G E O IVI O R P H I C 

CHANGES O N THE 

M I S S I S S I P P I RIVER 

F L O O D P LA I N 

A T 

MILLER CITY, ILLINOIS 

AS A RESULT OF 

TH E FLOOD OF 

9 9 3 

U .S . GEOLOGICAL SURVEY CIRCULAR I I 20-J 

Click here to return to USGS publications 
Front Cover-View of Miller City, Illinois levee break, 

August 1993 . (U.S . Geological Survey) 

Back Cover- Field Hydrologist making streamflow 

measurements (U.S . Geological Survey) 

View of Spirt of St . Louis Airport, 

Chesterfield, Mo. (Srenco Photography, 

St . Louis, Mo.) 
GEOMORPHIC CHANGES ON THE MISSISSIPPI RIVER


FLOOD PLAIN AT MILLER CITY, ILLINOIS, AS A RESULT OF


THE FLOOD OF 1993


By Robert B. Jacobson and Kevin A. Oberg 

Floods in the Upper Mississippi River Basin, 1993


U.S. GEOLOGICAL SURVEY CIRCULAR 1120-J 
U.S .DEPARTMENT Any BRUCE GORDON 

U

use 

.S 

of 

. GEOLOGICAL 

trade, BABBITT, product, 

P EATON, OF or firm THE Secretary
 SURVEY 

Director 

names INTERIOR in this 

publication is for descriptive purposes only and does 

not imply endorsement by the U.S . Government 

UNITED STATES GOVERNMENT PRINTING OFFICE : 1997 

Free on application to the


U.S. Geological Survey
 Branch of Information Services


Box 25286


Denver, Colorado 80225-0286


ISBN 0-607-87600-X 
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 Upper Mississippi 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. 

Gordon	P. Eaton 

Director 

Foreword III 
CONTENTS 

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

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

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

Purpose and Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 

Levee-Break Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

The Flood of 1993 and the Miller City, Illinois, Levee-Break Complex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

History of the Miller City Levee-Break Complex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 

Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 

Geomorphic Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 

Geomorphic Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 

Scour Morphology and Volume . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 

Processes, Magnitude, and Frequency of Geomorphic Changes in the Miller City, Illinois, 

Levee-Break Complex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 

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

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

FIGURES 

1 . Map showing part of the upper Mississippi and Missouri Rivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

2. Drawing showing definition of features in a typical levee-break complex. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

3.-5. Photographs showing: 

3. Examples of stripped surfaces from the Miller City, Illinois, levee-break complex. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

4. Sand dunes on the Missouri River flood plain between Kansas City and Hermann, 

Missouri, and in the Miller City, Illinois, levee-break complex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

5. Natural levee and crevasse splays on the Missouri River flood plain between 

Kansas City and Hermann, Missouri . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

6. Hydrograph of the daily mean discharge of the Mississippi River at Thebes, Illinois, 

April 1993-June 1994. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

7.-8. Map showing: 

7. Features of the Miller City, Illinois, levee-break complex area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 

8. Upstream end of the Miller City, Illinois, levee-break complex. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 

9. Photographs showing turbulence at the northern end, turbulence and vortices in the center, and 

the northern margin of main scour, Miller city, Illinois, levee-break complex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 

10 . Map showing historical channel positions and Holocene channel-position stages in the Miller City, 

Illinois, area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 

11 . Photograph of reentry scour east of Roth, Illinois . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 

12 .-14 . Graph showing: 

12 . Acoustic Doppler current profile transects measured in the main levee break, 

Miller City, Illinois . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 

13 . Levee break, Miller City, Illinois . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 

14 . Acoustic Doppler current profile transects along the main scour, Miller City, 

Illinois, levee-break complex. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 

15 . Map showing geomorphic features created in the Miller City, Illinois, levee-break complex. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 

16 . Graphs showing cross sections in the main and secondary scours, Miller City, Illinois, 

levee-breakcomplex. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 

Contents V 
17 .-18 . Photograph showing: 

17. Grooves and vertical walls at the peninsula, Miller City, Illinois, levee-break complex. . . . . . . . . . . . . . . . . . . . . . . . . . . 19 

18 . Typical pre-1993 flood sediment deposit, Miller City, Illinois, levee-break complex. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 

TABLES 

1 . Discharge and stage of the Mississippi River at Thebes, Illinois, upstream from the Miller City,

Illinois, levee-break complex, discharge upstream from and at the levee-break complex, and width 

of the levee break . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 

2. Aerial photographic information for documenting geomorphic changes at the Miller City, 

Illinois, levee-break complex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 

3. Characteristics of geomorphic map units used in preparation and interpretation of the Miller City,

Illinois, levee-break complex map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 

4. Summary of erosion and deposition estimates in the Miller City, Illinois, levee-break complex . . . . . . . . . . . . . . . . . . . . . . . 19 

CONVERSION FACTORS AND VERTICAL DATUM 

Multiply By To obtain 

kilometer (km) 0.6214 mile 

meter (m) 3 .281 foot 

centimeter (cm) 0.3937 inch 

square kilometer (km z)

cubic meter (m3) 

0.3861 

35 .31 

square mile 

cubic foot 

centimeter per second (cm/s) 

cubic meter per second (m3/s) 

0.0328 

35 .31 

foot per second 

cubic foot per second 

metric tons 1 .102 short tons 

Sea level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929)-a geodetic datum derived 

from a general adjustment of the first-order level nets of both the United States and Canada, formerly called Sea Level Datum of 1929 . 

VI Contents 
Geomorphic Changes on the Mississippi River Flood 

Plain at Miller City, Illinois, as a Result of the Flood 

of 1993 

By Robert B. Jacobson andKevin A. Oberg 

Abstract 

During the flood of 1993 on the upper Mississippi and Missouri Rivers, the most dramatic 

changes to flood plains resulted at levee-break 

complexes. At these sites, large discharges were 

concentrated through narrow levee breaks, which 

resulted in high water-surface slopes and 

intensely turbulent flow. These conditions created 

extensive, deep scours and permitted sedimentladen water to flow into areas behind the levees . 

As a result, large areas of formerly productive 

bottom land were eroded or covered by thick 

deposits of sand. 

The levee break at Miller City, Illinois, created a typical large levee-break complex. As much 

as 28 percent of the Mississippi River discharge 

flowed through the break during the peak of the 

flood. With velocities as great as 300 centimeters 

per second, the flow through the levee break was 

able to scour an area 2,200 meters long to a maximum depth of more than 20 meters . Transport of 

sediment from the main channel, through the 

break, resulted in net deposition of at least 8.2 

million cubic meters of sand. 

Although the history of this part of the Mississippi River Valley indicates that channel 

changes have been extensive and frequent, the 

geomorphic changes that resulted from the 1993 

flood seem to be unprecedented during the last 

10,000 years or so. The large geomorphic changes 

at Miller City during the 1993 floods are attributable to the magnitude of the flood, the hydraulic 

head artificially increased by the levee, and the 

lack of energy dissipation on the agricultural 

flood plain relative to presettlement, forested 

conditions . 

INTRODUCTION 

Large amounts of rainfall in the upper Mississippi River Basin through winter, spring, and summer 

1992-93 produced widespread and persistent flooding 

(Parrett and others, 1993; Wahl and others, 1993). The 

flood of 1993 on the upper Mississippi and Missouri 

Rivers set record discharges at many streamflow-gaging stations (Parrett and others, 1993). More than 

26,000 km2 of urban and agricultural land were inundated; total costs of the flood exceeded $12 billion 

(Interagency Floodplain Management Review Committee, 1994). 

More than $1 .2 billion in losses were attributed 

to actual flooding of agricultural bottomland (Interagency Floodplain Management Review Committee, 

1994) . Along the Missouri River, more than an estimated 1,800 km2 of prime agricultural land was 

severely affected by erosion and deposition (Interagency Floodplain Management Review Committee, 

1994). More than 300 km2 of this prime agricultural 

land was covered with sand deposits that were more 

than 60 cm thick. The costs to remove this sand and 

return the soil to its former productivity levels have 

been estimated to be about $1 .2 million per square 

kilometer (Vance, 1994) . The most severe erosion and 

sedimentation damages by far were associated with 

breaks of agricultural levees . 

Purpose and Scope 

This report documents the geomorphic changes 

associated with a levee break typical of those created 

Introduction 1 
by the 1993 flood. The geomorphic effects at levee 

breaks are of concern because erosion and sedimentation at levee-break complexes are substantial public 

safety and economic hazards. Furthermore, routing of 

sediment to and from storage in flood plains has the 

potential to redistribute sediment-borne contaminants . 

