Prepared in cooperation with the

Federal Emergency Management Agency

Flood of April 2007 and Flood-Frequency Estimates at 

Streamflow-Gaging Stations in Western Connecticut

Scientific Investigations Report 2009–5108

U.S. Department of the Interior

U.S. Geological Survey
Cover: The upstream side of the bridge crossing the Rippowam River at Bridge Street in Stamford, Connecticut. 
Flood of April 2007 and Flood-Frequency 

Estimates at Streamflow-Gaging 

Stations in Western Connecticut 

By Elizabeth A. Ahearn

Prepared in cooperation with the

Federal Emergency Management Agency

Scientific Investigations Report 2009–5108

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2009

For product and ordering information:

World Wide Web: http://www.usgs.gov/pubprod

Telephone: 1-888-ASK-USGS

For more information on the USGS—the Federal source for science about the Earth, 

its natural and living resources, natural hazards, and the environment:

World Wide Web: http://www.usgs.gov

Telephone: 1-888-ASK-USGS

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Ahearn, E.A., 2009, Flood of April 2007 and flood-frequency estimates at streamflow-gaging stations in western 

Connecticut: U.S. Geological Survey Scientific Investigations Report 2009-5108, 40 p. Available online only at 

http://pubs.usgs.gov/sir/2009/5108.
iii

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

Introduction.....................................................................................................................................................2

Previous Flood Investigation Studies .........................................................................................................2

Storm Characteristics ...................................................................................................................................6

Drainage-Basin Characteristics ..................................................................................................................6

Annual Peak-Discharge Data ....................................................................................................................12

Peak Discharges Affected by Flood-Control Dams ................................................................................13

Flood-Frequency Analyses of the April 2007 Flooding ...........................................................................17

Flood-Frequency Estimates for Streamflow-Gaging Stations .....................................................17

Record Extension Using Two Methods: MOVE-1 and Two-Station Comparison .....................21

Comparison of the April 2007 Flood Data to Flood Insurance Studies ................................................21

Summary........................................................................................................................................................26

Acknowledgments .......................................................................................................................................27

Selected References ..................................................................................................................................27

Appendix 1. Federal Emergency Management Agency Disaster Declaration Map .....................31

Appendix 2. National Climatic Data Center April 2007 Storm Events: Details for Fairfield 

and Litchfield Counties, Connecticut ..........................................................................................32

Appendix 3. Summary of Maximum Pool Levels for U.S. Army Corps of Engineers 

Flood-Control Dams........................................................................................................................35

Figures

1. Map showing locations of U.S. Geological Survey streamflow-gaging stations used 

to investigate the April 2007 flood in western Connecticut ...................................................3

2. Map showing rainfall totals for April 15–17, 2007, at selected National Weather 

Service and Connecticut Department of Environmental Protection Flood Alert 

Network rain gages, Long Island, New York and Connecticut. ............................................7

3. Maps showing next-Generation Radar (NEXRAD) 1-day rainfall totals in Connecticut 

ending (A) April 15, 2007, and (B) April 16, 2007 .......................................................................8

4. Map showing drainage basins for U.S. Geological Survey streamflow-gaging 

stations in western Connecticut ................................................................................................9

5. Map showing locations of U.S. Army Corps of Engineers flood-control dams in the 

Farmington and Naugatuck River Basins, U.S. Geological Survey streamflow-gaging 

stations, and their upstream drainage basins in western Connecticut .............................16

Contents
iv

Tables

 1. Descriptions of U.S. Geological Survey streamflow-gaging stations whose records 

were used to determine the magnitude and frequency of the April 2007 flood in 

western Connecticut ....................................................................................................................4

 2. Drainage-basin characteristics for the streamflow-gaging stations used to 

investigate the April 2007 flood in western Connecticut ......................................................10

 3. Summary of peak stages and peak discharges during the April 2007 flood and 

maximum peak discharges and gage heights for the period of record for 28 streamflow-gaging stations in western Connecticut ........................................................................14

 4. Flood-frequency estimates for selected exceedance probabilities for 24 streamflowgaging stations in western Connecticut .................................................................................18

 5. Flood-frequency estimates derived using the Maintenance of Variance Extension 

(MOVE-1) method and the two-station comparison extended-record analysis for five 

streamflow-gaging stations in western Connecticut ...........................................................22

 6. Magnitude and frequency of floods from flood insurance studies and current 

(2007) flood-frequency estimates for streamflow-gaging stations in western 

Connecticut ..................................................................................................................................24

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

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)

cubic foot per second per square

mile [(ft3

/s)/mi2]

 0.01093 cubic meter per second per square

kilometer [(m3/s)/km2]

Vertical coordinate information is referenced to “North American Vertical Datum of 

1988 (NAVD 88).”

Conversion Factors and Datums
Abstract 

A spring nor’easter affected the East Coast of the United 

States from April 15 to 18, 2007. In Connecticut, rainfall 

varied from 3 inches to more than 7 inches. The combined 

effects of heavy rainfall over a short duration, high winds, and 

high tides led to widespread flooding, storm damage, power 

outages, evacuations, and disruptions to traffic and commerce. 

The storm caused at least 18 fatalities (none in Connecticut). 

A Presidential Disaster Declaration was issued on May 11, 

2007, for two counties in western Connecticut―Fairfield and 

Litchfield. This report documents hydrologic and meteorologic aspects of the April 2007 flood and includes estimates of 

the magnitude of the peak discharges and peak stages during 

the flood at 28 streamflow-gaging stations in western Connecticut. These data were used to perform flood-frequency 

analyses. Flood-frequency estimates provided in this report are 

expressed in terms of exceedance probabilities (the probability of a flood reaching or exceeding a particular magnitude in 

any year). Flood-frequency estimates for the 0.50, 0.20, 0.10, 

0.04, 0.02, 0.01, and 0.002 exceedance probabilities (also 

expressed as 50-, 20-, 10-, 4-, 2-, 1-, and 0.2- percent exceedance probability, respectively) were computed for 24 of the 

28 streamflow-gaging stations. Exceedance probabilities can 

further be expressed in terms of recurrence intervals (2-, 5-, 

10-, 25-, 50-, 100-, and 500-year recurrence interval, respectively). Flood-frequency estimates computed in this study 

were compared to the flood-frequency estimates used to derive 

the water-surface profiles in previously published Federal 

Emergency Management Agency (FEMA) Flood Insurance 

Studies. The estimates in this report update and supersede previously published flood-frequency estimates for streamflowgaging stations in Connecticut by incorporating additional 

years of annual peak discharges, including the peaks for the 

April 2007 flood. 

In the southwest coastal region of Connecticut, the 

April 2007 peak discharges for streamflow-gaging stations 

with records extending back to 1955 were the second highest 

peak discharges on record; the 1955 annual peak discharges 

are the highest peak discharges in the station records. In the 

Housatonic and South Central Coast Basins, the April 2007 

peak discharges for streamflow-gaging stations with records 

extending back to 1930 or earlier ranked between the fourth 

and eighth highest discharges on record, with the 1936, 1938, 

and 1955 floods as the largest floods in the station records. 

The peak discharges for the April 2007 flood have 

exceedance probabilities ranging between 0.10 to 0.02 

(a 10- to 2-percent chance of being exceeded in a given year, 

respectively) with the majority (80 percent) of the stations 

having exceedance probabilities between 0.10 to 0.04. At three 

stations—Norwalk River at South Wilton, Pootatuck River at 

Sandy Hook, and Still River at Robertsville—the April 2007 

peak discharges have an exceedance probability of 0.02.

Flood-frequency estimates made after the April 2007 

flood were compared to flood-frequency estimates used to 

derive the water-surface profiles (also called flood profiles) in 

FEMA Flood Insurance Studies developed for communities. In 

general, the comparison indicated that at the 0.10 exceedance 

probability (a 10-percent change of being exceeded in a given 

year), the discharges from the current (2007) flood-frequency 

analysis are larger than the discharges in the FEMA Flood 

Insurance Studies, with a median change of about +10 percent. 

In contrast, at the 0.01 exceedance probability (a 1-percent 

change of being exceeded in a year), the discharges from the 

current flood-frequency analysis are smaller than the discharges in the FEMA Flood Insurance Studies, with a median 

change of about -13 percent. 

Several stations had more than + 25 percent change in 

discharges at the 0.10 exceedance probability and are in the 

following communities: Winchester (Still River at Robertsville, +50 percent change); Hamden (Mill River near Hamden, 

+46 percent change); Woodbury (Weekeepeemee River at 

Hotchkissville, +29 percent change); and Newtown (Pootatuck 

River at Sandy Hook , +28 percent change). Although the 

majority of the streamflow-gaging stations had discharges 

at the 0.01 exceedance probability smaller than in the Flood 

Insurance Studies, the (2007) flood-frequency estimates 

were larger than in the Flood Insurance Studies for stations 

in the following communities: Hamden (Mill River near 

Hamden, +53 percent change); Thomaston (Naugatuck River 

at Thomaston, +27 percent change); Newtown (Pootatuck 

River at Sandy Hook, +18 percent change); and Wallingford 

Flood of April 2007 and Flood-Frequency Estimates at 

Streamflow-Gaging Stations in Western Connecticut

By Elizabeth A. Ahearn
2 Flood of April 2007 and Flood-Frequency Estimates at Streamflow-Gaging Stations in Western Connecticut

(Quinnipiac River at Wallingford, +13 percent change). The 

1-percent exceedance probability (100-year flood) elevations 

at streamflow-gaging stations exceeded the FEMA projected 

100-year flood elevations by more than +0.5 feet in two 

Flood Insurance Studies in the communities of Wallingford 

(Quinnipiac River at Wallingford, +0.6 feet change) and 

Hamden (Mill River near Hamden, + 2.3 feet change). 

Introduction 

Major flooding occurred in Fairfield and Litchfield Counties, Connecticut, from April 15 to 18, 2007, resulting in substantial damage to public and personal property. The Federal 

Emergency Management Agency (FEMA) reported that flood 

damages in Connecticut exceeded an estimated $6.4 million 

(FEMA, 2007). The two counties have a combined land area 

of 1,545 mi2. Fairfield County is 625 mi2, the population density is about 1,430 people per square mile, and it is characterized by residential, industrial, commercial, and institutional 

development. Litchfield County is 920 mi2, the population 

density is 205 people per square mile, and, in contrast to Fairfield County, it is considerably less developed, with development characterized primarily as low-density residential. Since 

1990, both counties have experienced an 8.1 percent growth in 

population (USA Counties IN Profile, 2009). 

In response to the flooding, President George W. Bush 

issued a Disaster Declaration on May 11, 2007, for the counties of Fairfield and Litchfield (see appendix 1, Federal Emergency Management Agency Disaster Declaration map). FEMA 

requested that the U.S. Geological Survey (USGS) perform a 

flood-frequency analysis of recorded long-term streamflowgaging station data to determine the magnitude and frequency 

of flooding within the disaster-declaration areas and compare 

the current (2007) flood-frequency estimates to the estimates 

used to derive the water-surface profiles in Flood Insurance 

Studies developed for the local communities. This study provides FEMA and local communties with updated estimates of 

magnitude and frequency of floods for the streams studied and 

presents base data that can be used to identify deficiencies in 

floodplain delineations and to prepare any subsequent revisions to the Flood Insurance Studies.

The relation of discharge to frequency of occurrence at 

a streamflow-gaging station is generally expressed in terms 

of exceedance probability or recurrence interval. Exceedance 

probability is the percent chance or likelihood that a given 

discharge will be exceeded in any 1-year period. Recurrence 

interval is the average time interval (in years) during which a 

given flow is expected to be exceeded one time. Recurrence 

interval does not imply regular, predictable occurrences. A 

large flood in one year does not preclude the occurrence of 

an even larger flood the next year. It is possible that a flood 

with a recurrence interval of 100 years could occur during 

consecutive years because, statistically, there is a 1 percent 

chance or likelihood of occurrence in any year. For this study, 

the 0.50, 0.20, 0.10, 0.04, 0.02, 0.01, and 0.002 exceedance 

probabilities (also expressed as 50-, 20-, 10-, 4-, 2-, 1-, and 

0.2- percent exceedance probability, respectively) were 

determined for stations in western Connecticut. Exceedance 

probabilities can further be expressed in terms of recurrence 

intervals (recurrence interval of 2, 5, 10, 25, 50, 100, and 

500 years, respectively).

The study area in western Connecticut—primarily Fairfield and Litchfield Counties—includes 28 active streamflowgaging stations for reporting peak stages and peak discharges 

(fig. 1 and table 1). The data collected and analyzed from this 

storm enhance the scientific understanding of flood characteristics and are critical for reducing deaths, injuries, and property damages from floods. Current estimates of the magnitude 

and frequency of floods provide key information for updating 

Flood Insurance Studies, mitigating losses associated with 

future floods, and designing bridges and culverts. 

This report includes information on rainfall that led to 

the flooding, peak stages and peak discharges for the April 

2007 flood for rivers with active streamflow-gaging stations, 

and updated flood-frequency estimates for streamflow-gaging 

stations in western Connecticut. Also included in this report is 

a comparison of flood-frequency estimates using (1) reported 

data through water year 2007 (ending September 30, 2007) to 

(2) flood-frequency estimates in community Flood Insurance 

Studies prepared for communities by FEMA. The changes in 

flood stages at the stations attributed to changes in the recurrence interval values are described. 

Previous Flood Investigation Studies 

Several studies published by the USGS provide estimates 

of the magnitude and frequency of floods in Connecticut. 

These studies provide FEMA with a means to identify floodprone areas, enabling communities to guide development 

away from areas subject to flood damage, and to assess risk 

and set flood-insurance rates for properties subject to flooding. In 2003, the USGS published flood-frequency estimates 

for 138 streamflow-gaging stations in Connecticut (Ahearn, 

2003). In 2004, the USGS published a statewide floodfrequency study (Ahearn, 2004) that included (1) floodfrequency estimates for 70 rivers with streamflow-gaging 

stations (the estimates are weighted averages derived from the 

log-Pearson Type III distribution of the annual peak discharges 

at the gaging station and a regression equation for estimating the magnitude and frequency of the peak discharges); 

(2) regional regression equations for estimating the 0.50, 0.10, 

0.04, 0.02, 0.01, and 0.002 exceedance probabilities (2-, 10-, 

25-, 50-, 100-, and 500-year recurrence intervals, respectively) 

at ungaged locations; and (3) a method for combining the 

flood-frequency estimates derived from the log-Pearson Type 

III distribution of the annual peak discharges at the station 

and from a regression equation. In 2005, the USGS published 

flood-frequency estimates for stations on the main stem of the 
Previous Flood Investigation Studies 3

Kent

Sharon

Norfolk Salisbury

Litchfield

Newtown

Guilford

Suffield

Goshen

Granby

New Milford

Cornwall

Danbury

Oxford

Berlin

Avon

Greenwich

Wilton

Bristol

Madison

Canaan

Stamford

Shelton

Easton

Hartland

Torrington

Hamden

Southbury

Redding

Fairfield

Warren

Windsor

Middletown

Cheshire

Simsbury

Wallingford

Woodbury

Ridgefield

Canton

Monroe

Milford

Washington

Colebrook

Roxbury

Harwinton

Southington

Winchester

Burlington

Morris

Durham

Meriden

Barkhamsted

New Hartford

Wolcott

Enfield

Waterbury

Norwalk

Watertown

Portland

Weston

Trumbull

Bethel

Bethany

Haddam

Sherman

Branford

Farmington

Bloomfield

Killingworth

Plymouth

Orange

Hartford

Westport

Brookfield

Bethlehem

Seymour

New Haven

Prospect Naugatuck

North Canaan

Cromwell

Rocky Hill

Plainville

Glastonbury

Stratford

New Fairfield

Darien

North Branford

New Canaan

Middlebury

North Haven

West Hartford

Woodbridge

Bridgeport

Bridgewater

East Hartford

East Granby

South 

Windsor

East 

Windsor

Newington

Middlefield

New Britain

East Haven

Thomaston

Wethersfield

Derby

West Haven

Ansonia

Beacon 

Falls

Windsor Locks

Clinton

Housatonic Basin

Southwest Coast Basin

South-Central Coast Basin

#

#

#

#

#

#

#

# #

#

#

#

#

#

# #

#

#

#

#

#

#

#

#

#

#

#

#

73°30' W 73° W

42° N

41°30' N

41° N

01195490

01203000

01203510

01199050

01203805

01203600

01196500

01206900

01186500

01202501

01201487

01186000

01209761

01205500

01204000

01200000

01208500

01208950

01208925

01196620

01208990

01200500

01209700

01208873

01209901

012035055

012095493

01199000

01208873

LITCHFIELD COUNTY

NEW HAVEN COUNTY

HARTFORD COUNTY

FAIRFIELD COUNTY

CONNECTICUT

MASSACHUSETTS

NEW YORK

Connecticut Basin

EXPLANATION

Major drainage basins

Connecticut

Housatonic

Hudson

South-Central Coast

Southwest Coast

County boundary

State boundary

Town boundary

Basin boundary

# USGS streamflow-gaging station 

and identifier 

Base from U.S. Geological Survey, 1:24,000, 1969–1984

Connecticut State Plane Projection

0 5 10 MILES

0 10 20 KILOMETERS

Figure 1. Locations of U.S. Geological Survey streamflow-gaging stations used to investigate the April 2007 flood 

in western Connecticut. 
4 Flood of April 2007 and Flood-Frequency Estimates at Streamflow-Gaging Stations in Western Connecticut

Table 1. Descriptions of U.S. Geological Survey streamflow-gaging stations whose records were used to determine the magnitude 

and frequency of the April 2007 flood in western Connecticut.—Continued

[Station locations shown on figure 1. lat, latitude; long, longitude; mi, miles; ft, feet; right and left bank are referenced facing downstream]

Station 

number Station name Description of station location

01186000 West Branch Farmington River at 

Riverton

Lat 41°57'47", long 73°01'05", Litchfield County, Hydrologic Unit 01080207, on right 

bank at downstream side of bridge on State Route 20 at Riverton, 0.3 mi upstream from 

Still River, 2.0 mi downstream from Goodwin Dam of West Branch Reservoir, and at 

mile 55.

