U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2012–5175

Prepared in cooperation with the U.S. Department of Agriculture, 

Natural Resources Conservation Service

Method for Estimating Potential Wetland Extent by Utilizing 

Streamflow Statistics and Flood-Inundation Mapping 

Techniques: Pilot Study for Land Along the Wabash River 

Near Terre Haute, Indiana
Cover images: Background photograph of a flood plain of the Muscatatuck River in Jackson County, Indiana, 2002, by Gerald (Jerry) 

L. Roach, U.S. Department of Agriculture, Natural Resources Conservation Service; top left photograph of a great blue heron in a flood 

plain of the Muscatatuck River, 2008, by Bob J. Steiner, Soil and Water Conservation District, Jennings County, Indiana; center photograph of part of a map of approximate flood extents for the Wabash River near Terre Haute, Indiana (the entire map is figure 3 of this 

report); lower right photograph of a North American river otter on a beaver lodge by Chris R. Reidy, U.S. Department of Agriculture, 

Natural Resources Conservation Service.
Method for Estimating Potential Wetland 

Extent by Utilizing Streamflow Statistics 

and Flood-Inundation Mapping Techniques: 

Pilot Study for Land Along the Wabash 

River Near Terre Haute, Indiana

By Moon H. Kim, Christian T. Ritz, and Donald V. Arvin

Prepared in cooperation with the U.S. Department of Agriculture, 

Natural Resources Conservation Service

Scientific Investigations Report 2012–5175

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

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Suggested citation:

Kim, M.H., Ritz, C.T., and Arvin, D.V., 2012, Method for estimating potential wetland extent by utilizing streamflow 

statistics and flood-inundation mapping techniques—Pilot study for land along the Wabash River near Terre Haute, 

Indiana: U.S. Geological Survey Scientific Investigations Report 2012–5175, 15 p., 1 app.
iii

Acknowledgments

The authors thank the many agencies and individuals that have contributed invaluable 

information to this report. The following individuals are acknowledged for their extensive technical support, expertise, and advice during all phases of this study: Kenneth H. Collins, Claudia 

C. Hoeft, Christopher E. Morse, Gerald L. Roach, David A. Stratman, and Kenneth T. Streett of 

the U.S. Department of Agriculture, Natural Resources Conservation Service, and Suzanne R. 

Delay, David B. Knipe, and Darrin K. Miller of the Indiana Department of Natural Resources, 

Division of Water. 

The authors acknowledge Richard A. Weber of the U.S. Department of Agriculture, 

Natural Resources Conservation Service, and John B. Storm of the U.S. Geological Survey 

(USGS) for their thorough and timely review of technical aspects of this study. Mike Eberle 

of the USGS is acknowledged for editorial review of the report. The many hydrologists and 

hydrologic technicians from the USGS Indiana Water Science Center who assisted with various 

aspects of the project also are acknowledged.

v

Contents

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

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

Purpose and Scope ..............................................................................................................................2

Description of Study Area ...................................................................................................................3

Methods...........................................................................................................................................................3

Compilation of Streamflow Statistics ................................................................................................3

Estimation of Water-Surface Elevations...........................................................................................5

Mapping of Wetland Extents...............................................................................................................8

Utility of the Methods for Future Studies ................................................................................................10

Summary........................................................................................................................................................10

References Cited..........................................................................................................................................11

Glossary.........................................................................................................................................................13

Appendix 1.....................................................................................................................................................15

Figures

1. Map showing location of study area along the Wabash River near Terre Haute, 

Indiana............................................................................................................................................4

2. Graph showing example of historical flood profile along the Wabash River near 

Terre Haute, Indiana.....................................................................................................................6

3. Map showing estimated inundation zone within the study area along the 

Wabash River near Terre Haute, Indiana .................................................................................9

Tables

1. Calculated values of inundation flows and corresponding inundation elevations 

at selected U.S. Geological Survey streamgages along the Wabash River in and 

near Indiana...................................................................................................................................5

2. Inundation elevations within the study area along the Wabash River near 

Terre Haute, Indiana.....................................................................................................................8
vi

Conversion Factors and Datums

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

meter (m) 3.281 foot (ft)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter (m2

)

acre 0.4047 hectare (ha)

acre 0.004047 square kilometer (km2

)

square mile (mi2

) 259.0 hectare (ha)

square mile (mi2

) 2.590 square kilometer (km2) 

Slope

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

Flow rate

cubic foot per second (ft3

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

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 

(NGVD 29).

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

83).

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

7MQ2 Annual highest 7-consecutive-day mean discharge 

with a 2-year recurrence interval

DEM Digital Elevation Model

FEMA Federal Emergency Management Agency

FIS Flood Insurance Study

GIS Geographic Information System

HWM High-water mark

IDNR Indiana Department of Natural Resources

IWCP Indiana Wetlands Conservation Plan

NRCS Natural Resources Conservation Service

NWIS National Water Information System

USDA U.S. Department of Agriculture

USGS U.S. Geological Survey

WRP Wetland Reserve Program

WREP Wetlands Reserve Enhancement Program

Abbreviations and Acronyms
Method for Estimating Potential Wetland Extent by 

Utilizing Streamflow Statistics and Flood-Inundation 

Mapping Techniques: Pilot Study for Land Along the 

Wabash River Near Terre Haute, Indiana

By Moon H. Kim1, Christian T. Ritz2, and Donald V. Arvin1

Abstract 

Potential wetland extents were estimated for a 14-mile 

reach of the Wabash River near Terre Haute, Indiana. This 

pilot study was completed by the U.S. Geological Survey in 

cooperation with the U.S. Department of Agriculture, Natural 

Resources Conservation Service (NRCS). The study showed 

that potential wetland extents can be estimated by analyzing streamflow statistics with the available streamgage data, 

calculating the approximate water-surface elevation along the 

river, and generating maps by use of flood-inundation mapping 

techniques.