The levee-break complex chosen for documentation 

here occurred on the Mississippi River at river mile 34, 

which is near Miller City, Illinois (fig . 1). A chronology of events during the levee break, measurements of 

discharge and velocity in the main channel and the 

break, maps of erosion and deposition, descriptions of 

erosional features and sediments, and a comparison of 

the levee break to the Holocene history of the Mississippi channel are presented. Although it is one of the 

larger levee-break complexes created during the flood, 

the Miller City levee-break complex contains features 

typical of many of the levee-break complexes of the 

Mississippi and the Missouri Rivers. 

42° 

40-

38o 

36° 

96° 94° 920 90 , 88 , 86° 

-_~~rl~Keokuk ILLINOIS 

Area of 

figure7-L..1 t KENTUCKY Cairo 

0 100 200 MILES 

0 100 200 KILOMETERS 

Figure 1 . Part of the upper Mississippi and Missouri Rivers . 

Levee-Break Complexes 

Levees may break as a result of a variety of 

mechanisms, which include overtopping by floodwater, liquefaction of the levee or foundation material, or 

mass failure. The term "levee break" in this report 

describes levee sections where the levee was completely removed or eroded at least to ground level, 

regardless of the erosion mechanism. 

The 1993 flood resulted in breaks of 1,082 levees 

in the upper Mississippi and Missouri River Basins 

(Bhowmik and others, 1994). As many as 604 separate 

levee breaks were identified on the Missouri River 

between Kansas City and St. Louis, Missouri Q.C. 

Dohrenwend, U.S . Geological Survey, oral common., 

1994). Large hydraulic heads and flow constriction 

through narrow openings in the levee breaks created 

zones of intense scour downstream and upstream from 

typical breaks. Flow through the levee breaks also transported large quantities of sediment onto the flood plain. 

A levee-break complex consists of the levee 

break and associated erosional and depositional features. Although erosion and deposition patterns in levee 

break complexes can be quite complicated, they generally consist of scour, stripped, and depositional zones 

(fig . 2). The scour zone (locally called blue, blew, blow, 

or blown holes) is an area of deep erosion, usually more 

than 1 to 2 mdeep, with steep sides. Detached blocks of 

cohesive sediment are common along the margins of the 

scours, which attest to erosion by gravitational slumping 

and toppling. The planform of scours ranges from circular to elongated; many scours extend upstream and 

downstream from the break to form elongated channels . 

In cases where the scour channels connect with the main 

channel upstream, the scours are efficient structures for 

funneling water and sediment onto the flood plain (fig. 

2). Exit scours were formed when floodwater overtopped the levees or descended the banks as it reentered 

the main channel. 

EXPLANA770N 

Levee 

Stripped zone O Depositional zone 

(medium-thick and thick sand)

Mud 

Exit scourzone 

Figure 2. Definition of features in atypical levee-break 

complex. 

Downstream from the levee break, the scour 

zone is skirted with a zone where erosion generally is 

less than 50 cm deep. This zone is referred to as the 

"stripped zone" (figs . 2, 3) because the plow horizons 

and rooted horizons of the soil are typically stripped, 

2 Geomorphic Changes of the Mississippi River Flood Plain at Miller City, Illinois, as a Result of the Flood of 1993 
which leaves less erodible B-horizons and plow 

pans. Stripped zones commonly have shallow, 

rounded scours near their upstream margin and are 

characterized by abundant grooves (fig . 3) . Some 

grooves are parallel to flow directions and seem to 

have been carved by debris carried by the floodwaters, whereas other grooves are evidently related to 

preexisting weaknesses in the soil, such as those 

caused by plowing (fig . 3). 

B 

Figure 3. Examples of stripped surfaces from the Miller 

City, Illinois, levee-break complex. A, Flow was toward 

the viewer; B, Flow was from left to right ; with grooves. 

Downstream from the stripped zone, there is 

a transition into the depositional zone that consists 

of sediment deposits of varying thicknesses . 

Because of the sequence of flooding at any particular site, sand and mud (silt- and clay-sized sediment) may have been deposited over stripped zones 

or may partially fill scours. Generally, sand deposits are thickest and coarsest closer to the scour. Sedimentary features include sand waves, dunes, and 

current ripples ; commonly, areas of dunes and current ripples are draped with mud that was deposited 

during recession of the floodwaters or by subsequent floods (fig . 4). Extensive areas of the flood 

plain used for agriculture were covered with sand 

deposits, some of which were as much as 4 m thick; 

according to Vance (1994), some sand deposits 

were as much as 6 m thick on the Missouri River 

flood plain. Mud deposition seemed to be minor in 

extent. Mostly, mud was left as thin deposits in 

eddies or slackwater areas or was deposited during 

recession of the floodwaters in low areas that 

existed before the flood. The geomorphic effects of 

mud deposition were minor. 

B 

Figure 4. Sand dunes on the Missouri River flood plain 

between Kansas City and Hermann, Missouri, and in the 

Miller City, Illinois, levee-break complex. A, Missouri River 

Flood plain between Kansas City and Hermann, Missouri. 

View is to the north; flow was from left to right ; B, Miller City, 

Illinois, levee-break complex. View is to the northwest ; flow 

was from right to left . 

Introduction 3 
Scour zones and sand deposits in levee-break 

complexes are by far the most dramatic geomorphic 

effects of the 1993 flood . In contrast, geomorphic 

changes in nonleveed areas consisted of minor bank 

erosion and sedimentation in natural levees and 

crevasse-splay features (fig. 5). Natural-levee sedimentation was prominent in nonleveed bank areas 

with dense woody vegetation. Crevasse-splay features, which consist of a scour (crevasse) and associated sand deposits (splay), are similar but much 

smaller natural analogs for levee-break complexes . 

Crevasse splays occur where a weakness in the natural levee, or flow concentration, results in erosion 

of a crevasse channel through which sediment is 

transported from the channel onto the flood plain 

(Collinson, 1978). 

Figure 5. Natural levee and crevasse splays on the 

Missouri River flood plain between Kansas City and 

Hermann, Missouri . A, Natural levee; 8, Crevasse splay. 

Acknowledgments 

The authors thank Ronald Eastwood, Illinois 

Department of Transportation (IDOT), for logistical 

support and access to video and photographic data. 

The U.S. Army Corps of Engineers (COE), St. Louis 

District, provided aerial photography, discharge-measurement data, and levee-break width data. Roger 

Edinson, IDOT, Department of Water Resources, provided postflood survey data of the extent of erosional 

scour at the Miller City, Illinois, levee-break complex. 

THE FLOOD OF 1993 AND THE MILLER 

CITY, ILLINOIS, LEVEE-BREAK COMPLEX 

The flood of 1993 on the upper Mississippi and 

Missouri Rivers was notable for the extreme discharge 

and long duration. Discharges in excess of the esti 

mated 100-year flood were recorded on the Missouri 

River from Rulo, Nebraska, to Hermann, Missouri, 

and on the Mississippi River at Keokuk, Iowa, and St. 

Louis (Parrett and others, 1993; fig . 1) . At Thebes, Illinois, the peak discharge was 28,200 m3/s on August 7, 

1993 (Reed and others, 1994); the estimated recurrence interval was from 10 to 50 years (Parrett and others, 1993). The daily mean discharge at Thebes is 

shown in figure 6. 

30,000 

Z 28,000 

O 26,000 U 

w 24,000 

w 22,000 a 

w 20,000 

h 18,000 

w 16,000 

U 14,000 m 

U 12,000 

Z 10,000 

w 8,000 t7 

Q 6,000 

x 4,000 U

V7 2,000 

0 

A M J J A S 0 N D I J F M A M J 

1993 1994 

Figure 6. Daily mean discharge of the Mississippi River at 

Thebes, Illinois, April 1993-June 1994. 

In addition to large discharges, the flood of 

1993 was notable for its long duration . At St. Louis, 

the Mississippi River was above flood stage for more 

than 2 months. At Thebes, several smaller floods 

occurred from April 1993 through May 1994 in addition to the main flood of July through August 1993. 

Geomorphic Changes of the Mississippi River Flood Plain at Miller City, Illinois, as a Result of the Flood of 1993 
Downstream from the junction of the Ohio and the 

Mississippi Rivers (fig . 1), the magnitude of the 

flood was not severe because of low flows on the 

Ohio River in July and August 1993. 