01186500 Still River at Robertsville Lat 41°58'01", long 73°02'02", Litchfield County, Hydrologic Unit 01080207, on left 

bank 1,500 ft downstream from Sandy Brook, 1 mi southeast of Robertsville, 1 mi 

northwest of Riverton, and 1 mi upstream from mouth.

01195490 Quinnipiac River at Southington Lat 41°36'13", long 72°52'59", Hartford County, Hydrologic Unit 01100004, on west 

bank, 400 ft downstream from bridge on Mill Street, and 500 ft upstream from bridge 

on Center Street in Southington.

01196500 Quinnipiac River at Wallingford Lat 41°26'60", long 72°50'28", New Haven County, Hydrologic Unit 01100004, on right 

bank on Wilbur Cross Highway, 0.8 mi downstream from bridge on Quinnipiac Street 

in Wallingford, and 2 mi upstream from Wharton Brook.

01196620 Mill River near Hamden Lat 41°25'14", long 72°54'10", New Haven County, Hydrologic Unit 01100004, 150 ft 

downstream from bridge on Mount Carmel Avenue, 0.4 mi downstream from Eaton’s 

Brook, and 2.5 mi north of Hamden.

01199000 Housatonic River at Falls Village Lat 41°57'26", long 73°22'09", Litchfield County, Hydrologic Unit 01100005, on left 

bank at hydroelectric plant of Connecticut Light and Power Company at Falls Village, 

1.4 mi downstream from Hollenbeck River, and at mile 75.9.

01199050 Salmon Creek at Lime Rock Lat 41°56'32", long 73°23'28", Litchfield County, Hydrologic Unit 01100005, on left 

bank 300 ft upstream from bridge on Uptown Salisbury Road, 0.6 mi north of Lime 

Rock, and 3.0 mi upstream from mouth.

01200000 Tenmile River near Gaylordsville Lat 41°39'32", long 73°31'44", Dutchess County, Hydrologic Unit 01100005, on right 

bank 0.1 mi downstream from Deuel Hollow Brook, 1.2 mi upstream from New YorkConnecticut State line.

01200500 Housatonic River at Gaylordsville Lat 41°39'12", long 73°29'24", Litchfield County, Hydrologic Unit 01100005, on left 

bank 0.4 mi downstream from hydroelectric plant of Connecticut Light and Power Co., 

0.5 mi upstream from bridge on U.S. Route 7 at Gaylordsville, 1.5 mi downstream 

from Tenmile River, and at mile 50.6.

01201487 Still River ar Route 7 at 

Brookfield Center

Lat 41°27'58", long 73°24'13", Litchfield County, Hydrologic Unit 01100005, on bridge 

on upstream side of State Route 7 South, 800 ft upstream from Silvermile Rd.

01202501 Shepaug River at Peter’s Dam at 

Woodville

Lat 41°71'92", long 73°29'33", Litchfield County, Hydrologic Unit 01100005, 0.2 mi 

downstream from Shepaug Reservoir Dam, at end of Reservoir Rd., 1 mi north of 

Woodville.

01203000 Shepaug River near Roxbury Lat 41°32'59", long 73°19'47", Litchfield County, Hydrologic Unit 01100005, at Wellers 

Bridge, 1.2 mi southwest of Roxbury.

012035055 Pootatuck River at Berkshire Lat 41°24'22", long 73°16'22", Fairfield County, Hydrologic Unit 01100005, on property 

of Pootatuck Fish and Game club, near upstream side of bridge on Mile High Rd.

01203510 Pootatuck River at Sandy Hook Lat 41°25'12", long 73°16'56", Fairfield County, Hydrologic Unit 01100005, at bridge on 

Church Hill Road, at Sandy Hook.

01203600 Nonewaug River at Minortown Lat 41°34'33", long 73°10'43", Litchfield County, Hydrologic Unit 01100005, on right 

bank 1,000 ft downstream from bridge by State Routes 6 and 202 at Minortown, and 

2.5 mi northeast of Woodbury.

01203805 Weekeepeemee River at 

Hotchkissville

Lat 41°33'26", long 73°12'57", Litchfield County, Hydrologic Unit 01100005, on 

downstream left bank at Jack’s Bridge Rd., 500 ft upstream from confluence with 

Pomperaug River and 1 mi north of Woodbury.
Previous Flood Investigation Studies 5

Table 1. Descriptions of U.S. Geological Survey streamflow-gaging stations whose records were used to determine the magnitude 

and frequency of the April 2007 flood in western Connecticut.—Continued

[Station locations shown on figure 1. lat, latitude; long, longitude; mi, miles; ft, feet; right and left bank are referenced facing downstream]

Station 

number Station name Description of station location

01204000 Pomperaug River at Southbury Lat 41°28'54", long 73°13'29", New Haven County, Hydrologic Unit 01100005, on right 

bank 200 ft upstream from bridge on Poverty Rd., 800 ft downstream from Bullet Hill 

Brook, 0.6 mi west of Southbury, and 5.8 mi upstream from mouth.

01205500 Housatonic River at Stevenson Lat 41°23'02", long 73°10'02", New Haven County, Hydrologic Unit 01100005, on left 

bank, 0.2 mi downstream from dam of Connecticut Light and Power Company at 

Stevenson, Fairfield County, 0.2 mi upstream from Eightmile Brook, and at mile 19.2.

01206900 Naugatuck River at Thomaston Lat 41°40'26", long 73°04'12", Litchfield County, Hydrologic Unit 01100005, on left 

bank at downstream side of bridge on U.S. Routes 6 and 202 at Thomaston, 1.5 mi 

downstream from Thomaston Reservoir, 2.5 mi upstream from Branch Brook, and at 

mile 29.5.

01208500 Naugatuck River at Beacon Falls Lat 41°26'31", long 73°03'47", New Haven County, Hydrologic Unit 01100005, on left 

bank at downstream side of bridge on Bridge Street at Beacon Falls, 0.4 mi upstream 

from Bronson Brook, and at mile 10.1.

01208873 Rooster River at Fairfield Lat 41°10'48", long 73°13'09", Fairfield County, Hydrologic Unit 01100006, on left bank, 

on floodwall, at corner of Renwick Drive and Renwick Place, Bridgeport.

01208925 Mill River near Fairfield Lat 41°09'55", long 73°16'13", Fairfield County, Hydrologic Unit 01100006, on right 

bank just downstream from bridge on Duck Farm Rd., 1.5 mi north of Fairfield, 

14.0 mi downstream from headwater of Mill River.

01208950 Sasco Brook near Southport Lat 41°09'10", long 73°18'21", Fairfield County, Hydrologic Unit 01100006, on left down 

stream abutment of bridge on Hulls Farm Rd., 1.5 mi northwest of Southport.

01208990 Saugatuck River near Redding Lat 41°17'40", long 73°23'42", Fairfield County, Hydrologic Unit 01100006, on right 

downstream side of bridge on State Route 53, 100 ft south of intersection of State 

Routes 53 and 107, 0.8 mi upstream from Saugatuck Reservoir, and 1.0 mi southwest 

of Redding.

012095493 Ridgefield Brook at Shields Lane 

near Ridgefield

Lat 41°09'50", long 73°25'11", Fairfield County, Hydrologic Unit 01100006, on right 

bank at end of Sheilds Lane in Town of Ridgefield, on western side of street, 1.4 mi 

west of Redding.

01209700 Norwalk River at South Wilton Lat 41°09'50", long 73°25'11", Fairfield County, Hydrologic Unit 01100006, on right 

bank at upstream side of bridge on Kent Rd. at South Wilton, 2.5 mi north of Norwalk.

01209761 Fivemile River near New Canaan Lat 41°10'28", long 73°30'43", Fairfield County, Hydrologic Unit 01100006, on right 

bank, 40 ft downstream from paved driveway leading to private residence, at end of 

cul-de-sac of Indian Rock Rd. 

01209901 Rippowam River at Stamford Lat 41°03'56", long 73°32'59", Fairfield County, Hydrologic Unit 01100005, on left bank 

100 ft upstream from bridge on Bridge St., 2.7 mi downstream from Holts Ice Pond 

Brook, and 1.7 mi upstream from Long Island Sound.
6 Flood of April 2007 and Flood-Frequency Estimates at Streamflow-Gaging Stations in Western Connecticut

Connecticut River in Connecticut (Ahearn, 2005). The floodfrequency estimates in this report update and supersede previously published flood-frequency estimates for streamflowgaging stations in Connecticut.

In addition to the various flood-frequency studies 

described above, the USGS summarized substantial floods 

in Connecticut in a series of Water-Supply Papers entitled 

“Summary of Floods in the United States” and in a series of 

“State Summaries of Floods” as part of the USGS National 

Water Summary that documents extreme hydrologic events 

on the basis of stream-discharge data (Perry, 2001 and 2005; 

Perry and others, 1998; Paulson and others, 1991). These 

reports detail the areal extent of the most notable floods and 

provide estimates of how frequently floods of such severity 

can be expected to recur. 

Storm Characteristics 

Major storms in Connecticut include nor’easters, which 

form along the East Coast, usually between October and April, 

although they can form any time of the year. A nor’easter 

is a macro-scale storm associated with two major systems 

colliding―a Gulf Stream low-pressure system and an arctic high-pressure system. Usually nor’easters bring massive 

amounts of precipitation, hurricane-force winds, and storm 

surges to coastal areas that create coastal flooding. Nor’easters 

can be devastating, especially in the winter months when most 

damage and deaths are cold-related. 

A late season nor’easter impacted Fairfield and Litchfield 

Counties primarily April 15–16, 2007, and persisted with trace 

amounts of rainfall through April 18. Initially, the precipitation fell as a mixture of wet snow, sleet, and rain across the 

higher elevations and changed to all rain by late Sunday 

afternoon (April 15) into Monday (April 16). The precipitation fell as rain across the lower elevations. Rainfall totals 

from the National Weather Service (NWS) rain gages and 

from the Connecticut Department of Environmental Protection 

(DEP) Flood Alert Network ranged from 3.31 to 7.81 inches 

(fig. 2) (National Climate Data Center, 2007b; M.K. Baribault, Connecticut Department of Environmental Protection, written commun., 2008). The heaviest rain fell across 

southwest and south-central Connecticut, where many small 

rivers and streams exceeded their banks within 12 hours of 

the heavy rainfall during Sunday, April 15 (National Climate 

Data Center, 2007b). Rainfall in Fairfield County ranged 

from 3.57 inches in Bridgeport to 7.81 inches in Ridgefield. 

Rainfall in Litchfield County ranged from 3.31 inches in Falls 

Village to 6.73 inches in New Hartford. Figures 3a and 3b 

show Next-Generation Radar (NEXRAD) 1-day rainfall totals 

ending April 15 and April 16, 2007 (National Weather Service, 

2007a, b). The NEXRAD data are multi-sensor (radar and 

rain) rainfall estimates obtained from NWS River Forecast 

Centers (National Weather Service, 2008a, b).

Conditions prior to a storm affect the amount of runoff that occurs during the storm. Streamflow prior to the 

April 15–18 flooding generally was in the normal range 

(between the 25th and 75th percentiles) of the seasonal high 

flow at the streamflow-gaging stations in the two counties 

examined. Antecedent moisture conditions are substantially 

affected by preceding rainfall amounts. Prior to the April 

storm, recent rainfall had saturated the soils in most of the 

State. Other variables such as air temperature, minimal tree 

canopy, and melting snow and ice also impacted antecedent 

moisture conditions. Air temperatures were below freezing 

in higher elevations. Tree canopy cover was minimal. Precipitation fell as a mixture of wet snow, which changed to all 

rain. The liquid equivalent precipitation total from this storm 

ranged from 3 to slightly more than 7 inches. 

In Ridgefield, rainfall intensity averaged 1 inch every 2 or 

3 hours over a 13-hour period and totaled 7 inches in 23 hours 

(DEP Flood Alert System, Ridgefield station). The 50-year, 

24-hour highest rainfall total for southwestern Connecticut is 

between 7 and 8 inches (DeGaetano, 1997). In Thomaston, 

rainfall intensity averaged 1 inch every 2 or 3 hours over a 5- 

to 6-hour period and totaled about 5 inches in about 15 hours 

(DEP Flood Alert System, Thomaston station). The 25-year, 

12-hour highest rainfall total is between 4.8 and 5.4 inches 

for northwestern Connecticut (DeGaetano, 1997). The recurrence intervals determined from the rainfall-frequency maps 

for Connecticut (DeGaetano, 1997) indicate the rainfall in 

southwestern Fairfield County was approximately equal to or 

greater than 50 years, and for other areas of Fairfield County, 

the recurrence interval was approximately equal to or greater 

than 25 years. 

Event record details from the National Climatic Data 

Center are provided in appendix 2 (National Climatic Data 

Center, 2008b). Additional storm data—information on areas 

affected by river, coastal, and urban flooding; and high winds 

for April 2007 for Fairfield, Litchfield, New Haven, New London, and Hartford counties—and statistics on personal injuries 

and damage estimates is provided in “Storm Data April 2007, 

Volume 49, Number 4,” by the National Weather Service 

(http://www1.ncdc.noaa.gov/pub/orders/04045B03-435CADE8-23F3-70793651897F.PDF) 

Drainage-Basin Characteristics 

Drainage-basin characteristics that could potentially 

affect the magnitude and frequency of floods and that are used 

in Connecticut’s peak-flow regression equations (Ahearn, 

2004) are summarized in table 2. Locations of streamflowgaging stations and the extent of their upstream drainage 

basins are shown in figure 4. Basin characteristics were 

compiled for all stations except three on the main stem of 

the Housatonic River where Connecticut’s peak-flow regression equations are not applicable. The drainage areas for the 
Drainage-Basin Characteristics 7

0 5 10 MILES

EXPLANATION

!. Connecticut Department of Environmental Protection rain gage

! National Weather Service rain gage

Precipitation, in inches

0 10 20 KILOMETERS

Base from U.S. Geological Survey, 1:24,000, 1969 to 1984Connecticut State Plane Projection

NEW YORK

CONNECTICTUT

NEW YORK

MASSACHUSETTS

CONNECTICTUT

RHODE ISLAND

CONNECTICTUT LITCHFIELD COUNTY HARTFORD COUNTY

FAIRFIELD COUNTY

TOLLAND COUNTY

WINDHAM COUNTY

NEW HAVEN COUNTY

NEW LONDON COUNTY MIDDLESEX COUNTY

LONG ISLAND

!.

!.

!.

!.

!.

!.

