Introduction

The city of Decorah, Iowa, has experienced severe flooding from the Upper Iowa River 

resulting in property damage to homes and businesses. Streamflow data from two U.S. Geological Survey (USGS) streamflow-gaging stations, the Upper Iowa River at Decorah, Iowa (station 

number 05387500), located upstream from the College Drive bridge; and the Upper Iowa River 

near Decorah, Iowa (station number 05388000), at the Clay Hill Road bridge (locally known as 

the Freeport bridge) were used in the study. The three largest floods on the Upper Iowa River 

at Decorah occurred in 1941, 1961, and 1993, for which the estimated peak discharges were 

27,200 cubic feet per second (ft3/s), 20,200 ft3/s, and 20,500 ft3/s, respectively. Flood-discharge 

information can be obtained from the World Wide Web at URL (uniform resource locator) 

http://waterdata.usgs.gov/nwis/. In response to the need to provide the City of Decorah and 

other flood-plain managers with an assessment of the risks of flooding to properties and facilities along an 8.5-mile (mi) reach of the Upper Iowa River, the USGS, in cooperation with the 

City of Decorah, initiated a study to map 100- and 500-year flood-prone areas.

Purpose and Scope

The purpose of this report is to describe a flood-plain study of the Upper Iowa River in the 

vicinity of Decorah, Iowa. Results of the study provide Decorah with sufficient data to describe 

flood hazards along an 8.5-mi reach of the Upper Iowa River. Community and local planners 

can use this report to evaluate hazards and manage the flood plain in Decorah. The study area 

and study reach are shown in figure 1. 

Study Area Description

The Upper Iowa River Basin is located in northeast Iowa and southeast Minnesota (fig. 1). 

The Upper Iowa River headwaters begin in southeast Minnesota, and the river flows through 

extreme northeast Iowa to the mouth at the Mississippi River. The study area of the Upper Iowa 

River near Decorah extends from about 0.75 mi upstream from the U.S. Highway 52 bridge to 

about 1 mi downstream from the Clay Hill Road bridge (fig. 1). 

The drainage area of the Upper Iowa River is 511 square miles (mi2) near the College Drive 

bridge, and is 568 mi2 at the Clay Hill Road bridge (Larimer, 1957). The flood plain near Decorah is characterized by narrow, bluff-walled valleys in which the flood plain is constricted to less 

than 300 feet (ft) in width (Iowa Natural Resources Council, 1958). 

The slope of the Upper Iowa River is 6.5 feet per mile (ft/mi) along the study reach. Channel banks along the study reach are steep-sloped and lined with trees and vegetation. Land use 

along the study-reach flood plain is predominately urban, with some agricultural fields at the 

upstream and downstream ends of the reach.

Hydrologic Analysis

Standard hydrologic analysis methods were used to determine the flood-hazard data 

required for the Upper Iowa River flood-plain study. Two flood events of a magnitude that are 

expected to be equaled or exceeded once, on average, during any 100- or 500-year period (recurrence interval) were selected as having special significance for flood-plain management. These 

two flood events, commonly termed the 100- and 500-year floods, have a 1 and 0.2 percent 

chance, respectively, of being equaled or exceeded during any year. Although the recurrence 

interval represents the average long-term period between floods of a specific magnitude, rare 

floods could occur at shorter intervals, or even within the same year. 

Hydrologic analyses were performed to estimate flood-frequency discharges for selected 

recurrence intervals for the Upper Iowa River flood-plain study. The 100- and 500-year floodfrequency discharge estimates computed for three sites along the Upper Iowa River are listed 

in table 1. Flood-frequency discharge estimates were computed from a statistical analysis of 

annual-peak discharge records collected during 84 years at two USGS streamflow-gaging stations (1952–2006 for station 05387500 and 1914, 1919–27, 1933–1952 for station 05388000). 

Statistical analysis followed the standard log-Pearson Type III method as outlined by the Interagency Advisory Committee on Water Data (1982). 

Hydraulic Analysis

The Hydrologic Engineering Center’s River Analysis System (HEC-RAS, version 4.0 

Beta), a one-dimensional, hydraulic-flow model developed by the U.S. Army Corps of Engineers (USACE) (U.S. Army Corps of Engineers, 2006), was used to model the study reach 

of the Upper Iowa River near Decorah. The HEC-RAS program was designed specifically 

for application in flood-plain management and flood-insurance studies to evaluate floodway 

encroachment and to simulate estimated flood inundation (U.S. Army Corps of Engineers, 

2006). The HEC-RAS model was used to compute water-surface elevations and develop 

flood-inundation areas along the Upper Iowa River study reach for the 100- and 500-year flood 

discharges (table 1). 