Planning successful restorations for Wetland Reserve 

Program (WRP) easements requires a determination of areas 

that show evidence of being in a zone prone to sustained or 

frequent flooding. Zone determinations of this type are used 

by WRP planners to define the actively inundated area and 

make decisions on restoration-practice installation. According 

to WRP planning guidelines, a site needs to show evidence 

of being in an “inundation zone” that is prone to sustained or 

frequent flooding for a period of 7 consecutive days at least 

once every 2 years on average in order to meet the planning 

criteria for determining a wetland for a restoration in agricultural land. By calculating the annual highest 7-consecutiveday mean discharge with a 2-year recurrence interval (7MQ2) 

at a streamgage on the basis of available streamflow data, one 

can determine the water-surface elevation corresponding to 

the calculated flow that defines the estimated inundation zone 

along the river.

By using the estimated water-surface elevation (“inundation elevation”) along the river, an approximate extent of 

potential wetland for a restoration in agricultural land can be 

mapped. As part of the pilot study, a set of maps representing 

the estimated potential wetland extents was generated in a 

geographic information system (GIS) application by combining (1) a digital water-surface plane representing the surface of 

inundation elevation that sloped in the downstream direction 

of flow and (2) land-surface elevation data. These map products from the pilot study will aid the NRCS and its partners 

with the onsite inundation-zone verification in agricultural 

land for a potential restoration and will assist in determining at 

what elevation to plant hardwood trees for increased survivability on ground above frequently flooded terraces.

Introduction

Wetlands are defined as a transitional environment 

between water bodies and dry land, and they represent a 

substantial part of the Nation’s natural resources. Over a 

period of about 200 years, from the 1780s to the 1980s, the 

total estimated loss of wetlands in the conterminous United 

States was about 53 percent, from about 221 million acres to 

about 103 million acres. Indiana lost about 85 percent of its 

wetlands, from about 5.6 million acres to about 813,000 acres, 

during the same time period (Dahl, 1990; Indiana Department 

of Natural Resources, 1996). For many years, wetlands were 

classified as wastelands that could be made useful by draining 

and filling for other purposes. In Indiana, the loss of wetlands was mostly due to drainage for agricultural production 

(Indiana Department of Natural Resources, 1996). To this day, 

interests in industrial, agricultural, and residential development in Indiana continue to drive the conversion of wetlands 

by draining, filling, dredging, diking, and damming of the 

landscape. Remaining wetlands in Indiana are being lost at a 

rate of about 1 to 3 percent each year, mainly because of drainage for agricultural purposes (U.S. Geological Survey, 1996). 

Wetlands serve many ecological and hydrological functions that benefit surrounding ecosystems. One of the best 

known functions of wetlands is to provide habitat for waterfowl, fish, and other terrestrial and aquatic animals. Wetlands 

support a wide variety of plant life, and they supply resting 

1

 U.S. Geological Survey, Water Science Center, Indianapolis, Indiana.

2

 U.S. Department of Agriculture, Natural Resources Conservation Service, 

Indianapolis, Indiana.
2 Method for Estimating Potential Wetland Extent by Utilizing Streamflow Statistics and Flood-Inundation Mapping Techniques

and feeding places for migrating birds, as well as food, shelter, 

breeding areas, and nurseries for many animals. By filtering many contaminants out of inflowing water and storing 

or transforming them, wetlands can prevent or reduce water 

pollution. Flood control can be achieved near riparian3

wetlands by temporary water storage and gradual release, 

which together reduce flow velocity and flood peaks. Vegetation in riparian wetlands helps to maintain stream channels 

by stabilizing the land surface. In Indiana, wetlands provide 

considerable value for recreational, educational, and economic 

activities. Common activities in and surrounding wetlands 

include birding, hiking, fishing, hunting, swimming, and 

boating. Economic benefits are provided by wetlands through 

such activities as fur trapping, lumbering, and tourism (U.S. 

Geological Survey, 1996).

During the past few decades, as the function of wetlands 

and their value to society and the environment have become 

better understood, the interest in conservation of wetlands has 

increased. In 1996, the Indiana Wetlands Conservation Plan 

(IWCP) was developed through coordination by the Indiana 

Department of Natural Resources (IDNR) and funding from 

the U.S. Environmental Protection Agency. The IWCP is 

intended as a guide for all wetland conservation efforts in the 

State and encourages participation from all levels of government, private organizations, and the general public (Indiana 

Department of Natural Resources, 1996). At the Federal level, 

several agencies, including the U.S. Army Corps of Engineers, 

U.S. Fish and Wildlife Service, and National Park Service, 

participate in many wetland-related activities. These activities 

include management, regulation, restoration, creation, delineation, and inventory of wetlands. 