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!5.6

5.9

3.71

4.02

4.66

3.03

3.57

4.08

7.04

7.04

4.14

4.67

3.98

4.19

4.12

3.31

5.31

6.16

3.57

6.73

4.79

4.39

2.46 2.29

7.81

6.73

6.46

5.12

4.88

4.33

Kent

Sharon

Stafford

Killingly

Norfolk

Salisbury

Litchfield

Newtown Lyme

Lebanon

Guilford

Suffield

Goshen

Tolland

Granby Woodstock

Haddam

New Milford

Cornwall

Danbury

Pomfret

Ashford

Hebron

Union

Montville

Enfield

Oxford Ledyard

Mansfield

Plainfield

Berlin

Groton

Salem

Colchester

Avon

Thompson

Greenwich

Wilton

Glastonbury Bristol

Coventry

Griswold

Canaan

Stamford

Shelton

East Haddam

Preston

Easton

Hartland

Torrington

Ellington

Hamden

Southbury

Voluntown

Redding

Fairfield

Warren

Windsor

Middletown

Somers

Cheshire

Simsbury

Canterbury

Wallingford

Norwich

Woodbury

Sterling

Stonington

Eastford

Waterford

Ridgefield

Canton

Monroe

Willington

Brooklyn

Milford

Washington

North Stonington

Colebrook

Roxbury

Harwinton

Southington

Winchester

Burlington

Morris

Durham

Portland

Meriden

Windham

Barkhamsted

New Hartford

Bozrah

Wolcott

Waterbury

Norwalk

Watertown

Weston

Trumbull

Bethel

Putnam

Bethany

Chaplin

Vernon

Farmington

Lisbon

Bloomfield

Franklin

Old Lyme

Manchester

Orange

Bolton

Clinton

Columbia

Hartford

Westport

Chester

Essex

Darien

Bethlehem

Cromwell

Madison

East Lyme Killingworth

Hampton

East Hampton

Sherman

Branford

Plymouth

Scotland

Stratford

Brookfield

East Windsor

South Windsor

New Fairfield

Andover

Marlborough

North Branford

New Canaan

Middlebury

North Haven Seymour

New Haven

Prospect

Sprague

West Hartford

Woodbridge

Bridgeport

Naugatuck

Westbrook

Bridgewater

East Hartford

East Granby

North Canaan

Rocky Hill

Deep River

Newington

Middlefield

New Britain

Old Saybrook

Plainville

East Haven

Thomaston

Wethersfield

Derby

West Haven

Beacon Falls

Ansonia

Windsor Locks

New London

72° W 72°30' W 73° W 73°30' W

42° N

41°30' N

41° N

Figure 2. Rainfall totals for April 15–17, 2007, at selected National Weather Service and Connecticut Department of Environmental Protection Flood Alert Network rain gages, Long Island, New York and Connecticut (National Climatic Data Center, 2007a; 2007b).
8 Flood of April 2007 and Flood-Frequency Estimates at Streamflow-Gaging Stations in Western Connecticut

Connecticut: 4/15/2007 1-Day Observed Precipitation

Valid at 4/15/2007 1200 UTC—Created 8/14/07 1:11 UTC

Connecticut: 4/16/2007 1-Day Observed Precipitation

Valid at 4/16/2007 1200 UTC—Created 8/14/07 1:09 UTC

A

B

Figure 3. Next-Generation Radar (NEXRAD) 1-day rainfall totals in Connecticut ending (A) April 15, 2007, and 

(B) April 16, 2007 (NOAA, National Weather Service). 
Drainage-Basin Characteristics 9

Kent

Sharon

Norfolk Salisbury

Litchfield

Newtown

Guilford

Suffield

Goshen

Granby

New Milford

Cornwall

Danbury

Enfield

Oxford

Berlin

Haddam

Avon

Greenwich

Wilton

Madison

Bristol

Canaan

Stamford

Shelton

Easton

Hartland

Torrington

Hamden

Southbury

Redding

Fairfield

Warren

Windsor

Middletown

Cheshire

Simsbury

Glastonbury

Wallingford

Woodbury

Ridgefield

Canton

Monroe

Milford

Washington

Colebrook

Roxbury

Killingworth

Harwinton

Southington

Winchester

Burlington

Morris

Durham

Portland

Meriden

Barkhamsted

New Hartford

Wolcott

Waterbury

Watertown

Weston

Trumbull

Bethel

Bethany

Sherman

Farmington

Bloomfield

Plymouth

Orange

Clinton

Hartford

Westport

Brookfield

East Windsor

South Windsor

Darien

Manchester

Bethlehem

Prospect Naugatuck

North Canaan

Cromwell

Rocky Hill

Norwalk

Branford

Stratford

New Fairfield

North Branford

East Hampton

New Canaan

Middlebury

North Haven Seymour

New Haven

West Hartford

Woodbridge

Bridgeport

Bridgewater

East Hartford

East Granby

Newington

Middlefield

New Britain Plainville

East Haven

Thomaston

Wethersfield

Derby

West Haven

Chester

Ansonia

Beacon Falls

Windsor Locks

Somers

Westbrook

Marlborough

73°30' W 73° W 72°30' W

42° N

41°30' N

41° N

EXPLANATION

Gaged drainage basin (variously colored)

County boundary

State boundary

Town boundary

# U.S. Geological Survey streamflow-gaging 

station and identifier

Base from U.S. Geological Survey, 1:24,000, 1969–1984

Connecticut State Plane Projection

01209700

##

#

#

#

#

# #

#

#

#

#

#

#

#

#

#

#

# # #

#

#

# #

#

#

#

LITCHFIELD COUNTY HARTFORD COUNTY

FAIRFIELD COUNTY

NEW HAVEN COUNTY

MIDDLESEX

COUNTY

MASSACHUSETTS

CONNECTICUT

NEW YORK

0 5 10 MILES

0 10 20 KILOMETERS

01195490

01209901

01208990

01208500

01206900

01204000

01203805

01202501

01201487

01200500

01199050

01196620

01196500

01186000

01209761

01209700

01208950

01208925

01208873

01205500

01203600

01203510

01203000

01200000

01199000

01186500

012035055

012095493

Figure 4. Drainage basins for U.S. Geological Survey streamflow-gaging stations in western Connecticut.
10 Flood of April 2007 and Flood-Frequency Estimates at Streamflow-Gaging Stations in Western Connecticut

Table 2. Drainage-basin characteristics for the streamflow-gaging stations used to investigate the April 2007 flood in western Connecticut.

[Station locations are shown in figure 1. Impervious cover and land cover derived from National Land Cover Data at a 30-meter resolution grid (U.S. Geological Survey, 

Data at a 30-meter resolution grid (U.S. Geological Survey, 2001); Drainage area, mean basin elevation, and 24-hour rainfall derived from Connecticut StreamStats (Ahearn, 

USGS 

station 

number

Station name

Drainage 

area 

(mi2)

Impervious cover 

(percentage 

of drainage 

area)

Water 

(percentage of 

drainage 

area)

Developed, 

open space 

(percentage 

of drainage 

area)

Developed, low 

intensity 

(percentage 

of drainage 

area)

Developed, 

medium 

intensity 

(percentage 

of drainage 

area) 

Farmington River Basin 

01186000 West Branch Farmington River at Riverton 131 0.4 4.4 4.7 0.5 0.1 

01186500 Still River at Robertsville 85 1.7 2.5 5.7 2.3 0.8 

South-Central Coast Basin 

01195490 Quinnipiac River at Southington 17.4 22.7 0.8 21.3 22.3 17.2 

01196500 Quinnipiac River at Wallingford 115 16.4 1.4 18.7 19.5 10.9 

01196620 Mill River near Hamden 24.5 8.1 0.2 16.0 14.2 2.8 

Housatonic River Basin 

01199000 Housatonic River at Falls Village 634 -- -- -- -- -- 

01199050 Salmon Creek at Lime Rock 29.4 0.8 3.9 4.7 1.0 0.4 

01200000 Tenmile River near Gaylordsville 203 1.2 1.2 4.7 1.5 0.6 

01200500 Housatonic River at Gaylordsville 996 -- -- -- -- -- 

01201487* Still River at Route 7 at Brookfield Center 62.3 14.4 2.3 13.8 14.0 10.0 

01202501 Shepaug River at Peters Dam at Woodville 38.1 0.5 4.8 4.6 0.5 0.0 

01203000 Shepaug River near Roxbury 132 1.1 2.9 5.9 1.4 0.3 

012035055 Pootatuck River at Berkshire 16 3.1 0.5 9.2 5.0 1.0 

01203510 Pootatuck River at Sandy Hook 24.8 5.4 0.3 11.0 8.1 2.5 

01203600 Nonewaug River at Minortown 17.7 1.9 1.1 6.4 3.8 0.4 

01203805 Weekeepeemee River at Hotchkissville 26.8 0.9 0.7 6.5 1.0 0.1 

01204000 Pomperaug River at Southbury 75.1 2.1 0.7 7.1 3.1 0.8 

01205500 Housatonic River at Stevenson 1,544 -- -- -- -- -- 

01206900 Naugatuck River at Thomaston 99.8 4.2 2.0 6.8 4.4 2.9 

01208500 Naugatuck River at Beacon Falls 260 8.6 1.9 9.1 8.4 6.8 

Southwest Coast Basin 

01208873 Rooster River at Fairfield 10.6 36.3 0.8 28.1 29.0 29.9 

01208925 Mill River near Fairfield 28.6 5.7 5.4 17.3 9.1 1.7 

01208950 Sasco Brook near Southport 7.38 5.4 0.0 30.0 7.2 0.5 

01208990 Saugatuck River near Redding 21.0 1.2 1.3 11.9 0.8 0.3 

012095493 Ridgefield Brook at Shields Lane near 

Ridgefield

3.39 11.8 0.0 18.1 10.6 6.4 

01209700 Norwalk River at South Wilton 30.0 5.2 0.8 14.8 4.5 2.9 

01209761 Fivemile River near New Canaan 1.00 1.7 2.3 7.2 3.1 0.0 

01209901 Rippowam River at Stamford 34.0 6.1 2.8 18.2 7.5 2.8 

*

 Basin characteristics for 01201487 Still River exclude the Candlewood Lake drainage area of 41.1 square miles.
Storm Characteristics 11

Land cover derived from National Land Cover Data at a 30-meter resolution grid (U.S. Geological Survey, 2001); Mean basin slope derived from the National Elevation 

2004); USGS, U.S. Geological Survey; mi2, square miles; GIS, geographic information systems; --, not determined]

Developed, 

high intensity 

(percentage of 

drainage area)

Forest 

(percentage 

of drainage 

area)

Main basin 

slope 

(feet)

GIS drainage 

area at 

selected 

point 

(mi2)

Mean basin 

elevation 

(feet)

2-year, 

24-hour 

rainfall 

(inches)

10-year, 

24-hour 

rainfall 

(inches)

25-year, 

24-hour 

rainfall 

(inches)

50-year, 

24-hour 

rainfall 

(inches)

100-year, 

24-hour 

rainfall 

(inches)

Farmington River Basin

0.0 81.6 10.8 129 1,380 3.2 5.0 6.4 7.8 9.4

0.1 78.9 11.5 85.5 1,210 3.4 5.2 6.6 7.9 9.4

South-Central Coast Basin

3.1 27.8 6.2 17.8 258 3.6 5.2 6.6 7.8 9.2

1.9 38.7 7.2 111 302 3.6 5.2 6.4 7.6 8.9

0.4 58.0 8.2 24.5 303 3.6 5.2 6.4 7.6 8.9

Housatonic River Basin

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

0.0 73.1 13.0 29.4 1,170 2.9 4.2 5.3 6.2 7.3

0.1 55.0 12.9 200 819 3 4.3 5.3 6.3 7.4

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

2.8 47.8 9.4 62.3 597 3.6 5.0 6.1 7.0 8.1

0.0 74.2 9.7 38.4 1,210 3.3 4.7 5.9 6.9 8.1

0.0 67.1 9.9 132 1,020 3.3 4.7 5.8 6.8 7.9

0.2 63.8 7.5 16.3 530 3.8 5.3 6.4 7.3 8.5

0.5 58.0 7.8 24.9 511 3.8 5.2 6.3 7.3 8.4

0.0 49.0 7.7 17.7 764 3.3 4.5 5.5 6.3 7.3

0.0 69.6 10.8 27 763 3.3 4.5 5.5 6.3 7.3

0.1 67.7 9.9 75.3 653 3.3 4.6 5.5 6.4 7.4

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

0.5 71.0 10.3 101 978 3.5 5.1 6.4 7.6 9.0

1.1 59.4 9.9 259 784 3.4 4.9 6.0 7.1 8.3

Southwest Coast Basin

5.7 6.2 5.1 11 244 3.6 5.3 6.7 7.9 9.5

0.3 56.1 7.0 28.6 349 3.6 5.3 6.5 7.7 9.1

0.0 50.7 5.3 7.38 230 3.6 5.2 6.5 7.7 9.1

0.1 75.3 12.0 22.3 571 3.7 5.2 6.3 7.3 8.4

3.1 39.0 6.8 3.39 642 3.7 5.2 6.4 7.4 8.7

0.9 67.1 8.56 29.9 477 3.7 5.2 6.3 7.4 8.6

0.3 66.4 4.4 1.2 504 3.6 5.2 6.5 7.6 8.9

0.6 61.9 7.1 34.2 402 3.6 5.3 6.5 7.7 9.0
12 Flood of April 2007 and Flood-Frequency Estimates at Streamflow-Gaging Stations in Western Connecticut

Housatonic River streamflow-gaging stations are not shown in 

figure 4 because of the limited extent of the map. 

Drainage-basin characteristics were determined from 

digital map data and included (1) drainage subbasins at 

1:24,000 scale from the DEP; (2) elevation from USGS Digital 

Elevation Models (DEMs) at 1:24,000 scale from the USGS 

National Elevation Dataset (U.S. Geological Survey, 2006); 

(3) land cover at 1:24,000 scale from the USGS National Land 

Cover Dataset (U.S. Geological Survey, 2000); (4) hydrography (stream network) for Connecticut at 1:24,000 scale 

(Danenberg and Bogar, 1994; U.S. Geological Survey, 2005); 

and (5) maximum 24-hour rainfall characteristics (Miller and 

others, 2003; B.N. Belcher, Northeast Regional Climate Center, written commun., 2003).

The drainage basins analyzed had a wide range of characteristics and represent various stream types—from steep, 

mountainous streams to low-gradient, meandering streams and 

from small streams to large rivers—and basin types—from 

mostly forested environments to urban environments. Land 

elevations range from zero feet above the North American 

Vertical Datum of 1988 (NAVD 88) at the coast in Fairfield 

County to greater than 2,300 ft in the northwestern part of 

Litchfield County. Litchfield County has the steepest topography in the State; land-surface elevations range from 500 to 

2,300 ft. For the streamflow-gaging stations studied, drainage 

areas range from 1.0 mi2 (Fivemile River near New Canaan) to 

1,544 mi2 (Housatonic River at Stevenson). Mean basin slopes 

range from 4.4 percent (Fivemile River near New Canaan) to 

13 percent (Housatonic River at Falls Village). Mean basin 

elevations range from 230 ft (Sasco River near Southport) to 

1,380 ft (Farmington River at Riverton). Basin characteristics 

were not compiled for the three stations on the main stem of 

the Housatonic River because the Housatonic River Basin 

extends into Massachusetts, and Connecticut’s peak-flow 

regression equations do not apply. 

Streams in urbanized basins tend to rise more quickly 

during storms and have higher peak discharges than those 

in less urbanized basins. To determine the degree to which 

a basin has been urbanized, impervious area was evaluated 

(U.S. Geological Survey, 2000). The amount of impervious area for the basins with streamflow-gaging stations 

ranged from 0.4 percent (Farmington River at Riverton) to 

36.3 percent (Rooster River at Fairfield). Basins that have 

more than 20 percent of their drainage area designated as 

impervious include Rooster River in Fairfield (36.3 percent) 

and Quinnipiac River at Southington (22.7 percent). The 

flood-frequency estimates for these basins were adjusted for 

urbanization using the nationwide seven-parameter urban 

regression equation (Sauer and others, 1983).

Annual Peak-Discharge Data

Annual peak-discharge data are used to characterize the 

magnitude and frequency of floods, such as the flood from 

the April 2007 storm. The annual peak discharge is defined 

as the maximum flow occurring in a water year (October 

1–September 30). Annual peak-discharge data, along with 

the corresponding gage heights, date of occurrence, information on the cause of the annual peak (for example, hurricane, ice jam, dam failure), and known effects of flood 

control, urbanization, or basin changes are stored in the 

USGS National Water Information System (NWIS) database 

(http://waterdata.usgs.gov/nwis/peak).

The maximum peak stages and peak discharges recorded 

at the 28 streamflow-gaging stations for the April 2007 flood 

and for the entire period of record are summarized in table 3. 

The streamflow-gaging station locations are shown on figure 1 

and described in table 1. The peak discharges at streamflowgaging stations for the April 2007 flood are the maximum 

flows occurring in water year 2007, except for one station—

Rooster River at Fairfield—where the maximum flow occurred 

on March 2 (maximum peak discharge, 2,040 ft3/s; gage 

height, 10.78 ft). 

During the April flood, all the rivers studied for which 

flood stages have been established by the NWS rose to or 

exceeded flood stage (table 3). The NWS defines flood stage 

as the river level that begins to impact life and (or) property. 

The main stem of the Quinnipiac River at Southington (USGS 

streamflow-gaging station 01195490) and at Wallingford 

(01196500) rose 6.97 ft and 1.51 ft above flood stage on 

April 16, respectively. The lower Housatonic River crested 

8.96 ft above flood stage at Stevenson (01205500) and 4.97 ft 

above flood stage at Gaylordsville (01200500) on April 16; 

the upper Housatonic River crested 3.93 ft above flood stage 

at Falls Village (01199000) on April 18. The main stem of the 

Naugatuck River at Beacon Falls (01208500) rose to 12.1 ft 

on April 16; 0.1 ft above flood stage. Two other rivers with 

established flood stages (Tenmile River near Gaylordsville 

(01200000) and Pomperaug River at Southbury (01204000)) 

also rose above flood stage by 2.23 ft and 4.85 ft, respectively, 

on April 16. 

During the 20th century, the most severe floods in terms 

of magnitude in western Connecticut occurred in March 1936, 

September 1938, August and October 1955, January 1979, and 

June 1982. The April 2007 peak discharges were compared 

in magnitude to all the historical annual peak discharges for 

streamflow-gaging stations with records extending back to 

1955 or earlier. Nine of the 28 streamflow-gaging stations had 

records extending back to 1955 or earlier and are not affected 

by flood-control regulation (only three stations in the western 

region have records extending back to 1936). The ranking 

of the April 2007 peak discharges in relation to other annual 

peak discharges in the station records is listed in table 3. The 

April 2007 peak discharges in the Southwest Coast Basin 

(fig. 1) ranked between the highest and fourth highest in the 

streamflow-gaging records (table 3); the 1955 annual peak discharges are the highest peak discharges in the station records. 