Table 1. Flood-frequency estimates for the Decorah flood-plain study.

[ft3/s, cubic feet per second]

Peak discharges (ft3/s)

Flooding source and location Drainage area in square miles 100-Year 500-Year

Upper Iowa River at U.S. Highway 

52, Decorah

473 22,600 29,400

Upper Iowa River at Decorah 

(station number 05387500) near 

College Drive bridge, Decorah

511 23,400 30,400

Upper Iowa River near Decorah 

(station number 05388000) 

Clay Hill Road bridge (Freeport 

bridge), Decorah

568 24,600 32,000

The City of Decorah provided topographic data for the flood study that included 41 stream 

channel cross sections, 2-ft and 1-ft elevation contours within the study area (determined by 

aerial photogrammetric surveys and land surveys through the river channel), and bridge plans 

(Clay Hill Road, 5th Street, College Drive, and U.S. Highway 52) for this study. WHKS & Co, 

Mason City, Iowa, provided all cross section and model data for the 5th Avenue bridge (L.A. 

Narigon, P.E., WHKS & Co., written commun., 2007).

The HEC-GeoRas (U.S. Army Corps of Engineers, 2005) computer program was used to 

extract flood-plain cross sections from the elevation-contour data provided by the City of Decorah. HEC-GeoRas is a package of geographic information system (GIS) computer code specifically designed to view and manipulate geospatial data for use in HEC-RAS computer modeling. 

A total of 186 cross sections were extracted about every 200 ft along the study reach. Extracted 

cross sections located at or near the measured cross sections, were compared to the measured 

sections for quality assurance purposes. Extracted cross sections were extended to the lateral 

limit of the study area using the photogrammetric survey data. A typical HEC-RAS cross section 

and profile are shown in figures 2 and 3. Results from the HEC-RAS model were imported into 

ArcGIS (Environmental Systems Research Institute, Inc., 2004) using HEC-GeoRAS post processor routines. The 100- and 500-year flood-inundation areas and 100-year floodway boundary were then created using water-surface elevations interpolated from the HEC-GeoRas post 

processor in ArcGIS. The floodway is defined as the channel of a river, or other watercourse, 

and the adjacent land areas that must be reserved in order to discharge the 100-year flood without cumulatively increasing the water-surface elevation by more than a designated height of 1 ft 

(Federal Emergency Management Agency (FEMA), 2003).

Water-surface elevations for the 100- and 500-year floods and the 100-year floodway 

boundary, computed at all cross sections from the HEC-RAS model, were used by HECGeoRAS routines to intersect with the elevation-contour data provided by City of Decorah. The 

results of the intersection of the water-surface elevations and elevation-contour data are the 100- 

and 500-year flood-inundation areas and the 100-year floodway boundary (fig. 1). 

Limitations and errors associated with developing flood-inundation areas are dependent 

on the topographic data, hydrologic data, and one-dimensional hydraulic modeling used in a 

flood-plain study. A USGS review of the elevation-contour data provided by the City of Decorah 

determined that the FEMA elevation accuracy requirement of 1.2 ft, at a 95-percent confidence 

level, may be met for the 2-ft contour data (Federal Emergency Management Agency (FEMA), 

2003).

Model results indicate that the present levee system through Decorah retains the 100- 

and 500-year floods throughout the main sections of the city. Model results indicate that the 

500-year flood may overtop the levee reach on the east bank upstream of the 5th Avenue bridge. 

The 100- and 500-year flood-inundation areas may be better defined by using more detailed 

topographic data, such as LIDAR (Light Detection and Ranging), in the future (Bales and others, 2007).

