)epartment of the Interior

ieological Survey

EFFECTS OF THE 1993 FLOOD

ON THE DETERMINATION OF

FLOOD MAGNITUDE AND

FREQUENCY IN IOWA

FLOODS I N THE

UPPER MISSISSIPPI

RIVER BASIN

1 9 - 9 - 3

U .S . GEOLOGICAL SURVEY CIRCULAR 1120-K
Front Cover-View looking west along Interstate 80 crossing the Des Moines River 

north of Des Moines, Iowa, July 12, 1993 (U .S . Geological Survey). 

Back Cover-View looking west along U.S . Highway 30 south of Ames, Iowa, July 9, 

1993, of hydrologist making a road-overflow streamflow measurement for 

the South Skunk River (U .S . Geological Survey). 

View of the Iowa River in Iowa County, Iowa, July 12, 1993 

(U .S . Geological Survey). 
EFFECTS OF THE 1993 FLOOD ON THE 

DETERMINATION OF FLOOD MAGNITUDE AND 

FREQUENCY IN IOWA 

By David A. Eash 

Floods in the Upper Mississippi River Basin, 1993 

U .S. GEOLOGICAL SURVEY CIRCULAR 1120-K 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

Mark Schaefer, Acting Director 

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 

1997 

Library of Congress Cataloging in Publication Data Free on application to the 

U.S . Geological Survey Eash, David A. 

Effects of the 1993 Flood on the Determination of Flood Magnitude Information Services 

and Frequency in Iowa / by David A. Eash. Box 25286, Federal Center 

p. 24 cm. - (Floods in the upper Mississippi River basin, 1993 Denver, CO 80225 K) (U .S . Geological Survey circular ; 1120) 

Includes bibliographical references . 

Supt. of Docs. no. : 119.4/3 :1120-K 

ISBN 0-607-88700-1 (alk. paper) 

1 . Floods-Iowa. I . Title . II . Series . III . Series : U.S . Geological 

Survey circular ; 1120. 

GB1217.F58 1993 Ch. K 

[GB1225 .18] 

551 .48'9'0977 s--dc21 

[551 .48'9'09777] 97-38698 

CIP 
FOREWORD


During spring and summer 1993, record flooding inundated much of the upper Mississippi River 

Basin. The magnitude of the damages-in terms of property, disrupted business, and personal traumawas unmatched by any other flood disaster in United States history. Property damage alone is expected 

to exceed $10 billion. Damaged highways and submerged roads disrupted overland transportation 

throughout the flooded region . The Mississippi and the Missouri Rivers were closed to navigation 

before, during, and after the flooding. Millions of acres of productive farmland remained under water for 

weeks during the growing season . Rills and gullies in many tilled fields are the result of the severe erosion that occurred throughout the Midwestern United States farmbelt. The hydrologic effects of extended 

rainfall throughout the upper Midwestern United States were severe and widespread . The banks and 

channels of many rivers were severely eroded, and sediment was deposited over large areas of the basin's 

flood plain. Record flows submerged many areas that had not been affected by previous floods . Industrial and agricultural areas were inundated, which caused concern about the transport and fate of industrial chemicals, sewage effluent, and agricultural chemicals in the floodwaters. The extent and duration 

of the flooding caused numerous levees to fail . One failed levee on the Raccoon River in Des Moines, 

Iowa, led to flooding of the city's water treatment plant. As a result, the city was without drinking water 

for 19 days. 

As the Nation's principal water-science agency, the U.S . Geological Survey (USGS) is in a unique 

position to provide an immediate assessment of some of the hydrological effects of the 1993 flood. The 

USGS maintains a hydrologic data network and conducts extensive water-resources investigations nation 

wide. Long-term data from this network and information on local and regional hydrology provide the 

basis for identifying and documenting the effects of the flooding. During the flood, the USGS provided 

continuous streamflow and related information to the National Weather Service (NWS), the U.S . Army 

Corps of Engineers, the Federal Emergency Management Agency (FEMA), and many State and local 

agencies as part of its role to provide basic information on the Nation's surface- and ground-water 

resources at thousands of locations across the United States . The NWS has used the data in forecasting 

floods and issuing flood warnings . The data have been used by the Corps of Engineers to operate water 

diversions, dams, locks, and levees . The FEMA and many State and local emergency management agencies have used USGS hydrologic data and NWS forecasts as part of the basis of their local flood-response 

activities . In addition, USGS hydrologists are conducting a series of investigations to document the 

effects of the flooding and to improve understanding of the related processes . The major initial findings 

from these studies will be reported in this Circular series as results become available. 

U.S . Geological Survey Circular 1120, Floods in the UpperMississippi River Basin, 1993, consists of individually published chapters that will document the effects of the 1993 flooding. The series 

includes data and findings on the magnitude and frequency of peak discharges ; precipitation; water-qual 

ity characteristics, including nutrients and man-made contaminants ; transport of sediment ; assessment of 

sediment deposited on flood plains ; effects of inundation on ground-water quality; flood-discharge volume; effects of reservoir storage on flood peaks; stream-channel scour at selected bridges; extent of floodplain inundation ; and documentation of geomorphologic changes. 

Mark Schaefer 

Acting Director 

Foreword III 
CONTENTS


Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III


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


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


Purpose and scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . 3


Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . 3


Flood-frequency analyses for Iowa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3


Selection of streamflow data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . 3


Data analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
 3 

Assumptions of data analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
 5 

Flood-recurrence interval . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5


Comparison of 1992 and 1993 flood-frequency discharges for Iowa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5


Factors that affect the computed flood-frequency discharges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8


Effect of 1993 flood magnitude. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8


Effect of 1993 record length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10


Combined effect of the 1993 flood magnitude and record length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10


Effect of flood-frequency discharges on water-surface elevations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12


Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
 15

References cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15


FIGURES 

1-3 . Map showing: 

1 . The Mississippi River Basin and general area of flooding streams, March through September 1993. . . . . . 2 

2. Areal distribution of total precipitation in the area of flooding streams in the upper Mississippi 

River Basin, April 1 through September 30, 1993 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
 2 

3 . Location of selected streamflow-gaging stations in Iowa and ranges in recurrence 

intervals for the 1993 peak discharges. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4


4. Graphs showing ranges in differences between the 1992 and the 1993 flood-frequency analyses 

for selected recurrence-interval discharges at selected streamflow-gaging stations in Iowa . . . . . . . . . . . . . . . . . . . 6


5 . Map showing location of selected streamflow-gaging stations in Iowa and ranges in 

differences between the 1992 and the 1993 flood-frequency analyses for 100-year 

recurrence-interval discharges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7


6. Graphs showing frequency distributions of recurrence interval, effective record length, 

and selected categories of recurrence interval and effective record length for selected 

streamflow-gaging stations in Iowa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9


7. Maps showing location of selected streamflow-gaging stations in Iowa grouped on the 

basis of ranges in differences between the 1992 and the 1993 flood-frequency analyses for 

100-year recurrence-interval discharge and by selected categories of recurrence interval and 

effective record length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11


8-10. Graphs showing : 

8. Ranges in differences between the 1992 and the 1993 flood-frequency analyses for 100-year 

recurrence-interval discharges and selected categories of recurrence interval and effective 

record length for selected streamflow-gaging stations in Iowa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12


9. Effects of the 1993 flood on the determination of flood magnitude and frequency and 

100-year flood elevation for the Upper Iowa River near Dorchester streamflow-gaging 

station . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13


10. Effects of the 1993 flood on the determination of flood magnitude and frequency and 100-year 

flood elevation for the Squaw Creek at Ames streamflow-gaging station . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14


Contents V 
TABLES 

1 . Flood-frequency data for selected streamflow-gaging stations in Iowa. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17


2. Summary statistics for differences between the 1992 and the 1993 flood-frequency analyses for 

selected recurrence-interval discharges at selected streamflow-gaging stations in Iowa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6


3. Summary statistics for five selected factors for selected streamflow-gaging stations in Iowa . . . . . . . . . . . . . . . . . . . . . . 8


4. Correlations of percentage differences between the 1992 and the 1993 flood-frequency analyses for 

100-year recurrence-interval discharges and five selected factors for selected streamflow-gaging 

stations in Iowa. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8


5. Summary statistics for differences between the 1992 and the 1993 flood-frequency analyses at selected 

recurrence-interval discharges at selected streamflow-gaging stations in Iowa grouped on the 

basis of recurrence intervals for the 1993 peak discharges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9


6 . Summary statistics for differences between the 1992 and the 1993 flood-frequency analyses for selected 

recurrence-interval discharges at selected streamflow-gaging stations in Iowa grouped on the 

basis of effective record lengths for 1993 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
 10 

7 . Summary statistics for differences between the 1992 and the 1993 flood-frequency analyses for 100-year 

recurrence-interval discharges at selected streamflow-gaging stations in Iowa grouped on the 

basis of selected categories of recurrence intervals for the 1993 peak discharges and effective record 

lengths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


CONVERSION FACTORS 

Multiply By To obtain 

inch 25.4 millimeter 

foot 0.3048 meter 

mile 1 .609 kilometer 

square mile 2.590 square kilometer 

cubic foot per second 0 .02832 cubic meter per second 

cubic foot per second per square mile 0.01093 cubic meter per second per square kilometer 

VI Effects of the 1993 Flood on the Determination of Flood Magnitude and Frequency in Iowa 
Effects of the 1993 Flood on the Determination of Flood 

Magnitude and Frequency in Iowa 

By David A. Eash 

Abstract 

To evaluate the effects of the 1993 flood in 

the upper Mississippi River Basin on the determination of flood magnitude and frequency, dis 

charges that had recurrence intervals of 10, 25, 50, 

and 100 years computed from data through the 

1992 water year were compared with those computed from data through the 1993 water year for 62 

selected streamflow-gaging stations in Iowa. On 

the basis of the flood-frequency analysis computed 

from data through the 1993 water year, a flood that 

was greater than or equal to a 10-year recurrenceinterval discharge occurred during 1993 at all 62 

gaging stations, and a flood greater than or equal to 

a 100-year recurrence-interval discharge occurred 

at 11 of the gaging stations . 

Results of the comparison indicated that 

inclusion of the 1993 flood in the data base 

resulted in an increase in the magnitude of dis 

charges for all selected recurrence intervals at the 

62 streamflow-gaging stations in Iowa. A larger 

percentage increase in the magnitude of discharge 

was computed for the larger recurrence intervals 

than for the smaller recurrence intervals for most 

of the selected gaging stations . As a result of 

including the 1993 peak discharge in the flood-frequency analysis, three gaging stations had an 

increase in the 100-year recurrence-interval discharge that was greater than 30 percent. 