Only 2 of the 8 streamflow-gaging stations in the Southwest 

Coast Basin have records back to 1955. The April 2007 peak 

discharges in the Housatonic and South Central Coast Basins 
Peak Discharges Affected by Flood-Control Dams 13

(fig. 1) ranked between the fourth and tenth highest peak discharges with years of records greater than 50 and 30 years for 

the Housatonic and South Central Coast Basins, respectively 

(table 3). Generally, the 1938 and 1955 floods are the largest 

floods on record in the Housatonic and South Central Coast 

Basins. Other major floods—January 1979 and June 1982—

impacted several of the stations in the Housatonic and South 

Central Coast Basins. For the majority of these stations, the 

April 2007 peak discharges were less than the January 1979 

and June 1982 peak discharges. Streamflow-gaging stations 

in the Connecticut River Basin are located downstream from 

flood-control dams. The April 2007 peak discharges for these 

stations ranked between the highest and 3rd highest since the 

flood-control dams were completed between 1962 and 1969. 

Peak discharges on the Rippowam River at Stamford (01209901) and the Quinnipiac River at Southington 

(01195490) were the highest recorded discharges for these stations; both stations have less than 20 years of record. Peak discharges on the Norwalk River (01209700) and the Saugatuck 

River (01208990) were the second highest in the records for 

these stations. Although the April peak discharges at Mill 

River (01208925) and Rooster River (01208873) were not the 

highest for the 2007 water year, peak discharges on the Mill 

River and Rooster River in Fairfield were the third highest in 

35 and 30 years of record, respectively. Peak discharge on the 

Housatonic River at Gaylordsville (1200500) was the seventh 

largest peak in 95 years of record. 

Peak discharges were determined by applying the peak 

stage to the most current stage-discharge relation developed 

by taking flow measurements at different water elevations. At 

seven streamflow-gaging stations (Tenmile River, Still River, 

Pootatuck River at Sandy Hook, Pootatuck at Berkshire, Saugatuck River, Ridgefield Brook, and Fivemile River), the current stage-discharge relations are not yet developed (highest 

discharge measurement is less than 2 times the maximum peak 

discharge recorded) for the April 2007 flood peak. In such 

cases, the stage-discharge relation was graphically extended to 

the peak stage by using the trend of the upper end of the peak 

discharge relation and the stage-discharge relation from watersurface profiles. The error introduced to the peak-discharge 

relation is unknown when the stage-discharge relation is 

extended without manually measured discharges; this introduces additional error in the flood-frequency estimates.

Peak Discharges Affected by FloodControl Dams 

The U.S. Army Corp of Engineers (USACE) constructed 

10 flood-control dams in and near Litchfield County after the 

1955 floods in New England: seven dams in the Naugatuck 

River Basin, which control 152 mi2 or about 50 percent of the 

basin; and three dams in the Farmington River Basin, which 

control 139 mi2 or about 25 and 50 percent of the Still River 

and West Branch of the Farmington River Basins, respectively 

(fig. 5). The flood-control dams in the Naugatuck River Basin 

are located on tributaries to the Naugatuck River, except for 

Thomaston Dam, which is located on the main stem. Thomaston Dam is the largest flood-control dam in the Naugatuck 

River Basin and helps to minimize flooding in the cities of 

Naugatuck, Seymour, and Ansonia. The Colebrook River 

Lake Dam is the largest flood-control dam in the Farmington 

River Basin and helps to minimize flooding on the main stem 

of the Farmington River. The Mad River Dam and Sucker 

Brook Dam have comparatively small drainage areas and 

can minimize flooding on the Mad River in Winsted. Since 

the flood-control dams were constructed, peak discharges 

for the April 2007 flood on the Still River (tributary of the 

Farmington River, 01186500) and West Branch Farmington 

River (01186000) were the highest and third highest, respectively. To maximize flood protection, a minimum amount of 

water is maintained in the flood-control pool prior to flooding. Specific reductions of peak discharges resulting from 

these dams are tabulated by USACE after noteworthy storms 

and are available on the USACE Web site (https://rsgis.

crrel.usace.army.mil/nae/pls/nae/nae_web.nae_webmenu.

displaymenu?menu=main, accessed September 29, 2008).

During the April 2007 flood, the USACE dams stored 

substantial runoff. Flood-control storage at the 10 USACE 

dams ranged from 9 to 55 percent of the dam capacities. All 

discharge releases (at dams with controlled gates) were considered controlled releases; no water spilled over the spillway 

crests. The highest pool stage recorded occurred during the 

April 2007 flood at three dams—Mad River Dam, Sucker 

Brook Dam, and East Branch Dam. The Colebrook River 

Lake Dam reached 55 percent capacity and peaked 34 ft below 

the spillway crest. In contrast, Sucker Brook Dam reached 9 

percent capacity and peaked 39 ft below the spillway crest. 

A summary of maximum pool levels reached and the percent 

of storage for individual dams during the April 2007 flood is 

included in appendix 3—Summary of Maximum Pool Levels 

for U.S. Army Corps of Engineers Flood-Control Dams. Peak 

discharges also are affected by storm runoff stored in watersupply reservoirs. However, water-supply reservoirs in the 

Mill River Basin (Hemlock Reservoir and Easton Reservoir) 

and the Rippowam River Basin (Laurel Reservoir) were near 

or at capacity at the start of the April 2007 flood, and consequently, the flood-storage capacity was negligible. Information on the activities of the USACE New England District’s 

Reservoir Regulation Section, which was responsible for the 

Corps flood-control reservoirs and hurricane barriers during 

the April 2007 flood, is provided in “Water Control Management, Annual Report FY 2007, April 16 (Patriots Day), 2007 

Flood,” U.S. Army Corp of Engineers, New England District 

http://reservoircontrol.com/
14 Flood of April 2007 and Flood-Frequency Estimates at Streamflow-Gaging Stations in Western Connecticut

Table 3. Summary of peak stages and peak discharges during the April 2007 flood and maximum peak discharges and gage heights for 

[USGS, U.S. Geological Survey; NWS, National Weather Service; mi2, square miles; ft, feet; ft3/s, cubic feet per second; ft3/s/mi2, cubic feet per second per 

roads and or damage man-made features and generally applies to a localized point.]

USGS 

station 

number 

(figure 1)

Station name

Drainage 

area 

(mi2)

NWS flood 

stage 

(ft)

Flood of April 15–16, 2007 

Peak stage, 

local datum 

(ft)

Peak 

discharge 

(ft3/s)

Peak 

discharge

(ft3/s/mi2)

Date Time 

(hr)

Farmington River Basin 

01186000 West Branch Farmington River at 

Riverton

131 8.13 3,040 23 04/24/07 10:15 AM 

01186500 Still River at Robertsville 85 9.7 8,130 96 04/16/07 9:00 AM 

South-Central Coast 

01195490 Quinnipiac River at Southington 17.4 3.5 10.47 949 55 04/16/07 9:15 AM 

01196500 Quinnipiac River at Wallingford 115 10.0 11.51 4,220 37 04/16/07 05:30 PM 

01196620 Mill River near Hamden 24.5 5.2 1,940 79 04/15/07 011:00 PM 

Housatonic River Basin 

01199000 Housatonic River at Falls Village 634 7.0 10.93 10,900 17 04/18/07 1:15 AM 

01199050 Salmon Creek at Lime Rock 29.4 5.18 1,400 48 04/16/07 12:45 PM 

01200000 Tenmile River near Gaylordsville 203 9.0 11.23 9,650 48 04/16/07 05:45 PM 

01200500 Housatonic River at Gaylordsville 996 8.0 12.97 23,400 23 04/16/07 011:00 PM 

01201487 Still River at Route 7 at 

Brookfield Center4

62.3 18.69 3,580 57 04/16/07 12:45 PM 

01202501 Shepaug River at Peters Dam at 

Woodville

38.1 6.51 4,160 109 04/16/07 8:45 AM 

01203000 Shepaug River near Roxbury 132 10.37 8,000 61 04/16/07 10:45 AM 

012035055 Pootatuck River at Berkshire 16 7.6 e3,000 194 04/16/07 7:45 AM 

01203510 Pootatuck River at Sandy Hook 24.8 10.05 4,090 165 04/16/07 7:30 AM 

01203600 Nonewaug River at Minortown 17.7 6.83 4,320 244 04/16/07 1:00 AM 

01203805 Weekeepeemee River at 

Hotchkissville

26.8 9.19 3,670 137 04/16/07 2:30 AM 

01204000 Pomperaug River at Southbury 75.1 9.0 13.85 7,350 98 04/16/07 7:00 AM 

01205500 Housatonic River at Stevenson 1,544 11.0 19.96 50,300 33 04/16/07 02:00 PM 

01206900 Naugatuck River at Thomaston 99.8 5.91 3,210 32 04/20/07 9:00 AM 

01208500 Naugatuck River at Beacon Falls 260 12.0 12.1 13,300 51 04/16/07 -- 

Southwest Coast Basin 

01208873 Rooster River at Fairfield 10.6 10.08 1,730 163 04/15/07 05:00 PM 

01208925 Mill River near Fairfield 28.6 6.99 3,010 105 04/16/07 4:15 AM 

01208950 Sasco Brook near Southport5 7.38 6.28 1,300 176 04/15/07 07:00 PM 

01208990 Saugatuck River near Redding6 21.0 6.01 2,280 109 04/16/07 6:45 AM 

012095493 Ridgefield Brook at Shields Lane 

near Ridgefield

3.39 5.21 373 110 04/16/07 10:15 AM 

01209700 Norwalk River at South Wilton 7 30.0 8.37 3,490 116 04/16/07 5:00 AM 

01209761 Fivemile River near New Canaan 1.00 -- e200 -- 04/16/07 -- 

01209901 Rippowam River at Stamford 34.0 7.85 2,490 73 04/16/07 2:15 AM 

1

 Regulated during April 2007 flood: Y, yes; N, no.

2

 Period of record in bold italics indicates annual peak discharges affected by flood-control regulation.

3

 For stations with flood-control regulation, rankings based on regulated years.

4

 Annual peak discharges from 1932–1966 from station 01201500, Still River near Lanesville, and annual-peak discharges from 1967–1984 from station 01201510, Still 

5

 Historic record used to estimate October 16, 1955, flood peak for Sasco River by averaging Norwalk River and Saugatuck River peak discharges per square mile. An 

6

 October 16, 1955, peak discharge estimated from downstream peak discharge on Saugatuck River at drainage area 33.5 mi2.

7

 October 16, 1955, peak discharge estimated from upstream peak discharge on Norwalk River at drainage area 7.54 mi2.
Peak Discharges Affected by Flood-Control Dams 15

the period of record for 28 streamflow-gaging stations in western Connecticut. 

square mile; hr, hours; WY, water year; e, estimated; -- unknown; National Weather Service flood stage refers to flood water levels that actually flood 

Maximum stage and discharge for period 

of record through water year 2006 Regulated 

during 

April 2007 

flood 1

Period of record 

(WY) 2

Number of 

peaks in 

record

Rank of April 

peak 

3

Peak stage 

(ft)

Peak 

discharge 

(ft3/s)

Date

Farmington River Basin

-- 57,200 08/19/1955 Y 1955–1968, 1969–2007 53 3

16.48 44,000 08/19/1955 Y 1936, 1938, 1949–1961, 1962–2007 61 1

South-Central Coast

10.04 876 05/13/2006 N 1988–2007 20 1

14.02 8,200 06/06/1982 N 1931–2007 77 6

9.53 5,580 06/06/1982 N 1969–1970, 1979–2007 31 6

Housatonic River Basin

19.4 23,900 01/01/1949 Y 1913–2007 95 10

13.5 6,300 08/19/1955 N 1949, 1955, 1962–2007 48 8

14.9 17,400 08/19/1955 N 1930–1987, 1992–2007 74 5

18.58 51,800 08/19/1955 Y 1901–1914, 1924, 1928–2007 95 7

14.11 7,980 10/16/1955 N 1932–1984, 2002–2007 59 4

6.22 3,740 10/15/2005 N 2001–2007 7 1

17.2 50,300 8/19/1955 N 1931–1984, 2007 55 5

-- -- -- N 2007 1 1

8.47 2,720 1/25/1979 N 1966–1984, 2007 20 1

13.2 10,000 8/19/1955 N 1955, 1963–1976, 1979, 2002–2007 23 4

9.33 3,790 4/23/2006 N 2001–2007 7 2

21.8 29,400 8/19/1955 N 1933–2007 75 8

24.5 75,800 10/16/1955 Y 1924–1925, 1928–2007 82 7

27.0 53,400 8/19/1955 Y 1955, 1960–2007 49 10

25.7 106,000 8/19/1955 Y 1920–1959, 1960–2007 88 4

Southwest Coast Basin

11.65 2,170 4/9/1980 N 1978–2007 30 3

7.15 1,800 4/10/1980 N 1973–2007 35 3

7.00 1,640 6/19/1972 N 1960–2007 49 4

-- 5,000 10/16/1955 N 1956, 1962–2007 47 2

4.17 149 10/4/2005 N 2003–2007 4 1

13.5 9,000 10/16/1955 N 1956, 1963–2007 46 2

3.28 56 6/19/1999 N 1998–2007 10 1

6.56 1,890 4/10/1980 N 1978–1982, 2002–2007 11 1

River at Lanesville adjusted to site (data transferred using transfer equation 6.12, Drainage Manual, Connecticut Department of Transportation, January 2000).

estimate of 2,000 ft3/s used in frequency analysis.
16 Flood of April 2007 and Flood-Frequency Estimates at Streamflow-Gaging Stations in Western Connecticut

Figure 5. Locations of U.S. Army Corps of Engineers flood-control dams in the Farmington and Naugatuck River Basins, U.S. 

Geological Survey streamflow-gaging stations, and their upstream drainage basins in western Connecticut. 

0 5 10 MILES

0 10 20 KILOMETERS

MASSACHUSETTS

CONNECTICUT

NEW YORK

EXPLANATION

!>

#

01190050

Base from U.S. Geological Survey, 1:24,000, 1969 to 1984

Connecticut State Plane Projection

County boundary

State boundary

Gaged drainage basin

Farmington River Basin

Naugatuck River Basin

Town boundary

U.S. Army Corp of Engineers Dam

U.S. Geological Survey streamflow-gaging 

station and number

Farmington River Basin

Naugatuck River Basin

LITCHFIELD COUNTY

MIDDLESEX

COUNTY

NEW HAVEN COUNTY

HARTFORD COUNTY

FAIRFIELD COUNTY

MAD RIVER DAM

BLACK ROCK DAM

HANCOCK BROOK DAM

COLEBROOK RIVER DAM

NORTHFIELD BROOK DAM

HALL MEADOW BROOK DAM

EAST BRANCH RESERVOIR DAM

SUCKER BROOK RESERVOIR DAM

HOP BROOK FLOOD CONTROL DAM

THOMASTON DAM

!>

!>

!>

!>

!>

!>

!>

!>

!>

!>

01195490

01209901

01209761

01209700

01209700

01208990

01208950

01208925

01208873

01208500

01206900

01205500

01204000

01203805

01203600

01203510

01203000

01202501

01201487

01200500 01200000

01199000

01196620

01196500

01186500

01186000

012035055

012095493

##

#

#

##

# #

#

#

#

#

#

#

#

#

#

#

# # #

#

#

# #

#

#

#

73°30' W 73° W 72°30' W

42° N

41°30' N

41° N
Flood-Frequency Analyses of the April 2007 Flooding 17

Flood-Frequency Analyses of the April 

2007 Flooding

Flood-frequency estimates for 24 of 28 streamflowgaging stations were calculated using the guidelines in 

Bulletin 17B of the Interagency Advisory Committee on Water 

Data (1982). These guidelines present a uniform technique 

for estimating flood frequencies for streamflow-gaging 

stations. Guidelines in Bulletin 17B recommend determining 

the flood-frequency distribution by fitting a log-Pearson, 

Type III frequency distribution to the log transformations of 

the annual peak discharges. Fitting the distribution requires 

calculating the mean, standard deviation, and skew coefficient 

of the logarithms of annual peak-dischcarge series. The mean, 

standard deviation, and skew coefficient describe the midpoint, slope, and curvature of the peak-discharge frequency 

curve, respectively. Flood-frequency estimates are computed 

by inserting the three statistics of the frequency distribution 

into the equation.

LogQ = + X KS, (1)

where

LogQ = the logarithm of the flood-frequency 

estimate for a given exceedance 

probability, in cubic feet per second;

X = the mean of the logarithms of annual 

peak discharges at the streamflow-gaging 

station;

K = a factor dependent on the skew coefficient 

and exceedance probability; and 

S = the standard deviation of the means of the 

logarithms of annual peak discharges.

The USGS computer program PeakFQ, version 5.2 

(Flynn, Kirby, and Hummel, 2006), which is based on Bulletin 17B guidelines, was used to compute the flood-frequency 

statistics for the streamflow-gaging stations in this report. 

