribution, are included in the program for comparative evaluations. All 

statistical parameters are estimated with the method of moments (Rao and Hamed, 2000). However, the program 

does not include, at present (2004), the detection of outliers or the adjustment for historical events. With the study 

emphasis on analyzing the lower flood quantiles, it was found that both GP and LP3 distributions can reasonably fit 

the PDS data, except that probability plots showed the GP distribution fitted the data better, probably because the 

sample skew (the weighted skew approach is not used at present) may not represent the data distribution. The tests 

performed in selecting a suitable probability distribution for the Illinois data and the parameter r are described below. 

The GP distribution was selected in this study for estimating at-site flood quantiles with r = 1.6. A brief description 

of the GP distribution is presented first.

Generalized Pareto distribution function

The GP is a three-parameter Wakeby distribution. The three parameters are: α, a scale parameter (standard 

deviation of the GP distribution); ε, the lower bound of the data sample; and k, the shape parameter (kurtosis of the 

GP distribution). The following description of the GP distribution is cited from Rao and Hamed (2000).

The probability density function of the GP distribution can be written as

 (2-1)

where the variable x is either ε ≤ x < ∞ when k ≤ 0, or ε ≤ x ≤ ε + α/k when k > 0. This distribution function refers to 

the population of PDS flood events. The special case, k = 0, yields the exponential distribution, whereas k = 1 yields 

the uniform distribution on [ε,ε+α]. The parameters in equation 2-1 are estimated from samples using the method of 

moments as

 , (2-2)

 (2-3)

f(x) = 1

 

1 – k (x – ε)

 1 –1

 – – , α α 

(k )

α = m2 (1 + k)

2 (1 + 2k)

 1

 

^ 2 ^ ^

ε = m1 – α / (1 + k) , ^ ^ ^  ́
where the estimated quantities are expressed with a symbol ^, m ́r is the rth moment about origin and mr is the rth central moment (the moment about the mean) calculated from samples. The moments are estimated as

 , (2-4)

 (2-5)

 (2-6)

The k is solved numerically using the relation of the coefficient of skewness Cs as

 , (2-7)

with the Cs approximated by

 (2-8)

The flood quantile, QT, for a given recurrence interval T (see note below) is computed by

 (2-9)

or when using the frequency factor method, the frequency factor is obtained as

 . (2-10)

Equation 2-9 comes from inversion of the cumulative distribution function of the GP distribution that is solving 

for the flood quantile with an exceedance probability, P, where P = 1–F(x). For PDS, the recurrence interval, T, in 

units of years, is referred to in that there are r events per year, on average. Hence, the recurrence interval of a QT for 

PDS is approximated by dividing the annual T corresponding to equation 2-10 with the average events per year, r.

 . (2-11)

Thus, for a PDS recurrence interval of T years, the annual event probability is P = 1/(rT), or one in rT events 

(William Kirby, U.S. Geological Survey, written commun., 2003). For example, if the T calculated in equation 2-10 

is for an annual T of 3.2 years, then the QT calculated is for the 2-year event for the PDS with r = 1.6.

Appendix 2. At-Site Analysis of Flood-Peak Series 55

T = ——— = — (in years) r rP

 1

—P 1 annual

event ( )

m1 = 1 xi = x n  ́ n

 Σi=1

KT = (1 + 2k)

1

 (1 + k)(1 – T –k) – k 2

———– k

Cs = 2(1 – k)(1 + 2k)

1

 

2

 

——————–

 (1 + 3k)

m^ 2 = 1 (xi – x)2, and n–1

n

 Σi=1

m^ 3 = n (xi – x)

3

 . (n – 1)(n – 2)

n

 Σi=1

QT = ε + α (1 – T –k) , k

^ ^

^

^

Cs =

 m3 

 —— .

 m2

3/2

^

^

^
56 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Tests Performed

Average number of flood peaks per year, r

The r is calculated from the sorted PDS peaks (in descending order) by dividing the total number of PDS peaks 

retained with the number of years of record. The larger the value of r, the more lower flood peaks are included in the 

analysis. When r = 3, where 3 has been the criterion specified in the retrieval of the PDS data from peak-flow files 

(see appendix 1), it is near the critical value that could render the assumptions of the PDS model invalid (Cunnane, 

1989). The proper value for parameter r needs to be tested for the Illinois data. Note that an annual exceedance 

series (AES) is a special case of PDS with the number of peaks equals the number of years (thus, r = 1). The meaning of “annual” indicates “one”, on average, not one peak for each year. 

Because the Weibull formula (Chow, 1964b) is used to approximate the recurrence intervals of systematic data, 

the Weibull formula for TPDS can be written as

 TPDS = N + 1 , (2-12)

 nr

where N is the number of events and n is the rank of an event in ascending order. Raising or lowering the r-value 

within an allowable range increases or decreases the sample points in the flood series; therefore, this increase or 

decrease affects the sample variance and the frequency structure of the samples and, therefore, the fit of distributions. Cunnane (1989) discussed the values of r = 1.65, 1.8, 1.9 in the context of efficiency of the PDS model for 

estimating QT’s. When the station data are representative, minor variations in the r-value would not affect appreciably the flood quantiles. However, for stations with less representative records (missing data or mixed types, appendix 1), the r-value could affect the flood quantiles or the success of data fitting. The r-value may be better determined for individual stations; however, the data processing for both input and outputs become more involved and 

requires re-evaluation as data are incorporated into the model. In this study, one r is applied to all the stations. 

Various r-values, ranging from 1 to 2 with an increment of 0.2 have been examined during the study in various 

tests such as that illustrated in figure 2-1 (r = 1.6). The model fit in the moment ratio diagram is not sensitive to the 

differences in the r-values.

Figure 2-1. Cs-Ck moment ratio diagram for the partial duration series of flood-peak discharges in Illinois streams.
Moment ratio diagrams 

For a given probability distribution, a theoretical relation between statistical parameters, such as Cs, the skew 

coefficient, and Ck, the kurtosis coefficient, can be developed using the conventional moments. By plotting the 

theoretical curves and sample values, a visual means to evaluate their agreement can be derived; figure 2-1 is developed for this purpose. In figure 2-1, coefficients Cs and Ck are computed from all PDS station (active and inactive 

stations, urban and rural watersheds) and the six theoretical distribution curves are obtained from Rao and Hamed 

(2000, p. 35), where the Gumbel and exponential distributions are represented by point values because they are twoparameter functions. The theoretical values are accurate for Ck < 40. 

The calculated station Cks are less than 30 (fig. 2-1); and the station values are best fit by the Gamma (Pearson 

3) distribution followed by the GP distribution. The PDS data used in this figure are re-sampled for r = 1.6.

Chi Square and Kolmogorov-Smirnov tests

Most of the methods available for distribution selection from small sample sizes are not sensitive enough to 

discriminate among distributions (Rao and Hamed, 2000). In an attempt to examine the suitability between GP

and LP3 distributions for PDS data in Illinois, two common and well-known statistical tests for the goodness of fit 

are applied. These are the Chi-Square and Kolmogorov-Smirnov tests (Kite, 1977; Davis, 1973; and others). The 

tests are conducted using a 10-interval classification. At the 10-percent significance level, the null hypothesis that 

the station is a GP (or LP3) distribution is rejected if the sample results are larger than the respective critical values. 

For example, for approximately 100 sample points for a station, the critical Chi-Square value is 14.68 (Davis, 1973) 

and the Dcritical for the Kolmogorov-Simirnov test is 0.12067. A range of derived Dcritical values are available (Lyon 

Research Center for Images and Intelligent Information Systems, 2002). The number of stations and test results 

are summarized in table 2-1. Note that all the 222 stations were used in the test; but not all data could be applied to 

the distribution test for a given r because the data structure changed with r. However, observing that more stations 

passed the Kolmogorov-Smirnov test than the Chi-Square test does not indicate that either test is more appropriate 

for the analysis, which is consistent with the findings of Rao and Hamed (2000). A possible explanation for this 

result is that the confidence limits of these two tests are large (Bedient and Huber, 1992), especially when smaller 

samples of data are used; thus, it could not be determined from application of these tests whether the data fit a particular distribution. For the present study, the tests help identify a proper range of r-values for the study. 

Appendix 2. At-Site Analysis of Flood-Peak Series 57

Table 2-1 Results of Chi-Square and Kolmogorov-Smirnov (K-S) tests on GP and LP3 distributions with given r-values for partial duration 

series (PDS) data for Illinois streams (*, all PDS data points are included).

Distribution tested 

with data re-sampled 

by given r-value

Number of stations 

that pass the analysis

Number of stations 

that pass the 

Chi-Square test

Percentage of 

passes of the 

Chi-Square test

Number of 

stations that 

pass the K-S test

Percentage of 

passes of the 

K-S test

LP3_1.0 173 85 49 145 84

LP3_1.2 171 74 43 138 81

LP3_1.4 163 73 44 134 82

LP3_1.6 157 65 41 129 82

LP3_1.8 147 54 37 115 78

LP3_2.0 132 48 36 98 74

LP3_all* 133 41 31 86 65

GP_1.0 171 109 64 137 80

GP_1.2 166 107 64 141 85

GP_1.4 165 104 63 137 83

GP_1.6 160 90 56 129 81

GP_1.8 150 78 52 113 75

GP_2.0 134 62 46 99 74

GP_all 134 51 38 84 63
58 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Comparison to the annual maximum series estimates and probability plots

It generally is accepted that the differences in estimated flood quantiles between the AMS and PDS models 

diminish after T of 10 years. Chow (1964a, b) showed that the recurrence interval TAES (r = 1) is related to TAMS

through the relation

 (2-13)

The relation is plotted in figure 2-2. Although the GP distribution with r = 1.6 is used in this study, the convergence of flood quantiles estimated by both PDS and AMS are observed in many datasets used in the study. 

Probability plots, such as the one shown in figures 2-3 and 2-4, are prepared for visual examination of the goodness-of-fit between sample data and the distribution. The PDS data from actual streamflow records are illustrated 

in figures 2-3 and 2-4. Station 03336500, Bluegrass Creek at Potomac, Ill., contains record from 1950 to 1982 but 

only with CSG data from 1972 to 1982. Station 05595820, Casey Fork at Mount Vernon, Ill., has complete record 

from 1986 to 1999. However, because floods of lower magnitudes cluster in the dataset of station 05595820, the fit 

is better at 03336500 than at 05595820. All probability plots done for this study are given in the attached CD-ROM 

(appendix 8). 

Figure 2-2. Relation between recurrence intervals of annual maximum series and annual exceedance series (Chow, 1964b). 

TAES = ln ( ) TAMS

 –1

 ——— .

 TAMS – 1
Appendix 2. At-Site Analysis of Flood-Peak Series 59

DISCHARGE, IN CUBIC FEET PER SECOND

RECURRENCE INTERVAL, IN YEARS

10,000

1,000

100

0.1 1 10 100 1,000

PDS DATA

PDS ESTIMATES BY GP

Figure 2-3. Probability plot of flood-peak discharge magnitudes estimated by Generalized Pareto (GP) distribution and partial duration 

series (PDS) data for Bluegrass Creek at Potomac, Ill., in Vermilion County (03336500). DISCHARGE, IN CUBIC FEET PER SECOND

RECURRENCE INTERVAL, IN YEARS

100,000

10,000

1,000

0.1 1 10 100 1,000

PDS DATA

PDS ESTIMATES BY GP

Figure 2-4. Probability plot of flood-peak discharge magnitudes estimated by Generalized Pareto (GP) distribution and partial duration 

series (PDS) data for Casey Fork at Mount Vernon, Ill., in Jefferson County (05595820).
60 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

References

Bedient, P.B., and Huber, W.C., 1992, Hydrology and floodplain analysis: Addison-Wesley Reading, Massachusetts, 692 p.

Chow, V.T., 1964a, Statistical and probability analysis of hydrologic data, Section 8-I, Handbook of Applied Hydrology, edited 

by V.T. Chow: McGraw-Hill, New York, p. 8-1 to 8-42.

Chow, V.T., 1964b, Editor-in-Chief, Handbook of Applied Hydrology: McGraw-Hill, New York, variously paginated.

Cunnane, C., 1989, Statistical distributions for flood-frequency analysis: World Meteorological Organization, Operational 

Hydrology Report No. 33. Secretariat of the World Meteorological Organization–No. 718, Geneva, Switzerland, 61 p. plus 

appendixes. 

Davis, J.C., 1973, Statistics and Data Analysis in Geology: John Wiley & Sons, New York, 646 p.

Hosking, J.R.M., and Wallis, J.R., 1987, Parameter and quantile estimation for the Generalized Pareto Distribution: Technometrics, 29(3), p. 339-349. Kirby, W.H., 1981, Annual flood-frequency analysis using U.S. Water Resources Council guidelines 

(program J407): U.S. Geological Survey Open-File Report 79-1336-I, WATSTORE User’s Guide, v. 4, chap. I, sec. C, 56 p.

Kite, G.W., 1977, Frequency and Risk Analysis in Hydrology: Water Resources Publications, Fort Collins, Colo., 224 p.

Lyon Research Center for Images and Intelligent Information Systems, 2002, Probabilities and Statistics: accessed February 18, 

2002, at URL http://rfv.insa-lyon.fr/~jolion/STAT/node158.htm

Rao, A.R., and Hamed, K.H., 2000, Flood-frequency analysis: CRS Press, Washington, D.C., 350 p. 

Thomas, W.O. Jr, Lumb, A.M., Flynn, K.M., and Kirby, W.H., 1998, Users manual for program PEAKFQ annual flood-frequency analysis using Bulletin 17B guidelines, 89 p.: accessed on March 13, 2001, at URL: http://water.usgs.gov/software/

peakfq.html
Appendix 3. Generalized Skew Coefficients for Illinois

The skew coefficient is used in determining the frequency factor in the LP3 distribution. This relation is 

depicted as in equation 3-1 (Chow, 1964a)

 QT = μ + Kσ , (3-1)

where QT is the flood quantile of recurrence interval T; μ and σ are population mean and standard deviation, respectively; and K is the frequency factor. When the population skew is estimated from station data, the sample skew 

(as estimated from available station data), GS, is sensitive to extreme events and it is difficult to obtain an accurate 

estimate from a small set of samples. The high bias and uncertainty in the skew coefficient cause greater uncertainty 

in the flood quantiles especially for those of larger T’s. Bulletin 17B (Interagency Advisory Committee on Water 

Data, 1982) recommends the use of a weighted skew, GW, to smooth out an erratic estimate of skew coefficients 

from insufficient samples. The weight is calculated with a generalized skew coefficient, G, as shown in equation 3-2 

(Interagency Advisory Committee on Water Data, 1982)

 (3-2)

where MSEG is the mean square error of generalized skew, and MSEGS is the mean square error of station skew. All 

computations are carried in log values. The station parameters GS and MSEGS are determined from station samples 

after the outliers and historical data are treated. Wallis and others (1974) showed that MSEGS is a function of record 

length and population skew. Depending on the record length and coverage of the peak-flow data, the procedure 

gives more weight to the generalized skew if the station record contains non-representative data. Oppositely, more 

weight is given to the station skew if the station records are long and/or covers sufficient wet/dry periods.

The current G and MSEG were developed in the 1970s (U.S. Water Resources Council, 1976). Conducting a 

detailed study of the G for the study region is recommended in Bulletin 17B. This appendix describes the update of 

skew coefficients for Illinois.

Procedures

Station selection

Streamflow-gaging stations that are located in Illinois and within 100 mi outside of the State line, have 25 or 

more years of record, and have drainage areas between 0.5 and 2,000 mi2 were used in determining the generalized skew coefficients. USGS offices in States adjacent to Illinois provided data for this analysis. After screening, 

15 stations from adjacent States were excluded, either because their data showed significant trend or because their 

drainage area could not be delineated and, therefore, basin centroid could not be determined, (one basin in Wisconsin 

and six basins in Missouri). Locations of the 372 stations used to determine the generalized skew coefficients are 

shown in figure 3-1.

Methodology

Three methods listed below, suggested in Bulletin 17B, are used for developing the generalized skew coefficients. 

1. Draw skew isolines on a map.

2. Develop a skew prediction equation by regression analysis with basin characteristics as explanatory variables.

3. Use the mean of station skew values in a region.

Appendix 3. Generalized Skew Coefficients for Illinois 61

GW = MSEG X GS + MSEGS X G –——————————— , MSEG + MSEGS
62 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Figure 3-1. Locations of basin centroids for streamflow-gaging stations in Illinois and adjacent States used in developing the generalized skew-coefficient map (fig. 4).
Oberg and Mades (1987) investigated the generalized skew coefficients for Illinois. The database for station 

skews in their report consisted of 730 stations with more than 10 years of record in Illinois and surrounding States 

as considered in the present study. Neither areal trends for drawing isolines of the skew coefficients nor reasonable regression equations could be identified, and the regional mean skew coefficient was derived in evaluating the 

effects on station estimates. Even with further analysis of the regional mean skew coefficient by grouping stations, 

they concluded that the mean skew approach only was slightly more accurate than the generalized skew coefficients 

(U.S. Water Resources Council, 1976) in the 30 stations tested. 

The regional mean value and regression analysis are evaluated with the 372 station data directly. However, the 

isoline map is developed with a kriging technique described in the next section.

Kriging technique

Kriging is a geostatistical method (Isaaks and Srivastava, 1989) that statistically determines optimal weights 

for values at unsampled locations based on spatial autocorrelation and the assumption that points closer together are 

more similar than those farther apart. Kriging compares the values of pairs of sampling points and considers the 

distance the points are from each other. The steps used in this study for kriging the station skew values include the 

following.

1. Determine skew values at the centroid of the watershed

2. Analyze station data

3. Define grid for kriging

4. Conduct semivariogram analysis

5. Determine isolines

The skew values among the 372 points of station data ranged from –1.483 to 1.716. A histogram indicated that 

these station skew coefficients were normally distributed, and trend plots did not indicate obvious global spatial 

trends. ArcMap Geostatistical Analyst (Environmental Systems Research Institute, 1998) was used to krige the skew 

values at the basin centroids. Kriged data then were input into Arc/Info as a lattice with a resolution of 500 m (1 m = 

3.28 ft) that was resampled to 25,000 m. Contours of skew with an interval of 0.1 were created with the LATTICECONTOUR command.

A uniform grid was used to estimate the skew coefficients. A spherical spatial model was fit to the station skew 

points based on a semivariogram where the difference in values squared and the distance that separates each pair of 

points is graphed. Modeling the semivariogram is a technique that defines the linear weighting functions to krige the 

grids and was performed prior to kriging. The best spatial model was determined to have a nugget1 of 0.14, a range 

of 150,000 m, a minor range of 260,400 m, a partial sill2 of 0.06, and 12 lags3 at 24,000 m (Johnston and others, 

2001). There was minor anisotropy4 so a search direction of 107 degrees was used. The search neighborhood was 

an ellipse with a major semi-axis of 150,000 m and a minor semi-axis of 260,400 m with four angular sectors. Five 

points were used to determine the prediction, with a minimum of two points required within one angular sector. The 

semivariogram plot for residuals of skew coefficient values in Illinois and adjacent States is shown in figure 3-2. 

Results and Comparisons

The generalized skew coefficients obtained by the three methods (skew isolines, skew prediction equation, and 

mean station skew) described previously are compared using the mean square error (MSE) of station skew coefficients and the generalized skew coefficients are derived from the three methods (table 3-1). The corresponding MSE

of the generalized skew map (U.S. Water Resources Council, 1976) is shown in table 3-1. 

Appendix 3. Generalized Skew Coefficients for Illinois 63

1

 The nugget is a measure of error at distances smaller than the sampling interval and includes measurement and independent 

error, and micro-scale variation too fine to detect. At distances closer than the range, the points are considered to be

autocorrelated; beyond the range, there is no measurable correlation between points.

2

 The sill is equal to the variance among correlated points, in this case 0.10. The partial sill is the sill minus the nugget.

3

 A lag is the vector that spatially separates any two sample points and has both a direction and a distance.

4

 Anisotropy is a spatial trend that shows higher autocorrelation in one direction than in another.
64 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Mean value of station skews 

Mean value of station skews has the highest MSE in the updated data. Therefore, this approach was not used.

Regression analysis 

The multiple regression of station skew with three significant explanatory variables identified by stepwise 

regression analysis yields

 GREG = 3.115 + log10(BS).511 + log10(LAT)

-2.27 + log10(FOREST)-.622 , (3-3)

where GREG is the generalized skew based on regression, BS is the average basin slope, LAT is the latitude of at 

the basin centroid, and FOREST is the percentage of areas classified as forest in the drainage basin (see appendix 

5). The regression yielded R2 = 0.08, overall F = 4.06, and p = 0.008. These values indicate that the regression is 

Figure 3-2. Variogram used to krig estimates of generalized skew coefficients for Illinois.

Table 3-1. Comparisons of mean square errors of regional and Illinois skew coefficients.

[The mean square errors are determined based on two groups of stations: the 372 stations with 25 or more years of records and within 100 miles of the State line 

(region) and the 140 stations in basins that drain into Illinois only; N/A, not available.]

Method Mean square errors (dimensionless)

Region Illinois

Generalized skew coefficient map from 1976 0.245 0.307

Mean value of station skew coefficients .190 .245

Isoline map from kriging techniques .121 .140

Regression equation N/A1 .205

1

 The regression equation approach was not applied for the region because it would involve obtaining other ancillary data from neighboring States. This work 

is beyond the scope of this study.
slightly better than no regression, and the selected variables barely can explain any variance of the observed data 

from the mean. Low R2 also was reported by Eash (2001).

Isoline map approach 

The isoline map approach has the lowest MSE (0.14) and was used in the at-site frequency analysis. The MSE

value is lower than 0.302 that was derived using the entire U.S. skew map (Interagency Advisory Committee on 

Water Data, 1982, p. 13). The updated generalized skew coefficients for Illinois was determined using this method. 

A map of isolines of generalized skew coefficient for the State is given in figure 4 in the main text. 

References

Chow, V.T., 1964a, Statistical and probability analysis of hydrologic data, Section 8-I, Handbook of Applied Hydrology, edited 

by V.T. Chow: McGraw-Hill, New York, p. 8-1 to 8-42.

Eash, D.A., 2001, Techniques for estimating flood-frequency discharges for streams in Iowa: U.S. Geological Survey WaterResources Investigations Report 00-4233, 88 p.

Environmental Systems Research Institute (ESRI), 1998, ARC/INFO user’s guide, version 7.2.1: Redlands, California, Environmental Systems Research Institute, variously paginated.

Interagency Advisory Committee on Water Data, 1982, Guidelines for determining flood flow frequency, Bulletin 17-B of the 

Hydrology Subcommittee: Reston, Virginia, U.S. Geological Survey, Office of Water Data Coordination, 183 p. [Available 

from National Technical Information Service, Springfield, Va. 22161, as report no. PB 86 157 278. Also available in PDF format (28 Mbyte file size) from the Federal Emergency Management Agency Web site http://www.fema.gov/mit/tsd/dl_flow.htm]

Isaaks, E.H., and Srivastava, R.M., 1989, Applied Geostatistics: Oxford University Press, New York, N.Y., 561 p.

Johnston, K., Jer Hoef, J.M., Kricorachko, K., and Lucas, N. 2001, Using ArcGIS Geostatistical Analyst: Environmental Systems Research Institute, Relands, Calif., 300 p.

Oberg, K.A., and Mades, D.M., 1987, Estimating generalized skew of the Log-Pearson Type III distribution for annual peak 

floods in Illinois: U.S. Geological Survey Water-Resources Investigations Report 86-4008, 42 p.

U.S. Water Resources Council, 1976, Guidelines for determining flood flow frequency. Bulletin 17: U.S. Water Resources Council, Washington, D.C.

Wallis, J.R., Matalas, N.C., and Slack, J.R., 1974, Just a moment!: Water Resources Research, 10(2), p. 211-219. 

Appendix 3. Generalized Skew Coefficients for Illinois 65
66 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Appendix 4. Effects of Additional Flood Records and Updated Generalized Skew 

Coefficients on At-Site Flood Quantiles Based on Annual Maximum Series

Additional record length and better coverage of major flood event(s) could improve the statistical reliability of 

flood-frequency analysis. In this study, the generalized skew coefficients also are updated for Illinois (appendix 3). 

The effects of additional flood records (since WY 1986) and updated generalized skew coefficient on at-site AMS 

flood frequencies are examined. The evaluation first examines the new annual maximum flood records collected 

from WY 1986 to WY 1999 in terms of their temporal and spatial distributions, then compares the effects of additional flood records and updated generalized skew coefficients on at-site AMS flood frequencies for stations used in 

the regional analysis. 

Record Length

Update in record length

Curtis (1987) used 268 rural streamflow-gaging stations with data up to WY 1985 in the regional regression 

analysis. A total of 288 streamflow-gaging stations with data up to WY 1999 were used in the regional regression 

analysis for this study. Among the 268 stations used by Curtis (1987), additional flood records were available at 116 

stations between WY 1986 and WY 1999. Also, 10 rural-watershed stations became available for use in the regional 

flood-frequency analysis after 1985. The changes in record length for stations used by Curtis and the present study 

are examined by a comparison for the number of stations in the following three groups: records < 15 years, 15 years 

≤ records < 25 years, and records ≥ 25 years. The 15 and 25 years are selected arbitrarily for this analysis, but, in 

general, station statistics estimated with record length between 10 and 15 years are most sensitive to extreme events, 

and record length of 25 years has been used as a criterion in selecting stations for updating the regional skew coefficient. The comparison shown in table 4-1 indicates that more stations appeared in the ≥ 25 years group. Also, for the 

remaining two groups (records < 15 years, and 15 ≤ records < 25 years), additional flood records only were collected 

at 10 out of the 119 stations (table 4-1).

For the 10 new stations, 4 have records less than 15 years and the other 6 have records between 15- and 25- 

years long. However, these newly added stations might fall within the short-record group because of major floods 

in the 1990’s. Furthermore, if small watersheds are defined arbitrarily as those with drainage areas less than 5 mi2, 

83 of the 288 stations fall in this category. Among the 83 stations, 64 stations have less than 25 years of record; 18 

stations have 25 years of record, 1 with 26 years, and 1 with 45 years of record (Hurricane Creek near Roodhouse, 

Ill., 5586500). Data-collection programs for most small watersheds ended prior to 1980. Data collection at Hurricane Creek at Roodhouse ended in 1995. The lack of data for small watersheds since 1980 could result in biases in 

frequency estimates for small watersheds.

Record length of streamflow-gaging stations in hydrologic regions

Information concerning the record length of streamflow-gaging stations that are used in developing regional 

equations is presented in table 4-2. Such information is organized with four categories and presented for individual 

hydrologic regions. Category A is the number of stations that received new data since Curtis (1987), and categories 

Table 4-1. Comparisons of record length for stations used in the regional analysis of Curtis (1987) and this study in Illinois.

[<, less than; ≤, less than or equal to; ≥, greater than or equal to; yrs, years; WY, water year]

Studies Year ends Number of stations < 15 yrs 15 yrs ≤ x < 25 yrs ≥ 25 yrs

Curtis (1987) WY 1985 268 36 118 114

Present WY 1999 288 28 91 168
B, C, and D are the same as record-length groups discussed above. The number of streamflow-gaging stations in 

each hydrologic region available for regression analysis is obtained by summing the stations under record categories 

B, C, and D. With streamflow data up to WY 1999, it can be seen that all regions have the largest number of stations 

in category D (the ≥ 25 years group) except for region 2. In general, the update and distribution of station records 

are similar in each hydrologic region.

Major flood events in the additional streamflow records 

Between WY 1986 and WY 1999, large flood events occurred in various parts of Illinois. For the 116 stations 

with additional flood records, new maximums were established at 40 stations; 16 of the stations exceeded the Q100

and 1 station matched the Q100 estimated in 1987 (Curtis, 1987). 

Distribution of major flood events in the additional streamflow records

The number of events and the years recorded are reported in table 4-3.

Effects of Additional Flood Records and Updated Generalized Skew Coefficients on At-Site Flood 

Quantiles and Width of Confidence Intervals

Better estimate of T’s for infrequent events can be obtained with additional flood records thus improving the 

determination of QT -T relations. Additional flood records also help reduce the uncertainty in estimating the statistical parameters of the distribution. On the other hand, the generalized skew coefficient is used in the weighted skew 

approach to smooth out potential erratic estimates of skew coefficients from the systematic data (appendix 3). In 

this study, two estimated flood statistics are used for evaluating the effects of additional record length and updated 

generalized skew coefficient on the at-site AMS flood-frequency analysis; these statistics are the flood quantiles and 

Appendix 4. Effects of Additional Flood Records and Updated Generalized Skew Coefficients on At-Site Flood Quantile Based on Annual Maximum Series 67

Table 4-2. Spatial distributions of record length for stations used in the regional analysis of Curtis (1987) and this study in Illinois.

[<, less than; ≤, less than or equal to; ≥, greater than or equal to; Record category: A, number of stations with new data since water year 1986; B, number of 

stations in the < 15 years group; C, number stations in the 15 years ≤ records < 25 years group; D, number of stations in the ≥ 25 years group.]

Hydrologic Regions (fig. 5) Record Category

A B C D

1 12 2 6 18

2 24 7 36 29

3 31 5 21 49

4 20 3 11 34

5 21 5 10 23

6 4 4 7 9

7 4 2 0 6

Table 4-3. Distributions of major flood events in Illinois from water year 1986 to water year 1999.

Hydrologic Regions (fig. 5) Number of Events Water Year

1 2 1993, 99

2 12 1986, 91, 93, 94, 96, 97, 99

3 9 1990, 94, 96, 97

4 2 1993, 96

5 9 1990, 94, 95, 96

6 6 1990, 94, 96

7 0 None recorded
68 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

the width of confidence intervals. The evaluation consists of comparison of the selected parameters computed for 

the following three cases. 

A. Parameter estimates using data up to WY 1985 and weighted with the previous generalized skew coefficients 

(S1985_old_skew), 

B. Parameter estimates using data up to WY 1999 and weighted with the previous generalized skew coefficients 

(S1999_old_skew), and

C. Parameter estimates using data up to WY 1999 and weighted with the updated generalized skew coefficients 

(S1999_new_skew).

The effects of additional flood records and updated generalized skew coefficients are evaluated as follows. 

Letting S be the statistical parameter that either can be the magnitude of at-site flood quantiles or the width of confidence intervals, and ∆% be the change in S either because of additional records or updated regional skew coefficients, then ∆%, in percent, can be computed as

 Δ% = Ѕ1999_old_skew – Ѕ1985_old_skew X 100 , (4-1)

 ——————––————

 Ѕ1999_old_skew

which is an evaluation of the effect of additional flood records, or

					Δ% = Ѕ1999_old_skew – Ѕ1999_old_skew X 100 , (4-2)

 ————————––——

 Ѕ1999_old_skew

which is an evaluation of the effect of updated generalized skew coefficients. In the evaluations, Case B is used as 

the base conditions for comparison. Further, only the changes associated with 2- and 100-year recurrence intervals 

are evaluated in the present study. Therefore, S either could be the magnitudes or the width of confidence intervals 

for Q2 or Q100 at each station. After changes in S for all stations are computed, they are categorized according to 

whether they are increased (positive ∆% values), unchanged (zero ∆%), or decreased (negative ∆% values); and are 

organized by the hydrologic regions. Results for other recurrence intervals between 2 and 100 years could reasonably be expected to fall between the two T’s presented here.

Effects of additional flood records

The effects of additional flood records on percentage of changes of at-site flood frequencies and width of confidence intervals are presented in figure 4-1. In this figure, the x-axis is the seven hydrologic regions and the y-axis 

is the percentage of stations that fall in the increased, unchanged, and decreased categories. The percentage for each 

category is calculated by dividing the number of stations in “increased”, “unchanged”, or “decreased” category in a 

region by the total number of stations in that region. Note that only 126 of the 288 stations had additional flood data 

since WY 1986. For the purpose of presenting the comparison in a consistent basis when the updated skew is evaluated, all 288 stations are used in the presentation of figure 4-1. Therefore, besides the 126 stations with additional 

flood records, all other stations fall into the unchanged category.

In regions 2, 3, 4, 5, and 6, more stations have increased magnitudes of Q2 and Q100 than that of decreased 

because of the additional flood records. Region 1 has more stations with decreased Q2 but has more stations with 

increased Q100. Region 7, on the other hand, has decreased values of Q2 and Q100 for stations with additional flood 

record. The major floods in the 1990’s have caused increases in estimated Q100 for most of the regions. The addition of large flood events may result in changes in the station statistics; in particular, skew values may increase. The 

estimated QT’s at larger T’s (such as T = 100 year) will be larger for a positively skewed dataset and smaller for a 

negatively skewed dataset, when compared to a normally distributed dataset. The increases in flood-peak discharge 

magnitude at larger T’s potentially increase the overall slope of the frequency curves and result in decreases in lower 

flood quantiles for some stations.

The width of confidence interval decreases with additions in flood data and record length, as would be expected. 

All regions, except for region 6, have higher percentages of decreasing width of confidence interval for stations with 

additional flood records. The decreases in the width of confidence intervals indicate an improvement in the accuracy of the estimates in those regions. Regions 6 and 7 contain fewer stations with additional flood records than 
other regions since WY 1986 (table 4-2). Also, either fewer number of major storm events or no major storm event 

were recorded at stations in regions 6 and 7 than other regions since WY 1986. Data evaluation also indicates that 

increase in width of confidence interval is associated with positive changes in the skew coefficient.

Effects of updated generalized skew coefficients

The effects of updated generalized skew coefficients on the flood quantile and width of confidence intervals are 

presented in figure 4-2. Similar to the analysis conducted for additional flood records, records from the 288 stations are used in this evaluation. Note that equations 4-1 and 4-2 are designed to evaluate the deviations from the 

common basis (Case B). Therefore, a decreased value in figure 4-2 indicates an increase in the flood quantile or the 

width of confidence interval for Case C. With the updated generalized skew coefficients, almost all regions had a 

trend of decreases in magnitudes and width of confidence intervals for Q2, but a trend of increases magnitudes and 

width of confidence intervals for Q100. This result could be due to the range of flood-peak discharge data added to 

Appendix 4. Effects of Additional Flood Records and Updated Generalized Skew Coefficients on At-Site Flood Quantile Based on Annual Maximum Series 69

Figure 4-1. Changes in flood magnitudes and confidence intervals because of additional station records in Illinois. Percentage changes 

are calculated using equation 4-1; Q2 and Q100 are the peak-flood discharge with 2- and 100-year recurrence interval, respectively.
70 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

the flood files in the 1990’s. Besides observing the increased, unchanged, or decreased trend categories in the magnitudes and width of confidence intervals, the relative magnitudes in these trends are discussed in the next section.

Comparing the effects of additional flood records to updated skew coefficient

The range (minimum to maximum) of changes in the magnitudes and width of confidence intervals of Q2 and 

Q100, evaluated using equations 4-1 and 4-2, are listed in table 4-4 (minimum and maximum values are listed). It can 

be observed that the ranges in the “decrease in width of confidence interval” group are larger than the range in the 

“increase in width of confidence interval” group because of additional flood records. The ranges of changes resulting from additional flood records are larger than those resulting from updated skew coefficients. 

Figure 4-2. Changes in flood magnitudes and confidence intervals because of updated skew coefficients in Illinois. Percentage 

changes are calculated using equation 4-1; Q2 and Q100 are the peak-flood discharge with 2- and 100-year recurrence interval, respectively.
References

Curtis, G.W., 1987, Technique for estimating flood-peak discharges and frequencies on rural streams in Illinois: U.S. Geological 

Survey Water-Resources Investigations Report 87-4207, 79 p.

Appendix 4. Effects of Additional Flood Records and Updated Generalized Skew Coefficients on At-Site Flood Quantile Based on Annual Maximum Series 71

Table 4-4. Changes in Q2, Q100, and the width of confidence intervals at 2 and 100 years because of additional flood records and updated 

generalized skew coefficients for Illinois.

[∆Q2, changes in 2-year flood quantile, in percent; minimum, minimum value of the specified quantity from the 288 stations, in percent; maximum, maximum 

value of the selected quantity from the 288 stations, in percent; ∆Q100, changes in 100-year flood quantile, in percent; ∆ConF2, changes in the width of confidence interval for the 2-year flood quantile, in percent; ∆ConF100, changes in the width of confidence interval for the 100-year flood quantile, in percent]

∆Q2

minimum

∆Q2

maximum

∆Q100

minimum

∆Q100

maximum

∆ConF2

minimum

∆ConF2

maximum

∆ConF100

minimum

∆ConF100

maximum

Additional records -12.4 17.4 -24.3 47.7 -58.6 45.5 -99.8 67

Updated skew coefficient -4 5.7 -31.8 14.8 -8.3 11.1 -51.4 21.6
72 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Appendix 5. Determination of Basin Characteristics

Basin characteristics, including geometric and topographic parameters, soil variables, land uses, and rainfall 

intensities, have been used for interpreting flows from a watershed resulting from the rainfall-runoff processes. 

Determining and/or updating these basin characteristics require appreciable resources and are time consuming; therefore, only limited basin characteristics have been determined and used in previous flood-frequency studies in Illinois. 