Several factors, which included recurrence 

intervals for the 1993 peak discharges and the 

effective record lengths for 1993, were 

investigated for the 62 selected streamflowgaging stations to evaluate their possible effect on 

the computed flood-frequency discharges. The 

combined effect of these two factors on the computed 100-year recurrence-interval discharges was 

significant . Gaging stations were grouped into four 

discrete categories on the basis ofrecurrence intervals for the 1993 peak discharges and the effective 

record lengths for 1993 . Of the 28 gaging stations 

that had small flood magnitudes in 1993 and long 

record lengths, the difference between the 1992 

and the 1993 flood-frequency analyses for 100-

year recurrence-interval discharges at 22 gaging 

stations was less than 5 percent. Of the 10 gaging 

stations that had large flood magnitudes in 1993 

and short record lengths, the increase in 100-year 

recurrence-interval discharges at 9 gaging stations 

was greater than 15 percent. 

INTRODUCTION 

A nine-State area of the upper Mississippi River 

Basin was flooded from late March through September 

1993 (fig . 1) . From mid-June through early August 

1993, the flooding was severe as a result of the wide 

areal extent, large peak discharges, long duration, and 

overall destructiveness . Record or near-record peak 

discharges were recorded in 1993 at streamflowgaging stations in Illinois, Iowa, Kansas, Minnesota, 

Missouri, Nebraska, North Dakota, South Dakota, and 

Wisconsin after intense and persistent rainfall on soils 

saturated from excessive precipitation (Parrett and others, 1993). Spring 1993 was wetter than average, and 

weather patterns that persisted from early June through 

July caused an unusually large amount of precipitation 

to fall in the upper Midwest (Wahl and others, 1993). 

From April through September, more than 50 inches of 

rain fell in parts of Iowa, Kansas, and Missouri (fig . 2) . 

Introduction 1 
720. 

EXPLANATION 

Area of flooding streams 

-. .-Boundary of Mississippi River Basin 

Figure 1 . The Mississippi River Basin and general area of flooding streams, March through September 1993. 

95° 90° 

-30- Line of equal total 

precipitation for April 1 

through September 30, 1993 -Interval 10 inches 0 100 200 MILES 

- "" - Boundary of upper Mississippi River Basin 0 100 200 KILOMETERS 

Figure 2. Areal distribution of total precipitation in the area of flooding streams in the upper Mississippi River Basin, April 1 

through September 30, 1993. (Precipitation data from David Miscus, National Weather Service, written commun ., 1993.) 

2 Effects of the 1993 Flood on the Determination of Flood Magnitude and Frequency in Iowa 
The intense rain from the storm systems that tracked 

across the general area of flooding from April through 

September resulted in sustained high-flood volumes. 

Flood volumes at many gaging stations in the flooded 

area were significantly larger than previous maximums 

and more than twice the mean flow-volumes for April 

through September (Southard, 1995). 

Knowledge of the magnitude and frequency of 

floods is essential for the effective management of 

flood plains and for the economical planning and safe 

design of bridges, dams, levees, and other structures 

located in flood plains . Flood-frequency analyses are 

computed for streamflow-gaging stations by using 

annual peak discharges . As each additional annual peak 

discharge is added to the record of a gaging station, an 

updated flood-frequency analysis can be computed, 

and revised discharges for various frequencies of 

exceedance or recurrence intervals can be determined. 

Thus, flood-frequency statistics can be recalculated 

each year, and as additional annual peak discharges are 

collected and used in the analyses, these statistics 

become more reliable . One important aspect of the analysis and description of the 1993 flooding in the upper 

Mississippi River Basin must be considered-the effect 

the addition of 1993 peak discharges to the records of 

gaging stations will have on the determination of flood 

magnitude and frequency and, subsequently, on estimates of recurrence intervals of the 1993 flood. 

Purpose and Scope 

The purpose of this report is to compare quantitatively discharges that had recurrence. intervals of 10, 

25, 50, and 100 years computed from data through the 

1992 water year to those computed from data through 

the 1993 water year and to evaluate the effects of the 

1993 flooding on the computed flood-frequency statistics in Iowa. Iowa was selected for this study because a 

large number of streamflow-gaging stations recorded 

significant flooding in 1993, and a large data set could, 

therefore, be compiled for comparative floodfrequency analyses. 

Acknowledgments 

The information contained herein is based on 

data collected by the National Weather Service, the 

U.S . Army Corps of Engineers, the U.S . Geological 

Survey, and several State and local agencies. Appreciation is expressed to the personnel in these agencies 

who were involved in the collection of data . The flood 

data used in this study often were collected during 

adverse conditions, and the efforts of these individuals 

made this analysis possible . 

FLOOD-FREQUENCY ANALYSES FOR 

IOWA 

Engineers and planners often design structures 

or regulate development on flood plains for which damage may be incurred by occasional floods of varying 

magnitude (Dalrymple, 1960). By using floodfrequency analyses to design structures or to regulate 

development on flood plains to either a specific probability or a specific calculated risk, such as a 1- or a 2-

percent chance that a given flood magnitude will be 

exceeded in any one year, engineers and planners are 

able to standardize the risk factors involved with estimating flood-frequency discharges . 

Selection of Streamflow Data 

In Iowa, 83 unregulated streamflow-gaging stations that had at least 11 years of systematic, continuous-record data through the 1993 water year were 

initially considered for use in this study. Peak discharges in 1993 at these gaging stations were initially 

compared with previously published flood-frequency 

discharges computed from data through the 1990 water 

year (Eash, 1993); on the basis of this comparison, 62 

of these gaging stations that had a greater than or equal 

to 10-year recurrence-interval flood in 1993 were 

selected for this study. 

Data Analyses 

In this study, the method described in the Interagency Advisory Committee on Water Data (1982) was 

used to compute the magnitude and frequency of floods 

at each ofthe 62 selected streamflow-gaging stations in 

Iowa. Two separate flood-frequency curves were developed for each gaging station by fitting a Pearson TypeIII distribution to the logarithms (base 10) of the annual 

peak discharges by means of the U.S . Geological Survey's WATSTORE flood-frequency-analysis program 

(Kirby, 1981). The flood-frequency analysis, which 

was based on data through the 1992 water year, is hereafter termed the "1992 analysis," and the flood-freFlood-frequency analyses for Iowa 3 
quency analysis, which was based on data through the 

1993 water year, is hereafter termed the "1993 analysis ." 

Table 1 (at end of report) lists the computed discharges that had recurrence intervals of 10, 25, 50, and 

100 years for the 1992 and the 1993 analyses; the loca 

tions of these streamflow-gaging stations and ranges in 

recurrence intervals for their 1993 peak discharges, 

which were based on the 1993 analysis, are shown in 

figure 3 . Table 1 also lists information on the drainage 

area; the discharge, date, and unit runoff of the 1993 

peak discharge (Southard and others, 1994); the differences between discharges from the 1992 and the 1993 

analyses; the recurrence interval for the 1993 peak discharge, which was interpolated from the 1992 and the 

1993 analyses ; the annual peak discharge period of 

record (listed as the WATSTORE peak flow record) ; the 

previous annual maximum discharge and date ; use of 

historical data ; and the effective record length of the 

gaging station for the 1993 analysis . The effective 

record length is an estimate of the record length when 

historical flood data are weighted with the systematic 

record length . 

Base from U .S . Geological Survey digital data, 

1 ,2,000,000, 1979 0 

Universal Transverse Mercator projection, 

None 15 0 25 50 KILOMETERS 

EXPLANATION 

Streamflow-gaging station and site number used in table 1 

and ranges of recurrence intervals for 1993 peak discharges 

computed by using data through the 1993 water year 

3zL 

10 to less than 25 years 50 to less than 100 years 

4A 14-

25 to less than 50 years Greater than or equal to 100years 

Figure 3. Location of selected streamflow-gaging stations in Iowa and ranges in recurrence intervals for the 1993 peak 

discharges . 

4 Effects of the 1993 Flood on the Determination of Flood Magnitude and Frequency in Iowa 
The record of annual peak discharges for a 

streamflow-gaging station includes the water years during which it was operated, which is termed the "period 

of systematic record ." This record also may include 

historical peak discharges during water years outside 

the period of systematic record . Annual peak discharges, which are maintained in theWATSTORE Peak 

Flow File data base (Lepkin and others, 1979), were 

used to perform the flood-frequency analyses described 

in this report . 

For the 1992 and the 1993 analyses, extremely 

small discharge values (low outliers) were censored 

and adjusted for, historical data were used to make 

adjustments for extremely large discharge values (high 

outliers), and the coefficient of skew was weighted for 

each streamflow-gaging station with skew values 

obtained from a statewide skew analysis . Whenever 

possible, historical flood data were used to extend the 

flood record for gaging stations. Flood-frequency analyses for 41 of the 62 selected gaging stations were 

adjusted for historical data, whereas flood-frequency 

analyses for the other 21 were based only on their 

period of systematic record . 

Assumptions of Data Analyses 

The accuracy and reliability of flood-frequency 

analyses are dependent on several assumptions about 

the data (Interagency Advisory Committee on Water 

Data, 1982) . The fundamental assumption is that the 

record of past flood discharges is an accurate and reliable indicator of the range of flood discharges that 

could occur in the future . This assumption implies the 

following assumptions: 

The flood-generating mechanism is time-stationary, 

this is, the meteorologic and hydrologic processes 

that generate future floods will be the same as they 

were during the period of the past flood record . 

This implies that the climatic and meteorologic 

conditions, hydrologic conditions created by land 

use and land cover in the watershed, and hydraulic 

conditions of the stream channel and flood plain 

remain constant through time. 

The flood record is an accurate and representative 

depiction of the floods that occurred during the 

period of record . This implies that measurements 

of flood discharges included in the record are 

accurate and that any special flood-risk conditions 

associated with a site were properly identified and 

considered in the flood-frequency analysis . 

The hydrologic process of flood occurrence can be 

represented mathematically as a sequence of independent annual peak discharges that are randomly 

sampled from a population of all possible flood 

discharges . 

Flood-Recurrence Interval 

The magnitude and frequency of floods are computed for a streamflow-gaging station by relating 

annual peak discharges to either annual exceedance 

probability or recurrence interval. Annual exceedance 

probability is expressed as the chance that a selected 

flood magnitude will be exceeded in any one year . 

