Flooding in the Northeastern United States, 2011

Professional Paper 1821

U.S. Department of the Interior

U.S. Geological Survey
Outside cover. U.S. Geological Survey (USGS) streamgage 01388000 Ramapo River at Pompton Lakes, New Jersey, 

showing flood damages along Paterson-Hamburg Turnpike in Pompton Lakes on August 29, 2011, after major flooding 

associated with rainfall from the remnants of Hurricane Irene. Photograph by John Trainor, USGS.

Inside cover. Ice jam flooding on the Pemigewasset River at Plymouth, New Hampshire, March 2011. Photograph by 

Richard Kiah, USGS.
Flooding in the Northeastern United States, 

2011

By Thomas P. Suro, Mark A. Roland, and Richard G. Kiah

Professional Paper 1821

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015

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Suggested citation:

Suro, T.P., Roland, M.A., and Kiah, R.G., 2015, Flooding in the Northeastern United States, 2011: U.S. Geological Survey Professional Paper 1821, 32 p., http://dx.doi.org/10.3133/pp1821.

ISSN 2330-7102 (online)
iii

Contents

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

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

Purpose and Scope .......................................................................................................................................3

Collection and Application of Streamflow Data during Floods ..............................................................4

2011 Flooding—Causes, Chronology, and Magnitude.............................................................................5

Antecedent Conditions for the 2011 Eastern United States Flooding ..........................................6

Chronology and Magnitude of Flooding during 2011 ......................................................................8

2011 Flooding—Comparison with Historic Floods..................................................................................17

Annual Exceedance Probability Analysis for 2011.................................................................................23

Annual Exceedance Probability using Expected Moments Algorithm...............................................25

Summary........................................................................................................................................................28

Acknowledgments.......................................................................................................................................28

References Cited..........................................................................................................................................28

Glossary.........................................................................................................................................................31

Figures

1. Map showing the Northeastern United States and drainage area studied of

flooding during 2011 ....................................................................................................................2

2. Photograph showing U.S. Geological Survey hydrographer determining the 

outside stage at the Passumpsic River at Passumpsic, Vt., USGS streamgage, 

May 27, 2011 ..................................................................................................................................4

3. Photograph showing U.S. Geological Survey hydrographer making a streamflow

measurement using an acoustic Doppler current profiler at the Mohawk River at

Freeman’s Bridge at Schenectady, N.Y., USGS streamgage.................................................5

4. Map showing monthly-average streamflow conditions across the United States 

for February 2011 ..........................................................................................................................6

5. Graphs showing cumulative precipitation totals from October 1, 2010, to 

September 30, 2011, in relation to historic average cumulative precipitation for 

selected sites in the Northeastern United States ...................................................................7

6. Map showing cumulative precipitation totals for February 28, 2011, and locations of

U.S. Geological Survey streamgages in Ohio with peak streamflows that had an

annual exceedance probability of less than 4 percent and cumulative precipitation

totals for March 6–7, 2011 and locations of U.S. Geological Survey streamgages

in Connecticut and Massachusetts with peak streamflows that had an annual

exceedance probability of less than 4 percent .......................................................................9

7. Map showing cumulative precipitation totals for April 26–27, 2011 and locations of

U.S. Geological Survey streamgages in New Hampshire, New York, and Vermont

with peak streamflows that had an annual exceedance probability of less than 4

percent .........................................................................................................................................11

8. Map showing streamflow conditions at U.S. Geological Survey streamgages on

May 25, 2011 ................................................................................................................................12

9. Map showing cumulative precipitation totals for May 26–27, 2011 and locations 

of U.S. Geological Survey streamgages in Vermont with peak streamflows that 

had an annual exceedance probability of less than 4 percent..........................................13

Flooding on the Sleepers River 

near St. Johnsbury, Vermont, 

May 27, 2011. Photograph by 

Richard Kiah, U.S. Geological 

Survey.
iv

10. Geostationary Operational Environmental Satellite east image of Hurricane Irene

making landfall near New York City on August 28, 2011.......................................................14

11. Map showing cumulative precipitation totals for August 27–29, 2011 and locations

of U.S. Geological Survey streamgages in Delaware, Maine, Massachusetts,

Maryland, New Hampshire, New Jersey, New York, and Vermont with peak

streamflows that had an annual exceedance probability of less than 4 percent............15

12. Map showing cumulative precipitation totals for September 6–9, 2011 and locations

of U.S. Geological Survey streamgages in Maryland, New Jersey, New York, and

Pennsylvania with peak streamflows that had an annual exceedance probability of

less than 4 percent.....................................................................................................................16

13. Graphs showing annual peak streamflows through 2011 and the 4-, 2-, 1-, and

0.2-percent annual exceedance probability streamflows for selected U.S.

Geological Survey streamgages in the Northeast................................................................18

14. Graphs showing streamflow for selected U.S. Geological Survey streamgages in

the northeast for the 2011 flood period and selected previous major floods ...................20

15. Photograph showing rescuers use a boat to navigate the flooded streets of

Pompton Lakes, N.J., as the Ramapo River overflows its banks because of the

heavy rain on August 29, 2011 ..................................................................................................22

16. Map showing annual exceedance probabilities computed for peak streamflows

during 2011 at selected USGS streamgages in the Northeastern United States and

major drainage basins studied during 2011 flooding ............................................................26

17. Map showing selected U.S. Geological Survey streamgages in the Northeastern

United States with annual exceedance probabilities computed using the expected

moments algorithm method ......................................................................................................27

Tables

1. Peak streamflows and flood probability estimates at selected streamgages 

in the Northeastern United States during the 2011 floods available at 

http://dx.doi.org/10.3133/pp1821

2. Peak streamflows and flood probability estimates computed using the expected 

moments algorithm at selected streamgages in the Northeastern United States 

during the 2011 floods available at http://dx.doi.org/10.3133/pp1821
v

Conversion Factors

Inch/Pound to International System of Units

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

mile, nautical (nmi) 1.852 kilometer (km)

Area

square mile (mi2) 259.0 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2) 

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)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as 

°F = (1.8 × °C) + 32.

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as 

°C = (°F – 32) / 1.8.

Datums

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).
U.S. Geological Survey (USGS) hydrographer flagging a high-water 

mark in a tree after the flooding associated with Tropical Storm Lee in 

September 2011, near Bloomsburg, Pennsylvania. Photograph taken by 

Clinton D. Hittle, USGS.
Flooding in the Northeastern United States, 2011

By Thomas P. Suro, Mark A. Roland, and Richard G. Kiah

Abstract

Flooding in the Northeastern United States during 2011 

was widespread and record setting. This report summarizes 

peak streamflows that were recorded by the U.S. Geological 

Survey (USGS) during separate flooding events in February, 

March, April, May, July, August, and September. The flooding 

of late April, which combined snowmelt and heavy rain and the 

floods associated with the tropical storms of late August and 

September, were the most severe and widespread. Precipitation 

totals from March to May for Pennsylvania, New York, and 

Vermont were documented as being the highest totals in 117 

years of record. In late August, the heavy rains associated with 

Hurricane Irene produced widespread flooding in many parts 

of the Northeastern United States, which resulted in damage 

estimates in excess of $7 billion and approximately 45 deaths. 

In September, Tropical Storm Lee produced 6–12 inches of 

rain in parts of the Northeastern United States adding to the 

growing total of record peak streamflows set in 2011.

The annual exceedance probability (AEP) for 

327 streamgages in the Northeastern United States were 

computed using annual peak streamflow data through 2011 

and are included in this report. The 2011 peak streamflow 

for 129 of those streamgages was estimated to have an AEP 

of less than or equal to 1 percent. Almost 100 of these peak 

streamflows were a result of the flooding associated with 

Hurricane Irene in late August 2011. More extreme than the 

1-percent AEP, is the 0.2-percent AEP. The USGS recorded 

peak streamflows at 31 streamgages that equaled or exceeded 

the estimated 0.2-percent AEP during 2011. Collectively, 

the USGS recorded peak streamflows having estimated 

AEPs of less than 1 percent in Connecticut, Delaware, 

Maine, Maryland, Massachusetts, Ohio, Pennsylvania, New 

Hampshire, New Jersey, New York, and Vermont and new 

period-of-record peak streamflows were recorded at more than 

180 streamgages resulting from the floods of 2011.

Introduction

Flooding in the Northeastern United States (hereafter 

referred to as the Northeast) was documented on numerous 

rivers during the general time frame of February to September 

2011 (fig. 1). The most severe flooding was associated with 

Flooding associated with Tropical Storm Lee in September 2011. The 

Susquehanna River overflowing the right (west) bank looking north 

(upstream) near the intersection of Pine Street and Front Street in the 

Borough of Wormleysburg, Pennsylvania. Photograph taken by Scott A. 

Hoffman (U.S. Geological Survey).
2 Flooding in the Northeastern United States, 2011

Study area

65° 70° 75° 80° 85° 90°

46°

44°42°40°38°36°

Lake 

Champlain

ATLANTIC OCEAN

Lake Erie

Lake Ontario

Lake Huron

Lake

Michigan

vi

R

n

o

sd

u H

re

Potomac River

Raquette R

Passaic River

Black River

ennoC

cit c

R

t u

vi

re

Mohawk R

Schuylkill R Raritan R

Delaware River

CANADA

ONTARIO

QUEBEC

NEW 

BRUNSWICK

PENNSYLVANIA

VERMONT

MAINE

MASSACHUSETTS

NEW HAMPSHIRE

CONNECTICUT

RHODE ISLAND

NEW YORK

NEW JERSEY

DELAWARE

MARYLAND

WEST VIRGINIA

VIRGINIA

OHIO

MICHIGAN

WISCONSIN

ILLINOIS

KENTUCKY

INDIANA

TENNESSEE

Albany

Boston

New York

Trenton

Binghamton

Washington

Philadelphia

Burlington

Williamsport

Norfolk

euqs

uS

ah nna River

Little Egg Inlet

Hackensack River

Base from U.S. Geological Survey digital data, 1:2,000,000Universal Transverse Mercator projection, zone 18North American Datum of 1983

Drainage area studied

National Weather Service 

precipitation site

EXPLANATION

St Lawrence River

0 100 200 MILES0 100 200 KILOMETERS

Figure 1. The Northeastern United States and drainage area studied of flooding during 2011.
Purpose and Scope 3

Hurricane Irene and the remnants of Tropical Storm Lee, 

which were back-to-back storms in late August and early 

September. Persistent spring precipitation combined with 

snowmelt produced earlier major flooding in parts of New 

Hampshire, New York, and Vermont (fig.1). Localized 

intense precipitation of short duration caused varying levels 

of flooding in parts of the Northeast in the early summer 

months, whereas excessive precipitation on saturated soils 

associated with two tropical cyclones produced record 

setting flooding during August and September in parts of 

Connecticut, Delaware, Maine, Maryland, Massachusetts, 

New Hampshire, New Jersey, New York, Pennsylvania, and 

Vermont. Precipitation totals from March to May for much 

of Pennsylvania, New York, and Vermont ranged from about 

16 inches to almost 20 inches, and the totals were documented 

as being the highest totals in 117 years of record (National 

Climate Data Center, 2012a). Record flood levels on Lake 

Champlain during April and May of 2011 exceeded the 

previous maximum known elevation since 1827 (Lumia and 

others, 2014).