PEAKFQ automates many of the analysis procedures recommended in Bulletin 17B, such as outlier detection, historical 

periods, and weighting of station skews with a generalized 

skew based on the skews of other stations within the region. 

The frequency curves generated from PeakFQ for this study 

were reviewed to determine how well the annual peak discharges fit the theoretical (log-Pearson Type III) distribution 

and were adjusted for outliers using historic flood information when appropriate. The assignment of a longer historic 

period is based upon examination of longer flood records from 

nearby, similar streamflow-gaging stations. The computer program and user manual for PeakFQ are available at http://water.

usgs.gov/software/peakfq.html, accessed April 15, 2008.

Flood-Frequency Estimates for StreamflowGaging Stations 

Flood-frequency estimates for the 0.50, 0.20, 0.10, 0.04, 

0.02, 0.01, and 0.002 exceedance probabilities (2-, 5-, 10-, 

25-, 50-, 100-, and 500-year recurrence interval, respectively) 

were computed for 24 of 28 active gaging stations in Connecticut (table 4). Four of the 28 gaging stations—Shepaug 

River at Peters Dam at Woodville (01202501), Pootatuck 

River at Berkshire (102035055), Ridgefield Brook at Shields 

Lane near Ridgefield (012095493), and Fivemile River near 

New Canaan (01209761)—had insufficient data on which 

to perform a flood-frequency analysis. The flood-frequency 

estimates are weighted averages determined by combining the 

frequency estimates from a log-Pearson Type III analysis with 

the frequency estimates from the peak-discharge regression 

equations. The weight applied to the frequency estimates is 

based on the years of record at the station and the equivalent 

years of record for the regression equation. Generally, a floodfrequency estimate derived by combining and weighting estimates is more reliable than one derived from the log-Pearson 

Type III analysis, particularly for stations with a short length 

of record relative to the recurrence interval evaluated. A longer 

record than the period of the flood frequency to be determined 

is necessary for the frequency estimate to be highly dependable (within 10 percent of the true, long-term value), as 

described in Dalrymple (1960). 

A regional regression equation for estimating the 

peak discharge for the 0.2 exceedance probability (5-year 

recurrence interval) does not exist for Connecticut; therefore, 

a weighted average for the 0.2 exceedance probability was not 

derived. Flood-frequency estimates for the West Branch of 

the Farmington River (01186000), Still River at Robertsville 

(01186500), Naugatuck River at Thomaston (01206900), and 

Naugatuck River at Beacon Falls (01208500) (streams with 

flood-control dams) and for the main stem of the Housatonic 

River (01199000, 01200500, and 01205500) (streams with 

regulation) are based on a log-Pearson Type III analysis of 

the post-flood control or regulated record using the individual 

station skew.

The results of the flood-frequency analysis indicate that 

the April 2007 peak discharges equaled a 0.02 exceedance 

probability (50-year recurrence interval) at three stations—

Norwalk River at South Wilton (01209700), Pootatuck River 

at Sandy Hook (01203510), and Still River at Robertsville 

(01186500). These stations had the smallest probabilities of 

exceedance in Connecticut. The majority of the stations had 

peak discharges between 0.1 to 0.04 exceedance probabilities (10 to 25-year recurrence interval): seven stations with 

0.04 exceedance probability (25-year recurrence interval); 

six stations between a 0.10 and 0.04 exceedance probability (10- and 25-year recurrence interval); and seven stations 

with a 0.10 exceedance probability (10-year recurrence 

interval) (table 4). One station—Naugatuck River at Thomaston (01206900)—had a 0.2 exceedance probability (5-year 

recurrence interval). 
18 Flood of April 2007 and Flood-Frequency Estimates at Streamflow-Gaging Stations in Western Connecticut Table 4. Flood-frequency estimates for selected exceedance probabilities for 24 streamflow-gaging stations in western Connecticut.—Continued [Station locations are shown on figure 1. USGS, U.S. Geological Survey; ft3/s, cubic feet per second; yr, years; e, estimated; LP3, log-Pearson Type III; WGT, weighted average; REG EQ, Reqression equation; MOVE-1, Maintenance of Variance Extension; --, not determined] USGS station number Station name April 2007 peak discharge (ft3/s) Exceedance probability of April peak discharge (percent) Discharges for selected exceedance probabilities (ft3/s) Frequency determination method 0.5 0.2 0.1 0.04 0.02 0.01 0.002 Farmington River Basin 01186000 West Branch Farmington River at Riverton 3,040 0.04 1,490 2,240 2,680 3,180 3,500 3,790 4,370 LP3 01186500 Still River at Robertsville 8,130 0.02 2,870 4,550 5,650 7,010 7,990 8,930 11,000 LP3 South Central Coast Basin 01195490 Quinnipiac River at Southington 949 0.10 510 -- 1,040 1,360 1,640 1,940 2,510 WGT (LP3 and urban REG EQ) 467 508 841 1,040 1,180 1,340 1,700 LP3 851 -- 1,760 2,230 2,740 3,270 4,250 REG EQ 01196500 Quinnipiac River at Wallingford 4,220 0.10 2,150 -- 4,200 5,420 6,420 7,480 9,960 WGT (LP3 and REG EQ) 2,140 3,280 4,110 5,220 6,100 7,020 9,300 LP3 2,390 -- 5,230 7,020 8,780 10,500 14,200 REG EQ 01196620 Mill River near Hamden 1,940 0.10 906 -- 1,990 2,690 3,320 3,980 5,860 WGT (LP3 and REG EQ) 926 1,570 2,100 2,910 3,630 4,440 6,800 LP3 750 -- 1,620 2,150 2,660 3,150 4,290 REG EQ Housatonic River Basin 01199000 Housatonic River at Falls Village 10,900 0.10 6,250 9,050 11,400 15,200 18,500 22,400 34,400 LP3 01199050 Salmon Creek at Lime Rock 1,400 0.10–0.04 626 -- 1,620 2,420 3,140 4,010 7,010 WGT (LP3 and REG EQ) 624 1,150 1,630 2,440 3,200 4,140 7,160 LP3 650 -- 1,560 2,330 2,920 3,610 6,540 REG EQ 01200000 Tenmile River near Gaylordsville 9,650 0.04 3,090 -- 6,900 9,550 11,900 14,600 22,700 WGT (LP3 and REG EQ)

3,100 5,200 6,920 9,540 11,800 14,400 21,800 LP32,860 -- 6,680 9,620 12,600 15,700 27,600 REG EQ
Flood-Frequency Analyses of the April 2007 Flooding 19

Table 4. Flood-frequency estimates for selected exceedance probabilities for 24 streamflow-gaging stations in western Connecticut.—Continued[Station locations are shown on figure 1. USGS, U.S. Geological Survey; ft3/s, cubic feet per second; yr, years; e, estimated; LP3, log-Pearson Type III; WGT, weighted average; REG EQ, Reqression equation; MOVE-1, Maintenance of Variance Extension; --, not determined]

USGS 

station 

number

Station name

April 2007 

peak 

discharge 

(ft3/s)

Exceedance 

probability 

of April peak 

discharge 

(percent)

Discharges for selected exceedance probabilities 

(ft3/s) Frequency 

determination 

method 0.5 0.2 0.1 0.04 0.02 0.01 0.002

Housatonic River Basin—Continued

01200500 Housatonic River at Gaylordsville 23,400 0.10–0.04 10,900 16,600 21,000 27,300 32,500 38,200 53,600 LP301201487 Still River at Route 7 at 

Brookfield Center

3,580 0.10–0.04 1,230 -- 2,950 4,180 5,250 6,480 10,200 WGT (LP3 and 

REG EQ)

1,200 2,100 2,870 4,080 5,170 6,440 10,200 LP31,836 -- 3,620 4,770 5,620 6,660 10,300 REG EQ

01202501 Shepaug River at Peters Dam at 

Woodville

4,160 Flood-frequency estimates not determined

01203000 Shepaug River near Roxbury 8,000 0.10 3,010 -- 7,300 10,700 13,800 17,500 30,100 WGT (LP3 and 

REG EQ)

3,012 5,300 7,440 11,100 14,600 18,900 33,200 LP32,920 -- 6,400 8,830 11,000 13,200 20,800 REG EQ

012035055 Pootatuck River at Berkshire e3,000 Flood-frequency estimates not determined01203510 Pootatuck River at Sandy Hook 4,090 0.02 1,180 -- 2,490 3,390 4,200 5,120 7,840 Record extension 

using MOVE-1

01203600 Nonewaug River at Minortown 4,320 0.10–0.04 1,620 -- 3,870 5,550 7,100 8,940 14,600 Record extension 

using MOVE-1

01203805 Weekeepeemee River at 

Hotchkissville

3,670 0.10 1,570 -- 3,540 4,910 6,110 7,510 12,400 Record extension 

using MOVE-1

01204000 Pomperaug River at Southbury 7,350 0.10–0.04 2,760 -- 6,290 8,760 11,000 13,700 22,600 WGT (LP3 and 

REG EQ)

2,820 4,880 6,700 9,630 12,300 15,600 25,600 LP31,680 -- 3,490 4,570 5,660 6,830 12,200 REG EQ

01205500 Housatonic River at Stevenson 50,300 0.10 21,100 35,500 46,700 62,700 75,900 90,100 128,000 LP301206900 Naugatuck River at Thomaston 3,210 0.20 2,470 3,280 3,750 4,270 4,610 4,920 5,560 LP301208500 Naugatuck River at Beacon Falls 13,300 0.04 6,060 9,150 11,500 14,900 17,600 20,700 28,800 LP3
20 Flood of April 2007 and Flood-Frequency Estimates at Streamflow-Gaging Stations in Western Connecticut Table 4. Flood-frequency estimates for selected exceedance probabilities for 24 streamflow-gaging stations in western Connecticut. —Continued [Station locations are shown on figure 1. USGS, U.S. Geological Survey; ft3/s, cubic feet per second; yr, years; e, estimated; LP3, log-Pearson Type III; WGT, weighted average; REG EQ, Reqression equation; MOVE-1, Maintenance of Variance Extension; --, not determined] USGS station number Station name April 2007 peak discharge (ft3/s) Exceedance probability of April peak discharge (percent) Discharges for selected exceedance probabilities (ft3/s) Frequency determination method 0.5 0.2 0.1 0.04 0.02 0.01 0.002 South West Coast Basin 01208873 Rooster River at Fairfield 1,730 0.10–0.04 1,070 -- 2,160 2,890 3,520 4,250 6,350 Record extension using MOVE-1 1,190 1,610 1,850 2,110 2,280 2,430 2,720 LP3 01208925 Mill River near Fairfield 3,010 0.04 729 -- 1,990 2,850 3,620 4,480 6,750 WGT (LP3 and REG EQ) 716 1,390 1,990 2,940 3,790 4,780 7,680 LP3 876 -- 1,990 2,590 3,190 3,820 4,990 REG EQ 01208950 Sasco Brook near Southport 1,300 0.04 282 - - 782 1,190 1,580 2,060 3,640 WGT (LP3 and REG EQ) 282 550 818 1,300 1,790 2,430 4,700 LP3 277 -- 595 810 991 1,170 1,570 REG EQ 01208990 Saugatuck River near Redding 2,280 0.04 607 -- 1,490 2,090 2,610 3,180 4,880 WGT (LP3 and REG EQ) 590 1,050 1,450 2,060 2,593 3,210 5,000 LP3 894 -- 1,780 2,260 2,670 3,080 4,540 REG EQ 012095493 Ridgefield Brook at Shields Lane near Ridgefield 373 Flood-frequency estimates not determined 01209700 Norwalk River at South Wilton 3,490 0.02 998 -- 2,170 2,900 3,530 4,180 6,030 WGT (LP3 and REG EQ) 993 1,650 2,180 2,940 3,580 4,280 6,210 LP3 1,070 -- 2,130 2,720 3,310 3,880 5,520 REG EQ 01209761 Fivemile River near New Canaan e200 Flood-frequency estimates not determined 01209901 Rippowam River at Stamford 2,490 0.04 793 -- 1,750 2,370 2,900 3,500 5,130 Record extension using MOVE-1
Comparison of the April 2007 Flood Data to Flood Insurance Studies 21

Record Extension Using Two Methods: MOVE-1 

and Two-Station Comparison

Often, streamflow-gaging stations have too short a 

length of record to contain a sufficient range of annual peak 

discharges to estimate the long-term flood probability for 

the basin. Consequently, estimates of the larger exceedance probabilities, such as the 0.01 exceedance probability (100-year recurrence interval), can be very uncertain. 

Hydrologic records of the annual peak discharges are often 

extended by mathematical procedures that relate the annual 

peak discharges at the short-term stations to those from nearby 

long-term stations. Record extensions provide better estimates 

of statistical parameters that describe the random variation 

of a set of observations (annual peak discharges) and can be 

used to estimate flow values that have not been measured at a 

short-term station. 

For this study, records were extended at five streamflowgaging stations—Pootatuck River at Sandy Hook (01203510), 

Nonewaug River at Minortown (01203600), Weekeepeemee River at Hotchkissville (01203805), Rooster River at 

Fairfield (01208873), and Rippowam River at Stamford 

(01209901)—using a mathematical procedure developed by 

Hirsch (1982) and referred to as Maintenance of Variance 

Extension (MOVE-1). The MOVE-1 method allows users to 

estimate annual peak discharges to fill in the missing peaks at 

a short-term station. The MOVE-1 technique produces peak 

discharge estimates at the short-term station with a statistical distribution similar to that expected if the streamflow had 

actually been measured (Helsel and Hirsch, 1992, p. 277). The 

frequency curve used to obtain the exceedance probabilities is 

fitted to the population of estimated and observed annual peak 

discharges at the record-extension station. The 95-percent confidence intervals were computed using PeakFQ for the four 

stations (table 5). Although the confidence intervals for the 

record-extension stations do not account for all of the uncertainty in the estimated peaks, the uncertainty is assumed to 

be evenly distributed, and therefore, reflected in the PeakFQ 

uncertainty analysis. A MOVE-1 relation was developed for 

stations where the correlation coefficients were greater than 

0.8. The correlation coefficients for the stations with record 

extensions range from 0.87 for 20 years of concurrent record 

(Pootatuck River at Sandy Hook) to 0.98 for 7 years of 

concurrent record (Weekeepeemee River at Minortown). The 

long-term station used to extend the record of the short-term 

station was selected on the basis of the correlation coefficient 

of the concurrent record, record length (concurrent and periodof-record), and proximity to the short-term station.

The two-station comparison method of record extension 

(Interagency Advisory Committee on Water Data, 1982) also 

was used to extended records for the same short-term stations. 

The two-station comparison method involves adjusting the 

logarithmic mean and standard deviation of a short-term station with statistics from a long-term station. The two-station 

comparison method is used to model the relation between the 

two stations and not to generate missing peaks. Minimum 

allowable correlation coefficients varied depending on the 

length of the concurrent record. The minimum requirement 

set for the correlation coefficient by the Interagency Advisory 

Committee on Water Data (1982) for improving the standard 

deviation of peaks from a short-record station ranged from 

0.65 for 10 years of concurrent record to 0.47 for 20 years 

of concurrent record. The minimum requirement set for the 

correlation coefficient for improving the mean of peaks from a 

short-record station ranged from 0.35 for 10 years of concurrent record to 0.24 for 20 years of concurrent record. 

The flood-frequency estimates for the 0.20, 0.10, 0.04, 

0.02, 0.01, 0.002, and 0.005 exceedance probability (5-, 

10-, 25-, 50-, 100-, 200-, and 500-year recurrence intervals, 

respectively) for the five short-term stations with record extensions are shown in table 5. The flood-frequency estimates 

determined from the record-extension techniques were lower 

than estimates from the frequency analysis of the observed 

annual peak discharges, except for the Pootatuck River at 

Sandy Hook (01203510) and the Rooster River at Fairfield 

(01208873). Generally, the short-term records contain several 

large peak discharges or are missing several large peak discharges that can be affecting the shape of the peak-dischargefrequency curve resulting in flood-frequency estimates for 

selected recurrence intervals that may not be representative 

of the long-term frequency. The peak discharges determined 

from the record-extension methods for the Pootatuck River at 

Sandy Hook (01203510) are comparable to the peak discharges from the frequency analysis of the observed annual 

peak discharges, which indicates that the observed record is 

similar to the extended record and may cover a wider range of 

hydrologic conditions. 

The record extension for the Pootatuck River at Sandy 

Hook provides a much narrower range of discharges within 

the 95-percent confidence limits than the estimates from the 

frequency analysis of the observed annual peak discharges 

(1966–1984, 2007). The flood-frequency estimates for the 

0.01 exceedance probability (100-year recurrence interval) 

range from 4,580 to 12,600 ft3/s based on observed data 

(no extension) and 6,170 to 10,700 ft3/s based on the record 

extension. The MOVE-1 record-extension method may 

provide improved estimates of the exceedance probabilities; 

however, as of 2009, the method has not been incorporated 

into the Bulletin 17B national guidelines for estimation of 

flood frequency.