Recurrence interval, which is the reciprocal of the 

annual exceedance probability, is the average number 

of years between exceedances of a selected flood magnitude . For example, if a theoretical flood magnitude. i s 

exceeded once on the average during any 100-year 

period (recurrence interval), then it has a 1-percent 

chance (annual exceedance probability equals 0.01) of 

being exceeded during any one year. This flood, which 

is commonly termed the. "100-year flood," is the theoretical peak discharge against which actual flood peak 

discharges generally are compared to measure their 

severity. Although the recurrence interval represents 

the long-term average period between floods of a specific magnitude, rare floods could occur at shorter intervals or even within the same year. For example, the 

South Skunk River near Ames streamflowgaging station (fig . 3, site 26) has a theoretical 100-

year recurrence-interval discharge of 9,090 cubic feet 

per second as computed from the 1993 analysis (table 

1) . During 1993, however, a flood peak discharge of 

11,100 cubic feet per second occurred on July 9, and 

another flood peak discharge of 11,200 cubic feet per 

second occurred on August 16. Thus, two floods that, 

theoretically, each had less than a 1-percent chance of 

occurring during any one year occurred at this site in 

the same year. 

COMPARISON OF 1992 AND 1993 FLOODFREQUENCY DISCHARGES FOR IOWA 

Differences between discharges that had recurrence intervals of 10, 25, 50, and 100 years for the 1992 

and the 1993 analyses were evaluated to determine the 

Comparison of 1992 and 1993 flood-frequency discharges for Iowa 5 
effects of the 1993 flood on the computed flood magni- Figure 4 shows the number of streamflow-gaging statude and frequency (table 1) . Differences were calcu- tions that had differences of greater than or equal to 15 

lated as follows: the difference between the 1993 percent for the selected recurrence intervals-3,10 year 

recurrence-interval discharge and that of 1992 was (fig . 4A); 6, 25 year (fig . 4B) ; 10, 50 year (fig . 4C) ; and 

divided by the 1992 recurrence-interval discharge, and 13,100 year (fig . 4D). The spatial distribution of ranges 

this value was multiplied by 100. The ranges in differ- in differences between the 1992 and the 1993 analyses 

ences between the discharges are shown in figure 4, and for 100-year recurrence-interval discharges is shown in 

the summary statistics of the ranges are listed in table figure 5 . 

2. The overall trend indicated by the differences in disTable 2. Summarystatistics for differences between the charges for selected recurrence intervals (fig . 4; table 2) 1992 and the 1993 flood-frequency analyses for selected is that smaller differences were computed for dis- recurrence-interval discharges at selected streamflowcharges that had smaller recurrence intervals, and gaging stations in Iowa 

larger differences were computed for discharges that 

had larger recurrence intervals. Difference in discharge (percent) for indicated 

Summary recurrence interval (years) 

The mean and median statistic statistics for differences 

listed in table 2 for each recurrence interval show this 10 25 50 100 

trend of larger differences for larger recurrence Maximum . . . . . . 19 .6 25 .6 29.9 34 .9 

intervals. The mean differences range from 5.9 percent 1 .5 1 .9 1 .6 for the 10-year recurrence interval to 9 .6 percent for the Minimum. . . . . . . 1 .7 

100-year recurrence interval. The median differences Mean. . . . . . . . . . . . . . 5.9 7.3 8.4 9.6 

range from 4.6 percent for the 10-year recurrence interval to 6.2 percent for the 100-year recurrence Median. . . . . . . . . . . 4.6 5.2 5.6 6.2 interval. 

40 40 

10-year recurrence 50-year recurrence 

30 interval 30 interval 

z z

0 0 

a 20 a 20 

F F N V) 

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z 10 z 10 

C7 3 Q Q O c7 

0 I I I 3 O L_ 

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Q Q W 

25-year recurrence W

H m 100-year recurrenceF- interval 30 

interval W 

O O 

W W m 20 m 20 

z z 

10 10 

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B D 

RANGES IN PERCENTAGE DIFFERENCES 

Figure 4. Ranges in differences between the 1992 and the 1993 flood-frequency analyses for selected recurrence-interval 

discharges at selected streamflow-gaging stations in Iowa. A, 10-year recurrence intervals ; B, 25-year recurrence intervals; 

C, 50-year recurrence intervals; D, 100-year recurrence intervals. 

6 Effects of the 1993 Flood on the Determination of Flood Magnitude and Frequency in Iowa 
Acomparison ofthe computed discharges for the 

1992 and the 1993 analyses listed in table 1 indicates 

that inclusion of the 1993 flood resulted in larger dis 

charges for all selected recurrence intervals at the 62 

selected streamflow-gaging stations. Including the 

1993 flood in the flood-frequency analysis had a greater 

effect on the larger recurrence-interval discharges for 

the majority of these gaging stations. Differences 

between the 1992 and the 1993 analyses were slightly 

larger at the 10-year recurrence interval than at the 100-

year recurrence interval at five of the gaging stations 

listed in table 1 (sites 18, 19, 38, 51, 56) . On the basis 

of the 1993 analysis, recurrence intervals for 1993 peak 

Base from U .S . Geological Survey digital data, 

1 :2,000,000,1979 

3 Universal Transverse Mercatorprojection, 

None 15 

discharges at these five gaging stations ranged from 10 

to 14 years. 

At three streamflow-gaging stations (sites 1, 29, 

59) listed in table 1, the increase in the 100-year recurrence-interval discharge was greater than 30 percent as 

a result of including the 1993 flood in the flood-fre 

quency analysis . The maximum difference between the 

1992 and the 1993 analyses (tables 1, 2) was 34.9 percent for the 100-year recurrence-interval discharge for 

the Upper Iowa River near Dorchester (site 1) . This site 

had a large flood magnitude in 1993 (recurrence interval of 70 years based on the 1993 analysis) and a fairly 

short record length (computed effective record length 

of 25 years) . 

0 

F 

0 

EXPLANATION 

Streamflow-gaging station and site number used in table 1 and 

ranges of differences between 1992 and 1993 flood-frequency 

analyses for 100-year recurrence-interval discharges 

2 

z~s Less than 5 percent 

5 to less than 15 percent 

1A Greater than or equal to 15 percent 

Figure 5. Location of selected streamflow-gaging stations in Iowa and ranges in differences between the 1992 and the 1993 

flood-frequency analyses for 100-year recurrence-interval discharges. 

Comparison of 1992 and 1993 flood-frequency discharges for Iowa 
FACTORSTHAT AFFECTTHE COMPUTED 

FLOOD-FREQUENCY DISCHARGES 

The following factors were investigated to evaluate their possible effect on the differences between 

discharges computed from the 1992 and the 1993 anal 

yses (table 1) : drainage area, 1993 peak discharge, 

1993 peak unit runoff (the 1993 peak discharge divided 

by the drainage area), recurrence interval of the 1993 

peak discharge based on the 1993 analysis, and effective record length based on the 1993 analysis . Summary statistics for these factors are listed in table 3. 

Table 3. Summary statistics for five selected factors for 

selected streamflow-gaging stations in Iowa 

[mi`, square miles ; ft3/s, cubic feet per second] 

RecurSummary 

statistic 

Drainage 

area 

(mil ) 

1993 

peak 

discharge 

(ft3/s) 

1993 

peak 

unit 

runoff 

[(ft 3/S)/ 

mil] 

rence 

interval 

of the 

1993 

peak 

discharge 

1993 

effective 

record 

length 

(years) 

(years) 

Maximum . . . . . 12,499 111,000 624.8 1 1 .8 91 

Minimum . . . . . 25 .3 4,380 6.8 10 15 

Mean . . . . . . . . . . . . 1,640 27,438 51 .0 '77 53 

Median . . . . . . . . . 746 22,500 26.8 32 52 

'Recurrence intervals for 1993 peak discharges that are greater than 

100 years are herein reported as a ratio of the 1993 peak discharge to the 

100-year recurrence-interval discharge for the 1993 flood-frequency analysis . 

`For computation of the mean values, recurrence intervals of greater 

than 100 years were interpolated and rounded to the nearest 10 years for 

intervals between 100 and 200 years and to the nearest 25 years for inter 

vals between 200 and 500 years . Recurrence intervals of greater than 500 

years were rounded to 500 years . 

Differences between the 1992 and the 1993 

analyses for 100-year recurrence-interval discharges 

were correlated with the factors. The 100-year recur 

rence interval was selected because the differences 

between the 1992 and the 1993 analyses were largest 

(fig . 4; table 2). Results of the correlations are listed in 

table 4 as Pearson's product-moment correlation 

coefficients and Spearman's rank correlation coefficients . The Pearson's product-moment correlation coefficient is computed by using a parametric correlation 

analysis on the data, and the Spearman's rank correlation coefficient is computed by using a nonparametric 

correlation analysis on the ranks of the data. 

Table 4. Correlations of percentage differences between 

the 1992 and the 1993 flood-frequency analyses for 100-

year recurrence-interval discharges and five selected factors 

for selected streamflow-gaging stations in Iowa 

Factor 

Drainage area . . . . . . . . . . . . . . . . . . . . . . . . . . . 

1993 peak discharge . . . . . . . . . . . . . . . . 

1993 peak unit runoff . . . . . . . . . . . . . . 

Pearson 

product 

moment 

correlation 

coefficient 

Spearman 

rank 

correlation 

coefficient 

. . . . -0.275 -0.438 

. . . . - .060 - .006 

. . . . .437 .513 

Recurrence interval of 1993 peak .507 .721 

discharge . 

1993 effective record length . . . . . . . . - .583 - .651 

Correlation coefficients are statistics that provide 

a measure of the strength of the linear relation between 

two variables. The correlation coefficient ranges 

between -1 .0 and +1 .0, and the closer the value is to 

±1 .0, the stronger is the linear relation. Apositive

value for the correlation coefficient indicates that as 

one variable increases, the other variable also 

increases. Anegative value for the correlation 

coefficient indicates that as one variable increases, the 

other variable decreases. 

The two factors with the strongest correlations 

with differences between the 1992 and the 1993 analyses for 100-year recurrence-interval discharges were 

selected for further investigation . Results of the correlation analyses listed in table 4 indicate that the two 

factors with the strongest correlations are recurrence 

interval of the 1993 peak discharge and 1993 effective 

record length. These two factors were investigated further to evaluate their possible effect on the computed 

flood-frequency discharges . 

Effect of 1993 Flood Magnitude 

Of the five factors investigated in the correlation 

analyses, 1993 peak discharge, 1993 peak unit runoff, 

and recurrence interval of the 1993 peak discharge are 

related to the magnitude of the 1993 flood. Of these 

three, the recurrence interval of the 1993 peak discharge most strongly correlated with the difference 

between the 1992 and the 1993 analyses for 100-year 

recurrence-interval discharges (table 4) . Thus, on the 

basis of the 1993 analysis (table 1), recurrence 

intervals of the 1993 flood peaks were used to evaluate the effect of 1993 flood magnitude on the computed flood-frequency discharges in Iowa. 