In late August, Hurricane Irene tracked up the east 

coast of the United States and weakened from a category 1 

hurricane (or cyclone) to a strong tropical storm as it made 

landfall in New Jersey. The heavy rains associated with 

this storm produced widespread flooding in many parts of 

the Northeast resulting in damage estimates in excess of 

$7 billion and approximately 45 deaths. In New Jersey, 

about 1 million people were evacuated (NBC News, 2014) 

and initial damage estimates were approximately $1 billion 

(Watson and others, 2014). In New York, 31 counties were 

declared disaster areas and damage estimates were more 

than $1.3 billion (Lumia and others, 2014) because of the 

flooding associated with rains from Hurricane Irene. As 

a result of this storm, nearly 100 U.S. Geological Survey 

(USGS) streamgages recorded a peak streamflow exceeding 

the 1-percent annual exceedance probability (AEP) and 

more than 70 USGS streamgages recorded a new periodof-record peak streamflow. The AEP is the probability, or 

chance, that a peak streamflow will be equaled or exceeded 

in any given year. In September 2011, another storm, the 

remnants of Tropical Storm Lee, tracked northward toward 

the still recovering Northeast delivering more heavy rains 

and widespread flooding. Rainfall totals were 6–12 inches 

throughout the Susquehanna River Basin and upwards of 

12 inches in parts of southern New York.

The floods of 2011 in the Northeast stand out with respect 

to the cost of human life, property damage, and environmental 

effect but also stand out with respect to the persistence of 

the 2011 floods. The documentation of the severity and 

unusual repetitiveness of the flooding in the Northeast is 

important for the future protection of life and property. Other 

recently published USGS reports provide additional details 

and analyses of the 2011 flooding in New Jersey (Watson 

and others, 2014), New York (Lumia and others, 2014), 

Massachusetts (Olson and Bent, 2013; Bent and others, 2013), 

and Vermont (Medalie and Olson, 2013).

The flooding of 2011 was spread throughout the year 

and the region and resulted in about 300 USGS streamgages 

documenting peak streamflows that exceeded the 4-percent 

AEP streamflow, and of those, 129 streamgages documented 

peak streamflows that were less than or equal to the 1-percent 

AEP streamflow. The USGS recorded new period-of-record 

peak streamflows at more than 180 streamgages as a result 

of the floods of 2011. The National Weather Service (NWS) 

Flood Loss Report for the United States during 2011 indicated 

that direct freshwater damages from Hurricane Irene and 

Tropical Storm Lee totaled at least $3.9 billion from Virginia 

to Vermont (National Weather Service, 2014b).

The USGS is a national science agency and within 

its mission areas is charged with documenting the water 

resources and assessing natural hazards of the United 

States. Documenting flood peaks, along with the antecedent 

conditions, flood chronology, and AEP, will help to put 

the floods of 2011 into historic perspective and facilitate 

public and private awareness of the flooding potential and 

considerations of land use and flood-insurance regulations 

by local and regional citizens and elected officials. The 

data collected by the USGS to document these major floods 

will also assist other Federal, State, and local agencies with 

emergency management planning for future floods, hazard 

mitigation plans, and resiliency planning.

Purpose and Scope

The purpose of this report is to provide a broad overview 

of the flood magnitudes and frequencies in the Northeast 

during 2011. The USGS maintains a national database of 

water information and has flood documentation dating 

back to the 1800s. Documenting the magnitude, extent, and 

frequency of major floods preserves this important information 

that is necessary in protecting human life, property, and 

infrastructure. The streamgages included in this report 

were selected on the general basis of documenting a peak 

streamflow having an AEP of about 4 percent or less. The 

AEPs for all 327 streamgages presented in this report were 

computed using the log-Pearson type III (LPIII) method, and 

AEPs for 122 of these 327 streamgages also were computed 

by the Expected Moments Algorithm method. Other USGS 

flood reports providing more detailed analysis of the flooding 

in the Northeastern United States during 2011 are available at 

the USGS publications warehouse (http://pubs.er.usgs.gov/).

This report consolidates and summarizes the flooding 

information and documents the flood peaks (stage and streamflow) for many States in the Northeast that were affected by 

the floods of 2011. In this report, the 8-digit number listed in 

parentheses after a streamgage name is the USGS station number for that streamgage. Additionally, each USGS streamgage 

listed in table 1 (available at http://dx.doi.org/10.3133/pp1821)

contains a map number that allows cross reference from the 

table to the respective map figure for that flood period.
4 Flooding in the Northeastern United States, 2011

Collection and Application of 

Streamflow Data during Floods

The organizational structure of the USGS is based on 

the following seven science mission areas: Climate and Land 

Use Change, Core Science Systems, Ecosystems, Energy and 

Minerals, Environmental Health, Natural Hazards, and Water. 

Specific to the Water Mission Area, one of the objectives 

is to provide tools that managers and policymakers can 

use for understanding, predicting, and mitigating waterrelated hazards such as floods (Evenson and others, 2013). 

To help accomplish this objective, the USGS collects and 

disseminates reliable, impartial, and timely information 

about the Nation’s rivers and streams. The following are 

several examples from Holmes and others (2010) that 

include data collection and scientific interpretation of 

streamflow data as the data relate to flooding: the operation 

of a nationwide network of streamgages, the development 

of flood-inundation maps, the documentation of high-water 

mark elevations, and the determination of annual exceedance 

flood probabilities.

The USGS operates and maintains a network of more 

than 9,000 streamgages across the Nation that provides 

valuable streamflow information over the internet on a near 

real-time basis (http://waterdata.usgs.gov/usa/nwis/rt). This 

information may be related to water quality (such as pH, 

temperature, or turbidity); water level (commonly referred 

to as stage); or water quantity (typically reported as a rate 

of streamflow or discharge). Stage and streamflow data are 

of particular interest during times of flooding, or predicted 

flooding, to a variety of agencies, organizations, and municipalities, as well as the general public. The USGS streamgage 

network is continuously maintained and relations between 

river stage and streamflow are regularly checked to define 

constantly changing conditions to ensure 

quality data are available at all times. A 

photograph of a USGS hydrographer determining the outside stage at the Passumpsic River at Passumpsic, Vt. (01135500) 

streamgage on May 27, 2011, is shown 

in figure 2. The stage is determined by 

reading the staff gage on the outside of the 

streamgage shelter as an independent check 

on the hydrologic recording equipment 

inside of the streamgage shelter. The NWS 

uses information such as precipitation and 

historical streamflow data to forecast flood 

stages at select streamgages (also known 

as flood forecast sites) using mathematical models that simulate the movement of 

floodwater as it flows downstream. Flood 

forecast information typically is available 

3–5 days in advance of events in many 

locations, with typical reporting intervals 

of 6 hours (National Weather Service, 

2014a). The U.S. Army Corps of Engineers, the Federal 

Emergency Management Agency, and many other state and 

local agencies and organizations rely on streamflow data to 

assist in their respective operational duties such as water 

control, emergency management, and mitigation during times 

of flooding.

Digital flood-inundation maps, which can be accessed 

through the USGS Flood Inundation Mapping Science Web 

site at http://water.usgs.gov/osw/flood_inundation/, depict 

the estimated areal extent and depth of flooding corresponding to stages at select USGS streamgages. These maps, in 

conjunction with the near real-time stage data from USGS 

streamgages and National Weather Service flood-stage 

forecasts, can help guide the general public in taking individual safety precautions and provide local officials with a 

tool to efficiently manage emergency flood operations. The 

documentation of high-water marks as a result of flooding is 

also an important piece of information that can be useful for 

validating the accuracy of data being recorded at streamgages 

and for developing water-surface profiles through a reach. 

Water-surface profiles may be used to assist in the calibration 

of mathematical models for flood simulation and many associated applications such as flood insurance studies, development 

of flood-mapping products, and (or) estimating flow quantities. Estimates of the frequency and magnitude of floods are 

essential for flood insurance studies, flood-plain management, 

and the design of bridges and flood-control structures (Roland 

and Stuckey, 2008). The estimation of flood frequencies and 

magnitudes is based on an analysis using annual peak streamflow data. The reliability of these data is contingent upon the 

accuracy of the data collected at a streamgage.

A primary function of a streamgage is to record the 

stage of the river, or stream, at the established location of 

the streamgage on a preset interval (for example, every 

15 minutes) and transmit that data to a central USGS database 

Staff gage

Figure 2. U.S. Geological Survey (USGS) hydrographer determining the outside 

stage at the Passumpsic River at Passumpsic, Vt. (01135500) USGS streamgage, 

May 27, 2011. Photograph by Richard Kiah, USGS.
2011 Flooding—Causes, Chronology, and Magnitude 5

on a near real-time basis where the data are processed and 

made available online to the public. Continuous real-time 

data are extremely valuable because the data allow for remote 

monitoring of stage at many rivers and streams across the 

United States. Stage data can be related to streamflow, or 

discharge (typically reported in units of cubic feet per second 

[ft3/s]) by establishing a stage-discharge relation typically 

referred to as a rating curve. A rating curve is unique to each 

streamgage and is established by making a series of discrete 

streamflow measurements over a range of stages. These 

streamflow measurements and associated stages are plotted 

and used to define a relation (or curve) between stage and 

streamflow. Changes to channel geometry and drainage basin 

characteristics require continuous monitoring and adjustment 

of rating curves to accurately represent stage-discharge 

relationships. Defining a rating curve at extreme high stages 

can be challenging because of the relative infrequency of 

extreme flood events and, in some cases, not being able to 

physically access or measure a river or stream that is flooding. 

A photograph of a USGS hydrographer making a streamflow 

measurement at the Mohawk River at Freeman’s Bridge at 

Schenectady, N.Y. (01354500) streamgage during the flooding 

associated with Hurricane Irene in August 2011 is shown in 

figure 3. The hydrographer is using an acoustic doppler current 

profiler (ADCP) mounted in a small boat and tethered by a 

rope to collect water depth and velocity data. The data are 

continuously transmitted by radio link to a nearby laptop, not 

shown in the photograph. During times of flooding, additional 

emphasis typically is placed on maintaining streamgage 

operations to ensure uninterrupted data delivery and on 

obtaining sufficient high-flow measurements for defining and 

adjusting rating curves.

Rating curves that have been well defined generally 

translate to increased confidence in the forecasted stages, 

resulting in improved reliability of stage and streamflow 

data during flood events. Reliable streamflow data provide 

emergency management agencies, first responders, and the 

general public with critical information about river conditions; 

thus, allowing better informed operational and response 

decisions for the protection of life and property. The ability 

of USGS field personnel to respond rapidly allows for the 

collection and dissemination of accurate and timely data 

to those who rely on the data. The USGS made more than 

2,000 discrete measurements of streamflow to verify or define 

rating curves at many streamgages throughout the Northeast 

during the floods of August and September 2011.