Comparison of the April 2007 Flood 

Data to Flood Insurance Studies

FEMA Flood Insurance Studies contain information on 

flood elevations and corresponding discharges for selected 

exceedance probabilities for each stream studied in detail in a 

given community. For rivers with streamflow-gaging stations, 

the discharges for the 0.1, 0.02, 0.01, and 0.002 exceedance 

probabilities (10-, 50-, 100-, and 500-year recurrence intevals, 
22 Flood of April 2007 and Flood-Frequency Estimates at Streamflow-Gaging Stations in Western Connecticut

Table 5. Flood frequency estimates derived using the Maintenance of Variance Extension (MOVE-1) method and the two-station comparison 

extended-record analysis for five streamflow-gaging stations in western Connecticut.—Continued

[USGS, U.S. Geological Survey; ft3/s, cubic feet per second; R, correlation coefficient]

Pootatuck River (USGS station 01203510), Newtown

Exceedance 

probability

Recurrence 

interval 

(years)

Period-of-record analysis 

(1966–1984, 2007)

Extended-record analysis 

(1933–2007)a

Two-station comparisonb MOVE-1c

Peak 

flow 

(ft3/s)

95-percent confidence limit Peak

flow 

(ft3/s)

95-percent confidence limit Peak

flow 

(ft3/s)

95-percent confidence limit

Lower Upper Lower Upper Lower Upper

0.2 5 2,140 1,740 2,840 2,010 1,720 2,430 1,890 1,700 2,140

0.1 10 2,730 2,160 3,850 2,620 2,190 3,300 2,490 2,190 2,900

0.04 25 3,550 2,700 5,400 3,510 2,840 4,690 3,390 2,910 4,100

0.02 50 4,210 3,120 6,750 4,280 3,380 5,960 4,200 3,530 5,220

0.01 100 4,910 3,550 8,270 5,150 3,960 7,440 5,120 4,220 6,550

0.005 200 5,650 3,980 9,990 6,110 4,590 9,180 6,180 4,990 8,130

0.002 500 6,715 4,580 12,600 7,570 5,520 11,900 8,590 6,170 10,700

a

 Records extended using Pomperaug River (USGS station 01204000); R=0.87.

Nonnewaug River (USGS station 01203600), Woodbury

Exceedance 

probability

Recurrence 

interval 

(years)

Period-of-record analysis 

(1955, 1963–1979, 2001–2007)

Extended-record analysis 

(1933–2007)a

Two-station comparisonb MOVE-1c

Peak 

flow 

(ft3/s)

95-percent confidence limit Peak

flow 

(ft3/s)

95-percent confidence limit Peak

flow 

(ft3/s)

95-percent confidence limit

Lower Upper Lower Upper Lower Upper

0.2 5 2,850 2,120 4,180 2,560 2,210 3,030 2,450 2,140 2,860

0.1 10 4,070 2,930 6,510 3,540 2,990 4,340 3,300 2,830 3,980

0.04 25 5,980 4,090 10,600 5,070 4,150 6,540 4,600 3,830 5,760

0.02 50 7,690 5,060 14,700 6,470 5,160 8,630 5,720 4,670 7,380

0.01 100 9,650 6,110 19,700 8,090 6,310 11,200 6,980 5,590 9,260

0.005 200 11,900 7,270 26,000 9,990 7,610 14,200 8,400 6,600 11,400

0.002 500 15,350 8,970 36,300 13,000 9,600 19,300 10,600 8,090 14,900

a

 Records extended using Pomperaug River (USGS station 01204000); R=0.97.

Weekeepeemee River (USGS station 01203805), Woodbury

Exceedance 

probability

Recurrence 

interval 

(years)

Period-of-record analysis 

(2001–2007)

Extended-record analysis 

(1933–2007)a

Two-station comparisonb MOVE-1c

Peak 

flow 

(ft3/s)

95-percent confidence limit Peak

flow 

(ft3/s)

95-percent confidence limit Peak

flow 

(ft3/s)

95-percent confidence limit

Lower Upper Lower Upper Lower Upper

0.2 5 3,240 2,060 7,240 2,910 2,470 3,530 2,810 2,470 3,270

0.1 10 4,480 2,730 12,600 3,990 3,310 5,060 3,870 3,320 4,640

0.04 25 6,340 3,620 23,400 5,690 4,550 7,620 5,550 4,630 6,970

0.02 50 7,960 4,300 35,300 7,210 5,620 10,100 7,100 5,780 9,220

0.01 100 9,770 5,010 51,400 8,990 6,820 13,100 8,940 7,100 12,000

0.005 200 11,800 5,760 72,900 11,100 8,170 16,700 11,100 8,630 15,400

0.002 500 14,800 6,800 112,000 14,300 10,200 22,600 14,600 11,000 21,000

a

 Records extended using Pomperaug River (USGS station 01204000); R=0.98.

b

 Extended-record using two-station comparison technique following Interagency Advisory Committee on Water Data (1982).

c

 Extended-record using the Maintenance of Variance Extension (MOVE-1) method.
Comparison of the April 2007 Flood Data to Flood Insurance Studies 23

Table 5. Flood frequency estimates derived using the Maintenance of Variance Extension (MOVE-1) method and the two-station comparison 

extended-record analysis for five streamflow-gaging stations in western Connecticut.—Continued

[USGS, U.S. Geological Survey; ft3/s, cubic feet per second; R, correlation coefficient]

Rooster River (USGS station 01208873), Fairfield

Exceedance 

probability

Recurrence 

interval 

(years) 

Period-of-record analysis 

(1978–2007)

Extended-record analysis 

(1955, 1962–2007)a

Two-station comparisonb MOVE-1c

Peak

flow 

(ft3/s)

95-percent confidence limit Peak 

flow 

(ft3/s)

95-percent confidence limit Peak 

flow 

(ft3/s)

95-percent confidence limit

Lower Upper Lower Upper Lower Upper

0.2 5 1,610 1,410 1,900 1,920 1,430 2,890 1,670 1,470 1,940

0.1 10 1,850 1,600 2,230 2,700 1,940 4,500 2,160 1,860 2,600

0.04 25 2,110 1,800 2,620 3,940 2,670 7,510 2,890 2,410 3,660

0.02 50 2,280 1,930 2,880 5,080 3,290 10,700 3,520 2,880 4,630

0.01 100 2,430 2,040 3,110 6,430 3,980 14,800 4,250 3,390 5,790

0.005 200 2,560 2,140 3,330 8,030 4,750 20,200 5,080 3,960 7,150

0.002 500 2,720 2,250 3,590 10,600 5,910 29,700 6,350 4,800 9,340

a

 Records extended using Fivemile River near Norwalk (USGS station 01209770); R=0.91.

Rippowam River (USGS station 01209901), Stamford

Exceedance 

probability

Recurrence 

interval 

(years)

Period-of-record analysis

(1978–1982, 2002–2007)

Extended-record analysis

(1963–2007)a

Two-station comparisonb MOVE-1c

Peak

flow 

(ft3/s)

95-percent confidence limit Peak 

flow 

(ft3/s)

95-percent confidence limit Peak 

flow 

(ft3/s)

95-percent confidence limit

Lower Upper Lower Upper Lower Upper

0.2 5 1,780 1,290 2,910 1,430 1,170 1,830 1,320 1,130 1,600

0.1 10 2,370 1,660 4,370 2,000 1,560 2,650 1,750 1,460 2,190

0.04 25 3,220 2,130 6,900 2,780 2,130 4,060 2,370 1,920 3,130

0.02 50 3,930 2,500 9,330 3,530 2,610 5,430 2,900 2,300 3,980

0.01 100 4,700 2,890 12,300 4,390 3,150 7,120 3,500 2,700 4,960

0.005 200 5,550 3,280 15,900 5,390 3,750 9,190 4,150 3,140 6,090

0.002 500 6,800 3,830 21,700 6,960 4,660 12,600 5,130 3,780 7,840

a

 Records extended using Norwalk River at South Wilton (USGS station 01209700); R=0.93.

b

 Extended-record using two-station comparison technique following Interagency Advisory Committee on Water Data (1982).

c

 Extended-record using the Maintenance of Variance Extension (MOVE-1) method.
24 Flood of April 2007 and Flood-Frequency Estimates at Streamflow-Gaging Stations in Western Connecticut

Table 6. Magnitude and frequency of floods from flood insurance studies and current (2007) flood-frequency estimates for streamflow-gaging 

[USGS, U.S. Geological Survey; FIS, Federal Emergency Management Agency Flood Insurance Study; NGVD29; National Geodetic Vertical Datum of 1929; 

USGS 

station 

number

Station name FIS 

community

FIS report 

date

Drainage 

area 

(mi2)

Peak 

discharge 

of April 

2007 

(ft3/s)

Recurrence interval of 

April 2007 

(yrs)

Magnitude of discharge for X-yr recurrence interval at streamflow-gaging 

station location from effective FIS1

(ft3/s)

Based 

on frequency 

analyses 

using 

data 

water 

year 2007

Based on 

FIS 10-yr 50-yr 100-yr 500-yr 

Farmington River Basin 

01186000 West Branch Farmington River at 

Riverton

Barkhamsted 17-Aug-81 131 3,040 25 50 2,440 3,350 4,370 22,800 

01186500 Still River at Robertsville Winchester Jan-78 85 8,130 50 ~100 2,800 4,600 9,500 -- 

South Central Coast 

01195490 Quinnipiac River at Southington Hartford 26-Sep-08 17.4 949 10 10 902 1,620 1,880 2,480 

01196500 Quinnipiac River at Wallingford Wallingford 18-Mar-99 115 4,220 10 10 3,900 5,700 6,500 7,380 

01196620 Mill River near Hamden Hamden Dec-78 24.5 1,940 10 > 100 1,080 1,670 1,870 2,460 

Housatonic River Basin 

01199000 Housatonic River at Falls Village Canaan 2-Sep-88 634 10,900 10 10 11,900 18,400 21,900 31,400 

01199050 Salmon Creek at Lime Rock Salisbury 5-Jan-89 29.4 1,400 10–25 10 1,400 2,920 3,870 7,100 

01200000 Tenmile River near Gaylordsville Dover, NY 4-Jul-98 203 9,650 25 ~10 and 50 7,240 13,100 16,100 25,000 

01200500 Housatonic River at Gaylordsville Sherman 18-Jun-87 996 23,400 10–25 10 23,500 40,400 50,400 81,400 

01201487 Still River at Route 7 at 

Brookfield Center

Brookfield Dec-78 62.3 3,580 10–25 ~10 and 50 2,600 6,200 8,580 17,400 

01202501 Shepaug River at Peters Dam at 

Woodville

Litchfield 2-Jan-92 38.1 4,160 -- -- -- 7,000 -- 

01203000 Shepaug River near Roxbury Roxbury 3-Dec-87 132 8,000 10 -- -- 24,100 -- 

012035055 Pootatuck River at Berkshire Newtown 16-Apr-03 15.5 e3000 -- >100 1,200 2,220 2,810 4,620 

01203510 Pootatuck River at Sandy Hook Newtown 16-Apr-03 24.8 4090 50 100 1,800 3,320 4,200 6,930 

01203600 Nonewaug River at Minortown Woodbury Mar-77 17.7 4,320 10~25 ~10 and 50 3,020 6,460 8,620 16,200 

01203805 Weekeepeemee River at 

Hotchkissville

Woodbury Mar-77 26.8 3,670 10 ~10 and 50 2,500 5,500 8,000 17,000 

01204000 Pomperaug River at Southbury Southbury Sep-79 75.1 7,350 10–25 ~10 and 50 5,680 12,300 17,000 34,100 

01205500 Housatonic River at Stevenson Oxford 18-Mar-91 1,544 50,300 10 ~10 and 50 42,000 87,000 126,000 196,000 

01206900 Naugatuck River at Thomaston Thomaston 5-Jan-82 99.8 3,210 5 10 3,500 3,500 3,600 6,760 

01208500 Naugatuck River at Beacon Falls Beacon Falls Sep-78 260 13,300 25 ~10 and 50 9,070 20,700 28,800 66,900 

South West Coast 

01208873 Rooster River at Fairfield Fairfield 6-Oct-98 10.6 2,040 10–25 ~10 and 50 1,600 2,600 3,500 5,900 

01208925 Mill River near Fairfield Fairfield 6-Oct-98 28.6 3,010 25 10 2,720 5,340 7,520 14,000 

01208950 Sasco Brook near Southport Fairfield 6-Oct-98 7.38 1,300 25 10 1,350 2,100 2,600 4,300 

01208990 Saugatuck River near Redding Redding 15-Dec-81 21.0 2,280 25 ~10 and 50 1,560 2,930 3,740 5,920 

012095493 Ridgefield Brook at Shields Lane 

near Ridgefield

Ridgefield 23-Aug-99 3.39 373 -- 125 185 235 410 

01209700 Norwalk River at South Wilton Wilton 18-Feb-98 30.0 3,490 50 ~10 and 50 2,980 5,840 7,460 12,500 

01209761 Fivemile River near New Canaan New Canaan 4-Jun-90 1.00 e 200 -- 150 260 310 550 

01209901 Rippowam River at Stamford Stamford 17-Nov-93 34.0 2,490 25 10 2,730 5,470 6,980 8,770 

1

 The date the Flood Insurance Study (FIS) is effective in the National Flood Insurance Program.
Comparison of the April 2007 Flood Data to Flood Insurance Studies 25

stations in western Connecticut.

mi2, square miles; ft3/s, cubic feet per second; yr, years; e, estimated; --, not determined; ~, approximately; >, greater than]

Magnitude of discharge for X-yr recurrence 

interval at streamflow-gaging location from 

hydrologic analyses 2008 

(ft3/s)

Differences between 2008 flood-frequency 

estimates and effective FIS discharge values 

(in percent) (positive percents indicate higher discharges (in bold) and negative percents indicate 

lower discharges)

100-year flood elevation at USGS streamflowgaging location (note-elevation differences 

are for sites where updated flood-frequnecy 

estimates are greater than the discharges in 

the FIS report)

10-yr 50-yr 100-yr 500-yr 10-yr 50-yr 100-yr 500-yr 

100-year FIS 

report flood 

elevation 

(NGVD29)

100-year flood 

elevation from 

current stagedischarge 

relation at gage 

(NGVD29)

Flood 

elevation 

difference

Farmington River Basin

2,680 3,500 3,790 -- 9.0 4.3 -15.3 --

5,650 7,990 8,930 -- 50.4 42.4 -6.4 --

South Central Coast

1,040 1,640 1,940 2,510 13.3 1.2 3.1 1.2 -- Undefined --

4,200 6,420 7,480 9,960 7.1 11.2 13.1 25.9 32.2 32.9 0.7

1,990 3,320 3,980 5,860 45.7 49.7 53.0 58.0 88.0 90.4 2.4

Housatonic River Basin

11,400 18,500 22,400 34,400 -4.4 0.5 2.2 8.7 547.0 547.4 0.4

1,620 3,140 4,010 7,010 13.6 7.0 3.5 -1.3 629.5 629.8 0.3

6,900 11,900 14,600 22,700 -4.9 -10.1 -10.3 -10.1

21,000 32,500 38,200 53,600 -11.9 -24.3 -31.9 -51.9

2,950 5,250 6,480 10,200 11.9 -18.1 -32.4 -70.6

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

7,300 13,800 17,500 30,100 -- -- -37.7 --

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

2,490 4,200 5,120 7,840 27.7 21.0 18.0 11.6 230.0 Undefined --

3,870 7,100 8,940 14,600 22.0 9.0 3.6 -11.0 366.0 Undefined --

3,540 6,110 7,510 12,400 29.4 10.0 -6.5 -37.1

6,290 11,000 13,700 22,600 9.7 -11.8 -24.1 -50.9

46,700 75,900 90,100 128,000 10.1 -14.6 -39.8 -53.1

3,750 4,610 4,920 -- 6.7 24.1 26.8 -- 363.2 361.3 -1.9

11,500 17,600 20,700 -- 21.1 -17.6 -39.1 --

South West Coast

2,160 3,520 4,250 6,350 25.9 26.1 17.6 7.1 19.5 Undefined --

1,990 3,620 4,480 6,750 -36.7 -47.5 -67.9 -107.4

782 1,580 2,060 3,640 -72.6 -32.9 -26.2 -18.1

1,490 2,610 3,180 4,880 -4.7 -12.3 -17.6 -21.3

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

2,170 3,530 4,180 6,030 -37.3 -65.4 -78.5 -107.3

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

1,750 2,900 3,500 5,130 -56.0 -88.6 -99.4 -71.0
26 Flood of April 2007 and Flood-Frequency Estimates at Streamflow-Gaging Stations in Western Connecticut

respectively) were compiled from these studies and are listed 

in table 6. The flood-frequency data obtained from the FEMA 

studies often differ from updated estimates of the magnitude 

and frequency of floods because in the updated estimates more 

data are available for statistical analysis and (or) the methods 

used for determining the magnitude and frequency of floods 

may be different. The additional data allow for more accurate 

estimates of magnitude and frequency of floods for streamflow-gaging stations. 