8 Effects of the 1993 Flood on the Determination of Flood Magnitude and Frequency in Iowa 
z 

O C 

30 r QH Category-{

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0 

z_ 0 20 Category 

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aa

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Hess 

10

Hess 

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less 

~o

Hess 

doless do

less 

do

less

90, . 

ess 

01 10 90 00 h0 00 1p 0p 

Figure 6. Frequency distributions of recurrence interval, effective record length, and selected categories of recurrence 

interval and effective record length for selected streamflow-gaging stations in Iowa. A, Ranges in recurrence intervals for the 

1993 peak discharges, in years; B, Ranges in effective record lengths for 1993, in years; C, Selected categories of recurrence 

interval (RI) and effective record length (ERL), in years. 

Figure 3 shows the spatial distribution . and fig- Table 5. Summary statistics for differences between the 

1992 and the 1993 flood-frequency analyses for selected 

ure 6A shows the frequency distribution of ranges in recurrence-interval discharges at selected streamflowrecurrence intervals for the 1993 peak discharges for gaging stations in Iowa grouped on the basis of recurrence 

the selected streamflow-gaging stations . In table 5, intervals for the 1993 peak discharges 

Difference in discharge (percent) for summary statistics for each of the selected recurrence Summary indicated recurrence interval (years) intervals for the 62 selected gaging stations are grouped statistic 

on the basis of recurrence intervals for the 1993 flood 10 25 50 100 

of greater than or equal to 50 years and recurrence 

1993 recurrence interval of greater than or equal to 50 yearsl 

intervals of less than 50 years . The mean and median Maximum. . . . . . . . . . . . . 19 .6 25 .6 29.9 34.9 

statistics for the two groups indicate that gaging sta- Minimum . . . . . . . . . . . . . 2.7 3 .6 4.0 4.9 

tions that had large flood magnitudes (recurrence inter- Mean . . . . . . . . . . . . . . . . . . . . 9.1 12 .5 15 .1 17 .8 

vals of greater than 50 years) in 1993 generally had Median . . . . . . . . . . . . . . . . . 8 .0 11 .3 14 .4 16 .4 

1993 recurrence interval of less than 50 yearsl larger differences between the 1992 and the 1993 

24.6 27.7 30 .8 flood-frequency analyses for discharges of all selected Maximum. . . . . . . . . . . . . 19 .2 

recurrence intervals. Minimum. . . . . . . . . . . . . 1 .7 1 .5 1 .9 1 .6 

Mean . . . . . . . . . . . . . . . . . . . . 4.5 5 .2 5 .7 6.2 

Median . . . . . . . . . . . . . . . . . 3 .5 4.2 4.6 4.8 

Number of sites is 18 . 

'Number of sites is 44 . 

Factors that affect the computed flood-frequency discharges 
Effect of 1993 Record Length 

Effective record lengths that were based on the 

1993 analysis were used to evaluate the effect of record 

lengths on the computed flood-frequency discharges in 

Iowa. The effective record length is an estimate of the 

record length for a streamflow-gaging station when historical flood data are weighted with the systematic 

record length. The increase in the effective record 

length over the. systematic record length provides an 

estimate of the value of the historical data . If a gaging 

station record only contains systematic data, then the 

effective record length is the same as the systematic 

record length . Effective record lengths calculated for 

gaging stations that have historical flood data can, 

therefore, be compared with the record lengths ofthose 

that have systematic data . 

The effective record length of a streamflowgaging station is based on an empirical analysis made 

by Gary D. Tasker (U.S . Geological Survey, written 

commun., 1992) of results reported in Tasker and Thomas (1978) and Stedinger and Cohn (1986) . Table 1 

lists the effective record lengths that were calculated 

for each gaging station. 

Figure 6B shows the frequency distribution of the 

effective record lengths for the selected streamflow-gaging stations . As indicated in table 4, the correlation is neg 

ative between effective record length and the difference 

between the 1992 and the 1993 flood-frequency analyses 

for 100-year recurrence-interval discharges. This correlation is negative because as the effective record length of a 

gaging station increases, the influence of any given 

annual peak discharge decreases even though an annual 

peak discharge may be very large. In table 6, summary 

statistics for each ofthe selected recurrence intervals for 

the 62 selected gaging stations are grouped on the basis 

of effective record lengths for 1993 of greater than or 

equal to 50 years and effective record lengths for 1993 

of less than 50 years. The mean and median statistics 

for the two groups indicate that gaging stations that had 

short record lengths (effective record lengths of less 

than 50 years) through 1993 generally had larger differences between the 1992 and the 1993 flood-frequency 

analyses for discharges of all selected recurrence intervals . 

Combined Effect of the 1993 Flood 

Magnitude and Record Length 

To evaluate the combined effect of the 1993 

flood magnitude and the record length on the computed 

flood-frequency discharges in Iowa, the 62 selected 

streamflow-gaging stations were grouped into four discrete categories on the basis of recurrence intervals of 

the 1993 flood and effective record lengths through 

1993 as follows : A, the recurrence interval of the 1993 

flood was greater than or equal to 50 years and the 

effective record length was less than 50 years ; B, the 

recurrence interval of the 1993 flood was greater than 

or equal to 50 years and the effective record length was 

greater than or equal to 50 years; C, the recurrence 

interval ofthe 1993 flood was less than 50 years and the 

effective record length was less than 50 years; and D, 

the recurrence interval for the 1993 flood was less than 

50 years and the effective record length was greater 

than or equal to 50 years. The frequency distributions 

of recurrence intervals for the 1993 flood, effective 

record lengths for 1993, and the above four categories 

are shown in figure 6 . Table 7 lists summary statistics 

for differences between the 1992 and the 1993 analyses 

for 100-year recurrence-interval discharges for each of 

these four categories . 

Table 6. Summary statistics for differences between the 

1992 and the 1993 flood-frequency analyses for selected 

recurrence-interval discharges at selected streamflowgaging stations in Iowa grouped on the basis of effective 

record lengths for 1993 

Difference in discharge (percent) for 

Summary indicated recurrence interval (years)

statistic 

10 25 50 100 

1993 effective record length of greater than or equal to 50 years 

Maximum. . . . . . . . . . . . 8.1 11 .7 15 .1 17 .8 

Minimum . . . . . . . . . . . . . 1.7 1 .5 1.9 1 .6 

Mean. . . . . . . . . . . . . . . . . . . . 3.6 4.4 5.0 5.5 

Median . . . . . . . . . . . . . . . . . 3.1 3.8 4.0 4.4 

1993 effective record length of less than 50 yearsz 

Maximum. . . . . . . . . . . . 19 .6 25 .6 29 .9 34.9 

Minimum. . . . . . . . . . . . . 3.8 3.8 3.6 4.0 

Mean . . . . . . . . . . . . . . . . . . . . 9.0 11 .5 13 .2 15 .2 

Median . . . . . . . . . . . . . . . . . 8.2 10 .2 11 .5 13 .0 

'Number of sites is 36 . 

'Number of sites is 26 . 

1 0 Effects of the 1993 Flood on the Determination of Flood Magnitude and Frequency in Iowa 
Table 7. Summary statistics for differences between the 

1992 and the 1993 flood-frequency analyses for 100-year

recurrence-interval discharges at selected streamflowgaging stations in Iowa grouped on the basis of selected 

categories of recurrence intervals for the 1993 peak 

discharges and effective record lengths 

Difference in 100-year recurrenceinterval discharges (percent) for 

Summary selected categories of recurrence 

statistic interval and effective record length 

A1 B2 C3 D4 

Number of sites . . . . . . . . . 10 8 16 28 

Maximum . . . . . . . . . . . . . . . . . . 34.9 17 .8 30 .8 10 .7 

Minimum. . . . . . . . . . . . . . . . . . . 9.0 4.9 4.0 1 .6 

Mean. . . . . . . . . . . . . . . . . . . . . . . . . . 23 .5 10 .7 10 .0 4.0 

Median. . . . . . . . . . . . . . . . . . . . . . . 24.2 10 .6 8.0 3.4 

'Recurrence interval of greater than or equal to 50 years and effective record length of less than 50 years. 

2Recurrence interval of greater than or equal to 50 years and effective record length of greater than or equal to 50 years . 

3 Recurrence interval of less than 50 years and effective record length 

of less than 50 years . 

4Recurrence interval of less than 50 years and effective record length

of greater than or equal to 50 years . 

As indicated by the differences in the mean and 

median statistics listed in tables 5 to 7 for the 100-year

recurrence-interval discharge, the combined effect of 

the 1993 flood magnitude and record length is greater

than the effect of either individual factor on the computed flood-frequency discharges in Iowa. Categories

A and Dbest indicate the combined effect of these two 

factors on the computed 100-year recurrence-interval 

discharges . Large flood magnitudes coupled with short 

record lengths (category A) have mean and median statistics of 23 .5 and 24.2 percent, respectively (table 7). 

Conversely, small flood magnitudes coupled with long 

record lengths (category D) have mean and median statistics of4.0 and 3 .4 percent, respectively (table 7). The 

contrast between the mean and median statistics of 

these two categories of streamflow-gaging stations 

indicates that the 1993 flood magnitudes and record 

lengths that were used in the 1993 analysis had a significant combined effect on the computed 100-year

recurrence-interval discharges . 

The spatial and frequency distributions of the 62 

streamflow-gaging stations were grouped on the basis 

of those that had a less than 5-percent difference, a 5 

to less than 15-percent difference, and a greater than or 

equal to 15-percent difference between the 1992 and 

the 1993 analyses for 100-year recurrence-interval discharges, as shown in figures 7 and 8, respectively . The 

spatial and frequency distributions shown in these two 

figures are identified according to the previously 

defined categories in figure 6C and table 7 . 

Base from U .S . Geological Survey digital data,

1 :2,000,000, 1979 0 50 100 MILES 

Universal Transverse Mercator projection, I-- -

Zone 15 0 50 100 KILOMETERS 

EXPLANATION 

Streamflow-gaging station and site number used in table 1 and 1993 

recurrence-interval and effective-record-length categories used in figure 6 

and table 7 

14 4®

1 Category A Category B Category C 20 Category D 

Figure 7. Location of selected streamflow-gaging stations in 

Iowa grouped on the basis of ranges in differences between 

the 1992 and the 1993 flood-frequency analyses for 100-

year recurrence-interval discharges and by selected 

categories of recurrence interval and effective record length . 

A, Streamflow-gaging stations that have a less than 5-

percent difference ; B, Streamflow-gaging stations that have 

a 5 -to less than 15-percent difference ; C, Streamflowgaging stations that have a greater than or equal to 15-

percent difference . 