2011 Flooding—Causes, Chronology, 

and Magnitude

Floods are caused when meteorological processes deliver 

more precipitation or runoff to a region than can be retained 

or stored in a watershed. Climate, geology, topography, 

and antecedent conditions all play a role in the delivery 

and retention of precipitation in a 

watershed. In a region as diverse 

as the Northeast, flood-causing 

processes can range from orographic 

lifting and thermal convection, 

to lake-effect precipitation along 

the Great Lakes, to extra tropical 

or tropical cyclones. Typically, 

localized flooding is caused by 

severe, convective storms; whereas, 

widespread flooding is caused by 

frontal systems that move into the 

region from the south and west or 

by tropical storms that can include 

cyclones (Paulson and others, 1991).

The 2011 flooding in the 

Northeast was on small streams and 

large rivers. Much of the flooding 

was widespread and included parts 

of Connecticut, Delaware, Maine, 

Maryland, Massachusetts, New 

Hampshire, New Jersey, New York, 

Ohio, Pennsylvania, and Vermont. 

During February to September of 

2011, each month recorded at least 

one separate flood event somewhere 

in the Northeast.

Figure 3. U.S. Geological Survey (USGS) hydrographer making a streamflow measurement 

using an acoustic Doppler current profiler (ADCP) at the Mohawk River at Freeman’s Bridge 

at Schenectady, N.Y. (01354500) USGS streamgage. Photograph by Alicia Gearwar, USGS.
6 Flooding in the Northeastern United States, 2011

Antecedent Conditions for the 2011 Eastern 

United States Flooding

As noted by Holmes and others (2010) the genesis of 

most major widespread flooding is not just one particular 

storm but the result of frequent and consistently abundant 

precipitation over the same geographic area for an extended 

period. As soils become saturated, infiltration capacities 

decrease and stream levels can rise to bankfull conditions or 

beyond. Additional precipitation and associated runoff will 

result in progressively severe flooding. In the Northeast, the 

presence of frozen soils or accumulated snow cover may be 

factors of great importance in determining the nature and 

extent of flooding. Flooding in 2011 began in late winter with 

February monthly streamflows ranging from 25 to 75 percent 

of normal for most of the Northeast (fig. 4). During the 

winter of 2010–11, above-average snowfall was reported 

in the northern States. The March 2011 snowpack was the 

seventh largest in the last 46 years (National Climatic Data 

Center, 2012b). In mountainous regions of New York and 

New England, the snowpack in some locations contained 

the equivalent of 18–20 inches of water (National Weather 

Service, 2012a), of which the melting contributed to flooding 

by saturating the soils and filling the streams to near bankfull 

conditions in numerous locations.

A dichotomy of conditions existed in the northeast 

during 2011 as evidenced when comparing October 2010 

through September 2011 (The 12-month period from 

October 1, 2010 to September 30, 2011 is referred to as water 

year 2011) cumulative precipitation with historic average 

cumulative precipitation (1981–2010) for four selected 

precipitation gages (National Climatic Data Center, 2012a; 

fig. 5A–D). The October 2010 through September 2011 

cumulative precipitation and the historic average cumulative 

precipitation at four selected sites in the northeast are shown 

in figure 5A–D. Water year 2011 was the wettest on record 

From U.S. Geological Survey WaterWatch, accessed 

December 12, 2012 (http://waterwatch.usgs.gov)

EXPLANATION

Streamflow conditions, in percentile classes

High

Greater than 90, much above normal

76 to 90, above normal

25 to 75, normal

10 to 24, below normal

Less than 10, much below normal

Low

Figure 4. Monthly-average streamflow conditions across the United States for February 2011 (U.S. Geological Survey, 

2012a).
2011 Flooding—Causes, Chronology, and Magnitude 

7rol15-HWCR-0022_fig 05

A. Williamsport, Pennsylvania

0

10203040506070

EXPLANATION

October 2010 through September 2011 cumulative precipitationHistoric cumulative precipitation (1981–2010)

B. Burlington, Vermont

C. Trenton, New Jersey

0

10203040506070

D. Norfolk, Virginia

Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept.

2010 2011 2010 2011

Cumulative precipitation, in inches

Water year Water year

Figure 5. Cumulative precipitation totals from October 1, 2010, to September 30, 2011, in relation to historic average cumulative precipitation (1981–2010) for selected sites in the Northeastern United States (National Climatic Data Center, 2012b).
8 Flooding in the Northeastern United States, 2011

for hydrologic units in New England, New Jersey, New York, 

Ohio, and Pennsylvania (U.S. Geological Survey, 2012) as 

precipitation was consistently above normal. In contrast, 

some hydrologic units in Delaware, Maryland, and Virginia 

reported below normal to much-below normal runoff in water 

year 2011 as precipitation was normal to below normal during 

much of the year.

Chronology and Magnitude of Flooding during 

2011

The 2011 Northeast floods recorded from February 

through September were caused by persistent spring 

precipitation combined with snowmelt, localized intense 

precipitation of short duration, and excessive precipitation 

amounts on saturated soils resulting from tropical cyclones. 

Well-above average precipitation amounts were observed 

in many areas of the Northeast. Parts of Pennsylvania, New 

England, New Jersey, and New York received precipitation 

from 4–20 inches above average (National Weather Service, 

2012b). The 12-month total precipitation was composed of 

numerous discrete storm sequences that induced multiple 

flooding events in different geographic locations.

Peak streamflows for 327 streamgages in the northeast 

that had estimated AEPs of about 4 percent or less were 

included in table 1. Of these 327 streamgages, 183 recorded 

peak-of-record streamflows during the 2011 floods. A few 

selected streamgages that reported peak streamflows with 

estimated AEPs greater than 4 percent were also included in 

table 1 to indicate the extent of major flooding. Approaches 

used in computing AEPs are described in the “Annual 

Exceedance Probability Analysis, 2011” section of this 

report. To minimize figure clutter, only the major rivers (for 

example, Connecticut, Hudson, Susquehanna Rivers) and 

selected small rivers mentioned in the report text for that 

particular flood period are shown on the figures. The 2011 

peak-stage and streamflow data, previous peak-of-record 

flood data, estimated AEP for the 2011 peak streamflow, and 

estimates of the magnitude of the streamflow corresponding to 

the 4-percent, 2-percent, 1-percent, and 0.2-percent AEP are 

listed in table 1. For each figure corresponding to a particular flood period, the size of the symbol for each streamgage 

represents the estimated AEP that corresponds to the magnitude of the computed peak streamflow—the less probable (less 

frequent) the peak streamflow, the larger the symbol. Daily 

NWS rainfall observations for the United States were used to 

represent cumulative precipitation totals for individual storm 

events (National Weather Service, 2012c).

The first flood-inducing precipitation events began on 

February 28, 2011 (fig. 6A) and March 6, 2011 (fig. 6B). These 

snowmelt-enhanced events caused major flooding in parts 

of Ohio, Connecticut, and Massachusetts. Flooding resulted 

from increased streamflow as well as localized ice jamming 

(U.S. Geological Survey, 2013).

In Ohio, more than 4 inches of snow-water equivalent 

was present on February 27, 2011, and was reduced to trace 

amounts by March 1, 2011 (National Weather Service, 

2012d). Peak-of-record streamflow was recorded at the USGS 

streamgages Ottawa River at Lima, Ohio (04187100); Honey 

Creek at Melmore, Ohio (04197100); and Old Woman Creek 

at Berlin Road near Huron, Ohio (04199155). In this report, 

the 8-digit number listed in parentheses after a streamgage 

name is the USGS station number for that streamgage. In 

Southern New England upwards of 10 inches of snow-water 

equivalent was present on March 5, 2011, and was reduced to 

trace amounts by March 8, 2011 (National Weather Service, 

2012d). Peak-of-record streamflow was recorded at the USGS 

streamgage Saugatuck River near Redding, Conn. (01208990; 

table 1). Rain and rapidly melting snow combined to mobilize 

the ice cover on some northern rivers causing ice jams and 

localized flooding in some locations. The late February and 

early March events did not result in severe flooding outside 

of Ohio and southern New England; however, the widespread 

rainfall and snowmelt contributed to increased soil-moisture 

levels and streamflows in other areas.

Precipitation and snowmelt contributed to flooding in 

late April and early May of 2011. During April 26–29, 2011, 

above normal temperatures, snowmelt in the mountains, and 

bands of rainfall and thunderstorms (fig. 7) caused flooding in 

northern areas of New Hampshire, New York, and Vermont. 

The snow-water equivalent in the mountains of New York 

and Vermont held upwards of 10 inches of water on April 26, 

2011(National Weather Service, 2012e). Substantial flooding 

occurred along the Hudson River in New York as peak-ofrecord streamflows were recorded at the USGS streamgages 

in North Creek (01315500), Hadley (01318500), Fort Edward 

(01327750), and Stillwater (01331095). The peak streamflow 

recorded at the USGS streamgage Hudson River at Newcomb, 

NY (01312000), was the second highest peak on record and 

was only exceeded by the peak of January 1998 (table 1). 

Other peak-of-record streamflows were recorded at USGS 

streamgages in New Hampshire and Vermont (table 1).

In the winter and spring of 2011, record rainfall 

and snowmelt for much of the Northeast contributed to 

streamflows that were above normal at numerous USGS 

streamgages in New England, New York, New Jersey, 

Delaware, and Maryland (fig. 8). Precipitation totals for March 

through May in New York (16.68 inches), Pennsylvania 

(19.13 inches), and Vermont (17.14 inches) were the highest 

in 117 years of record (National Climate Data Center, 2012a). 

Record rainfall contributed to record flooding on Lake 

Champlain during the spring of 2011. Lake Champlain was 

above the National Weather Service flood stage of 100 feet (ft) 

for 68 days from April 13 to June 19, 2011. The peak watersurface elevation of Lake Champlain, 103.27 ft, was recorded 

on May 6, 2011, at the Lake Champlain at Burlington 

(04294500) lake gage (Kiah and others, 2013). This elevation 

surpassed the maximum known elevation since at least 1827 

of 102.1 ft on May 4, 1869.
2011 Flooding—Causes, Chronology, and Magnitude 9

rol15-HWCR-0022_fig 06a

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""" -81°11' -83°12' -85°13' -87°14'

41°9'40°8'

Flood annual exceedance probability (AEP), in percent—Number 

is map number (table 1). 