History of the Miller City Levee-Break 

Complex 

The levee near Miller City, which is locally 

called the Len Small levee, was designed and constructed by local landowners to protect agricultural 

89°22'30" 

37°07'30" 

37° 

U .S . Geological Survey digital data 

Universal Transverse Mercator projection 

Zone 16 

land in Dogtooth Island bend of the Mississippi 

River (fig . 7) . The levee is continuous along the left 

bank (facing downstream) of the Mississippi River 

from Fayville, Illinois, to past the apex of Dogtooth 

Island bend. The levee was designed to deflect highvelocity floodwaters of the Mississippi River away 

from agricultural land at the upstream part of the 

bend. The downstream part of Dogtooth Island bend 

is not protected by a levee, so low-velocity backflooding has occurred in the area south of the Miller 

City Road during many floods . The Len Small levee 

89°15' 

EXPLANATION 

Approximate low-water 

channel 

Approximate maximum area 

of inundation 

Levee 

Secondary road 

Gravelroad 

Location of levee break 

+ 25 River mile and number 

U.S. Geological Survey 

gaging station 

4 MILES i

ITrT1 

0 2 4 KILOMETERS 

Figure 7. Features of the Miller City, Illinois, levee-break complex area . 

The Flood of 1993 and the Miller City, Illinois, Levee-Break Complex 5 
was constructed from locally available materials by 

using draglines and scrapers . Most of the levee 

material is uncompacted top soil . Internal stratigraphy of levee remnants showed that black top soil 

was interbedded with clean, white sand units. The 

river side of the levee was protected with riprap at 

the site of the break and for some distance upstream 

and downstream. The riprap and numerous wingdams located in this reach of the Mississippi River 

are maintained by the COE to improve navigation . 

The levee broke on July 15, 1993, at about 

1 :00 p .m. local time . According to anecdotal evidence from several witnesses, the levee was 

inspected 15 minutes before the break. At the time 

of the break, the water level on the channel side of 

the levee was estimated to be as much as 2 m below 

the levee crest. [For comparison, at the U.S . Geological Survey (USGS) streamflow-g. aging station 

at Thebes, which is about 16 km upstream, the daily 

mean stage was 104.68 m on July 15 ; on July 20 and 

August 7, the daily mean stage at Thebes peaked at 

105 .24 m (table 1)] . An eyewitness related that a 

loud noise drew his attention at 12:50 p.m. When he 

looked at the levee from a distance of about 500 m, 

he saw a near-horizontal, 15- to 30-cm-diameter 

fountain of water gushing out of the side of the 

levee . The hole quickly enlarged, and a section of 

the levee failed . As the levee eroded and widened, 

water began to fill the area between the main levee 

and a cross levee (fig . 8A) . During the first few days 

after the break, flow through the break was 

extremely turbulent (fig . 9) with standing waves 

that were about 1 m high. 

The area between the main levee and the cross 

levee is the upstream end of a channel that the Mississippi River occupied during 1820 (fig . 10) . The 

extension of the 1820 channel formed an arcuate 

trough to the east and south with about 1 .5 m of 

relief before the 1993 flood . The 1820 channel was 

bounded to the north and east by a terrace scarp that 

cut laterally into alluvial deposit of late Wisconsin 

and presumed early Holocene age (Fisk, 1944). The 

scarp ranged from about 2 to 3 m high. As the low 

area between the two levees filled, water flowed 

northward and eastward over the scarp, and followed low areas of former channels that extended 

Standing waves are stationary wave forms indicative of critical flow, which occurs in flows with large ratios of velocity to 

depth . 

toward Horseshoe Lake (figs . 8A, 10) . As water levels rose in the Horseshoe Lake area, flow began to 

move southward along former channels, descended 

the scarp, and reentered the 1820 channel to the east 

and downstream of the cross levee (fig . 8A). By July 

17, water that reentered the 1820 channel had 

eroded a secondary scour at least 400 m long by 

upstream (northward) migration of a steep head cut 

(location A, fig . 8A) . Most of the area of Dogtooth 

Island bend south of the Miller City Road was inundated by backflooding or seepage by this time. 

Much of the early floodwater followed low 

areas directly to Horseshoe Lake. Subsequent overflow of Horseshoe Lake to the south and east caused 

water to flow across the Miller City Road between 

Willard and Roth, Illinois (fig . 7), and down 0.5- to 

1-m-high scarps that separate paleochannels of the 

Mississippi River, notably the scarp between deposits of Holocene channel-position stage 2 and those 

of channel-position stages 5 and 9 (fig . 10). Observations during the flood indicated that flow over the 

road and down the terrace scarps typically was fast, 

highly turbulent, and characterized by standing 

waves. 

Aerial videotape from July 17 indicated small 

areas of flow over the cross levee at the extreme 

northern and southern ends. Larger quantities of 

overflow were noted on July 19. Because the area 

had been evacuated, the precise time and mechanism of failure of the cross levee are not known. 

However, by July 20, a substantial part of the cross 

levee had broken, and by July 31, the entire 400 m 

of the cross levee was removed. 

After the cross levee was breached, flow 

through the main levee was concentrated in the 1820 

channel, and floodwater that moved through the sec 

ondary scour immediately downstream of the cross 

levee (location A, fig . 8A) reversed direction and 

flowed northeastward. By July 26, flow was 5,570 

m3/s from the Mississippi River main channel into 

the break, and 8,000 m3/s of flow was measured near 

the peak discharge on August 7 . From July 17 

through August 10, the northern (left) bank of the 

main scour eroded laterally more than 60 m (location B, fig. 8A). In the process, more than 680 m of 

a county road was removed, as well as a house, a 

machine shed, and the front yard and front porch of 

another house (fig . 9) . 

6 Geomorphic Changes of the Mississippi River Flood Plain at Miller City, Illinois, as a Result of the Flood of 1993 
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Table 1 . Daily mean discharge and stage of the Mississippi River at Thebes, Illinois, upstream from the Miller City,

Illinois, levee-break complex, discharge upstream from and at the levee-break complex, and width of the levee break, 

July 15 to August 25, 1993


[m3 /s, cubic meters per second; m, meters ; --, not measured]


Daily mean 

Date discharge at 

Thebes Illinois 

(mi/s) 

07-15 24,451 

07-16 23,687 

07-17 23,065 

07-18 23,659 

07-19 24,508 

07-20 25,159 

07-21 25,159 

07-22 25,215 

07-23 24,734 

07-24 25,046 

07-25 24,480 

07-26 22,980 

07-27 23,602 

07-28 24,423 

07-29 25,102 

07-30 24,621 

07-31 24,649 

08-01 25,074 

08-02 25,442 

08-03 25,923 

08-04 26,008 

08-05 26,432 

08-06 27,338 

08-07 27,677 

08-08 27,253 

08-09 26,913 

08-10 26,149 

08-11 25,555 

08-12 24,904 

08-13 24,083 

08-14 23,489 

08-15 22,895 

08-16 22,272 

08-17 21,819 

08-18 21,423 

08-19 20,829 

08-20 20,008 

08-21 19,386 

08-22 18,735 

08-23 17,942 

08-24 17,291 

08-25 16,782 

Daily mean stage 

at Thebes, Illinois 

(meters above 

sea level) 

104.68 

104.81 

104.88 

105.01 

105.15 

105.24 

105.15 

105.09 

104.93 

104.92 

104.78 

104.53 

104.67 

104.74 

104.79 

104.75 

104.83 

104.95 

105.00 

105.03 

104.97 

105.04 

105.18 

105.24 

105.14 

105 .09 

104.95 

104.85 

104.71 

104.58 

104.45 

104.34 

104.23 

104.12 

104.04 

103 .91 

103.75 

103.60 

103.41 

103.22 

103.03 

102.87 

Discharge at river 

mile 35, immediately 

upstream of the Miller 

City levee break (m3/s) 1 

24,000 

22,900 

19,500 

18,200 

Discharge

measured in the Surveyed 

width of the levee break at levee break river mile 34 (m3/s) (m) 

113 

120 

5,570 472 

4,920 478 

5,480 484 

5,580 532 

5,550 

6,720 539 

6,930 558 

7,100 587 

8,000 

6,600 

7,560 

6,090 618 

5,130 

4,730 

2,890 625 

Discharges measured in main channel upstream from the levee break are somewhat lower than those measured at the U.S . Geological 

Survey gage at Thebes, Illinois, possibly because of unmeasured flow to the west of Powers Island (fig . 7). 

The Flood of 1993 and the Miller City, Illinois, Levee-Break Complex 7 
A 89°22'30" 89°21' 

37°06'30" 

EXPLANATION 

Deposits and areas eroded by

the 1993 flood 

Initial major flow direction 

prior to failure of cross levee 

Major flow direction after 

failure of cross levee 

= = = Levee eroded July - August 1993 

37°05" 

Universal Transverse Mercator, Zone 16. 

Map features based on photography dated 

January 8, 1994 

8 

Universal Transverse Mercator, Zone 16. 