The magnitude and frequency of discharges determined 

in this study using data through water year 2007 were compared to the magnitude and frequency of discharges in the 

community Flood Insurance Studies for streamflow-gaging 

stations in western Connecticut. The comparison indicated 

that, in general, the discharges at the 0.10 exceedance probability (10-year recurrence interval) from the flood-frequency 

analysis are larger than in the Flood Insurance Studies, but 

the discharges at the 0.01 exceedance probability (100-year 

recurrence interval) from the flood-frequency analysis are 

smaller than in the Flood Insurance Studies. For example, the 

discharges at the 0.10 exceedance probability in the Flood 

Insurance Study and frequency analysis are 5,680 ft3/s and 

6,290 ft3/s, respectively (data from station 01204000). For the 

same station, the discharges at the 0.01 exceedance probabities 

in the Flood Insurance Study and flood-frequency analysis are 

17,000 ft3/s and 13,700 ft3/s, respectively. Differences in the 

flood-frequency estimates at the 0.10 exceedance probability 

(10-year recurrence interval) ranged from -72.6 to -4.4 percent 

(8 stations) and +6.7 to +50.5 percent (15 stations) with an 

overall median change of about +10 percent. At the 0.01 

exceedance probability (100-year recurrence interval), differences in the flood-frequency estimates ranged from -99.5 to 

-6.4 (15 stations) to +2.2 to +53.0 (9 stations) with an overall 

median change of about -13 percent (table 5). 

At the 0.10 exceedance probability (10-year recurrence 

interval), the largest (+) changes—where the flood-frequency 

estimates are greater than the estimates in the FEMA Flood 

Insurance Studies—were for Winchester (Still River at 

Robertsville, +50 percent change); Hamden (Mill River near 

Hamden, +46 percent change); Newtown (Pootatuck River at 

Sandy Hook, +28 percent change); and Woodbury (Weekeepeemee River at Hotchkissvile, +29 percent change). At the 

0.01 exceedance probability (100-year recurrence interval), 

the largest (+) changes—where the flood-frequency estimates 

are larger than the published estimates in the FEMA Flood 

Insurance Studies—were for Wallingford (Quinnipiac River at 

Wallingford, +13 percent change); Hamden (Mill River near 

Hamden, +53 percent change); Newtown (Pootatuck River 

at Sandy Hook, +18 percent change); and Thomaston (Naugatuck River at Thomaston, +27 percent change).

The 0.01 exceedance probability (100-year) flood elevations—determined at streamflow-gaging station locations from 

the current (water year 2007) USGS stage-discharge relation 

using the flood-frequency estimates—were compared to the 

0.01 exceedance probability (100-year) flood elevations in the 

Flood Insurance Studies. The comparison shows that the flood 

elevations reported by the USGS are higher at the streamflowgaging stations than the flood elevations in the FEMA studies by more than 0.5 ft for Wallingford (Quinnipiac River at 

Wallingford, 0.6 ft difference) and Hamden (Mill River near 

Hamden, 2.3 ft difference). 

The 0.01 exceedance probability (100-year) flood elevations were obtained from the water-surface profiles of the 

Flood Insurance Studies. It is not known if the differences 

in the exceedance probability discharges or flood elevations 

translate to a large change in the delineation of the regulatory 

floodplain. In relatively flat topography, a change of 0.5 ft 

can represent a significant change to the regulatory floodplain 

boundary. Several stations that showed increases in discharges 

at the 0.01 exceedance probability could not be evaluated for 

changes in the flood elevations because the relation between 

the stage and discharge at the streamflow-gaging stations is 

not defined for higher flows. These stations included Quinnpiac River at Southington (01195490), Pootatuck River at 

Sandy Hook (01203510), Nonewaug River at Minortown 

(01203600), and Rooster River at Fairfield (01208873).

Summary 

From April 15 to 18, 2007, Fairfield and Litchfield Counties in Connecticut experienced moderate to severe flooding 

caused by a late season nor’easter that dropped more than 

7 inches of rainfall in the region. A Presidential Disaster 

Declaration was issued on May 11, 2007, for the two counties. 

Because communities need up-to-date estimates of the magnitude and frequency of flooding in order to make sound decisions about community planning, zoning, and development, 

the Federal Emergency Management Agency requested that 

the U.S. Geological Survey conduct a study to determine the 

magnitude and frequency of discharges at streamflow-gaging 

stations in and near the declared disaster areas. Records of 

annual maximum flows and information on the magnitude and 

frequency of floods are used extensively in Flood Insurance 

Studies to determine flood zones and promote sound land use 

and flood-plain development. In addition, information on the 

magnitude and frequency of floods is important in the design 

of hydraulic structures such as bridges and culverts.

Peak discharges for exceedance probabilities of 0.50, 

0.20, 0.10, 0.04, 0.02, 0.01, and 0.002 (recurrence intervals of 

2, 5, 10, 25, 50, 100, and 500 years, respectively) were determined for 24 streamflow-gaging stations in western Connecticut. Flood-frequency curves were derived from data through 

water year 2007 by fitting the annual series of peak-discharge 

data to a log-Pearson Type III frequency distribution. The 

frequencies were computed following the guidelines recommended by the Interagency Advisory Committee on Water 

Data (1982) in Bulletin 17B. Annual peak-discharge data at 

several stations (Pootatuck River, Nonewaug River, Weekeepeemee River, Rooster River, and Rippowam River) were 

determined to have too short a record to contain a sufficient 
Selected References 27

range of hydrologic conditions; consequently, record extension methods were applied to improve the flood-frequency 

estimates for these stations. The estimates in this report update 

and supersede previously published flood-frequency estimates 

for streamflow-gaging stations in Connecticut by incorporating additional years of annual peak discharges, including the 

peaks for the April 2007 flood. 

During the April 2007 flood, all streamflow-gaging 

stations at which flood stages have been established by the 

NWS rose to or exceeded flood stage. Peak discharges on the 

Norwalk River and the Saugatuck River were the highest since 

the historical New England flood of 1955. Discharges on the 

Rippowam River at Stamford and the Quinnipiac River at 

Southington were the maximum discharges for the period of 

record for those stations. For the majority (80 percent) of the 

streamflow-gaging stations, the April 2007 peak discharges 

range between a 0.10 and 0.04 exceedance probability of 

being equaled or exceeded in any one year (recurrence intervals of 10- and 25-years, respectively). At three streamflowgaging stations—Norwalk River at South Wilton, Pootatuck 

River at Sandy Hook, and Still River at Robertsville—the 

April 2007 peak discharges have a 0.02 exceedance probability of being equaled or exceeded in any one year (recurrence 

interval of 50 years). 

Flood-frequency estimates determined using peakdischarge data through water year 2007 were compared to 

those used to calculate water-surface profiles in Federal 

Emergency Management Agency Flood Insurance Studies 

prepared for communities. The comparison indicated that 

for the majority of the streamflow-gaging stations—at the 

0.10 exceedance probability (10-year recurrence interval)—

the peak discharges determined in the flood-frequency analysis 

are larger than the peak discharges in the FEMA Flood 

Insurance Studies. The differences in the peak discharges 

determined using data through water year 2007 compared to 

the flood-frequency estimates in the Flood Insurance Studies 

ranged from -72.6 to -4.4 percent (8 stations) and +6.7 to 

+50.5 percent (15 stations), with an overall median change 

of about +10 percent. At the 0.10 exceedance probability 

(10-year recurrence interval), several stations for which 

discharges from the frequency analysis using data through 

water year 2007 exceeded discharges in the Flood Insurance 

Studies had more than +25 percent change: Winchester 

(Still River at Robertsville, +50 percent change); Hamden 

(Mill River near Hamden, +46 percent change); Woodbury 

(Weekeepeemee River at Hotchkissville, +29 percent 

change); and Newtown (Pootatuck River at Sandy Hook, 

+28 percent change). 

The comparison also indicated that for the majority 

of the streamflow-gaging stations—at the 0.01 exceedance 

probability (recurrence interval of 100 years)—the discharges 

determined in the flood-frequency analysis using data through 

water year 2007 are smaller than the discharges reported in the 

FEMA Flood Insurance Studies. The differences in the magnitude of discharges ranged from -99.5 to -6.4 (15 stations) to 

+2.2 to +53.0 (9 stations) with an overall median change of 

about -13 percent. Although the majority of the streamflowgaging stations studied had discharges at the 0.01 exceedance 

probability smaller than in the Flood Insurance Studies, the 

(2007) flood-frequency estimates were larger than in the Flood 

Insurance Studies for four stations in the following communities: Hamden (Mill River near Hamden, +53 percent change); 

Thomaston (Naugatuck River at Thomaston, +27 percent 

change); Newtown (Pootatuck River at Sandy Hook, 

+18 percent change); and Wallingford (Quinnipiac River at 

Wallingford, +13 percent change). 

A comparison of the 0.01 exceedance probability (recurrence interval of 100 years) flood elevations determined at 

streamflow-gaging stations from the current U.S. Geological 

Survey stage-discharge relation to the 0.01 exceedance probability flood elevations in the Flood Insurance Studies indicated that the flood elevations determined from the frequency 

analysis exceeded the flood elevations in the Flood Insurance 

Study water-surface elevation by more than 0.5 ft for Wallingford (Quinnipiac River at Wallingford, 0.6 ft difference) 

and Hamden (Mill River near Hamden, 2.3 ft difference). 

It is unknown whether the differences in the 100-year peak 

discharges or flood elevations translate to a large change in the 

delineation of the regulatory 100-year floodplain. 

Acknowledgments

The author thanks Ronald Lansverk and Ivy Frances 

from FEMA and Arthur Christian from Connecticut Department of Environmental Protection for identifying areas for 

flood investigation and providing preliminary reports of flood 

damages incurred by the local communities. Thanks are due 

to Mark Baribault, Connecticut Department of Environmental 

Protection, for compilation of the rainfall data from the State’s 

flood warning system network; to USGS coworkers Jonathan 

Morrison, James Norris, John Organek, Jason Pollender, and 

Timothy Sargent for field assistance and data compilation; and 

to Phillip Zarriello (MA) and Scott Olson (NH) for providing 

colleague reviews of the report. 

Selected References 

Ahearn, E.A., 2003. Peak-flow frequency estimates for U.S. 

Geological Survey streamflow-gaging stations in 

Connecticut: U.S. Geological Survey Water-Resources 

Investigations Report 2003–4196, 29 p., available online at 

http://pubs.er.usgs.gov/usgspubs/wri/wri034196.

Ahearn, E.A., 2004, Regression equations for estimating flood 

flows for the 2-, 10-, 25-, 50-, 100-, and 500-year recurrence 

intervals in Connecticut: U.S. Geological Survey Scientific 

Investigations Report 2004–5160, 62 p., available online at 

http://pubs.er.usgs.gov/usgspubs/sir/sir20045160.
28 Flood of April 2007 and Flood-Frequency Estimates at Streamflow-Gaging Stations in Western Connecticut

Ahearn, E.A., 2005, Estimates of the Magnitude and 

Frequency of Flood Flows in the Connecticut River in 

Connecticut: U.S. Geological Survey Open File Report 

2005–1369, 12 p., available online at http://pubs.usgs.gov/

of/2005/1369/pdf/ofr2005-1369.pdf. 

Ahearn, E.A., Ries, K.G., III, and Steeves, P.A. 2006, Streamstats—U.S. Geological Survey web application for streamflow statistics for Connecticut: U.S. Geological Survey Fact 

Sheet 2006–3129, 4 p., available online at 

http://pubs.er.usgs.gov/usgspubs/fs/fs20063129.

Bogart, D.B., 1960, Floods of August–October 1955, New 

England to North Carolina: U.S. Geological Survey WaterSupply Paper 1420, 854 p.

Connecticut Department of Environment Protection, Office of 

Information Management, 1995, Subregional basin boundaries datalayer, available online at http://www.ct.gov/dep/

cwp/view.asp?a=2698&depNav_GID=1707&q=323072.

Connecticut Department of Transportation, 2000, Drainage 

Manual, Chapter 6, Section 11, Transferring Gaged Data, 

available online at http://www.ct.gov/dot/lib/dot/

documents/ddrainage/6.11.pdf

Danenberg, Diana, and Bogar, James, 1994, Hydrography 

subset: Hartford, Conn., Natural Resources Center, Department of Environmental Protection, datalayer, accessed 

February 5, 2008, available online at http://www.ct.gov/dep/

cwp/view.asp?A=2698&Q=323194. 

Dalrymple, Tate, 1960, Flood-frequency analyses: U.S. 

Geological Survey Water-Supply Paper 1543–A, Manual of 

hydrology; Part 3. Flood-flow techniques, 80 p. 

DeGaetano, A.T., 1997, Extreme precipitation analysis for 

the State of Connecticut: Ithaca, N.Y., Northeast Regional 

Climate Center report, unpaginated. 

FEMA, 1978, Flood insurance study, town of Beacon Falls, 

Connecticut, New Haven County: U.S. Department of 

Housing and Urban Development, Federal Insurance 

Administration, 20 p.

FEMA, 1978, Flood insurance study, town of Brookfield, 

Connecticut, Fairfield County: U.S. Department of Housing 

and Urban Development, Federal Insurance Administration, 

18 p.

FEMA, 1978, Flood insurance study, town of Hamden, 

Connecticut, New Haven County: U.S. Department of 

Housing and Urban Development, Federal Insurance 

Administration, 24 p.

FEMA, 1978, Flood insurance study, town of Winchester, 

Connecticut, Litchfield County: U.S. Department of 

Housing and Urban Development, Federal Insurance 

Administration, 15 p. 

FEMA, 1980, Flood insurance study, town of Southbury, 

Connecticut, New Haven County: Federal Emergency 

Management Agency, Community number 090089, 15 p.

FEMA, 1981, Flood insurance study, town of Redding, 

Connecticut, Fairfield County: Federal Emergency 

Management Agency, Community number 090141, 19 p.

FEMA, 1982, Flood insurance study, town of Thomaston, 

Connecticut, Litchfield County: Federal Emergency 

Management Agency, Community number 090055, 22 p.

FEMA, 1987, Flood insurance study, town of New Milford, 

Connecticut, Litchfield County: Federal Emergency 

Management Agency, Community number 090049, 44 p.

FEMA, 1987, Flood insurance study, town of Roxbury, 

Connecticut, Litchfield County: Federal Emergency 

Management Agency, Community number 090051, 12 p.

FEMA, 1987, Flood insurance study, town of Sherman, 

Connecticut, Fairfield County: Federal Emergency 

Management Agency, Community number 090166, 16 p. 

FEMA, 1988, Flood insurance study, town of Canaan, 

Connecticut, Litchfield County: Federal Emergency 

Management Agency, Community number 090044, 18 p.

FEMA, 1988, Flood insurance study, town of Dover, New 

York, Dutchess County: Federal Emergency Management 

Agency, Community number 361335, 17 p.

FEMA, 1989, Flood insurance study, city of Bridgeport, 

Connecticut, Fairfield County: Federal Emergency 

Management Agency, Community number 090002, 34 p. 

FEMA, 1989, Flood insurance study, town of Salisbury, 

Connecticut, Litchfield County: Federal Emergency 

Management Agency, Community number 090052, 18 p.

FEMA, 1990, Flood insurance study, town of New Canaan, 

Connecticut, Fairfield County: Federal Emergency 

Management Agency, Community number 090010, 

23 p.

FEMA, 1990, Flood insurance study, town of Warren, Connecticut, Litchfield County: Federal Emergency 

Management Agency, Community number 090175, 16 p.

FEMA, 1991, Flood insurance study, town of Monroe, 

Connecticut, Fairfield County: Federal Emergency 

Management Agency, Community number 090009, 25 p.

FEMA, 1991, Flood insurance study, town of Oxford, 

Connecticut, New Haven County: Federal Emergency 

Management Agency, Community number 090150, 25 p.

FEMA, 1992, Flood insurance study, town of Litchfield, 

Connecticut, Litchfield County: Federal Emergency Management Agency, Community number 090047, 22 p.
Selected References 29

FEMA, 1993, Flood insurance study, city of Stamford, 

Connecticut, Fairfield County: Federal Emergency 

Management Agency, Community number 090015, 32 p. 

FEMA, 1998, Flood insurance study, town of Fairfield, 

Connecticut, Fairfield County: Federal Emergency 

Management Agency, Community number 090007, 30 p.

FEMA, 1998, Flood insurance study, town of Westport, 

Connecticut, Fairfield County: Federal Emergency 

Management Agency, Community number 090019, 30 p.

FEMA, 1998, Flood insurance study, town of Wilton, 

Connecticut, Fairfield County: Federal Emergency 

Management Agency, Community number 090020, 27 p.

FEMA, 1999, Flood insurance study, town of Ridgefield, 

Connecticut, Fairfield County: Federal Emergency 

Management Agency, Community number 090013, 24 p.

FEMA, 1999, Flood insurance study, town of Wallingford, 

Connecticut, New Haven County: Federal Emergency 

Management Agency, Community number 090090, 26 p.

FEMA, 2003, Flood insurance study, town of Newtown, 

Connecticut, Fairfield County: Federal Emergency 

Management Agency, Flood Insurance Study number 

090011V000A, 26 p.

FEMA, 2007, Connecticut severe storms and flooding: 

Federal Emergency Management Agency Web page, 

accessed September 8, 2008, at http://www.fema.gov/news/

event.fema?id=8070. 