Carns (1973), Curtis (1977a, b; 1987) used a maximum of nine explanatory variables from the basin characteristics 

in the development of regional regression equations for Illinois. Whether the regional regression equations could be 

improved by evaluating additional explanatory variables in basin characteristics has been a question. Computer programs, such as the BASINSOFT (Harvey and Eash, 1996), now provide an efficient and consistent way to determine 

basin characteristics in conjunction with the use of digital spatial data (Eash, 2001, 1993). For Illinois, spatial digital 

databases including a DEM, STATSGO (Natural Resources Conservation Service, 1993), NLCD (http://landcover.

usgs.gov/nationallandcover.html), and precipitation frequency (Huff and Angel, 1992) became available after Curtis 

(1987). Thus, determining a set of basin characteristics for Illinois using BASINSOFT and Arc/Info procedures on 

these digital databases is one of the objectives of this study, and these determined basin characteristics are used in the 

regression analysis. 

Methodology

BASINSOFT is a GIS computer program developed using the Arc/Info (Environmental System Research 

Institute, 1998) Arc Macro Language (AML). The BASINSOFT program used in this study is an internal USGS 

version (version 1.0, 2001) not publicly disseminated. For each drainage basin, four data layers, including drainage divide, stream-network, and two separate types of elevation data (contour and lattice), were used to calculate 

variables for input into equations used to quantify the basin-morphometric characteristics (see list below). All data 

layers were derived from available 1:100,000-scale digital data except in some small basins where it was necessary 

to manually digitize 1:24,000-scale topographic quadrangles in order to obtain enough features to allow BASINSOFT to run properly. It also was necessary to use 1:250,000-scale elevation data for various basins with drainages 

partially extending into Indiana where 1:100,000-scale digital data were not available. The 1:100,000-scale data 

sources were: 1) National Hydrography Dataset (NHD) for stream network data (http://www.nhd.gov/); 2) digital 

line graph (DLG) hypsography for elevation contour data (http://edc.usgs.gov/doc/edchome/ndcdb/ndcdb.html); and 

3) digital elevation model (DEM) derived from the 1:100,000-scale hypsography data using the Arc/Info command, 

TOPOGRID.

A complete list of BASINSOFT characteristics with definitions has been compiled by Harvey and Eash (1996). 

Definitions for selected explanatory variables pertinent to this study are given as follows. Harvey and Eash (1996) 

used CDA, contributing drainage area, in defining most of the morphometric characteristics. The CDA is the more 

hydrologically relevant variable than TDA (total drainage area) for surface-water-flow studies. However, a generally 

accepted method for determining CDA has not been defined, and TDA is measured and used instead of CDA in this 

Illinois study. This modification is reflected in the regional equations presented in the main text.

Basin-Morphometric Characteristics

TDA—Total drainage area, in square miles, includes all area within the drainage-basin boundary.

CDA—Contributing drainage area, in square miles, defined as the total area that contributes to surface runoff at the

basin outlet. By definition, CDA is a portion of TDA as computed by CDA = TDA – NCDA, where NCDA

is the noncontributing area. Because a recognized way for computing CDA has not been determined, it is 

assumed that CDA = TDA for all basins in Illinois in this study.

BL—Basin length, in miles, measured along a line areally centered through the drainage-basin boundary data layer 

from the basin outlet to the intersection of the main channel (extended) and the basin boundary.

BP—Basin perimeter, in miles, measured along the entire drainage-basin boundary.

BS—Average basin slope, in feet per mile, quantified using the “contour-band” method, which is computed as

BS=(total length of all selected elevation contours within the CDA)(contour interval)/CDA.

BR—Basin relief, in feet, measured as the elevation difference in the digital elevation model between the highest 

grid cell and grid cell at the basin outlet.
BA—Basin azimuth, in compass degrees of a line defined from where the main-channel extension meets the basin

divide downslope to the basin outlet. Measured clockwise from north at 0°.

BW—Effective basin width, in miles, BW=CDA/BL.

SF—Shape factor, dimensionless, as the ratio of basin length to effective basin width, SF=BL/BW.

ER—Elongation ratio, dimensionless; as the ratio of (1) the diameter of a circle of area equal to that of the basin to 

(2) the length of the basin, ER=(4CDA/π(BL)2)0.5=1.13(1/SF)0.5.

RB—Rotundity of basin, dimensionless; RB=(π(BL)2)/(4CDA)=0.785 SF.

CR—Compactness ratio, dimensionless; as the ratio of the perimeter of the basin to the circumference of a circle of 

equal area, CR=BP/2(πCDA)0.5.

RR—Relative relief, in feet per mile, RR=BR/BP.

MCL—Main-channel length, in miles; as measured along the main channel from the basin outlet to where the mainchannel extension meets the basin divide.

TSL—Total stream length, in miles; as computed by summing the length of all stream segments within the CDA.

MCS—Main-channel slope, in feet per mile; an index of the slope of the main channel computed from the difference 

in streambed elevations at points 10 percent and 85 percent of the distance along the main channel from the 

basin outlet to the basin divide, MCS=(E85-E10)/(0.75MCL).

MCSR—Main-channel sinuosity ratio, dimensionless, MCSR=MCL/BL.

SD—Stream density, in miles per square mile, as within the CDA, SD=TSL/CDA.

CCM—Constant of channel maintenance, in square miles per mile, as within the CDA, CCM=CDA/TSL=1/SD.

MCSP—Main-channel slope proportion, MCSP=MCL/(MCS0.5). Note that MCSP is not a non-dimensional term.

RN—Ruggedness number, in feet per mile, RN=(TSL)(BR)/CDA=(SD)(BR).

SR—Slope ratio of main-channel slope to basin slope, dimensionless; as within the CDA, SR=MCS/BS.

FOS—Number of first-order streams within the CDA, dimensionless. FOS is computed using Strahler’s method of 

ordering streams (Strahler, 1964, 1957).

BSO—Basin stream order, dimensionless, stream order of the main channel at the basin outlet. BSO is computed 

using Strahler’s method of ordering streams (Strahler, 1964, 1957).

DF—Drainage frequency, in number of first-order streams per square mile within the CDA, DF=FOS/CDA.

RSD—Relative stream density, dimensionless, as within the CDA, RSD=(FOS)(CDA)/(TSL)2=DF/(SD)2.

Soil and precipitation characteristics

The following explanatory variables are quantified from STATSGO (Natural Resources Conservation Service, 

1993) database and precipitation frequency estimates (Huff and Angel, 1992). Note that the variable values presented are area-weighted for the basin studied, and are computed using the area-weighting program of BASINSOFT. 

Description of the variables given in STATSGO is given first and in parenthesis.

PermL— (The minimum value for the range in permeability rate for the soil layer or horizon, in in/hr). Value 

presented is area-weighted average, minimum permeability rate of soil aggregated by soil layer and 

component, as a low value in the permeability range.

PermH— (The maximum value for the range in permeability rate for the soil layer or horizon, in in/hr). Value 

presented is area-weighted average, maximum permeability rate of soil aggregated by soil layer and 

component as a high value in the permeability range.

PermAvg— Average of the area-weighted PermH and PermL.

AWCL— (The minimum value for the range of available water capacity for the soil layer or horizon, in in/hr). Value 

presented is area-weighted average, minimum available water capacity of soil aggregated by soil layer.

AWCH— (The maximum value for the range of available water capacity for the soil layer or horizon, in in/hr). 

Value presented is area-weighted average, maximum available water capacity of soil aggregated by soil 

layer. 

Appendix 5. Determination of Basin Characteristics 73
74 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

SlopeL— (The minimum value for the range of slope of a soil component within a map unit). Value presented is 

area-weighted average, minimum slope of soil, in percent, aggregated by soil component.

SlopeH— (The maximum value for the range of slope of a soil component within a map unit). Value presented is 

area-weighted average, maximum slope of soil, in percent, aggregated by soil component. 

TTF—2-year, 24-hour precipitation depth, in inches, defined as the maximum 24-hour precipitation expected to be

exceeded, on average, once every 2 years.

Land-use characteristics

Basin centroids, given as latitude-longitude coordinate pairs, were determined using the Arc/Info command 

CENTROIDLABELS with the INSIDE option and the ADDLATLONG1 command with the DD (Decimal Degrees) 

option. Land-cover variables were determined from the Multi-Resolution Land Characteristics Consortium’s 

National Land Cover Data (NLCD) (http://www.epa.gov/mrlc/nlcd.html).

LAT—Latitude of the basin centroid above the station or location of interest, in decimal degrees.

LONG—Longitude of the basin centroid above the station or location of interest, in decimal degrees.

Open water (%Water)—For a basin, the percentage of area classified as open water and herbaceous wetland (areas

that are 75-100 percent grassy-type vegetation with periodic saturation).

Forest—For a basin, the percentage of area classified as forest plus the forested wetland. 

Factors to be Considered in Determining Basin Characteristics

The following factors should be considered in determining basin characteristics. 

•	 Care should be taken to convert datasets to a uniform unit system before running BASINSOFT. For example,

digital elevation dataset can be specified in Standard International or English unit systems.

•	 The grid resolution of the 1:100,000-scale DEM was approximately 98 ft. When determining basin characteristics for small basins, 1:24,000-scale data should be digitized, including hypsography and hydrography. 

Typically, smaller-scale datasets (1:100,000 to 1:250,000) lack the detail necessary to run BASINSOFT for 

small basins.

•	 Until more refined procedures are developed, the basin boundaries should be determined with a reliable

method. In this study, river basin boundaries delineated previously from other studies are used. These boundary delineations are digitized from the 7.5-minute quadrangle maps into GIS layers before running BASINSOFT for other parameters.

•	 Definition for some parameters needs to be specified by the user for the BASINSOFT analysis. The MCL, for 

example, can result in different values if a different definition for the main channel is used. Before calculations begin, the user is prompted to extend the main channel up to the basin divide and then select this new 

segment and the outlet point of the basin to highlight the main channel. The user can select the main channel 

as the set of stream segments that drain the most area, or select the main channel as identified by the named 

segment at the basin outlet and following this named feature to the basin divide. The latter definition is used 

in this study. 

•	 In using STATSGO to determine PermL or PermH, the approximate minimum area delineated is 625 hectares 

(1,544 acres), which is represented on a 1:250,000 scale map by an area approximately 1 cm by 1 cm (0.33 

inch by 0.33 inch). Linear delineations should not be less than 0.5 cm in width. The number of delineations 

per 1:250,000 quadrangle should range from 100 to 200, but a range up to 400 is allowed (www.ftw.nrcs.usda.

gov/pdf/statsgo_db.pdf). When combining map units, the four steps listed in the manual should be followed 

(www.essc.psu.edu/soil_info/index.cgi?soil_data&counus&data_cov&perm&methods; p. 18-19). 

1

 ADDLATLONG is an AML designed by USGS personnel, which may not be included in the general public version of 

ArcINFO.
Two basin-morphometric parameters, MCL and MCS, determined from BASINSOFT procedures are compared 

to available data (Curtis, 1987) for evaluating the differences in these parameters from the two datasets; hand-delineated (Curtis, 1987) and BASIINSOFT delineated. First, the MCS values are compared to those reported in Curtis 

(1987). In general, values from the two sources are similar but some discrepancies are present especially for values 

greater than approximately 150 ft/mi (fig. 5-1). It is considered that measurement errors could arise from interpretations of contour values, stream-network delineation, or local effects that require engineering judgment. To examine 

what basin types are associated with larger measurement errors, the MCS data used in figure 5-1 are used to compute 

the percent difference, as (BASINSOFT value – Curtis value)/(Curtis value), as shown in figure 5-2. It can be seen 

that the MCS values determined by BASINSOFT are mostly within the band of +50 percent (arbitrarily defined 

value above this limit is considered to be an overestimate) and –50 percent (an underestimate) from those used in the 

1987 study (Curtis, 1987). Underestimation of the MCS with BASINSOFT occurs mostly at small drainage areas 

(lesser than 10 mi2), overestimation by BASINSOFT occurs mostly at drainage areas approximately between 10 and 

1,000 mi2, but larger MCS errors occur in watersheds with drainage areas about 10 mi2.

Various MCL values determined in this study were checked with published data (Healy, 1979a, b) and the results 

were similar. Eash (U.S. Geological Survey, written commun., 2002) also cautioned that users might encounter 

mixed map scales or measurement methods. Some possible problems in determining the basin geometric parameters 

are listed below. 

•	 Manual MCS measurements could have been derived from maps with different scales. Examples are those 

stored in WATSTORE dated back to mid 1970’s.

•	 The sources of elevation and channel length data are in different scales. For example, elevation is derived

from 1:24,000 data but the MCL is determined from 1:100,000 data.

•	 MCL measurements could be made using dividers set at 0.1-mi increments and the dividers were used to 

“walk” along the river to measure the MCL. This measurement method has been used previously and all the 

channel sinuosity usually is not measured with this method. Because the program measures along the stream 

centerline and, therefore, includes the sinuosity, the MCL values from BASINSOFT usually are greater than 

determined using graph paper.

Appendix 5. Determination of Basin Characteristics 75

Figure 5-1. Comparisons of main-channel slope values used in Curtis (1987) and determined with the BASINSOFT program for selected 

watersheds in Illinois.
76 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

The last problem listed above indicates why the MCS determined by BASINSOFT could be lower than values 

determined manually. Besides the 1:100,000 digital-scale data used, 1:24,000 digital-scale data for deriving hydrography data and 1:24,000 digital-scale data for deriving elevation were used for 16 stations. Also, 1:250,000 digitalscale data for deriving elevation were used but 1:100,000 digital-scale data for deriving hydrography data were used 

for six stations where the drainage areas partially are located in Indiana.

References

Carns, J.M., 1973, Magnitude and frequency of floods in Illinois: Illinois Department of Transportation, Division of Water 

Resources Management, 599 p.

Curtis, G.W., 1987, Technique for estimating flood-peak discharges and frequencies on rural streams in Illinois: U.S. Geological 

Survey Water-Resources Investigations Report 87-4207, 79 p.

Curtis, G.W., 1977a, Frequency analysis of Illinois Floods using observed and synthetic streamflow records: U.S. Geological 

Survey Water-Resources Investigations Report 77-104, 32 p.

Curtis, G.W., 1977b, Techniques for estimating magnitude and frequency of floods in Illinois: U.S. Geological Survey WaterResources Investigations Report 77-117, 70 p.

Eash, D.A., 2001, Techniques for estimating flood-frequency discharges for streams in Iowa: U.S. Geological Survey WaterResources Investigations Report 00-4233, 88 p.

Eash, D.A. 1993, Estimating design-flood discharges for streams in Iowa using drainage-basin and channel-geometry characteristics: U.S. Geological Survey Water-Resources Investigations Report 93-4062, 96 p.

Environmental Systems Research Institute (ESRI), 1998, ARC/INFO user’s guide, version 7.2.1: Redlands, California, Environmental Systems Research Institute, variously paginated.

Harvey, C.A., and Eash, D.A., 1996, Description, instructions, and verification for BASINSOFT, a computer program to quantify drainage-basin characteristics: U.S. Geological Survey Water-Resources Investigations Report 95-4287, 25 p.

Figure 5-2. The difference, in percent, of main-channel slope between the BASINSOFT results and those used in Curtis (1987) for 

selected watersheds in Illinois (horizontal lines delineate the ±50 percent difference).
Healy, R.W., 1979a, River mileages and drainage areas for Illinois Streams—Volume 1, Illinois except Illinois River Basin: U.S. 

Geological Survey Water-Resources Investigations Report 79-110, 350 p.

Healy, R.W., 1979b, River mileages and drainage areas for Illinois Streams—Volume 2, Illinois River Basin: U.S. Geological 

Survey Water-Resources Investigations Report 79-111, 303 p.

Huff, F.A., and Angel, J.R., 1992, Rainfall frequency atlas of the Midwest: Bulletin 71, Illinois State Water Survey, Champaign, 

Ill., 141 p.

Illinois Department of Natural Resources, 1996, Illinois land cover – an atlas: Springfield, Ill., Critical Trends Assessment Project Phase II: Illinois Department of Natural Resources, 157 p. and CD-ROM.

Natural Resources Conservation Service (formerly Soil Conservation Services), 1993, State Soil Geographic (STATSGO) database : U.S. Department of Agriculture, Miscellaneous Publication No. 1492, 110 p. and computer data.

Strahler, A.N., 1964, Quantitative geomorphology of drainage basins and channel networks, in Handbook of Applied Hydrology, 

V.T. Chow (editor in chief): McGraw Hill, New York, p. 4-39 to 4-76.

Strahler, A.N., 1957, Quantitative analysis of watershed geomorphology: Transactions of the American Geophysical Union, 38: 

913-920.

Appendix 5. Determination of Basin Characteristics 77
78 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Appendix 6. Hydrologic Regions for Illinois

Regionalization is the development of techniques to extend knowledge of at-site flood-frequency relations to 

other stream locations within a region, where the region is defined as a collection of river basins such that the occurrence of flood-peak discharge magnitudes at any site in the region can be described with a single frequency distribution. That is, the flood-peak discharge magnitudes of each river basin in the region are considered to be sub-samples 

from a common population. Various methods have been designed to test and define a “statistically homogenous 

region” for flood-frequency analysis (for example, Chow, 1964; Kite, 1977; Nguyen, 2000; Hosking and Wallis, 

1997). However, delineation of hydrologic regions remains as a state-of-the-art science and a generally accepted 

method for regionalization currently has not been defined (Nguyen, 2000).

Geographical closeness among stations is not necessarily an indicator of similarity of the frequency distribution 

(Hosking and Wallis, 1997). However, maintaining geographical closeness has advantages in practical applications. 

Methods for forming hydrologic regions can be developed with judgment based on basin characteristics (Acreman 

and Sinclair, 1986), by geographical locations (National Environmental Research Council, 1975), by analyzing 

skew coefficients (Interagency Advisory Committee on Water Data, 1982), or by analyzing residuals (Stedinger and 

Tasker, 1985; Tasker, 1989). Methods for testing homogeneity of the delineated hydrologic regions also have been 

developed (for example, Darlymple, 1960; Hosking and Wallis, 1993; Nguyen and others, 1997). Even if a region 

is moderately heterogeneous, regional analysis still will yield much more accurate flood-frequency estimates than 

at-site analysis (Hosking and Wallis, 1997).

Two general sets of hydrologic regions can be identified from the previous flood-frequency analyses in Illinois 

according to their analytical approaches (Mitchell, 1954; Carns, 1973; Curtis, 1977b; Singh, 1981; Curtis, 1987). 

The general hydrologic regions are based primarily on 1) physiographic characteristics, and 2) residual analysis. For 

Illinois, the residual analysis has been used mainly with the LP3 distribution for AMS analysis. With the inclusion 

of PDS analysis and expanded basin-morphmetric characteristics, the previous development in hydrologic regions 

was reviewed for identifying an approach to reach delineation that is physically based and can maintain or improve 

the accuracy of frequency predictions for both AMS and PDS models. 

Previous Development

Mitchell (1954) outlined 15 hydrologic regions in Illinois. In 11 of the 15 regions where data were available, 

Mitchell computed and illustrated the distinctions in parameters k (a physiographic factor), j (a climatologic factor) 

and c (a flood-potential factor, c = j/k) among these hydrologic regions. Mitchell’s hydrologic regions were developed by slightly modifying the 15 physiographic regions defined by Leighton and others (1948). Probably restricted 

by data availability, Mitchell predicted occasional floods (large floods, for recurrence intervals roughly larger than 

50 years) using 3 hydrologic divisions (northern, central, and southern) that were formed by combining the 15 

hydrologic regions.

Carns (1973) considered residual patterns from the regression analysis and watershed boundaries in defining 

four hydrologic regions of Illinois. Residuals were the differences in flood quantiles estimated by at-site analysis 

and a statewide regression analysis, where TDA, MCS, and TTF were used as explanatory variables in the regression 

equations and the LP3 distribution was used to fit station data. These hydrologic regions are different from those 

outlined by Mitchell (1954). Carns’ hydrologic regions were modified by Curtis (1977a, b). Curtis used the LP3

distribution and TDA, MCS, and (TTF-2.5) as explanatory variables but with updated station data and analysis techniques. The same hydrologic regions and explanatory variables were used in a later update (Curtis, 1987) but with 

refined regional factors.

Singh (1981) classified the State with eight hydrologic regions when he studied the parameters for unit-hydrograph analysis for Illinois. Initially, 12 regions were demarcated on the basis of physiographic regions (Leighton and 

others, 1948), model flow duration, and hydrologic and climatologic homogeneity. The grouping or transferring of 

river basins from regions was tested using regression analysis with combinations of explanatory variables in TDA, 

MCS, and MCL. 

Current Approach

Analyzing residuals from a regression model can identify regional differences effectively but the results depend 

on the analytical techniques and explanatory variables used. Also, when additional flood data were available for a 

station, the residual is likely to change. In order to determine hydrologic region delineation that will not be subject 
to change because of changes in methodology and/or additional data, a reasonable approach is to analyze variables 

that are relatively invariant with time in the watershed hydrologic processes. If such an approach can be proven successful, the hydrologic regions outlined in this report could be useful in other studies, such as improving the progress 

in flood-frequency analysis, evaluating data needs, identifying the unit-hydrograph parameters, and others.

Two main steps were used to determine hydrologic regions in this study: 1) delineate the general outlines of the 

hydrologic boundary using major river basins, and 2) adjust the boundaries along basin divides of smaller watersheds. Three methods were tested for delineating the general boundaries of hydrologic regions in Illinois: 1) the 

updated regional skew coefficients; 2) cluster analysis (for example, Kachigan, 1986) using physiographic parameter 

groups such as BS, MCS, PermH, PermL, %Water, DF, BS, SF; and 3) hydrologic regions referenced to the regions 

delineated by Mitchell (1954) and Singh (1981). The delineation based on Mitchell and Singh showed the most 

consistent results. In delineating regional boundaries, a newly developed detailed surface-topography map of Illinois 

(Luman and others, 2003) was referenced.

Adjusting/refining the hydrologic boundaries is conducted using residual analysis and evaluated using the sum 

of squares of errors (SSE) for each region in each regression equation. Various regression models (with dummy 

variables) tested are described in appendix 7. Through multiple comparisons, a regional delineation that results in 

the lowest SSE is selected as the final hydrologic region for analysis. At this step, the consistency of at-site estimates 

and various regional equation estimates could be analyzed and the regional boundary adjusted. Uncertainty may 

result in adjusting boundaries at finer scales using this approach if an available subbasin has limited representation of 

the entire watershed. For example, the Mazon River Basin is represented by only one streamflow-gaging station at 

Coal City. However, the basin has a flat upland but steep slopes are present downstream especially near the Illinois 

River. The lower Fox River and Kankakee River below Momence have similar topographic characteristics as the 

Mazon River Basin.

River basins re-assigned to adjacent regions are those in the Green River Lowland, Mazon and Vermilion (in 

northern Illinois) River Basins, and bluff watersheds in the American Bottoms Lowlands in the southwestern part 

of the State. The Clark unit hydrograph storage coefficients study (Graf and others, 1982) was evaluated to justify 

the regional delineation of the Skillet Fork of the Little Wabash River in southeastern Illinois. There also are three 

stations that are considered anomalous and deleted from the regional analysis after examining their flow records 

(appendix 7). 

Description of Current Hydrologic Regions for Illinois 

The final delineation of the hydrologic regions is presented in figure 5 in the main text. An Adobe Acrobat 

image file (pdf extension) is given in the attached CD-ROM. The text below describes the river basins in each 

hydrologic region with a brief reference to the hydrologic characterization of Mitchell (1954) and physiographic 

divisions of Leighton and others (1948). However, only general information concerning the features of the regions 

was given in these reports. More specific information on physiographic and other features in various river basins has 

been collected since these studies but a comprehensive documentation compiling the newly identified physiographic 

characteristics is not yet available. A brief description of the hydrologic regions defined during this study are listed 

below.

Region 1: River basins in region 1 include the Apple River Basin, Rock River Basin, and Kishwaukee River Basin 

of the Rock River. The Wisconsin Driftless area and Rock River Hill Country dominate the region. The Wisconsin 

Driftless area is characterized by flat upland areas but channels are steep sloped with narrow valleys. The Rock 

River Hill Country is characterized by deep and permeable soils. 

Region 2: River basins in region 2 include the Des Plaines River, Fox River, Green River, and Kankakee River 

excluding the Iroquois River. The Chicago Lake Plain, Wheaton Morainal Region, Green River Lowland, a portion 

of the Kankakee Plain, and a portion of the Bloomington Ridged Plain above the Illinois River cover this region. 

The Chicago Lake Plain has swampy and poorly drained soils; the Wheaton Morainal Region has flat slopes, long, 

narrow basins, and large storage in lakes and swamp areas; the Green River Basin has low and poorly drained plains; 

the Fox River Basin contains many lakes; and the Kankakee Plain is characterized as a level to gently undulatory 

plain. Hydrographs of streams in these regions generally have low, flat crests with long recession limbs. 

Region 3: River basins in the region include the Bureau Creek Basin, Mazon River Basin, Vermilion River Basin, 

Iroquois River Basin, Upper Embarras River Basin, Upper Sangamon River Basin, Mackinaw River Basin, and 

Appendix 6. Hydrologic Regions for Illinois 79
80 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Macoupin River Basin. The region primarily is composed of the lower portion of the Kankakee and Bloomington 

Ridged Plains, and the upper portion of the Springfield Plain. Leighton and others (1948) described it as “It was 

in this district more than in any other that the grass-covered stretches of rolling prairie and extensive swamps, ...“ 

Region 3 is characterized by thick glacial deposits.

Region 4: The region includes the Edward River Basin, Pope Creek Basin, Spoon River Basin, La Moine River 

Basin, Bear Creek Basin, Bay Creek Basin, and the Cahokia River Basin. The region consists of the entire Galesburg Plain Region, Lincoln Hills Region, and Upper Salem Plateau Section. The Galesburg Plain contains steeply 

sloping channels and sharply incised valleys. Lincoln Hills and Salem Plateau are unglaciated with some loess 

deposits or underlain by limestones.

Region 5: River basins in the region include the Kaskaskia River Basin below Lake Shelbyville, Lower Embarras River Basin, and Upper Little Wabash River Basin. The region consists of Springfield Plain Region and Little 

Wabash River Basin of the Mt. Vernon Hill Region. The Springfield Plain Region is characterized by flat topography but well-developed drainage systems. The uplands are relatively low with respect to the main stream and contain shallow valleys. Streams have low-gradients and occupy broad alluviated and terraced valleys. The Mt. Vernon 

Hill Region contains low-gradient streams, long and narrow basins, and wide floodplains for potential storage.

Region 6: This region includes the Big Muddy River Basin, Skillet Fork of the Little Wabash River Basin, Saline 

River Basin, Wabash River Basin, Blue River Basin, and Bonpas Creek Basin. The region consists of the Mt. Vernon Hill Region and the northern portion of the Shawnee Hill Region. The Shawnee Hill Region lies between the 

southern limits of glacial drift and the northern limits of Coastal Plain sediment. The regional structure is complicated by faulting and folding over a large part of the basin (Leighton and others, 1948). The northern Shawnee Hill 

Region is composed of largely Pennsylvanian rocks, where in most places, the ridge is maturely dissected by youthful valleys, but remnants of flat upland are preserved locally on narrow ridge crests throughout the length of the 

escarpment (Leighton and others, 1948). The physiographic features seem appreciably different from those of region 

2, but analysis of streamflow records showed some similarities to those features in region 2. It is likely that the lowgradient and broad alluviated valleys for possible storage, as well as swamp areas, characterize the locations of the 

gaging stations located in the Shawnee Hill Region.

Region 7: The region includes the Cache River Basin. The region consists of the Coastal Plain Province and Shawnee Hill Region. The Coastal Plain Province consists of alluvial plain of the Cache and Mississippi Valleys and the 

Cretaceous hills between the Cache Valley and the Ohio River. The alluvial plains are characterized by terraces and 

recent floodplain features, and the Cretaceous hills have eroded into a low upland of gently sloping knolls and ridges 

(Leighton and others, 1948). Similar to region 6, the regional structure is complicated and the flow characteristics 

observed at streamflow-gaging stations reflect largely the local features. For example, swamp areas are known to be 

present along Cache River above Forman, but the basin above Wetaug is long and narrow (Mitchell, 1954).

References

Acreman, M.C., and Sinclair, C.D., 1986, Classification of drainage basins according to their physical characteristics: an 

approach for flood-frequency analysis in Scotland: Journal of Hydrology, 84, p. 365-380.

Carns, J.M., 1973, Magnitude and frequency of floods in Illinois: Illinois Department of Transportation, Division of Water 

Resources Management, 599 p.

Chow, V.T., 1964, Editor-in-Chief, Handbook of Applied Hydrology: McGraw-Hill, New York, variously paginated.

Curtis, G.W., 1987, Technique for estimating flood-peak discharges and frequencies on rural streams in Illinois: U.S. Geological 

Survey Water-Resources Investigations Report 87-4207, 79 p.

Curtis, G.W., 1977a, Frequency analysis of Illinois Floods using observed and synthetic streamflow records: U.S. Geological 

Survey Water-Resources Investigations Report 77-104, 32 p.

Curtis, G.W., 1977b, Techniques for estimating magnitude and frequency of floods in Illinois: U.S. Geological Survey WaterResources Investigations Report 77-117, 70 p.
Dalrymple, T., 1960, Flood-frequency analyses: U.S. Geological Survey Water-Supply Paper 1543-A, 80 p.

Graf, J.B., Garklavs,G., and Oberg, K.A., 1982, Time of concentration and storage coefficient values for Illinois streams: U.S. 

Geological Survey Water-Resources Investigations Report 82-13, 35 p.

Hosking, J.R.M., and Wallis, J.R., 1993, Some statistics useful in regional frequency analysis: Water Resources Research, 29(2), 

p. 271-281.

Hosking, J.R.M., and Wallis, J.R., 1997, Regional Frequency Analysis: Cambridge University Press, Cambridge CB2 2RU, U.K. 

Interagency Advisory Committee on Water Data, 1982, Guidelines for determining flood flow frequency, Bulletin 17-B of the 

Hydrology Subcommittee: Reston, Virginia, U.S. Geological Survey, Office of Water Data Coordination, 183 p. [Available 

from National Technical Information Service, Springfield, Va. 22161, as report no. PB 86 157 278. Also available in PDF 

format (28 Mbyte file size) from the Federal Emergency Management Agency Web site http://www.fema.gov/mit/tsd/dl_flow.

htm]

Kachigan, S.K., 1986, Statistical Analysis: Radius Press, New York, N.Y., 589 p.

Kite, G.W., 1977, Frequency and Risk Analysis in Hydrology: Water Resources Publications, Fort Collins, Colo., 224 p.

Leighton, M.M., Ekblaw, G.E,., and Horberg, L., 1948, Physiographic Divisions of Illinois: Report of Investigations–No. 129, 

State Geological Survey, Illinois Department of Registration and Education, 33 p.

Luman, D.E., L.R., Smith, and C.C., Goldsmith, 2003, Illinois Surface Topography: Illinois State Geological Survey, Champaign, Ill., Illinois Map Series 11, scale 1:500,000.

Mitchell, W.D., 1954, Floods in Illinois: Magnitude and Frequency: Prepared in cooperation with the U.S. Geological Survey 

and Division of Waterways, Department of Public Works and Buildings, 386 p. 

National Environmental Research Council, 1975, Flood study report: London, p. 1-5.

Nguyen, V.T.V., 2000, Recent advances in the modeling of extreme rainfalls and floods: In Ecosystem and Flood 2000, Hanoi, 

Vietnam, June 27-29, 2000.

Nguyen, V.T.V, Pandey, G., and Wang, H., 1997, Scaling approach to regional estimation of extreme hydrologic variables: Proceedings of the 1997 Congress of the Canadian Society of Civil Engineering. v. 3, p. 81-90.

Singh, K.P., 1981, Derivation and regionalization of unit hydrograph parameters for Illinois (Dam Safety Program): Illinois 

Institute of Natural Resources, State Water Survey Division, SWS Contract Report 258, 84 p.

Stedinger, J.R., and Tasker, G.D., 1985, Regional hydrologic analysis, 1. ordinary, weighted, and generalized least squares compared: Water Resources Research, 21(9):1421-1432.

Tasker, G.D., and Stedinger, J.R., 1989, An operational GLS model for hydrologic regression: Journal of Hydrology, 111, 

p. 61-375.

Appendix 6. Hydrologic Regions for Illinois 81
82 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Appendix 7. Regression Analysis

The regression analysis is used to identify the relations between the at-site estimate QT (response variable) and 

subset of basin characteristics (explanatory variables) for a specified T among stations in the hydrologic regions. 

The determined relations then can be used for estimating QT at ungaged streams or improving estimates of QT at 

gaged streams. Various factors can affect the accuracy of the analysis, including the errors and uncertainties in the 

estimated at-site QT’s and the basin characteristics, the grouping of selected explanatory variables to explain the 

variations in QT’s, and the techniques of regression analysis utilized. 

Multiple linear regression (MLR) analysis is used when two or more explanatory variables are considered; the 

response and explanatory variables are transformed into log-10 units in the analysis to derive the final equations 

in non-linear (power-law) forms. In exploring which group of explanatory variables can best predict the QT’s, the 

ordinary least squares (OLS) regression technique is used. However, OLS assigns equal weights to all at-site QT’s 

regardless of differences in record length at various stations and cannot account for inter-site correlations. Stedinger 

and Tasker (1985) developed a weighted least squares (WLS) technique that can account for different record length 

of stations. Stedinger and Tasker (1986) further developed the regression techniques, obtaining a generalized least 

squares (GLS) technique. The GLS technique accounts for differences in record lengths, differences in flood-peak 

discharge variances, and cross-correlations of concurrent flood-peak discharges among stations used in the regression analysis, and, therefore, improves the accuracy of regression equations. This appendix explains the procedures 

used in identifying suitable grouping of explanatory variables and the evaluation of the accuracy of selected regression equations. 

Analytical procedures used in this study can be described as follows.

1. Identify potential groupings of explanatory variables. 

2. Apply MLR with OLS regression technique to identify the most suitable variable grouping for each region 

and for each QT.

3. Evaluate the delineation of hydrologic regions and re-adjust regional boundaries by reassigning subbasins to 

adjacent regions (see appendix 6).

4. Apply the estimated generalized least squares (EGLS) technique in the GLSNET program (Tasker and Stedinger, 1989) to AMS data. Evaluate and remove stations that are clearly outliers, and derive the final GLS 

regression equations for the AMS model. For PDS, the GLSNET program is not applicable because the GP

distribution and r = 1.6 are used. Therefore, the regression equations for the PDS model are derived with the 

OLS technique. 

Grouping of Explanatory Variables

Step-wise regression techniques (forward, backward, and step-wise) were used to detect suitable variables 

to be included in the analysis. However, these techniques might not lead to a unique combination of explanatory 

variables (Helsel and Hirsch, 1992). For example, highly correlated variables (collinearity) may be selected. For 

analyses such as making inferences about coefficients, undesirable consequences can result when selected explanatory variables have high multi-collinearity. However, if the purpose of the regression analysis is for prediction, such 

concerns could are reduced (Helsel and Hirsch, 1992). 

Multi-collinearity among variables was analyzed first with a correlation matrix analysis. From the definitions of 

basin-morphometric characteristics (appendix 5), one could expect that various variables would be highly correlated 

(to TDA, for example; see appendix 5) according to how they were measured or derived. Clearly, TDA is the predominant explanatory variable in estimating flood hydrology from a river basin. The multi-collinearity analysis indicated that most basin geometric variables are highly correlated to TDA except the variables of MCS, TTF, PermH, 

PermL, and %Water. Also alternatively, BL and BW could be used in place of TDA in grouping the variables. 

Variables with high correlations to TDA were removed from the subsequent step-wise regression analysis presented 

in the following sections. Note that the average permeability rate, PermAvg (computed as the arithmetic average of 

PermH and PermH), is used in place of PermL or PermH.
Potential Regression Equations

Results from step-wise regression analysis

Two regression equations resulting from step-wise analysis are given below.

 QT = ƒstep-wise(1) (TDA, MCS, Water%, PermAvg) and (7-1)

 QT = ƒstep-wise(2) (BL, BW, MCS, Water%, PermAvg), (7-2)

where ƒstep-wise stands for the function derived from the step-wise regression analysis and the function is expressed 

with the explanatory variables in the parenthesis. Similar ƒ terms are used in the following. The explanatory variables have been defined in appendix 6. 

Curtis (1987, 1977) and Carns (1973)

The general form of the Curtis (1987, 1977) and Carns (1973) equations can be described as

 QT = ƒCurtis-Carns [TDA, MCS, (TTF-constant), RF], (7-3)

where the constant is 0 in Carns’ and 2.5 in Curtis’ equations, respectively; and RF stands for regional factors resulting from the use of dummy variables.

Singh (1981)

The regional equations for flood-peak discharge of the normalized unit hydrograph, QP, can be described as

 QP = ƒSingh (TDA, MCS, MCL) . (7-4)

The final regional equations for each region contain all or subsets of the variables. 

Clearly, TDA and MCS are two primary explanatory variables relating to flood-peak discharge magnitudes in 

Illinois. Variable BL is an analogy of the travel time of flow in the main stem, variable BW is an analogy of the 

travel time of lateral inflows, and variables PermAvg and Water% are analogies of storage characteristics of the 

drainage basin. The updated TTF is not included probably because the rainfall depth has little variation across the 

State, as shown in figure 7-1.

Use of Dummy Variables

Dummy variables are used as the surrogate for factors affecting QT’s that could not be properly expressed as 

variables (for example, Helsel and Hirsch, 1992). At the end of analysis, the values of the dummy variable are converted to a scale factor that varied among different hydrologic regions where the same group of explanatory variables is used. Use of the dummy variables is appropriate in this analysis because the physiographic contrast among 

hydrologic regions of Illinois is not substantial but different hydrologic characteristics in flood-producing mechanisms are expected. Subsequently, the intercepts and/or slopes in the regional-regression equations will vary when 

the same group of variables is used. 

