U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2007–1146

Prepared in cooperation with the Federal Emergency Management Agency

Estimated Magnitudes and Recurrence Intervals of 

Peak Flows on the Mousam and Little Ossipee Rivers 

for the Flood of April 2007 in Southern Maine
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Estimated Magnitudes and Recurrence 

Intervals of Peak Flows on the Mousam 

and Little Ossipee Rivers for the 

Flood of April 2007 in Southern Maine

By Glenn A. Hodgkins, Gregory J. Stewart, Timothy A. Cohn, and 

Robert W. Dudley

Prepared in cooperation with the Federal Emergency Management Agency

Open-File Report 2007–1146

U.S. Department of the Interior

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

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2007

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Suggested citation:

Hodgkins, G.A., Stewart, G.J., Cohn, T.A., and Dudley, R.W., 2007, Estimated magnitudes and recurrence intervals of 

peak flows on the Mousam and Little Ossipee Rivers for the flood of April 2007 in southern Maine: U.S. Geological 

Survey Open-File Report 2007–1146, 5 p.
iii

Contents

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

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

Methods of Estimating the Magnitudes and Recurrence Intervals of Peak Flows ............................1

Estimation of Magnitudes....................................................................................................................1

Estimation of Recurrence Intervals ...................................................................................................3

Estimated Magnitudes and Recurrence Intervals of Peak Flows for the Flood of April 2007...........3

Summary..........................................................................................................................................................5

Acknowledgments.........................................................................................................................................5

References Cited............................................................................................................................................5

Figure

1. Map showing Mousam River and Little Ossipee River Basins in southern Maine 

and data-collection locations.....................................................................................................2

Tables

1. Estimated peak flows for selected recurrence intervals, Mousam River near 

West Kennebunk (USGS station number 01069500)................................................................4

2. Estimated peak flows for selected recurrence intervals, Little Ossipee River 

near South Limington (USGS station number 01066500)........................................................4

3. Estimated peak flows and recurrence intervals for flood of April 2007...............................4
iv

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

Area

square mile (mi2) 259.0 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2) 

Volume

cubic foot (ft3) 0.02832 cubic meter (m3) 

Flow rate

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

Conversion Factors

Inch/Pound to SI
Abstract

Large amounts of rain fell on southern Maine from 

the afternoon of April 15, 2007, to the afternoon of 

April 16, 2007, causing substantial damage to houses, roads, 

and culverts. This report provides an estimate of the peak 

flows on two rivers in southern Maine—the Mousam River 

and the Little Ossipee River—because of their severe flooding. The April 2007 estimated peak flow of 9,230 ft3

/s at the 

Mousam River near West Kennebunk had a recurrence interval 

between 100 and 500 years; 95-percent confidence limits for 

this flow ranged from 25 years to greater than 500 years. The 

April 2007 estimated peak flow of 8,220 ft3/s at the Little 

Ossipee River near South Limington had a recurrence interval 

between 100 and 500 years; 95-percent confidence limits for 

this flow ranged from 50 years to greater than 500 years.

Introduction

Flood-related data is useful for many purposes. For example, the Federal Emergency Management Agency (FEMA) and 

Maine Emergency Management Agency (MEMA) need timely 

information on the magnitudes and recurrence intervals of 

floods to help respond to flood damage. Peak-flow magnitudes 

are used to help delineate floodplain boundaries for land-use 

planning, and to design bridges, culverts, and other structures in the floodplain. Flood data are also used for scientific 

purposes, including the study of the effects of climate changes 

and land-use changes on hydrologic regimes, which in turn 

affect ecosystems.

On April 9, 2007, a thin, wet snowpack covered the 

ground in the Mousam and Little Ossipee River Basins. Snowpack was measured at four locations in or near the basins on 

April 9 (West Kennebunk, Newfield, Hollis, Cornish, fig. 1) as 

part of the Maine Cooperative Snow Survey program. Snowpack depths ranged from 2.4 to 5.3 in. and had an equivalent 

water content of 0.8 to 1.5 in. There was no snowpack at the 

sites in West Kennebunk, Newfield, and Hollis on April 19 (no 

data available for Cornish) (Maine Cooperative Snow Survey, 

2007). A storm on April 12 and 13 may have changed the 

equivalent water content in the snowpack prior to the 

April 2007 flood, either by adding to the snowpack or by 

melting and then replacing the snowpack (Thomas Hawley, 

National Weather Service, written commun., 2007).