Precipitation, in inches—Interval 

0.5 and 1 inches

"""

0 to 0.50.5 to 11 to 2

""

2 to 3

3 to 5

EXPLANATION

Greater than 42 to 41 to 2

0.2 to 1

0 60 30 MILES0 60 30 KILOMETERS

Base from U.S. Geological Survey digital data, 1998, 1:5,000,000

Great Lakes BasinAlbers Equal-Area Conic projection

North American Datum of 1983

OHIO

INDIANA

MICHIGAN

283

279 286

280

284

285

287

288

289

290

293

294 Lake Erie

Lake

Michigan

Oh oi

R vi

er

Figure 6. Cumulative precipitation totals for 

A, February 28, 2011, and locations of U.S. Geological Survey streamgages in Ohio with peak streamflows that had an 

annual exceedance probability of less than 4 percent and 

B, cumulative precipitation totals for March 6–7, 2011 (National Weather Service, 2012c) and locations of 

U.S. Geological Survey streamgages in Connecticut and Massachusetts with peak streamflows that had an annual exceedance probability of less than 4 percent.
10 Flooding in the Northeastern United States, 2011

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""

44°10'

43°20'

42°30'

41°40'

5

51

50

38

35

34

33

25

Connecticut River

Hudson River

NEW YORK

VERMONT NEW HAMPSHIRE

MASSACHUSETTS

CONNECTICUT

RHODE

ISLAND

MAINE

PENNSYLVANIA

0 30 60 MILES

0 30 60 KILOMETERS

Flood annual exceedance probability (AEP), in percent—Number 

is map number (table 1)

Precipitation, in inches—Interval 

1 inch

"

"

"

0 to 1

1 to 2

2 to 3

"

"

"

3 to 4

4 to 5

5 to 6

EXPLANATION

Greater than 4

2 to 4

1 to 2

Base from U.S. Geological Survey digital data, 1998, 1:5,000,000

Albers Equal-Area Conic projection

Standard parallels 29°50’N and 45°50’N

Central meridan 96°00’W

North American Datum of 1983

Figure 6. Cumulative precipitation totals for A, February 28, 2011, and locations of U.S. Geological Survey streamgages in Ohio 

with peak streamflows that had an annual exceedance probability of less than 4 percent and B, cumulative precipitation totals 

for March 6–7, 2011 (National Weather Service, 2012c) and locations of U.S. Geological Survey streamgages in Connecticut and 

Massachusetts with peak streamflows that had an annual exceedance probability of less than 4 percent.—Continued
2011 Flooding—Causes, Chronology, and Magnitude 11

rol15-HWCR-0022_fig 07

75°50' 73°45' 71°40'

45°25'

43°20'

CANADA

NEW

HAMPSHIRE

VERMONT

MASSACHUSETTS

CONNECTICUT

NEW YORK

Hudson River

Connecticut River

Susquehanna

River

7

54 55

56 57

58

59 60

62

70

71

72

298

299

300

301

302

303

304

305 306

307

308

309

321

323 324

327

6

0 30 60 MILES

0 30 60 KILOMETERS

Lake Ontario

Flood annual exceedance probability (AEP), in percent—Number 

is map number (table 1)

Precipitation, in inches—Interval 

1 inch

"

"

"

0 to 1

1 to 2

2 to 3

"

"

"

3 to 4

4 to 5

5 to 6

EXPLANATION

Greater than 4

2 to 4

1 to 2

0.2 to 1

Less than 0.2

Base from U.S. Geological Survey digital data, 1998, 1:5,000,000

Albers Equal-Area Conic projection

Standard parallels 29°50’N and 45°50’N

Central meridan 96°00’W

North American Datum of 1983

Figure 7. Cumulative precipitation totals for April 26–27, 2011 (National Weather Service, 2012c) and locations of U.S. Geological 

Survey streamgages in New Hampshire, New York, and Vermont with peak streamflows that had an annual exceedance probability of 

less than 4 percent.
12 Flooding in the Northeastern United States, 2011

An isolated weather system in late May contributed 

precipitation amounts as much as 7 inches as a band of 

storms tracked across northern Vermont on May 26–27, 2011 

(fig. 9). Localized thunderstorms, hail, and heavy rainfall were 

associated with several miniature supercells (National Weather 

Service, 2012f). Substantial flooding was limited mostly to 

streams with drainage areas less than 100 square miles (mi2), 

such as Sleepers River near St. Johnsbury, Vt. (01135300), 

where a peak-of-record streamflow was recorded on May 27, 

2011 (table 1).

Hurricane Irene made landfall in New Jersey on 

August 28, 2011, as a category 1 hurricane, according to initial 

reports, weakened to a tropical storm as it moved towards 

New York and to an extra tropical cyclone over Vermont. 

Winds and widespread flooding resulted in damage estimates 

in excess of $7 billion and approximately 45 deaths (National 

Oceanic and Atmospheric Administration, 2012). Additional 

information about the origins and life cycle of Hurricane Irene 

can be found in the NWS Hurricane Irene Tropical Cyclone 

Report (Avila and Cangialosi, 2011). The NWS rainfall 

observations for the Northeast from August 27–29, 2011, 

document cumulative precipitation of more than 6 inches over 

an area of about 20,000 mi2, with some areas receiving two to 

three times as much precipitation (fig. 11). The intense rainfall 

associated with Hurricane Irene contributed to new August 

total precipitation records for New Hampshire, New Jersey, 

New York, and Vermont (National Climatic Data Center, 

2012b). The record precipitation produced streamflows with 

estimated AEPs less than 4 percent at 186 USGS streamgages, 

of which 119 were period-of-record peaks. Particularly hard hit 

were New York (46 peak-of-record streamflows), New Jersey 

(32 peak-of-record streamflows), and Vermont (12 peak-ofrecord streamflows). Peak-of-record streamflows were recorded 

at 31 additional USGS streamgages in Delaware, Maine, 

Maryland, Massachusetts, New Hampshire, and Pennsylvania. 

In the Green Mountains of Vermont, 13 communities were 

isolated for days as floodwaters damaged or destroyed 

more than 480 bridges, 960 culverts, and 500 miles of State 

rol15-HWCR00-0022_fig 08

From U.S. Geological Survey WaterWatch, accessed 

December 12, 2012 (http://waterwatch.usgs.gov)

EXPLANATION

Streamflow conditions, in percentile classes

High

Greater than 90, much above normal

76 to 90, above normal

25 to 75, normal

10 to 24, below normal

Less than 10, much below normal

Low

Figure 8. Streamflow conditions at U.S. Geological Survey streamgages on May 25, 2011 (U.S. Geological Survey, 2012).
2011 Flooding—Causes, Chronology, and Magnitude 13

rol15-HWCR-0022_fig 09

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""

46°

45°

44°

10 9

11

12

MAINE

NEW

HAMPSHIRE

VERMONT

NEW

YORK

0 15 30 60 MILES

0 15 30 60 KILOMETERS

CANADA

Connecticut River

Flood annual exceedance probability (AEP), in percent—Number 

is map number (table 1). Not all ranges are represented on map

Precipitation, in inches—Interval 

1 inch

"

"

"

0 to 1

1 to 2

2 to 3

"

"

"

3 to 4

4 to 5

5 to 6

EXPLANATION

Greater than 4

2 to 4

1 to 2

0.2 to 1

Less than 0.2

Base from U.S. Geological Survey digital data, 1998, 1:5,000,000

Albers Equal-Area Conic projection

Standard parallels 29°50’N and 45°50’N

Central meridan 96°00’W

North American Datum of 1927

Figure 9. Cumulative precipitation totals for May 26–27, 2011 (National Weather Service, 2012c) and locations of 

U.S. Geological Survey streamgages in Vermont with peak streamflows that had an annual exceedance probability of 

less than 4 percent.
14 Flooding in the Northeastern United States, 2011

highways (Vermont Emergency Management, 2014). Runoff 

greater than 700 cubic feet per second per square mile was 

computed in high-elevation, small drainage tributaries in 

the Green Mountains (USGS streamgages Whetstone Brook 

Tributary near Marlboro, Vt. [01156300] and Kent Brook near 

Killington, Vt. [01150800]). The USGS streamgage at Saxtons 

River at Saxtons River, Vt. (01154000), recorded a peak 

streamflow of 21,600 ft3/s on August 28 that was 155 percent 

greater than the previous peak-of-record streamflow 

(8,460 ft3/s) set in 1976 (table 1), and the peak stage of 19.58 ft 

was the maximum stage since at least 1869.

Recovery efforts for Hurricane Irene were still underway 

when remnants of Tropical Storm Lee tracked across the 

Northeast drenching already saturated areas of Maryland, New 

Jersey, New York, and Pennsylvania. During September 8–9, 

substantial precipitation and subsequent flooding took place 

as 6–12 inches were reported throughout the Susquehanna 

River Basin and upwards of 12 inches of rain was reported 

in parts of southern New York, and greater than 20 inches 

was reported in parts of Virginia (fig. 12). Peak streamflows 

having an estimated AEP of less than 4 percent were recorded 

at 59 USGS streamgages, of which 31 were period-of-record 

peaks (table 1). In the upper reaches of the Susquehanna 

River, near peak-of-record streamflows were recorded at the 

streamgages Susquehanna River at Unadilla, N.Y. (01500500) 

and Susquehanna River at Conklin, N.Y. (01503000). Farther 

downstream along the Susquehanna River in New York, peakof-record streamflows were recorded at the USGS streamgages 

in the communities of Vestal (01513500), Owego (01513831), 

and Waverly (01515000). In Binghamton, N.Y., about 20,000 

residents were evacuated (Associated Press, 2011) as the 

streamgage Susquehanna River at Vestal, N.Y. (01513500) 

reached a peak-of-record stage of 35.26 ft with an associated 

peak streamflow of 129,000 ft3/s on September 8. The peak 

stage of 35.26 ft was 1.6 ft higher than the previous peak-ofrecord stage of 33.66 ft set in 2006. Farther downstream at the 

streamgage Susquehanna River at Waverly, N.Y. (01515000), 

located 1 mi. downstream from the New York-Pennsylvania 

State line, a peak-of-record stage of 26.67 ft and discharge of 

167,000 ft3/s were recorded. The 2011 peak stage exceeded 

the previous peak-of-record stage set during the flood of 

June 2006 by more than 4 ft and exceeded the historic peak 

stage set back in March of 1936 by more than 5 ft (Suro and 

others, 2009).

rol15-HWCR-0022_fig 10

Figure 10. Geostationary Operational Environmental Satellite (GOES) east image of Hurricane Irene making 

landfall near New York City on August 28, 2011. Image is courtesy of the National Oceanic and Atmospheric 

Administration, 2011.
2011 Flooding—Causes, Chronology, and Magnitude 15

15-HWCR-0022_fig 11

75° 74° 73° 72° 71° 70°

45°

44°

43°

42°

41°

40°

39°

Connecticut River

Connecticut River

Susquehanna River

Susquehanna River

Hudson River

1

2 3

4

8

13 14

15

16 17 18

19

20

21

22 23

24

26 27

28 29

30

31

32

37

39

40

41

42 43

44

45

46

48

49

52

53

61

63 64

65

66

68 67

74 69

75

76

79 78

81

82 83

84 85

86

87

88

89

90

91

92

94

95

96

97

98

99

100

101 102

103

104

105 106

107 108

109

110

112

114

115

116

117

119

121

124

126 125

129

130

131

132 133

134 135

136

137

138

139

141

142

143

144 145

146

147

148

149

150

151

152 153

154

155

156 157

158 159

160

162

163

164

165

167

168

169

170 171

172

173

174

175

176

179 180

182

183 184

185 186

187 188 189

190

195

196 197

201

202

203 205

206

208

209

210

211

212

213

214

215 216

268

275

276

310

311 312 313

314

315

316

317

318

319

320

322

325 326

77

80

93

111

113

120

122

123

166

36

118 127

204 207

MAINE

NEW

HAMPSHIRE

VERMONT

NEW YORK

MASSACHUSETTS

PENNSYLVANIA

CONNECTICUT

RHODE

ISLAND

NEW

JERSEY

DELAWARE

MARYLAND

DISTRICT OF

COLUMBIA

0 30 60 120 MILES

0 30 60 120 KILOMETERS

EXPLANATION

Precipitation, in inches—Intervals of 1, 2, 

and 4 inches

0 to 1

1 to 2

2 to 3

3 to 4

"