Factors that affect the computed flood-frequency discharges 1 1 
25 

-Category A 

20 ®Category B <5% 

Category C 

15 =Category D 

10 

5 

z

0 

QF 

C7 

a 

0 

aw 

F 

0 e 

m 25 

z 0 

15 

10 

Figure 8. Ranges in differences between the 1992 and the 

1993 flood-frequency analyses for 100-year recurrenceinterval discharges and selected categories of recurrence 

interval (RI) and effective record length (ERL), in years, for 

selected streamflow-gaging stations in Iowa . A, Streamflowgaging stations that have a less than 5-percent difference ;

B, Streamflow-gaging stations that have a 5-to less than 

15-percent difference ; C, Streamflow-gaging stations that 

have a greater than or equal to 15-percent difference . 

Of the 28 streamflow-gaging stations in category D(fig . 6C), 22 had a less than 5-percent difference between 100-year recurrence-interval 

discharges for 1992 and 1993 (fig . 8A), and of the 25 

gaging stations that had a less than 5-percent difference between 100-year recurrence-interval discharges for 1992 and 1993, 22 were in category D 

(figs . 7A and 8A) . Conversely, of the 10 gaging stations in category A (fig . 6C), 9 had a greater than or 

equal to15-percent difference between 100-year 

recurrence-interval discharges for 1992 and 1993 

(fig . 8C), and of the 13 gaging stations that had a 

greater than or equal to 15-percent difference 

between 100-year recurrence-interval discharges for 

1992 and 1993, 9 were in category A(figs.7C and 

8C) . 

EFFECT OF FLOOD-FREQUENCY 

DISCHARGES ON WATER-SURFACE 

ELEVATIONS 

Associated with a specified flood-frequency discharge is a water-surface elevation that can be determined by using hydraulic principles of open-channel 

flow or from a stage-discharge rating curve. Water-surface elevations determined for flood-frequency discharges are used for the effective management of flood 

plains and for the safe design of bridges, dams, levees, 

and other structures located in flood plains . 

To evaluate the effect that changes in flood-frequency discharges may have on water-surface elevations, two of the streamflow-gaging stations in Iowa 

that had large differences between the 1992 and the 

1993 flood-frequency discharges were selected for 

investigation . The Upper Iowa River near Dorchester 

(fig . 3, site 1) and the Squaw Creek at Ames (fig . 3, site 

27) had differences between the 1992 and the 1993 

analyses for 100-year recurrence-interval discharges of 

34.9 and 26.2 percent, respectively (table 1) . 

The records of annual peak discharges for these 

two streamflow-gaging stations are shown in figures 9A 

and 10A. The 1992 and the 1993 flood-frequency 

curves that were computed by fitting a Pearson TypeIII distribution to the logarithms of the annual peak discharges for each of these gaging stations (table 1) are 

shown in figures 9B and 10B. These figures show a 

larger difference in estimated discharges between the 

1992 and the 1993 flood-frequency curves at the smaller 

annual exceedance probabilities or larger recurrence 

intervals. 

The computation of discharge records at a 

streamflow-gaging station is dependent upon the development of a stage-discharge relation, or rating curve, 

between water-surface elevations, or stages, and the 

corresponding flow rates, or discharges . A rating curve 

is developed by measuring the discharge at a variety of 

stages, graphing the stage versus discharge points, and 

drawing a best-fit curvilinear line through the points . 

1 2 Effects of the 1993 Flood on the Determination of Flood Magnitude and Frequency in Iowa 
----

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

40,000 

1941 peak discharge = 30,400 

30,000 100-year 

cubic feet per second 1993 peak discharge = 22,000 

flood cubic feet per second 

v 20,000 

0 

z 

O 10,000 w

in 

w

d 

F 

w 

w 

0 

1920 1940 1960 1980 2000 

u_ 

U YEAR 

m 

U 

Z 40,000 

ui 

0 EXPLANATION 

Q

2 

U

in 

1992 flood-frequency curve 

1993 flood-frequency curve 

993 peak disc harge 

r r V. 

10,000 " 19 annual peak discharges used 

in 1993 flood-frequency analysis 

11111111!:1 

5,000 

t 

2,000 

99.9 99 98 95 90 80 70 60 50 40 30 20 10 5 2 1 0.1 

B ANNUAL EXCEEDANCE PROBABILITY, IN PERCENT 

gage height for 1993 100-year flood discharge = 20 .40 feet 

w~ gage height for 1992 100-year flood discharge = 18 .63 feet w~ 

u- F 

Z Q EXPLANATION 

1992-93 rating curve 1992 100-year 

C7

2 (~

Q flood discharge = 

w (7 1992 and 1993 100-year 17,500 cubic recurrence-interval discharges = w feet per second w> and interpolated gage heights 

Qm 1993 100-year flood 

C7 Q discharge = 23,600 cubic ! 

feet per second I \j 

I I I I 

200 500 1,000 2,000 5,000 10,000 20,000 40,000 

DISCHARGE, IN CUBIC FEET PER SECOND 

Figure 9. Effects of the 1993 flood on the determination of flood magnitude and frequency and 100-year flood 

elevation for the Upper Iowa River near Dorchester streamflow-gaging station (sitel, fig . 3) . A, Annual peak 

discharges ; B. Flood-frequency curves; C, Stage-discharge rating curve and interpolated gage heights for 100-year 

recurrence-interval discharges . 

Effect of flood-frequency discharges on water-surface elevations 1 3 

00
---- -----------

30,000 

1993 peak discharge = 

24,300 cubic feet 

persecond 

w 

100-year 20,000 flood 

0 10,000 

z 

0

U

w 

W

CL 0 

i JUL

1900 1920 1940 1960 1980 2000 w 

w 

u_ YEAR

U A 

m 

U 

z 30,000 1 I I I I I I I I I I I I I 1993 peak discharge 

EXPLANATION 

10,000 ---- 1992 flood-frequency curve 111111 81011 U... .. MMM1111M MINIMUM 

0 1993 flood-frequency curve 111111111111////////111111111!SiICG"//=11/ 1Ell NIIIIIIIIIIIIIIIIIIIII 

38 annual peak discharges used 

in 1993 flood-frequency analysis 1111111111111Dllllll II 11 

1r" __ 

111~~~_ ..dl Ills 

1,000 IIIIIIIIIII~~"~I nunooo~~~~i 

iiiiiiissommi"mil 

110"11" 

ommmmm 

500 

99.9 

n 

99 98 95 90 80 70 60 50 40 30 20 10 5 2 1 0.1 

8 ANNUAL EXCEEDANCE PROBABILITY, IN PERCENT 

gage height for 1993 100-year flood discharge = 15 .92 feet 

gage height for 1992100-year . flood discharge 14 .87 feet 

! 1993100-

w EXPLANATION i year flood 

u_ H Z Q i discharge = _ 

O i 13,500 cubic 

H w 1992 and 1993 2 /feet per 

(9 Q 100-year recurrence- second 

w (9 interval discharges = w and interpolated w > gage heights (9 O 

m 

Cal Q

10,000 30,000 

C DISCHARGE, IN CUBIC FEET PER SECOND 

Figure 10. Effects of the 1993 flood on the determination of flood magnitude and frequency and 100-year flood 

elevation for the Squaw Creek at Ames streamflow-gaging station (site 27, fig . 3) . A, Annual peak discharges ; 8, 

Flood-frequency curves ; C, Stage-discharge rating curve and interpolated gage heights for 100-year recurrenceinterval discharges . 

1 4 Effects of the 1993 Flood on the Determination of Flood Magnitude and Frequency in Iowa 
The rating curves that were in use during the 

1992 and the 1993 water years at the two selected 

streamflow-gaging stations are shown in figures 9 Cand 

1OC. Discharges that were computed for the 100-year 

recurrence interval for 1992 and 1993 (table 1) were 

used to interpolate gage heights (stages) from the 

respective rating curve for each gaging station. Differences in gage heights between the 1992 and the 1993 

analyses for 100-year recurrence-interval discharges 

were interpolated to have increased 1 .77 feet for the 

Upper Iowa River near Dorchester gaging station (fig . 

9C) and 1 .05 feet for the Squaw Creek at Ames gaging 

station (fig . 1OC). 

The gage heights shown in figures 9C and IOC 

for 100-year recurrence-interval discharges are considered to be only approximate because they do not take 

into consideration adjustments that may have been 

made to the rating curves during the flood; consequently, they are presented only for illustrative purposes. The differences in gage heights between the 

1992 and the 1993 analyses for 100-year recurrenceinterval discharges for these two streamflow-gaging 

stations illustrate how changes in flood-frequency discharges, which result from the inclusion of the 1993 

peak discharges, can affect water-surface elevations . It 

should be noted that the differences in gage heights that 

are illustrated for these two gaging stations are larger 

than anticipated for most gaging stations given their 

large differences in 100-year recurrence-interval discharges. 

SUMMARY 

To evaluate the effects of the 1993 flood on the 

determination of flood magnitude and frequency in 

Iowa, discharges that had recurrence intervals of 10, 

25, 50, and 100 years computed from data through the 

1992 water year were compared with discharges that 

had the same recurrence intervals computed from data 

through the 1993 water year for 62 selected streamflow-gaging stations in Iowa. On the basis of the 1993 

flood-frequency analysis, a flood that was greater than 

or equal to a 10-year recurrence-interval discharge 

occurred during 1993 at all 62 gaging stations, and a 

flood that was greater than or equal to a l 00-year recurrence-interval discharge occurred at 11 of the gaging 

stations. 

Results of the comparison indicated that inclusion ofthe 1993 peak discharges in the flood-frequency 

analysis caused an increase in the magnitude of discharges for all selected recurrence intervals at the 62 

selected streamflow-gaging stations in Iowa. A larger 

percentage increase in the magnitude of discharge was 

computed for the larger recurrence intervals than for 

the smallerrecurrence intervals for most of the selected 

gaging stations. As a result ofincluding the 1993 flood 

in the flood-frequency analysis, three gaging stations 

had an increase in the 100-year recurrence-interval 

discharge that was greater than 30 percent. 

Several factors, which included recurrence intervals for 1993 peak discharges and effective record 

lengths for 1993, were investigated for the 62 selected 

streamflow-gaging stations to evaluate their possible 

effect on differences between 1992 and 1993 flood-frequency discharges . This investigation indicated that the 

1993 flood magnitudes and record lengths that were 

used in the 1993 flood-frequency analysis had a significant combined effect on the computed 100-year recurrence-interval discharges . Gaging stations were 

grouped into four discrete categories on the basis of 

recurrence intervals for 1993 peak discharges and 

effective record lengths for 1993. Of the 28 gaging stations that had small flood magnitudes in 1993 and long 

record lengths, the difference between the 1992 and the 

1993 flood-frequency analyses for 100-year recurrence-interval discharges at 22 gaging stations was less 

than 5 percent. Of the 10 gaging stations that had large 

flood magnitudes in 1993 and short record lengths, the 

increase in 100-year recurrence-interval discharges at 9 

gaging stations was greater than or equal to 15 percent. 