Most of the rainfall that caused the April 2007 flood 

occurred from the afternoon of April 15, 2007, to the afternoon of April 16, 2007. Maximum 24-hour total precipitation 

at continuous-record precipitation gages in Sanford, Hollis, 

and Cornish (fig. 1) were 7.58 in., 4.97 in., (Thomas Hawley, 

National Weather Service, Gray, Maine) and 4.60 in., respectively (Cornish data from USGS continuous-rainfall gage).

This report presents results of a preliminary study 

conducted by the U.S. Geological Survey (USGS) to collect, 

compute, and compile flood-related data for the Mousam 

and Little Ossipee Rivers, which were substantially impacted 

by the storm. Estimates are presented of the magnitudes and 

associated recurrence intervals of peak flows for the Mousam 

River and the Little Ossipee River, for the flood of April 2007. 

Although the Mousam River Basin has reservoirs with sufficient storage capacity to potentially affect the magnitude of 

peak flows, the actual effect of reservoirs on April 2007 peak 

flows was not analyzed in this report.

Methods of Estimating the Magnitudes 

and Recurrence Intervals of 

Peak Flows

Estimation of Magnitudes

The 2007 peak flow at the Mousam River near 

West Kennebunk was estimated from surveyed April 2007 

high watermarks by extension of the historical rating curve 

(the relation between river height and flow). This is believed to 

be a reasonable approach based on the stability and type of the 

river height/flow control for high flows at this location.

Estimated Magnitudes and Recurrence Intervals of 

Peak Flows on the Mousam and Little Ossipee Rivers 

for the Flood of April 2007 in Southern Maine

By Glenn A. Hodgkins, Gregory J. Stewart, Timothy A. Cohn, and Robert W. Dudley
Figure 1. Mousam River and Little Ossipee River Basins in southern Maine and data-collection locations.

!

!

!

!

X

X

X

#

#

Canada

95

LAKE, POND, OR OCEAN

SHADED-RELIEF TOPOGRAPHY

RIVER OR STREAM

MAJOR ROAD

INTERSTATE HIGHWAY

WATERSHED BOUNDARY

STATE BOUNDARY

COUNTY BOUNDARY

MUNICIPAL BOUNDARY

U.S. GEOLOGICAL SURVEY

STREAMFLOW-GAGING STATION

SNOW SURVEY SITE

PRECIPITATION STATION

EXPLANATION

L i t t l e O s s i p e e B a s i n

M o u s a m B a s i n

(

(

(

(

Hollis

Cornish

Newfield

West Kennebunk

HOLLIS

SANFORD

01069500

01066500

71°00' 70°30'

43°30'

43°45'

CORNISH

L i t t l e O s s i p e e B a s i n

M o u s a m B a s i n

Base from USGS and Maine Office of GIS digitial files

NAD83 UTM Zone 19 New Hampshire Maine

Maine

Area of

detail map

CANADA

UNITED 

STATES

New Hampshire

Atlantic Ocean

MILES

KILOMETERS

0 10 20

0 10 20

01068500

Cornish

CORNISH

   Peak Flows on the Mousam and Little Ossipee Rivers for the Flood of April 2007 in Southern Maine
The 2007 peak flow at the Little Ossipee River near 

South Limington was estimated using surveyed April 2007 

high watermarks and hydraulic principles for contracted 

openings (Matthai, 1967) at the Sand Pond Road bridge. 

The geometry of the channel and bridge was defined by field 

surveys. The geometric data were entered in the U.S. Army 

Corps of Engineers (USACE) step-backwater computer 

program HEC-RAS (U.S. Army Corps of Engineers, 2004). 