Dummy variables were applied to the regression constant (intercept) for all regions in the AMS analysis, but to 

the regression constant and/or an explanatory variable (slope of the variable, primarily on TDA) for regions in the 

PDS analysis. A partial residual test (Helsel and Hirsch, 1992) was used to test the necessity for applying a dummy 

variable to TDA. 

Besides as an additional explanatory variable, use of the dummy variables provided an opportunity for reviewing the reasonableness of delineated hydrologic regions. If the values of a dummy variable for two different regions 

were similar, the result could indicate that additional evaluation of the two hydrologic regions was needed. Use 

of the dummy-variable technique also was helpful to a systematic evaluation of regression equations in this study 

Appendix 7. Regression Analysis 83
84 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Figure 7-1. Two-year, 24-hour rainfall intensity for Illinois and adjacent States (modified from Huff and Angel, 1991).
because the sum of squares of errors for each equation was calculated with the entire station data (either AMS or 

PDS), not merely station data in each region. Also, the same upper and lower limits of the parameter space for 

explanatory variables could be used for various hydrologic regions when their regional equations used the same 

group of explanatory variables. 

Evaluation of Regression Equations

Regression equations with explanatory variables, such as those described in equations 7-1 to 7-4 or subgroups of 

the explanatory variables in equations 7-1 and 7-2 and with dummy variables, were evaluated with their corresponding sum of square of error (SSE) The SSE is defined as

 SSE = N residual2= N (Qpredicted – Qobserved)

2 , (7-5)

 Σ i Σ

 

i=1

 

i=1

where N is the total number of stations in a hydrologic region. The regression equation with the least SSE was first 

selected to represent the region. The procedures then were repeated for each selected T’s. However, different groups 

of explanatory variables might result for different T’s in the same hydrologic region, and for the same T’s in different 

regions. Such outcomes could be expected because physical processes involved in producing flood-peak discharges 

of different magnitudes would be different even if in the same watershed, not to mention in other watersheds in other 

hydrologic regions. However, using regression equations with different groups of explanatory variables for different 

T’s in a watershed might result in a discontinuity in the estimated flood-frequency curve. That is, higher QT’s could 

be predicted at lower T’s. This situation was observed at a few small watersheds especially for those near the boundary of hydrological regions. To prevent such erroneous estimates, one group of explanatory variables was used in 

the regression equations for all QT’s in a hydrologic region in this study, but different regression equations might be 

used in different hydrologic regions. The procedures used for determining a regression equation for each hydrologic 

region are described in the following steps.

1. Conduct regression analysis with each selected group of explanatory variables.

2. Calculate SSE for each regression equation for each region.

3. Evaluate SSE’s for all regression equations and all T’s in a region, identify the equation with the lowest SSE

for all the T’s.

4. If different regression equations result from step 3, identify the equation where SSE’s are within 10 percent of 

the lowest SSE for all the T’s.

By setting a tolerance of 10 percent within the best-fit regression equation (step 4), one regression equation 

could be identified for all QT’s of a region for AMS analysis. On the other hand, in order to use only one regional 

regression equation in the PDS analysis, the 10-percent tolerance had to be relaxed to 25 percent for region 2 and 17 

percent for region 3.

For peak flows in Illinois, the parameters TDA (for some regions the parameters are BL and BW) and MCS form 

the basic group of explanatory variables for all the regions. However, an additional explanatory variable, such as 

PermAvg or %Water, could improve regression relations in separate regions for different QT’s. The final regression 

equations for each hydrologic region for the AMS and PDS model are presented in the main text. 

Adjusting Regional Boundaries

Refining hydrologic regions was conducted based on the AMS results. The general forms of AMS equations 

used in the study were as follows.

For regions 1, 3, and 5:

 QT = ƒ(TDA, MCS, PermAvg, βiT Z), (7-6)

Appendix 7. Regression Analysis 85
86 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

where βiT is the constant for dummy variable Z for region i and recurrence interval T.

For regions 2, 6, and 7:

 QT = ƒ(TDA, MCS, %water+5, βiT Z). (7-7)

For region 4 (note that the final equation was modified with BW=TDA/BL):

 QT = ƒ(BL, BW, MCS, βiT Z). (7-8)

After the regression analysis, if a βiT variable was significant, then it meant that there was a difference between stations in region i and stations that are not in region i for that T. If the βiT’s of two or three regions were approximately 

the same, these regions possibly could be combined into a single region. When testing if two regions could be combined, the test should be done for each T years (Gary Tasker, U.S. Geological Survey, written commun. 2002). The 

current region 4 resulted from combining two regions initially designated as two separate regions. 

During the SSE analysis, if station(s) in a subbasin of a region indicated that their QT’s were better predicted by 

variables from the adjacent regions, the station records and basin-morphometric characteristics were examined. If 

the results indicated that the subbasin could be incorporated into an adjacent region and the subbasin was located 

near the region boundaries, the subbasin was assigned to the adjacent region. The analysis resulted in reassigning 

the Green River Lowland from region 1 to region 2, readjusting the basin boundaries between region 1 and region 2 

along the Upper Fox River (switch to region 2), and reassigning the Upper Kankakee River subbasin from region 1 

to region 2. Strip-mine areas in central and southern Illinois were kept in their assigned regions. Zuehls and others (1981) found that flood-peak discharge magnitudes in the strip-mine areas could be predicted with similar basin 

characteristics used in this study.

Identify Station Outliers

Regression equations 7-6 to 7-8 were tested against the proposed hydrologic regions of Illinois using the 

GLSNET program in the final stage of the analysis. From the GLSNET output, station records showing large residuals and large Cook’s D (Helsel and Hirsch, 1992) were examined. In general, large residuals resulted at stations 

either with non-representative data (either record length or coverage of events) or at locations with unique physiographic features. Some of these features include the driftless area in northwestern Illinois or low-gradient areas near 

the confluence of the Illinois and Sangamon Rivers or the Cache River Basin in southern Illinois. Initially, 291 rural 

station records were selected and used for the regional regression analysis. After examining for possible errors in 

the observed flood-peak discharge magnitudes and in basin characteristics at stations, and reviewing the probability 

plots from the PEAKFQ (Gary Tasker, U.S. Geological Survey, written commun. 2002), the following three stations 

were considered to be outliers and, therefore, excluded from the rest of the stations used in the regression analysis. 

1. Normandy Ditch at Normandy, Ill., (5447200): The lower half of the main channel has been channelized.

2. Diamond Lake Drain at Mundelein, Ill., (5528170): This streamflow-gaging station is located downstream of 

Diamond Lake.

3. Little Calumet River at Harvey, Ill., (5536325): This station has an extremely flat MCS. 

Uncertainty and Accuracy of the Annual Maximum Series Regression Equations

Various parameters for evaluating accuracy and uncertainty of the regression equations have been reported 

in tables 3, 4, and 5. All these parameters are obtained from the GLSNET program (Tasker and Stedinger, 1989). 

Among them, the average prediction error and equivalent years of record (Hardison, 1971) quantify the accuracy 

of regression equations. A method for estimating the model uncertainty has been developed and widely utilized in 

various recent flood-frequency analysis reports (for example, Hodge and Tasker, 1995; Wiley and others, 2000). The 

following is a brief summary of how to use the GLSNET output information to calculate uncertainty. Definitions of 

the variables are given below.

AEYR: average equivalent years of record
MSE: mean square error (this and all the following terms are in log-10 units, unless specified otherwise)

SEE: standard error of estimate

APE: average prediction error for a site, i, or for a region, r

γ2: model error variance

γ: standard error of the model

SEi: standard error of prediction, sample error at site i

ASE: average standard error of prediction, average of sample error for the region

The AEYR describes the accuracy of the regression equation. It is an estimate of the number of years of streamflow 

records that must be collected at a streamflow-gaging station to estimate the flood magnitude for a selected frequency with accuracy equivalent to that of the regression equation (Hardison, 1971, p. C232). The AEYR is used as 

a weighting factor (along with the years of record at the station in question) in improving the flood-quantile estimate 

at gaged sites (equation 11).

The method for using APE to evaluate model accuracy and uncertainty are briefly presented here. Full descriptions of the method are presented in Tasker (1987) and Tasker and Steinger (1989) and reports mentioned earlier.

The linear form of a regression model can rewritten as (in log-10 units)

 Y = Xβ + e, (7-9)

where Y is a (n × 1) matrix of at-site estimates of the T-year flood, where n is the number of sites in the region under 

study, X is a (n × m) matrix of basin characteristics (m-1 explanatory variables) augmented by a column of ones, 

β is a (m × 1) matrix of regression coefficients, and e is a (n × 1) matrix of random errors. In the GLS model, the 

assumptions of equal variance of the T-year events and zero cross-correlation for concurrent flows are relaxed, and 

the GLS estimator for β is (Stedinger and Tasker, 1985)

β = (XT Λ-1 X)-1 XT Λ-1Y , (7-10)

where it is assumed that the errors have zero mean E[e] = 0, and covariance E[e eT] = Λ. Stedinger and Tasker 

(1985) proposed an estimator for this error covariance matrix Λ as

						Λ = γ2I + Σ , (7-11)

where γ2 is an estimate of the model-error variance because of an imperfect model (a measure of the precision of the 

true regression model), I is an (n × n) identity matrix, and Σ is a (n × n) matrix of sampling covariance. The diagonal elements of Λ, therefore, are the sum of γ2

 and a sampling error because of estimating the true model parameters 

from observed flows for site i, and i = 1, 2, ...n. On the other hand, the off-diagonal elements are the estimated 

cross-correlations between flood peaks at sites i and j. The model-error variance is defined by

 γ2 = E[Y – Xβ)2] , (7-12)

where the β estimator by GLS is given by equation 7-10. Once the γ2 is calculated, the standard error of the model, 

γ, can be transformed from log 10 unit to percent by 

 . (7-13)

The γ2 and γ% values for each model and each T were given in tables 1, 2, and 3.

Appendix 7. Regression Analysis 87

γ% = 100 e

(5.30119γ2

)

– 1 0.5
88 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

For a GLS regression model, the SE at a site i is computed as

 , (7-14)

where the xo is a row matrix containing basin characteristics determined at the study site i. By treating each gaged 

site in the region as if it were an ungaged site, the ASE for the region can be calculated as 

 , (7-15)

for all sites i where i is counted from 1 to the nth sites in the region. For each regression model, the matrix 

{XT Λ-1 X}-1 is given in the attached CD-ROM. 

In applying the GLS regression model to estimate flood frequencies at an ungaged site, the uncertainty or error 

in prediction is estimated by computing the standard error of estimate, SEE. The SEE for a site i or for a region is 

estimated, respectively, as

 or (7-16)

The average prediction error of the model for a region, APE, in percent, can be computed as

 . (7-17)

These APE values are reported in tables 3, 4 and 5. The prediction error at a site can be computed as

 . (7-18)

Computing model error at a ungaged site

In the following example, it is assumed that the Blackberry Creek at Yorkville station, as described in the main 

text, was an ungaged site. Therefore, the χo matrix, containing the explanatory variables for region 2, can be written 

as

Note that RF for region 2 is 1. The {XT Λ-1 X}-1 matrix for the 100-year recurrence interval, for example, is 

Coefficient log(TDA) log(MCS) Log(%Water+5) RF(1) RF(3) RF(4) RF(5) RF(6) RF(7)

1 log(69.4) log(5.9) log(6.04) 0 0 0 0 0 0

COEFF. TDA MCS % WATER REGION 1 REGION 3 REGION 4 REGION 5 REGION 6 REGION 7

3.38E-02 -2.54E-03 -6.71E-03 -2.83E-02 -1.08E-03 -2.98E-03 -1.64E-03 -2.76E-03 -1.63E-04 -5.50E-04

-2.54E-03 7.09E-04 1.35E-03 2.22E-04 -2.62E-04 -2.78E-05 -1.43E-04 2.12E-05 -6.07E-05 -1.91E-04

-6.71E-03 1.35E-03 3.63E-03 2.03E-03 -4.43E-04 1.64E-04 -4.62E-04 1.35E-04 -2.04E-04 -8.93E-04

-2.83E-02 2.22E-04 2.03E-03 3.30E-02 1.20E-03 2.34E-03 1.35E-03 1.82E-03 -9.92E-04 2.97E-04

-1.08E-03 -2.62E-04 -4.43E-04 1.20E-03 3.46E-03 1.06E-03 1.17E-03 1.04E-03 9.68E-04 1.08E-03

-2.98E-03 -2.78E-05 1.64E-04 2.34E-03 1.06E-03 2.32E-03 1.36E-03 1.46E-03 1.15E-03 1.16E-03

-1.64E-03 -1.43E-04 -4.62E-04 1.35E-03 1.17E-03 1.36E-03 2.84E-03 1.47E-03 1.34E-03 1.53E-03

-2.76E-03 2.12E-05 1.35E-04 1.82E-03 1.04E-03 1.46E-03 1.47E-03 3.15E-03 1.62E-03 1.49E-03

-1.63E-04 -6.07E-05 -2.04E-04 -9.92E-04 9.68E-04 1.15E-03 1.34E-03 1.62E-03 4.76E-03 2.07E-03

-5.50E-04 -1.91E-04 -8.93E-04 2.97E-04 1.08E-03 1.16E-03 1.53E-03 1.49E-03 2.07E-03 8.68E-03

SEi = xo {XT Λ-1 X}-1 xT √ o

SEEi = γ2 +SEi

2 √ ,

SEE = γ2 +ASE2 . √

APE% = 100 e

(5.30119SEE) – 1 0.5

PE% = 100 e

(5.30119SEEi)

– 1 0.5

ASE = n

 xo {XT Λ-1 X}-1 xT

 

Σ

 n 

o

 

√i=1
After removing the zero terms, the matrix reduces to

and the result is 0.0396 in log value. The prediction error, in percent, is calculated in the following steps.

(1) Obtain γ2 from table 4; compute (SEEi

)2

 using equation 7-16 as

 (SEEi

)2 = 0.0381 + 0.0396 = 0.0777; SEEi = 0.279.

(2) Convert to percent using equation 7-18 as

References

Carns, J.M., 1973, Magnitude and frequency of floods in Illinois: Illinois Department of Transportation, Division of Water 

Resources Management, 599 p.

Curtis, G.W., 1987, Technique for estimating flood-peak discharges and frequencies on rural streams in Illinois: U.S. Geological 

Survey Water-Resources Investigations Report 87-4207, 79 p.

Curtis, G.W., 1977, Techniques for estimating magnitude and frequency of floods in Illinois. U.S. Geological Survey WaterResources Investigations Report 77-117, 70 p.

Draper, N.R., and Smith, H., 1981, Applied Regression Analysis, 2nd edition: John Wiley & Sons, New York, 709 p.

Hardison, Clayton H., 1971, Prediction error of regression estimates of streamflow characteristics at ungaged sites: U.S. Geological Survey Professional Paper 750-C, p. C228-C236.

Helsel, D.R., and Hirsch, R.M., 1992, Statistical methods in water resources: Studies in Environmental Science 49: Elsevier 

Publishing. New York, 522 p.

Hodge, .S.A., and Tasker, G.D., 1995, Magnitude and frequency of floods in Arkansas: U.S. Geological Survey Water-Resources 

Investigations Report 95-4224, 52 p. plus 4 appendixes.

Huff, F.A., and Angel, J.R., 1992, Rainfall frequency atlas of the Midwest: Bulletin 71, Illinois State Water Survey, Champaign, 

Ill., 141 p.

Singh, K.P., 1981, Derivation and regionalization of unit hydrograph parameters for Illinois (Dam Safety Program): Illinois 

Institute of Natural Resources, State Water Survey Division, SWS Contract Report 258, 84 p.

Stedinger, J.R., and Tasker, G.D., 1986, Correction to Regional hydrologic analysis, 1. ordinary, weighted, and generalized least 

squares compared: Water Resources Research, 22(5):844.

Stedinger, J.R., and Tasker, G.D., 1985, Regional hydrologic analysis, 1. ordinary, weighted, and generalized least squares compared: Water Resources Research, 21(9):1421-1432.

Tasker, G.D., and Stedinger, J.R., 1989, An operational GLS model for hydrologic regression: Journal of Hydrology, 111, 

p. 361-375.

Tasker, G.D., 1987, Regional analysis of flood frequencies: Regional Flood-frequency Analysis, edited by V.P. Singh: Proceedings of the International Symposium on Flood-frequency and Risk Analysis, May 14-17, 1986, Louisiana State University, 

Baton Rouge, p. 1-9.

Appendix 7. Regression Analysis 89

3.38E-02 -2.54E-03 -6.71E-03 -2.83E-02 1

1 log(69.4) log(5.9) log(6.04) × -2.54E-03 7.09E-04 1.35E-03 2.22E-04 × log(69.4)

-6.71E-03 1.35E-03 3.63E-03 2.03E-03 log(5.9)

-2.83E-02 2.22E-04 2.03E-03 3.30E-02 log(6.04)

PE% = 100 e

5.30119(0.279) – 1 0.5 

= 69.1 percent .
90 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Wiley, J.B., Atkins, J.T., and Tasker, G.D., 2000, Estimating magnitude and frequency of peak discharges for rural, unregulated, 

streams in West Virginia: U.S. Geological Survey Water-Resources Investigations Report 00-4080, 98 p.

Zuehls, E.E., Ryan, G.L., Peart, D.B., and Fitzgerald, K.K., 1981, Hydrology of area 35, Eastern Region, interior coal province, 

Illinois and Kentucky – Big Muddy River, Saline River, Ohio River: U.S. Geological Survey Water-Resources Investigations, 

Open-File Report 81-403, 68 p.
Appendix 8. Digital Data and Plots

The CD-ROM (in pocket), contains input data, output results, and plots produced for the flood-frequency 

analysis for the State of Illinois. The eight sub-directories in the CD-ROM are described in three major groups: the 

input and output files for the annual maximum series (AMS), the input and output files for the partial duration series 

(PDS), and basin characteristics derived from the BASINSOFT program. The input data for the AMS and PDS 

are obtained from the peak-flow files and are presented in space-delimited American Standard Code for Information Interchange (ASCII) text files. The covariance matrix {XT Λ-1 X}-1 for each regression model for each recurrence interval obtained from the EGLS analysis using GLSNET program also is presented in the ASCII file. The 

ASCII text files can be opened with a text editor for a quick view or imported into a spreadsheet or database for data 

analysis. The basin characteristics are presented in the Microsoft Excel file and can be opened with Excel or similar 

programs. The outputs are plots from the AMS and PDS analysis and are presented with postscript files. These files 

can be opened with Adobe Acrobat reader. For explanations of variable fields and station option records, users can 

refer to Novak (1985) for the AMS and PDS input files, and appendix 5 for basin characteristics. The locations of 

the AMS and PDS stations are given in figures 2A, 2B, and 3 in the main text.

Descriptions of Files Stored on CD

Files stored on the CD-ROM are organized in directories (folders). The eight directories and associated file 

names, and contents are:

Specific content descriptions of the directories are given below.

AMS_PEAKFQ_INPUT is a directory containing the data file AMS_424_1999.inp for the 419 streamflow-gaging

stations with more than 10 years of annual maximum series data with WY 1999 as the ending year for data 

retrieval. The last five stations at the end of the file contain records modified to remove the period affected

by reservoir operations. Overall, these 419 stations either are in rural or in urbanized areas, or subject to 

flow alterations. Among these 419 stations, 288 stations are selected for regression analysis (appendix 1). 

The file can be run with the frequency-analysis program PEAKFQ.

AMS_PEAKFQ_OUTPUT is a directory containing output of the PEAKFQ analysis of the input data 

(AMS_424_1999.inp). The file name is AMS_424_1999.out. 

AMS_PLOT_OUTPUT is a directory containing a postscript file (Adobe Acrobat readable) AMS_plots.pdf for all

the AMS stations included in AMS_424_1999.out. The plots are obtained from the PEAKFQ program. 

Appendix 8. Digital Data and Plots 91

Directory name File name Content

AMS_PEAKFQ_INPUT AMS_424_1999.inp AMS input data for PEAKFQ program

AMS_PEAKFQ_OUTPUT AMS_424_1999.out AMS output from PEAKFQ program

AMS_PLOT_OUTPUT AMS_plots.pdf Postscript plots of AMS curves

AMS_GLS_OUTPUT Cov_matrix.xls {XT Λ-1 X}-1 matrices

PDS_INPUT PDS_1999.inp PDS input data

PDS_OUTPUT PDS_142_1999.xls PDS output data

PDS_PLOT_OUTPUT Various pdf files Postscript files for plots of PDS curves

BASIN_CHARACTERISTICS Basin_char_288_xls Basin characteristics of the watersheds

(same) AMS_fig2a.pdf Digital image of figure 2A

(same) AMS_fig2b.pdf Digital image of figure 2B

(same) PDS_fig3.pdf Digital image of figure 3

(same) Hydrologic_regions.pdf Digital image of figure 5

(same) PermAvg_fig8a.pdf Digital image of figure 8A

(same) PermAvg_fig8b.pdf Digital image of figure 8B

(same) PermAvg_fig8c.pdf Digital image of figure 8C

(same) PermAvg_fig8d.pdf Digital image of figure 8D
92 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

AMS_GLS_OUTPUT is a directory containing an Excel file Cov_matrix.xls, which contains the {XT Λ-1 X}-1 matrix 

for each regression model at each recurrence interval. 

PDS_INPUT is a directory containing the data file PDS_1999.inp that includes all the partial duration series records

retrieved from peak-flow files with WY 1999 as the ending year. The format is the same as the AMS data

file. 

PDS_OUTPUT is a directory containing the output file PDS_142_1999.xls for the 142 streamflow-gaging stations

that are fit with the Generalized Pareto distribution. The reported estimates are for recurrence intervals of

0.8, 1.01, 1.5, 2. 3, and 5 years and based on the average number of years equal to 1.6.

PDS_PLOT_OUTPUT is a directory containing the Adobe Acrobat readable postcript files for the GP-fitted floodfrequency curves (for example, figure 6) for the 142 streamflow-gaging stations. 

BASIN_CHARACTERISTICS directory contains an Excel file Basin_char_288.exl for the 288 streamflow-gaging

stations used in the regression analysis, and eight Adobe Acrobat readable files that are digital images of

figures 2A, 2B, 3, 7, 8A, B, C, and D that users can use the zoom features to identify their sites or parameter

values. These eight files are: AMS_fig2a.pdf, location of active and inactive U.S. Geological Survey

streamflow-gaging stations in Illinois and adjacent States where annual maximum series were retrieved and

flood quantiles were estimated, and stations used in the regression analysis.

AMS_fig2b.pdf, location of active and inactive U.S. Geological Survey streamflow-gaging stations in

northern Illinois and adjacent States where annual maximum series were retrieved and flood quantiles were 

estimated, and stations used in the regression analysis.

PDS_fig3.pdf, Location of active and non-active U.S. Geological Survey streamflow-gaging stations in

Illinois and adjacent States where partial duration series were retrieved, and stations used in the regression 

analysis.

Hydrologic_Regions.pdf, Hydrologic regions for flood-frequency analysis of rural streams in Illinois.

PermAvg_fig8a.pdf, Average soil permeability (from 0.2 to less than 1.0 inch per hour) for Illinois. Average

soil permeability is obtained by taking the arithmetic average of the high and low soil permeability values 

from the STATSGO database (Natural Resources Conservation Service, 1993).

PermAvg_fig8b.pdf, Average soil permeability (from 1.0 to less than 2.0 inches per hour) for Illinois. 

PermAvg_fig8c.pdf, Average soil permeability (from 2.0 to less than 3.0 inches per hour) for Illinois. 

PermAvg_fig8d.pdf, Average soil permeability (from 3.0 to greater than 8.0 inches per hour) for Illinois. 

References

Natural Resources Conservation Service (formerly Soil Conservation Services), 1993, State Soil Geographic (STATSGO) database: U.S. Department of Agriculture, Miscellaneous Publication No. 1492, 110 p. and computer data.

Novak, C.E., 1985, Water Resources Division data report preparation guide: U.S. Geological Survey Water-Resources Division, 

199 p. plus appendixes.
Table 1. Flood-peak discharges for recurrence intervals, T, of 2, 5, 10, 25, 50, 100, and 500 years estimated from the annual maximum 

series at streamflow-gaging stations in Illinois and adjacent States.

[T, recurrence interval in years, QT, instantaneous peak-flood discharge, in cubic feet per second, for a given T of 2-, 5-, 10-, 25-, 50-, 100-, and 500-year flood. 

Three estimates are listed for each station: the values in the top row are QT from at-site frequency curves; values in the middle row are QT from regional regression equations; values in the bottom row are QT obtained by weighting the at-site and regional regression frequency curves; NA, not assigned; dashes (---) given 

in any QT row indicates that the corresponding frequency curves are not computed. Station noted by an asterisk (*) have anomalous characteristics and are omitted from the regional analysis]

Station 

number

(figs. 2A 

and 2B)

Station Name Hydrologic 

Region

Flood Quantiles of Selected Recurrence Interval

Q2 Q5 Q10 Q25 Q50 Q100 Q500

03336100 Big Four Ditch Tributary near Paxton, Ill. 3 115 183 228 284 324 363 450

132 251 342 467 565 667 913

117 192 245 315 368 420 542

03336500 Bluegrass Creek at Potomac, Ill. 3 1,840 2,870 3,580 4,490 5,180 5,870 7,500

1,310 2,350 3,130 4,160 4,970 5,780 7,750

1,780 2,810 3,520 4,440 5,150 5,870 7,570

03336645 Middle Fork Vermilion River above Oakwood, Ill. 3 6,500 9,470 11,400 13,800 15,500 17,200 21,000

5,560 9,510 12,300 16,000 18,800 21,700 28,300

6,380 9,470 11,500 14,200 16,100 18,100 22,600

03336900 Salt Fork near St. Joseph, Ill. 3 2,530 3,810 4,740 5,990 6,970 8,000 10,600

2,580 4,560 6,010 7,940 9,420 10,900 14,600

2,530 3,880 4,870 6,220 7,280 8,390 11,200

03337000 Boneyard Creek at Urbana, Ill. NA 533 705 815 951 1,050 1,150 1,370

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

03337500 Saline Branch at Urbana, Ill. 3 1,300 2,110 2,680 3,410 3,970 4,530 5,850

1,070 1,840 2,380 3,090 3,630 4,170 5,440

1,280 2,100 2,650 3,380 3,930 4,480 5,790

03338000 Salt Fork near Homer, Ill. 3 3,780 6,120 7,880 10,330 12,300 14,400 19,800

4,140 7,120 9,250 12,000 14,200 16,300 21,400

3,800 6,200 8,000 10,500 12,500 14,600 20,100

03338100 Salt Fork Trib near Catlin, Ill. 3 189 375 515 703 847 991 1,330

188 354 481 655 792 934 1,280

189 371 509 693 836 980 1,320

03338500 Vermilion River near Catlin, Ill. 3 8,570 15,000 20,300 27,900 34,500 41,700 61,400

9,600 16,400 21,200 27,600 32,400 37,300 48,900

8,700 15,200 20,400 27,900 34,000 40,600 58,100

03338780 North Fork Vermilion River near Bismarck, Ill. 3 8,010 13,300 17,200 22,500 26,600 30,900 41,500

3,900 6,780 8,870 11,600 13,700 15,800 20,900

6,900 11,300 14,300 18,200 21,200 24,300 32,000

03338800 N F Vermilion River Tributary near Danville, Ill. 3 292 539 737 1,030 1,270 1,530 2,220

199 387 535 740 904 1,080 1,500

274 505 685 941 1,150 1,380 1,960

03339000 Vermilion River near Danville, Ill. 3 14,200 22,200 27,600 34,600 39,900 45,200 57,600

12,200 20,800 27,000 35,100 41,300 47,500 62,400

14,200 22,100 27,600 34,700 40,000 45,300 58,000

Table 1. Flood-peak dishcarges for recurrence intervals, T, of 2, 5, 10, 25, 50, 100, and 500 years estimated from the annual maximum series 93
94 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Station 

number

(figs. 2A 

and 2B)

Station Name Hydrologic 

Region

Flood Quantiles of Selected Recurrence Interval

Q2 Q5 Q10 Q25 Q50 Q100 Q500

03341700 Big Creek Tributary near Dudley, Ill. 3 183 293 375 488 579 675 922

157 309 429 597 731 873 1,230

177 297 389 520 627 740 1,030

03341900 Raccoon Creek Trib near Annapolis, Ill. 5 18 32 42 57 69 82 114

14 27 37 51 63 74 104

17 31 41 56 68 80 112

03343400 Embarras River near Camargo, Ill. 3 3,200 4,850 5,870 7,060 7,880 8,630 10,200

2,620 4,520 5,890 7,680 9,040 10,400 13,700

3,150 4,820 5,880 7,140 8,030 8,870 10,700

03344000 Embarras River near Diona, Ill. 3 9,200 13,800 17,000 20,900 23,800 26,600 33,300

6,940 11,600 14,900 19,100 22,300 25,400 32,900

8,990 13,600 16,700 20,600 23,500 26,400 33,200

03344250 Embarras River Tributary near Greenup, Ill. NA 23 38 49 65 78 92 127

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

03344425 Muddy Creek Tributary at Woodbury, Ill. 5 23 49 73 111 146 188 313

58 118 166 234 289 348 498

28 60 90 139 183 232 371

03344500 Range Creek near Casey, Ill. 5 896 1,620 2,220 3,110 3,870 4,720 7,050

603 1,100 1,460 1,950 2,330 2,720 3,670

868 1,560 2,120 2,920 3,600 4,330 6,320

03345500 Embarras River at Ste. Marie, Ill. 5 14,600 23,900 30,500 39,000 45,500 52,000 67,500

13,800 22,900 29,100 37,100 43,100 49,100 63,500

14,600 23,900 30,500 39,000 45,500 52,000 67,500

03346000 North Fork Embarras River near Oblong, Ill. 5 8,180 14,200 18,600 24,400 28,700 33,100 43,500

6,490 11,200 14,500 18,800 22,100 25,500 33,600

8,090 14,000 18,300 23,900 28,100 32,400 42,600

03378000 Bonpas Creek at Browns, Ill. 5 3,110 4,360 5,190 6,240 7,020 7,810 9,670

3,950 6,640 8,520 10,900 12,800 14,600 19,000

3,140 4,450 5,350 6,510 7,380 8,260 10,300

03378635 Little Wabash River near Effingham, Ill. 5 6,190 9,400 11,700 14,900 17,300 19,900 26,400

6,140 10,800 14,100 18,500 21,900 25,400 33,900

6,180 9,510 12,000 15,300 17,900 20,600 27,500

03378650 Second Creek Tributary at Keptown, Ill. 5 227 385 509 687 836 998 1,430

168 313 422 570 685 804 1,100

216 369 487 654 790 935 1,310

03378900 Little Wabash River at Louisville, Ill. 5 11,500 18,800 24,300 31,900 38,200 44,800 62,100

11,200 18,900 24,400 31,500 36,900 42,300 55,300

11,500 18,800 24,300 31,900 38,000 44,400 60,800

03378980 Little Wabash River Trib at Clay City, Ill. 5 127 241 332 460 564 674 956

118 235 328 457 562 672 950

125 240 330 459 563 673 953
Table 1. Flood-peak dishcarges for recurrence intervals, T, of 2, 5, 10, 25, 50, 100, and 500 years estimated from the annual maximum series 95

Station 

number

(figs. 2A 

and 2B)

Station Name Hydrologic 

Region

Flood Quantiles of Selected Recurrence Interval

Q2 Q5 Q10 Q25 Q50 Q100 Q500

03379500 Little Wabash River below Clay City, Ill. 5 13,600 23,900 31,800 42,800 51,700 61,100 85,200

14,000 23,400 30,000 38,500 44,900 51,400 66,800

13,700 23,900 31,700 42,600 51,300 60,400 83,700

03379650 Madden Creek near West Salem, Ill. 5 411 664 860 1,140 1,380 1,630 2,320

292 559 764 1,050 1,270 1,510 2,090

389 642 838 1,120 1,350 1,590 2,250

03380300 Dums Creek Tributary near Iuka, Ill. NA 42 66 83 104 121 138 179

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

03380350 Skillet Fork near Iuka, Ill. 6 4,940 9,000 12,100 16,500 20,000 23,600 32,900 

5,430 8,930 11,500 14,900 17,500 20,200 26,800

4,990 8,990 12,000 16,200 19,500 22,900 31,500 

03380400 Horse Creek Tributary near Cartter, Ill. 6 221 366 474 621 738 862 1,170

264 486 661 905 1,100 1,310 1,840

229 390 517 696 842 995 1,380

03380450 White Feather Creek near Marlow, Ill. 6 134 205 254 317 364 412 524

153 289 397 550 674 807 1,150

136 215 273 351 412 475 627

03380475 Horse Creek near Keenes, Ill. 6 4,030 6,790 8,830 11,600 13,800 16,000 21,600 

3,540 5,940 7,690 10,100 11,900 13,800 18,500

4,000 6,720 8,710 11,400 13,500 15,700 21,100 

03380500 Skillet Fork at Wayne City, Ill. 6 8,700 16,000 22,100 31,200 38,900 47,600 71,500 

7,930 12,700 16,100 20,600 24,100 27,600 36,100

8,670 15,900 21,900 30,600 38,000 46,300 68,800

03381500 Little Wabash River at Carmi, Ill. 5 15,600 23,500 29,100 36,500 42,200 48,100 62,700 

22,600 36,700 46,300 58,500 67,600 76,800 98,100

15,800 24,000 29,900 37,600 43,700 49,900 65,000

03381600 Little Wabash River Tributary nr New Haven, Ill. 5 88 157 212 293 362 437 643

62 125 175 245 302 362 515

83 149 202 278 342 411 595

03382025 Little Saline Creek Tributary near Goreville, Ill. 6 271 354 408 475 525 574 688

185 351 483 672 827 992 1,410

258 354 421 511 581 653 827

03382100 South Fork Saline River nr Carrier Mills, Ill. 6 2,790 3,790 4,440 5,260 5,860 6,460 7,850

3,470 5,580 7,110 9,150 10,700 12,300 16,300

2,820 3,870 4,590 5,500 6,190 6,870 8,490

03382170 Brushy Creek near Harco, Ill. 6 1,130 1,550 1,840 2,220 2,500 2,800 3,530

1,030 1,780 2,340 3,120 3,740 4,390 6,000

1,110 1,580 1,930 2,400 2,770 3,150 4,090

03382500 Saline River near Junction, Ill. NA 10,800 17,300 22,200 29,000 34,500 40,400 55,600

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----
96 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Station 

number

(figs. 2A 

and 2B)

Station Name Hydrologic 

Region

Flood Quantiles of Selected Recurrence Interval

Q2 Q5 Q10 Q25 Q50 Q100 Q500

03382510 Eagle Creek near Equality, Ill. 6 520 599 642 690 722 751 812

1,030 1,860 2,500 3,390 4,110 4,870 6,780

575 727 845 1,000 1,120 1,240 1,500

03382520 Black Branch Tributary near Junction, Ill. NA 154 304 434 634 809 1,010 1,570

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

03384450 Lusk Creek near Eddyville, Ill. 7 5,180 7,690 9,460 11,800 13,700 15,600 20,400

3,630 5,680 7,150 9,120 10,600 12,200 16,100

5,090 7,540 9,260 11,600 13,400 15,200 19,800

03385000 Hayes Creek at Glendale, Ill. 7 2,170 3,610 4,690 6,160 7,320 8,540 11,600

2,310 3,670 4,670 6,010 7,060 8,150 10,800

2,170 3,610 4,690 6,150 7,310 8,520 11,600

03385500 Lake Glendale Inlet near Dixon Springs, Ill. 7 569 932 1,200 1,550 1,820 2,110 2,810

495 849 1,120 1,500 1,810 2,130 2,950

563 924 1,190 1,540 1,820 2,110 2,830

03386500 Sugar Creek near Dixon Springs, Ill. NA 1,440 1,890 2,200 2,600 2,910 3,220 4,000

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

03612000 Cache River at Forman, Ill. 7 3,640 5,870 7,480 9,650 11,300 13,100 17,400

6,090 8,710 10,500 12,800 14,500 16,200 20,300

3,670 5,910 7,540 9,720 11,400 13,200 17,500

03612200 Q Ditch Tributary near Choat, Ill. NA 134 224 296 401 489 587 854

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

03614000 Hess Bayou Tributary near Mound City, Ill. 7 449 594 692 819 915 1,010 1,250

367 589 751 967 1,140 1,310 1,740

439 594 702 845 957 1,070 1,350

04087300 Lake Michigan Tributary at Winthrop Harbor, Ill. 2 82 148 199 270 327 388 544

111 194 253 330 387 444 575

86 154 207 281 339 400 551

04087400 Kellogg Ravine at Zion, Ill. 2 239 412 542 721 864 1,010 1,390

228 389 503 647 754 860 1,100

237 409 535 705 837 973 1,300

05414820 Sinsinawa River near Menominee, Ill. 1 2,720 5,840 8,810 13,800 18,500 24,100 41,800

1,610 2,870 3,820 5,100 6,110 7,160 9,760

2,610 5,460 8,000 12,000 15,700 19,900 32,600

05415000 Galena River at Buncombe, Wis. 1 4,190 7,030 9,320 12,700 15,600 18,800 27,700

3,030 5,250 6,880 9,060 10,800 12,500 16,800

4,130 6,910 9,130 12,400 15,100 18,100 26,400

05415500 East Fork Galena River at Council Hills, Ill. 1 1,930 4,140 6,280 9,930 13,500 17,800 31,700

1,180 2,150 2,900 3,920 4,730 5,590 7,720

1,850 3,850 5,680 8,620 11,300 14,500 24,300
Table 1. Flood-peak dishcarges for recurrence intervals, T, of 2, 5, 10, 25, 50, 100, and 500 years estimated from the annual maximum series 97