REFERENCES CITED 

Dalrymple, Tate, 1960, Flood frequency analyses, pt . 3 of 

Flood flow techniques in Manual of hydrology: U.S . 

Geological Survey Water-Supply Paper 1543-A, 

p.1-180 . 

Eash, D.A., 1993, Estimating design-flood discharges for 

streams in Iowa using drainage-basin and channelgeometry characteristics : U.S . Geological Survey 

Water-Resources Investigations Report 93-4062, 96 p. 

Interagency Advisory Committee on Water Data, 1982, 

Guidelines for determining flood flow frequency: 

Hydrology Subcommittee Bulletin 17B, 28 p. and 

appendixes . 

References cited 1 5 
Kirby, WH., 1981, Annual flood frequency analysis using 

U.S . Water Resources Council guidelines (Program 

J407): U.S . Geological Survey National Water Data 

Storage and Retrieval System (WATSTORE), v. 4, 

chap. 1, sec. C, 57 p. 

Lepkin, W.D., DeLapp, M.M., Kirby, WH., and Wilson, 

T.A., 1979, Instructions for peak flow file, in WATSTORE user's guide: U.S . Geological Survey Open 

File Report 79-1336-1, v. 4, chap. 1, 191 p. 

Parrett, Charles, Melcher, N.B ., and James, R.W., Jr., 1993, 

Flood discharges in the upper Mississippi River Basin, 

in Floods in the upper Mississippi River Basin, 1993 : 

U.S . Geological Survey Circular l 120-A, 14 p. 

Southard, Rodney, 1995, Flood volumes in the upper Mississippi River Basin, April 1 through September 30, 1993, 

in Floods in the upper Mississippi River Basin, 1993 : 

U.S . Geological Survey Circular 1120-H, 32 p. 

Southard, Rodney, Sneck-Fahrer, D.A., Anderson, C.J ., and 

Goodrich, R.D., 1994, Water Resources data, Iowa, 

water year 1993: U.S . Geological Survey Water-Data 

Report IA-93-1, 388 p. 

Stedinger, JR., and Cohn, T.A., 1986, Flood frequency analysis with historical and paleoflood information : Water 

Resources Research, v. 22, no. 5, p. 785-793. 

Tasker, G.D., and Thomas, W.O., Jr., 1978, Flood frequency 

analysis withpre-recordinformation : American Society 

of Civil Engineers, Journal of Hydraulics Division, v. 

104, no. 12, p. 249-259. 

Wahl, K.L., Vining, K.C., and Wiche, G.J ., 1993, Precipitation in the upper Mississippi River Basin, January 

through July 31, 1993, in Floods in the upper Missis 

sippi River Basin, 1993: U.S . Geological Survey 

Circular 1120-B, 13 p. 

1 6 Effects of the 1993 Flood on the Determination of Flood Magnitude and Frequency in Iowa 

1 
Table 1 . Flood-frequency data for selected streamflow-gaging stations in Iowa 

[mi`, square mile ; ft 3/s, cubic feet per second ; (ft3/s)/mil , cubic feet per second per square mile ; FF, flood-frequency analyses computed through the 1992 and the 1993 water years ; RI, approximate recurrence 

interval interpolated from the 1992 and the 1993 flood-frequency analyses, rounded to nearest 5 years for 20- to 50-year recurrence intervals and to nearest 10 years above the 50-year recurrence interval ; WATSTORE, National Water Data Storage and Retrieval System data base used to perform flood-frequency analyses ; ERL, 1993 effective record length, indicates systematic record length used in the 1993 floodfrequency analysis when no value is listed for HST; HST, historically adjusted record length used in the 1993 flood-frequency analysis ; *, ratio of 1993 peak discharge to 100-year recurrence-interval discharge 

for the 1992 and the 1993 flood-frequency analyses ; % DIFF, percentage difference between recurrence-interval discharges for 1992 and 1993 ; --, historically adjusted record length was not used in flood-frequency analysis] 

Flood-peak discharge estimates (ft3/s) Previous 

1993 peak for indicated recurrence interval Record maximum 

Site Drain- (years) discharge 

number 

(fig .3) 

SiS taton taton 

number name 

age 

area 

(mi l ) Discharge 

(ft 3/s) 

Date 

(month/ 

day) 

Unit 

runoff 

[(ft3/s)/ 

mil] 

FF 10 25 50 100 RI 

WAT 

STORE 

peak 

flow 

record 

(years) 

ERL 

(years) 

HST 

(years) 

Maximum 

discharge 

(ft3/s) 

Date 

(month/ 

year) 

1 05388250 Upper Iowa 770 22,000 08/17 28 .6 1992 10,700 13,300 15,400 17,500 *1 .3 1941, 25 80 30,400 05/1941 

River near 1993 12,800 16,700 20,000 23,600 70 1976-93 

Dorchester. % DIFF + 19 .6 +25.6 +29.9 +34.9 

05418500 Maquoketa 1,553 35,300 07/06 22.7 1992 29,500 37,300 43,200 49,000 20 1903, 82 91 48,000 06/1944 

River near 1993 30,100 38,000 44,000 50,100 19 1914-93 

Maquoketa. % DIFF + 2 .0 +1 .9 +1 .9 +2.2 

3 05422000 Wapsipinicon 2,330 22,300 07/08 9.6 1992 19,800 25,000 29,000 32,900 16 1935-93 59 60 31,100 06/1990 

River near 1993 20,200 25,600 29,600 33,700 15 

De Witt. % DIFF + 2.0 +2.4 +2.1 +2.4 

4 05449000 East Branch 133 4,380 03/31 32.9 1992 2,560 3,680 4,640 5,700 40 1944, 44 - - 5,960 06/1954 

Iowa River 1993 2,740 4,000 5,080 6,280 35 1949-76, 

near Klemme. % DIFF + 7.0 +8.7 +9.5 +10.2 1978-93 

5 05449500 Iowa River 429 6,140 04/01 14.3 1992 4,970 6,640 7,930 9,250 20 1941-76, 52 - - 8,460 06/1954 

near Rowan. 1993 5,140 6,890 8,240 9,630 17 1978-93 

%DIFF +3.4 +3 .8 +3 .9 +4.1 

6 05451500 Iowa River at 1,564 20,400 08/17 13.0 1992 18,000 23,100 26,800 30,400 16 1903, 80 112 42,000 06/1918 

Marshalltown . 1993 18,300 23,500 27,300 31,100 15 1915-27, 

% DIFF + 1 .7 + 1 .7 +1 .9 +2.3 1929-30, 

1933-93 

7 05451700 Timber Creek 118 8,870 07/09 75.2 1992 6,380 8,540 10,200 11,900 30 1947, 45 47 12,000 08/1977 

near 1993 6,690 9,030 10,800 12,700 25 1950-93 

Marshalltown . °Io DIFF + 4.9 +5 .7 +5.9 +6.7 

8 05451900 Richland Creek 56.1 4,900 07/17 87.3 1992 3,970 5,460 6,710 8,070 18 1950-93 44 - - 12,200 04/1991 

near Haven. 1993 4,130 5,690 7,000 8,430 16 

% DIFF + 4.0 +4.2 +4.3 +4.5 
--

- -

--

- -

Table 1 . Flood-frequency data for selected streamflow-gaging stations in Iowa-Continued 

�Previous Flood-peak discharge estimates (ft3/s)

1993 peak for indicated recurrence interval Record maximum 

w Site Drain- (years) discharge 

°. 

w 

0

0' 

0 

number 

(fig .3) 

9 

Station Station 

number name 

05452000 Salt Creek 

near Elberon. 

age 

(ml 

area 

) 

201 

Discharge 

(ft 3/s) 

Date 

(month/ 

day) 

36,600 07/09 

Unit 

runoff 

[(ft3/s)/ 

mil] 

182.1 

FF 10 25 

1992 13,400 20,000 

1993 14,800 22,600 

%DIFF +10.4 +13.0 

50 

25,900 

29,900 

+15.4 

100 

32,800 

38,500 

+17.4 

WAR 

RI 

STORE 

peak 

flow 

record 

(years) 

*1 .1 1944, 

90 1946-93 

ERL 

(years) 

49 

HST 

(years) 

50 

Maximum 

discharge

(ft3/s) 

Date 

(month/ 

year) 

35,000 06/1947 

v 

10 05453100 Iowa River 

at Marengo. 

2,794 38,000 07/19 13 .6 1992 

1993 

23,900 28,800 

25,200 30,800 

32,300 

34,800 

35,400 

38,600 

*1 .1 

90 

1957-93 39 46 30,800 03/1960 

d % DIFF + 5.4 +6.9 +7.7 +9.0 

3 

0, 

?i0

0 

11 05454000 Rapid Creek 

near Iowa 

City . 

25 .3 6,700 08/10 264.8 1992 

1993 

% DIFF 

4,110 

4,320 

+5.1 

5,630 

5,990 

+6.4 

6,790 

7,280 

+7.2 

7,960 

8,580 

+7.8 

50 

35 

1938-93 56 6,100 05/1965 

12 05454300 Clear Creek 

near 

Coralville . 

98 .1 6,760 07/06 68 .9 1992 

1993 

% DIFF 

4,760 

5,030 

+ 5.7 

6,680 

7,110 

+6.4 

8,270 

8,840 

+6.9 

10,000 

10,700 

+7.0 

25 

20 

1953-93 41 1 10,200 06/1990 

c 

A 

a 

m 

13 05455100 Old Mans Creek 

near Iowa 

City . 

201 13,000 07/06 64 .7 1992 

1993 

% DIFF 

6,800 9,720 

7,360 10,700 

+ 8? + 10 .1 

12,200 

13,600 

+ 11 .5 

14,900 

16,800 

+12.8 

60 

45 

1951-87, 

1989-93 

41 43 13,500 06/1982 

m 

c 

14 05455500 English River 

at Kalona. 

573 36,100 07/06 63 .0 1992 

1993 

13,600 17,900 

14,700 20,000 

21,200 

24,400 

24,700 

29,100 

*1 .5 1930, 

*1 .2 1940-93 

57 64 20,000 09/1965 

%DIFF +8.1 +11.7 +15.1 +17 .8 

0 

d 15 05457700 Cedar River at 

Charles City . 