The one-dimensional steady-flow water-surface profile 

computation component of HEC-RAS was used in the 

analysis. Boundary conditions and energy-loss parameters, 

including channel roughness and contraction and expansion 

coefficients, were measured and(or) estimated in the field and 

further adjusted during calibration of the model. The model 

was fully calibrated to match 2007 flood high-watermark 

elevations at the upstream approach and downstream exit 

sections of the bridge to within ±0.01 ft of the highest-quality 

high-watermark elevations. In addition to the calibrated 2007 

flood flow, a range of flows (with recurrence intervals from 

2 years to 500 years) were modeled as part of the calibration 

to test that model functionality was well conditioned.

Estimation of Recurrence Intervals

The recurrence interval is the statistically computed 

average period of time between peak flows that are greater 

than, or equal to, a specified magnitude. For example, the 

50-year peak flow is the flow that would be exceeded or 

equaled, on long-term average, once in 50 years. This does 

not imply that flooding will happen at regular intervals. Two 

50-year peak flows could be experienced in 2 consecutive 

years. Conversely, a 50-year peak flow might not be 

experienced for 100 years. The reciprocal of the recurrence 

interval is the annual exceedance probability, which is the 

probability that a given peak flow will be exceeded or equaled 

in any given year. For example, the annual exceedance 

probability of the 50-year peak flow is 0.02. In other words, 

there is a 2-percent chance that the 50-year peak flow will be 

exceeded or equaled in any given year.

The U.S. Geological Survey (USGS) collected continuous streamflow data on the Mousam River near West Kennebunk (USGS station number 01069500) from October 1939 

to September 1984 and on the Little Ossipee River near South 

Limington (station number 01066500) from August 1940 to 

October 1982 (fig. 1). Annual peak-flow data for these stations 

were obtained from the USGS National Water Information 

System (U.S. Geological Survey, 2007).

For the Mousam River, the largest peak flow recorded 

from 1939 to 1984 was 4,020 ft3/s on March 20, 1983. Known 

peak flows occurred outside of the period of continuous 

record; on October 22, 1996, the peak flow was 3,600 ft3/s and 

on May 15, 2006, the peak flow was 6,100 ft3/s. For the Little 

Ossipee River, the largest peak flow recorded from 1940 to 

1982 was 5,760 ft3/s on March 15, 1977; on March 19, 1936, 

the peak flow was 8,530 ft3/s; and on October 22, 1996, the 

peak flow was 5,800 ft3/s.

Peak flows for selected recurrence intervals were 

estimated for this report by use of the Expected Moments 

Algorithm (EMA) (Cohn and others, 1997; Cohn and others, 

2001; Griffis and others, 2004). EMA is a generalization of the 

procedures in Bulletin 17B (Interagency Advisory Committee on Water Data, 1982) and was designed to better accommodate historical peak-flow data—known peak flows outside 

the period of continuous streamflow data collection. Although 

the procedures in Bulletin 17B can employ historical data, 

EMA makes more efficient use of historical data and the EMA 

confidence intervals are more accurate than the ones given in 

Bulletin 17B. Peak flows from the April 2007 flood were used 

in the calculations.

Regional information was not used to estimate the 

peak flows for the Mousam River because of its substantial 

historical flow regulation. Reservoirs (Square Pond, Mousam 

Lake, Estes Lake) above the USGS streamflow-gaging 

station near West Kennebunk were believed in 1984 to have 

a combined capacity of about 700,000,000 ft3 (Haskell 

and others, 1985). Flow-storage capacity above the station 

could affect recorded peak flows by more than 10 percent 

(Hodgkins, 1999; Benson, 1962).

Regional information was used to compute a weighted 

skew for the Little Ossipee River following the procedures 

in Griffis and others (2004). The generalized skew used at 

this site was 0.029 with a standard error of 0.297 (Hodgkins, 

1999). Regional regression equations were not used to weight 

the at-site data; Hodgkins (1999) found that the weighted 

estimates for the 2-year to 500-year peak flows differed by 

less than 3 percent from station estimates for the Little 

Ossipee River.