Station 

number

(figs. 2A 

and 2B)

Station Name Hydrologic 

Region

Flood Quantiles of Selected Recurrence Interval

Q2 Q5 Q10 Q25 Q50 Q100 Q500

05418750 South Fork Apple River near Nora, Ill. 1 202 360 480 644 774 910 1,250

214 399 541 737 893 1,060 1,470

204 366 490 663 800 944 1,300

05418800 Mill Creek Tributary near Scales Mound, Ill. 1 247 431 577 788 964 1,160 1,670

184 357 496 692 851 1,020 1,450

236 417 559 763 932 1,120 1,600

05419000 Apple River near Hanover, Ill. 1 5,140 7,610 9,360 11,700 13,500 15,500 20,200

4,550 7,770 10,100 13,200 15,700 18,100 24,200

5,120 7,620 9,410 11,800 13,700 15,700 20,600

05420000 Plum River bl Carroll Creek nr Savanna, Ill. 1 3,510 5,930 7,730 10,200 12,200 14,200 19,400

4,070 6,920 8,990 11,700 13,800 16,000 21,300

3,550 6,000 7,840 10,400 12,300 14,400 19,600

05430500 Rock River at Afton, Wis. 1 6,390 8,840 10,400 12,200 13,400 14,600 17,300

6,630 9,400 11,100 13,100 14,400 15,700 18,300

6,390 8,840 10,400 12,200 13,400 14,600 17,300

05431486 Turtle Creek at Carvers Rock Road nr Clinton, Wis. 1 1,840 3,580 5,050 7,260 9,170 11,300 17,200

2,070 3,400 4,340 5,550 6,470 7,400 9,650

1,850 3,570 5,010 7,150 8,960 11,000 16,400

05434500 Pecatonica River at Martintown, Wis. 1 5,070 8,330 10,800 14,100 16,700 19,500 26,600

7,550 12,100 15,300 19,400 22,400 25,500 32,900

5,140 8,450 10,900 14,300 17,100 19,900 27,000

05435000 Cedar Creek near Winslow, Ill. 1 100 292 497 859 1,210 1,630 2,930

160 299 405 552 669 792 1,100

105 292 480 790 1,070 1,400 2,340

05435500 Pecatonica River at Freeport, Ill. 1 5,470 8,920 11,600 15,300 18,400 21,800 30,600

7,700 12,200 15,200 19,100 22,000 24,900 31,800

5,520 8,990 11,700 15,500 18,600 21,900 30,700

05435650 Lost Creek Tributary near Shannon, Ill. 1 285 401 477 571 639 706 861

199 367 496 674 815 962 1,330

268 394 480 594 682 771 984

05436500 Sugar River near Brodhead, Wis. 1 3,280 5,930 8,040 11,100 13,600 16,300 23,400

4,600 7,580 9,670 12,400 14,500 16,600 21,800

3,310 5,970 8,090 11,100 13,600 16,300 23,300

05436900 Otter Creek Tributary near Durand, Ill. 1 51 97 136 192 240 293 436

116 226 313 436 536 642 913

58 113 161 234 296 363 542

05437000 Pecatonica River at Shirland, Ill. 1 8,280 12,300 14,900 18,300 20,800 23,300 29,100

9,360 14,400 17,800 22,100 25,200 28,400 35,700

8,340 12,400 15,100 18,600 21,200 23,800 29,800

05437500 Rock River at Rockton, Ill. 1 14,700 20,800 24,600 29,200 32,500 35,700 42,900

17,900 27,500 34,000 42,200 48,300 54,400 68,800

14,800 20,900 24,900 29,700 33,100 36,500 44,000
98 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Station 

number

(figs. 2A 

and 2B)

Station Name Hydrologic 

Region

Flood Quantiles of Selected Recurrence Interval

Q2 Q5 Q10 Q25 Q50 Q100 Q500

05437600 Rock River Tributary near Rockton, Ill. 1 142 259 344 455 539 623 818

235 439 596 813 987 1,170 1,630

155 286 389 529 638 750 1,020

05437950 Kishwaukee River near Huntley, Ill. 2 128 158 175 194 206 216 238

320 519 654 820 941 1,060 1,320

142 187 218 258 285 311 364

05438250 Coon Creek at Riley, Ill. 2 1,250 2,120 2,690 3,400 3,890 4,370 5,370

1,330 2,170 2,740 3,450 3,970 4,470 5,600

1,250 2,110 2,690 3,400 3,900 4,380 5,420

05438300 Lawrence Creek Tributary near Harvard, Ill. 2 77 130 168 219 258 299 398

96 174 232 309 367 426 564

79 135 177 234 278 323 434

05438390 Piscasaw Creek below Mokeler Creek nr Capron, Ill. 2 1,800 2,530 3,010 3,630 4,090 4,550 5,630

1,320 2,150 2,700 3,400 3,900 4,390 5,490

1,710 2,450 2,940 3,570 4,030 4,500 5,580

05438500 Kishwaukee River at Belvidere, Ill. 2 3,890 6,760 8,850 11,600 13,700 15,900 21,000

4,080 6,400 7,940 9,820 11,200 12,500 15,400

3,890 6,740 8,800 11,500 13,600 15,700 20,600

05438850 M Br of So Br Kishwaukee R nr Malta, Ill. 2 136 246 320 411 476 537 665

111 193 251 326 381 436 562

134 239 310 397 460 519 649

05439000 South Branch Kishwaukee River at Dekalb, Ill. 2 908 1,440 1,810 2,300 2,680 3,070 4,020

927 1,470 1,820 2,250 2,560 2,860 3,510

909 1,440 1,810 2,290 2,660 3,040 3,930

05439500 South Branch Kishwaukee River nr Fairdale, Ill. 2 4,170 7,030 9,060 11,700 13,700 15,700 20,400

2,610 4,030 4,950 6,050 6,830 7,580 9,200

4,110 6,960 8,850 11,320 13,200 15,100 19,400

05439550 South Branch Kishwaukee River Trib nr Irene, Ill. 2 81 191 287 433 556 690 1,040

151 271 358 472 559 646 849

88 201 299 441 558 681 992

05440000 Kishwaukee River near Perryville, Ill. 2 7,490 12,500 15,800 20,100 23,200 26,200 32,900

6,610 10,300 12,700 15,700 17,800 19,800 24,300

7,450 12,400 15,700 19,800 22,700 25,600 32,000

05440500 Killbuck Creek near Monroe Center, Ill. 2 2,400 4,320 5,590 7,110 8,160 9,130 11,100

1,610 2,610 3,290 4,130 4,730 5,330 6,650

2,360 4,200 5,400 6,820 7,810 8,730 10,600

05440650 Stillman Creek Tributary near Holcomb, Ill. 2 82 144 188 248 293 339 447

79 139 181 236 277 317 410

81 143 187 245 290 334 439

05440900 Leaf River Tributary near Forreston, Ill. 1 56 104 143 199 246 298 436

55 107 149 207 254 305 433

56 104 144 201 249 300 436
Table 1. Flood-peak dishcarges for recurrence intervals, T, of 2, 5, 10, 25, 50, 100, and 500 years estimated from the annual maximum series 99

Station 

number

(figs. 2A 

and 2B)

Station Name Hydrologic 

Region

Flood Quantiles of Selected Recurrence Interval

Q2 Q5 Q10 Q25 Q50 Q100 Q500

05441000 Leaf River at Leaf River, Ill. 1 2,770 5,580 7,730 10,600 12,800 15,100 20,200

2,620 4,570 6,000 7,930 9,430 11,000 14,800

2,760 5,500 7,600 10,300 12,400 14,500 19,500

05441500 Rock River at Oregon, Ill. 1 21,700 33,300 41,800 53,400 62,600 72,200 96,800

22,400 34,600 42,800 53,100 60,800 68,600 86,800

21,800 33,500 42,000 53,000 62,200 71,300 94,000

05442000 Kyte River near Flagg Center, Ill. 2 1,270 1,740 2,030 2,370 2,610 2,840 3,330

1,330 2,110 2,630 3,260 3,710 4,150 5,110

1,280 1,800 2,130 2,540 2,840 3,100 3,740

05443500 Rock River at Como, Ill. 1 24,800 36,200 43,200 51,500 57,300 62,800 74,500

23,500 36,300 44,800 55,600 63,700 71,800 91,000

24,800 36,100 43,200 51,600 57,500 63,100 75,200

05444000 Elkhorn Creek near Penrose, Ill. 1 2,980 4,670 5,710 6,900 7,700 8,430 9,910

2,200 3,660 4,690 6,040 7,070 8,120 10,600

2,950 4,620 5,650 6,860 7,690 8,460 10,100

05444100 Spring Creek Tributary near Coleta, Ill. 1 272 480 634 841 1,000 1,160 1,560

216 411 564 777 948 1,130 1,590

260 464 615 822 985 1,150 1,570

05445500 Rock Creek near Morrison, Ill. 1 2,260 3,320 4,050 5,000 5,720 6,460 8,250

2,410 4,040 5,210 6,740 7,910 9,120 12,000

2,270 3,370 4,140 5,160 5,950 6,750 8,690

05446000 Rock Creek at Morrison, Ill. 1 2,120 2,960 3,520 4,250 4,800 5,360 6,710

2,380 3,980 5,120 6,610 7,750 8,920 11,700

2,130 3,000 3,600 4,380 4,980 5,590 7,060

05446500 Rock River near Joslin, Ill. 1 23,500 34,200 41,100 49,400 55,400 61,100 73,900

26,200 40,400 50,100 62,300 71,400 80,600 102,000

23,600 34,400 41,400 50,000 56,100 62,100 75,500

05446950 Green River Tributary near Amboy, Ill. 2 91 202 300 452 585 733 1,140

74 132 175 231 273 316 414

88 188 270 388 485 589 860

05447000 Green River at Amboy, Ill. 2 2,730 4,460 5,520 6,740 7,550 8,280 9,720

1,650 2,560 3,150 3,870 4,370 4,860 5,910

2,670 4,330 5,340 6,490 7,260 7,960 9,380

05447050 Green River Tributary No 2 near Ohio, Ill. 2 148 233 292 368 425 483 618

231 395 510 657 766 874 1,120

158 254 325 420 491 563 730

05447200 Normandy Ditch at Normandy, Ill. NA 47 72 88 108 122 136 165

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05447350 Mud Creek Tributary near Atkinson, Ill. 4 192 327 434 590 721 864 1,250

191 356 483 659 798 944 1,300

192 332 445 607 741 886 1,270
100 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Station 

number

(figs. 2A 

and 2B)

Station Name Hydrologic 

Region

Flood Quantiles of Selected Recurrence Interval

Q2 Q5 Q10 Q25 Q50 Q100 Q500

05447500 Green River near Geneseo, Ill. 2 6,260 8,690 10,100 11,600 12,600 13,400 15,200

5,240 8,030 9,840 12,000 13,600 15,100 18,300

6,230 8,660 10,100 11,600 12,600 13,600 15,400

05448000 Mill Creek at Milan, Ill. 4 2,640 4,580 5,960 7,750 9,090 10,400 13,500

2,350 4,100 5,400 7,140 8,490 9,870 13,200

2,630 4,560 5,930 7,700 9,040 10,400 13,500

05448050 Sand Creek near Milan, Ill. 4 36 79 120 186 247 317 526

66 126 174 241 295 351 491

38 85 128 196 256 325 518

05466000 Edwards River near Orion, Ill. 4 3,420 4,610 5,350 6,230 6,860 7,470 8,820

3,550 5,970 7,710 10,000 11,700 13,500 17,800

3,430 4,660 5,460 6,430 7,130 7,820 9,360

05466500 Edwards River near New Boston, Ill. 4 4,170 6,450 7,990 9,950 11,400 12,800 16,200

6,340 10,000 12,600 15,800 18,300 20,700 26,500

4,210 6,540 8,130 10,200 11,700 13,200 16,700

05467000 Pope Creek near Keithsburg, Ill. 4 2,440 4,010 5,190 6,820 8,130 9,520 13,100

3,520 5,580 7,010 8,840 10,200 11,600 14,800

2,470 4,060 5,260 6,920 8,240 9,640 13,200

05467500 Henderson Creek near Little York, Ill. 4 2,320 4,230 5,940 8,720 11,300 14,400 23,800

3,610 5,970 7,650 9,850 11,500 13,200 17,300

2,370 4,310 6,050 8,800 11,300 14,200 23,000

05468000 North Henderson Creek near Seaton, Ill. 4 1,130 1,570 1,870 2,240 2,530 2,820 3,510

2,020 3,300 4,210 5,390 6,280 7,190 9,340

1,230 1,780 2,190 2,750 3,180 3,610 4,650

05468500 Cedar Creek at Little York, Ill. 4 2,140 4,480 6,650 10,200 13,500 17,500 29,700

3,100 5,130 6,570 8,460 9,890 11,300 14,800

2,170 4,500 6,650 10,100 13,300 17,000 28,000

05469000 Henderson Creek near Oquawka, Ill. 4 4,800 8,450 11,600 16,600 21,100 26,400 42,300

7,410 12,500 16,100 21,000 24,700 28,400 37,500

4,880 8,600 11,800 16,800 21,300 26,500 41,900

05469500 South Henderson Creek at Biggsville, Ill. 4 1,690 3,230 4,600 6,790 8,800 11,400 18,200

2,550 4,220 5,420 6,990 8,190 9,400 12,300

1,720 3,270 4,650 6,800 8,700 11,000 17,500

05469750 Ellison Creek Tributary near Roseville, Ill. 4 50 92 125 172 210 250 354

92 173 235 321 389 460 637

54 100 138 192 237 284 405

05495200 Little Creek near Breckenridge, Ill. 4 393 743 1,010 1,380 1,670 1,980 2,720

204 381 517 706 854 1,010 1,390

364 679 909 1,220 1,460 1,710 2,320

05495500 Bear Creek near Marcelline, Ill. 4 9,550 16,100 20,800 26,800 31,300 35,800 46,300

5,730 9,810 12,800 16,700 19,800 22,800 30,200

9,340 15,700 20,100 25,800 30,100 34,400 44,500
Table 1. Flood-peak dishcarges for recurrence intervals, T, of 2, 5, 10, 25, 50, 100, and 500 years estimated from the annual maximum series 101

Station 

number

(figs. 2A 

and 2B)

Station Name Hydrologic 

Region

Flood Quantiles of Selected Recurrence Interval

Q2 Q5 Q10 Q25 Q50 Q100 Q500

05496900 Homan Creek Tributary near Quincy, Ill. 4 248 465 627 846 1,020 1,190 1,600

155 305 426 598 737 884 1,260

231 431 579 779 935 1,100 1,490

05501500 Burton Creek Tributary near Burton, Ill. 4 163 345 497 721 908 1,110 1,630

112 211 289 398 485 576 804

152 312 438 613 755 906 1,290

05502020 Hadley Creek near Barry, Ill. 4 4,240 6,300 7,590 9,130 10,200 11,200 13,400

2,170 3,900 5,220 7,020 8,440 9,910 13,500

4,070 6,070 7,330 8,860 9,960 11,000 13,400

05502040 Hadley Creek at Kinderhook, Ill. 4 6,650 11,100 14,200 18,000 20,900 23,700 30,000

3,060 5,400 7,140 9,500 11,400 13,300 17,900

6,390 10,600 13,400 17,000 19,600 22,200 28,200

05502120 Kiser Creek Trib near Barry, Ill. 4 373 638 836 1,110 1,330 1,550 2,130

319 630 884 1,250 1,540 1,860 2,660

366 637 845 1,140 1,370 1,620 2,260

05512500 Bay Creek at Pittsfield, Ill. 4 4,470 8,460 11,600 15,800 19,300 22,800 31,600

1,740 3,040 3,990 5,280 6,270 7,300 9,780

4,300 8,060 10,900 14,700 17,700 20,800 28,400

05513000 Bay Creek at Nebo, Ill. 4 7,120 12,200 15,700 20,300 23,600 26,900 34,300

3,860 6,490 8,390 10,900 12,800 14,800 19,500

6,930 11,800 15,100 19,300 22,400 25,400 32,400

05513200 Salt Spring Creek near Gilead, Ill. 4 282 511 687 934 1,130 1,340 1,870

368 731 1,030 1,450 1,800 2,180 3,140

292 539 739 1,030 1,260 1,510 2,150

05518000 Kankakee River at Shelby, Ind. 2 4,390 5,320 5,830 6,380 6,740 7,060 7,720

5,070 7,390 8,830 10,500 11,700 12,700 15,000

4,400 5,350 5,890 6,480 6,870 7,220 7,940

05519000 Singleton Ditch at Schneider, Ind. NA 1,230 1,770 2,120 2,550 2,850 3,140 3,800

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05519500 West Creek near Schneider, Ind. 2 954 1,370 1,630 1,950 2,170 2,380 2,830

572 886 1,090 1,330 1,500 1,660 2,010

921 1,320 1,570 1,860 2,070 2,260 2,690

05520000 Singleton Ditch at Illinoi, Ill. 2 1,710 2,070 2,250 2,450 2,570 2,680 2,880

1,800 2,790 3,450 4,240 4,800 5,340 6,530

1,720 2,100 2,320 2,560 2,720 2,870 3,160

05520500 Kankakee River at Momence, Ill. 2 6,740 8,890 10,100 11,500 12,400 13,300 15,000

5,900 8,550 10,200 12,100 13,400 14,600 17,200

6,730 8,880 10,100 11,500 12,500 13,300 15,100

05524500 Iroquois River near Foresman, Ind. 2 2,870 3,910 4,520 5,220 5,690 6,130 7,050

2,660 4,060 4,960 6,030 6,790 7,510 9,070

2,870 3,910 4,530 5,250 5,740 6,200 7,170
102 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Station 

number

(figs. 2A 

and 2B)

Station Name Hydrologic 

Region

Flood Quantiles of Selected Recurrence Interval

Q2 Q5 Q10 Q25 Q50 Q100 Q500

05525000 Iroquois River at Iroquois, Ill. 2 3,910 5,530 6,550 7,750 8,610 9,420 11,200

3,410 5,160 6,270 7,600 8,530 9,410 11,300

3,900 5,520 6,540 7,760 8,610 9,440 11,200

05525050 Eastburn Hollow near Sheldon, Ill. 3 270 518 724 1,030 1,290 1,580 2,360

360 651 868 1,160 1,380 1,610 2,170

281 539 751 1,060 1,310 1,590 2,300

05525500 Sugar Creek at Milford, Ill. 3 6,610 11,500 15,000 19,700 23,300 26,900 35,600

6,300 11,000 14,500 19,100 22,600 26,200 34,800

6,600 11,400 15,000 19,600 23,200 26,900 35,600

05526000 Iroquois River near Chebanse, Ill. 3 13,200 18,900 22,600 27,000 30,100 33,100 39,800

8,380 13,600 17,300 21,900 25,300 28,700 36,700

13,000 18,700 22,300 26,700 29,800 32,800 39,600

05526150 Kankakee River Tributary near Bourbonnais, Ill. 2 32 82 131 213 289 379 648

26 46 61 80 94 109 142

31 77 118 182 239 302 480

05526500 Terry Creek near Custer Park, Ill. 2 148 268 364 503 619 746 1,080

342 568 723 918 1,060 1,200 1,510

158 286 392 543 668 801 1,140

05527050 Prairie Creek near Frankfort, Ill. 2 82 132 172 230 279 334 485

69 121 158 205 241 276 357

80 130 169 225 271 320 450

05527500 Kankakee River near Wilmington, Ill. 2 24,710 37,200 45,400 55,500 62,900 70,100 86,300

12,600 18,500 22,200 26,500 29,600 32,400 38,500

24,400 36,700 44,600 54,300 61,400 6,820 83,600

05527800 Des Plaines River at Russell, Ill. 2 700 1,270 1,670 2,180 2,550 2,910 3,700

995 1,520 1,870 2,270 2,560 2,830 3,420

710 1,280 1,680 2,180 2,550 2,910 3,700

05527840 Des Plaines River at Wadsworth, Ill. 2 832 1,620 2,180 2,910 3,430 3,950 5,060

1,090 1,660 2,030 2,460 2,770 3,060 3,690

854 1,620 2,160 2,840 3,320 3,780 4,800

05527870 Mill Creek at Wedges Corner, Ill. NA 84 139 179 232 274 316 419

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05527900 North Mill Creek at Hickory Corners, Ill. 2 210 306 369 447 504 559 686

303 472 583 718 814 906 1,110

217 320 391 480 545 608 751

05527950 Mill Creek at Old Mill Creek, Ill. NA 535 856 1,060 1,290 1,450 1,590 1,890

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05528000 Des Plaines River near Gurnee, Ill. 2 1,310 2,000 2,420 2,900 3,230 3,530 4,150

1,270 1,900 2,290 2,760 3,090 3,390 4,050

1,310 2,000 2,420 2,910 3,240 3,550 4,180
Table 1. Flood-peak dishcarges for recurrence intervals, T, of 2, 5, 10, 25, 50, 100, and 500 years estimated from the annual maximum series 103

Station 

number

(figs. 2A 

and 2B)

Station Name Hydrologic 

Region

Flood Quantiles of Selected Recurrence Interval

Q2 Q5 Q10 Q25 Q50 Q100 Q500

05528150 Indian Creek at Diamond Lake, Ill. 2 282 522 700 939 1,120 1,310 1,760

261 423 534 674 775 874 1,100

280 510 677 894 1,060 1,220 1,610

05528170 Diamond Lake Drain at Mundelein, Ill. NA 54 76 90 107 119 131 157

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05528200 Hawthorn Drainage Ditch near Mundelein, Ill. 2 211 318 392 486 557 629 800

125 204 257 323 371 418 522

201 302 369 454 518 582 732

05528230 Indian Creek at Prairie View, Ill. NA 547 885 1,120 1,430 1,660 1,890 2,450

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05528360 Aptakisic Creek at Aptakisic, Ill. 2 111 192 253 334 398 463 623

90 146 184 230 263 295 366

108 186 241 313 367 422 553

05528400 Des Plaines River at Wheeling, Ill. NA 1,750 2,640 3,200 3,850 4,290 4,710 5,570

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05528440 Buffalo Creek near Lake Zurich, Ill. 2 81 131 165 210 244 278 359

85 149 196 256 300 345 447

82 133 171 219 256 294 382

05528470 Buffalo Creek at Long Grove, Ill. 2 228 367 460 575 657 737 915

229 377 478 606 698 789 993

228 368 462 580 666 749 934

05528500 Buffalo Creek near Wheeling, Ill. NA 384 581 699 834 925 1,010 1,170

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05529000 Des Plaines River near Des Plaines, Ill. NA 2,280 3,250 3,830 4,490 4,940 5,350 6,210

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05529300 Mc Donald Creek near Wheeling, Ill. NA 178 315 415 547 647 748 987

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05529500 Mc Donald Creek near Mount Prospect, Ill. NA 200 361 478 631 747 863 1,130

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05529900 Weller Creek at Mount Prospect, Ill. NA 485 779 986 1,250 1,460 1,670 2,160

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05530000 Weller Creek at Des Plaines, Ill. NA 749 1,080 1,290 1,520 1,690 1,840 2,160

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----
104 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Station 

number

(figs. 2A 

and 2B)

Station Name Hydrologic 

Region

Flood Quantiles of Selected Recurrence Interval

Q2 Q5 Q10 Q25 Q50 Q100 Q500

05530400 Higgins Creek near Mount Prospect, Ill. NA 138 246 327 438 525 615 838

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05530480 Willow Creek at Orchard Place, Ill. NA 564 1,050 1,420 1,940 2,350 2,790 3,880

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05530600 Des Plaines River at River Grove, Ill. NA 2,560 3,380 3,850 4,360 4,700 5,010 5,640

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05530700 Silver Creek at Melrose Park, Ill. NA 444 557 622 698 749 798 901

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05530800 Des Plaines River at Forest Park, Ill. NA 2,760 3,680 4,200 4,780 5,170 5,510 6,200

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05530940 Salt Creek at Palatine, Ill. NA 204 290 348 422 478 535 670

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05530960 Salt Creek near Palatine, Ill. NA 303 467 580 724 831 939 1,190

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05530990 Salt Creek at Rolling Meadows, Ill. NA 783 1,060 1,230 1,440 1,590 1,730 2,060

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05531000 Salt Creek near Arlington Heights, Ill. NA 453 701 864 1,070 1,210 1,360 1,680

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05531050 Salt Creek near Wood Dale, Ill. NA 667 1,000 1,230 1,520 1,750 1,970 2,510

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05531080 Spring Brook at Bloomingdale, Ill. NA 175 247 296 359 406 455 571

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05531100 Meacham Creek at Medinah, Ill. NA 60 90 111 140 163 187 247

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05531130 Spring Brook at Walnut Ave at Itasca, Ill. NA 241 339 404 485 544 604 742

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05531200 Salt Creek at Addison, Ill. NA 814 1,090 1,270 1,490 1,650 1,810 2,190

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----
Table 1. Flood-peak dishcarges for recurrence intervals, T, of 2, 5, 10, 25, 50, 100, and 500 years estimated from the annual maximum series 105

Station 

number

(figs. 2A 

and 2B)

Station Name Hydrologic 

Region

Flood Quantiles of Selected Recurrence Interval

Q2 Q5 Q10 Q25 Q50 Q100 Q500

05531300 Salt Creek at Elmhurst, Ill. NA 1,060 1,400 1,600 1,850 2,020 2,180 2,550

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05531380 Salt Creek at Oak Brook, Ill. NA 999 1,290 1,460 1,650 1,780 1,900 2,160

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05531500 Salt Creek at Western Springs, Ill. NA 1,260 1,710 1,990 2,340 2,580 2,820 3,360

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05531800 Addison Creek at Northlake, Ill. NA 311 376 411 448 473 494 537

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05532000 Addison Creek at Bellwood, Ill. NA 464 629 732 855 941 1,020 1,210

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05532500 Des Plaines River at Riverside, Ill. NA 4,020 5,400 6,220 7,150 7,790 8,380 9,620

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05533000 Flag Creek near Willow Springs, Ill. NA 836 1,340 1,700 2,180 2,550 2,930 3,860

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05533200 Sawmill Creek Tributary near Tiedtville, Ill. 2 232 279 305 334 352 369 403

150 261 341 443 520 596 771

220 276 310 352 382 410 471

05533300 Wards Creek near Woodridge, Ill. 2 80 113 134 159 177 194 230

122 202 258 327 378 428 539

84 123 149 183 207 231 283

05533400 Sawmill Creek near Lemont, Ill. NA 656 1,120 1,460 1,920 2,280 2,660 3,580

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05533500 Des Plaines River at Lemont, Ill. NA 3,110 4,190 4,810 5,510 5,980 6,410 7,290

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05534300 North Branch Chicago River at Lake Forest, Ill. NA 192 262 303 351 383 413 475

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05534400 North Branch Chicago River at Bannockburn, Ill. 2 246 320 362 408 438 465 520

263 415 515 636 723 806 987

248 329 379 437 478 515 592

05534500 North Branch Chicago River at Deerfield, Ill. NA 338 506 615 749 845 939 1,150

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----
106 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Station 

number

(figs. 2A 

and 2B)

Station Name Hydrologic 

Region

Flood Quantiles of Selected Recurrence Interval

Q2 Q5 Q10 Q25 Q50 Q100 Q500

05534600 North Branch Chicago River at Northfield, Ill. NA 345 433 483 538 574 607 674

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05534900 Skokie River at Lake Bluff, Ill. NA 204 314 385 472 534 594 726

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05535000 Skokie River at Lake Forest, Ill. NA 234 338 401 476 527 575 676

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05535070 Skokie River near Highland Park, Ill. NA 458 628 735 862 953 1,040 1,230

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05535150 Skokie River at Northfield, Ill. NA 378 449 488 533 563 591 648

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05535200 North Branch Chicago River at Glenview, Ill. NA 696 872 977 1,100 1,190 1,270 1,450

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05535300 WF of Nb Chicago River at Bannockburn, Ill. NA 210 285 332 386 424 459 537

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05535400 WF of Nb Chicago River at Deerfield, Ill. NA 365 453 504 560 598 632 705

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05535500 WF of Nb Chicago River at Northbrook Il NA 493 691 809 945 1,040 1,120 1,300

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05535700 WF of Nb Chicago River at Glenview, Ill. NA 633 845 971 1,120 1,220 1,310 1,510

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05535800 N Branch Chicago River at Morton Grove, Ill. NA 1,030 1,340 1,530 1,750 1,910 2,060 2,390

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05536000 North Branch Chicago River at Niles, Ill. NA 1,210 1,580 1,820 2,100 2,300 2,500 2,940

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05536178 Plum Creek near Dyer, Ind. 2 1,160 1,710 2,060 2,460 2,740 3,000 3,570

589 949 1,190 1,490 1,700 1,910 2,370

1,070 1,570 1,860 2,210 2,460 2,700 3,210

05536190 Hart Ditch at Munster, Ind. 2 1,520 2,150 2,540 3,000 3,330 3,640 4,310

908 1,440 1,800 2,230 2,550 2,850 3,530

1,500 2,120 2,500 2,950 3,270 3,580 4,250
Table 1. Flood-peak dishcarges for recurrence intervals, T, of 2, 5, 10, 25, 50, 100, and 500 years estimated from the annual maximum series 107

Station 

number

(figs. 2A 

and 2B)

Station Name Hydrologic 

Region

Flood Quantiles of Selected Recurrence Interval

Q2 Q5 Q10 Q25 Q50 Q100 Q500

05536195 Little Calumet River at Munster, Ind. 2 757 943 1,060 1,190 1,280 1,370 1,570

1,040 1,640 2,030 2,520 2,860 3,200 3,940

767 969 1,100 1,250 1,370 1,470 1,710

05536201 Thorn Creek at Park Forest, Ill. NA 322 601 831 1,170 1,460 1,780 2,660

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05536207 Thorn Creek Tributary at Chicago Heights, Ill. NA 262 428 555 733 879 1,040 1,450

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05536210 Thorn Creek near Chicago Heights, Ill. NA 996 1,430 1,720 2,090 2,360 2,630 3,260

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05536215 Thorn Creek at Glenwood, Ill. NA 1,120 1,630 1,980 2,430 2,770 3,130 3,990

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05536235 Deer Creek near Chicago Heights, Ill. NA 553 731 845 983 1,080 1,180 1,410

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05536238 Butterfield Creek near Lincoln Estates, Ill. NA 141 265 362 498 607 722 1,010

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05536255 Butterfield Creek at Flossmoor, Ill. NA 709 1,170 1,520 2,000 2,390 2,800 3,860

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05536265 Lansing Ditch near Lansing, Ill. NA 180 264 319 385 433 479 583

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05536270 North Creek near Lansing, Ill. NA 354 512 614 738 827 913 1,110

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05536275 Thorn Creek at Thornton, Ill. NA 2,040 2,990 3,610 4,410 5,000 5,590 6,970

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05536290 Little Calumet River at South Holland, Ill. NA 2,520 3,350 3,840 4,420 4,820 5,200 6,010

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05536310 Calumet Union Drainage Ca near Markham, Ill. NA 300 389 440 498 538 574 650

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05536325 Little Calumet River at Harvey, Ill. NA 1,970 2,970 3,620 4,420 5,000 5,570 6,840

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----
108 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Station 

number

(figs. 2A 

and 2B)

Station Name Hydrologic 

Region

Flood Quantiles of Selected Recurrence Interval

Q2 Q5 Q10 Q25 Q50 Q100 Q500

05536335 Midlothian Creek near Tinley Park, Ill. NA 223 295 342 401 445 489 594

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05536340 Midlothian Creek at Oak Forest, Ill. NA 244 339 403 486 550 614 771

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05536460 Tinley Creek near Oak Forest, Ill. NA 421 634 778 961 1,100 1,230 1,550

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05536500 Tinley Creek near Palos Park, Ill. NA 597 953 1,220 1,590 1,890 2,210 3,030

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05536510 Navajo Creek at Palos Heights, Ill. NA 236 313 363 424 468 511 611

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05536560 Melvina Ditch near Oak Lawn, Ill. NA 140 234 301 390 458 527 691

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05536570 Stony Creek (West) at Worth, Ill. NA 421 713 922 1,200 1,400 1,610 2,110

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05536620 Mill Creek near Palos Park, Ill. NA 131 251 355 515 657 818 1,280

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05536630 Mill Creek at Palos Park, Ill. NA 213 447 664 1,010 1,340 1,720 2,860

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05536995 Chicago Sanitary and Ship Ca at Romeoville, Ill. NA 15,800 18,200 19,500 20,900 21,800 22,600 24,100

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05537500 Long Run near Lemont, Ill. NA 602 1,090 1,510 2,140 2,700 3,340 5,160

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05538000 Des Plaines River at Joliet, Ill. NA 15,400 18,100 19,600 21,300 22,400 23,400 25,500

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05538440 Spring Creek near Orland Park, Ill. NA 41 60 73 89 100 111 137

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05539000 Hickory Creek at Joliet, Ill. NA 2,910 5,040 6,820 9,520 11,900 14,600 22,300

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----
Table 1. Flood-peak dishcarges for recurrence intervals, T, of 2, 5, 10, 25, 50, 100, and 500 years estimated from the annual maximum series 109

Station 

number

(figs. 2A 

and 2B)

Station Name Hydrologic 

Region

Flood Quantiles of Selected Recurrence Interval

Q2 Q5 Q10 Q25 Q50 Q100 Q500

05539870 West Branch Du Page River at Ontarioville, Ill. 2 311 498 620 768 873 973 1,190

228 367 461 576 659 739 917

302 482 597 736 835 929 1,140

05539890 West Branch Du Page River near Wayne, Ill. 2 488 775 977 1,240 1,440 1,650 2,140

422 675 845 1,050 1,200 1,350 1,670

483 765 962 1,220 1,410 1,600 2,050

05539900 W Branch Du Page River nr West Chicago, Ill. NA 509 707 823 954 1,040 1,120 1,280

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05539950 Klein Creek at Carol Stream, Ill. 2 184 281 352 448 523 602 800

244 400 507 640 737 831 1,040

189 293 370 474 555 639 845

05540030 West Br Du Page River at West Chicago, Ill. 2 724 1,010 1,190 1,390 1,530 1,660 1,950

748 1,180 1,460 1,810 2,060 2,300 2,830

726 1,030 1,220 1,440 1,600 1,760 2,090

05540060 Kress Creek at West Chicago, Ill. NA 278 419 526 676 799 932 1,290

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05540080 Spring Brook at Wheaton, Ill. NA 149 207 245 296 334 372 466

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05540095 West Br Du Page River near Warrenville, Ill. NA 1,260 1,840 2,230 2,730 3,100 3,470 4,360

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05540110 Ferry Creek at Warrenville, Ill. NA 87 128 156 192 219 246 310

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05540130 West Branch Du Page River near Naperville, Ill. NA 2,150 2,870 3,310 3,820 4,180 4,520 5,260

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05540140 East Br Du Page River nr Bloomingdale, Ill. NA 59 102 133 173 204 234 304

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05540150 East Br Du Page River at Glen Ellyn, Ill. NA 243 398 506 643 744 845 1,080

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05540160 E Br Du Page River near Downers Grove, Ill. NA 597 862 1,030 1,250 1,400 1,550 1,900

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05540190 St. Joseph Creek at Belmont, Ill. NA 343 517 630 768 867 963 1,180

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----
110 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Station 

number

(figs. 2A 

and 2B)

Station Name Hydrologic 

Region

Flood Quantiles of Selected Recurrence Interval

Q2 Q5 Q10 Q25 Q50 Q100 Q500

05540195 St. Joseph Creek at U.S. Route 34 at Lisle, Ill. NA 617 861 1,020 1,210 1,350 1,490 1,800

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05540240 Prentiss Creek near Lisle, Ill. 2 193 304 385 497 585 678 913

283 484 626 807 941 1,070 1,380

201 322 413 540 639 742 998

05540250 East Branch Du Page River at Bolingbrook, Ill. NA 1,050 1,520 1,840 2,270 2,590 2,920 3,720

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05540275 Spring Brook at 87th Street near Naperville, Ill. NA 195 366 514 746 954 1,190 1,900

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05540500 Du Page River at Shorewood, Ill. NA 3,750 5,930 7,540 9,730 11,500 13,300 17,900

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05541750 Mazon River Tributary near Gardner, Ill. 2 106 145 167 191 206 221 249

150 248 314 396 455 513 640

109 153 180 212 234 254 297

05542000 Mazon River near Coal City, Ill. 2 9,080 14,000 17,000 20,500 22,900 25,100 29,600

3,250 5,050 6,240 7,690 8,720 9,720 11,900

8,830 13,500 16,300 19,600 21,800 23,800 28,100

05543500 Illinois River at Marseilles, Ill. NA 45,000 63,500 75,200 89,300 99,400 109,000 131,000

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05545750 Fox River near New Munster, Wis. 2 2,690 3,860 4,660 5,680 6,460 7,240 9,100