Map features based on photography dated 

January 8, 1994 

0 0.5 1 MILE 

I I r 

0 0 .5 1 KILOMETER 

89°21' 

0 0 .5 1 MILE 

I J 

0 0.5 1 KILOMETER 

Levee, January 1994 

Road 

.mar	 Rock dam installed between 

August 1993 and January 1994 

Wing-dam 

A Secondary channel 

B North bank of main scour 

C Peninsula of resistant sediment 

D Scoured and stripped area 

EXPLANATION 

Thick sand-Well-sorted, fine to 

medium sand greater than 60 

centimeters thick 

j Medium-thick sand-Well-sorted, 

fine to medium sand, 30 to 60 

centimeters thick 

Thin sand-Sand and mud less than 

30 centimeters thick 

Scour-Steep-sided erosional scour 

Stripped surface-Planar, eroded 

surface 

Levee eroded July-August 1993 

Levee, January 1994 

70	 Line of surveyed section and 

number 

® Wing-dam 

Figure 8. Upstream end of the Miller City, Illinois, levee-break complex . A, Flow directions and locations ; B, Erosional and 

depositional features. 

8 Geomorphic Changes of the Mississippi River Flood Plain at Miller City, Illinois, as a Result of the Flood of 1993 
Figure 9. Miller City, Illinois, levee-break complex. A, 

Turbulence at northern end (Illinois Department of 

Transportation) ; 8, Turbulence and vortices in center (Illinois 

Department of Transportation), and; C, Northern margin of 

main scour, October 1993. 

Observations from water and aerial reconnaissance from July 26 through August 8 indicated that the 

main-flow vectors were oriented along the 1820 channel 

with divergence to the northeast and southeast (fig . 8A). 

To the north of the Miller City Road, in the area between 

Willard and Roth, flow vectors were to the east and 

northeast; secondary flow diverged southward across the 

Miller City Road (fig . 7). All the Dogtooth Island bend 

area south of the Miller City Road was inundated at this 

time, although water that flowed at higher velocities 

seemed to be confined to the northern one-half of the 

area. Floodwater diverged to the south and followed the 

1820 channel until it encountered the levee. Then it concentrated in a low area defined by channel-position stage 

12 (Fisk, 1944) and flowed to the southeast until ponded 

water forced the flow to the east and back into the main 

channel near river mile 16 (fig . 7). 

Because of the wide angle of divergence of flow, 

it is impossible to define the degree to which flow around 

Dogtooth Island bend was cut off as a result of flow 

through the levee break. The location of flow back into 

the Mississippi River channel ranged from about river 

mile 20 as determined by the downstream extent of the 

Len Small levee, to about river mile 13.3, which is 

immediately upstream from the Cache River cutoff (fig . 

7). 

Floodwaters slowly receded after the peak on 

August 7 (table 1) . As higher areas became dry, flow 

concentrated in the 1820 channel and the newly scoured 

areas. Flow in secondary scours oriented to the north and 

northeast once again reversed and moved into the 1820 

channel. Probably the most dramatic change during this 

time was about 3 km east of Roth where large volumes 

of water were continuing to drain from Horseshoe Lake 

(fig . 7). As the water level dropped in the main channel 

of the Mississippi River, a reentry scour was created by 

upstream migration of the headcut (fig . 11). Erosion processes were still continuing during the last week of 

August 1993. 

Subsequent to the summer flooding during 1993, 

the Miller City area was affected substantially by three 

additional floods (fig . 6). During late September and 

early October 1993, intense rainfall in Missouri resulted 

in another rise on the Mississippi River; the peak discharge of 20,600 m3/s was at Thebes. In mid-November 

1993, intense rain in Missouri and southern Illinois created a flood with a peak discharge of about 13,500 m3 / 

s at the streamflow-gaging station at Thebes. Both floods 

were sufficient to create a high volume of flow through 

the levee break with subsequent inundation of most of 

Dogtooth Island bend south of the Miller City Road. 

Before the November flood, construction had begun on 

a new levee alignment, and rock dams had been built 

across the main scour and one of the secondary scours 

(fig . 8B). The November flood caused increased erosion 

around the southwestern corner ofthe dam that had been 

constructed across the main scour. Another flood in April 

1994 had a peak discharge of 18,500 m3/s at Thebes. 

This flood created extensive changes, which included 

erosion of as much as 100 m of the southern margin of 

The Flood of 1993 and the Miller City, Illinois, Levee-Break Complex 

C 
the main scour and extensive reworking of sand deposits in the area of the 1820 channel. The April 1994 

flood also destroyed about 1,000 m of newly reconstructed levee. 

Measurements 

Measurements of discharge, velocity, and bathymetry in the levee break were made by the U.S . Army 

89°30' 89°22'30" 

37°07'30" 

37° 

Base from U .S. Geological Survey digital data . 

Universal Transverse Mercator projection, Zone 16 

Corp of Engineers (COE) and the USGS from July 26 

through September 9, 1993. The USGS measurements 

were made in cooperation with the Illinois Department 

of Transportation (IDOT) . Discharge and velocity 

were measured in the Mississippi River upstream from 

the levee break and in the levee break. The COE measured discharge by the moving-boat technique (Smoot 

and Novak, 1969) and bathymetry by use of a digital 

echo-sounder and range-range positioning system. 

The range of the COE boat was limited by its draft; 

89°15' 

EXPLANATION 

Historical channel positions 

1944 

1880 

1820 

- 1765 

Holocene channel-position stages 

. 1 16 

15 

14 

® 13 

12 

11 

10 

9 

s 5 

1993 bankline 

Levee-break flow vectors 

From Fisk, 1944 

0 

0 2 4 KILOMETERS 

Figure 10. Historical channel positions and Holocene channel-position stages in the Miller City, Illinois, area . 

10 Geomorphic Changes of the Mississippi River Flood Plain at Miller City, Illinois, as a Result of the Flood of 1993 
therefore, it could be used to measure velocity and 

bathymetry only in the immediate vicinity of the 

break. 

The USGS used an acoustic Doppler current 

profiler (ADCP) on an 8-m boat to measure discharge 

and velocity in and near the levee break. An ADCP 

may be used to measure vertical profiles of water 

velocity in three dimensions from a moving boat. The 

technique used for ADCP measurements is similar to 

the moving-boat technique except that the ADCP can 

measure water velocity every 25 cm in the water column, rather than at a single depth, and requires no 

external positioning system for computing boat velocity. Gordon (1989), Simpson and Oltman (1993), and 

Oberg and Mueller (1994) provided detailed descriptions of the ADCP and its application to streamflow 

measurements. Bathymetry measurements also were 

made by combining depths measured by using the 

ADCP with concurrent horizontal position data from a 

global positioning system (Oberg and Mueller, 1994). 

An ADCP equipped with a 300-kilohertz (kHz) 

transducer was used to measure discharge and velocity 

in the Mississippi River upstream from the levee break, 

downstream from the levee break, and, in some 

instances, in the levee break. However, most measurements of discharge and velocity in the levee break 

were made by using an ADCP equipped with a 1,200-

kHz transducer because the 300-kHz ADCP could not 

be used to measure water velocities in less than 5 m 

depth; the minimum recordable depth for the 1,200-

kHz ADCP was about 1 .3 m. Discharges measured by 

the COE and the USGS upstream from and in the levee 

break are shown in table 1 . 

The 1,200-kHz ADCP also was used to collect 

velocity and bathymetry data downstream from the 

levee break along the scour. These measurements were 

used to determine changes in water velocities along the 

main scour and the extent of scour formation . A small 

subset of the velocity data collected by using the 

ADCP is presented here to illustrate the range of 

velocities measured in and downstream from the levee 

break. Velocity profiles in the cross section for 

transects made in the levee break on July 31, August 9, 

August 19, and September 26, 1993, are shown in figure 12. These transects were located approximately 

along the former levee alignment . 

Turbulence in and near the levee break made 

some measurements of water velocities and discharge 

difficult. Large (1- to 3-m diameter) vortices were 

present in the levee break, especially in the area immediately riverward of the original levee (fig . 9). This turbulence may have been associated with flow over the 

remains of riprap . In some instances, the ADCP was 

not able to measure velocities because of the turbulent 

flow conditions. Velocities measured by the ADCP in 

the levee break may be subject to errors because of the 

Figure 11 . Reentry scour east of Roth, Illinois . (Aerial photograph January 8, 1994;U.S . Army Corps of 

Engineers, St. Louis) . 

The Flood of 1993 and the Miller City, Illinois, Levee-Break Complex 
unsteady, turbulent flow between the northern and 

southern end of the levee. High turbulence also was 

noted in an area about 200 m downstream of the break. 

When the water receded, it was revealed that this second area was above a peninsula of extremely compact, 

cohesive, and resistant sediment (location C, fig . 8A). 