"

"

"

" 4 to 5

5 to 6

6 to 8

8 to 10

10 to 12

"

"

"

"

" 12 to 14

14 to 16

16 to 20

"

"

" 20 to 24

Greater than 4.0 or not estimated

2.0 to 4.0

1.0 to 2.0

0.2 to 1.0

Less than 0.2

Flood annual exceedance probability (AEP),

in percent—Number is map number (table 1)

Base from U.S. Geological Survey digital data, 1998, 1:5,000,000

Albers Equal-Area Conic projection

Standard parallels 29°50’N and 45°50’N

Central meridan 96°00’W

North American Datum of 1983

Figure 11. Cumulative 

precipitation totals for 

August 27–29, 2011 

(National Weather Service, 

2012c) and locations of 

U.S. Geological Survey 

streamgages in Delaware, 

Maine, Massachusetts, 

Maryland, New Hampshire, 

New Jersey, New York, 

and Vermont with peak 

streamflows that had 

an annual exceedance 

probability of less than 

4 percent.
16 Flooding in the Northeastern United States, 2011

rol15-HWCR-0022_fig 12

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VERMONT

MASSACHUSETTS

CONNECTICUT

NEW YORK

PENNSYLVANIA

NEW

JERSEY

WEST

VIRGINIA

DELAWARE

MARYLAND

VIRGINIA

DISTRICT OF

COLUMBIA

Base from U.S. Geological Survey digital data, 1998, 1:5,000,000

Albers Equal-Area Conic projection

North American Datum of 1983

Susquehanna R

Potomac River

Hudson River

78° 77° 76° 75° 74°

43°

42°

41°

40°

39°

Ohio

0 25 50 KILOMETERS

0 25 50 MILES

EXPLANATION

Precipitation, in inches—Intervals of 1, 2, 

and 4 inches

0 to 1

1 to 2

2 to 3

3 to 4

"

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5 to 6

6 to 8

8 to 10

10 to 12

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14 to 16

16 to 20

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Greater than 4.0 or not estimated

2.0 to 4.0

1.0 to 2.0

0.2 to 1.0

Less than 0.2

Flood annual exceedance probability (AEP),

in percent—Number is map number (table 1)

River

Figure 12. Cumulative precipitation totals for September 6–9, 2011 (National Weather Service, 2012c) and locations of 

U.S. Geological Survey streamgages in Maryland, New Jersey, New York, and Pennsylvania with peak streamflows that had an 

annual exceedance probability of less than 4 percent.
2011 Flooding—Comparison with Historic Floods 17

2011 Flooding—Comparison with 

Historic Floods

In 2011, record-breaking rainfall events produced 

widespread flooding across the Northeast resulting in peak 

streamflows being recorded at 327 streamgages that generally 

equaled or exceeded the estimated 4-percent AEP streamflow 

(table 1), of which 263 were recorded during the months of 

August and September. As previously discussed, some of the 

storm events in 2011 tended to be more isolated in nature 

leading to more localized flooding; however, the storms that 

had the most effect on a regional basis were two tropical 

systems, Tropical Storm Irene and the remnants of Tropical 

Storm Lee. Arriving only a week apart from each other 

in late August and early September, tropical storms Irene 

and Lee caused catastrophic flooding. The NWS reported 

37 fatalities directly related to freshwater flooding and direct 

freshwater flood damages of $3.9 billion from Virginia to 

Vermont (National Weather Service, 2014a). Other reports 

summarize 45 fatalities and more than $7 billion in total 

damages (National Oceanic and Atmospheric Administration, 

2012). Tropical Storm Irene made landfall at Little Egg Inlet, 

N.J., (fig. 1) on August 28, 2011, depositing 6–7 inches of 

precipitation across most of the State causing record-breaking 

floods at many streams. In New Jersey alone, about 1 million 

people across the State were evacuated, and every county was 

declared a Federal disaster area (Watson and others, 2014).

As stated by Holmes and others (2010, 2013) in 

discussing 2008 flooding in the Midwest United States and 

2011 floods in the central United States, when significant 

rainfall events produce flooding, it is important to document 

the magnitude of a flood so it can be put into context for 

comparison with previous floods (table 1). Eight USGS 

streamgages in the Northeast were selected to illustrate 

the comparison of historic flood peaks to the flood peaks 

recorded during the 2011 floods. A timeline of annual peak 

streamflows for each of these eight streamgages was plotted 

to show a comparison of the magnitude of 2011 flood 

peaks with previous annual peak streamflows (fig. 13). The 

estimated values of the 4-, 2-, 1-, and 0.2-percent AEP flood 

quantiles are also included for each streamgage to provide 

perspective among recorded annual peaks. Additionally, 

hydrographs for a subset of these selected USGS streamgages 

in the Northeast are presented in figure 14A–D that depict 

daily mean streamflow over time (represented by a solid 

line) as well as the annual peak streamflows for select years 

(represented by an independent marker), thus allowing 

comparison of the 2011 flood events to previous major floods. 

The annual peak streamflows are associated with a peak stage 

at a specific time (or instant) when a river crested; therefore, 

the values will be equal to, or higher than the daily mean 

streamflows. Comparatively, the hydrographs presented in 

figure 14A–D show that streamgages monitoring streamflow 

on a larger river, such as the streamgages Susquehanna River 

at Waverly, N.Y. (01515000; fig. 14A) and Susquehanna River 

at Danville, Pa. (01540500; fig. 14B), which have drainage 

areas of approximately 4,770 and 11,200 mi2 respectively, 

can have a daily mean streamflow that is of relatively similar 

magnitude as the instantaneous peak; whereas, streamgages 

monitoring streamflow on relatively smaller rivers, such as the 

streamgages Saco River at Conway, N.H. (01064500; fig. 14C) 

or North River at Shattuckville, Mass. (01169000; fig. 14D), 

which have drainage areas of approximately 385 and 90 mi2, 

respectively, may have daily mean streamflows significantly 

less than the instantaneous peak. Several factors and basin 

characteristics may contribute to the ratio of peak streamflow 

to daily mean streamflow, such as rainfall duration and 

intensity, percentage of impervious surfaces within a basin, 

and channel slope; however, for larger drainage basins the 

time required for precipitation to drain through the basin is 

probably a large part of the reason why the ratio is lower.

At many USGS streamgages, the 2011 peak streamflows 

were the largest peaks in many decades. Major flooding 

because of a combination of rain with significant snowmelt 

during April 26–May 4, 2011, resulted in new peak-of-record 

streamflows being recorded at Hudson River at Hadley, 

N.Y. (01318500, fig. 1); at Raquette River at Raymondville, 

N.Y. (04268000, fig. 1) and at 10 additional streamgages in 

the Hudson and St. Lawrence River Basins in northeastern 

New York. In western New York, streamgages on Flint 

Creek and North Branch Salmon River, tributaries to Lake 

Ontario, also recorded new period-of-record streamflows 

during April 26– May 4, 2011. A new period-of-record peak 

streamflow of 34,900 ft3/s was recorded on April 28, 2011, 

at the streamgage Hudson River at North Creek, N.Y. 

(01315500; table 1), which has been in operation since 

1907 (fig. 13), exceeding the previous peak streamflow of 

28,900 ft3/s on December 31, 1948. Other select streamgages 

within the Hudson River Basin experiencing new peaks-ofrecord were the streamgages Indian River near Indian Lake, 

N.Y. (01315000; table 1); West Canada Creek at Kast Bridge, 

N.Y. (01346000; table 1); and Hudson River at Hadley, N.Y. 

(01318500; table 1), which have periods of record dating back 

to 1913, 1921, and 1922, respectively. In the St. Lawrence 

River Basin, the streamgage Raquette River at Piercefield, 

N.Y. (04266500; table 1) has peak streamflow records dating 

back to 1909, and a new period-of-record peak streamflow of 

10,400 ft3/s was set on May 1, 2011. New period-of-record 

peak streamflows also were recorded farther downstream at 

the Raquette River at South Colton, N.Y. (04267500; table 1) 

and Raquette River at Raymondville, N.Y. (04268000; table 1; 

Suro, 2011) streamgages.

In 1955, the mid-Atlantic and Northeast United States 

experienced massive amounts of rainfall from hurricanes 

Connie and Diane, which were within about a week of each 

other. The hurricanes caused severe flooding across the region 

resulting in an estimated 180 deaths and $680 million dollars 

in property damage (National Weather Service, 2014d). 

The USGS streamgages recorded record flood peaks across 

the Northeast as a result of precipitation associated with 

hurricanes Connie and Diane. Flood peaks from the 1955 
18 Flooding in the Northeastern United States, 2011

rol15-HWCR00-0022_fig 13a

EXPLANATION

0.2

1

2

4

Estimated annual exceedance probability 

 streamflow (graphs)—In percent

U.S. Geological Survey (USGS) streamgage (map)

78° 75° 72° 69°

45°

42°

39°

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

1900 1920 1940 1960 1980 2000 Annual peak streamflow, in cubic feet per second

Water year

Hudson River at North Creek, New York, USGS streamgage 01315500

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

North River at Shattuckville, Massachusetts, USGS streamgage 01169000

1900 1920 1940 1960 1980 2000 Annual peak streamflow, in cubic feet per second

Water year

1900 1920 1940 1960 1980 2000 Annual peak streamflow, in cubic feet per second

Water year

0

5,000

10,000

15,000

20,000

25,000

30,000 Mad River near Moretown, Vermont, USGS streamgage 04288000

1900 1920 1940 1960 1980 2000 Annual peak streamflow, in cubic feet per second

Water year

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000 Saco River near Conway, New Hampshire, USGS streamgage 01064500

CANADA

ATLANTIC OCEAN

State boundaries from National Atlas, 2014

Streams from EPA River Reach Files, Level 1, 2004

Universal Transverse Mercator projection, Zone 18

North American Datum of 1983

0 50 100 MILES

0 50 100 KILOMETERS

NEW YORK

VERMONT MAINE

NEW

HAMPSHIRE

MASSACHUSETTS

CONNECTICUT

PENNSYLVANIA

NEW JERSEY

MARYLAND

DELAWARE

RHODE

ISLAND

WEST

VIRGINIA

VIRGINIA

D.C.