2,670 3,870 4,620 5,490 6,090 6,660 7,860

2,690 3,860 4,650 5,670 6,430 7,190 9,010

05547755 Squaw Creek at Round Lake, Ill. 2 188 254 294 340 372 402 466

248 386 477 585 662 736 896

195 271 322 383 427 468 558

05548150 North Br Nippersink Crk nr Genoa City, Wis. 2 181 270 332 410 470 530 672

290 468 587 734 840 942 1,170

185 279 346 432 497 562 716

05548280 Nippersink Creek near Spring Grove, Ill. NA 1,270 2,030 2,540 3,170 3,630 4,080 5,080

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05549000 Boone Creek near Mc Henry, Ill. 2 126 199 247 308 353 396 494

357 581 734 924 1,060 1,200 1,500

131 209 264 334 385 436 549

05549700 Mutton Creek at Island Lake, Ill. 2 80 161 230 332 419 514 771

246 397 500 627 719 808 1,010

89 179 255 368 460 559 813
Table 1. Flood-peak dishcarges for recurrence intervals, T, of 2, 5, 10, 25, 50, 100, and 500 years estimated from the annual maximum series 111

Station 

number

(figs. 2A 

and 2B)

Station Name Hydrologic 

Region

Flood Quantiles of Selected Recurrence Interval

Q2 Q5 Q10 Q25 Q50 Q100 Q500

05549850 Flint Creek near Fox River Grove, Ill. 2 275 380 449 535 599 663 812

507 794 987 1,220 1,390 1,560 1,920

285 401 481 584 660 736 911

05549900 Fox River Tributary near Cary, Ill. 2 11 25 37 56 72 90 139

18 32 44 58 69 81 107

12 26 38 56 71 88 131

05550000 Fox River at Algonquin, Ill. NA 3,270 4,520 5,280 6,190 6,820 7,420 8,730

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05550300 Tyler Creek at Elgin, Ill. 2 342 457 531 622 688 753 902

773 1,270 1,610 2,040 2,350 2,650 3,340

370 514 618 754 855 955 1,180

05550430 East Branch Poplar Creek near Palatine, Ill. NA 87 125 151 184 210 235 297

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05550450 Poplar Creek near Ontarioville, Ill. NA 211 290 340 398 440 480 567

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05550470 Poplar Creek Trib near Bartlett, Ill. 2 162 240 295 365 419 474 606

155 257 327 414 477 539 678

161 242 299 373 429 486 620

05550500 Poplar Creek at Elgin, Ill. NA 431 643 781 951 1,070 1,190 1,460

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05551030 Brewster Creek at Valley View, Ill. 2 236 411 535 695 815 933 1,210

308 498 627 787 903 1,020 1,270

242 419 546 709 830 949 1,220

05551050 Norton Creek near Wayne, Ill. 2 97 201 295 443 576 730 1,180

192 311 392 492 565 635 791

103 210 305 450 574 713 1,090

05551060 Norton Creek near St. Charles, Ill. 2 123 216 290 397 486 584 845

292 478 604 762 876 988 1,240

133 235 319 439 537 641 908

05551200 Ferson Creek near St. Charles, Ill. 2 929 1,530 1,910 2,370 2,680 2,970 3,560

1,120 1,850 2,370 3,010 3,490 3,960 5,030

938 1,550 1,940 2,420 2,750 3,070 3,730

05551520 Indian Creek near North Aurora, Ill. 2 133 217 275 350 406 463 595

92 148 186 232 264 296 365

129 208 262 329 379 429 543

05551530 Indian Creek at Aurora, Ill. NA 525 659 733 814 866 913 1,010

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----
112 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Station 

number

(figs. 2A 

and 2B)

Station Name Hydrologic 

Region

Flood Quantiles of Selected Recurrence Interval

Q2 Q5 Q10 Q25 Q50 Q100 Q500

05551620 Blackberry Creek near Kaneville, Ill. 2 430 529 585 647 689 727 806

552 917 1,170 1,490 1,720 1,940 2,460

442 569 653 756 829 898 1,050

05551650 Lake Run Trib near Batavia, Ill. 2 52 116 175 268 352 447 717

158 278 365 478 563 648 845

60 133 201 304 393 490 749

05551700 Blackberry Creek near Yorkville, Ill. 2 686 1,200 1,570 2,070 2,460 2,850 3,800

976 1,560 1,950 2,440 2,780 3,120 3,870

697 1,210 1,590 2,100 2,480 2,870 3,800

05551800 Fox River Tributary No 2 near Fox, Ill. 2 68 165 254 392 512 645 1,010

64 116 155 206 245 284 376

67 156 233 344 435 533 786

05551900 East Branch Big Rock Creek near Big Rock, Ill. 2 643 950 1,150 1,400 1,580 1,760 2,160

713 1,180 1,490 1,890 2,180 2,470 3,110

652 980 1,200 1,490 1,700 1,900 2,360

05551930 Welch Creek near Big Rock, Ill. 2 320 460 547 651 724 794 946

543 899 1,140 1,450 1,680 1,900 2,390

341 504 616 759 862 962 1,180

05552500 Fox River at Dayton, Ill. NA 12,600 19,200 23,800 29,700 34,100 38,600 49,100

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05554000 North Fork Vermilion River near Charlotte, Ill. 3 2,420 3,740 4,560 5,510 6,160 6,760 8,000

3,130 5,490 7,210 9,490 11,200 13,000 17,300

2,450 3,820 4,700 5,760 6,510 7,220 8,740

05554500 Vermilion River at Pontiac, Ill. 3 5,730 8,650 10,500 12,700 14,200 15,700 18,800

4,510 7,480 9,560 12,200 14,200 16,200 20,800

5,670 8,580 10,400 12,700 14,200 15,700 19,000

05554600 Mud Creek Tributary near Odell, Ill. 3 49 90 120 158 186 213 273

30 57 78 106 129 152 207

45 82 108 142 167 192 250

05555000 Vermilion River at Streator, Ill. 3 7,760 12,400 15,500 19,500 22,400 25,300 32,100

7,450 12,300 15,700 20,100 23,300 26,600 34,200

7,720 12,300 15,500 19,600 22,600 25,700 32,700

05555300 Vermilion River near Leonore, Ill. 3 12,400 20,000 25,100 31,300 35,700 40,000 49,600

8,630 14,300 18,300 23,400 27,200 31,100 40,000

12,300 19,700 24,700 30,800 35,200 39,500 49,000

05555400 Vermilion River Tributary at Lowell, Ill. 3 26 68 111 187 261 351 638

98 195 274 383 472 565 798

33 84 137 227 310 406 686

05555500 Vermilion River at Lowell, Ill. 3 10,700 17,900 23,100 29,900 35,000 40,300 52,700

8,970 14,900 19,100 24,400 28,500 32,500 41,900

10,600 17,700 22,800 29,300 34,300 39,400 51,400
Table 1. Flood-peak dishcarges for recurrence intervals, T, of 2, 5, 10, 25, 50, 100, and 500 years estimated from the annual maximum series 113

Station 

number

(figs. 2A 

and 2B)

Station Name Hydrologic 

Region

Flood Quantiles of Selected Recurrence Interval

Q2 Q5 Q10 Q25 Q50 Q100 Q500

05555775 Vermilion Creek Tributary at Meriden, Ill. 3 43 69 86 108 124 140 176

75 147 203 280 341 405 564

48 83 110 147 177 206 277

05556500 Big Bureau Creek at Princeton, Ill. 3 4,470 7,320 9,110 11,200 12,500 13,800 16,300

3,750 6,650 8,800 11,700 13,900 16,100 21,600

4,420 7,270 9,080 11,200 12,700 14,100 16,900

05557000 West Bureau Creek at Wyanet, Ill. 3 2,660 4,810 6,470 8,820 10,700 12,700 17,900

2,480 4,530 6,070 8,170 9,810 11,500 15,700

2,650 4,780 6,440 8,750 10,600 12,600 17,600

05557100 West Bureau Creek Tributary near Wyanet, Ill. 4 81 159 222 316 395 480 708

123 229 310 424 514 608 844

85 167 235 334 417 506 739

05557500 East Bureau Creek near Bureau, Ill. 3 2,340 4,120 5,410 7,110 8,420 9,730 12,800

2,980 5,450 7,310 9,840 11,800 13,900 18,900

2,370 4,190 5,530 7,320 8,700 10,100 13,400

05558000 Big Bureau Creek at Bureau, Ill. 3 8,070 11,800 14,100 17,000 19,000 21,000 25,300

7,530 13,300 17,600 23,300 27,700 32,300 43,200

7,950 12,100 15,000 18,800 21,600 24,500 31,100

05558050 Coffee Creek Tributary near Florid, Ill. 3 19 39 56 83 106 131 203

19 38 55 78 97 117 168

19 39 56 81 103 126 190

05558075 Coffee Creek Tributary near Hennepin, Ill. 3 61 117 163 234 296 365 557

54 112 160 229 286 346 503

60 116 163 233 294 360 542

05558300 Illinois River at Henry, Ill. NA 64,800 89,800 106,000 125,000 138,000 152,000 182,000

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05558500 Crow Creek (West) near Henry, Ill. 4 1,500 2,680 3,630 5,050 6,250 7,590 11,300

2,260 4,010 5,330 7,100 8,490 9,920 13,400

1,550 2,770 3,780 5,270 6,520 7,880 11,600

05559000 Gimlet Creek at Sparland, Ill. 4 810 1,300 1,640 2,090 2,430 2,780 3,600

745 1,410 1,940 2,670 3,260 3,880 5,440

800 1,310 1,680 2,170 2,550 2,940 3,890

05559500 Crow Creek near Washburn, Ill. 3 2,170 3,350 4,190 5,290 6,140 7,020 9,150

2,570 4,560 6,040 8,000 9,520 11,100 14,800

2,200 3,440 4,350 5,580 6,530 7,510 9,900

05560000 Illinois River at Peoria, Ill. NA 38,000 48,700 54,700 61,400 65,800 69,800 78,000

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05560500 Farm Creek at Farmdale, Ill. NA 578 818 998 1,250 1,460 1,680 2,290

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----
114 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Station 

number

(figs. 2A 

and 2B)

Station Name Hydrologic 

Region

Flood Quantiles of Selected Recurrence Interval

Q2 Q5 Q10 Q25 Q50 Q100 Q500

05561000 Ackerman Creek at Farmdale, Ill. 3 630 1,370 2,060 3,170 4,180 5,360 8,840

976 1,890 2,610 3,620 4,430 5,290 7,420

666 1,440 2,150 3,260 4,240 5,340 8,460

05561500 Fondulac Creek near East Peoria, Ill. NA 249 362 443 550 634 722 942

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05562000 Farm Creek at East Peoria, Ill. NA 3,410 6,970 10,200 15,400 20,100 25,700 42,400

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05563000 Kickapoo Creek near Kickapoo, Ill. 4 7,110 13,800 19,400 28,100 35,700 44,200 68,300

3,920 6,920 9,170 12,200 14,600 17,000 23,000

6,910 13,200 18,500 26,200 32,900 40,300 60,700

05563100 Kickapoo Creek Tributary near Kickapoo, Ill. 4 27 65 102 166 228 303 544

30 59 83 115 142 170 239

28 64 98 152 202 260 429

05563500 Kickapoo Creek at Peoria, Ill. 4 7,700 13,200 17,700 24,300 30,000 36,200 53,500

6,020 10,200 13,300 17,400 20,600 23,800 31,600

7,630 13,100 17,500 23,800 29,100 35,000 51,000

05564400 Money Creek near Towanda, Ill. 3 867 1,380 1,760 2,290 2,720 3,180 4,370

1,200 2,130 2,810 3,710 4,400 5,110 6,800

897 1,450 1,890 2,490 2,980 3,490 4,800

05564500 Money Creek above Lake Bloomington, Ill. 3 935 1,520 1,980 2,660 3,240 3,880 5,650

1,250 2,220 2,930 3,860 4,580 5,320 7,070

965 1,590 2,100 2,840 3,460 4,140 5,960

05565000 Hickory Creek above Lake Bloomington, Ill. 3 493 991 1,410 2,040 2,570 3,150 4,730

471 860 1,150 1,550 1,850 2,160 2,920

490 968 1,360 1,910 2,370 2,860 4,130

05566000 East Branch Panther Creek near Gridley, Ill. 3 152 285 402 589 760 961 1,570

450 841 1,140 1,550 1,860 2,190 2,990

176 338 488 727 939 1,180 1,860

05566500 East Branch Panther Creek at El Paso, Ill. 3 573 1,100 1,580 2,370 3,120 4,010 6,790

845 1,490 1,970 2,600 3,080 3,560 4,720

592 1,130 1,620 2,410 3,110 3,940 6,390

05567000 Panther Creek near El Paso, Ill. 3 2,310 4,660 6,590 9,390 11,700 14,200 20,600

2,060 3,640 4,790 6,320 7,490 8,680 11,500

2,290 4,580 6,430 9,040 11,200 13,400 19,200

05567500 Mackinaw River near Congerville, Ill. 3 8,940 16,200 22,000 30,400 37,400 45,100 65,600

7,390 12,500 16,200 21,000 24,600 28,200 36,900

8,860 15,900 21,500 29,400 36,000 43,100 61,800

05567800 Indian Creek Tributary near Hopedale, Ill. 3 185 328 442 609 749 903 1,320

142 278 384 532 650 773 1,080

173 312 422 579 708 846 1,210
Table 1. Flood-peak dishcarges for recurrence intervals, T, of 2, 5, 10, 25, 50, 100, and 500 years estimated from the annual maximum series 115

Station 

number

(figs. 2A 

and 2B)

Station Name Hydrologic 

Region

Flood Quantiles of Selected Recurrence Interval

Q2 Q5 Q10 Q25 Q50 Q100 Q500

05568000 Mackinaw River near Green Valley, Ill. 3 8,330 15,100 21,000 30,500 39,000 49,000 79,200

9,480 16,000 20,700 26,800 31,400 36,100 47,100

8,370 15,100 21,100 30,300 38,500 48,100 76,500

05568500 Illinois River at Kingston Mines, Ill. NA 49,000 64,900 74,200 84,700 91,800 98,300 112,000

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05568650 Duck Creek near Canton, Ill. 4 66 103 130 167 198 230 313

93 172 232 314 378 444 605

70 112 146 194 233 274 379

05568800 Indian Creek near Wyoming, Ill. 4 1,660 2,660 3,440 4,540 5,460 6,450 9,110

2,130 3,590 4,650 6,040 7,110 8,190 10,800

1,680 2,710 3,520 4,670 5,610 6,630 9,320

05568850 Forman Creek Tributary near Victoria, Ill. 4 101 192 270 388 491 608 937

79 151 208 287 350 415 577

97 183 254 357 444 539 796

05569500 Spoon River at London Mills, Ill. 4 9,750 15,000 19,200 25,200 30,300 36,000 51,500

10,900 17,700 22,400 28,600 33,200 37,800 48,800

9,780 15,100 19,300 25,400 30,500 36,100 51,400

05569825 Cedar Creek Tributary at St. Augustine, Ill. 4 382 613 799 1,070 1,310 1,570 2,300

451 834 1,130 1,530 1,850 2,190 3,010

389 637 841 1,140 1,400 1,680 2,450

05570000 Spoon River at Seville, Ill. 4 12,600 19,600 24,600 31,200 36,300 41,600 54,500

13,900 22,200 28,000 35,400 41,000 46,500 59,700

12,700 19,700 24,700 31,300 36,500 41,800 54,700

05570350 Big Creek at St. David, Ill. NA 859 1,210 1,460 1,790 2,050 2,320 2,990

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05570360 Evelyn Branch near Bryant, Ill. NA 75 150 216 315 403 501 775

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05570370 Big Creek near Bryant, Ill. 4 761 1,030 1,200 1,400 1,540 1,680 1,980

1,710 2,890 3,760 4,900 5,780 6,670 8,830

823 1,160 1,410 1,720 1,960 2,180 2,700

05570380 Slug Run near Bryant,Ill. NA 138 306 455 687 891 1,120 1,760

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05570500 Illinois River at Havana, Ill. NA 44,700 60,300 69,700 80,600 88,100 95,200 110,000

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05570910 Sangamon River at Fisher, Ill. 3 3,930 6,490 8,350 10,800 12,800 14,800 19,600

4,040 7,080 9,300 12,300 14,500 16,800 22,400

3,940 6,580 8,510 11,100 13,100 15,200 20,300
116 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Station 

number

(figs. 2A 

and 2B)

Station Name Hydrologic 

Region

Flood Quantiles of Selected Recurrence Interval

Q2 Q5 Q10 Q25 Q50 Q100 Q500

05571000 Sangamon River at Mahomet, Ill. 3 4,080 7,040 9,330 12,500 15,200 18,000 25,200

4,880 8,440 11,000 14,400 17,000 19,600 25,800

4,140 7,170 9,510 12,800 15,400 18,200 25,300

05572000 Sangamon River at Monticello, Ill. 3 5,400 8,950 11,500 15,000 17,800 20,600 27,600

5,940 10,100 13,100 17,100 20,000 23,000 30,200

5,420 8,980 11,600 15,100 17,900 20,700 27,800

05572100 Wildcat Creek Tributary near Monticello, Ill. NA 28 46 58 75 88 101 134

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05572450 Friends Creek at Argenta, Ill. 3 1,630 2,760 3,670 4,990 6,110 7,340 10,700

2,420 4,280 5,640 7,460 8,850 10,300 13,700

1,730 3,000 4,030 5,530 6,760 8,090 11,600

05572500 Sangamon River near Oakley, Ill. 3 5,610 9,190 12,000 16,100 19,500 23,200 33,300

7,110 12,000 15,500 20,000 23,500 26,900 35,000

5,720 9,450 12,400 16,600 20,000 23,800 33,600

05573540 Sangamon River at Rt 48 at Decatur, Ill. NA 8,610 12,800 15,500 18,900 21,500 24,000 29,700

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05574000 South Fork Sangamon River near Nokomis, Ill. 3 956 1,830 2,640 3,980 5,260 6,820 11,800

691 1,280 1,730 2,340 2,820 3,310 4,510

925 1,750 2,480 3,640 4,700 5,940 9,680

05574500 Flat Branch near Taylorville, Ill. 3 3,920 6,590 8,510 11,000 13,000 14,900 19,600

3,250 5,520 7,120 9,200 10,800 12,300 16,100

3,870 6,490 8,360 10,800 12,700 14,600 19,100

05575500 South Fork Sangamon River at Kincaid, Ill. 3 4,290 7,860 10,700 15,000 18,500 22,500 33,000

6,330 10,800 14,000 18,100 21,300 24,400 31,900

4,360 7,960 10,900 15,100 18,700 22,600 32,800

05575800 Horse Creek at Pawnee, Ill. 3 1,870 2,970 3,720 4,660 5,350 6,030 7,600

1,540 2,740 3,620 4,790 5,690 6,600 8,790

1,820 2,930 3,700 4,690 5,440 6,190 7,940

05576000 South Fork Sangamon River nr Rochester, Ill. NA 5,620 9,810 12,900 17,000 20,200 23,500 31,300

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05576500 Sangamon River at Riverton, Ill. 3 15,800 25,200 31,400 39,100 44,600 49,900 61,600

15,900 26,300 33,700 43,100 50,200 57,300 73,900

15,700 25,200 31,400 39,200 44,900 50,300 62,500

05577500 Spring Creek at Springfield, Ill. 3 1,730 3,760 5,620 8,570 11,200 14,300 23,100

2,030 3,540 4,650 6,110 7,220 8,350 11,100

1,740 3,740 5,530 8,290 10,700 13,500 21,200

05577700 Sangamon River Tributary at Andrew, Ill. 3 214 386 518 702 849 1,000 1,400

226 447 622 869 1,070 1,280 1,800

215 395 537 737 899 1,070 1,500
Table 1. Flood-peak dishcarges for recurrence intervals, T, of 2, 5, 10, 25, 50, 100, and 500 years estimated from the annual maximum series 117

Station 

number

(figs. 2A 

and 2B)

Station Name Hydrologic 

Region

Flood Quantiles of Selected Recurrence Interval

Q2 Q5 Q10 Q25 Q50 Q100 Q500

05578500 Salt Creek near Rowell, Ill. 3 3,380 6,350 8,920 12,900 16,500 20,600 32,500

4,230 7,250 9,430 12,300 14,400 16,600 21,800

3,410 6,390 8,950 12,900 16,300 20,200 31,300

05579500 Lake Fork near Cornland, Ill. 3 2,140 4,070 5,840 8,720 11,400 14,600 24,700

3,460 6,050 7,940 10,400 12,300 14,300 18,900

2,190 4,170 5,970 8,870 11,500 14,600 24,000

05579750 Kickapoo Creek Tributary at Heyworth, Ill. 3 385 674 917 1,290 1,620 1,990 3,070

283 541 740 1,010 1,230 1,460 2,020

365 645 873 1,210 1,500 1,810 2,680

05580000 Kickapoo Creek at Waynesville, Ill. 3 4,350 8,000 11,200 16,200 20,700 26,000 41,700

4,280 7,570 10,000 13,200 15,700 18,300 24,400

4,350 7,970 11,100 15,900 20,200 25,000 39,100

05580500 Kickapoo Creek near Lincoln, Ill. 3 3,980 7,020 9,600 13,600 17,100 21,200 33,000

5,030 8,840 11,600 15,300 18,200 21,100 28,000

4,030 7,130 9,750 13,800 17,200 21,200 32,300

05580700 Salt Creek Tributary at Middletown, Ill. 3 95 307 551 1,000 1,460 2,020 3,820

116 228 316 439 538 643 904

98 288 479 789 1,070 1,390 2,310

05580950 Sugar Creek near Bloomington, Ill. NA 2,580 3,910 4,850 6,090 7,050 8,040 10,500

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05581500 Sugar Creek near Hartsburg, Ill. 3 5,510 11,300 16,900 26,500 35,800 47,300 84,600

5,610 9,880 13,000 17,200 20,400 23,700 31,600

5,510 11,200 16,500 25,300 33,600 43,500 74,500

05582000 Salt Creek near Greenview, Ill. 3 12,500 20,800 27,000 35,600 42,500 49,700 68,200

13,500 22,700 29,300 37,900 44,400 50,900 66,400

12,600 20,900 27,100 35,700 42,500 49,700 67,800

05582200 Cabiness Creek Tributary near Petersburg, Ill. 3 124 312 501 824 1,130 1,500 2,640

98 188 257 351 426 503 693

120 286 437 674 882 1,120 1,790

05582500 Crane Creek near Easton, Ill. 3 217 400 538 728 877 1,030 1,410

642 1,140 1,500 1,980 2,350 2,720 3,620

237 442 605 834 1,020 1,200 1,660

05583000 Sangamon River near Oakford, Ill. 3 22,500 38,300 49,500 64,100 75,100 86,200 112,000

23,500 38,400 48,800 62,100 72,100 82,100 105,000

22,500 38,200 49,400 64,000 75,000 86,000 112,000

05584400 Drowning Fork at Bushnell, Ill. 4 576 1,170 1,670 2,420 3,070 3,780 5,720

1,030 1,780 2,330 3,060 3,630 4,200 5,570

601 1,210 1,730 2,500 3,140 3,840 5,700

05584450 Wigwam Hollow Creek near Macomb, Ill. NA 208 343 439 566 664 763 1,000

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----
118 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Station 

number

(figs. 2A 

and 2B)

Station Name Hydrologic 

Region

Flood Quantiles of Selected Recurrence Interval

Q2 Q5 Q10 Q25 Q50 Q100 Q500

05584500 La Moine River at Colmar, Ill. 4 8,490 15,700 21,300 28,900 35,000 41,300 57,000

9,190 15,600 20,300 26,500 31,200 36,000 47,600

8,510 15,700 21,200 28,700 34,600 40,800 56,000

05584950 West Creek at Mount Sterling, Ill. 4 231 369 468 600 702 807 1,060

262 478 642 867 1,040 1,230 1,680

236 391 508 670 798 931 1,260

05585000 La Moine River at Ripley, Ill. 4 9,530 15,400 19,600 25,100 29,400 33,700 44,200

11,200 18,100 23,000 29,300 34,000 38,800 50,000

9,570 15,500 19,700 25,300 29,600 34,000 44,600

05585220 Indian Creek Tributary near Sinclair, Ill. 5 387 722 978 1,330 1,610 1,890 2,600

369 691 934 1,270 1,530 1,800 2,470

384 717 970 1,320 1,590 1,870 2,570

05585500 Illinois River at Meredosia, Ill. NA 62,200 85,700 100,000 117,000 129,000 140,000 164,000

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05585700 Dry Fork Tributary near Mount Sterling, Ill. 4 32 51 64 80 92 104 132

46 87 119 163 197 234 323

34 55 71 93 110 127 168

05586000 N Fk Mauvaise Terre Cr nr Jacksonville, Ill. 5 957 2,270 3,480 5,350 6,990 8,830 13,800

1,220 2,170 2,860 3,770 4,480 5,200 6,940

970 2,260 3,410 5,160 6,650 8,300 12,700

05586100 Illinois River at Valley City, Ill. NA 78,500 91,200 98,500 107,000 112,000 118,000 129,000

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05586200 Illinois River Tributary at Florence, Ill. 4 320 561 728 937 1,090 1,230 1,560

179 356 499 703 870 1,050 1,500

297 524 681 883 1,040 1,190 1,550

05586350 Little Sandy Creek Tributary nr Murrayville, Ill. 5 402 791 1,100 1,520 1,860 2,220 3,090

338 657 906 1,250 1,530 1,820 2,560

385 750 1,030 1,420 1,720 2,050 2,850

05586500 Hurricane Creek near Roodhouse, Ill. 5 209 427 610 880 1,110 1,350 2,010

263 498 677 921 1,120 1,320 1,820

213 433 617 885 1,110 1,350 1,980

05586800 Otter Creek near Palmyra, Ill. NA 2,480 5,580 8,360 12,700 16,500 20,800 32,800

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05586850 Bear Creek Tributary near Reeders, Ill. NA 13 22 28 37 45 52 72

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05587000 Macoupin Creek near Kane, Ill. 5 9,920 17,500 23,000 30,100 35,400 40,700 52,800

10,900 18,400 23,600 30,300 35,500 40,600 53,000

10,000 17,600 23,000 30,100 35,400 40,700 53,000
Table 1. Flood-peak dishcarges for recurrence intervals, T, of 2, 5, 10, 25, 50, 100, and 500 years estimated from the annual maximum series 119

Station 

number

(figs. 2A 

and 2B)

Station Name Hydrologic 

Region

Flood Quantiles of Selected Recurrence Interval

Q2 Q5 Q10 Q25 Q50 Q100 Q500

05587850 Cahokia Creek Tributary No 2 nr Carpenter, Ill. NA 158 299 404 542 647 751 994

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05587900 Cahokia Creek at Edwardsville, Ill. 4 5,030 6,800 7,730 8,690 9,270 9,760 10,700

4,660 7,700 9,870 12,700 14,900 17,100 22,300

4,990 6,860 7,910 9,080 9,850 10,500 11,900

05588000 Indian Creek at Wanda, Ill. 4 1,870 3,280 4,340 5,810 6,980 8,210 11,300

1,570 2,620 3,370 4,360 5,110 5,880 7,730

1,860 3,240 4,280 5,700 6,830 8,000 10,900

05589500 Canteen Creek at Caseyville, Ill. 4 1,830 3,260 4,350 5,870 7,080 8,350 11,600

1,210 2,100 2,750 3,620 4,300 4,990 6,680

1,790 3,170 4,210 5,620 6,740 7,910 10,800

05589780 Little Canteen Creek Trib near Collinsville, Ill. 5 172 379 557 827 1,060 1,310 1,980

298 582 804 1,110 1,360 1,630 2,290

194 421 620 913 1,160 1,420 2,100

05590000 Kaskaskia Ditch at Bondville, Ill. 3 370 665 908 1,270 1,590 1,940 2,920

683 1,280 1,740 2,370 2,870 3,390 4,660

386 701 968 1,370 1,710 2,090 3,130

05590400 Kaskaskia River near Pesotum, Ill. 3 1,810 2,450 2,860 3,360 3,730 4,080 4,900

1,570 2,700 3,500 4,540 5,330 6,120 8,000

1,770 2,490 2,980 3,620 4,090 4,570 5,660

05590500 Kaskaskia River at Ficklin, Ill. 3 1,930 3,220 4,150 5,390 6,350 7,330 9,710

1,610 2,730 3,530 4,560 5,330 6,110 7,940

1,870 3,100 3,980 5,130 6,000 6,900 9,040

05590800 Lake Fork at Atwood, Ill. 3 2,230 3,010 3,470 3,980 4,330 4,650 5,320

1,580 2,630 3,370 4,310 5,020 5,720 7,350

2,170 2,970 3,450 4,020 4,420 4,810 5,630

05591200 Kaskaskia River at Cooks Mills, Ill. 3 5,350 7,600 8,970 10,600 11,700 12,700 14,900

3,800 6,320 8,080 10,300 12,000 13,700 17,600

5,210 7,470 8,870 10,500 11,700 12,900 15,300

05591500 Asa Creek at Sullivan, Ill. 3 333 655 915 1,290 1,590 1,920 2,750

253 447 587 770 909 1,050 1,380

326 631 869 1,200 1,470 1,750 2,450

05591550 Whitley Creek near Allenville, Ill. NA 1,100 1,840 2,420 3,230 3,900 4,630 6,560

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05591700 West Okaw River near Lovington, Ill. 3 3,190 5,040 6,390 8,210 9,650 11,100 14,900

2,060 3,590 4,710 6,180 7,300 8,440 11,200

3,020 4,790 6,050 7,730 9,050 10,400 13,800

05591750 Stringtown Branch Tributary near Lake City, Ill. 3 51 86 111 143 167 191 248

65 125 170 233 282 333 458

53 92 121 161 191 223 297
120 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Station 

number

(figs. 2A 

and 2B)

Station Name Hydrologic 

Region

Flood Quantiles of Selected Recurrence Interval

Q2 Q5 Q10 Q25 Q50 Q100 Q500

05592000 Kaskaskia River at Shelbyville, Ill. 3 9,330 16,200 21,100 27,500 32,200 36,900 47,900

6,560 10,800 13,700 17,400 20,200 23,000 29,400

9,120 15,700 20,300 26,100 30,500 34,800 44,900

05592000 Kaskaskia River at Shelbyville, Ill. NA 3,820 5,120 5,880 6,770 7,370 7,930 9,110

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05592025 Mud Creek Tributary near Tower Hill, Ill. NA 112 223 320 471 604 757 1,190

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05592100 Kaskaskia River near Cowden, Ill. NA 7,710 11,500 14,000 17,200 19,600 21,900 27,400

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05592300 Wolf Creek near Beecher City, Ill. 5 2,930 4,990 6,540 8,650 10,300 12,100 16,500

2,070 3,680 4,850 6,400 7,590 8,820 11,800

2,810 4,790 6,220 8,160 9,690 11,300 15,200

05592500 Kaskaskia River at Vandalia, Ill. NA 15,700 23,300 28,100 33,900 38,100 42,000 50,600

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05592575 Hickory Creek nr Brownstown, Ill. 5 3,820 5,230 6,120 7,200 7,980 8,730 10,400

2,340 4,240 5,640 7,530 9,000 10,500 14,200

3,410 4,940 5,960 7,310 8,340 9,380 11,900

05592700 Hurricane Creek Tributary near Witt, Ill. NA 71 98 115 135 149 163 193

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05592800 Hurricane Creek near Mulberry Grove, Ill. 5 8,120 12,200 14,800 17,900 20,100 22,200 26,700

4,770 8,410 11,000 14,500 17,200 20,000 26,700

7,720 11,700 14,200 17,300 19,600 21,800 26,800

05592900 East Fork Kaskaskia River near Sandoval, Ill. 5 4,050 7,370 9,970 13,600 16,600 19,800 27,900

3,340 5,830 7,610 9,950 11,700 13,600 18,000

3,950 7,100 9,480 12,700 15,300 18,000 24,900

05593000 Kaskaskia River at Carlyle, Ill. 5 13,700 24,600 33,200 45,300 55,200 65,700 93,000

22,500 37,100 47,100 60,000 69,700 79,400 103,000

14,000 25,200 34,000 46,400 56,300 66,900 93,800

05593520 Crooked Creek near Hoffman, Ill. 5 6,120 11,100 14,900 20,200 24,400 28,900 40,200

5,160 8,800 11,400 14,700 17,300 19,800 26,000

6,010 10,800 14,300 19,100 22,900 26,800 36,700

05593575 Little Crooked Creek near New Minden, Ill. 5 4,430 7,570 9,860 12,900 15,200 17,600 23,400

2,810 4,930 6,450 8,460 10,000 11,600 15,400

4,260 7,250 9,360 12,100 14,300 16,400 21,700

05593520 Crooked Creek near Hoffman, Ill. 5 6,120 11,100 14,900 20,200 24,400 28,900 40,200

5,160 8,800 11,400 14,700 17,300 19,800 26,000

6,010 10,800 14,300 19,100 22,900 26,800 36,700
Table 1. Flood-peak dishcarges for recurrence intervals, T, of 2, 5, 10, 25, 50, 100, and 500 years estimated from the annual maximum series 121

Station 

number

(figs. 2A 

and 2B)

Station Name Hydrologic 

Region

Flood Quantiles of Selected Recurrence Interval

Q2 Q5 Q10 Q25 Q50 Q100 Q500

05593575 Little Crooked Creek near New Minden, Ill. 5 4,430 7,570 9,860 12,900 15,200 17,600 23,400

2,810 4,930 6,450 8,460 10,000 11,600 15,400

4,260 7,250 9,360 12,100 14,300 16,400 21,700

05593600 Blue Grass Creek near Raymond, Ill. 5 954 1,360 1,620 1,930 2,150 2,360 2,830

834 1,460 1,910 2,500 2,950 3,410 4,500

943 1,370 1,650 2,000 2,260 2,510 3,080

05593700 Blue Grass Creek Tributary near Raymond, Ill. 5 147 213 257 313 355 396 492

110 210 287 393 476 563 777

137 212 266 339 396 454 594

05593900 East Fork Shoal Creek near Coffeen, Ill. 5 2,250 3,420 4,220 5,240 6,010 6,780 8,590

2,110 3,710 4,870 6,400 7,570 8,770 11,600

2,240 3,440 4,280 5,380 6,210 7,050 9,040

05594000 Shoal Creek near Breese, Ill. 5 9,240 15,700 20,300 26,300 30,800 35,200 45,600

11,100 18,700 24,100 31,100 36,400 41,800 54,600

9,310 15,900 20,500 26,600 31,200 35,700 46,400

05594090 Sugar Creek at Albers, Ill. 5 3,730 6,540 8,530 11,100 13,000 14,900 19,200

3,170 5,480 7,140 9,300 11,000 12,700 16,700

3,600 6,250 8,090 10,500 12,200 14,000 18,200

05594100 Kaskaskia River near Venedy Station, Ill. NA 22,600 37,000 46,900 59,400 68,600 77,700 98,500

---- ---- ---- ---- ---- ---- ----

---- ---- ---- ---- ---- ---- ----

05594200 Williams Creek near Cordes, Ill. 5 338 598 801 1,090 1,330 1,580 2,240

299 561 759 1,030 1,240 1,460 2,000

331 590 791 1,070 1,300 1,540 2,160

05594330 Mud Creek near Marissa, Ill. 5 1,870 3,230 4,310 5,850 7,130 8,510 12,200

2,280 3,980 5,200 6,810 8,050 9,310 12,300

1,940 3,390 4,530 6,130 7,420 8,780 12,200

05594450 Silver Creek near Troy, Ill. 5 3,870 6,770 8,650 10,900 12,400 13,700 16,500

4,040 7,050 9,220 12,100 14,300 16,500 22,000

3,880 6,790 8,710 11,000 12,700 14,200 17,500

05594800 Silver Creek near Freeburg, Ill. 5 5,220 9,110 11,700 14,700 16,900 18,900 23,000

7,170 12,200 15,700 20,200 23,700 27,200 35,600

5,360 9,350 12,100 15,400 17,800 20,100 25,000

05595000 Kaskaskia River at New Athens, Ill. 5 22,700 41,100 55,200 74,700 90,200 106,000 147,000

33,300 54,100 68,200 86,200 99,700 113,000 145,000

23,100 41,700 55,900 75,500 91,000 107,000 147,000

05595200 Richland Creek near Hecker, Ill. 5 5,580 9,400 12,200 16,000 19,000 22,000 29,600

3,750 6,630 8,720 11,500 13,700 15,900 21,300

5,380 9,060 11,700 15,200 18,000 20,800 27,900

05595500 Marys River near Sparta, Ill. 4 1,450 2,890 4,150 6,130 7,890 9,910 15,700

1,110 2,010 2,690 3,620 4,340 5,090 6,940

1,410 2,750 3,880 5,560 6,990 8,590 13,000
122 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Station 

number

(figs. 2A 

and 2B)

Station Name Hydrologic 

Region

Flood Quantiles of Selected Recurrence Interval

Q2 Q5 Q10 Q25 Q50 Q100 Q500

05595510 Lick Branch near Eden, Ill. 4 160 322 468 700 911 1,160 1,890

206 377 508 688 829 976 1,340

167 332 476 696 885 1,100 1,680

05595550 Marys River Tributary at Chester, Ill. 4 265 383 463 566 643 720 904

180 352 490 686 844 1,010 1,440

246 376 470 598 701 807 1,070

05595730 Rayse Creek near Waltonville, Ill. 6 6,170 13,100 19,200 28,400 36,300 45,000 68,900

3,620 6,110 7,950 10,400 12,400 14,400 19,400

5,840 12,000 16,900 23,900 29,700 36,000 52,600

05595800 Sevenmile Creek near Mt. Vernon, Ill. 6 1,020 1,560 1,940 2,430 2,810 3,190 4,130

1,740 3,070 4,080 5,490 6,610 7,780 10,700

1,080 1,700 2,160 2,800 3,300 3,820 5,080

05595820 Casey Fork at Mount Vernon, Ill. 6 5,250 9,890 13,600 19,000 23,400 28,100 40,500

2,910 4,860 6,300 8,240 9,760 11,300 15,200

4,850 8,840 11,800 15,800 19,000 22,400 31,100

05596000 Big Muddy River near Benton, Ill. 6 7,450 14,200 19,800 28,400 35,900 44,300 67,900

6,140 9,460 11,800 14,900 17,200 19,600 25,300

7,370 13,800 19,000 26,600 33,100 40,300 59,900

05596100 Andy Creek Tributary at Valier, Ill. 6 248 454 615 842 1,030 1,220 1,730

275 513 702 968 1,190 1,420 2,000

252 464 631 870 1,060 1,270 1,800

05597000 Big Muddy River at Plumfield, Ill. 6 7,570 12,800 16,400 21,100 24,600 28,100 36,100

10,200 16,000 20,100 25,500 29,700 33,900 44,000

7,640 12,900 16,600 21,400 24,900 28,500 36,700

05597450 Crab Orchard Creek Tributary near Pittsburg, Ill. 6 210 257 289 329 359 389 460

162 297 403 552 674 802 1,130

201 265 312 377 429 483 615

05597500 Crab Orchard Creek near Marion, Ill. 6 1,650 2,910 3,900 5,310 6,480 7,740 11,100

1,710 2,910 3,810 5,020 5,980 6,970 9,420

1,650 2,910 3,890 5,290 6,440 7,660 10,900

05599000 Beaucoup Creek near Matthews, Ill. 6 4,710 8,970 12,500 17,800 22,400 27,500 41,400

4,960 7,880 9,940 12,700 14,700 16,800 21,900

4,720 8,900 12,300 17,300 21,600 26,200 38,700

05599500 Big Muddy River at Murphysboro, Ill. 6 12,500 20,400 25,800 32,700 37,800 42,800 54,300

15,700 23,800 29,400 36,600 42,100 47,600 60,700

12,600 20,500 26,000 33,000 38,100 43,300 55,100

05599560 Clay Lick Creek near Makanda, Ill. 6 731 1,300 1,780 2,500 3,140 3,860 5,910

479 897 1,230 1,700 2,090 2,500 3,560

685 1,220 1,650 2,280 2,820 3,420 5,090

05599580 Big Muddy River Tributary near Gorham, Ill. 4 38 65 86 117 143 171 247

115 231 328 467 581 704 1,020

47 85 121 175 221 273 408
Table 1. Flood-peak dishcarges for recurrence intervals, T, of 2, 5, 10, 25, 50, 100, and 500 years estimated from the annual maximum series 123

Station 

number

(figs. 2A 

and 2B)

Station Name Hydrologic 

Region

Flood Quantiles of Selected Recurrence Interval

Q2 Q5 Q10 Q25 Q50 Q100 Q500

05599640 Green Creek Tributary near Jonesboro, Ill. 7 259 416 529 680 797 917 1,210

207 350 458 606 724 848 1,160

254 409 520 669 784 904 1,200

05599800 Orchard Creek near Fayville, Ill. 7 60 99 130 176 214 256 371

63 107 140 186 222 260 355

60 100 132 178 216 257 366

05600000 Big Creek near Wetaug, Ill. 7 2,090 2,770 3,270 3,930 4,460 5,020 6,470

2,680 4,160 5,220 6,620 7,710 8,820 11,500

2,100 2,810 3,330 4,030 4,600 5,190 6,730
124 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois
Table 2. Flood-peak dishcarges for recurrence intervals, T, of 0.8, 1.01, 1.5, 2, 3, and 5 years estimated from the partial duration series 125

Table 2. Flood-peak discharges for recurrence intervals, T, of 0.8, 1.01, 1.5, 2, 3, and 5 years estimated from the partial duration series at 

streamflow-gaging stations in Illinois and adjacent States.