On July 31, 1993, an ADCP transect was made 

through the approximate center ofthe levee break from 

west (Mississippi River) to east (flood plain) . The lon 

gitudinal transect of the scour and vertical components 

of velocities measured by the ADCP are shown in figure 13. This transect shows the flow up and over the 

riprap that lined the low-water channel and the large 

scour hole that formed immediately east (downstream) 

of the riprap . The vertical velocities measured with the 

ADCP generally correspond to the scour morphology. 

The vertical velocity is predominantly positive 

(upward) as flow moves up and over the riprap and 

predominantly negative as flow descends into the scour 

hole. About 300 m downstream from the levee, flow 

ascends over an area of resistant sediment referred to 

as the "peninsula" (location C, fig . 8A). Velocity components that fluctuate upward and downward are indicative of high turbulence in this area. Velocity vectors 

veer from west/east to southwest/northeast around the 

peninsula, which indicates that it also was effective in 

steering flow in planform. 

W 

W 

z 

uwU

Q 

25 

The ADCP velocity and bathymetry measurements also were made downstream (inland) from the 

break on August 1, 1993. Cross sections and location 

of velocity-profile transects are shown in figure 14. 

Results of these measurements indicate that water 

velocities tended to increase downstream of the levee 

break until the downstream end of the scour channel 

was reached. This increase in velocity may be 

explained by the narrowing of the scour channel downstream of the levee break and the consequent loss of 

conveyance. 

GEOMORPHIC CHANGES 

Geomorphic changes on the flood plain include 

erosional and depositional alterations of the surface . 

Erosion and deposition are primary evidence for the 

spatial distribution of energy and sediment transport 

related to the levee break. Therefore, geomorphic 

changes can be used to evaluate the pattern of energy 

and sediment transport on the flood plain. Furthermore, 

the economic effect of the substantive geomorphic 

changes that occurred as a result of the 1993 flood was 

large. Many areas with deep scours and thick sand 

deposits may be uneconomical to repair, and natural 

EXPLANATION 

O /- 220 

w 193
 JULY 31, 1993, AUGUST 19, 1993, U . 166


W 5,550 CUBIC METERS PER SECOND 4,730 CUBIC METERS PER SECOND [[ 140
 a 113 3

3

OJ 

W 60 w 33 W 

LD 5 6 

S -20 H

a 

0 10 

-47 

-74 

-100 

15 

Current projected

into vertical plane

oriented north 10° W 

0 86 

20 

25 

0 100 200 300 400 500 600 700 0 100 200 300 400 500 600 700 

AUGUST9, 1993, SEPTEMBER 26, 1993,

6,600 CUBIC METERS PER SECOND 2,870 CUBIC METERS PER SECOND 

DISTANCE ALONG TRANSECT, IN METERS 

Figure 12. Acoustic Doppler current profile transects measured in the main levee break, Miller City, Illinois . 

12 Geomorphic Changes of the Mississippi River Flood Plain at Miller City, Illinois, as a Result of the Flood of 1993 
W 

z 

0

z 

Z 

W 

0

Q 

a 

0 

w 

LLI 

Z 

w 

0 

a

3 

400 

Horizontal flow vector at 2 .14 300 meter depth . This length 

equivalent to 50 centimeters persecond 

200 

100 

0 

-100 

-200 

-300 

-400 0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 

DISPLACEMENT EAST, IN METERS 

LONGITUDINAL. 

IRANSLCT 

wl rH PROFIT F 

OF VLHI IC ;AL 

COMPnNCNTS 

.\ OF VFI () I:ITY 

Variable depth 

error -lerence 

24 1 1 -- 1 i I ~ I 

0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 

DISTANCE ALONG MAIN SCOUR CHANNEL, IN METERS 

Figure 13. Levee break, Miller City, Illinois . A, Boat track 

and flow vectors ; 8, Longitudinal transect with profile of 

vertical components of velocity . 

geomorphic processes would require hundreds to thousands of years to erase the damage. 

Geomorphic Features 

Geomorphic features in the Miller City leveebreak complex were documented through aerial photographic interpretation, supplemented with onsite vis 

its to obtain ground-truth information. The magnitude 

of the changes described in this report are determined, 

in part, by the scale of aerial photography, date of photography, and methods used to transfer photographic 

data to a map. Information on aerial photography is 

given in table 2. 

The map of geomorphic features (fig . 15) was 

prepared by using the following procedure. First, control points identifiable on preflood photographs and 

USGS 1 :24,000 scale topographic maps of the area 

(Cache, Thebes SW, Thebes, Charleston, and Tamms) 

were digitized . These data were projected into a universal transverse mercator, zone 16, and then used for 

absolute orientation of the preflood photographs. The 

absolute accuracy of the location of the control points 

and all subsequent points is constrained by the accuracy of the information on the topographic maps from 

which the data were digitized, or about 12 m horizontal and 0.8 m vertical . However, additional control 

points were digitized from the preflood photography 

and used for absolute orientation of the postflood photography, thereby diminishing the estimated interphotographic accuracy to about 2 m horizontal. Once control points were established, they were used for absolute orientation of stereomodels by using a 

photogrammetric stereoplotter and a photogrammetric 

computer software program. 

For preflood conditions, 16 photogrammetric 

stereomodels were used to map channel margins, 

wooded areas, ponded water, roads, levees, and struc 

tures. For postflood conditions, 12 stereomodels were 

used to map 6 geomorphic mapping units (table 3) and 

navigation structures that were more apparent at the 

lower water level during January 1994. Digital map 

files for each stereomodel were imported to a geographic information system format where they were 

edge matched, edited, plotted, and analyzed. 

The geomorphic units (table 3) were developed 

through onsite checking and evaluation of how much 

interpretation was possible given the scale and resolution of the postflood photography. Onsite checks 

involved spot determinations of thickness, stratigraphy, and sedimentology of sediments and descriptions 

of eroded areas. Vertical resolution of the photography 

was not adequate for mapping topography of the sand 

units, but it was sufficient to distinguish between 

thicker and thinner deposits . The thickness values 

given in table 3 are conservative to avoid overestimation of sediment volume . 

The main scour channel (figs . 8, 15) follows the 

1820 channel mapped by Fisk (1944; fig . 10) . Total 

scour-channel length is about 2,200 m; the maximum 

width is about 400 m. Thick sand within the scour was 

deposited either on the receding limb of the 

hydrograph or during subsequent floods before the 

January 8, 1994, photography. 

Geomorphic Changes 1 3 
w 

w 

Z 

ww0 

w 

F 

Q 

EXPLANATION 

Water, January 1994 2 Acoustic Doppler current profile 

transect and number 

Deposits and areas eroded 

by the 1993 flood WE West-east transect 

= Road 

EXPLANATION 

r 250 

w 

0 w W 200 

1 zO I I 150 w0 2 0 UJ 

_ w

3 Transect 15	 zU) I 1100

~ 

_ 4	 . . . . . ow 150 

100 200 300 400 00U UVU /UU uuu 9uu 0 1,000 1,100 O 

w 

DISTANCE ALONG TRANSECT, IN METERS 

Figure 14. Acoustic Doppler current profile transects along the main scour, Miller City, Illinois, leve-break complex . 

1 4 Geomorphic Changes of the Mississippi River Flood Plain at Miller City, Illinois, as a Result of the Flood of 1993 
Table 2. Aerial photographic information for documenting 

geomorphic changes at the Miller City, Illinois, levee-break 

complex 

[U .S . Army Corps of Engineers, St . Louis District] 

Approximate Date of Application of scale and type of photography photography photography 

July 7, 1993 Mapprelevee-break con- 1 :24,000, 

ditions in Dogtooth black and 

Island bend . white. 

August 7, 1993 Evaluate flow vectors and Do. 

inundated area . 

January 8, 1994 Map geomorphic effects 1 :24,000, 

in levee-break com- color. 

plex, low-water channel, and navigation 

structures . 

or during subsequent floods before the January 8, 1994, 

photography. 

Additional areas of scour occur as narrow secondary channels on the margins of the main scour and as 

elongated features downstream from the main scour. 

These smaller scours usually are surrounded by stripped 

areas (fig . 8B). Scoured and stripped areas are most 

extensive to the east and southeast of the downstream 

end of the main scour (locations D and E, fig . 8A). 

Smaller areas of scouring and stripping are described as 

follows : 

" A long, narrow, southeast-trending region (1 .7-4.3 km) 

directly south of the downstream end of the main 

channel . This was an area of concentrated flow, and 

the scour seems to have started where the flow overtopped a road embankment (location A, fig . 15) that 

crossed a preexisting channel. 

" North of and crossing the Miller City Road where 

floodwaters flowed southward and down a prominent scarp (location B, fig . 15) . 

" At an exit scour, immediately upstream of the Cache 

River cutoff (location C, fig. 15) . 