LAKE ONTARIO

Figure 13. Annual peak streamflows through 2011 and the 4-, 2-, 1-, and 0.2-percent annual exceedance probability streamflows 

for selected U.S. Geological Survey streamgages in the Northeast.
2011 Flooding—Comparison with Historic Floods 19

rol15-HWCR00-0022_fig 13b

EXPLANATION

0.2

1

2

4

Estimated annual exceedance probability 

 streamflow (graphs)—In percent

U.S. Geological Survey (USGS) streamgage (map)

78° 75° 72° 69°

45°

42°

39°

CANADA

ATLANTIC OCEAN

State boundaries from National Atlas, 2014

Streams from EPA River Reach Files, Level 1, 2004

Universal Transverse Mercator projection, Zone 18

North American Datum of 1983

0 50 100 MILES

0 50 100 KILOMETERS

NEW YORK

VERMONT MAINE

NEW

HAMPSHIRE

MASSACHUSETTS

CONNECTICUT

PENNSYLVANIA

NEW JERSEY

MARYLAND

DELAWARE

RHODE

ISLAND

WEST

VIRGINIA

VIRGINIA

D.C.

LAKE ONTARIO

1900 1920 1940 1960 1980 2000 Annual peak streamflow, in cubic feet per second

Water year

Susquehanna River near Waverly, New York, USGS streamgage 01515000

0

20,000

40,000

60,000

80,000

100,000

120,000

140,000

160,000

180,000

200,000

Susquehanna River at Danville, Pennsylvania, USGS streamgage 01540500

0

50,000

100,000

150,000

200,000

250,000

300,000

350,000

400,000

1900 1920 1940 1960 1980 2000 Annual peak streamflow, in cubic feet per second

Water year

1900 1920 1940 1960 1980 2000 Annual peak streamflow, in cubic feet per second

Water year

0

4,000

8,000

12,000

16,000

20,000

24,000

Ramapo River near Mahwah, New Jersey, USGS streamgage 01387500

1900 1920 1940 1960 1980 2000 Annual peak streamflow, in cubic feet per second

Water year

0

2,000

4,000

6,000

8,000

10,000

12,000

Christina River at Coochs Bridge, Delaware, USGS streamgage 01478000

Figure 13. Annual peak streamflows through 2011 and the 4-, 2-, 1-, and 0.2-percent annual exceedance probability streamflows 

for selected U.S. Geological Survey streamgages in the Northeast.—Continued
20 Flooding in the Northeastern United States, 2011 Daily mean and annual peak streamflow, in cubic feet per second

March April May June July August September October

Month

2011 Peak

2011 Daily mean

2006 Peak

2006 Daily mean

1972 Peak

1972 Daily mean

EXPLANATION

0

20,000

40,000

60,000

80,000

100,000

120,000

140,000

160,000

180,000

A. Susquehanna River at Waverly, New York, U.S. Geological Survey streamgage 01515000

Daily mean and annual peak streamflow, in cubic feet per second

B. Susquehanna River at Danville, Pennsylvania, U.S. Geological Survey streamgage 01540500

0

50,000

100,000

150,000

200,000

250,000

300,000

350,000

400,000

March April May June July August September October

Month

2011 Peak

2011 Daily mean

2006 Peak

2006 Daily mean

1972 Peak

1972 Daily mean

EXPLANATION

Figure 14. Streamflow for selected U.S. Geological Survey streamgages in the northeast for the 2011 

flood period and selected previous major floods.
2011 Flooding—Comparison with Historic Floods 21

Daily mean and annual peak streamflow, in cubic feet per second

2011 Peak

2011 Daily mean

1987 Peak

1987 Daily mean

1953 Peak

1953 Daily mean

EXPLANATION

0

10,000

20,000

30,000

40,000

50,000

60,000

C. Saco River near Conway, New Hampshire, U.S. Geological Survey streamgage 01064500

March April May June July August September October

Month

Daily mean and annual peak streamflow, in cubic feet per second

2011 Peak

2011 Daily mean

2005 Peak

2005 Daily mean

1987 Peak

1987 Daily mean

EXPLANATION

March April May June July August September October

Month

D. North River at Shattuckville, Massachussetts, U.S. Geological Survey streamgage 01169000

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

Figure 14. Streamflow for selected U.S. Geological Survey streamgages in the northeast for the 2011 

flood period and selected previous major floods.—Continued
22 Flooding in the Northeastern United States, 2011

storms are still the highest recorded peaks for streamgages 

such as West Branch Farmington River near New Boston, 

Mass. (01185500); Rutgers Creek at Gardnerville, N.Y. 

(01368500); and Delaware River at Trenton, N.J. (01463500). 

For other streamgages that were in operation during 1955 and 

2011, such as Mill River at Northampton, Mass. (01171500); 

Roundout Creek at Rosendale, N.Y. (01367500); and Flat 

Brook near Flatbrookville, N.J. (01440000); flooding from 

Tropical Storm Irene produced comparatively higher peaks 

(table 1). In the Passaic and Hackensack River Basins in 

New Jersey (fig. 1), the streamgages Whippany River at 

Morristown, N.J. (01381500); Wanaque River at Awosting, 

N.J. (01383500); Ramapo River near Mahwah, N.J. 

(01387500; fig. 13); and Ramapo River at Pompton Lakes, 

N.J. (01388000) recorded the highest flood peaks in more 

than 90 years of record (table 1; Watson and others, 2014). A 

photograph of rescuers using a boat to navigate the flooded 

streets of Pompton Lakes, N.J., after the Ramapo River 

overflowed its banks because of the heavy rain associated 

with Tropical Storm Irene on August 29, 2011, is shown in 

figure 15.

Heavy rainfall from Hurricane Floyd in September 1999 

resulted in record flooding in parts of Maryland, Delaware, 

New Jersey, Pennsylvania and New York. The flood peaks 

from September 1999 at several streamgages in the northeast 

such as Wissahickon Creek at Mouth, Philadelphia, Pa. 

(01474000), White Clay Creek near Newark, Del. (01479000), 

and the Raritan River at Manville, N.J. (01400500), which 

has 100 years of streamflow record, were not surpassed by the 

flood peaks of 2011. However, for other streamgages such as 

Sawmill Creek at Glen Burnie, Md. (01589500); Hackensack 

River at West Nyack, N.Y. (01376800); and Christina River at 

Coochs Bridge, Del. (01478000; fig. 13), all of which were in 

operation in 1999, flooding from Tropical Storm Irene in 2011 

did surpass the 1999 flood peaks, establishing new period-ofrecord flood peaks (table 1).

In the New England region, flooding associated with 

Tropical Storm Irene in late August 2011 resulted in new 

period-of-record peak streamflows being recorded at 

37 streamgages across Vermont, western Massachusetts, 

northern New Hampshire, and parts of eastern New York 

(Olson and Bent, 2013). The 2011 peak-of-record streamflow 

at the streamgage North River at Shattuckville, Mass. 

(01169000) was 30,300 ft3/s, which exceeded the previously 

known maximum streamflow of 18,800 ft3/s, which was 

recorded in 2005 (figs. 13, 14D, table 1). Peak-of-record 

streamflows were also recorded at the streamgages Saco River 

near Conway, N.H. (01064500; figs. 13, 14C, table 1) and 

Mad River near Moretown, Vt. (04288000; fig. 13, table 1), 

which have periods of record dating back to 1904 and 1928, 

respectively.

In September 2011, the Susquehanna River main 

stem experienced record-setting (or near record-setting) 

streamflows at USGS streamgages in New York and 

Pennsylvania where water levels topped levees along 

the river, which inundated several cities in New York 

including Waverly, Owego, Vestal, Endicott, Johnson City, 

and downtown Binghamton (National Weather Service, 

2014a). The streamgage Susquehanna River at Waverly, 

N.Y. (01515000; figs. 13, 14A, table 1), situated near the 

New York-Pennsylvania border, has been in operation since 

1936, and the September 2011 flood peak is the largest 

streamflow recorded at this site and exceeds the previous 

peak-of-record stage set in 2006 by more than 4 ft. The peak 

streamflows associated with the September 2011 event at 

the streamgages along the Susquehanna River at Towanda 

(01531500; table 1); at Wilkes-Barre (01536500; table 1); 

and at Danville, Pa. (01540500; figs. 13, 14B, table 1); were 

second only to the peak-of-record streamflows associated 

with Hurricane Agnes in 1972. In Harrisburg, Pennsylvania, 

where the forecast of flooding led to the evacuation of about 

100,000 people, including 10,000 people and the Governor’s 

residence in the downtown area (National Weather Service, 

2014a), the September 2011 recorded peak streamflow 

at the streamgage Susquehanna River at Harrisburg, Pa. 

(01570500; table 1) was 590,000 ft3/s, well below the 1972 

peak-of-record streamflow of 1,020,000 ft3/s. Conversely, on 

Swatara Creek (a tributary joining the Susquehanna River 

approximately 10 miles downstream from the Susquehanna 

River at Harrisburg streamgage), the September 2011 peak 

streamflow recorded at the streamgage Swatara Creek at 

Harper Tavern, Pa. (01573000; table 1) exceeded the 1972 

peak by approximately 12 percent. The difference in annual 

peak streamflow relations between the Susquehanna River 

at Harrisburg streamgage and the Swatara Creek at Harper 

Tavern streamgage for the 1972 and 2011 storm events may 

be attributed to relative size of basins at the gaged locations 

(24,100 and 337 mi2, respectively) and the geographic and 

temporal distribution of the rainfall associated with each 

storm event.

Figure 15. Rescuers use a boat to navigate the flooded streets 

of Pompton Lakes, N.J., as the Ramapo River overflows its banks 

because of the heavy rain on August 29, 2011. Photograph by 

Robert Atkinson, U.S. Geological Survey. 
Annual Exceedance Probability Analysis for 2011 23

The 1972 flooding was a result of heavy and persistent 

rainfall associated with the remnants of Hurricane Agnes and 

resulted in major flooding on many of the tributaries and much 

of the main stem of the Susquehanna River in Pennsylvania 

and the southern tier of New York. Nationwide, 122 deaths 

were attributed to Agnes, of which 50 were in the State of 

Pennsylvania, and total damages from the storm reached more 

than $3 billion dollars nationwide, with more than $2 billion 

dollars in losses in the Susquehanna River Basin (National 

Weather Service, 2014c).

Annual Exceedance Probability 

Analysis for 2011

This report provides the results of exceedance probability analyses of annual peak streamflow for 327 selected 

streamgages in the Northeast that experienced moderate to 

major flooding during 2011. The probability analyses were 

computed using annual peak streamflow data through 2011. 

Current methods outlined in Bulletin 17B (Interagency 

Advisory Committee on Water Data, 1982) were used for the 

analysis, but the results of newer experimental methods of 

flood frequency analysis are presented in table 2 (available at 

http://dx.doi.org/10.3133/pp1821) for comparison. An explanation of these newer experimental methods is provided in the 

section titled Annual Exceedance Probability using Expected 

Moments Algorithm.

The evaluation of future flood risk is an integral part 

of why the probability of annual maximum streamflow 

is analyzed. Knowing the flood ranking at a streamgage 

or the maximum elevation at selected locations helps in 

understanding local flooding conditions, but an analysis of the 

exceedance probability of an annual peak streamflow allows 

for a better evaluation of the flood risk and provides data 

for better infrastructure design and flood damage mitigation 

planning. Annual peak streamflows are used to estimate a 

probability distribution. The probability distribution relates 

the probability to the magnitude of a specific peak streamflow 

being equaled or exceeded in any given year.