[T, recurrence interval in years, QT, instantaneous peak-flood discharge, in cubic feet per second, for a given T of 0.8-, 1.01-, 1.5-, 2-, 3-, and 5-year flood. Two 

estimates are listed for each station: the values in the top row are QT from at-site frequency curves; values in the bottom row are QT from regional regression 

equations]

Station

number

(fig. 3)

Station Name Hydrologic

 Region

Flood Quantiles of Selected Recurrence Interval

Q0.8 Q1.01 Q1.5 Q2 Q3 Q5

3336500 Bluegrass Creek at Potomac, Ill. 3 1,290 1,560 2,000 2,300 2,720 3,200 

683 879 1,190 1,400 1,700 2,060 

3336645 Middle Fork Vermilion River above Oakwood, Ill. 3 4,900 5,610 6,780 7,600 8,720 10,100 

3,600 4,300 5,470 6,310 7,470 8,900 

3336900 Salt Fork near St. Joseph, Ill. 3 1,810 2,100 2,590 2,950 3,450 4,090 

1,660 2,050 2,680 3,130 3,740 4,510 

3337500 Saline Branch at Urbana, Ill. 3 864 1,010 1,260 1,440 1,700 2,030 

1,040 1,310 1,730 2,030 2,440 2,950 

3338000 Salt Fork near Homer, Ill. 3 2,330 2,770 3,500 4,030 4,760 5,660 

3,080 3,710 4,750 5,500 6,530 7,800 

3338500 Vermilion River near Catlin, Ill. 3 6,850 7,810 9,520 10,800 12,800 15,400 

6,080 7,100 8,880 10,200 12,000 14,000 

3338780 North Fork Vermilion River near Bismarck, Ill. 3 4,910 6,170 8,240 9,720 11,700 14,200 

2,630 3,200 4,140 4,800 5,720 6,850 

3343400 Embarras River near Camargo, Ill. 3 2,450 2,800 3,360 3,760 4,300 4,960 

2,080 2,550 3,320 3,860 4,610 5,540 

3344000 Embarras River near Diona, Ill. 3 6,200 7,500 9,530 10,900 12,600 14,600 

5,970 6,990 8,760 10,000 11,800 14,000 

3344500 Range Creek near Casey, Ill. 5 550 730 1,020 1,240 1,530 1,900 

386 506 695 829 1,010 1,240 

3345500 Embarras River at Ste. Marie, Ill. 5 10,700 12,800 16,300 18,800 22,000 25,800 

12,800 14,600 17,800 20,000 23,000 26,900 

3346000 North Fork Embarras River near Oblong, Ill. 5 5,750 6,900 8,830 10,200 12,100 14,500 

5,270 6,340 8,080 9,310 11,000 13,100 

3378900 Little Wabash River at Louisvile, Ill. 5 8,270 9,880 12,300 13,900 15,900 18,100 

8,850 10,360 12,900 14,600 17,100 20,100 

3379500 Little Wabash River below Clay City, Ill. 5 8,710 10,900 14,700 17,300 21,000 25,500 

11,000 12,700 15,600 17,600 20,400 23,800 

3380350 Skillet Fork near Iuka, Ill. 6 3,610 4,290 5,480 6,360 7,640 9,300 

4,300 5,230 6,790 7,920 9,540 11,600 

3380475 Horse Creek near Keenes, Ill. 6 3,150 3,570 4,330 4,910 5,780 6,950 

2,670 3,230 4,160 4,810 5,740 6,910 

3380500 Skillet Fork at Wayne City, Ill. 6 5,410 6,820 9,330 11,200 14,100 17,900 

6,490 7,920 10,360 12,200 14,800 18,200 
126 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois 

Station

number

(fig. 3)

Station Name Hydrologic

 Region

Flood Quantiles of Selected Recurrence Interval

Q0.8 Q1.01 Q1.5 Q2 Q3 Q5

3382100 South Fork Saline River nr Carrie Mills, Ill. 6 2,370 2,660 3,100 3,400 3,780 4,210 

2,840 3,290 4,070 4,620 5,400 6,350 

3382170 Brushy Creek near Harco, Ill. 6 960 1,040 1,180 1,290 1,440 1,630 

726 841 1,020 1,140 1,300 1,490 

3382510 Eagle Creek near Equality, Ill. 6 490 510 550 570 590 610 

698 834 1,040 1,180 1,370 1,590 

3384450 Lusk Creek near Eddyville, Ill. 7 4,400 4,880 5,710 6,330 7,220 8,360 

2,290 2,810 3,670 4,280 5,140 6,220 

3385000 Hayes Creek at Glendale, Ill. 7 1,450 1,720 2,190 2,540 3,060 3,720 

1,390 1,700 2,180 2,530 3,000 3,580 

3385500 Lake Glendale Inlet near Dixon Springs, Ill. 7 242 341 501 611 758 930 

263 312 384 431 490 558 

5414820 Sinsinawa River near Menominee, Ill. 1 1,480 2,170 3,350 4,230 5,480 7,090 

1,110 1,450 1,990 2,360 2,860 3,470 

5419000 Apple River near Hanover, Ill. 1 4,050 4,710 5,750 6,470 7,400 8,470 

3,080 3,840 5,050 5,880 7,000 8,340 

5420000 Plum River bl Carroll Creek nr Savanna, Ill. 1 2,430 3,000 3,950 4,620 5,530 6,640 

2,710 3,360 4,390 5,110 6,070 7,210 

5435000 Cedar Creek near Winslow, Ill. 1 17 53 116 162 227 309 

102 144 207 250 310 385 

5435500 Pecatonica River at Freeport, Ill. 1 3,550 4,370 5,730 6,700 8,040 9,680 

5,090 5,950 7,420 8,440 9,810 11,400 

5438250 Coon Creek at Riley, Ill. 2 794 936 1,185 1,370 1,650 2,020 

625 760 974 1,130 1,340 1,610 

5438500 Kishwaukee River at Belvidere, Ill. 2 3,140 3,660 4,530 5,130 5,960 6,950 

2,720 3,240 4,080 4,670 5,480 6,470 

5439000 South Branch Kishwaukee River at Dekalb, Ill. 2 699 778 919 1,030 1,190 1,420 

795 950 1,200 1,380 1,630 1,940 

5439500 South Branch Kishwaukee River nr Fairdale, Ill. 2 3,170 3,600 4,360 4,960 5,860 7,110 

3,260 3,770 4,620 5,230 6,070 7,110 

5440000 Kishwaukee River near Perryville, Ill. 2 6,170 7,090 8,610 9,680 11,200 13,000 

5,720 6,680 8,250 9,350 10,900 12,700 

5440500 Killbuck Creek near Monroe Center, Ill. 2 1,560 1,950 2,580 3,000 3,550 4,180 

1,190 1,400 1,740 1,990 2,340 2,770 

5441000 Leaf River at Leaf River, Ill. 1 1,710 2,280 3,190 3,800 4,580 5,460 

1,890 2,420 3,250 3,820 4,600 5,530 

5442000 Kyte River near Flagg Center, Ill. 2 1,010 1,110 1,280 1,400 1,570 1,780 

909 1,110 1,420 1,640 1,950 2,330 
Table 2. Flood-peak dishcarges for recurrence intervals, T, of 0.8, 1.01, 1.5, 2, 3, and 5 years estimated from the partial duration series 127

Station

number

(fig. 3)

Station Name Hydrologic

 Region

Flood Quantiles of Selected Recurrence Interval

Q0.8 Q1.01 Q1.5 Q2 Q3 Q5

5444000 Elkhorn Creek near Penrose, Ill. 1 2,380 2,720 3,260 3,620 4,100 4,650 

1,470 1,820 2,390 2,780 3,300 3,920 

5445500 Rock Creek near Morrison, Ill. 1 1,450 1,680 2,080 2,360 2,750 3,240 

1,720 2,150 2,820 3,280 3,900 4,640 

5447000 Green River at Amboy, Ill. 2 1,740 2,070 2,610 2,990 3,520 4,160 

1,360 1,630 2,050 2,360 2,780 3,300 

5447500 Green River near Geneseo, Ill. 2 5,390 5,930 6,790 7,370 8,110 8,960 

6,040 6,830 8,210 9,200 10,600 12,300 

5448000 Mill Creek at Milan, Ill. 4 2,280 2,700 3,400 3,900 4,610 5,480 

1,870 2,330 3,080 3,610 4,340 5,240 

5466000 Edwards River near Orion, Ill. 4 2,890 3,120 3,500 3,780 4,170 4,680 

2,570 3,100 3,960 4,570 5,410 6,450 

5466500 Edwards River near New Boston, Ill. 4 3,840 4,160 4,740 5,190 5,850 6,750 

3,580 4,140 5,080 5,750 6,710 7,930 

5467000 Pope Creek near Keithsburg, Ill. 2,180 2,540 3,150 3,580 4,180 4,930 

4 1,970 2,320 2,880 3,280 3,850 4,570 

5467500 Henderson Creek near Little York, Ill. 831 1,280 2,090 2,730 3,710 5,070 

4 2,370 2,850 3,620 4,170 4,940 5,910 

5468000 North Henderson Creek near Seaton, Ill. 900 1,030 1,190 1,290 1,390 1,490 

4 1,260 1,530 1,950 2,250 2,670 3,200 

5468500 Cedar Creek at Little York, Ill. 1,120 1,590 2,420 3,050 3,980 5,240 

4 2,090 2,510 3,190 3,670 4,330 5,170 

5469000 Henderson Creek near Oquawka, Ill. 3,470 4,180 5,470 6,470 7,970 10,000 

4 5,320 6,350 8,020 9,220 10,900 13,000 

5469500 South Henderson Creek at Biggsvile, Ill. 1,020 1,350 1,910 2,350 2,980 3,830 

4 1,640 1,980 2,550 2,950 3,510 4,220 

5495500 Bear Creek near Marcelline, Ill. 8,230 9,430 11,500 12,900 15,000 17,600 

4 4,720 5,680 7,220 8,300 9,790 11,600 

5502020 Hadley Creek near Barry, Ill. 2,710 3,300 4,230 4,840 5,630 6,510 

4 1,850 2,350 3,190 3,780 4,610 5,630 

5502040 Hadley Creek at Kinderhook, Ill. 4 6,270 7,180 8,560 9,440 10,500 11,700 

2,380 2,980 3,960 4,660 5,650 6,880 

5512500 Bay Creek at Pittsfield, Ill. 4 3,380 4,240 5,610 6,540 7,750 9,130 

1,380 1,730 2,290 2,690 3,250 3,930 

5513000 Bay Creek at Nebo, Ill. 4 5,920 6,810 8,300 9,360 10,800 12,600 

2,660 3,210 4,120 4,770 5,680 6,810 

5525000 Kankakee River at Shelby, Ind. 2 2,290 2,800 3,620 4,170 4,900 5,740 

3,390 3,960 4,900 5,560 6,470 7,590 
128 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois 

Station

number

(fig. 3)

Station Name Hydrologic

 Region

Flood Quantiles of Selected Recurrence Interval

Q0.8 Q1.01 Q1.5 Q2 Q3 Q5

5525500 Sugar Creek at Milford, Ill. 3 5,100 6,070 7,680 8,830 10,400 12,300 

3,770 4,510 5,760 6,660 7,890 9,420 

5526000 Iroquois River near Chebanse, Ill. 3 9,370 10,900 13,400 15,100 17,400 20,000 

10,300 11,800 14,500 16,400 19,200 22,600 

5526500 Terry Creek near Custer Park, Ill. 2 58 97 167 222 305 421 

79 101 136 161 196 241 

5527500 Kankakee River near Wilmington, Ill. 2 16,900 19,800 24,600 28,100 32,800 38,700 

18,400 20,700 24,700 27,500 31,400 36,000 

5542000 Mazon River near Coal City, Ill. 2 7,130 8,120 9,750 10,880 12,400 14,200 

3,560 4,140 5,100 5,780 6,720 7,880 

5547755 Squaw Creek at Round Lake, Ill. 2 149 167 194 213 237 265 

277 336 430 498 594 716 

5551200 Ferson Creek near St. Charles, Ill. 2 747 855 1,033 1,159 1,330 1,540 

470 575 741 860 1,030 1,230 

5551700 Blackberry Creek near Yorkvile, Ill. 2 418 522 712 861 1,090 1,400 

743 881 1,104 1,266 1,490 1,770 

5554000 North Fork Vermilion River near Charlotte, Ill. 3 1,270 1,800 2,530 2,960 3,450 3,920 

2,110 2,580 3,360 3,910 4,670 5,610 

5554500 Vermilion River at Pontiac, Ill. 3 5,540 6,010 6,780 7,310 8,010 8,850 

4,430 5,260 6,670 7,670 9,070 10,800 

5555000 Vermilion River at Streator, Ill. 3 6,160 7,460 9,430 10,700 12,200 13,900 

6,700 7,800 9,750 11,100 13,100 15,500 

5555300 Vermilion River near Leonore, Ill. 3 9,860 11,400 14,000 15,700 17,900 20,500 

7,370 8,550 10,700 12,200 14,300 16,900 

5555500 Vermilion River at Lowell, Ill. 3 9,000 10,400 12,700 14,300 16,600 19,300 

7,470 8,660 10,800 12,300 14,400 17,100 

5556500 Big Bureau Creek at Princeton, Ill. 3 3,190 3,690 4,540 5,150 6,010 7,080 

2,160 2,640 3,440 4,000 4,770 5,730 

5557500 East Bureau Creek near Bureau, Ill. 3 1,590 1,950 2,560 3,000 3,610 4,360 

1,370 1,710 2,250 2,630 3,160 3,810 

5558000 Big Bureau Creek at Bureau, Ill. 3 6,170 7,030 8,450 9,450 10,830 12,510 

3,920 4,680 5,950 6,860 8,120 9,670 

5558500 Crow Creek (West) near Henry, Ill. 4 1,080 1,370 1,870 2,230 2,740 3,390 

1,930 2,430 3,240 3,810 4,600 5,560 

5559500 Crow Creek near Washburn, Ill. 3 1,480 1,760 2,220 2,540 2,980 3,500 

1,490 1,860 2,440 2,850 3,410 4,110 

5561000 Ackerman Creek at Farmdale, Ill. 3 400 519 734 901 1,152 1,500 

303 399 547 651 792 966 
Table 2. Flood-peak dishcarges for recurrence intervals, T, of 0.8, 1.01, 1.5, 2, 3, and 5 years estimated from the partial duration series 129

Station

number

(fig. 3)

Station Name Hydrologic

 Region

Flood Quantiles of Selected Recurrence Interval

Q0.8 Q1.01 Q1.5 Q2 Q3 Q5

5563000 Kickapoo Creek near Kickapoo, Ill. 4 2,670 4,340 7,070 8,980 11,600 14,600 

3,210 3,990 5,270 6,170 7,430 9,000 

5563500 Kickapoo Creek at Peoria, Ill. 4 4,100 5,240 7,270 8,820 11,100 14,300 

4,490 5,410 6,920 7,990 9,480 11,300 

5564400 Money Creek near Towanda, Ill. 3 798 893 1,050 1,170 1,330 1,540 

839 1,070 1,430 1,690 2,040 2,470 

5564500 Money Creek above Lake Bloomington, Ill. 3 750 864 1,070 1,220 1,440 1,730 

883 1,120 1,500 1,770 2,140 2,590 

5565000 Hickory Creek above Lake Bloomington, Ill. 3 471 558 698 796 928 1,090 

296 394 545 651 797 977 

5566500 East Branch Panther Creek at El Paso, Ill. 3 373 490 700 860 1,110 1,450 

620 798 1,080 1,270 1,540 1,880 

5567500 Mackinaw River near Congervile, Ill. 3 7,060 8,280 10,400 12,100 14,500 17,700 

5,250 6,170 7,750 8,900 10,500 12,400 

5568000 Mackinaw River near Green Valley, Ill. 3 5,950 7,310 9,700 11,500 14,200 17,700 

6,570 7,650 9,540 10,900 12,800 15,100 

5568800 Indian Creek near Wyoming, Ill. 4 1,270 1,460 1,790 2,030 2,390 2,860 

1,500 1,830 2,380 2,760 3,300 3,960 

5569500 Spoon River at London Mills, Ill. 4 7,480 8,590 10,500 12,000 14,100 16,900 

7,130 8,240 10,100 11,400 13,200 15,500 

5570000 Spoon River at Seville, Ill. 4 10,300 11,700 14,100 15,800 18,100 21,000 

8,640 9,870 11,900 13,400 15,500 18,100 

5570370 Big Creek near Bryant, Ill. 4 676 751 860 926 1,000 1,080 

1,200 1,480 1,940 2,270 2,720 3,290 

5570910 Sangamon River at Fisher, Ill. 3 2,690 3,150 3,930 4,520 5,360 6,460 

2,460 2,990 3,860 4,480 5,330 6,380 

5571000 Sangamon River at Mahomet, Ill. 3 3,130 3,750 4,780 5,530 6,560 7,840 

3,240 3,890 4,990 5,760 6,840 8,160 

5572000 Sangamon River at Monticello, Ill. 3 4,060 4,730 5,870 6,700 7,860 9,330 

4,260 5,070 6,420 7,390 8,740 10,400 

5572450 Friends Creek at Argenta, Ill. 3 1,060 1,350 1,820 2,170 2,660 3,270 

1,510 1,880 2,470 2,890 3,470 4,180 

5572500 Sangamon River near Oakley, Ill. 3 3,690 4,580 6,060 7,100 8,540 10,300 

5,380 6,330 7,970 9,150 10,800 12,800 

5574000 South Fork Sangamon River near Nokomis, Ill. 3 620 860 1,280 1,600 2,080 2,730 

313 415 574 685 837 1,030 

5574500 Flat Branch near Taylorville, Ill. 3 2,690 3,310 4,330 5,070 6,070 7,300 

2,720 3,290 4,240 4,920 5,850 7,000 
130 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois 

Station

number

(fig. 3)

Station Name Hydrologic

 Region

Flood Quantiles of Selected Recurrence Interval

Q0.8 Q1.01 Q1.5 Q2 Q3 Q5

5575800 Horse Creek at Pawnee, Ill. 3 1,650 1,910 2,290 2,510 2,760 3,000 

900 1,140 1,530 1,800 2,170 2,630 

5576500 Sangamon River at Riverton, Ill. 3 9,490 11,300 14,600 17,000 20,400 24,900 

11,500 13,000 15,800 17,800 20,700 24,200 

5577500 Spring Creek at Springfield, Ill. 3 1,000 1,460 2,230 2,800 3,600 4,600 

1,400 1,750 2,300 2,690 3,230 3,890 

5578500 Salt Creek near Rowell, Ill. 3 2,320 3,000 4,210 5,130 6,470 8,250 

2,570 3,020 3,770 4,300 5,030 5,900 

5579500 Lake Fork near Cornland, Ill. 3 1,160 1,610 2,440 3,090 4,090 5,500 

2,300 2,810 3,650 4,250 5,070 6,080 

5580000 Kickapoo Creek at Waynesville, Ill. 3 3,060 3,830 5,180 6,200 7,670 9,610 

2,350 2,860 3,690 4,280 5,100 6,100 

5580500 Kickapoo Creek near Lincoln, Ill. 3 2,070 2,880 4,300 5,370 6,940 9,000 

2,880 3,470 4,460 5,150 6,120 7,310 

5581500 Sugar Creek near Hartsburg, Ill. 3 1,850 3,190 5,540 7,310 9,920 13,400 

3,050 3,680 4,710 5,450 6,480 7,740 

5582000 Salt Creek near Greenview, Ill. 3 10,490 12,000 14,550 16,400 19,010 22,290 

9,120 10,430 12,830 14,570 17,000 20,000 

5582500 Crane Creek near Easton, Ill. 3 167 213 281 324 377 431 

584 760 1,030 1,210 1,470 1,790 

5583000 Sangamon River near Oakford, Ill. 3 13,900 16,700 21,500 25,200 30,600 37,800 

18,200 20,200 24,200 27,200 31,500 36,700 

5584400 Drowning Fork at Bushnell, Ill. 4 401 496 664 792 982 1,240 

890 1,110 1,470 1,710 2,040 2,440 

5584500 La Moine River at Colmar, Ill. 4 7,330 8,530 10,600 12,100 14,300 17,100 

7,120 8,470 10,700 12,200 14,400 17,100 

5585000 La Moine River at Ripley, Ill. 4 7,510 8,840 11,000 12,600 14,700 17,200 

7,750 8,940 10,900 12,200 14,200 16,500 

5586000 N Fk Mauvaise Terre Cr nr Jacksonvile, Ill. 5 287 547 1,000 1,340 1,840 2,480 

1,000 1,270 1,690 1,990 2,390 2,890 

5586500 Hurricane Creek near Roodhouse, Ill. 5 110 151 224 281 365 480 

208 283 404 491 612 765 

5587000 Macoupin Creek near Kane, Ill. 5 8,700 9,920 12,000 13,600 15,800 18,800 

9,340 10,900 13,400 15,200 17,700 20,700 

5587900 Cahokia Creek at Edwardsville, Ill. 4 4,650 4,950 5,430 5,750 6,150 6,600 

3,010 3,590 4,550 5,230 6,200 7,410 

5588000 Indian Creek at Wanda, Ill. 4 1,330 1,570 2,010 2,340 2,830 3,480 

1,020 1,250 1,630 1,900 2,280 2,750 
Table 2. Flood-peak dishcarges for recurrence intervals, T, of 0.8, 1.01, 1.5, 2, 3, and 5 years estimated from the partial duration series 131

Station

number

(fig. 3)

Station Name Hydrologic

 Region

Flood Quantiles of Selected Recurrence Interval

Q0.8 Q1.01 Q1.5 Q2 Q3 Q5

5589500 Canteen Creek at Caseyville, Ill. 4 1,200 1,490 2,020 2,430 3,030 3,840 

921 1,160 1,550 1,820 2,200 2,670 

5590000 Kaskaskia Ditch at Bondville, Ill. 3 287 354 468 550 664 805 

334 440 604 719 876 1,070 

5590400 Kaskaskia River near Pesotum, Ill. 3 1,440 1,620 1,900 2,100 2,340 2,630 

1,440 1,790 2,350 2,740 3,290 3,950 

5590500 Kaskaskia River at Ficklin, Ill. 3 1,240 1,550 2,040 2,370 2,830 3,360 

1,590 1,960 2,570 3,000 3,580 4,310 

5590800 Lake Fork at Atwood, Ill. 3 1,800 1,980 2,270 2,480 2,740 3,060 

1,830 2,260 2,950 3,440 4,120 4,960 

5591200 Kaskaskia River at Cooks Mills, Ill. 3 3,930 4,560 5,530 6,150 6,940 7,790 

3,860 4,610 5,860 6,760 8,000 9,530 

5591500 Asa Creek at Sullivan, Ill. 3 195 260 371 451 563 703 

252 337 469 562 689 846 

5591700 West Okaw River near Lovington, Ill. 3 2,050 2,420 3,070 3,560 4,270 5,200 

1,500 1,860 2,450 2,870 3,440 4,150 

5592050 Robinson Creek near Shelbyville, Ill. 5 2,630 3,130 3,970 4,590 5,480 6,600 

2,660 3,320 4,370 5,120 6,160 7,460 

5592300 Wolf Creek near Beecher City, Ill. 5 2,230 2,790 3,660 4,230 4,950 5,750 

1,500 1,890 2,500 2,940 3,540 4,290 

5592575 Hickory Creek nr Brownstown, Ill. 5 2,430 2,920 3,660 4,130 4,710 5,310 

1,670 2,120 2,860 3,380 4,110 5,020 

5592800 Hurricane Creek near Mulberry Grove, Ill. 5 6,000 7,370 9,410 10,700 12,300 13,900 

3,600 4,420 5,780 6,730 8,070 9,720 

5592900 East Fork Kaskaskia River near Sandoval, Ill. 5 2,740 3,370 4,460 5,270 6,430 7,950 

2,470 3,030 3,920 4,540 5,400 6,460 

5593520 Crooked Creek near Hoffman, Ill. 5 4,150 5,060 6,650 7,830 9,560 11,830 

3,750 4,440 5,560 6,340 7,400 8,700 

5593575 Little Crooked Creek near New Minden, Ill. 5 2,520 3,260 4,480 5,330 6,480 7,860 

2,010 2,480 3,220 3,750 4,460 5,350 

5593600 Blue Grass Creek near Raymond, Ill. 5 820 900 1,040 1,140 1,280 1,450 

590 750 990 1,160 1,380 1,660 

5593900 East Fork Shoal Creek near Coffeen, Ill. 5 1,900 2,130 2,520 2,810 3,200 3,690 

1,540 1,920 2,520 2,940 3,530 4,240 

5594000 Shoal Creek near Breese, Ill. 5 7,090 8,070 9,820 11,200 13,200 16,000 

8,300 9,630 11,800 13,400 15,600 18,200 

5594090 Sugar Creek at Albers, Ill. 5 2,640 3,060 3,790 4,330 5,110 6,110 

2,530 3,080 3,960 4,580 5,430 6,480 
132 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Station

number

(fig. 3)

Station Name Hydrologic

 Region

Flood Quantiles of Selected Recurrence Interval

Q0.8 Q1.01 Q1.5 Q2 Q3 Q5

5594330 Mud Creek near Marissa, Ill. 5 1,750 2,030 2,480 2,810 3,270 3,830 

1,710 2,100 2,710 3,140 3,740 4,460 

5594450 Silver Creek near Troy, Ill. 5 2,970 3,410 4,150 4,680 5,420 6,320 

3,330 4,070 5,280 6,130 7,300 8,750 

5594800 Silver Creek near Freeburg, Ill. 5 4,320 4,850 5,750 6,410 7,340 8,510 

5,990 7,040 8,760 9,970 11,600 13,700 

5595200 Richland Creek near Hecker, Ill. 5 4,060 4,690 5,810 6,660 7,910 9,590 

3,010 3,660 4,730 5,490 6,530 7,820 

5595500 Marys River near Sparta, Ill. 4 1,020 1,380 1,990 2,430 3,050 3,840 

1,010 1,300 1,770 2,100 2,560 3,110 

5595730 Rayse Creek near Waltonville, Ill. 6 3,770 5,330 7,830 9,550 11,800 14,500 

2,750 3,330 4,300 4,990 5,960 7,190 

5595800 Sevenmile Creek near Mt. Vernon, Ill. 6 753 893 1,120 1,280 1,490 1,740 

1,240 1,490 1,890 2,170 2,550 3,010 

5595820 Casey Fork at Mount Vernon, Ill. 6 2,930 3,820 5,320 6,400 7,930 9,830 

2,210 2,630 3,330 3,820 4,500 5,350 

5597500 Crab Orchard Creek near Marion, Ill. 6 1,120 1,390 1,880 2,250 2,790 3,510 

1,240 1,460 1,810 2,050 2,390 2,790 

5599000 Beaucoup Creek near Matthews, Ill. 6 3,100 4,020 5,560 6,670 8,200 10,100 

4,010 4,750 5,990 6,900 8,180 9,790 

5600000 Big Creek near Wetaug, Ill. 7 1,810 1,930 2,140 2,310 2,550 2,890 

1,650 2,000 2,560 2,950 3,490 4,160 
Table 6. Selected basin characteristics and equivalent years of record for the 288 streamflow-gaging stations in Illinois and adjacent States 133

Table 6. Selected basin characteristics and equivalent years of record for the 288 streamflow-gaging stations in Illinois and adjacent

States.

[TDA, basin drainage area in square miles, mi2; MCS, main-channel slope in feet per mile, ft/mi; BL, basin length in miles, mi; BW, basin width in miles, mi; 

PermAvg, averaged permeability in inches per hour, in/hr; (%Water +5) percentage of open water and herbaceous wetland plus a constant 5, in percent; equivalent years of record for various QT (flood quantiles of specific recurrence intervals), in years]

Station

number

(figs. 2A 

and 2B)

TDA

(mi 2)

MCS

(ft/mi)

BL

(mi)

BW

(mi)

PermAvg

(in/hr)

(%water + 5)

(percent)

Hydrologic

Region

 (fig. 7)

Equivalent years of record (years)

Q2 Q5 Q10 Q25 Q50 Q100 Q500

3336100 1.03 15.66 1.91 0.54 0.452 5.00 3 3.8 4.6 5.6 6.7 7.4 8.0 8.9

3336500 35.0 6.97 8.03 4.36 .514 5.02 3 3.3 4.0 4.9 5.9 6.5 7.0 7.8

3336645 432 2.75 52.77 8.18 .679 5.35 3 2.8 3.4 4.1 5.0 5.5 6.0 6.6

3336900 133 5.19 20.25 6.55 1.242 5.15 3 3.0 3.6 4.4 5.3 5.9 6.3 7.0

3337500 67.5 2.14 13.44 5.02 1.368 5.28 3 2.6 3.2 3.9 4.7 5.2 5.6 6.2

3338000 337 2.93 30.83 10.94 1.232 5.23 3 2.7 3.3 4.0 4.9 5.4 5.8 6.4

3338100 2.22 14.87 3.12 .71 1.140 5.00 3 3.5 4.3 5.2 6.3 7.0 7.5 8.3

3338500 957 2.98 55.05 17.38 .980 5.46 3 2.7 3.3 4.0 4.9 5.4 5.8 6.4

3338780 262 3.84 26.29 9.97 1.126 5.08 3 2.9 3.4 4.2 5.1 5.6 6.1 6.7

3338800 1.32 32.12 1.65 .80 .610 5.00 3 4.1 4.9 6.0 7.2 8.0 8.6 9.5

3339000 1,289 3.15 58.18 22.15 1.000 5.53 3 2.7 3.3 4.0 4.9 5.4 5.8 6.4

3341700 1.03 39.18 1.61 .64 1.083 5.00 3 4.0 4.8 5.9 7.1 7.9 8.5 9.4

3341900 .04 27.21 .32 .11 1.279 5.00 5 3.6 4.3 5.3 6.4 7.1 7.6 8.4

3343400 186 3.01 22.85 8.12 1.363 5.11 3 2.8 3.3 4.0 4.9 5.4 5.8 6.4

3344000 916 1.66 60.55 15.13 1.267 5.34 3 2.5 3.0 3.7 4.5 4.9 5.3 5.9

3344425 0.11 82.43 0.51 0.22 0.760 5.00 5 4.4 5.3 6.5 7.9 8.7 9.4 10.3

3344500 7.13 8.71 4.18 1.71 .398 5.02 5 3.4 4.1 5.0 6.1 6.7 7.2 8.0

3345500 1,510 1.67 92.71 16.29 1.061 5.37 5 2.4 2.9 3.6 4.3 4.8 5.1 5.7

3346000 318 3.73 38.63 8.23 .716 5.14 5 2.8 3.4 4.2 5.0 5.6 6.0 6.6

3378000 228 1.98 28.50 8.01 .699 5.27 5 2.6 3.2 3.9 4.7 5.2 5.6 6.2

3378635 240 5.87 27.72 8.66 0.800 6.43 5 3.0 3.6 4.4 5.3 5.8 6.3 7.0

3378650 1.20 12.04 2.08 .58 .549 5.03 5 3.5 4.2 5.2 6.3 6.9 7.4 8.2

3378900 746 2.83 52.89 14.10 .662 5.64 5 2.7 3.3 4.0 4.8 5.3 5.7 6.3

3378980 .35 54.11 1.17 .30 .647 5.98 5 4.3 5.1 6.2 7.5 8.3 9.0 9.9

3379500 1,131 2.23 64.26 17.60 .638 5.81 5 2.6 3.2 3.8 4.7 5.2 5.5 6.1

3379650 1.63 26.03 1.46 1.12 0.517 5.11 5 3.9 4.7 5.7 7.0 7.7 8.3 9.1

3380350 209 3.61 20.43 10.23 .598 5.67 6 2.1 2.5 3.1 3.8 4.2 4.5 5.0

3380400 1.14 30.92 1.35 .84 .619 5.21 6 2.9 3.5 4.3 5.2 5.8 6.2 6.9

3380450 .44 50.74 1.04 .42 1.001 5.40 6 3.1 3.7 4.5 5.5 6.1 6.6 7.3

3380475 97.1 4.88 17.49 5.55 .912 5.34 6 2.2 2.7 3.3 4.0 4.5 4.8 5.3

3380500 463 1.95 35.08 13.20 0.698 5.54 6 1.9 2.4 2.9 3.5 3.9 4.2 4.6

3381500 3,102 1.17 109.43 28.35 .696 5.71 5 2.4 2.8 3.5 4.2 4.7 5.0 5.5

3381600 .17 61.51 .40 .44 1.279 6.21 5 4.1 4.9 6.0 7.3 8.0 8.6 9.5

3382025 .51 64.30 1.00 .51 1.405 5.48 6 3.1 3.8 4.6 5.6 6.2 6.7 7.4

3382100 147 4.99 22.32 6.57 .936 10.71 6 1.8 2.1 2.6 3.2 3.5 3.8 4.2
134 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Station

number

(figs. 2A 

and 2B)

TDA

(mi 2)

MCS

(ft/mi)

BL

(mi)

BW

(mi)

PermAvg

(in/hr)

(%water + 5)

(percent)

Hydrologic

Region

 (fig. 7)

Equivalent years of record (years)