City of Decorah

Clay Hill Road

5th Street

5th Avenue

College

Drive

U.S. Highway 52

Highway 9

Clay Hill Road

5th Street

5th Avenue

College

Drive

U.S. Highway 52

Highway 9

Cross Section No. 87

#

#

05387500

05388000 05387500

91°49’ 91°48’ 91°47’ 91°46’ 91°45’ 91°44’

43°20’

43°19’

43°18’

43°17’

100-year floodway

100-year flood inundation area

500-year flood inundation area

Study area boundary

Study reach

Base from U.S. Department of Agriculture, 

National Agricultural Imagery Program (NAIP), 2006

Universal Transverse Mercator projection

Zone 15

05387500#

EXPLANATION

0 1 KILOMETER

0 1 MILE

U.S. Geological Survey stream-flow

 gaging station and number

Upper Iowa River Basin

IOWA

Decorah

Summary

Flood-frequency discharges, water-surface elevation profiles, flood-inundation areas, and a 

floodway boundary were computed in cooperation with the City of Decorah, Iowa, for an 8.5-mi 

reach of the Upper Iowa River in the vicinity of Decorah. The City of Decorah can use this flood 

study to manage flood-prone areas along the Upper Iowa River in the vicinity of Decorah. Maps 

depicting the 100- and 500-year flood-inundation areas and the 100-year floodway boundary 

were estimated for the study reach using land-surveyed and aerially-surveyed elevation data, and 

one-dimensional HEC-RAS modeling.

References Cited

Bales, J.D., Wagner, C.R., Tighe, K.C., and Terziotti, Silvia, 2007, LiDAR-derived flood-inundation 

maps for real-time flood-mapping applications, Tar River basin, North Carolina: U.S. Geological 

Survey Scientific Investigations Report 2007–5032, 42 p.

Environmental Systems Research Institute, Inc., 2004, Getting to know ArcGIS Desktop: Redlands, 

California, various pagination. 

Federal Emergency Management Agency (FEMA), April 2003, Guidelines and specifications for 

flood hazard mapping partners: available online at URL < http://www.fema.gov/hazard/flood/

info.shtm >.

Interagency Advisory Committee on Water Data, 1982, Guidelines for determining flood flow frequency: Hydrology Subcommittee Bulletin 17B, 28 p. and appendixes. 

Iowa Natural Resources Council, 1958, An Inventory of Water Resources and Water Problems, northeastern Iowa River Basins: Des Moines, Iowa, Iowa Natural Resources Council Bulletin No 7, 

74 p.

Larimer, O.J., 1957, Drainage Areas of Iowa Streams: Iowa Research Board Bulletin No. 7 (reprinted 

1974), 421 p.

U.S. Army Corps of Engineers, 2005, Hydrologic Engineering Center HEC-GeoRAS, an application 

for support of HEC-RAS using ARC/INFO: User Manual, version 4.0, various pagination.

U.S. Army Corps of Engineers, 2006, Hydrologic Engineering Center HEC-RAS river analysis system: Hydrologic User Manual, version 4.0 beta, various pagination.

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

Prepared in cooperation with the

CITY OF DECORAH, IOWA

SCIENTIFIC INVESTIGATIONS MAP 3005

Christiansen, D.E., and Eash, D.A., Flood-plain study of the 

Upper Iowa River in the vicinity of Decorah, Iowa

Figure 1. Location of the study area, 100-year floodway, and 100- and 500-year flood boundaries on the Upper Iowa River, near Decorah, Iowa.

Flood-Plain Study of the Upper Iowa River in the Vicinity of Decorah, Iowa

By

Daniel E. Christiansen and David A. Eash

2008

Figure 2. Typical HEC-RAS cross section 87, looking downstream.

Figure 3. Typical HEC-RAS profile.

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2) 2.590 square kilometer (km2) 

Flow rate

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

Hydraulic gradient

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

Conversion Factors

0 200 400 600 800 1,000

840

845

850

855

860

865

870

DISTANCE FROM LEFT EDGE LOOKING DOWNSTREAM, IN FEET

ELEVATION, IN FEET

Main

channel

Cross Section No. 87

Levee

Calculated water surface for 500-year flood discharge

Calculated water surface for 100-year flood discharge

Ground

0 10,000 20,000 30,000 40,000 50,000

810

820

830

840

850

860

870

880

890

MAIN CHANNEL FROM DOWNSTREAM (RIGHT), TO UPSTREAM (LEFT), IN FEET 

ELEVATION, IN FEET 

West East

Calculated water surface for 500-year flood discharge

Calculated water surface for 100-year flood discharge

Ground

Cross Section No. 87

Levee

Bridge

Upper 

Iowa River

U.S. Highway 525th AvenueCollege DriveTwin Bridges at 5th StrteetFreeport Bridge at Clay Hill Road

Map Jacket Photo. Upper Iowa River looking upstream of 5th Street bridge. Photograph by D.E. 

Christiansen, U.S. Geological Survey.