" Directly south and parallel to the main scour. These are 

long, narrow scours that were partially refilled with 

sand after the flood. Such a scour can be seen on the 

right side ofADCP profile 7 (fig . 15) . 

Medium-thick and thick sand deposits are distributed in a complex pattern . They generally consist ofelongated, somewhat streamlined bodies aligned parallel to 

flow direction . Thick sand units are most common close 

to the main scour and at the eastern edge of Dogtooth 

Island bend (location D, fig. 15) where much of the flow 

reentered the main channel of the Mississippi River. At 

this location, the main-channel flow apparently caused a 

hydraulic damming effect, which allowed the deposition 

of a large quantity of sand; thicknesses of greater than 4 

m were measured. Some locations of deposition of 

medium-thick and thick sand seemed to be controlled by 

fencelines and woody vegetation. Thick sand deposits 

were common in densely vegetated fencelines along the 

upstream margins of wooded areas. Several mediumthick sand deposits were mapped along the channel margin of the nonleveed part of Dogtooth Island bend (location E, fig . 15) ; these were interpreted to be natural 

crevasse-splay deposits from water that overtopped the 

natural levee and flowed from northeast to southwest 

onto Dogtooth Island bend. 

Mud deposits were limited in extent and detected 

mainly in the southern one-third of Dogtooth Island 

bend. This was an area of low-velocity flow, situated 

between the levee-break and the main flows, and protected by the remains of the Len Small levee. Mud generally accumulated in low areas of paleochannels and 

typically attained thicknesses that ranged from 20 to 

40 cm. 

Scour Morphology and Volume 

Cross sections of the main scour and two smaller 

scours were surveyed onsite by the IDOT during February 1994 (fig . 16) . These surveys were completed after 

the flood of November 17 to 22, 1993, and before the 

flood of April 13 to 21, 1994. Surveyed depths are minimum, or net, scour depths because of probable redeposition of sediment by the receding floodwaters at the end 

of the 1993 summer floods and by the November 1993 

flood. 

The main scour channel is 2,200 m long and forms 

an arc that follows the preexisting 1820 channel . The 

channel ranges in width from about 100 m to more than 

600 mand averages about 350 m. In the cross section, the 

scour is wide and deep at the levee break where the maximum net depth is 19.1 m (section 1, fig . 16) . Immediately downstream of the break, a prominent shelf is 

evident on the left bank (sections 2, 3, fig . 16) . This shelf 

was located near an area of intense turbulence during the 

levee break. Onsite observations indicate that it was 

formed of extremely dense, cohesive, and resistant silt 

and clay that eroded to form vertical scarps and long, 

narrow grooves (fig . 17). Subsequent to scour of the main 

channel, thick sand was deposited downstream and 

toward the left bank of the shelf (fig . 8B; section 4, fig . 

16). Apparently, the less-erodible sediment of the shelf 

deflected the flow to the right bank, which caused greater 

velocity and deepening on that side (figs . 13, 14) . 

Downstream of the shelf, the deepest part of the channel shifts to the left bank (sections 5 through 8) . During 

Geomorphic Changes 1 5 
Table 3. Characteristics of geomorphic map units used in preparation and interpretation of the Miller City, Illinois, 

levee-break complex map 

[km2, square kilometers ; cm, centimeters; m, meters ; >, greater than; <, less than ; NA, data not applicable] 

Area Geomorphic map unit (km2) 

Thick sand deposits . . . 7.1 

Medium-thick sand 15 .2 

deposits . 

Thin sand deposits . . . . 52.5 

Muddeposits . . . . . . . . 2.5 

Stripped surfaces . . . . . 2.1 

Scours . . . . . . . . . . . . . .5 

Unit characteristics 

Forms sand waves and dunes with amplitudes of 0.5 to 3 m; wavelengths of 

2 to 10 m. Thickness is sufficient to obscure all underlying, preflood 

topography on the scale of crop rows to road embankments. Usually little 

or no muddrape is present, and reflectivity on photography is high . Generally well-sorted, fine to medium sand . May have discontinuous, thin, 

interbedded mud layers . Internal sedimentary structures include wave ripple cross lamination, climbing ripple cross lamination, dune-scale crossbedding, and planar bedding. 

Forms low-amplitude dunes and extensive areas with little or no surface 

topography. Amplitudes of dunes range from 10 to 30 cm; wavelengths 

of 2 to 4 cm. Thickness is sufficient to obscure all underlying preflood 

topography on the scale of crop rows. Mud drapes are common and 

tend to obscure underlying sand ; reflectivity is often low, but mud is 

usually dry and light brown. Overlying mud drapes may be as much as 

30 cm thick. Generally well-sorted, fine to medium sand with common 

interbedded mud layers . Internal sedimentary structures include wave 

ripple cross lamination, climbing ripple cross lamination, planar bedding, and minor dune-scale crossbedding . 

Forms extensive areas of sand conforming to preexisting topography. Identification for aerial photography depends on presence of areas of highly 

reflective sediment that does not obscure crop rows . Muddrapes are 

extremely common and tend to obscure this unit, although because of 

underdraining by the sand, the mud tends to be dry and a lighter brown on 

photographs. Generally well-sorted, fine to medium sand with common 

interbedded mudlayers . Internal sedimentary structures include wave ripple cross lamination, climbing ripple cross lamination with minor planar 

bedding, and dune-scale crossbedding . 

Forms dark, low-reflective areas in low spots. Does not obscure crop-row 

topography and is often associated with standing water and patchy ice in 

January 1994 photography. 

Has distinctive grooved topography, has ledges with 5 to 100 cm of relief, 

and lacks preflood crop stubble and vegetation . Sharp boundaries at edge 

of stripped units indicate 30 to 50 cm of stripping. Red colors of B-horizon are prominent (where older terrace sediments are stripped). May have 

subsidiary shallow scours . Some areas have been covered with subsequently deposited sand and mud. 

Equant to elongate, steep-sided features with evident depths of greater than 

about 1 m. Often partially filled with water; commonly partially filled 

with sand deposited subsequent to erosion. 

Estimated 

thickness 

(cm) 

> 60 

30-60 

0-30 

< 60 

NA 

Do. 

1993 flood, an additional area was scoured to the main flood, so the cross section is eroded almost


south (right) of the rock dam constructed for the entirely in cohesive silt and clay.


new levee alignment, and additional sand was The volume of sediment eroded from the main


deposited just downstream from the rock dam (sec- scour was estimated by approximating the channel


tion 7, fig . 16) . segments between cross sections (fig . 16) as solid,


rectangular bodies and multiplying the area of The secondary scour represented in sections 9 stripped surfaces by an estimated mean erosion depth through 14 (fig . 16) was much narrower than the scour of 30 cm. The estimated volume (table 4) ignores of the main channel . The maximum depth at the down some minor scours and is based on the net depth of the 

stream end is about 5 m. A small amount of sand was two surveyed scours; hence, the calculation may 

deposited in the secondary scour subsequent to the underestimate the total eroded volume. 

1 6 Geomorphic Changes of the Mississippi River Flood Plain at Miller City, Illinois, as a Result of the Flood of 1993 
89°22'30" 89ol8'45" 

37 003'45" 

I i I i 

r 

0 1 2 KILOMETERS 

EXPLANATION 

Fine to medium sand-Well-sorted, greate Mud-Less than 60 centimeters thick A Scour associated with gravel road D Area of extensive, thick sand deposits 

than 60 centimeters thick 

Scour-Steep-sided erosional scour 

embankment Natural crevasse-splay deposits in 

Fine to medium sand-Well-sorted, 30 to _ . B Scour associated with Miller City non-leveed area 

60 centimeters thick Stripped surface-Planar, eroded Road embankment Rock dam surface 

Sand and mud-Less than 30 centimeters C Exit scour 

thick Levee 

Figure 15. Geomorphic features created in the Miller City, Illinois, levee-break complex. 

Geomorphic Changes 1 7 
110 

105 

100 

95 

90 

85 

80 

75 

70 

110 

105 

100 w 95 ww 

90

Z 85

J

W 80 

w 75 J 70 

wW 110 

w 105 

O 100 

m 

Q 95 

Z 90 O 

h 85 - Q 

w 80 j 

J 75 _ w 

70 

110 

105 

100 

95 

90 

85 

80 

75 

70 

0 100 200 300 400 500 

110


105 SECTION 5


100 ---A Water surface, spring 1994


95


90 ~-


85


80 `


75


70


110


105 SECTION 6


100 . Water surface, spring 1994


95


90


85


80


75


70 -


110


105 SECTION 7


100 _,~ , Water surface, spring 1994
 / 

95 

90 

85 

80 

75 

70 

110 

105 F SECTION 8 

100 ~. /Water surface . spring 1994 

95 

90 

85 f 

80 1-

75 

70 

600 700 800 0 100 200 300 400 500 600 700 800 

DISTANCE ALONG TRANSECT, IN METERS 

104 

SECTION 12 

102 Water surface, spring 1994 

100 

98 

96 

94 

104 

102 

SECTION 13 Water surface, spring 1994 

100 

98 

96 

94 

104 

SECTION 14 

102 Water surface, spring 1994 \ 

100 

98 

96 

94 

0 20 40 60 80 100 120 140 160 180 200 0 20 40 60 80 100 120 140 160 180 200 

DISTANCE ALONG TRANSECT, IN METERS 

Figure 16. Cross sections in the main and secondary scours, Miller City, Illinois, levee-break complex. 