The selection of the probability distribution and the 

process of fitting the parameters of the distribution can vary 

depending on the underlying characteristics of the data. The 

Interagency Advisory Committee on Water Data produced 

standard guidelines adopted by many Federal agencies for 

consistent computations of these probabilities known as Bulletin 17B (Interagency Advisory Committee on Water Data, 

1982). These guidelines recommend the use of the LPIII 

distribution and the “method of moments” for estimating the 

distribution parameters (mean, standard deviation, and skewness of the data). The analysis is based on annual peak streamflow data at USGS streamgages, which are available online 

from the USGS National Water Information System (NWIS) 

at http://waterdata.usgs.gov/nwis (U.S. Geological Survey, 

2013). In many previously published USGS reports flood 

probabilities have been expressed as flood frequencies using 

a T-year recurrence interval for a particular flood quantile (for 

example, the “100-year flood”). Use of the T-year recurrence 

interval to describe flood probability is now discouraged by 

many agencies (including USGS) because it tends to confuse 

the general public. A T-year recurrence interval is sometimes 

interpreted to imply that there is a set time interval between 

floods of a specific magnitude when, in fact, floods are random 

processes that are best understood using probabilistic terms. 

The use of an AEP percentage to describe the estimated probability of selected flood magnitudes is recommended as a way 

to better communicate the annual chance of occurrence. The 

reader can convert from the AEP to the previously used T-year 

recurrence interval by simply taking the reciprocal of the AEP. 

For example, the 1-percent AEP flood corresponds to a streamflow magnitude that has a 1-percent chance of being equaled 

or exceeded in any given year. A 1-percent chance relates to a 

probability of 0.01 when expressed as a decimal. The reciprocal of 0.01 is 100; thus, the T-year recurrence interval for the 

1-percent AEP flood is referred to as the 100-year flood.

Aerial views of damage in West Bridgewater, Vermont, as a result of floodwaters during Tropical Storm Irene in August 2011. Photograph courtesy of Lars Gange, 

Mansfield Heliflight.
24 Flooding in the Northeastern United States, 2011

AEP flood quantiles can be computed in many different 

ways, including the above referenced Bulletin 17B methods 

that were used for this report. The reliability of the flood 

quantiles computed using Bulletin 17B methods may be 

expressed as a “variance of prediction” that is computed 

by using the asymptotic formula given by Cohn and others 

(2001), with the addition of the mean-squared error of 

generalized skew (Griffis and others, 2004). The variance 

of prediction varies as a function of record length, the fitted 

flood-probability distribution parameters (mean, standard 

deviation, and weighted skew), and the accuracy of the 

method used to determine the regional skew component of the 

weighted skew.

An additional method is the use of regional regression 

techniques to develop regional regression equations (RRE) to 

estimate the AEP flood quantiles. These techniques are used 

to develop a relation between flood-probability data at many 

streamgages throughout a region and basin characteristics of 

those streams (Jennings and others, 1994). Once developed, 

the RRE can be used to estimate various streamflow 

characteristics, such as the 1-percent AEP flood quantile, 

for almost any location along a stream (gaged or ungagged) 

within the reliable boundaries of the RREs. The variance of 

prediction from the regional regression is a function of the 

RRE and values of independent variables used to develop the 

streamflow estimate from the RRE. The variance generally 

increases with departure of actual values from mean values of 

the independent variables.

The USGS National Streamflow Statistics (NSS) Program has software (http://water.usgs.gov/ software/nss.html) 

that can be used to apply the RRE by allowing the user to 

enter basin and land-use characteristics (drainage area, basin 

slope, area of wetlands, surficial geology, and so on) that are 

used in the equations as independent (explanatory) variables 

into RRE and compute various streamflow characteristics. The 

USGS StreamStats Program has developed a more comprehensive online tool called “StreamStats”, which allows the 

user to work in an interactive map environment to select a 

site of interest and have basin and streamflow characteristics 

computed and displayed in a report format (http://water.usgs.

gov/osw/streamstats).

Holmes and others (2010) provided detailed discussions 

about how to express the accuracy of RREs in terms of equivalent years of record and suggested that weighting by variance 

provides a more natural characterization of the underlying 

uncertainty of the various peak streamflow estimates. At a 

gaged location, the optimal estimate of the AEP flood quantile 

is determined by weighting the AEP flood-quantile estimate 

determined from the Bulletin 17B methods with the AEP flood 

quantile estimate determined from the RRE. The weights are 

inversely proportional to the variances of prediction (equation 1), yielding the weighted estimator:

(1)

where

QP,OPT is the optimal estimate of AEP flood quantile 

for a particular probability of flooding (p; 

Interagency Advisory Committee on Water 

Data, 1982, appendix 8),

Var[RRE] is the variance of the RRE estimate of 

the AEP flood quantile for a particular 

probability of flooding (p),

QP,LPIII is the Bulletin 17B method estimate of 

the AEP flood quantile for a particular 

probability of flooding (p), 

Var[LPIII] is the variance of the Bulletin 17B estimate 

of the AEP flood quantile for a particular 

probability of flooding (p), and 

QP,RRE is the RRE estimate of the AEP flood quantile 

for a particular probability of flooding (p).

Previous USGS reports have expressed the accuracy 

of RREs in terms of equivalent years of record and used 

these estimates with the length of record at the streamgage 

to combine RRE and LPIII AEP flood quantile estimates (for 

example, Hodge and Tasker, 1995; Ries and Dillow, 2006). 

Although this method was presented in the Bulletin 17B 

guideline, the length of record can fail to account for the true 

variance of LPIII flood-probability estimates. For all records 

of the same length, the accuracy is assumed to be of the same 

reliability, which may not be accurate. The flood-probability 

distributions computed from two different streamgage records 

of the same length may not be of equal reliability because 

of differences in underlying variances of the streamflow 

records for each site. For example, one streamgage may 

have less stable control conditions and more highly varied 

records making it more difficult to accurately measure the 

streamflow than another. The LPIII distributions at the first 

streamgage, therefore, could have larger variances than the 

second streamgage in the example. More importantly, the 

equivalent years-of-record concept, although relatively easy to 

understand, misrepresents the relation between the AEP flood 

quantile estimates and the variances. Using estimated variances 

provides a more natural characterization of the underlying 

uncertainty of the various streamflow estimates. The weighted 

estimates of the AEP flood quantiles corresponding to the 

Log

Log Log Q

Var RRE Q Var LPIII Q

Var R P OPT

P LPIII P RRE

,

, , [ ] [ ]

[ = ( ) × + ×

( ) RE] [ +Var LPIII]

Debris catches on snowmobile bridge near Waterbury, Vermont, as a result 

of floodwaters from the Winooski River during Tropical Storm Irene, in August 

2011. Photograph courtesy of Lars Gange, Mansfield Heliflight.
Annual Exceedance Probability using Expected Moments Algorithm 25

4-, 2-, 1-, and 0.2-percent AEP, along with their respective 

95-percent confidence limits, for selected streamgages in the 

Northeast, are listed in table 1. Streamgages that generally 

recorded an annual peak streamflow that exceeded the 

4-percent AEP during 2011 were included in table 1, with a few 

exceptions to help define the extent of major flooding.

Another statistic included in table 1 that was typically not 

included in previous USGS reports is the “typical range” for 

the 66.7-percent confidence level for the computed AEPs for 

the 2011 flood peaks. In table 1, the values under the column 

heading of “AEP for observed 2011 flood” are the estimated 

AEPs for each 2011 flood peak and the corresponding typical 

range of the AEP. The AEPs of the 2011 floods were calculated 

using the methods outline in USGS Office of Surface Water 

Technical Memorandum 2013.01 (U.S. Geological Survey, 

2012b). As previously stated, the AEP is estimated from 

a LPIII statistical model fit to the at-site peak streamflow 

data. The typical range is presented to report the lower and 

upper bounds on 66.7-percent confidence level for the true 

AEP. These bounds, unlike the AEP estimate, are based on 

order statistics and do not assume a model. The distinction 

is important because the model-based AEP estimates may 

occasionally lie outside the nonparametric confidence bounds, 

which intuitively may not make sense to the reader. The 

confidence bounds, or typical range of the true AEP, are 

intended to communicate the magnitude of the uncertainty in 

estimates of the AEP by providing the likely range of the AEP 

corresponding to the k-th largest event in a sample of size N. 

For example, if the exceedance probability corresponding to 

a particular flood at 2 percent is estimated, the understanding 

is that the uncertainty in this statistic is large, and, in general, 

a 66.7-percent confidence level for the true exceedance 

probability of the 2-percent flood extends from 0.4 to 

4 percent.

The Streamflow of 2011—Water Year Summary shows 

that statewide streamflow in the northeast was above average 

to wet using a statistical period from 1930 to 2011(Jian 

and others, 2012). During 2011, the USGS recorded peak 

streamflows that were generally equal to or less than a 

4-percent AEP at 327 streamgages in the northeast. These 

327 streamgages that generally recorded moderate to major 

flooding during 2011 in the northeast are listed in table 1 and 

shown in figure 16. More than 180 of the 327 streamgages 

listed recorded a new period-of-record peak streamflow in 

2011. Peak streamflows greater than or equal to the 1-percent 

AEP discharge were recorded at 129 streamgages across 

Connecticut, Delaware, Maine, Maryland, Massachusetts, 

Ohio, Pennsylvania, New Hampshire, New Jersey, New 

York, and Vermont. Additionally, nearly 100 of the 129 peaks 

with an AEP of 1 percent or less were a result of the flooding 

associated with Hurricane Irene in late August 2011, and at 24 

of these streamgages the recorded peak was extreme enough 

to exceed the 0.2-percent AEP during the August storm. A 

total of 31 of the streamgages listed in table 1 recorded peak 

streamflows that equaled or exceeded the 0.2-percent AEP 

during 2011.

Annual Exceedance Probability using 

Expected Moments Algorithm

The Hydrology Subcommittee of the Interagency 

Advisory Committee on Water Data (1982) developed the 

Bulletin 17B guidelines for determining flood frequency 

studies in the United States. These guidelines include methods 

for improving skew estimates and testing for outliers, but 

the authors of Bulletin 17B also identified the need for 

methodological improvements and recommended further 

study. The Hydrologic Frequency Analysis Work Group is 

considering modest changes to Bulletin 17B guidelines to 

address those recommendations for improved methods. The 

work group is proposing the adoption of the expected moments 

algorithm (EMA; Cohn and others, 1997) with Multiple 

Grubbs-Beck low outlier screening (Cohn and others, 2013).

AEPs using the EMA method for 122 of the 

327 streamgages included in table 1 are listed in table 2 

(available at http://dx.doi.org/10.3133/pp1821). The peak 

streamflows that are reported in table 1 are also included in 

table 2 as a reference. The 122 streamgages included in table 2 

that have AEP estimates using the EMA method are also 

shown in figure 17 as an additional geographic reference.