FEMA, 2008, Flood insurance study, Connecticut, Hartford 

County, Connecticut (All Jurisdictions): Federal 

Emergency Management Agency, Flood Insurance Study 

number 09003CV001A, 155 p.

Flynn, K.M., Kirby, W.H., and Hummel, P.R., 2006, User’s 

manual for program PeakFQ, annual flood frequency analysis using Bulletin 17B guidelines: U.S. Geological Survey 

Techniques and Methods, book 4, chap. B4, 42 p., available 

online at http://pubs.usgs.gov/tm/2006/tm4b4/.

Flynn, K.M., Kirby, W.H., Mason, Robert, and Cohn, T.A., 

2006, Estimating magnitude and frequency of floods using 

the PeakFQ program: U.S. Geological Survey Fact Sheet 

2006–3143, 2 p., available online at http://pubs.usgs.gov/

fs/2006/3143/.

Helsel, D.R., and Hirsch, R.M., 1992, Statistical methods in 

water resources: Amsterdam, the Netherlands, Elsevier 

Science Publishers, 522 p.

Hirsch, R.M., 1982, A comparison of four streamflow record 

extension techniques: Water Resources Research, v. 18, 

no. 4, p. 1081–1088.

Interagency Advisory Committee on Water Data, 1982, 

Guidelines for determining flood-flow frequency: Bulletin 

17B of the Hydrology Subcommittee, Office of Water Data 

Coordination, U.S. Geological Survey, Reston,Va., 183 p., 

available online at http://water.usgs.gov/osw/bulletin17b/

bulletin_17B.html.

Miller, D.R., Warner, G.S., Ogden, F.L., and DeGaetano, A.T., 

2003, Rainfall in Connecticut: accessed October 20, 2003 

at http://www.ctiwr.uconn.edu/PrecipinCT/precip.pdf.

Moody, D.W., Fischer, J.N., and Chase, E.B., 1985, National 

water summary 1884—hydrologic events, selected waterquality trends, and ground-water resources: U.S. 

Geological Survey Water-Supply Paper 2275, 467 p.

National Climatic Data Center, 2007a, Climatological data, 

New England, April 2007: National Climatic Data Center, 

v. 119, no. 04, ISSN 0364-533. 

National Climatic Data Center, 2007b, Hourly precipitation 

data, New England, April 2007: National Climatic Data 

Center, v. 57, no. 04, ISSN 0364-6270.

National Climatic Data Center, 2007c, Storm data April 2007: 

National Climatic Data Center, v. 49, no. 4, accessed 

June 30, 2008 from http://www1.ncdc.noaa.gov/

National Climatic Data Center, 2008a, Climate data inventories, accessed April 25, 2008, http://www.ncdc.noaa.gov/oa/

climate/climateinventories.html

National Climatic Data Center, 2008b, Storm events for Connecticut: accessed August 25, 2008, at http://www4.ncdc.

noaa.gov/cgi-win/wwcgi.dll?wwevent~ShowEvent~656879. 

National Weather Service, 2007a, Connecticut; 4/15/2007 

1-day observed precipitation: NOAA, National Weather 

Service Web page, accessed August 14, 2008, from 

http://water.weather.gov/. 

National Weather Service, 2007b, Connecticut; 4/16/2007 

1-day observed precipitation: NOAA, National Weather 

Service Web page, accessed August 14, 2008, from 

http://water.weather.gov/.

National Weather Service, 2008a, Precipitation analysis pages, 

accessed June 30, 2008, at http://water.weather.gov/about.

php.

National Weather Service, 2008b, Advanced hydrologic prediction service: Digital data coverages, accessed June 30, 

2008, at http://water.weather.gov/download.php.

Paulson, R.W., Chase, E.B., Roberts, R.S., and Moody, D.W., 

1991, National water summary 1988–89, hydrologic events 

and floods and droughts: U.S. Geological Survey WaterSupply Paper 2375, 591 p.
30 Flood of April 2007 and Flood-Frequency Estimates at Streamflow-Gaging Stations in Western Connecticut

Perry, C.A., 2001, Summary of significant floods in the United 

States, Puerto Rico, and the Virgin Islands, 1970 through 

1989: U.S. Geological Survey Water-Supply Paper 2202, 

598 p.

Perry, C.A., 2005, Summary of significant floods in the United 

States and Puerto Rico, 1994 through 1998 water years: 

U.S. Geological Survey Scientific Investigations Report 

2005–5194, 327 p.

Perry, C.A., and Combs, L.J., 1998, Summary of floods in the 

United States, January 1992 through September 1993: U.S. 

Geological Survey Water-Supply Paper 1998–2499, 286 p.

Sauer, V.B., Thomas, W.O., Jr., Stricker, V.A., and Wilson, 

K.V., 1983, Flood characteristics of urban watersheds in the 

United States: U.S. Geological Survey Water-Supply Paper 

2207, 63 p. 

Thompson, M.T., Gannon, W.B., Thomas, M.P., Hayes, G.S., 

and others, 1964, Historical floods in New England: U.S. 

Geological Survey Water-Supply Paper 1779–M, 105 p.

USA Counties IN Profile, 2007, Overview for Fairfield 

County, CT: STATS Indiana Web service, accessed 

February 6, 2009, at http://www.stats.indiana.edu/uspr/a/

usprofiles/09/us_over_sub_pr09001.html. 

U.S. Geological Survey, 1954, Summary of floods in the 

United States during 1950: U.S. Geological Survey WaterSupply Paper 1137-I, p. 957–991.

U.S. Geological Survey, 2000, National land cover dataset: 

U.S. Geological Survey Fact Sheet 108-00, accessed 

February 6, 2009, at http://pubs.er.usgs.gov/usgspubs/fs/

fs10800.

U.S. Geological Survey, 2005, _National hydrography dataset, 

accessed February 5, 2008 at http://www.ct.gov/dep/cwp/

view.asp?a=2698&q=323188&depNav_GID=1707 

U.S. Geological Survey, 2006, National elevation dataset, 

accessed February 5, 2008, at http://gisdata.usgs.gov/NED/.

U.S. Geological Survey, 2007, Water resources application 

software, PeakFQ, version 5.2, accessed April 15, 2008, at 

http://water.usgs.gov/software/PeakFQ/03). 
Appendix 1. Federal Emergency Management Agency Disaster 

Declaration Map.

Appendix 1 31

Litchfield Hartford

Fairfield

Tolland Windham

New London

New Haven

Middlesex

NY

RI

MA

NJ

0 3

26 12 18

miles

ITS Mapping & Analysis Center

Washington, DC

PA

NY

OH

ME

WV

VT

NH

VA

NJ

MD

MA

MI CT

DE

RIRI

DC

Location Map

FEMA-1700-DR, Connecticut

Disaster Declaration as of 06/13/2007

MapID 0a7975025f0

Legend

06/13/07 -- 04:50 PM EDT

Source: Disaster Federal Registry Notice

Amendment No. 1 - 06/13/2007

Designated Counties

No Designation

Individual Assistance

Individual Assistance and Public Assistance

All counties are eligible

for Hazard Mitigation

Figure 1. Federal Emergency Management Agency Disaster Declaration map.
32 Flood of April 2007 and Flood-Frequency Estimates at Streamflow-Gaging Stations in Western Connecticut

Event: Flood State: Connecticut

Map of Counties

Begin Date: 15 Apr 2007, 14:00:00 PM EST County: Fairfield 

Begin Location: Stamford

Begin LAT/LON: 41°03’N / 73°31’W

End Date: 16 Apr 2007, 07:00:00 AM EST

End Location: Shelton

End LAT/LON: 41°19’N / 73°00’W

Magnitude: 0

Fatalities: 0

Injuries: 0

Property Damage: $ 0.0K

Crop Damage: $ 0.0K

Map of Counties

Description:

A Northeaster occurred during Sunday and Monday, 

April 15th and 16th. It brought heavy rain and high winds 

that caused widespread and significant river, stream, and 

urban flooding of low lying and poor drainage areas. High 

winds downed many trees and power lines across Southeast 

Connecticut. The combination of high winds, heavy rain, 

and high water tables produced widespread moderate tidal 

flooding across portions of the Long Island Sound Shorelines 

through Thursday, April 19th. Significant river flooding 

lasted through Monday, April 23rd. Storm Total Rainfall 

amounts ranged from 1.76 inches at New London Airport 

in Groton to 7.83 inches in New Canaan. Fairfield County 

rainfall ranged from 3.49 inches at Bridgeport to 7.83 inches 

at New Canaan. New Haven County rainfall ranged from 3.32 

at Oxford Airport in Waterbury to 4.50 inches at Hamden. 

Middlesex County rainfall at Middletown was 5.25 inches. 

New London County rainfall at New London Airport was 

1.76 inches. Flooding from Rainfall: The heaviest rain 

fell across southwest and southcentral Connecticut, where 

many small rivers, streams, and brooks rose over their banks 

within 12 hours of the heavy rainfall during Sunday, April 

15th. River Flood Observations include: Long stem larger 

rivers, such as the Housatonic and the Connecticut Rivers 

rose over their banks the following day, on Monday, April 

16th. The Yantic River rose above its flood stage of 9.0 feet 

at 3:45 am on April 16th. It crested at 10.42 feet at 10:42 

am, then fell below flood stage at 9:45 pm on April 16th. 

Urban Flood Observations include: Trained spotters reported 

sewage drains overflowing in Greenwich, street flooding 

in Trumbull, Old Greenwich, and Hamden; and flooding 

along I-95 in Fairfield and New Haven Counties. High Wind 

Observations include: The highest winds occurred across 

southeast Connecticut during Saturday night from April 15th 

to the 16th. East winds increased during the day, Sunday as 

the low approached. Wind speeds gusted from 35 to 55 mph. 

As the low moved toward Long Island, another period of high 

winds occurred during Sunday night with peak wind speeds 

from 45 to 55 mph. The National Weather Service (NWS) 

Appendix 2. National Climatic Data Center April 2007 Storm Events: Details for 

Fairfield and Litchfield Counties, Connecticut

These event record details are from the National Climatic Center Web site at

http://www4.ncdc.noaa.gov/cgi-win/wwcgi.dll?wwevent~storms, accessed August 25, 2008. 

Event Record Details
Appendix 2 33

Automated Surfacing Observing System (ASOS) measured 

a peak wind gust of 53 mph from the Southeast at Meriden 

Markham Airport in Meriden on April 16th. The NWS ASOS 

measured a peak wind gust of 52 mph from the Southeast at 

New London Airport on April 15th. Based on the receipt of 

many downed trees and power lines in New London County 

Sunday night, instantaneous peak wind gusts of 60 to 70 mph 

were estimated. Tidal Flooding: Coastal flooding occurred 

along the Long Island Sound shores of Connecticut around 

Event: Coastal Flood State: Connecticut

Map of Counties

Begin Date: 15 Apr 2007, 22:00:00 PM EST Forecast Zones affected: Southern Fairfield, Southern New Haven

Begin Location: Not Known

End Date: 16 Apr 2007, 00:00:00 AM EST

End Location: Not Known

Magnitude: 0

Fatalities: 0

Injuries: 0

Property Damage: $ 0.0K

Crop Damage: $ 0.0K

Description:

A strong late season Nor’easter impacted the region with 

a period of moderate coastal flooding. The low orignated over 

the southern plains on Friday, April 13, and then reached the 

mid Atlantic coast Sunday morning, April 15. From there, 

it underwent rapid deepening and tracked northeast, reaching the waters off southern New England Sunday night. The 

combination of a strong high off the New England coast and a 

period of higher than normal spring tides resulted in tidal piling across Long Island Sound. Tidal departures were highest 

on western Long Island Sound from Sunday evening through 

Monday morning, ranging from 2.5 to 3.5 feet. At Stamford, 

Event Record Details

the times of high tide starting Sunday evening, and lasting 

through Thursday, April 19th. The highest surges of 3 to 4 

feet occurred across the Southeast shores of Connecticut 

Sunday evening. Because of the new moon on April 17th, the 

highest water levels (and highest tidal departures) occurred 

during Tuesday, April 17th. Water slowly receded during the 

remainder of the week, when mainly minor tidal flooding 

occurred during Wednesday and Thursday.

Connecticut, there was moderate coastal flooding with the 

highest tide level 11.34 feet (MLLW) at 10:30 p.m. on Sunday, 

April 15. This was comparable to tide levels reached during 

the March 1993 Nor’easter and Hurricane Donna. Several 

days prior to this event, model storm surge forecasts were as 

high as 4.5 feet. For Long Island Sound, such a surge would 

have produced flooding approaching the level of the December 

1992 Nor’easter. While these levels were not realized due to 

changes in the forecast track, this storm resulted in considerable damage to property. 
34 Flood of April 2007 and Flood-Frequency Estimates at Streamflow-Gaging Stations in Western Connecticut

Event: Flood State: Connecticut 

Map of Counties

Begin Date: 15 Apr 2007, 21:00:00 PM EST County: Litchfield

Begin Location: New Hartford

Begin LAT/LON: 41°52’N / 72°58’W

End Date: 18 Apr 2007, 10:00:00 AM EST

End Location: Not Known

Magnitude: 0

Fatalities: 0

Injuries: 0

Property Damage: $ 750.0K

Crop Damage: $ 0.0K

Description:

Heavy rain led to widespread flooding across Litchfield 

County from Sunday evening on the 15th, and persisted 

through Wednesday morning on the 18th. Several streams and 

creeks exceeded bankful as a result of this heavy rain, including the Still River in Winsted, and the Nepaug Brook and 

River in New Hartford. This led to numerous road closures 

across the county, as well as some evacuations. Some of the 

roads that were closed included Carpenter Road in new Hartford, where a foot of water was reported covering the bridge 

just north of Route 202, South Main Street and Highland Lake 

Road in Torrington, where some debris also covered portions 

of Highland Lake Road, Route 47 and Weekeepeemee Road in 

Woodbury, Torrington Road between Torrington and Winsted, 

Cross Road and Youngsfield Road in New Milford, as well as 

several roads in Washington and Winsted that were washed 

out. In addition, several evacuations also occurred, including residents on Standard Avenue in Winsted, as well as at a 

40 unit apartment complex in Winsted. A mudslide also was 

reported at Grove Street in New Milford by an Emergency 

Manager. This resulted in the evacuation of 5 homes. The 

runoff from this heavy rainfall also led to moderate flooding 

on the Housatonic River. At Falls Village, the Housatonic 

Event Record Details

River crested at 11.14 feet at 0115 EST on the 18th, while at 

Gaylordsville, the river crested at 12.97 feet at 2300 EST on 

the 16th. Further south, the Housatonic River at Stevenson 

Dam crested at 19.96 feet at 1400 EST on the 16th. Low pressure developed over the lower Mississippi Valley on Saturday 

April 14th, and then moved northeast while intensifying, 

reaching the southern Appalachians by Sunday morning, April 

15th, and then just south of western Long Island by Monday 

morning, April 16th. This low became very intense, with a 

central barometric pressure falling below 970 millibars upon 

reaching just south of Long Island Monday morning. The low 

then headed off the New England coast by Tuesday morning. 

This intense coastal storm spread heavy precipitation across 

northwest Connecticut, starting on Sunday, and persisting into 

late Monday. Initially, the precipitation fell as a mixture of 

wet snow, sleet and rain, with snow and sleet more prevalent 

across the higher elevations. The precipitation then changed 

to plain rain by late Sunday afternoon into Monday. Liquid 

equivalent precipitation total from this storm ranged from 3 

to 7 inches. This led to widespread flooding across northwest 

Connecticut from late Sunday into Monday evening. 
Appendix 3 35

Appendix 3. Summary of Maximum Pool Levels for U.S. Army Corps of Engineers 

Flood-Control Dams

Flood-Control Dams

Summary of maximum pool levels, in feet

[From U.S. Army Corps of Engineers, Water Control Management, Annual Report FY 2007, Exhibit 3, December 2007]

Reservoir Fiscal year 2007 Highest pool of record

Pool level Percent full Date Pool level Percent full Date

Colebrook River Lake 739.9 55 Apr 07 757.5 90 Jun 84

Mad River Dam 79.2 29 Apr 07 79.2 29 Apr 07

Sucker Brook Dam 15.2 9 Apr 07 25.2 24 Dec 73

East Branch Dam 44.7 39 Apr 07 44.7 39 Apr 07

Hall Meadow Brook Dam 25.6 30 Apr 07 25.6 30 Apr 07

Thomaston Dam 78.9 38 Apr 07 87.2 50 Jun 84

Northfield Brook Lake 64.8 36 Apr 07 67.4 40 Jun 84

Black Rock Lake 80.5 45 Apr 07 93.4 65 Jun 84

Hancock Brook Lake 12.7 13 Apr 07 23.4 58 Jun 82

Hop Brook Dam 47.8 31 Apr 07 57.7 53 Jun 82
Prepared by the Pembroke Publishing Service Center.

For more information concerning this report, contact:

Director

U.S. Geological Survey

Connecticut Water Science Center

101 Pitkin St.

East Hartford, CT 06108

or visit our Web site at:

http://ct.water.usgs.gov
Ahearn—Flood of April 2007 and Flood-Frequency Estimates at Streamflow-Gaging Stations in Western Connecticut—Scientific Investigations Report 2009–5108

Printed on recycled paper