The Wetland Reserve Program (WRP), created by the 

1990 Food, Agriculture, Conservation, and Trade Act, is a voluntary program that provides technical and financial assistance 

to private landowners to restore, protect, and enhance wetlands 

in exchange for retiring eligible land from agriculture (U.S. 

Department of Agriculture, 2012). Since 1994, Indiana has 

been involved in the WRP, which is administered under the 

U.S. Department of Agriculture (USDA), Natural Resources 

Conservation Service (NRCS). To assist with this program, 

in 2005, funding was awarded for use in Indiana through the 

Wetlands Reserve Enhancement Program (WREP), a partnership program with the NRCS. The WRP and the WREP operate in Indiana with a high level of partnership involvement. 

These partnerships are forming between groups and organizations with a great diversity of backgrounds and purposes and 

have provided technical support, easement acquisition, and 

restoration work. For example, with the WREP, the Nature 

Conservancy has partnered with the NRCS to develop the 

Wabash River Floodplain Corridor for six counties. In Indiana, 

these programs have three focus areas: the northern one-third 

of the State, the counties along the Wabash and White Rivers, and the Muscatatuck River Basin. These areas have been 

identified historically as having the best potential for wetland restoration. Common restoration practices include tree 

planting, direct seeding, ditch plugs, dikes, tile breaks, water 

control structures, and macrotopography restoration (U.S. 

Department of Agriculture, 2008b). 

Landowners interested in participating in the WRP can 

contact their local NRCS office. The land may be eligible for 

participation if it is agricultural land that contains degraded or 

converted wetlands that have a high probability for successful restoration. If the site is selected for participation, then the 

NRCS develops a restoration plan and makes an offer to the 

landowner (U.S. Department of Agriculture, 2012). Existing 

natural wetlands or adjacent uplands also can be enrolled if 

they are ecologically important to the restoration of the site. 

Under the WRP planning guidelines, before a site in 

an agricultural land area can be determined to be a wetland 

and be considered for restoration, several criteria must be 

satisfied regarding vegetation, soils, and hydrology. Planning successful restorations for WRP easements requires 

determination of a frequency-duration parameter, which is 

a key parameter relative to low-lying riparian areas subject 

to inundation (U.S. Department of Agriculture, 1994, 2007). 

This parameter is defined as the annual highest 7-consecutiveday mean discharge with a 2-year recurrence interval (or 

50-percent chance annual probability) at a streamgage, based 

on available streamflow data. Sites that are in a zone showing evidence of sustained or frequent flooding, as described 

above, typically support herbaceous hydrophytic vegetation 

and can be considered for restoration-practice installation 

(U.S. Department of Agriculture, 2008a). Sites located above 

such a zone would be deemed suitable for plantings of hardwood seedlings or other similar restorations. In this report, the 

water-surface elevation that defines the upper limit of this zone 

hereafter is referred to as the “inundation elevation.” The flow 

that is related to the inundation elevation hereafter is referred 

to as the “inundation flow.” The area that is at or below this 

inundation elevation hereafter is referred to as the “inundation 

zone.”

Purpose and Scope

The purpose of this report is to describe and document 

the methods used to estimate the inundation elevation and the 

inundation zone that meet the planning criteria for vegetative 

establishment in the river flood plains by utilizing streamflow 

data and statistics available from the USGS streamgagenetwork database and available flood-inundation mapping 

techniques. The estimation of inundation elevation and 

inundation zone can further assist the NRCS and its partners in 

onsite determination and verification of potential wetlands for 

restoration in agricultural lands and determination of the zone 

above the frequently flooded area that would provide increased 

3

 Terms defined in the glossary are in bold print where first used in the main 

body of the report. 
Methods 3

survivability of planted hardwood trees (U.S. Department of 

Agriculture, 2008a). For the pilot study, the statistical and 

mapping analyses were limited to land along the Wabash River 

near Terre Haute, Indiana (Ind.). The results from the statistical analyses which included data from additional streamgages 

along the Wabash River were made available for future mapping analyses. These analyses relied on existing sources of 

information, with no additional field data collection involved.

Description of Study Area

The study area consisted of a reach of the Wabash River 

about 14 mi in length, along with the contiguous flood plain, 

near the city of Terre Haute (fig. 1). The upstream extent of 

the study area was the State Route 63 crossing of the Wabash 

River (river mile (RM) 217.4), and the downstream extent 

was near the confluence of Honey Creek and the Wabash 

River (RM 203.5). USGS streamgage 03341500 Wabash River 

at Terre Haute, Ind., is within the study area (RM 215.0); 

more than 83 years of streamflow record was available for this 

streamgage at the time of the study. 

The total drainage area of the Wabash River at the 

downstream study extent is about 12,508 mi2. The average 

slope of the channel through the study reach is about 0.6 ft/

mi; channel widths range from about 300 to 600 ft. The mean 

annual streamflow at the streamgage (through water year

2010) was 11,440 ft3/s (water year 2010 is the period October 

1, 2009, through September 30, 2010). The peak streamflow 

for the period of record was 189,000 ft3/s on May 20, 1943. 

Outside the period of record, the flow reached an estimated 

245,000 ft3/s on March 27, 1913. This study area was chosen 

because of availability of high-resolution land-surface elevation data, availability of USGS streamgage data, and potential 

sites for a wetland restoration.