Estimated Magnitudes and Recurrence 

Intervals of Peak Flows for the 

Flood of April 2007

The estimated April 2007 peak flow for the Mousam 

River near West Kennebunk was 9,230 ft3/s. The largest previously known peak flow from 1939 to 1984, 1996, and 2006 

was 6,100 ft3/s on May 15, 2006. The computed April 2007 

peak flow for the Little Ossipee River near South Limington 

was 8,220 ft3/s. The largest known peak flow from 1936, 1940 

to 1982, and 1996 was 8,530 ft3/s on March 19, 1936.

Estimated peak flows for the Mousam River and Little 

Ossipee River and their associated recurrence intervals are 

shown in tables 1 and 2, along with 95-percent confidence 

intervals. The April 2007 flood at the Mousam River near 

West Kennebunk had a recurrence interval between 100 and 

500 years; 95-percent confidence limits ranged from 25 years 

to greater than 500 years. The April 2007 flood at the Little 

Ossipee River near South Limington had a recurrence interval 

between 100 and 500 years; 95-percent confidence limits 

ranged from 50 years to greater than 500 years (table 3).

Estimated Magnitudes and Recurrence Intervals of Peak Flows for the Flood of April 2007 
Table 1. Estimated peak flows for selected recurrence intervals, Mousam River near West Kennebunk 

(USGS station number 01069500).

[USGS, U.S. Geological Survey; ft3/s, cubic feet per second; confidence intervals are 95-percent confidence intervals]

Recurrence interval

(years)

Estimated peak flow

(ft3/s)

Lower confidence interval

(ft3/s)

Upper confidence interval

(ft3/s)

2 1,390 1,120 1,650

5 2,230 1,750 2,890

10 3,010 2,370 4,400

25 4,340 3,210 9,830

50 5,630 3,930 18,300

100 7,240 4,730 32,500

500 12,700 6,990 126,000

Table 2. Estimated peak flows for selected recurrence intervals, Little Ossipee River near 

South Limington (USGS station number 01066500).

[USGS, U.S. Geological Survey; ft3/s, cubic feet per second; confidence intervals are 95-percent confidence intervals]

Recurrence interval

(years)

Estimated peak flow

(ft3/s)

Lower confidence interval

(ft3/s)

Upper confidence interval

(ft3/s)

2 2,090 1,760 2,480

5 3,330 2,770 4,080

10 4,280 3,490 5,450

25 5,640 4,430 7,700

50 6,770 5,160 9,850

100 7,990 5,910 12,500

500 11,300 7,690 21,400

Table 3. Estimated peak flows and recurrence intervals for flood of April 2007.

[USGS, U.S. Geological Survey; mi2, square miles; ft3/s, cubic feet per second; confidence intervals are 95-percent confidence intervals]

River name USGS station 

number

Drainage 

area

(mi2)

Estimated 

peak flow

(ft3/s)

Recurrence 

interval

(years)

Confidence intervals

(years)

Mousam River near West Kennebunk, Maine 01069500 99.0 9,230 100 to 500 25 to greater than 500

Little Ossipee River near South Limington, Maine 01066500 168 8,220 100 to 500 50 to greater than 500

 Peak Flows on the Mousam and Little Ossipee Rivers for the Flood of April 2007 in Southern Maine
Summary

The Federal Emergency Management Agency (FEMA) 

and Maine Emergency Management Agency (MEMA) need 

timely information on the magnitude and recurrence intervals 

of floods to help respond to flood damage. Large amounts of 

rain fell on southern Maine in April 2007, causing substantial 

damage to houses, roads, and culverts. Most of the rainfall 

occurred from the afternoon of April 15, 2007, to the afternoon of April 16, 2007; maximum 24-hour total precipitation 

at continuous-record precipitation gages in Sanford, Hollis, 

and Cornish was 7.58 in., 4.97 in., and 4.60 in., respectively.