Q2 Q5 Q10 Q25 Q50 Q100 Q500

3382170 13.3 16.52 7.42 1.79 0.860 8.26 6 2.3 2.8 3.4 4.1 4.6 4.9 5.5

3382510 8.51 25.05 4.51 1.89 1.331 5.51 6 2.7 3.3 4.0 4.9 5.4 5.8 6.5

3384450 42.8 17.45 8.92 4.80 1.001 5.16 7 1.8 2.1 2.6 3.1 3.5 3.7 4.1

3385000 19.1 25.97 8.07 2.37 1.001 5.18 7 1.8 2.2 2.7 3.3 3.6 3.9 4.3

3385500 1.08 164.76 1.22 .88 1.001 6.07 7 2.2 2.6 3.2 3.9 4.3 4.6 5.1

3612000 244 2.99 27.12 8.99 1.242 7.06 7 1.3 1.5 1.9 2.3 2.5 2.7 3.0

3614000 1.71 23.32 2.99 .57 1.404 5.69 7 1.9 2.2 2.7 3.3 3.7 3.9 4.4

4087300 1.51 30.41 2.32 .65 1.561 5.00 2 2.4 3.0 3.6 4.4 4.9 5.3 5.9

4087400 5.08 19.52 3.90 1.30 2.864 5.12 2 2.3 2.8 3.4 4.1 4.6 5.0 5.5

5414820 40.3 19.53 9.86 4.09 1.304 5.08 1 2.9 3.4 4.2 5.0 5.6 6.0 6.6

5415000 125 11.31 20.19 6.19 1.302 5.11 1 2.6 3.2 3.8 4.7 5.1 5.5 6.1

5415500 19.8 33.65 7.05 2.82 1.306 5.04 1 3.1 3.7 4.5 5.4 5.9 6.4 7.1

5418750 1.94 36.50 2.43 .80 1.280 5.00 1 3.2 3.8 4.7 5.6 6.2 6.7 7.4

5418800 .86 106.05 1.24 .69 1.123 5.00 1 3.6 4.4 5.3 6.4 7.1 7.6 8.4

5419000 246 8.76 24.39 10.09 1.295 5.34 1 2.5 3.0 3.7 4.5 4.9 5.3 5.9

5420000 230 7.51 21.85 10.53 1.285 5.47 1 2.5 3.0 3.6 4.4 4.9 5.2 5.8

5430500 3,343 1.02 86.69 38.57 3.677 10.69 2 1.2 1.4 1.7 2.1 2.4 2.5 2.8

5431486 196 2.89 15.19 12.90 2.804 7.22 1 2.1 2.5 3.0 3.7 4.1 4.4 4.8

5434500 1,034 2.11 46.25 22.36 1.288 5.23 1 2.1 2.5 3.1 3.7 4.1 4.4 4.8

5435000 1.30 35.51 1.92 .68 1.162 5.00 1 3.2 3.8 4.7 5.7 6.3 6.7 7.4

5435500 1,327 1.40 64.69 20.51 1.307 5.27 1 2.0 2.4 2.9 3.5 3.8 4.1 4.6

5435650 1.94 30.69 2.04 .95 1.307 5.00 1 3.1 3.7 4.6 5.5 6.1 6.5 7.2

5436500 521 3.59 36.02 14.46 3.051 5.26 1 2.1 2.5 3.1 3.7 4.1 4.4 4.9

5436900 .52 92.48 1.05 .50 1.267 5.00 1 3.6 4.3 5.2 6.3 7.0 7.5 8.3

5437000 2,548 .81 78.96 32.27 1.816 5.35 1 1.8 2.1 2.6 3.1 3.4 3.7 4.1

5437500 6,360 0.95 104.04 61.13 2.889 8.25 1 1.7 2.1 2.5 3.0 3.4 3.6 4.0

5437600 2.20 40.64 2.44 .90 1.555 5.00 1 3.2 3.8 4.6 5.6 6.2 6.7 7.4

5437950 14.4 8.98 6.44 2.24 3.602 7.12 2 1.9 2.3 2.8 3.4 3.8 4.1 4.6

5438250 84.9 8.74 13.95 6.08 2.585 5.22 2 2.0 2.4 3.0 3.6 4.0 4.3 4.8

5438300 .81 94.58 1.91 .43 1.069 5.90 2 2.6 3.1 3.8 4.7 5.2 5.6 6.2

5438390 88.4 8.49 12.96 6.82 2.626 5.53 2 1.9 2.4 2.9 3.5 3.9 4.3 4.7

5438500 542 5.18 22.15 24.46 2.976 5.85 2 1.8 2.1 2.6 3.2 3.5 3.8 4.2

5438850 1.68 24.34 2.01 .84 1.161 5.00 2 2.4 2.9 3.5 4.3 4.8 5.2 5.8

5439000 77.7 3.55 14.33 5.42 1.408 5.13 2 1.8 2.2 2.7 3.3 3.7 4.0 4.4

5439500 387 2.47 30.25 12.79 1.408 5.25 2 1.7 2.1 2.5 3.1 3.4 3.7 4.1

5439550 1.70 59.25 1.96 0.87 4.894 5.04 2 2.6 3.1 3.8 4.7 5.2 5.6 6.2

5440000 1,103 4.40 31.21 35.35 2.355 5.60 2 1.7 2.1 2.6 3.1 3.5 3.7 4.1

5440500 116 7.52 19.77 5.89 1.366 5.09 2 2.0 2.4 2.9 3.6 4.0 4.3 4.7

5440650 .98 28.81 2.17 .45 1.373 5.04 2 2.4 3.0 3.6 4.4 4.9 5.3 5.9

5440900 .20 85.77 .87 .23 1.305 5.00 1 3.6 4.3 5.2 6.3 7.0 7.5 8.3
Table 6. Selected basin characteristics and equivalent years of record for the 288 streamflow-gaging stations in Illinois and adjacent States 135

Station

number

(figs. 2A 

and 2B)

TDA

(mi 2)

MCS

(ft/mi)

BL

(mi)

BW

(mi)

PermAvg

(in/hr)

(%water + 5)

(percent)

Hydrologic

Region

 (fig. 7)

Equivalent years of record (years)

Q2 Q5 Q10 Q25 Q50 Q100 Q500

5441000 103 12.62 17.42 5.94 1.602 5.09 1 2.6 3.2 3.9 4.7 5.2 5.5 6.1

5441500 8,201 1.02 133.13 61.60 2.768 7.68 1 1.7 2.1 2.5 3.1 3.4 3.6 4.0

5442000 118 4.49 10.08 11.75 1.771 5.41 2 1.8 2.2 2.7 3.3 3.7 4.0 4.4

5443500 8,754 1.02 167.54 52.25 2.775 7.61 1 1.7 2.1 2.5 3.1 3.4 3.6 4.0

5444000 145 3.85 16.94 8.57 1.308 5.03 1 2.3 2.8 3.4 4.1 4.5 4.9 5.4

5444100 1.49 62.11 1.33 1.12 1.307 5.05 1 3.4 4.1 4.9 6.0 6.6 7.1 7.8

5445500 158 4.90 22.29 7.08 1.764 5.11 1 2.3 2.8 3.4 4.1 4.6 4.9 5.4

5446000 165 4.39 24.94 6.60 1.755 5.11 1 2.3 2.8 3.4 4.1 4.5 4.8 5.3

5446500 9,554 1.12 189.80 50.34 2.756 7.46 1 1.8 2.1 2.6 3.1 3.4 3.7 4.1

5446950 .66 54.09 .98 .67 2.856 5.00 2 2.6 3.2 3.9 4.7 5.2 5.7 6.3

5447000 198 2.69 18.20 10.88 2.293 5.28 2 1.7 2.1 2.6 3.1 3.5 3.8 4.2

5447050 5.12 23.24 3.65 1.40 5.904 5.71 2 2.2 2.7 3.3 4.1 4.5 4.9 5.4

5447350 1.16 29.08 1.75 .67 1.309 5.00 4 3.1 3.7 4.5 5.5 6.2 6.7 7.5

5447500 999 2.67 82.34 12.13 2.789 5.47 2 1.7 2.0 2.5 3.0 3.3 3.6 4.0

5448000 62.4 9.86 13.17 4.74 1.320 5.24 4 2.5 3.1 3.8 4.6 5.1 5.6 6.2

5448050 0.22 45.84 0.72 0.30 1.308 5.00 4 3.3 4.0 4.9 6.0 6.7 7.2 8.1

5466000 155 4.97 22.36 6.94 1.265 5.06 4 2.3 2.7 3.4 4.1 4.6 4.9 5.5

5466500 445 2.99 56.08 7.93 1.385 5.33 4 1.8 2.2 2.7 3.3 3.6 3.9 4.4

5467000 174 3.90 40.45 4.30 1.313 5.25 4 1.8 2.1 2.6 3.2 3.6 3.9 4.3

5467500 151 5.49 27.22 5.54 1.116 5.24 4 2.1 2.5 3.1 3.8 4.3 4.6 5.2

5468000 65.7 5.92 22.22 2.96 1.193 5.13 4 2.0 2.4 2.9 3.6 4.0 4.3 4.8

5468500 132 4.76 24.60 5.37 1.221 5.24 4 2.1 2.5 3.1 3.8 4.3 4.6 5.2

5469000 435 4.73 33.35 13.03 1.204 5.23 4 2.3 2.8 3.4 4.2 4.6 5.0 5.6

5469500 81.4 7.28 23.04 3.54 1.306 5.19 4 2.1 2.5 3.1 3.8 4.2 4.6 5.1

5469750 .38 37.11 1.14 .33 1.305 5.00 4 3.0 3.6 4.5 5.5 6.1 6.6 7.4

5495200 1.44 22.80 1.72 0.84 0.867 5.10 4 3.1 3.7 4.6 5.6 6.3 6.8 7.6

5495500 349 3.63 22.62 15.43 .873 5.27 4 2.5 3.0 3.6 4.5 5.0 5.4 6.0

5496900 .55 72.13 .99 .56 1.302 5.13 4 3.6 4.4 5.4 6.6 7.3 7.9 8.9

5501500 .38 61.84 1.24 .31 1.005 5.00 4 3.1 3.8 4.6 5.6 6.3 6.8 7.6

5502020 40.8 18.29 9.82 4.15 1.149 5.26 4 2.8 3.4 4.2 5.1 5.7 6.1 6.9

5502040 72.5 14.98 14.87 4.88 1.179 5.42 4 2.6 3.1 3.9 4.7 5.3 5.7 6.3

5502120 1.21 107.69 1.54 .78 1.092 5.17 4 3.7 4.4 5.4 6.6 7.4 8.0 8.9

5512500 39.5 10.77 11.24 3.52 1.192 5.24 4 2.5 3.0 3.7 4.6 5.1 5.5 6.2

5513000 148 6.83 24.76 5.98 1.234 5.17 4 2.2 2.7 3.3 4.1 4.5 4.9 5.5

5513200 1.23 153.11 1.72 .72 1.300 5.06 4 3.6 4.4 5.4 6.6 7.3 7.9 8.8

5518000 1,777 0.95 68.11 26.10 6.632 6.79 2 1.4 3.2 3.9 4.7 5.3 5.7 6.3

5519500 55.0 2.19 18.36 3.00 .760 5.93 2 1.7 2.0 2.5 3.0 3.4 3.6 4.0

5520000 218 4.11 26.57 8.20 3.066 6.89 2 1.7 2.0 2.5 3.1 3.4 3.7 4.1

5520500 2,301 .84 85.62 26.87 6.436 6.72 2 1.4 1.7 2.0 2.5 2.8 3.0 3.3

5524500 449 2.14 27.06 16.59 5.014 5.74 2 1.6 2.0 2.4 3.0 3.3 3.6 4.0
136 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Station

number

(figs. 2A 

and 2B)

TDA

(mi 2)

MCS

(ft/mi)

BL

(mi)

BW

(mi)

PermAvg

(in/hr)

(%water + 5)

(percent)

Hydrologic

Region

 (fig. 7)

Equivalent years of record (years)

Q2 Q5 Q10 Q25 Q50 Q100 Q500

5525000 687 1.74 43.97 15.63 3.758 5.61 2 1.6 1.9 2.4 2.9 3.2 3.5 3.9

5525050 10.2 6.41 4.43 2.31 2.294 5.01 3 3.0 3.6 4.3 5.2 5.8 6.2 6.9

5525500 447 5.12 25.73 17.39 1.331 5.09 3 2.9 3.5 4.2 5.1 5.7 6.1 6.8

5526000 2,089 .95 48.27 43.28 3.194 5.45 3 2.1 2.5 3.1 3.8 4.2 4.5 4.9

5526150 .18 39.77 .66 .27 .708 5.00 2 2.6 3.1 3.8 4.6 5.2 5.6 6.2

5526500 12.1 10.20 6.98 1.73 5.855 5.14 2 2.1 2.6 3.1 3.8 4.2 4.6 5.1

5527050 .81 28.16 1.79 .45 .311 5.00 2 2.4 3.0 3.6 4.4 4.9 5.3 5.9

5527500 5,149 1.22 123.47 41.71 4.570 6.08 2 1.4 1.7 2.1 2.6 2.9 3.1 3.5

5527800 123 2.42 16.24 7.55 .718 6.80 2 1.6 2.0 2.4 2.9 3.3 3.5 3.9

5527840 145 2.18 20.35 7.11 .726 6.74 2 1.6 1.9 2.4 2.9 3.2 3.5 3.9

5527900 21.3 5.37 6.66 3.20 0.766 10.15 2 1.6 1.9 2.3 2.8 3.1 3.4 3.8

5528000 228 1.75 27.58 8.26 .773 8.31 2 1.4 1.8 2.1 2.6 2.9 3.1 3.5

5528150 10.6 18.40 4.13 2.56 .587 11.45 2 1.7 2.0 2.5 3.0 3.4 3.6 4.0

5528200 4.12 12.14 3.63 1.14 .840 9.39 2 1.8 2.2 2.7 3.2 3.6 3.9 4.3

5528360 2.83 5.30 2.72 1.04 .957 5.85 2 1.9 2.3 2.8 3.5 3.8 4.1 4.6

5528440 1.02 33.35 1.91 0.53 0.595 5.10 2 2.5 3.0 3.7 4.5 5.0 5.4 5.9

5528470 7.89 15.10 5.01 1.57 .624 8.29 2 1.9 2.3 2.8 3.4 3.8 4.1 4.6

5533200 2.32 31.83 2.32 1.00 .619 5.31 2 2.4 2.9 3.6 4.3 4.8 5.2 5.8

5533300 3.19 15.38 2.47 1.29 .633 7.93 2 2.0 2.4 2.9 3.6 3.9 4.3 4.7

5534400 15.8 4.48 11.54 1.37 .337 7.53 2 1.7 2.1 2.6 3.1 3.5 3.7 4.1

5536178 34.7 6.65 12.31 2.82 0.636 6.10 2 1.9 2.3 2.8 3.4 3.8 4.1 4.6

5536190 69.9 6.62 19.96 3.50 4.037 7.16 2 1.8 2.2 2.6 3.2 3.6 3.9 4.3

5536195 91.9 6.29 22.88 4.02 5.569 7.90 2 1.7 2.1 2.5 3.1 3.4 3.7 4.1

5539870 10.1 7.03 4.07 2.48 .712 7.10 2 1.8 2.2 2.8 3.4 3.7 4.0 4.5

5539890 23.9 7.45 7.62 3.14 2.016 7.57 2 1.8 2.2 2.7 3.3 3.6 3.9 4.4

5539950 8.80 12.17 3.96 2.22 2.686 7.39 2 1.9 2.4 2.9 3.5 3.9 4.2 4.7

5540030 60.2 5.36 13.52 4.45 2.696 7.38 2 1.7 2.1 2.6 3.1 3.5 3.8 4.2

5540240 6.48 23.35 3.82 1.69 .904 5.38 2 2.3 2.8 3.4 4.1 4.6 5.0 5.5

5541750 4.65 7.09 4.81 .97 1.133 5.21 2 2.0 2.5 3.0 3.7 4.1 4.4 4.9

5542000 452 4.62 25.05 18.03 1.382 6.58 2 1.7 2.1 2.5 3.1 3.4 3.7 4.1

5545750 805 1.39 44.00 18.30 3.771 10.27 2 1.3 1.5 1.9 2.3 2.6 2.8 3.1

5547755 17.2 4.11 7.67 2.25 .848 9.13 2 1.6 1.9 2.4 2.9 3.2 3.4 3.8

5548150 13.7 6.29 4.79 2.85 3.817 6.26 2 1.9 2.3 2.8 3.4 3.8 4.1 4.6

5549000 15.5 9.84 2.38 6.50 4.443 6.76 2 1.9 2.4 2.9 3.5 3.9 4.2 4.7

5549700 10.8 12.18 4.44 2.42 1.153 9.86 2 1.7 2.1 2.6 3.1 3.5 3.7 4.2

5549850 36.9 9.11 7.44 4.96 0.794 11.63 2 1.5 1.9 2.3 2.8 3.1 3.3 3.7

5549900 .08 78.96 .31 .25 8.015 5.00 2 2.8 3.4 4.1 5.0 5.6 6.0 6.7

5550300 37.8 11.69 9.61 3.93 4.400 5.88 2 2.0 2.4 3.0 3.7 4.1 4.4 4.9

5550470 4.54 11.07 3.43 1.32 .623 6.45 2 2.0 2.5 3.0 3.7 4.1 4.4 4.9

5551030 14.0 12.07 5.58 2.50 3.612 9.13 2 1.8 2.2 2.6 3.2 3.6 3.8 4.3
Table 6. Selected basin characteristics and equivalent years of record for the 288 streamflow-gaging stations in Illinois and adjacent States 137

Station

number

(figs. 2A 

and 2B)

TDA

(mi 2)

MCS

(ft/mi)

BL

(mi)

BW

(mi)

PermAvg

(in/hr)

(%water + 5)

(percent)

Hydrologic

Region

 (fig. 7)

Equivalent years of record (years)

Q2 Q5 Q10 Q25 Q50 Q100 Q500

5551050 7.37 9.36 2.97 2.48 4.487 7.75 2 1.9 2.3 2.8 3.4 3.7 4.0 4.5

5551060 11.5 10.87 2.88 4.00 3.099 7.04 2 1.9 2.4 2.9 3.5 3.9 4.2 4.7

5551200 51.7 16.57 9.32 5.55 1.775 5.65 2 2.1 2.5 3.1 3.8 4.2 4.5 5.0

5551520 3.19 4.61 2.19 1.46 .702 6.09 2 1.8 2.2 2.7 3.4 3.7 4.0 4.5

5551620 21.5 12.75 8.90 2.42 1.279 5.38 2 2.1 2.6 3.1 3.8 4.2 4.6 5.1

5551650 2.13 43.06 1.82 1.17 1.150 5.20 2 2.5 3.0 3.7 4.5 5.0 5.4 6.0

5551700 70.2 6.43 19.65 3.57 1.585 6.06 2 1.9 2.3 2.8 3.4 3.8 4.1 4.5

5551800 .46 77.47 1.00 .46 1.086 5.08 2 2.7 3.3 4.0 4.9 5.4 5.8 6.5

5551900 32.6 10.61 7.40 4.40 1.246 5.17 2 2.1 2.5 3.1 3.8 4.2 4.5 5.0

5551930 22.0 11.32 11.78 1.87 1.389 5.29 2 2.1 2.5 3.1 3.8 4.2 4.6 5.1

5554000 188 4.66 15.57 12.08 1.409 5.08 3 2.9 3.5 4.2 5.1 5.7 6.1 6.8

5554500 580 1.26 30.07 19.31 1.154 5.24 3 2.5 2.9 3.6 4.4 4.8 5.2 5.7

5554600 .14 13.00 .53 .26 .310 10.76 3 3.7 4.5 5.4 6.6 7.3 7.8 8.6

5555000 1,086 1.33 51.38 21.13 1.095 5.33 3 2.5 3.0 3.6 4.4 4.8 5.2 5.8

5555300 1,256 1.46 60.93 20.62 1.094 5.35 3 2.5 3.0 3.6 4.4 4.9 5.3 5.8

5555400 0.39 51.23 1.03 0.38 0.569 5.00 3 4.4 5.2 6.4 7.7 8.6 9.2 10.2

5555500 1,282 1.54 66.23 19.36 1.087 5.36 3 2.5 3.0 3.7 4.5 4.9 5.3 5.9

5555775 .51 25.57 .96 .54 1.304 5.00 3 3.8 4.5 5.5 6.7 7.4 8.0 8.8

5556500 195 6.40 34.89 5.59 1.249 5.09 3 3.0 3.6 4.4 5.4 5.9 6.4 7.1

5557000 86.8 12.25 13.04 6.66 1.691 5.01 3 3.2 3.9 4.7 5.7 6.3 6.8 7.5

5557100 0.39 78.44 1.69 0.23 1.290 5.00 4 2.8 3.4 4.2 5.1 5.7 6.2 6.9

5557500 99.1 12.69 17.11 5.79 1.227 5.11 3 3.3 4.0 4.8 5.8 6.5 7.0 7.7

5558000 485 6.99 41.40 11.71 1.370 5.25 3 3.0 3.6 4.3 5.3 5.8 6.3 6.9

5558050 .04 86.06 .28 .15 1.368 5.00 3 4.4 5.3 6.4 7.8 8.6 9.2 10.2

5558075 .22 122.41 .75 .29 5.261 5.48 3 4.0 4.7 5.8 7.0 7.7 8.3 9.1

5558500 55.6 11.11 10.84 5.13 1.236 5.18 4 2.7 3.3 4.0 4.9 5.5 5.9 6.6

5559000 5.61 51.36 3.63 1.55 1.219 5.04 4 3.2 3.9 4.8 5.9 6.5 7.0 7.9

5559500 114 6.17 16.64 6.84 1.049 5.15 3 3.1 3.7 4.5 5.5 6.1 6.5 7.2

5561000 11.2 42.26 5.62 1.99 1.054 5.59 3 3.9 4.7 5.7 6.9 7.7 8.2 9.1

5563000 119 10.89 15.80 7.55 1.007 5.19 4 2.7 3.2 3.9 4.8 5.4 5.8 6.5

5563100 0.07 59.50 0.37 0.17 1.005 5.00 4 3.5 4.2 5.2 6.3 7.0 7.6 8.5

5563500 298 5.68 26.78 11.12 .996 5.79 4 2.4 2.9 3.5 4.3 4.8 5.2 5.8

5564400 47.6 4.88 17.99 2.65 1.109 5.05 3 3.0 3.6 4.4 5.4 5.9 6.4 7.1

5564500 51.4 4.71 19.51 2.63 1.107 5.05 3 3.0 3.6 4.4 5.3 5.9 6.4 7.0

5565000 10.0 8.36 5.38 1.87 1.038 5.01 3 3.3 4.0 4.8 5.8 6.5 7.0 7.7

5566000 6.26 13.94 2.63 2.38 0.652 5.03 3 3.6 4.4 5.3 6.5 7.1 7.7 8.5

5566500 30.7 4.17 7.97 3.85 .920 5.13 3 3.0 3.6 4.4 5.3 5.9 6.4 7.0

5567000 94.0 4.82 10.09 9.31 .945 5.09 3 3.0 3.6 4.4 5.4 6.0 6.4 7.1

5567500 765 2.46 56.07 13.65 1.066 5.43 3 2.7 3.2 3.9 4.8 5.3 5.7 6.3

5567800 .99 31.38 2.43 .41 .996 5.00 3 4.0 4.7 5.8 7.0 7.8 8.3 9.2
138 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Station

number

(figs. 2A 

and 2B)

TDA

(mi 2)

MCS

(ft/mi)

BL

(mi)

BW

(mi)

PermAvg

(in/hr)

(%water + 5)

(percent)

Hydrologic

Region

 (fig. 7)

Equivalent years of record (years)

Q2 Q5 Q10 Q25 Q50 Q100 Q500

5568000 1,072 2.45 77.71 13.79 1.067 5.45 3 2.7 3.2 3.9 4.7 5.2 5.6 6.2

5568650 .68 12.26 1.10 .62 1.002 5.46 4 2.9 3.5 4.3 5.3 5.9 6.3 7.1

5568800 63.2 7.40 17.25 3.66 1.000 5.03 4 2.2 2.7 3.3 4.1 4.5 4.9 5.5

5568850 .42 20.12 .69 .60 .645 12.44 4 3.3 4.0 5.0 6.1 6.7 7.3 8.1

5569500 1,069 2.27 56.21 19.03 1.064 5.65 4 2.0 2.4 3.0 3.7 4.1 4.4 4.9

5569825 4.34 21.91 3.02 1.44 0.996 5.00 4 3.0 3.7 4.5 5.5 6.1 6.6 7.4

5570000 1,635 1.89 73.09 22.37 1.056 5.78 4 1.9 2.3 2.9 3.5 3.9 4.2 4.7

5570370 41.1 9.48 14.71 2.79 .852 10.00 4 2.3 2.7 3.4 4.1 4.6 4.9 5.5

5570910 240 4.91 29.35 8.18 1.126 5.20 3 3.0 3.5 4.3 5.2 5.8 6.2 6.9

5571000 364 3.60 34.03 10.68 1.108 5.23 3 2.8 3.4 4.1 5.0 5.6 6.0 6.6

5572000 551 2.71 38.76 14.21 1.118 5.27 3 2.7 3.3 4.0 4.8 5.3 5.7 6.4

5572450 113 5.36 9.88 11.45 1.043 5.01 3 3.0 3.6 4.4 5.4 6.0 6.4 7.1

5572500 776 2.24 42.80 18.12 1.115 5.25 3 2.6 3.2 3.9 4.7 5.2 5.6 6.2

5574000 11.0 12.94 4.19 2.62 .548 5.04 3 3.6 4.3 5.3 6.4 7.1 7.6 8.4

5574500 277 2.02 22.34 12.42 .972 5.16 3 2.7 3.2 3.9 4.7 5.2 5.6 6.2

5575500 562 2.60 20.93 26.84 0.831 5.60 3 2.8 3.3 4.0 4.9 5.4 5.8 6.4

5575800 52.7 5.32 13.03 4.05 .599 5.05 3 3.2 3.8 4.6 5.6 6.2 6.7 7.4

5576500 2,617 1.62 73.34 35.69 1.005 5.97 3 2.5 3.0 3.7 4.4 4.9 5.3 5.8

5577500 103 3.97 21.81 4.74 .996 5.13 3 2.9 3.5 4.3 5.2 5.8 6.2 6.9

5577700 1.46 48.07 1.76 .83 .997 5.00 3 4.1 5.0 6.0 7.3 8.1 8.7 9.6

5578500 333 2.92 36.49 9.12 1.077 7.63 3 2.8 3.3 4.1 4.9 5.4 5.9 6.5

5579500 213 4.32 30.84 6.92 1.195 5.05 3 2.9 3.5 4.3 5.2 5.7 6.1 6.8

5579750 3.10 21.68 3.30 .94 1.080 5.00 3 3.7 4.5 5.5 6.6 7.3 7.9 8.7

5580000 228 6.14 27.60 8.28 1.092 5.31 3 3.0 3.6 4.4 5.4 6.0 6.4 7.1

5580500 308 5.27 41.85 7.37 1.098 5.31 3 3.0 3.6 4.3 5.3 5.8 6.3 6.9

5580700 0.90 37.57 1.47 0.61 2.514 5.50 3 3.7 4.5 5.4 6.6 7.3 7.8 8.7

5581500 332 5.86 37.45 8.86 1.049 5.22 3 3.0 3.6 4.4 5.3 5.9 6.3 7.0

5582000 1,804 2.42 71.36 25.27 1.237 5.72 3 2.6 3.1 3.8 4.6 5.1 5.5 6.1

5582200 .84 17.30 1.57 .54 1.046 5.00 3 3.6 4.4 5.3 6.5 7.2 7.7 8.5

5582500 27.6 4.24 5.40 5.12 2.282 5.01 3 2.8 3.3 4.1 4.9 5.5 5.9 6.5

5583000 5,091 1.34 87.46 58.21 1.164 5.83 3 2.4 2.9 3.5 4.2 4.7 5.0 5.6

5584400 26.9 5.55 7.87 3.42 .995 5.00 4 2.5 3.0 3.7 4.5 5.0 5.4 6.1

5584500 655 3.75 32.77 19.98 .986 5.34 4 2.4 2.9 3.5 4.3 4.8 5.2 5.8

5584950 1.97 24.20 2.60 .76 .995 5.07 4 2.8 3.4 4.2 5.2 5.7 6.2 6.9

5585000 1,295 1.65 52.23 24.80 .960 5.27 4 2.1 2.5 3.0 3.7 4.2 4.5 5.0

5585220 3.48 16.36 3.03 1.15 0.997 5.01 5 3.5 4.2 5.1 6.2 6.9 7.4 8.2

5585700 .15 35.29 .68 .22 .998 5.93 4 3.1 3.7 4.5 5.6 6.2 6.7 7.5

5586000 30.1 5.73 7.31 4.13 .996 5.07 5 3.0 3.6 4.4 5.3 5.9 6.4 7.0

5586200 .55 106.86 1.08 .51 1.309 5.06 4 3.7 4.4 5.4 6.6 7.4 8.0 8.9

5586350 1.87 40.37 2.21 .84 .922 5.11 5 3.9 4.7 5.8 7.0 7.7 8.3 9.2
Table 6. Selected basin characteristics and equivalent years of record for the 288 streamflow-gaging stations in Illinois and adjacent States 139

Station

number

(figs. 2A 

and 2B)

TDA

(mi 2)

MCS

(ft/mi)

BL

(mi)

BW

(mi)

PermAvg

(in/hr)

(%water + 5)

(percent)

Hydrologic

Region

 (fig. 7)

Equivalent years of record (years)

Q2 Q5 Q10 Q25 Q50 Q100 Q500

5586500 2.17 19.78 3.70 0.59 1.297 5.11 5 3.5 4.2 5.1 6.2 6.9 7.4 8.2

5587000 865 2.40 49.16 17.60 .898 5.63 5 2.6 3.1 3.8 4.6 5.1 5.5 6.1

5587900 211 5.65 32.31 6.53 .907 5.88 4 2.1 2.5 3.1 3.8 4.2 4.6 5.1

5588000 37.4 9.16 16.87 2.22 1.089 5.54 4 2.1 2.5 3.1 3.8 4.2 4.6 5.1

5589500 22.6 12.21 9.96 2.27 1.307 5.61 4 2.4 2.9 3.6 4.4 4.9 5.3 6.0

5589780 1.64 45.47 2.14 0.77 1.305 5.55 5 3.9 4.7 5.7 6.9 7.6 8.2 9.0

5590000 12.4 17.90 5.65 2.21 1.580 5.29 3 3.5 4.2 5.1 6.2 6.8 7.4 8.1

5590400 110 2.23 23.06 4.77 1.344 5.28 3 2.7 3.2 3.9 4.7 5.2 5.6 6.2

5590500 126 1.83 28.15 4.48 1.353 5.25 3 2.6 3.1 3.8 4.6 5.1 5.4 6.0

5590800 150 1.17 23.73 6.34 1.197 5.01 3 2.4 2.9 3.6 4.3 4.8 5.1 5.7

5591200 474 1.28 46.85 10.12 1.282 5.25 3 2.4 2.9 3.6 4.3 4.8 5.2 5.7

5591500 7.88 2.95 2.12 3.72 1.140 5.00 3 2.8 3.4 4.1 5.0 5.5 5.9 6.5

5591700 112 3.83 15.34 7.32 1.151 5.03 3 2.9 3.5 4.2 5.1 5.7 6.1 6.8

5591750 .53 15.57 2.15 .25 1.140 5.00 3 3.6 4.3 5.2 6.3 7.0 7.5 8.3

5592000 1,058 1.09 64.89 16.30 1.254 5.24 3 2.4 2.9 3.5 4.2 4.7 5.0 5.6

5592050 97.7 6.72 15.05 6.49 0.990 5.04 5 3.0 3.6 4.4 5.4 5.9 6.4 7.1

5592300 48.3 6.30 15.52 3.11 .545 5.07 5 3.2 3.8 4.6 5.6 6.2 6.7 7.4

5592500 1,953 1.69 103.09 17.98 1.021 6.33 3 2.5 3.0 3.7 4.5 5.0 5.3 5.9

5592575 44.2 10.43 10.48 4.21 .573 5.26 5 3.4 4.0 4.9 6.0 6.6 7.1 7.8

5592800 152 6.29 26.31 5.77 .602 5.24 5 3.1 3.7 4.5 5.5 6.1 6.5 7.2

5592900 113 4.43 24.61 4.58 0.582 5.37 5 3.0 3.6 4.4 5.3 5.9 6.3 7.0

5593000 2,723 1.69 132.67 20.52 .883 6.02 5 2.4 2.9 3.6 4.3 4.8 5.2 5.7

5593520 254 2.92 31.43 8.09 .611 6.18 5 2.8 3.4 4.1 5.0 5.5 5.9 6.5

5593575 83.6 4.85 14.52 5.75 .543 5.48 5 3.0 3.7 4.5 5.4 6.0 6.4 7.1

5593600 18.7 3.73 6.27 2.99 .518 5.01 5 3.0 3.6 4.4 5.3 5.9 6.3 7.0

5593700 0.47 20.92 1.23 0.38 0.425 5.00 5 3.9 4.7 5.7 6.9 7.6 8.2 9.0

5593900 56.1 5.05 12.88 4.36 .558 5.16 5 3.1 3.7 4.5 5.5 6.0 6.5 7.2

5594000 737 2.78 54.45 13.54 .650 6.37 5 2.7 3.3 4.0 4.8 5.3 5.7 6.3

5594090 124 3.81 22.57 5.51 .786 5.33 5 2.9 3.4 4.2 5.1 5.6 6.1 6.7

5594200 1.95 16.04 2.22 .88 .355 5.02 5 3.8 4.5 5.5 6.7 7.4 7.9 8.8

5594330 72.3 4.41 14.10 5.13 0.724 5.71 5 3.0 3.6 4.3 5.3 5.8 6.3 6.9

5594450 154 5.01 27.95 5.52 .897 5.28 5 2.9 3.5 4.3 5.2 5.7 6.2 6.8

5594800 466 2.75 50.27 9.27 .952 5.86 5 2.7 3.2 3.9 4.7 5.2 5.6 6.2

5595000 5,189 1.28 163.59 31.72 .802 6.83 5 2.3 2.8 3.4 4.2 4.6 4.9 5.5

5595200 129 6.65 21.06 6.11 1.121 6.28 5 3.0 3.6 4.4 5.3 5.8 6.3 7.0

5595500 17.8 15.58 6.31 2.82 0.798 7.91 4 2.8 3.4 4.2 5.2 5.7 6.2 6.9

5595510 1.27 29.94 2.23 .57 .887 6.09 4 2.9 3.4 4.2 5.2 5.8 6.2 7.0

5595550 .65 77.28 1.18 .55 1.210 5.32 4 3.5 4.3 5.3 6.4 7.2 7.7 8.6

5595730 91.4 6.06 15.70 5.82 .609 5.42 6 2.3 2.8 3.4 4.1 4.5 4.9 5.4

5595800 21.1 16.65 7.08 2.97 .850 5.51 6 2.6 3.1 3.8 4.6 5.1 5.5 6.1
140 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois

Station

number

(figs. 2A 

and 2B)

TDA

(mi 2)

MCS

(ft/mi)

BL

(mi)

BW

(mi)

PermAvg

(in/hr)

(%water + 5)

(percent)

Hydrologic

Region

 (fig. 7)

Equivalent years of record (years)

Q2 Q5 Q10 Q25 Q50 Q100 Q500

5595820 76.8 6.10 15.63 4.91 0.864 6.63 6 2.2 2.6 3.2 3.9 4.3 4.7 5.2

5596000 502 2.50 36.66 13.69 .693 12.94 6 1.5 1.8 2.2 2.7 3.0 3.2 3.5

5596100 1.03 44.80 1.71 .61 .496 5.37 6 3.0 3.6 4.4 5.4 6.0 6.4 7.1

5597000 792 1.86 45.53 17.40 .710 7.22 6 1.8 2.2 2.6 3.2 3.6 3.8 4.3

5597450 .63 55.66 1.59 .40 .664 8.97 6 2.6 3.2 3.9 4.7 5.2 5.6 6.2

5597500 31.6 8.76 8.79 3.60 0.702 6.72 6 2.3 2.8 3.4 4.1 4.5 4.9 5.4

5599000 288 2.02 30.33 9.49 .635 7.36 6 1.8 2.2 2.7 3.3 3.6 3.9 4.4

5599500 2,161 1.10 57.89 37.33 .749 9.22 6 1.5 1.9 2.3 2.8 3.1 3.3 3.6

5599560 1.96 61.34 2.33 .84 1.400 5.57 6 3.0 3.6 4.5 5.4 6.0 6.4 7.1

5599580 .17 316.86 .85 .20 2.020 5.00 4 3.6 4.4 5.4 6.5 7.3 7.9 8.8

5599640 0.43 82.30 1.13 0.38 1.676 5.72 7 2.2 2.6 3.2 3.8 4.3 4.6 5.1

5599800 .09 78.87 .37 .25 1.676 6.15 7 2.1 2.6 3.1 3.8 4.2 4.5 5.0

5600000 32.2 14.93 12.09 2.66 1.461 5.64 7 1.7 2.0 2.5 3.0 3.3 3.6 4.0
Table 7. Selected flood-peak information for the 288 streamflow-gaging stations in Illinois and adjacent States 141

Table 7. Selected flood-peak information for the 288 streamflow-gaging stations in Illinois and adjacent States.