Methods

Whereas the primary focus of this report is on a single 

selected stream reach, similar methods have been used on 

numerous stream reaches throughout the State. Through a 

coordinated effort between the Indiana NRCS and the U.S. 

Geological Survey (USGS) staff, a procedure was developed 

for determining streamflow and water-surface elevation along 

selected stream reaches for the purpose of estimating the 

inundation elevation and the inundation zone that meet the 

planning criteria of the WRP. This procedure consists of three 

phases: (1) compilation of streamflow statistics based on data 

from the available USGS streamgage-network database, (2) 

estimation of water-surface elevations based on the streamflow statistics, and (3) mapping of estimated potential wetland 

extents by use of the estimated water-surface elevations.

Compilation of Streamflow Statistics

The WRP planning guidelines state that in order to meet 

the planning criteria for determining a wetland for a restoration in agricultural land, a site needs to show evidence of 

being in a zone prone to sustained or frequent flooding for 

a period of 7 consecutive days at least once every 2 years 

on average. The evidence can be obtained through statistical analysis of daily mean discharges stored for USGS 

streamgages. The statistical value generated by this analysis 

is the annual highest 7-consecutive-day mean discharge with 

a 2-year recurrence interval. (A 2-year recurrence interval is 

sometimes referred to as having a 50-percent probability of 

occurrence.) This statistical value is referred to in this report 

as the “7MQ2.” The 7MQ2 at a streamgage serves as a determination of the inundation flow at that location. Daily mean 

discharges for the streamgages used in this study are stored in 

the USGS National Water Information System (NWIS). On the 

basis of those daily mean discharges, the set of annual highest 

7-consecutive-day mean discharges was calculated for each 

streamgage. Water year rather than calendar year was used 

as the annual time period for this flow statistic. By separating annual periods at the time of the year when discharges 

generally are low, the high-flow periods can be analyzed with 

a greater degree of continuity. 

The set of annual highest 7-consecutive-day mean 

discharges at each streamgage was used for the frequency 

analysis. Frequency curves relate the magnitude of a variable to the frequency of occurrence (Riggs, 1968). For this 

frequency analysis, the 7-consecutive-day mean discharge 

time-series data were analyzed by using a log-Pearson TypeIII distribution, as implemented in the USGS software package 

SWSTAT (U.S. Geological Survey, 2011). This frequency 

analysis produced the 7MQ2, which was used as the inundation flow. Resulting data for selected streamgages along the 

Wabash River in and near Indiana are listed in table 1. In order 

for the streamflow data to represent current conditions, the 

dataset included only those years after installation of floodcontrol reservoirs upstream of the study area (Ruddy and 

Hitt, 1990). Locations and other information regarding the 

streamgages can be found at http://waterdata.usgs.gov/in/nwis/

current/?type=flow.
4 Method for Estimating Potential Wetland Extent by Utilizing Streamflow Statistics and Flood-Inundation Mapping Techniques

87°30' 87°28' 87°26' 87°24'

39°30'

39°28'

39°26'

39°24'

0 0.5 1 MILE

0 0.5 1 KILOMETER

INDIANA Study

area

USGS streamgage and number

Interstate route marker

Flow arrow—Indicates direction

of water flow

03341500

EXPLANATION

Wabash

Figure 1.

Orthophotography from Indiana University, Indiana Spatial Data Portal, Vigo County, 2010 NAIP

Universal Transverse Mercator Zone 16 projection

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD83)

63

150

70

70

40

40

150

41

41

03341500

State route marker

U.S. route marker

70

40

63

Terre

Haute

West Terre

Haute River

Wabash

Clear

Creek

Honey

Creek

Figure 1. Location of study area along the Wabash River near Terre Haute, Indiana.
Methods 5

Table 1. Calculated values of inundation flows and corresponding inundation elevations at selected U.S. Geological Survey 

streamgages along the Wabash River in and near Indiana.

[ft, feet; ft3/s, cubic feet per second; NGVD 29, National Geodetic Vertical Datum of 1929]

USGS 

streamgage 

number

USGS 

streamgage 

name

Years 

of record

Period of record* 

(water year)

Inundation flow 

(ft3

/s)

Corresponding 

gage height (ft)

Inundation 

elevation (ft 

above NGVD 29)

03327500 Wabash River at 

Peru, Ind.

43 1968–2010 11,000 10.2 628.1

03329000 Wabash River at 

Logansport, Ind.

43 1968–2010 16,500 9.0 582.3

03335500 Wabash River at 

Lafayette, Ind.

43 1968–2010 34,300 17.1 521.2

03336000 Wabash River at 

Covington, Ind.

43 1968–2010 38,110 22.6 496.6

03340500 Wabash River at 

Montezuma, Ind.

43 1968–2010 55,500 23.6 481.3

03341500 Wabash River at 

Terre Haute, Ind.

43 1968–2010 56,800 21.3 467.1

03342000 Wabash River at 

Riverton, Ind.

43 1968–2010 61,700 22.2 436.8

03377500 Wabash River at 

Mt. Carmel, Ill.

43 1968–2010 137,000 26.6 396.1

*

 The period of record used in the data analysis represents the water years after the upstream reservoirs for flood control came online along the Wabash 

River in Indiana.