The resulting April 2007 peak flow at the Mousam River 

near West Kennebunk was estimated from surveyed flood 

marks by extension of the historical rating curve (the relation 

between river height and flow). The peak flow at the 

Little Ossipee River near South Limington was estimated 

using surveyed flood marks and hydraulic principles for 

contracted openings.

The estimated April 2007 peak flow for the Mousam 

River near West Kennebunk was 9,230 ft3/s. The largest known 

peak flow from 1939 to 1984, 1996, and 2006 was 6,100 ft3/s 

on May 15, 2006. The estimated April 2007 peak flow for the 

Little Ossipee River near South Limington was 8,220 ft3/s. The 

largest known peak flow from 1936, 1940 to 1982, and 1996 

was 8,530 ft3/s on March 19, 1936.

Peak flows for selected recurrence intervals were 

estimated for this report by use of the Expected Moments 

Algorithm (EMA). Recurrence intervals are the statistically 

computed long-term-average period of time between peak 

flows that are greater than, or equal to, a specified magnitude.

The April 2007 estimated peak flow at the Mousam 

River near West Kennebunk had a recurrence interval between 

100 and 500 years; 95-percent confidence limits ranged from 

25 years to greater than 500 years. The April 2007 flood at the 

Little Ossipee River near South Limington had a recurrence 

interval between 100 and 500 years; 95-percent confidence 

limits ranged from 50 years to greater than 500 years.

Acknowledgments

Thomas Hawley of the National Weather Service in Gray, 

Maine, provided most of the meteorological data analyzed in 

this report. Joshua Kempf and Nicholas Stasulis of the USGS 

Maine Water Science Center collected a substantial amount of 

field data that were required for this report.

References Cited

Benson, M.A., 1962, Factors influencing the occurrence of 

floods in a humid region of diverse terrain: U.S. Geological 

Survey Water-Supply Paper 1580–B, 64 p.

Cohn, T., Lane, W.L., and Stedinger, J.R., 2001, Confidence intervals for EMA flood quantile estimates: Water 

Resources Research, v. 37, no. 8, p. 1695–1706.

Cohn, T., Lane, W.L., and Baier, W.G., 1997, An algorithm for 

computing moments-based flood estimates when historical 

flood information is available: Water Resources Research, 

v. 33, no. 9, p. 2089–2096.

Griffis, V.W., Stedinger, J.R., and Cohn, T.A., 2004, LP3 

quantile estimators with regional skew information and 

low outlier adjustments: Water Resources Research, v. 40, 

doi:10.1029/2003WR002697.

Haskell, C.R., Bartlett, W.P., Higgins, W.B., and Nichols, W.J., 

1985, Water resources data, Maine, water year 1984: U.S. 

Geological Survey Water-Data Report ME-84-1, 144 p.

Hodgkins, G.A., 1999, Estimating the magnitude of peak 

flows in Maine for selected recurrence intervals: 

U.S. Geological Survey Water-Resources Investigations 

Report 99–4008, 45 p.

Interagency Advisory Committee on Water Data, 1982, Guidelines for determining flood flow frequency—Bulletin 17-B 

of the Hydrology Subcommittee, March 1982 (revised and 

corrected), 28 p. and appendices.

Maine Cooperative Snow Survey, 2007, accessed April 28, 

2007, at http://www.maine.gov/mema/weather/snow.htm

Matthai, 1967, Measurement of peak discharge at width 

contractions by indirect methods: Techniques of WaterResources Investigations of the U.S. Geological Survey, 

book 3, chap. A4, 44 p.

U.S. Army Corps of Engineers, 2004, HEC-RAS River Analysis System Version 3.1.2: Hydrologic Engineering Center, 

accessed April 29, 2007, at http://www.hec.usace.army.mil/

U.S. Geological Survey, 2007, National Water Information 

System, accessed April 28, 2007, at http://waterdata.usgs.

gov/me/nwis/sw

References Cited 
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Flood of April 2007 in Southern Maine—Open-File Report 2007–1146

Hodgkins, G.A. and others—Estimated Magnitudes and Recurrence Intervals of Peak Flows on the Mousam and Little Ossipee Rivers for the 

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