1,054 26,400 08/16 25 .0 1992 

1993 

% DIFF 

19,900 24,700 

20,800 26,000 

+ 4.5 +5.3 

28,100 

29,700 

+5.7 

31,200 

33,100 

+6.1 

35 

25 

1946-53, 

1961-62, 

1965-93 

39 29,200 03/1961 

16 05458000 Little Cedar 

River near 

Ionia. 

306 14,000 08/16 45 .8 1992 

1993 

% DIFF 

7,880 10,800 

8,530 11,900 

+ 8.2 +10.2 

13,000 

14,500 

+11.5 

15,300 

17,300 

+13.1 

70 

45 

1954-93 40 10,800 03/1961 

17 05458500 Cedar River 

at Janesville . 

1,661 35,000 08/18 21 .1 1992 

1993 

% DIFF 

23,400 30,200 

24,200 31,500 

+ 3 .4 +4.3 

35,100 

36,800 

+4.8 

39,900 

41,900 

+5.0 

50 

40 

1905-06, 

1915-21, 

1923-27, 

75 89 37,000 03/1961 

1933-42, 

1945-93 
--

Table 1 . Flood-frequency data for selected streamflow-gaging stations in Iowa-Continued 

Flood-peak discharge estimates (ft 3/s) Previous 

Site 

number 

(fig .3) 

Station 

number 

Station 

name 

Drainage 

area 

(mi l) 

1993 peak 

Discharge 

(ft3/s) 

Date 

(month/

day) 

Unit 

runoff 

[(ft3/s)/

mil] 

FF 

for indicated recurrence interval 

(years) 

10 25 50 100 RI 

WAT 

STORE 

peak

flow 

record 

(years) 

Record 

ERL 

(years) 

HST 

(years) 

maximum 

discharge 

Maximum 

discharge 

(ft 3/s) 

Date 

(month/

year) 

18 05458900 West Fork 

Cedar River 

at Finchford. 

846 17,600 04/01 20.8 1992 

1993 

% DIFF 

16,800 24,100 

17,400 24,900 

+ 3 .6 +3.3 

29,800

30,800 

+3.4 

35,600

36,800 

+3.4 

11 

10 

1929,

1946-93 

50 65 31,900 06/1951 

19 05459500 Winnebago 

River at 

Mason City. 

526 7,190 04/01 13 .7 1992 

1993 

% DIFF 

7,070 

7,190 

+ 1 .7 

9,160 

9,310 

+1.6 

10,700 

10,900 

+1 .9 

12,300 

12,500 

+1 .6 

11 

10 

1933-93 62 64 10,800 03/1933 

20 05463000 Beaver Creek 

at New 

Hartford . 

347 14,700 03/31 42.4 1992 

1993 

% DIFF 

12,100 17,000

12,600 17,800 

+ 4.1 +4.7 

20,800

21,800 

+4.8 

24,500

25,800 

+5.3 

17 

15 

1946-93 48 - - 18,000 06/1947 

21 05463500 Black Hawk 

Creekat 

Hudson. 

303 9,670 07/09 31.9 1992 

1993 

% DIFF 

9,050 13,200 

9,400 13,700 

+ 3 .9 +3.8 

16,600 

17,200 

+3.6 

20,200 

21,000

+4.0 

12 

11 

1952-93 42 19,300 07/1969 

22 05464000 CedarRiverat 

Waterloo . 5,146 68,100 04/02 13 .2 1992 1993 

% DIFF 

55,400 71,700 

56,400 73,200 

+ 1 .8 +2.1 

83,400 

85,300 

+2.3 

94,600 

96,900 

+2.4 

20 

19 

1929, 

1933, 

1941-93 

63 91 76,700 03/1961 

23 05464500 Cedar River at 

Cedar Rapids . 

6,510 71,000 04/04 10 .9 1992 

1993 

% DIFF 

51,100 64,300 

52,200 66,100 

+ 2.2 +2.8 

73,800 

76,000

+3.0 

82,800 

85,600

+3.4 

40 

35 

1851, 

1903-93 

91 - - 73,000 03/1961 

24 05465000 Cedar River 

near 

Conesville . 

7,785 74,000 04/06 9.5 1992 

1993 

%n DIFF 

54,300 67,700 

55,500 69,400 

+ 2.2 +2.5 

77,100 

79,300 

+2.9 

85,900 

88,600 

+3.1 

40 

35 

1940-93 62 91 70,800 04/1961 

25 05465500 Iowa River 

at Wapello. 

12,499 111,000 07/08 8.9 1992 

1993 

% DIFF 

69,900 84,300 

71,800 87,300 

+ 2.7 +3 .6 

94,100

97,900 

+4.0 

103,000 

108,000

+4.9 

*1.1 

100 

1903-93 91 - - 94,000 06/1947 

ar 

c 

26 05470000 South Skunk 

Rivernear 

Ames. 

315 11,200 08/16 35.6 1992 

1993 

% DIFF 

5,780 

6,050 

+ 4.7 

6,930 

7,360 

+6.2 

7,690 

8,260 

+7.4 

8,380 

9,090 

+8 .5 

*1 .3 1921-27, 

*1.2 1930 

1933-93 

70 75 8,630 06/1954 

i 

27 05470500 Squaw Creek 

at Ames. 

204 24,300 07/09 119.1 1992 5,740

1993 6,360 

% DIFF + 10.8 

7,590

8,870 

+16.9 

9,080

11,000 

+21 .1 

10,700

13,500 

+26.2 

*2 .3 1918,

*1 .8 1920-27, 

1965-93 

46 76 12,500 06/1990 
--

c Table 1 . Flood-frequency data for selected streamflow-gaging stations in Iowa-Continued 

Flood-peak discharge estimates (ft 3/s) Previous 

1993 peak for indicated recurrence interval Record maximum 

n 

0 Site Drain- (years) discharge 

°. 

5 

eo 

w 

m 

o

0 

number 

(fig .3) 

Station 

number 

Station 

name 

age 

area 

(mi)2 Discharge 

(ft3/s) 

Date 

(month/ 

day) 

Unit 

runoff 

[(ft3/s)/ 

mil] 

FF 10 25 50 100 RI 

STORE 

ppeak 

flow 

record 

(years) 

ERL 

(years) 

HST 

(years) 

Maximum 

discharge 

(ft 3/s) 

Date 

(month/ 

year) 

0 28 05471000 South Skunk 556 26.500 07/09 47.7 1992 10.200 11,900 13,000 14,000 *1 .9 1944, 39 75 14,700 06/1975 

River below 1993 10,800 13,200 15,100 17,100 *1 .5 1953-79, 

v

0.. 0 

29 

Squaw Creek 

near Ames. 

05471200 Indian Creek 276 18,600 07/09 67.4 

% DIFF 

1992 

+ 5 .9 +10.9 

8,180 10,200 

+16.2 

11,600 

+22.1 

13,000 

1990, 

1992-93 

*1 .4 1958-75, 30 49 23,500 06/1991 

0 

near Mingo. 1993 9,600 

% DIFF +17.4 

12,600 

+23.5 

15,000 

+29.3 

17,400 

+33.8 

*1 .1 1986-93 

30 05471500 South Skunk 

River near 

1,635 20,700 07/15 12 .7 1992 

1993 

15,500 19,000 

16,000 19,800 

21,500 

22,600 

24,000 

25,300 

40 

30 

1944, 

1946-93 

50 63 37,000 05/1944 

0 Oskaloosa. % DIFF + 3 .2 +4.2 +5.1 +5.4 a 

31 05472500 North Skunk 730 17,500 07/06 24.0 1992 13,900 18,400 21,800 25,200 20 1944, 50 63 27,500 03/1960 

River near 1993 14,500 19,200 22,800 26,400 18 1946-93 

a0 

Sigourney. % DIFF + 4.3 +4.3 +4.6 +4.8 

32 05473400 Cedar Creek 530 8,920 07/09 16.8 1992 8,350 9,140 9,650 10,100 20 1979-93 15 8,560 04/1983 

m near Oakland 

Mills. 

1993 

% DIFF 

8,670 

+ 3.8 

9,530 

+4.3 

10,100 

+4.7 

10,600 

+5 .0 

13 

c 

3 33 05474000 Skunk River 4,303 46,600 07/10 10 .8 1992 36,700 43,700 48,400 52,800 40 1903, 82 142 66,800 04/1973 

at Augusta. 1993 

% DIFF 

37,400 44,700 

+ 1 .9 +23 

49,600 

+2.5 

54,200 

+2.7 

35 1915-93 

0 

34 05476500 Des Moines 1,372 9,330 06/30 6 .8 1992 6,560 9,670 12,400 15,400 25 1952-93 44 55 16,000 04/1969 

Riverat 1993 6,950 10,300 13,300 16,600 20 

Estherville . % DIFF + 5.9 +6.5 +7 .3 +7.8 

35 05476750 Des Moines 

Riverat 

2,256 19,000 07/13 8 .4 1992 

1993 

10,300 

11,000 

13,600 

14,600 

15,900 

17,300 

18,100 

19,900 

100 

80 

1940-93 54 55 18,000 04/1969 

Humboldt. % DIFF + 6 .8 +7.4 +8.8 +9.9 

36 05479000 East Fork 1,308 16,200 04/01 12.4 1992 11,000 15,700 19,500 23,600 30 1938, 57 75 22,000 09/1938 

DesMoines 1993 11,600 16,500 20,600 25,100 25 1940-93 

River at % DIFF + 5 .5 +5.1 +5 .6 +6.4 

Dakota City. 
--

--

--

Table 1 . Flood-frequency data for selected streamflow-gaging stations in Iowa-Continued 

�Previous Flood-peak discharge estimates (ft 3/s)

1993 peak for indicated recurrence interval Record maximum 

(years) discharge Site Drainnumber 

(fig .3) 

Station 

number 

Station 

name 

age 

area 

(mil) 

Discharge 

(fts/s) 

Date 

(month/ 

day) 

Unit 

runoff 

[(ft3 /s)/ 

mil] 

FF 10 25 50 100 RI 

WAT 

STORE 

peak 

flow 

record 

(years) 

ERL 

(years) 

HST 

(years) 

Maximum 

discharge 

(ft3/s) 

Date 

(month/ 

year) 

37 05480500 Des Moines 4,190 31,200 04/01 7.4 1992 21,900 28,400 33,400 38,500 35 1905-06, 68 90 35,600 04/1965 

River at Fort 

Dodge. 