Estimation of Water-Surface Elevations

Once the inundation flow (the 7MQ2) was calculated for 

a selected streamgage, the water-surface elevation corresponding to that flow value was established as the inundation elevation at that streamgage location. The water-surface elevation 

was determined from the stage-discharge relation (referred to 

as the “rating”) at the streamgage, which was developed from 

periodic measurements of discharge and stage (Rantz and 

others, 1982). For this study, the most recent rating in effect 

was used for determining the inundation elevation; negligible 

benefit was expected from attempting to account for changes 

in the rating over time.

For sites in low-lying ground adjacent to a stream but not 

near a streamgage, additional resources were used to estimate 

the inundation elevation. A primary resource was the IDNR’s 

historical flood profiles (fig. 2), which are available for many 

of the streams throughout the State and which plot the elevation of surveyed high-water marks against the RM for specific 

historical floods. The graphs show the water-surface elevation 

profiles of those documented floods.

If data for a stream are available from both a streamgage 

and a historical flood profile, then (1) the inundation elevation can be determined at the streamgage location by using 

data from that streamgage, (2) that elevation can be plotted in 

context with the elevations of the high-water marks of the historical floods at the streamgage location, and (3) the relation 

between the inundation elevation and the historical floods can 

be extended downstream or upstream from the site of interest 

by extrapolating graphically by RM. This method basically 

draws an inundation-elevation profile that is approximately 

parallel to the historical flood-elevation profiles.

For example, the IDNR’s historical flood profiles for the 

Wabash River include water-surface elevation lines for the 

floods of March 1913, March 1939, and January 1969. An 

inundation elevation needed to be estimated for a site adjacent 

to the Wabash River at RM 203.5. The USGS streamgage 

03341500 Wabash River at Terre Haute, Ind., is at RM 215. 

The inundation flow was determined at RM 215 by calculating 

the 7MQ2, based on the daily mean discharge data from the 

streamgage, and the inundation elevation of the water surface 

was determined by identifying the elevation that corresponds 

with that discharge value, based on rating established for the 

streamgage. The inundation elevation at RM 215 was plotted 

on the historical flood profile to show the relation between that 

elevation and the elevations of the three documented historical 

floods. That relation shown at RM 215, where the streamgage 

is located, was then transferred downstream to RM 203.5, 

where the site of interest is located.
440 445 450 455 460 465 470 475 480 485 490180 185 190 195 200 205 210 215 220 

Elevation (feet, NGVD, 1929)

Miles From Mouth

May 1943January 1969May 1933March 1913March 1939February 1959

Departm

W

Figure 2.

Figure 2. Example of historical flood profile along the Wabash River near Terre Haute, Indiana.
Methods 7

For some WRP sites of interest, a historical flood profile 

may not be available. If a site is adjacent to a stream and 

between two streamgages, a linear interpolation method can be 

used to determine a general range for the inundation elevation. Two basic methods of linear interpolation were used in 

this study. One method involved assessing the rate of change 

in elevation per RM between the two streamgages and then 

applying that rate of change to assign an estimated elevation at 

the site of interest. This method can be described by using the 

following equation:

y

y yxx

x x

y site

site = ( ) − × −

− + 2 1 1

2 1

1

( )

( )

(1)

where

x1 is the RM at the downstream streamgage, 

x2 is the RM at the upstream streamgage, 

xsite is the RM at the site of interest, 

y1

 is the inundation elevation at the downstream 

streamgage, 

y2 is the inundation elevation at the upstream streamgage, 

and 

ysite is the resulting inundation elevation at the site of 

interest.

A second method that employed linear interpolation was 

particularly useful for estimating the upper range of elevation 

one might expect at the site of interest. In this second method,

the drainage area was used instead of RM. This method can be 

described by using the following equation:

y

y y d d

d d y site

site = ( ) − × −

− + 2 1 1

2 1

1

( )

( )

(2)

where

d1 is the drainage area at the downstream streamgage, 

d2 is the drainage area at the upstream streamgage, 

dsite is the drainage area at the site of interest, 

y1 is the inundation elevation at the downstream 

streamgage, 

y2

 is the inundation elevation at the upstream streamgage, 

and 

ysite is the resulting inundation elevation at the site of 

interest.

Use of a linear interpolation method has limitations as 

a stand-alone tool. Although such a method can be useful in 

obtaining a general range for the expected inundation elevation at a site of interest, especially if other supporting data are 

unavailable, the method does not necessarily reflect the actual 

hydraulic characteristics of the individual stream segments. 

Experience has shown that results derived from linear interpolation can differ from those derived by using the historical 

flood profile method. For this reason, the final results can be 

improved by making elevation adjustments based on Federal 

Emergency Management Agency (FEMA) Flood Insurance 

Study (FIS) flood maps and profiles. These maps are used in 

the administration of the National Flood Insurance Program 

and in the regulation of flood plains in Indiana.

The FIS flood maps and profiles, updated and improved 

in recent years by cooperative work between FEMA and the 

IDNR, show the elevation of the 1-percent probability flood 

(sometimes referred to as the 100-year recurrence interval). 

These maps are available for many of the major streams 

throughout Indiana. The elevation of the 1-percent probability 

flood shown in the FIS flood maps and profiles can be used to 

calibrate results of the linear interpolation method in a similar 

way that the historical flood profiles are used.