[No., number; The magnitude of flood peak is expressed in cubic feet per second, ft3/s; historically adjusted record length used in the flood-frequency analysis is 

obtained from Log Pearson III analysis (PEAKFQ output); the water year of maximum peak presented in parenthesis indicates that the peak represents a historical event; approximate recurrence interval of maximum peak is interpolated from flood-frequency curves (PEAKFQ output), rounded to the nearest 5 years for 

20- to 50-years, to the nearest 10 years for 50- to 100-years, to the nearest 20 years for 100- to 200-years, to the nearest 25 years for 200- to 500-years; >, greater 

than]

Station

number

(figs. 2A 

and 2B) Station name

Period

of

record 

(Water

Years)

Systematic

record

length

(No. of 

years)

Weighted

skew

Sample

skew

Historically

adjusted 

record

length

(No. of

years)

Maximum 

peakflow

(ft 3/s)

Water 

year of 

maximum 

peak

Approximate

recurrence

interval of

maximum

peak

(years)

3336100 Big Four Ditch Tributary near Paxton, Ill. 1956-80 25 -0.413 -0.671 25 249 1959 15

3336500 Bluegrass Creek at Potomac, Ill. 1950-82 33 -.223 -.206 33 5,160 1968 50

3336645 Middle Fork Vermilion River above Oakwood, Ill. 1979-99 21 -.261 -.263 21 15,500 1994 50

3336900 Salt Fork near St. Joseph, Ill. 1959-91 33 .057 .349 33 6,860 1968 50

3337500 Saline Branch at Urbana, Ill. 1937-75 39 -.276 -.502 43 4,080 1964 60

3338000 Salt Fork near Homer, Ill. 1945-82 37 0.017 -0.687 44 20,500 (1939) >500

3338100 Salt Fork Trib near Catlin, Ill. 1959-80 22 -.492 -1.299 22 640 1980 20

3338500 Vermilion River near Catlin, Ill. 1940-58 19 .072 .182 56 41,000 (1939) 100

3338780 North Fork Vermilion River near Bismarck, Ill. 1989-99 11 -.165 .353 11 20,100 1990/94 15

3338800 N F Vermilion River Tributary near Danville, Ill. 1956-76 21 -.087 .297 21 1,600 1974 60

3339000 Vermilion River near Danville, Ill. 1915-99 77 -0.225 -0.559 77 48,700 1939 200

3341700 Big Creek Tributary near Dudley, Ill. 1961-75 15 .017 .673 15 511 1961 30

3341900 Raccoon Creek Trib near Annapolis, Ill. 1956-80 25 -.178 -.483 25 48 1974 15

3343400 Embarras River near Camargo, Ill. 1961-99 39 -.556 -1.116 39 8,040 1994 60

3344000 Embarras River near Diona, Ill. 1939-92 27 -.242 -.178 27 20,400 1985 25

3344425 Muddy Creek Tributary at Woodbury, Ill. 1959-76 18 0.060 0.190 18 112 1974 30

3344500 Range Creek near Casey, Ill. 1951-91 41 .041 -.685 41 3,500 1961 35

3345500 Embarras River at Ste. Marie, Ill. 1908-99 88 -.271 -1.485 88 44,800 1950 50

3346000 North Fork Embarras River near Oblong, Ill. 1941-99 59 -.350 -1.533 59 27,100 1950 45

3378000 Bonpas Creek at Browns, Ill. 1941-99 59 -.052 -1.180 59 7,500 1961 80

3378635 Little Wabash River near Effingham, Ill. 1967-99 33 0.045 0.139 33 17,800 1996 60

3378650 Second Creek Tributary at Keptown, Ill. 1956-72 17 .071 .338 17 930 1970 60

3378900 Little Wabash River at Louisville, Ill. 1966-92 27 .026 -.277 43 42,800 (1950) 90

3378980 Little Wabash River Trib at Clay City, Ill. 1959-80 22 -.241 -.606 22 409 1971 20

3379500 Little Wabash River below Clay City, Ill. 1915-99 85 -.135 -.422 85 47,000 1950 40

3379650 Madden Creek near West Salem, Ill. 1956-76 21 0.167 0.562 21 1,550 1961 90

3380350 Skillet Fork near Iuka, Ill. 1966-83 18 -.223 -.267 18 19,000 1968 40

3380400 Horse Creek Tributary near Cartter, Ill. 1961-72 12 -.089 .228 12 570 1961/68 15

3380450 White Feather Creek near Marlow, Ill. 1956-80 25 -.198 -.272 25 323 1975 25

3380475 Horse Creek near Keenes, Ill. 1960-90 31 -.177 -.211 31 17,100 1961 150

3380500 Skillet Fork at Wayne City, Ill. 1909-99 82 0.018 -0.147 82 59,400 1990 250

3381500 Little Wabash River at Carmi, Ill. 1940-99 60 -.025 -.290 60 46,900 1961 100

3381600 Little Wabash River Tributary nr New Haven, Ill. 1960-76 17 .035 .314 17 484 1974 180

3382025 Little Saline Creek Tributary near Goreville, Ill. 1959-80 22 .046 -2.063 22 563 1969 90

3382100 South Fork Saline River nr Carrier Mills, Ill. 1966-99 34 -.031 -.465 34 5,160 1982 25

3382170 Brushy Creek near Harco, Ill. 1969-82 14 0.144 0.819 14 2,590 1977 60

3382510 Eagle Creek near Equality, Ill. 1967-82 16 -.218 -.644 16 668 1973 20

3384450 Lusk Creek near Eddyville, Ill. 1968-99 32 .050 .134 32 16,100 1985 120
142 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois 

Station

number

(figs. 2A 

and 2B) Station name

Period

of

record 

(Water

Years)

Systematic

record

length

(No. of 

years)

Weighted

skew

Sample

skew

Historically

adjusted 

record

length

(No. of

years)

Maximum 

peakflow

(ft 3/s)

Water 

year of 

maximum 

peak

Approximate

recurrence

interval of

maximum

peak

(years)

3385000 Hayes Creek at Glendale, Ill. 1950-99 50 -0.120 -0.206 50 9,450 1985 180

3385500 Lake Glendale Inlet near Dixon Springs, Ill. 1955-80 26 -.160 -.433 26 1,500 1958 30

3612000 Cache River at Forman, Ill. 1923-99 77 -.133 -.227 77 9,630 1935 25

3614000 Hess Bayou Tributary near Mound City, Ill. 1959-72 14 .212 -.406 14 754 1966 15

4087300 Lake Michigan Tributary at Winthrop Harbor, Ill. 1956-72 17 -.160 .608 17 355 1969 70

4087400 Kellogg Ravine at Zion, Ill. 1962-76 15 -0.171 0.651 15 937 1969 70

5414820 Sinsinawa River near Menominee, Ill. 1968-99 32 .130 .025 32 17,000 1999 40

5415000 Galena River at Buncombe, Wis. 1937-92 53 .200 .284 84 29,700 1969 >500

5415500 East Fork Galena River at Council Hills, Ill. 1940-69 30 .220 .342 30 16,600 1947 85

5418750 South Fork Apple River near Nora, Ill. 1961-80 20 -.229 -1.060 20 520 1974 15

5418800 Mill Creek Tributary near Scales Mound, Ill. 1956-75 20 0.008 -0.177 20 862 1965 45

5419000 Apple River near Hanover, Ill. 1935-99 65 .071 .086 65 12,000 1946 70

5420000 Plum River bl Carroll Creek nr Savanna, Ill. 1941-77 37 -.132 -.070 37 11,600 1946 45

5430500 Rock River at Afton, Wis. 1914-99 86 -.312 -.671 86 13,000 1929 50

5431486 Turtle Creek at Carvers Rock Road nr Clinton, Wis. 1938-99 60 -.051 -.046 63 16,500 1973 350

5434500 Pecatonica River at Martintown, Wis. 1916-99 60 -0.071 -0.140 84 15,100 1969 35

5435000 Cedar Creek near Winslow, Ill. 1952-76 25 -.230 -.470 25 698 1974 20

5435500 Pecatonica River at Freeport, Ill. 1914-99 86 .090 .097 86 18,400 1929 50

5435650 Lost Creek Tributary near Shannon, Ill. 1961-76 16 -.165 -1.399 16 660 1974 50

5436500 Sugar River near Brodhead, Wis. 1914-99 86 -.091 -.105 86 14,800 1915 90

5436900 Otter Creek Tributary near Durand, Ill. 1961-80 20 -0.072 0.258 20 187 1969 25

5437000 Pecatonica River at Shirland, Ill. 1940-71 32 -.182 -.345 43 16,600 1959 20

5437500 Rock River at Rockton, Ill. 1904-99 70 -.264 -.328 70 32,500 1916 50

5437600 Rock River Tributary near Rockton, Ill. 1961-76 16 -.432 -.859 16 308 1974 15

5437950 Kishwaukee River near Huntley, Ill. 1965-78 14 -.450 -.612 14 192 1972 25

5438250 Coon Creek at Riley, Ill. 1962-91 30 -0.578 -1.027 30 5,090 1978 350

5438300 Lawrence Creek Tributary near Harvard, Ill. 1961-80 20 -.209 -.163 20 180 1972 15

5438390 Piscasaw Creek below Mokeler Creek nr Capron, Ill. 1970-79 10 -.063 -1.265 10 4,000 1973 40

5438500 Kishwaukee River at Belvidere, Ill. 1940-99 60 -.319 -.301 60 11,900 1994 35

5438850 M Br of So Br Kishwaukee R nr Malta, Ill. 1956-80 25 -.638 -.930 25 393 1959 45

5439000 South Branch Kishwaukee River at Dekalb, Ill. 1926-99 28 -0.149 0.375 28 3,500 1983 225

5439500 South Branch Kishwaukee River nr Fairdale, Ill. 1940-99 60 -.336 -.243 63 25,400 1996 >500

5439550 South Branch Kishwaukee River Trib nr Irene, Ill. 1959-76 18 -.378 -.279 18 452 1971 30

5440000 Kishwaukee River near Perryville, Ill. 1940-99 60 -.450 -.480 62 24,200 1996 70

5440500 Killbuck Creek near Monroe Center, Ill. 1940-80 41 -.712 -1.043 41 6,100 1951/55 15

5440650 Stillman Creek Tributary near Holcomb, Ill. 1959-79 18 -0.348 -0.049 18 297 1971 50

5440900 Leaf River Tributary near Forreston, Ill. 1956-79 23 -.083 .455 23 212 1958 35

5441000 Leaf River at Leaf River, Ill. 1940-82 43 -.508 -.719 45 11,000 (1938) 25

5441500 Rock River at Oregon, Ill. 1940-49 10 .051 .338 35 47,000 (1937) 15

5442000 Kyte River near Flagg Center, Ill. 1940-51 12 -.310 .321 15 2,630 1951 60

5443500 Rock River at Como, Ill. 1915-99 80 -0.434 -0.827 80 59,700 1973 90

5444000 Elkhorn Creek near Penrose, Ill. 1940-99 60 -.648 -.995 62 6,800 (1938) 25

5444100 Spring Creek Tributary near Coleta, Ill. 1959-72 14 -.306 -.383 14 832 1965 25
Table 7. Selected flood-peak information for the 288 streamflow-gaging stations in Illinois and adjacent States 143

Station

number

(figs. 2A 

and 2B) Station name

Period

of

record 

(Water

Years)

Systematic

record

length

(No. of 

years)

Weighted

skew

Sample

skew

Historically

adjusted 

record

length

(No. of

years)

Maximum 

peakflow

(ft 3/s)

Water 

year of 

maximum 

peak

Approximate

recurrence

interval of

maximum

peak

(years)

5445500 Rock Creek near Morrison, Ill. 1940-71 32 -0.039 -0.014 35 6,500 (1937) 100

5446000 Rock Creek at Morrison, Ill. 1940-99 54 .048 .290 54 5,770 1946 150

5446500 Rock River near Joslin, Ill. 1940-99 60 -.321 -.465 60 46,500 1993 20

5446950 Green River Tributary near Amboy, Ill. 1961-76 16 -.207 .495 16 493 1967 30

5447000 Green River at Amboy, Ill. 1940-82 42 -.722 -1.075 42 7,600 1981 50

5447050 Green River Tributary No 2 near Ohio, Ill. 1959-72 14 -0.239 -0.801 14 431 1969 50

5447350 Mud Creek Tributary near Atkinson, Ill. 1961-76 16 .082 -.431 16 890 1967 150

5447500 Green River near Geneseo, Ill. 1936-99 64 -.627 -1.050 64 12,100 1974 40

5448000 Mill Creek at Milan, Ill. 1940-99 57 -.404 -.579 100 9,300 1973 50

5448050 Sand Creek near Milan, Ill. 1956-80 25 -.055 .181 25 168 1980 20

5466000 Edwards River near Orion, Ill. 1941-99 59 -0.208 -1.524 76 8,910 1951 >500

5466500 Edwards River near New Boston, Ill. 1935-99 65 -.272 -.383 65 18,000 1973 >500

5467000 Pope Creek near Keithsburg, Ill. 1935-99 60 -.030 -.061 60 8,900 1973 70

5467500 Henderson Creek near Little York, Ill. 1941-82 41 .389 .761 59 23,400 1982 500

5468000 North Henderson Creek near Seaton, Ill. 1941-51 11 .017 -.791 17 2,600 (1935) 50

5468500 Cedar Creek at Little York, Ill. 1941-99 56 0.126 0.123 76 18,100 1993 100

5469000 Henderson Creek near Oquawka, Ill. 1935-99 64 .372 .563 76 34,600 1982 250

5469500 South Henderson Creek at Biggsville, Ill. 1940-82 42 .220 -.159 59 10,500 1982 100

5469750 Ellison Creek Tributary near Roseville, Ill. 1956-80 25 -.175 -1.549 25 182 1958 30

5495200 Little Creek near Breckenridge, Ill. 1956-80 24 -.333 -.377 24 1,110 1958 15

5495500 Bear Creek near Marcelline, Ill. 1944-99 56 -0.353 -0.399 56 35,500 1996 100

5496900 Homan Creek Tributary near Quincy, Ill. 1956-76 21 -.390 -.585 21 616 1960 10

5501500 Burton Creek Tributary near Burton, Ill. 1961-76 15 -.298 -.238 15 796 1962 30

5502020 Hadley Creek near Barry, Ill. 1956-99 43 -.441 -1.080 43 9,000 1973/79 25

5502040 Hadley Creek at Kinderhook, Ill. 1940-86 46 -.419 -.864 48 24,000 (1939) 100

5502120 Kiser Creek Trib near Barry, Ill. 1956-80 25 -0.149 -0.572 25 1,330 1966 50

5512500 Bay Creek at Pittsfield, Ill. 1940-99 60 -.306 -.523 73 35,000 (1926) >500

5513000 Bay Creek at Nebo, Ill. 1940-86 47 -.439 -.802 70 24,000 (1916) 50

5513200 Salt Spring Creek near Gilead, Ill. 1956-80 24 -.199 .335 24 1,280 1960 90

5518000 Kankakee River at Shelby, Ind. 1923-99 77 -.418 -.745 77 7,650 1982 350

5519500 West Creek near Schneider, Ind. 1949-72 23 -0.370 -1.263 23 1,840 1955 20

5520000 Singleton Ditch at Illlinoi, Ill. 1945-77 33 -.577 -1.842 63 3,610 1976 >500

5520500 Kankakee River at Momence, Ill. 1915-99 85 -.459 -.583 85 16,000 1979 >500

5524500 Iroquois River near Foresman, Ind. 1949-99 51 -.436 -.526 51 5,930 1958 50

5525000 Iroquois River at Iroquois, Ill. 1945-99 55 -.336 -.337 55 10,400 1958 250

5525050 Eastburn Hollow near Sheldon, Ill. 1956-72 17 -0.084 0.494 22 1,950 1957 250

5525500 Sugar Creek at Milford, Ill. 1949-99 51 -.315 -.264 51 22,900 1951 50

5526000 Iroquois River near Chebanse, Ill. 1924-99 76 -.319 -.572 87 34,000 (1913) 100

5526150 Kankakee River Tributary near Bourbonnais, Ill. 1956-80 25 -.173 .092 25 233 1957 30

5526500 Terry Creek near Custer Park, Ill. 1950-75 26 -.051 .641 63 1,710 1970 >500

5527050 Prairie Creek near Frankfort, Ill. 1956-72 17 0.254 0.121 60 786 1957 >500

5527500 Kankakee River near Wilmington, Ill. 1915-99 85 -.313 -.277 117 75,900 1957 180

5527800 Des Plaines River at Russell, Ill. 1960-99 39 -.542 -.746 39 2,120 1979 20
144 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois 

Station

number

(figs. 2A 

and 2B) Station name

Period

of

record 

(Water

Years)

Systematic

record

length

(No. of 

years)

Weighted

skew

Sample

skew

Historically

adjusted 

record

length

(No. of

years)

Maximum 

peakflow

(ft 3/s)

Water 

year of 

maximum 

peak

Approximate

recurrence

interval of

maximum

peak

(years)

5527840 Des Plaines River at Wadsworth, Ill. 1962-76 15 -0.612 -1.155 15 2,170 1976 10

5527900 North Mill Creek at Hickory Corners, Ill. 1962-76 15 -.245 -.962 17 510 (1960) 50

5528000 Des Plaines River near Gurnee, Ill. 1946-99 53 -.603 -1.139 53 3,530 1986 100

5528150 Indian Creek at Diamond Lake, Ill. 1990-76 17 -.387 -.440 17 1,150 1960 60

5528200 Hawthorn Drainage Ditch near Mundelein, Ill. 1961-76 16 -.144 .409 16 543 1970 40

5528360 Aptakisic Creek at Aptakisic, Ill. 1961-76 16 -0.257 0.014 16 390 1972 50

5528440 Buffalo Creek near Lake Zurich, Ill. 1961-76 16 -.247 .060 16 203 1972 20

5528470 Buffalo Creek at Long Grove, Ill. 1961-76 16 -.441 -.626 16 539 1972 20

5533200 Sawmill Creek Tributary near Tiedtville, Ill. 1961-79 18 -.389 -2.668 18 315 1976 15

5533300 Wards Creek near Woodridge, Ill. 1962-76 15 -.339 -1.025 15 151 1966 20

5534400 North Branch Chicago River at Bannockburn, Ill. 1960-76 17 -0.501 -0.785 17 355 1967 10

5536178 Plum Creek near Dyer, Ind. 1966-77 12 -.455 -1.094 24 2,480 (1955) 25

5536190 Hart Ditch at Munster, Ind. 1943-99 57 -.353 -.401 57 3,010 1991 25

5536195 Little Calumet River at Munster, Ind. 1959-99 39 -.099 .059 39 1,510 1959 250

5539870 West Branch Du Page River at Ontarioville, Ill. 1961-79 19 -.495 -.735 19 630 1972 10

5539890 West Branch Du Page River near Wayne, Ill. 1961-79 19 -0.192 -0.101 25 1,620 (1955) 100

5539950 Klein Creek at Carol Stream, Ill. 1961-79 19 .030 1.390 32 888 1972 >500

5540030 West Br Du Page River at West Chicago, Ill. 1961-79 19 -.392 -.202 26 1,670 1972 100

5540240 Prentiss Creek near Lisle, Ill. 1961-80 20 .004 .710 20 532 1961 40

5541750 Mazon River Tributary near Gardner, Ill. 1959-80 22 -.587 -1.702 22 173 1979 15

5542000 Mazon River near Coal City, Ill. 1940-99 59 -0.595 -1.172 59 22,400 1983 50

5545750 Fox River near New Munster, Wis. 1940-99 60 -.049 .031 60 7,520 1960 150

5547755 Squaw Creek at Round Lake, Ill. 1990-99 10 -.351 -.335 10 312 1993 15

5548150 North Br Nippersink Crk nr Genoa City, Wis. 1962-99 38 -.140 -.031 38 517 1999 100

5549000 Boone Creek near Mc Henry, Ill. 1949-92 43 -.371 -.399 43 345 1986 50

5549700 Mutton Creek at Island Lake, Ill. 1962-76 15 -0.170 0.097 17 378 (1960) 40

5549850 Flint Creek near Fox River Grove, Ill. 1962-96 22 -.061 .616 22 690 1996 150

5549900 Fox River Tributary near Cary, Ill. 1956-79 23 -.227 .031 23 59 1972 30

5550300 Tyler Creek at Elgin, Ill. 1962-99 19 -.071 -.994 19 953 1999 >500

5550470 Poplar Creek Trib near Bartlett, Ill. 1961-79 19 -.075 .544 28 565 1967 400

5551030 Brewster Creek at Valley View, Ill. 1962-79 17 -0.418 -0.306 17 687 1967 25

5551050 Norton Creek near Wayne, Ill. 1962-79 18 -.002 .586 23 890 (1957) 160

5551060 Norton Creek near St. Charles, Ill. 1962-79 18 .002 1.054 23 954 1967 >500

5551200 Ferson Creek near St. Charles, Ill. 1961-99 39 -.636 -.887 39 2,580 1997 40

5551520 Indian Creek near North Aurora, Ill. 1961-79 19 -.298 -.084 19 402 1978 50

5551620 Blackberry Creek near Kaneville, Ill. 1961-79 17 -0.348 -0.713 17 640 1974 25

5551650 Lake Run Trib near Batavia, Ill. 1961-76 16 -.149 .626 16 346 1970 50

5551700 Blackberry Creek near Yorkville, Ill. 1961-99 39 -.301 .035 85 5,510 1996 >500

5551800 Fox River Tributary River No 2 near Fox, Ill. 1961-80 19 -.339 -.120 19 320 1978 15

5551900 East Branch Big Rock Creek near Big Rock, Ill. 1965-79 15 -.278 .174 15 1,580 1974 50

5551930 Welch Creek near Big Rock, Ill. 1965-80 16 -0.363 -0.197 16 694 1974 40

5554000 North Fork Vermilion River near Charlotte, Ill. 1943-99 57 -.581 -.773 57 4,900 1987/90 15

5554500 Vermilion River at Pontiac, Ill. 1943-99 56 -.469 -.595 56 13,100 1983 30
Table 7. Selected flood-peak information for the 288 streamflow-gaging stations in Illinois and adjacent States 145

Station

number

(figs. 2A 

and 2B) Station name

Period

of

record 

(Water

Years)

Systematic

record

length

(No. of 

years)

Weighted

skew

Sample

skew

Historically

adjusted 

record

length

(No. of

years)

Maximum 

peakflow

(ft 3/s)

Water 

year of 

maximum 

peak

Approximate

recurrence

interval of

maximum

peak

(years)

5554600 Mud Creek Tributary near Odell, Ill. 1959-76 18 -0.550 -0.719 18 163 1965 30

5555000 Vermilion River at Streator, Ill. 1915-30 15 -.319 -.380 15 17,100 1920 15

5555300 Vermilion River near Leonore, Ill. 1931-99 69 -.448 -.573 69 33,500 1958 35

5555400 Vermilion River Tributary at Lowell, Ill. 1956-76 21 -.066 .531 21 176 1958 20

5555500 Vermilion River at Lowell, Ill. 1931-71 41 -.285 -.257 41 33,500 1958 50

5555775 Vermilion Creek Tributary at Meriden, Ill. 1959-72 14 -0.390 -0.651 14 98 1960 15

5556500 Big Bureau Creek at Princeton, Ill. 1937-99 63 -.701 -1.046 63 12,500 1974 50

5557000 West Bureau Creek at Wyanet, Ill. 1937-91 54 -.169 -.016 61 20,100 1974 >500

5557100 West Bureau Creek Tributary near Wyanet, Ill. 1956-79 22 -.159 .036 22 261 1973 15

5557500 East Bureau Creek near Bureau, Ill. 1937-99 63 -.350 -.313 63 9,260 1997 90

5558000 Big Bureau Creek at Bureau, Ill. 1941-51 11 -0.340 -0.168 11 18,000 1951 35

5558050 Coffee Creek Tributary near Florid, Ill. 1956-76 21 -.100 .368 21 122 1958 100

5558075 Coffee Creek Tributary near Hennepin, Ill. 1956-80 24 -.005 .643 24 372 1958 100

5558500 Crow Creek (West) near Henry, Ill. 1950-82 33 .059 .257 33 6,930 1970 70

5559000 Gimlet Creek at Sparland, Ill. 1946-82 35 -.233 -.522 59 1,940 1974 20

5559500 Crow Creek near Washburn, Ill. 1945-82 37 -0.089 -0.076 37 5,750 1954 40

5561000 Ackerman Creek at Farmdale, Ill. 1954-80 27 -.032 -.222 27 5,100 1980 100

5563000 Kickapoo Creek near Kickapoo, Ill. 1945-99 53 .005 -.082 53 27,500 1967 30

5563100 Kickapoo Creek Tributary near Kickapoo, Ill. 1956-80 21 .050 .017 21 246 1959 60

5563500 Kickapoo Creek at Peoria, Ill. 1943-99 57 .132 .212 73 48,500 1974 320

5564400 Money Creek near Towanda, Ill. 1958-82 25 0.058 -0.030 25 2,600 1980 40

5564500 Money Creek above Lake Bloomington, Ill. 1934-58 25 .258 -.952 25 3,900 1947 100

5565000 Hickory Creek above Lake Bloomington, Ill. 1939-58 20 -.159 -.989 20 1,690 1951 15

5566000 East Branch Panther Creek near Gridley, Ill. 1950-72 23 .263 1.321 23 1,470 1951 400

5566500 East Branch Panther Creek at El Paso, Ill. 1950-82 33 .317 .989 33 5,300 1951 200

5567000 Panther Creek near El Paso, Ill. 1950-98 49 -0.263 -0.511 49 10,900 1951 35

5567500 Mackinaw River near Congerville, Ill. 1945-99 55 -.043 -.112 55 44,800 1983 100

5567800 Indian Creek Tributary near Hopedale, Ill. 1960-71 12 .007 -.918 12 446 1968 10

5568000 Mackinaw River near Green Valley, Ill. 1922-99 77 .311 .425 77 51,000 1983 100

5568650 Duck Creek near Canton, Ill. 1956-72 17 .094 .270 17 146 1968 15

5568800 Indian Creek near Wyoming, Ill. 1960-99 40 0.154 0.294 40 6,540 1974 100

5568850 Forman Creek Tributary near Victoria, Ill. 1961-76 16 .033 -.677 16 391 1975 25

5569500 Spoon River at London Mills, Ill. 1943-99 57 .352 .580 76 41,000 1974 180

5569825 Cedar Creek Tributary at St. Augustine, Ill. 1956-80 25 .318 -.191 25 1,460 1967 80

5570000 Spoon River at Seville, Ill. 1916-99 83 -.085 -.189 83 37,300 1924 60

5570370 Big Creek near Bryant, Ill. 1972-92 21 -0.269 -2.349 21 1,220 1974 10

5570910 Sangamon River at Fisher, Ill. 1979-99 21 -.190 -.422 21 13,000 1994 50

5571000 Sangamon River at Mahomet, Ill. 1948-78 31 -.073 -.073 31 14,600 1956 40

5572000 Sangamon River at Monticello, Ill. 1908-99 90 -.157 -.431 90 19,000 1927 70

5572450 Friends Creek at Argenta, Ill. 1967-82 16 .119 .678 16 5,660 1968 35

5572500 Sangamon River near Oakley, Ill. 1951-77 27 0.155 0.398 27 16,000 1974 25

5574000 South Fork Sangamon River near Nokomis, Ill. 1951-82 32 .393 .874 75 8,600 1957 200

5574500 Flat Branch near Taylorville, Ill. 1950-82 33 -.275 -.874 33 13,000 1957 50
146 Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois 

Station

number

(figs. 2A 

and 2B) Station name

Period

of

record 

(Water

Years)

Systematic

record

length

(No. of 

years)

Weighted

skew

Sample

skew

Historically

adjusted 

record

length

(No. of

years)

Maximum 

peakflow

(ft 3/s)

Water 

year of 

maximum 

peak

Approximate

recurrence

interval of

maximum

peak

(years)

5575500 South Fork Sangamon River at Kincaid, Ill. 1908-92 70 -0.037 -0.019 70 21,500 1957 90

5575800 Horse Creek at Pawnee, Ill. 1968-85 18 -.334 -.823 18 3,300 1983 15

5576500 Sangamon River at Riverton, Ill. 1908-99 86 -.442 -1.152 117 68,700 1943 >500

5577500 Spring Creek at Springfield, Ill. 1948-99 52 -.076 -.003 52 10,700 1996 50

5577700 Sangamon River Tributary at Andrew, Ill. 1956-80 24 -.206 -.310 24 660 1979 20

5578500 Salt Creek near Rowell, Ill. 1908-99 62 0.143 0.180 62 24,500 1968 180

5579500 Lake Fork near Cornland, Ill. 1948-99 52 .322 -.655 57 29,000 (1943) >500

5579750 Kickapoo Creek Tributary at Heyworth, Ill. 1956-73 18 .250 .609 31 2,400 1956 200

5580000 Kickapoo Creek at Waynesville, Ill. 1948-99 52 .245 .341 52 24,600 1981 80

5580500 Kickapoo Creek near Lincoln, Ill. 1945-92 47 .260 .427 93 23,300 1983 140

5580700 Salt Creek Tributary at Middletown, Ill. 1961-76 15 -0.242 -0.589 15 556 1974 10

5581500 Sugar Creek near Hartsburg, Ill. 1945-92 48 .312 .436 48 41,200 1983 70

5582000 Salt Creek near Greenview, Ill. 1942-99 58 -.069 -.158 58 41,200 1943 50

5582200 Cabiness Creek Tributary near Petersburg, Ill. 1956-76 21 -.097 .010 21 1,500 1965 100

5582500 Crane Creek near Easton, Ill. 1950-81 32 -.302 -.616 32 534 1979 10

5583000 Sangamon River near Oakford, Ill. 1910-99 82 -0.351 -0.544 82 123,000 1943 >500

5584400 Drowning Fork at Bushnell, Ill. 1961-92 31 -.149 -.287 31 3,500 1980 80

5584500 La Moine River at Colmar, Ill. 1945-99 55 -.284 -.358 55 38,900 1985 80

5584950 West Creek at Mount Sterling, Ill. 1961-72 12 -.147 .316 12 653 1961 30

5585000 La Moine River at Ripley, Ill. 1921-99 79 -.193 -.452 79 28,000 1985 35

5585220 Indian Creek Tributary near Sinclair, Ill. 1956-80 25 -0.321 -0.453 25 1,010 1958 10

5585700 Dry Fork Tributary near Mount Sterling, Ill. 1956-76 21 -.284 -.218 21 74 1961 20

5586000 N Fk Mauvaise Terre Cr nr Jacksonville, Ill. 1950-99 49 -.288 -.340 49 7,160 1994 50

5586200 Illinois River Tributary at Florence, Ill. 1956-80 25 -.528 -.905 25 730 1961 10

5586350 Little Sandy Creek Tributary nr Murrayville, Ill. 1961-72 12 -.353 -.606 12 1,130 1966 10

5586500 Hurricane Creek near Roodhouse, Ill. 1951-95 44 -0.225 -0.119 44 1,700 1957 200

5587000 Macoupin Creek near Kane, Ill. 1921-99 72 -.423 -.631 72 40,100 1994 100

5587900 Cahokia Creek at Edwardsville, Ill. 1969-99 31 -.823 -1.589 31 8,200 1979 15

5588000 Indian Creek at Wanda, Ill. 1941-99 59 -.184 .043 59 9,340 1946 160

5589500 Canteen Creek at Caseyville, Ill. 1939-84 46 -.199 -.026 46 10,200 1957 250

5589780 Little Canteen Creek Trib near Collinsville, Ill. 1959-72 14 -0.278 -0.028 14 613 1960 15

5590000 Kaskaskia Ditch at Bondville, Ill. 1924-90 46 .093 .286 46 1,490 1968 30

5590400 Kaskaskia River near Pesotum, Ill. 1965-79 15 -.120 -.632 15 3,310 1974 25

5590500 Kaskaskia River at Ficklin, Ill. 1954-64 11 -.221 -.170 11 4,400 1959 10

5590800 Lake Fork at Atwood, Ill. 1973-99 27 -.448 -.746 27 4,030 1979 30

5591200 Kaskaskia River at Cooks Mills, Ill. 1971-99 29 -0.439 -0.666 29 9,950 1994 20

5591500 Asa Creek at Sullivan, Ill. 1951-82 32 -.258 -1.571 32 1,460 1974 40

5591700 West Okaw River near Lovington, Ill. 1980-99 20 -.041 -.328 20 10,300 1996 70

5591750 Stringtown Branch Tributary near Lake City, Ill. 1961-80 20 -.336 -.576 20 150 1978 30

5592000 Kaskaskia River at Shelbyville, Ill. 1908-68 33 -.402 -1.152 33 25,900 1957 20

5592050 Robinson Creek near Shelbyville, Ill. 1980-99 20 0.203 0.150 43 26,400 (1957) 250

5592300 Wolf Creek near Beecher City, Ill. 1959-82 24 -.160 -.377 24 7,480 1970 20

5592500 Kaskaskia River at Vandalia, Ill. 1908-68 58 .190 -1.132 58 62,700 1957 100
Table 7. Selected flood-peak information for the 288 streamflow-gaging stations in Illinois and adjacent States 147

Station

number

(figs. 2A 

and 2B) Station name

Period

of

record 

(Water

Years)

Systematic

record

length

(No. of 

years)

Weighted

skew

Sample

skew

Historically

adjusted 

record

length

(No. of

years)

Maximum 

peakflow

(ft 3/s)

Water 

year of 

maximum 

peak

Approximate

recurrence

interval of

maximum

peak

(years)

5592575 Hickory Creek nr Brownstown, Ill. 1989-99 11 -0.188 -0.264 11 6,250 1994 10

5592800 Hurricane Creek near Mulberry Grove, Ill. 1971-99 29 -.429 -1.206 29 17,900 1983 25

5592900 East Fork Kaskaskia River near Sandoval, Ill. 1980-99 19 -.171 .023 19 17,000 1990 60

5593000 Kaskaskia River at Carlyle, Ill. 1908-67 44 -.137 -.973 44 54,400 1943 50

5593520 Crooked Creek near Hoffman, Ill. 1975-98 24 -.205 -.138 24 26,900 1990 80

5593575 Little Crooked Creek near New Minden, Ill. 1968-99 32 -0.270 -0.837 32 11,900 1995 20

5593600 Blue Grass Creek near Raymond, Ill. 1961-91 31 -.308 -.773 31 2,140 1973 50

5593700 Blue Grass Creek Tributary near Raymond, Ill. 1959-71 13 -.151 .161 13 356 1966 50

5593900 East Fork Shoal Creek near Coffeen, Ill. 1964-99 36 -.186 -.644 36 5,910 1967 50

5594000 Shoal Creek near Breese, Ill. 1910-99 59 -.365 -.662 59 52,000 1943 >500

5594090 Sugar Creek at Albers, Ill. 1973-82 10 -0.439 -0.414 10 10,500 1973 20

5594200 Williams Creek near Cordes, Ill. 1956-72 17 -.084 .169 17 966 1968 20

5594330 Mud Creek near Marissa, Ill. 1971-82 12 -.007 .500 12 5,520 1979 20

5594450 Silver Creek near Troy, Ill. 1967-99 33 -.725 -1.028 33 10,600 1979 25

5594800 Silver Creek near Freeburg, Ill. 1971-99 29 -.656 -1.001 29 15,300 1995 30

5595000 Kaskaskia River at New Athens, Ill. 1908-71 50 -0.235 -0.183 50 83,000 1943 40

5595200 Richland Creek near Hecker, Ill. 1970-99 30 -.189 -.113 30 23,400 1996 150

5595500 Marys River near Sparta, Ill. 1949-71 23 .029 .344 23 7,760 1968 50

5595510 Lick Branch near Eden, Ill. 1959-72 14 .099 .693 14 777 1969 30

5595550 Marys River Tributary at Chester, Ill. 1959-73 15 -.080 .022 15 572 1959 25

5595730 Rayse Creek near Waltonville, Ill. 1980-99 20 -0.197 -0.228 20 21,200 1994 10

5595800 Sevenmile Creek near Mt. Vernon, Ill. 1961-82 22 -.111 -.031 22 2,530 1961 30

5595820 Casey Fork at Mount Vernon, Ill. 1986-99 14 -.165 -.153 14 16,100 1990 15

5596000 Big Muddy River near Benton, Ill. 1946-70 25 .019 .272 25 38,600 1961 60

5596100 Andy Creek Tributary at Valier, Ill. 1956-72 17 -.181 -.332 17 835 1970 25

5597000 Big Muddy River at Plumfield, Ill. 1909-70 60 0.383 -0.961 60 42,900 1961 >500

5597450 Crab Orchard Creek Tributary near Pittsburg, Ill. 1960-72 13 .347 1.039 64 438 1961 250

5597500 Crab Orchard Creek near Marion, Ill. 1952-99 48 -.057 -.025 48 9,270 1996 200

5599000 Beaucoup Creek near Matthews, Ill. 1946-82 36 -.043 .070 36 18,800 1961 30

5599500 Big Muddy River at Murphysboro, Ill. 1916-70 43 -.357 -.956 43 33,300 1961 25

5599560 Clay Lick Creek near Makanda, Ill. 1960-76 17 0.175 0.614 17 3,000 1969 50

5599580 Big Muddy River Tributary near Gorham, Ill. 1961-76 16 .055 .230 16 114 1965 20

5599640 Green Creek Tributary near Jonesboro, Ill. 1956-80 25 -.147 -.521 25 605 1965 15

5599800 Orchard Creek near Fayville, Ill. 1961-72 12 .166 .291 12 148 1961 15

5600000 Big Creek near Wetaug, Ill. 1942-99 58 .476 .875 58 7,200 1943 >500
Soong and others—Estimating Flood-Peak Discharge Magnitudes and Frequencies for Rural Streams in Illinois—Scientific Investigations Report 2004-5103

Director, Illinois Water Science Center

U.S. Geological Survey

1201 W. University Ave., Suite 100

Urbana, IL 61801