1993 

% DIFF 

22,600 29,600 

+ 3.2 +4.2 

34,900 

+4.5 

40,400 

+4.9 

30 1914-27, 

1947-93 

38 05481000 Boone River 

nearWebster 

844 13,100 04/01 15.5 1992 

1993 

11,800 

12,100 

15,400 

15,800 

18,200 

18,600 

21,000 

21,500 

14 

13 

1918, 

1932, 

57 97 21,500 06/1918 

City. % DIFF + 2.5 +2.6 +2.2 +2.4 1941-93 

39 05481300 Des Moines 5,452 42,300 04/02 7.8 1992 31,200 40,700 48,000 55,500 30 1903, 88 91 57,400 06/1954 

River near 

Stratford. 

1993 

% DIF 

31,900 41,800 

+2.2 +2.7 

49,400 

+2.9 

57,200 

+3.1 

25 1905-29, 

1931, 

1933-93 

40 05481950 Beaver Creek 358 14,300 07/10 39.9 1992 5,670 7,140 8,210 9,250 *1.5 1960-93 34 7,980 06/1986 

near Grimes. 1993 6,490 

% DIFF + 14.5 

8,600 

+20.4 

10,200 

+24.2 

12,000 

+29.7 

*1.2 

41 05482500 NorthRaccoon 

River near 

1,619 16,900 07/10 10 .4 1992 

1993 

16,500 21,500 

16,800 21,900 

25,100 

25,700 

28,600 

29,300 

11 

10 

1940-93 54 29,100 06/1947 

Jefferson. % DIFF + 1 .8 +1.9 +2.4 +2.4 

42 05483450 Middle Raccoon 375 27,500 07/09 73.3 1992 8,590 11,800 14,500 17,300 *1.6 1973, 1) 1) 41 14,600 07/1973 

Rivernear 

Bayard. 

1993 9,870 

% DIFF + 14.9 

14,300 

+21 .2 

18,100 

+24.8 

22,400 

+29.5 

*1 .2 1979-93 

43 05483600 Middle 440 22,400 07/09 50.9 1992 10,300 13,400 15,900 18,500 *1 .2 1953, 38 41 15,300 06/1986 

Raccoon 1993 11,300 15,200 18,300 21,700 100 1958-93 

River at % DIFF + 9.7 + 13.4 + 15 .1 + 17.3 

Panora . 

44 05484000 SouthRaccoon 

River at 

994 44,000 07/10 44.3 1992 

1993 

20,300 25,100 

21,900 27,800 

28,500 

32,300 

31,800 

36,700 

*1 .4 1940-93 

*1 .2 

54 35,000 07/1958 

Redfield . % DIFF + 7 .9 +10.8 + 13 .3 +15.4 

45 05484500 Raccoon River 3,441 70,100 07/10 20.4 1992 29,500 37,400 43,300 49,100 *1 .4 1915-93 79 - - 41,200 06/1947 

at Van Meter. 1993 31,300 

%DIFF +6.1 

40,700 

+8.8 

48,000 

+10.9 

55,400 

+12.8 

*1 .3 

N 
--

- -

- -

Table 1 . Flood-frequency data for selected streamflow-gaging stations in Iowa-Continued 

m 

m

0 

y Site Drain1993 peak 

Flood-peak discharge estimates (ft3/s) 

for indicated recurrence interval 

(years) 

Record 

Previous 

maximum 

discharge 

m 

w 

m 

0

0 

°, 

ber 

(fig.3) 

numnumber Station 

name 

Station 

area 

(mil) 

age 

Discharge 

(ft3/s) 

Date 

(month/ 

day) 

Unit 

runoff 

((ft3/s)/ 

mil] 

FF 10 25 50 100 RI 

STORE 

peak 

flow 

record 

(years) 

WAT 

ERL 

(years) 

HST 

(years) 

Maximum 

discharge 

(ft3/s) 

Date 

(month/ 

year) 

0 

0 

46 05487470 South River 

near 

Ackworth. 

460 32,200 07/06 70 .0 1992 

1993 

% DIFF 

23,200 29,000 

23,900 30,100 

+ 3.0 +3 .8 

33,100 

34,500 

+4.2 

37,000 

38,700 

+4.6 

45 

35 

1930, 

1940-93 

57 64 38,100 06/1990 

v 

47 05487980 White Breast 

Creeknear 

342 25,500 07/06 74 .6 1992 

1993 

14,200 18,500 

15,600 20,700 

22,000 

25,000 

25,800 

29,800 

100 

50 

1962-93 34 49 37,300 07/1982 

m Dallas . %DIFF +9.9 +11 .9 +13.6 +15.5 

3 

°, 

m

0

0 

48 05489000 Cedar Creek 

near Bussey. 

374 36,100 07/05 96 .5 1992 20,000 28,800 

1993 21,500 31,300 

%DIFF +7.5 +8 .7 

36,700 

40,200 

+9.5 

45,900 

50,800 

+10.7 

50 

35 

1946, 

1948-93 

51 142 96,000 07/1982 

°' 3 

0 

c 

49 06483500 Rock River 

nearRock 

Valley. 

1592 , 29,300 05/09 18 .4 1992 

1993 

% DIFF 

19,400 28,200 

20,600 30,300 

+ 6.2 +7.4 

35,600 

38,400 

+7.9 

43,300 

47,000 

+8.5 

30 

25 

1948-93 51 96 40,400 04/1969 

a 

m

0 

50 06485500 Big Sioux River 

at Akron. 

8,424 66,700 05/10 7.9 1992 31,800 47,600 

1993 33,900 51,600 

%DIFF +6.6 +8.4 

61,100 

67,100 

+9.8 

76,000 

84,500 

+11 .2 

70 

50 

1929-93 65 80,800 04/1969 

3 

51 06605850 Little Sioux 

River at Linn 

Grove. 

1,548 16,100 07/02 10 .4 1992 

1993 

% DIFF 

13,100 19,000 

14,200 20,600 

+ 8.4 +8.4 

24,000 

26,000 

+8.3 

29,500 

31,900 

+8.1 

17 

14 

1953, 

1961-62, 

1965, 

1973-93 

31 102 22,500 06/1953 

0

d 52 06606600 Little Sioux 

River at 

Correction2,500 22,600 07/18 9.0 1992 

1993 

% DIFF 

15,700 21,500 

16,200 22,300 

+ 3.2 +3.7 

26,200 

27,400 

+4.6 

31,300 

32,800 

+4.8 

30 

25 

1919-25, 

1929-32, 

1937-93 

71 102 29,800 04/1965 

ville. 

53 06608500 Soldier River at 

Pisgah . 

407 23,400 07/09 57 .5 1992 

1993 

% DIFF 

18,700 24,000 

19,300 24,900 

+ 3.2 +3.8 

28,000 

29,100 

+3.9 

31,900 

33,200 

+4.1 

25 

20 

1940-93 54 22,500 06/1950 

54 06609500 Boyer River at 

Logan. 

871 26,200 07/09 30 .1 1992 

1993 

% DIFF 

22,400 26,900 

22,900 27,500 

+ 2.2 +2.2 

29,900 

30,600 

+2.3 

32,600 

33,400 

+2.5 

20 

20 

1918-25, 

1938-93 

64 30,800 06/1990 
--

--

Table 1 . Flood-frequency data for selected streamflow-gaging stations in Iowa-Continued 

Flood-peak discharge estimates (ft 3/s) Previous 

0 

Site Drain1993 peak for indicated recurrence interval 

(years) 

Record maximum 

discharge 

ro 

number 

(fig .3) 

Station 

number 

Station 

name 

age 

area (mil ) Discharge 

(ft3/s) 

Date 

(month/ 

day) 

Unit 

runoff 

[(ft /s)/ 

mil] 

FF 10 25 50 100 RI 

WAT 

STORE 

peak 

flow 

record 

(years) 

ERL 

(years) 

HST 

(years) 

Maximum 

discharge (ft3

/s) 

Date 

(month/ 

year) 

0 

"'M 

o 

55 06807410 West 

Nishnabotna 

River at 

609 30,100 07/10 49 .4 1992 19,500 24,600 

1993 21,500 27,800 

% DIFF + 10 .3 + 13 .0 

28,300 

32,300 

+14.1 

31,700 

36,700 

+ 15 .8 

70 

35 

1960-93 34 26,400 09/1972 

Hancock. 

56 06809500 East 

Nishnabotna 

River at Red 

894 21,600 08/31 24 .2 1992 21,000 26,600 

1993 21,400 27,000 

% DIFF + 1 .9 + 1 .5 

30,400 

31,000 

+2.0 

34,100 

34,700 

+ 1 .8 

11 

10 

1917-25, 

1936-93 

71 90 38,000 09/1972 

Oak. 

57 06810000 Nishnabotna 

River above 

Hamburg. 

2,806 37,700 07/25 13 .4 1992 

1993 

% DIFF 

28,700 

29,400 

+2.4 

33,900 

34,900 

+2.9 

37,300 

38,500 

+3.2 

40,300 

41,800 

+3.7 

60 

45 

1922-23, 

1929-93 

71 142 55,500 06/1947 

58 06817000 NodawayRiver 

at Clarinda. 

762 28,000 07/22 36 .7 1992 

1993 

25,800 32,700 

26,300 33,400 

37,400 

38,200 

41,800 

42,700 

14 

13 

1918-25, 

1936-93 

68 90 31,100 06/1947 

% DIFF + 1.9 +2.1 +2.1 +2.2 

59 06897950 Elk Creek 

nearDecatur 

52 .5 32,800 07/05 624.8 1992 

1993 

16,700 23,200 

19,900 28,900 

28,200 

36,000 

33,100 

43,300 

100 

35 

1967-93 27 18,000 07/1990 

City . % DIFF + 19 .2 +24.6 +27 .7 +30.8 

60 06898000 ThompsonRiver 

at 

701 30,300 07/05 43 .2 1992 

1993 

16,500 21,600 

17,000 22,500 

25,700 

26,900 

30,000 

31,600 

100 

80 

1885, 

1919-24, 

69 109 57,000 09/1992 

Davis City . % DIFF +3.0 +4.2 +4.7 +5.3 1926, 

1942-93 

61 06903400 Chariton River 

near Chariton . 

182 14,900 07/05 81 .9 1992 

1993 

9,680 14,000 

10,600 15,500 

17,800 

19,900 

22,100 

25,000 

30 

25 

1947, 

1960, 

33 47 37,700 09/1992 

% DIFF + 9.5 +10.7 + 11 .8 + 13 .1 1966-93 

62 06903700 South Fork 

Chariton 

River near 

168 16,900 07/05 100.6 1992 

1993 

% DIFF 

14,400 20,300 

15,400 21,900 

+ 6.9 +7.9 

25,400 

27,600 

+8.7 

31,200 

34,200 

+9.6 

15 

13 

1968-93 30 47 70,600 09/1992 

Promise City . 

i Discharge revised from a previously published value. 

N

W