Another useful source of information for this effort was 

Indiana StreamStats (http://water.usgs.gov/osw/streamstats/

indiana.html, accessed October 21, 2011). StreamStats is a 

national USGS program (http://streamstats.usgs.gov/index.

html) through which an Indiana statewide Web-based graphical application was developed as a cooperative effort between 

the USGS and the IDNR. This tool was used to determine 

the drainage area for selected sites of interest, the boundary 

delineations for those drainage basins, and flood-frequency 

discharge determinations for those basins. For the purposes of 

this study, the products of StreamStats found to be particularly 

applicable were the basin boundary delineations, the discharge 

of the 10-percent probability flood (sometimes referred to 

as the 10-year recurrence interval), and the discharge of the 

1-percent probability flood.
8 Method for Estimating Potential Wetland Extent by Utilizing Streamflow Statistics and Flood-Inundation Mapping Techniques

Mapping of Wetland Extents

Typically when a wetland map is prepared, combinations 

of hydrology, vegetation, and soils criteria are applied through 

aerial-photo interpretation and other sources of information 

such as satellite imagery, soils data, topographic data, and 

flood maps. For this study, maps of estimated wetland extents 

were prepared with the estimated inundation elevation data 

that meet the planning criteria for the WRP and flood-inundation mapping techniques. In previous studies, flood-inundation 

mapping techniques were used to develop flood maps by using 

high-water marks (HWMs) as the water-surface elevation data 

(Morlock and others, 2008). For this study, the HWMs were 

replaced by the inundation-elevation data (table 2, fig. 3). 

Once the inundation elevations were estimated for the 

study reach of interest, flood maps were developed in conjunction with digital elevation model (DEM) data. By using a GIS 

application, a water-surface plane was created to represent the 

surface of inundation elevation that sloped in the downstream 

direction of flow. After the surface of inundation elevation was 

generated, a map representing the inundation zone was created by subtracting the DEM from the surface of inundationelevation data. The inundation zone maps were produced in a 

GIS file format that provided estimated wetland extents. This 

format allows the maps to be overlain upon other maps and 

aerial photographs; an example is shown in figure 3. A set 

of estimated inundation-zone maps provided for use by the 

NRCS is available in appendix 1.

Table 2. Inundation elevations within the study area along the Wabash River near Terre Haute, Indiana.

[ft, feet; NGVD 29, National Geodetic Vertical Datum of 1929]

River mile

Inundation 

elevation 

(ft above NGVD 29)

Location description Latitude 

(decimal degrees)

Longitude 

(decimal degrees)

217.4 468.0 Wabash River at State Route 63 39.500 -87.422

215.0 467.1 USGS Streamgage 03341500 Wabash River 

near Terre Haute, Ind.

39.476 -87.419

212.8 463.5 Wabash River at Interstate 70 39.441 -87.436

206.2 459.5 Wabash River at Clear Creek outlet 39.406 -87.481

203.5 458.0 Wabash River at Honey Creek outlet 39.385 -87.499
Methods 9

468.0

458.0

459.5

463.5

467.1

87°30' 87°28' 87°26' 87°24'

39°30'

39°28'

39°26'

39°24'

0 0.5 1 MILE

0 0.5 1 KILOMETER

Location of inundation elevation

and its value in feet (NGVD 29)

Interstate route marker

Flow arrow—Indicates direction

of water flow

EXPLANATION

Figure 3.

Orthophotography from Indiana University, Indiana Spatial Data Portal, Vigo County, 2010 NAIP

Universal Transverse Mercator Zone 16 projection

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD83)

63

150

70

70

40

40

150

41

41

State route marker

U.S. route marker

70

40

63

Estimated inundation zone

468.0

Terre

Haute

West Terre

Haute River

Wabash

Clear

Creek

Honey

Creek

Figure 3. Estimated inundation zone within the study area along the Wabash River near Terre Haute, 

Indiana. The inundation zone and the inundation elevation are based on the inundation flow (annual highest 

7-consecutive-day mean discharge with a 2-year recurrence interval).
10 Method for Estimating Potential Wetland Extent by Utilizing Streamflow Statistics and Flood-Inundation Mapping Techniques

Utility of the Methods for Future 

Studies 

As a prerequisite for wetland inventory, regulation, management, protection, and restoration, wetland mapping products from the pilot study will aid the NRCS and its partners 

with the onsite inundation-zone verification for the wetland 

determination of agricultural land in Indiana through the WRP 

and WREP. The emphasis of these programs is to protect, 

restore, and enhance the functions and values of wetland 

ecosystems in order to attain increased habitat for migratory 

birds and wetland-dependent wildlife, including threatened 

and endangered species; increased protection and improvement of water quality; decreased peak streamflows associated 

with flooding; increased recharge of groundwater; increased 

protection and enhancement of open space and aesthetic quality; and increased protection of native flora and fauna contributing to the Nation’s natural heritage. These programs, in turn, 

enable eligible landowners to protect, restore, and enhance 

the original hydrology, native vegetation, and natural topography of eligible lands; restore and protect the functions and 

values of wetlands in the agricultural landscape; help achieve 

the national goal of no net loss of wetlands; and improve the 

general environment of the country. 

Data-driven methods, such as those described in this 

report, for assessing the optimal location for restorations could 

help more fully realize the benefits of wetlands. For example, 

by identifying zones of frequent inundation, appropriate 

plantings can be selected to fit the hydrologic conditions. 