18 Geomorphic Changes of the Mississippi River Flood Plain at Miller City, Illinois, as a Result of the Flood of 1993 
Figure 17. Groves and vertical walls at the peninsula, Miller 

City, Illinois, levee-break complex. View is to the Southeast, 

downstream along the main scour channel. 

Volumes of eroded soil and estimated areas of 

deposition in the thin, medium-thick, and thick geomorphic map units were used to calculate the net vol 

ume of sediment delivered to the flood plain (table 

4). The volumes of sand were conservatively estimated because the lower limit of the thickness range 

was used for the calculation . The mass of sediments 

eroded and deposited was calculated by using a range 

of typical bulk densities to cover the range of materials present. These estimates provide an indication 

of the importance of the Miller City levee-break 

complex as a net source or sink of sediment during 

the flood. 

Even with the range in uncertainties, the Miller 

City levee-break complex was clearly a sink for sediment in trans ort, thus accumulating a net volume of 

8 .2 million m and a mass from 11 .4 to 18.4 million 

metric tons . The mass of sediment deposited on the 

flood plain at Miller City is from 22 to 36 percent of 

the total sediment load that passed Thebes, during the 

flood (Holmes, 1966) . This mass of sediment also is 

equivalent from 8 to 13 percent of the mean annual 

suspended sediment load of the Mississippi River at 

Thebes. 

PROCESSES, MAGNITUDE, AND 

FREQUENCY OF GEOMORPHIC 

CHANGES IN THE MILLER CITY, ILLINOIS, 

LEVEE-BREAK COMPLEX 

A detailed analysis of the causes of the Miller 

City levee break is beyond the scope of this report . 

However, some discussion is warranted because geo 

metric aspects of the break are directly related to geomorphic processes in the levee-break complex. 

Anecdotal accounts of local residents attribute the 

break to poor levee maintenance that allowed weakening of the structure by groundhog burrows . On a 

broader scale, it may be significant that the Miller City 

Table 4. Summary of erosion and deposition estimates in the Miller City, Illinois, levee-break complex 

[km2 , square kilometers ; cm, centimeters ; m3 , cubic meters; kg/m 3 , kilograms per cubic meter; NA, not applicable] 

Estimated mass Estimated mass 

Geomorphic mapping unit Area 

(km2 ) 

Depth

used for 

calculation 

(cm) 

Estimated 

volume 

(millions

of m3 ) 

(millions of metric 

tons using

minimum bulk 

density of 

1,362 kg/m 3 ) 

(millions of metric 

tons using 

maximum bulk 

density of 

2,211 kg/m3) 

Thick sand deposits . . . . . . . . . 7 .1 60 4.2 5.8 9 .4 

Medium-thick sand deposits . . 15 .2 30 4.6 6.3 10.1 

Thin sand deposits . . . . . . . . . . 52.5 10 5.2 7.1 11 .5 

Stripped surfaces . . . . . . . . . . . 1 .8 30 .5 - .7 

Scours . . . . . . . . . . . . . . . . . . . 5 .8 Variable l 5.2 -7 .1 -11 .5 

Net deposition . . . . . . . . . . . . . NA NA 8 .2 11 .4 18.4 

Percentage of flood sediment . . . .do . . . . . . . do. . . . NA 22.3 36.1 

load at Thebes, Illinois . 2 

'Variable depths were used for surveyed cross sections to estimate scour volume . 

25 1,000,000 metric tons [estimated by Holmes (1996)] . 

Processes, Magnitude, and Frequency of Geomorphic Changes in the Miller City, Illinois, Levee-Break Complex 19 
levee broke downstream from Thebes Gap (fig . 1), 

which is an area where the Mississippi River Valley is 

constricted between bedrock bluffs . From the exit of 

Thebes Gap near Fayville to the downstream end of 

Goose Island, the agricultural levees on both sides of 

the channel converged to create a local constriction, or 

neck, exactly where the Len Small levee failed (fig . 7). 

In addition, the low-water channel of the Mississippi 

River, which carried a high percentage ofthe discharge 

during the flood, curves around Goose Island and into 

the levee at the site of the levee break. Constriction of 

the flow at the neck and channel geometry that directed 

the thread of high-velocity flow at the levee may have 

been partial factors in determining the location of the 

levee break. 

Once the break occurred, the geomorphic effectiveness of flow through the levee break was affected 

by several factors, which include the direction of flow 

through the break. From July 15 through August 25, 

1993, as much as 28 percent of the discharge of the 

Mississippi River flowed through the Miller City levee 

break. Depths in the upstream part of the levee break 

were comparable to those in the main channel . Local 

water-surface slope through the break exceeded 5 percent just after the break. During ADCP discharge measurements in late July, the water-surface drop through 

the break was estimated to be 1 m over a distance of 

about 20 m for a slope of about 5 percent . The slope 

across Dogtooth Island bend averaged about 0.04 percent, or twice the normal slope of 0.02 percent along 

the low-water channel. High flow concentrated in a 

narrow region with a large water-surface slope created 

conditions sufficient to erode the preexisting flood 

plain sediments . 

The characteristic steep walls and the presence 

of slumped and toppled blocks of cohesive sediment 

attest to erosion by a combination of hydraulic and 

gravitational forces . Observations at Miller City and 

from other locations on the Mississippi and the Missouri Rivers support a model of scour formation in 

which highly turbulent flow in or near the levee break 

erodes through a top stratum of cohesive muddy sediment. The initial piercing of the cohesive layer may 

result from vertical components of velocity in the turbulent flow or by scour around objects like tree trunks 

or both. Once the cohesive top-stratum is breached, 

noncohesive sediment under the top stratum is undermined, thus maintaining steep walls in the top stratum 

and leading to slumping or toppling. Undermining and 

toppling lead to extension of the scour upstream and 

downstream. In cases like the Miller City levee-break 

complex where the scour extends far enough upstream 

from the levee to intersect the main channel, the scour 

forms a ramp that expedites the transport of bedload 

and near-bed suspended load from the channel, 

through the levee break, and onto the flood plain. 

Vertical components of velocity are evident in 

the west-to-east ADCP transect through the levee 

break (fig . 13) . The overall ramp structure of the scour 

is interrupted by a ridge at the remains of the riprap . 

Upstream from the riprap, substantial components of 

velocity are directed vertically upward (positive values), and downstream, downward components are evident. The downward components probably caused the 

large scour hole between the riprap and the peninsula. 

Upward and downward fluctuating velocity components are evident around the peninsula, which indicated high turbulence in that area. 

In the break, velocities of as much as 300 cm/s 

were common on July 31 and August 9 (fig . 12) . 

Downstream from the break, velocities on August 1 

initially slowed in the area of expanding flow, then 

accelerated toward the downstream end (transects 2, 4, 

5, fig . 14) . Acceleration of flow probably was related to 

the narrower channel at the downstream end of the 

scour where channel width and depth were continuing 

to adjust to the imposed discharge by actively eroding 

banks and channel bed. Downstream from transect 5 

(fig . 14), ADCP data indicate transition through a more 

poorly defined channel (transect 7, fig . 14) to an area 

of multiple, shallow threads of highly variable velocity 

(transects 8, 15, fig . 14) . Bottom topography in ADCP 

profile 8, (fig . 14) shows several narrow channels from 

2 to 3 m deep; these probably became the isolated, 

small scours mapped in the stripped zone downstream 

from the main scour (figs . 8B, 15) . Some of the irregular bottom topography in transect 15 (fig . 14), however, probably resulted from deposition of longitudinal 

sand dunes (fig . 15) . 

The map pattern of the main scour, stripped 

zones, and depositional zones at Miller City indicates 

the effects of topography and structures on geomor 

phic changes in the levee-break complex. Initial flow 

directions were determined by the 1820 channel and 

the cross levee that blocked water from flowing to the 

south and east. After the cross levee was broken, flow 

directions were affected mainly by the scarp between 

the 1820 channel and older sediments . The larger secondary scours to the north and east of the main scour 

occur in preexisting low areas where flow was concentrated. Long, narrow scours to the south of the main 

scour (fig . 8B) are not clearly related to topography or 

20 Geomorphic Changes of the Mississippi River Flood Plain at Miller City, Illinois, as a Result of the Flood of 1993 
structure and may have been caused by scour around 

grounded tree boles or other debris carried by the flood. 