Certain tree species that cannot tolerate frequent inundation 

can be planted outside the inundation zone, saving money in 

lost trees. This project demonstrated that a science- and databased strategy can be applied to help administer important 

national programs to restore and protect riparian wetlands. 

Because data resources used for this project—including DEM 

data, flood profiles, and a national network of about 8,000 

USGS streamgages—are widespread, this project has great 

transferability potential to other States and regions across the 

county.

Summary

The U.S. Geological Survey, in cooperation with the U.S. 

Department of Agriculture, Natural Resources Conservation 

Service, conducted a pilot study to estimate the potential 

wetland extents for a 14-mi reach of the Wabash River near 

Terre Haute, Ind. The pilot study involved three phases: 

(1) compilation of streamflow statistics based on data from the 

available USGS streamgage-network database, (2) estimation 

of water-surface elevations based on the streamflow statistics, 

and (3) mapping of estimated potential wetland extents by use 

of the estimated water-surface elevations.

In phase 1, streamflow data were compiled and streamflow statistics were analyzed on the basis of WRP planning 

criteria and data from USGS streamgages along the Wabash 

River near Terre Haute, Ind. The WRP planning guidelines 

state that a site needs to show evidence of being in a zone 

prone to sustained or frequent flooding for a period of 7 

consecutive days at least once every 2 years on average 

(7MQ2) in order to meet the planning criteria for determining 

a wetland for a restoration in agricultural land. The inundation 

flow at a streamgage was determined by calculating the 7MQ2 

based on available daily mean discharge data. 

In phase 2, once the inundation flow was calculated, 

the corresponding inundation elevation was determined on 

the basis of the rating at the streamgage. For selected sites 

that were not near a streamgage, additional resources such as 

historical flood profiles and linear interpolation methods were 

used to estimate the inundation elevation.

In phase 3, a set of maps representing the inundation zone 

was created by use of a GIS application, inundation-elevation 

data, DEM data, and flood-inundation mapping techniques. 

Calculated and estimated inundation-elevation data from phase 

2 were used to develop a digital water-surface plane representing the surface of inundation elevation that sloped downward 

in the direction of flow. By subtracting the DEM data from the 

inundation-elevation data, GIS data were produced representing the extent of the inundation zone. By combining the GIS 

data, aerial photographs and other GIS layers, a final set of 

inundation-zone maps was created.
References Cited 11

References Cited

Dahl, T.E., 1990, Wetland losses in the United States 1780’s 

to 1980’s: Washington, D.C., U.S. Department of the 

Interior, Fish and Wildlife Service, 13 p., accessed 

October 7, 2011, at http://www.fws.gov/wetlands/

Documents/Wetlands-Losses-in-the-United-States1780s-to-1980s.pdf.

Indiana Department of Natural Resources, 1996, Indiana Wetlands Conservation Plan, accessed October 7, 2011, 

at http://www.in.gov/dnr/fishwild/3350.htm.

Morlock, S.E., Menke, C.D., Arvin, D.V., and Kim, M.H., 

2008, Flood of June 7–9, 2008, in central and 

southern Indiana: U.S. Geological Survey Open-File 

Report 2008–1322, 15 p., 3 app.

Rantz, S.E., and others, 1982, Measurement and computation 

of streamflow—Volume 1, Measurement of stage and 

discharge, and volume 2, Computation of discharge: 

U.S. Geological Survey Water-Supply Paper 2175, 

631 p.

Riggs, H.C., 1968, Frequency curves: U.S. Geological Survey 

Techniques of Water-Resources Investigations, 

book 4, chap. A2, 15 p. 

Ruddy B.C., and Hitt, K.J., 1990, Summary of selected characteristics of large reservoirs in the United States 

and Puerto Rico: U.S. Geological Survey 

Open-File Report 90–163, 295 p.

U.S. Department of Agriculture, 1994, Wetland mapping 

conventions for agricultural land and narrow band 

and small pocket inclusions of non-agricultural land: 

31 p., accessed August 9, 2009, at http://www.in.nrcs.

usda.gov/technical/Wetland%20Science/Wetland%20

Mapping%20Conventions.pdf.

U.S. Department of Agriculture, 2007, Wetland determination procedures for agricultural land: 8 p., 

accessed August 9, 2009, at http://www.in.nrcs.

usda.gov/technical/Wetland%20Science/Ag%20

Determinations%20Final.pdf.

U.S. Department of Agriculture, 2008a, Tree planting in flood 

plains: Forestry Technical Note, 5 p., accessed April 

4, 2012, at http://www.in.nrcs.usda.gov/intranet/

TechnicalNotes/Tree_Planting_in_Flood_Plains_

TechNote.pdf.

U.S. Department of Agriculture, 2008b, Wetland macrotopography: Biology Technical Note, version 

1.0, 9 p., accessed March 9, 2012, at http://www.

in.nrcs.usda.gov/intranet/TechnicalNotes/Wetland_

Macrotopography_TechNote.pdf.

U.S. Department of Agriculture, 2012, Restoring America’s 

wetlands—A private lands conservation success 

story: 16 p., accessed March 9, 2012, at http://

www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/

stelprdb1045079.pdf.