Stripped areas were formed preferentially at topographic breaks. Most stripping occurred where flow 

from the main channel ascended the scarp that separates 

the 1820 channel from older deposits. Stripping was 

extensive where shallow flow crossed the topographically high area along and north of the Miller City Road. 

Alarge area also was stripped during the flood recession 

where flow out of Horseshoe Lake descended the scarp 

into the low-water channel (location C, fig . 15). Also, 

stripping was commonly observed downstream, but not 

upstream, from fairly subtle topographic breaks, such as 

low road embankments or even wheel ruts . Association 

of stripped areas with topographic breaks indicates that 

turbulence induced by such features is a key control for 

initiation of shallow erosion . 

Flow diverged from the 1820 channel to the 

northeast and southeast and followed preexisting low 

spots defined by paleochannels and around topographic 

highs of older channel positions . Water that flowed 

southward parallel to the levee encountered ponded 

water in the southern one-half of Dogtooth Island bend 

and was forced to the northeast . Where flow that was 

eastward and northeastward across the bend converged 

with flow in the main channel (about river miles 15-17), 

decelerating current velocity resulted in particularly 

thick sand deposits . The patterns of thick sand deposits 

north ofthe Miller City Road indicate that wooded areas 

also slowed current velocities sufficiently to create deposition sites . 

The net amount of sediment deposited in the 

Miller City levee-break complex was conservatively 

estimated to be from 22 to 36 percent of the total flood 

sediment load. From this estimate, it is possible to conclude that levee-break complexes could be substantial 

sinks for sediment in transport during the flood. Holmes 

(1996) speculated that levee-break complexes on the 

Missouri River were, in part, responsible for a 21-percent decrease in flood-sediment load between Hermann 

and St. Louis. Onsite observations indicate that the 

effectiveness of levee-break complexes to extract sediment from transport is controlled, in part, by the extent 

to which the scour connects to the main channel at a 

favorable angle to funnel flow directly onto the flood 

plain. At Miller City, the levee break and main scour 

were almost perfectly aligned with the direction of flow 

guided by the low-water channel along the western side 

of Goose Island (fig . 7). 

Erosion from the 1993 flood created many extensive exposures of preexisting flood-plain sediments 

(figs . 17, 18) . Of the many exposures scattered through 

out the Miller City area, none contained thick sand 

deposits like those that resulted from the 1993 flood. 

Although the history of channel changes in the Miller 

City area attests to an extremely dynamic channel system (fig . 10), the apparent lack of thick sand in the 

stratigraphic record indicates that the geomorphic 

effects of the 1993 flood were unprecedented during the 

Holocene. The geomorphic effectiveness of the 1993 

flood in the levee-break complex was far greater than 

would be expected from either the estimated 100-year 

recurrence interval of the flood at St. Louis (Parrett and 

others, 1993) or the 75- to 300-year recurrence intervals 

estimated for the precipitation conditions that created 

the flood (Interagency Floodplain Management Review 

Committee, 1994; Kunkel and others, 1994). The large 

hydraulic head artificially maintained by the levee and 

the lack of energy dissipation on the dominantly agricultural land probably contributed to much greater geomorphic change in the levee-break complex than would 

have resulted from a flood of comparable magnitude on 

wooded, nonleveed bottomland. 

Figure 18. Typical pre-1993 flood sediment deposit, Miller 

City, Illinois, levee-break complex . 

SUMMARY 

Geomorphic changes at levee-break complexes 

were the most dramatic geomorphic effects of the 1993 

flood on the Mississippi and the Missouri Rivers. Erosion 

and sedimentation at the levee-break complexes severely 

damaged large areas of formerly productive agricultural 

land. Discharge through the Miller City levee-break complex was as much as 28 percent of the total Mississippi 

Summary 21 
River flow during the peak of the 1993 flood. Measured velocities in the levee break were as high as 

300 cm/s. 

The levee-break complex at Miller City is one of 

the larger features created during the 1993 flood and has 

many characteristics that are typical of levee-break complexes on the upper Mississippi and Missouri Rivers. 

Erosion created a steep-sided scour as much as 20 m 

deep that extended for more than 2,000 m downstream 

from the levee break. Deep, narrow secondary scours 

radiated outward in areas of concentrated flow. Concentric around the downstream end of the scour were areas 

where from 10 to 45 cm of soil had been stripped ; these 

areas accounted for about 1 .8 km2. Sand deposits in 

excess of 30 cm thickness covered slightly more than 22 

km2. 

Net deposition on the flood plain at Miller City 

was estimated conservatively to be from 11 .3 to 18.4 million metric tons, or from 22 to 36 percent of the 1993 

flood sediment load measured at Thebes. Although the 

history of the Mississippi River channel in this area 

attests to frequent, dynamic channel changes, the stratigraphic record does not seem to contain sand units comparable to those deposited during 1993 . The extensive 

erosion and even greater deposition may be attributable 

to the magnitude of the flood, the hydraulic head artificially increased by the levee, and the lack of energy dissipation on the agricultural flood plain relative to 

presettlement, forested conditions . 

REFERENCES CITED 

Bhowmik, N.G., Buck, A.G., Changnon, S.A., Dalton, R.H., 

Durgunoglu, A., Demissie, M., Juhl, A.R., Knapp, 

H.V, Kunkel, K.E ., McConkey, S .A., Scott, R.W., 

Singh, K.P., Soong, T.D., Sparks, R.E ., Visocky, A.P., 

Vonnahme, D.R., and Wendland, WM., 1994, The 

1993 flood on the Mississippi River in Illinois : Urbana, 

Illinois State Water Survey, Miscellaneous Publication 

151, 149 p. 

Collinson, J .D., 1978, Alluvial sediments, in Reading, H.G., 

ed., Sedimentary environments and facies : New York, 

Elsevier, p. 39. 

Fisk, H.N., 1944, Geological investigation of the alluvial 

valley of the lower Mississippi River: Vicksburg, Mississippi, U.S . Army Corps of Engineers, 78 p., 

27 maps. 

Gordon, R.L ., 1989, Acoustic measurement of river discharge : Journal of Hydraulic Engineering, American 

Society of Civil Engineers, v. 117, no. 7, p. 925-936. 

Holmes, R.R., Jr., 1996, Sediment transport in the lower 

Missouri and central Mississippi Rivers during the 

1993 flood, in Floods in the upper Mississippi River 

Basin, 1993: U.S . Geological Survey Circular 1120-I. 

23 p. 

Interagency Floodplain Management Review Committee, 

1994, Sharing the challenge-Floodplain management 

into the 21st century : U.S . Government Printing Office, 

191 p. 

Kunkel, K.E ., Changnon, S.A., and Angel, J .R., 1994, Climatic aspects of the 1993 upper Mississippi River 

Basin flood: Bulletin of the American Meteorological 

Society, v. 75, p. 811-822. 

Oberg, K.A., and Mueller, D.S ., 1994, Recent applications 

of acoustic Doppler current profilers, in Fundamentals and advancements in hydraulic measurements 

and experimentation: Hydraulic Engineering `94, 

American Society of Civil Engineers, Buffalo, N.Y, 

August 1-5, 1994, Proceedings, p. 341-350. 

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

Flood discharges in the upper Mississippi River Basin, 

1993, in Floods in the upper Mississippi River Basin, 

1993: U.S . Geological Survey Circular 1120-A, 14 p. 

Reed, H.K., Perkins, T.J ., and Gray, G.L., 1994, Water 

resources data for Missouri, water year 1993: U.S . Geological Survey Water-Data Report MO-93-1, 272 p. 

Simpson, M.R., and Oltman, R.N., 1993, Discharge measurement system using an acoustic Doppler current 

profiler with applications to large rivers and estuaries : 

U.S . Geological Survey Water-Supply Paper 2395, 

32 P. Smoot, G.E, and Novak, C.E., 1969, Measurement of discharge by moving boat methods: U.S . Geological Survey Techniques for Water-Resources Investigations, 

book 3, chap. A11, 22 p. 

Vance, J .M., 1994, Plugging up the drain : Missouri Conservationist, v. 55, no. 8, p. 22-27. 

Wahl, K.L., Vining, K.C., and Wiche, G.J., 1993, Precipitation in the upper Mississippi River Basin, January 1 

through July 31, 1993, in Floods in the upper Mississippi River Basin, 1993 : U.S . Geological Survey Circular 1120-B, 13 p. 

22 Geomorphic Changes of the Mississippi River Flood Plain at Miller City, Illinois, as a Result of the Flood of 1993