U.S. Geological Survey, 1996, National water summary on 

wetland resources: U.S. Geological Survey Water 

Supply Paper 2425, 431 p., accessed October 7, 2011, 

at http://pubs.usgs.gov/wsp/2425/report.pdf.

U.S. Geological Survey, 2011, SWSTAT, Surface-water statistics, accessed October 7, 2011, at http://water.usgs.

gov/software/SWSTAT/.

Glossary 13

Glossary

7MQ2 Annual highest 7-consecutive-day 

mean discharge with a 2-year recurrence 

interval.

datum A surface or point relative to which 

measurements of height and/or horizontal 

position are reported. A vertical datum is 

a horizontal surface used as the zero point 

for measurements of gage height, stage, or 

elevation; a horizontal datum is a reference 

for positions given in terms of latitude-lon-

gitude, State Plane coordinates, or Universal 

Transverse Mercator (UTM) coordinates.

discharge In its simplest concept, dis-

charge means outflow; therefore, the use 

of this term is not restricted as to course or 

location, and it can be applied to describe the 

flow of water from a pipe or from a drain-

age basin. If the discharge occurs in some 

course or channel, it is correct to speak of the 

discharge of a canal or of a river. It is also 

correct to speak of the discharge of a canal or 

stream into a lake, a stream, or an ocean (see 

also streamflow).

drainage area The drainage area of a 

stream at a specified location is that area, 

measured in a horizontal plane, which is 

enclosed by a drainage divide.

flood peakThe highest value of the stage 

or discharge attained by a flood; thus, peak 

stage or peak discharge. Flood crest has 

nearly the same meaning, but because it con-

notes the top of the flood wave, it is properly 

used only in referring to stage—thus, crest 

stage, but not crest discharge.

flood plain A strip of relatively smooth 

land bordering a stream, built of sediment 

carried by the stream and dropped in the 

slack water beyond the influence of the swift-

est current. It is called a living flood plain if 

it is overflowed in times of high water but a 

fossil flood plain if it is beyond the reach of 

the highest flood. 

flood profile A graph of elevation of the 

water surface of a river in flood, plotted as 

ordinate, against distance, measured in the 

downstream direction, plotted as abscissa. 

gage height The water-surface elevation 

referred to some arbitrary gage datum. Gage 

height is often used interchangeably with the 

more general term “stage”, although gage 

height is more appropriate when used with a 

reading on a gage.

macrotopography Shallow excavation in 

the flood plain or on terraces, up to 30 inches 

of depth, providing topographic changes in a 

flattened and tilled cropland to diversify the 

landscape, allowing favorable conditions for 

wetland species of amphibians, birds, and 

reptiles that thrive within varying hydroperi-

ods having complex wetland vegetation.

recurrence interval (return period) The 

average interval of time within which the 

given flood will be equaled or exceeded 

once.

riparian Pertaining to the banks of a 

stream.

river mileage The curvilinear distance, in 

miles, measured upstream from the mouth 

along the meandering path of a stream chan-

nel, and typically used to denote location 

along a river.

stage The height of a water surface above 

gage datum; same as gage height. 

stream A general term for a body of flow-

ing water. In hydrology the term is generally 

applied to the water flowing in a natural 

channel as distinct from a canal.

streamflow The discharge that occurs in 

a natural channel. Although the term “dis-

charge” can be applied to the flow of a canal, 

the word “streamflow” uniquely describes 

the discharge in a surface stream course.

streamgage A gaging station where a 

record of discharge of a stream is obtained. 

Within the U.S. Geological Survey, this term 

is used only for those gaging stations where a 

continuous record of gage-height is obtained.

water year In U.S. Geological Survey 

reports dealing with surface-water supply, 

the 12-month period, October 1 through Sep-

tember 30. The water year is designated by 

the calendar year in which it ends and which 

includes 9 of the 12 months. Thus, the year 

ended September 30, 1959, is called water 

year 1959.

Appendix 15

Appendix 1.

Flood-inundation area boundaries are intended to show the approximate extents of floods, with 

aerial photography and/or digital topographic map used as a base layer. Error sources in these 

boundaries can include, but are not limited to, digital-elevation-data errors and flood inundation 

mapping errors. Errors can result in inaccuracies in flood extent; thus, the flood-inundation area 

boundaries depicted here should be considered estimates.

Maps are in Transverse Mercator projection. Orthophotography is from Indiana University, 

Indiana Spatial Data Portal (http://gis.iu.edu/), National Agricultural Imagery Program (NAIP), 

2010. U.S. Geological Survey Topographic maps are from the National Map (http://nationalmap.

gov/ustopo/index.html). Vertical coordinate information is referenced to the National Geodetic 

Vertical Datum of 1929 (NGVD 29). Horizontal coordinate information is referenced to the North 

American Datum of 1983 (NAD 83).

The inundation zone and inundation elevation are based on the inundation flow (annual highest 

7-consecutive-day mean discharge with a 2-year recurrence interval).

Appendix Figures

Separate documents available on Web only, http://pubs.usgs.gov/sir/2012/5175/

Figures A1-1–A3-2. Estimated inundation-zone maps for the Wabash River near Terre Haute, 

Indiana.


Kim and others—Method for Estimating Potential Wetland Extent by Utilizing Streamflow Statistics and Flood-Inundation Mapping Techniques—SIR 2012–5175