Aerial view of damage along U.S. Route 4 near Killington, Vermont, as a 

result of floodwaters during Tropical Storm Irene in August 2011. Photograph 

courtesy of Lars Gange, Mansfield Heliflight.
26 Flooding in the Northeastern United States, 2011 70° 75° 80° 85° 44° 42°

40°38°

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VERMONT

NEW 

HAMPSHIRE

MAINE

MASSACHUSETTS

CONNECTICUT

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PENNSYLVANIA

NEW JERSEY

DELAWARE

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ISLAND

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Lake Erie

Lake Ontario

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Michigan Lake 

Base from U.S. Geological Survey digital data, 1983, 1:6,000,000 Universal Transverse Mercator projectionNorth American Datum of 1983 

0 50 100 MILES0 50 100 KILOMETERS

Lake 

Champlain

EXPLANATION

Drainage area studiedU.S. Geological Survey streamgages with 

annual exceedence probability(AEP)—Labeled with map number

4 percent to less than 2 percent2 percent to less than 1 percent1 percent to less than 0.5 percent0.5 percent to less than 0.2 percent

#*#*#*

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#*

200

Figure 16.

Annual exceedance probabilities computed for peak streamflows during 2011 at selected USGS streamgages in the Northeastern United States and major 

drainage basins studied during 2011 flooding. (Data are listed in table 1).
Annual Exceedance Probability using Expected Moments Algorithm 27

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Base from U.S. Geological Survey digital data, 1983, 1:6,000,000 

Universal Transverse Mercator projection

North American Datum of 1983 

75° 70°

44°

42°

40°

!

1

EXPLANATION

U.S. Geological Survey streamgages 

and map number

ATLANTIC OCEAN

ATLANTIC OCEAN

Figure 17. Selected U.S. Geological Survey streamgages in the Northeastern United States with annual exceedance 

probabilities computed using the expected moments algorithm method. (Data are listed in table 2).
28 Flooding in the Northeastern United States, 2011

Summary

The combination of several major flooding events in 2011 

caused damages that were estimated to exceed $7 billion in the 

Northeastern United States. The flooding in the Northeastern 

United States also resulted in approximately 45 deaths and 

more than 180 U.S. Geological Survey (USGS) streamgages 

recorded new period-of-record maximum. The flooding events 

began in late February and early March with snowmeltenhanced peak streamflows in northern Ohio, western 

Connecticut, and central Massachusetts. In late April, parts 

of northern New Hampshire, New York, and Vermont were 

flooded as a result of rainfall on heavy snowpack. Relatively 

wet conditions persisted throughout the year as a result of 

numerous small scale rain events (summer thunderstorms) 

from late May through mid-August, until record-setting 

precipitation associated with Hurricane Irene and the remnants 

of Tropical Storm Lee caused major flooding disasters 

throughout the Northeastern United States.

In the winter and spring of 2011, record rainfall and 

snowmelt throughout many areas of the Northeastern United 

States contributed to streamflows that were above normal at 

numerous USGS streamgages in New England, New York, 

New Jersey, Delaware, and Maryland. During the spring 

of 2011, the streamgage Lake Champlain at Burlington,Vt. 

(04294500) recorded record flooding as a result of the 

combined runoff from record rainfall and snowmelt. Lake 

Champlain was above the National Weather Service flood 

stage of 100 feet (ft) for 68 days from April 13 to June 

19, 2011, and peaked at 103.27 ft on May 6, 2011, surpassing 

the maximum known elevation since at least 1827 of 102.1 

ft on May 4, 1869. The intense rainfall associated with 

Hurricane Irene contributed to new August total precipitation 

records for New Hampshire, New Jersey, New York, 

and Vermont and resulted in new period-of-record peak 

streamflows being recorded at 119 USGS streamgages. In the 

Green Mountains of Vermont, 13 communities were isolated 

for days as floodwaters damaged or destroyed bridges, 

culverts, and 500 miles of State highways. New York reported 

similar damages to roads and bridges, and every county in 

New Jersey was declared a Federal disaster area. Several 

streamgages in New York and New Jersey, with more than 

90 years of data, recorded new peak-of-record maximums 

during the 2011 flood. In early September 2011, rainfall 

associated with the remnants of Tropical Storm Lee caused 

the main stem of the Susquehanna River and many tributaries 

to rise to record setting levels in southern New York and 

Pennsylvania. The peak streamflows associated with the early 

September 2011 event at the streamgages Susquehanna River 

at Towanda, Pa., Susquehanna River at Wilkes-Barre, Pa., and 

Susquehanna River at Danville, Pa., were second only to the 

peak-of-record streamflows associated with Hurricane Agnes 

in 1972.

Annual Exceedance Probabilities (AEPs) were estimated for 327 sites in the Northeastern United States using 

annual peak streamflows through 2011. The peak streamflows 

analyzed were estimated to have an AEP of 1 percent or less 

at 129 streamgages in Connecticut, Delaware, Maine, Maryland, Massachusetts, Ohio, Pennsylvania, New Hampshire, 

New Jersey, New York, and Vermont. Flooding associated 

with Hurricane Irene contributed to 100 of the 129 streamflow peaks with an AEP of 1 percent or less and 24 of the 

peak streamflows with an AEP of 0.2 percent or less. In 2011, 

31 streamgages recorded peak streamflows that equaled or 

exceeded the more extreme 0.2-percent AEP.

Acknowledgments

The authors would like to thank Robert Mason and 

Robert Holmes for their support and contributions to this 

report and all of the U.S. Geological Survey hydrologists and 

hydrologic technicians that worked extremely hard during and 

after these floods to collect and analyze the data used in this 

report. The authors would also like to acknowledge and thank 

Elizabeth Ahern, Gardner Bent, Jonathan Dillow, Gary Firda, 

Gregory Koltun, Rick Lumia, Scott Olson, Greg Stewart, and 

Kara Watson for providing flood and flood frequency data 

used in this report and Greg Sturgis and Kara Watson for 

their support in developing the tables and figures used in this 

report. 

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Nation: U.S. Geological Nation Water Information System 

Web Interface, accessed on May 14, 2013, at http://water.

usgs.gov/nwis/.

Vermont Emergency Management, 2014, Vermont Division of Emergency Management and Homeland Security, 

Georgetown Climate Center, “Lessons Learned from Irene”: 

accessed on April 20, 2014, at http://vem.vermont.gov/sites/

vem/files/Lessons%20Learned%20From%20Irene%20-%20

Final.pdf.

Watson, K.M., Collenburg, J.V., and Reiser, R.G., 2014, Hurricane Irene and associated floods of August 27–30, 2011, 

in New Jersey: U.S. Geological Survey Scientific Investigations Report 2013–5234, 149 p., http://dx.doi.org/10.3133/

sir20135234.
Glossary 31

Glossary

Note: Glossary definitions are taken from Langbein and Iseri (1960) whenever possible.

A

annual exceedance probability (AEP) The 

probability, or chance, of a flood of a given 

streamflow magnitude being equaled or 

exceeded in any given year. The probability 

can be expressed as a fraction, decimal, or 

percentage.

annual exceedance probability flood quantile 

(AEP flood quantile) The value of the peak 

streamflow that corresponds to a particular 

annual exceedance probability (for example, 

1-percent AEP flood quantile).

B

Bulletin 17B Report by the Interagency 

Advisory Committee on Water Data, published in 1982, that identifies guidelines 

delineates the recommended method for 

flood-probability analysis.

C

confidence limits To gage the accuracy 

of an approximation based on a probability 

distribution, upper and lower confidence 

limits can be estimated based on the properties of the probability distribution. This 

report includes the 95-percent confidence 

limits of the estimate of the flood quantiles as 

computed by the methods outlined in Bulletin 

17B.

confidence interval is based on order statistics and is intended to communicate the general magnitude of the uncertainty in estimates 

of the AEP by providing the likely range of 

the AEP corresponding to the k-th largest 

event in a sample of size N. AEP estimates 

may occasionally lie outside the nonparametric confidence intervals.

D

discharge In its simplest concept discharge 

means outflow; therefore, the use of this term 

is not restricted as to course or location, and 

it can be applied to describe the flow of water 

from a pipe or from a drainage basin.

E

expected moments algorithm is an alternative to maximum likelihood estimation (MLE) 

or the Bulletin 17 (Interagency Advisory 

Committee on Water Data, 1982) methods for 

incorporating historical information in flood 

frequency analysis or studies.

F

flood An overflow or inundation that comes 

from a river or other body of water, and 

causes or threatens damage.

flood peak The highest value of the stage 

or streamflow attained by a flood; generally 

designated as peak stage or peak streamflow, 

respectively. The annual flood peak is the 

largest flood peak for a given year or water 

year.

flood quantile See annual exceedance probability flood quantile.

flood stage The stage at which overflow of 

the natural banks of a stream begins to cause 

damage in the reach in which the water surface elevation is measured.

H

hydrograph A graph showing stage, streamflow, velocity, or other property of water with 

respect to time.

L

log-Pearson type III probability distribution 

(LPIII) One of the family of probability distributions developed by Karl Pearson that is 

commonly used in the United States as a bestfit for the distribution of annual peak flood 

streamflows and is suggested in Bulletin 17B 

as “the base method” for analysis procedures 

developed by the Interagency Advisory Committee on Water Data (1982).

O

orographic lifting The change in air flow 

when the topography of the land forces the air 

up the side of a mountain.
Debris catches on snowmobile bridge near 

Waterbury, Vermont, as a result of floodwaters 

from the Winooski River during Tropical Storm 

Irene, in August 2011. Photograph courtesy of 

Lars Gange, Mansfield Heliflight.

32 Flooding in the Northeastern United States, 2011

P

peak-of-record streamflow The largest 

instantaneous streamflow value for the period 

that data have been collected. 

peak stage See flood peak.

peak streamflow See flood peak.

precipitation As used in hydrology, precipitation is the discharge of water, in liquid or 

solid state, out of the atmosphere, generally 

upon a land or water surface. It is the common 

process by which atmospheric water becomes 

surface or subsurface water. The term “precipitation” is also commonly used to designate 

the quantity of water that is precipitated.

probability A means to express the likelihood of something occurring, also known as 

chance. The probability can be expressed as a 

fraction, decimal, or percentage.

probability distribution Describes the range of 

possible values that a random variable can attain 

and the probability that the value of the random 

variable is within any subset of that range.

R

rating curve A graph showing the relation between the stage (gage height), usually 

plotted as the ordinate, and amount of water 

flowing in the channel (streamflow) expressed 

as volume per unit time, plotted as abscissa.

recurrence interval The average interval of 

time within which the given flood is expected 

to be equaled or exceeded once.

regional regression equation Equation 

developed through use of regression techniques that relate the flood-probability data 

at many streamgages in a region to the basin 

characteristics of the streams monitored by 

the streamgages. For any location along a 

stream, a user can enter the basin characteristics (for example, drainage area, basin slope) 

as independent variables into the equations 

and compute various flow characteristics (for 

example, 1-percent AEP flood quantile, 

2-percent AEP flood quantile, and annual 

mean streamflow).

S

stage Height of a water surface above 

an established datum, also known as gage 

height.

streamflow The discharge that occurs in a 

natural channel. Although the term discharge 

can be applied to flow in a canal, the word 

streamflow uniquely describes the discharge 

in a surface stream course. The units of 

measurement generally are reported in cubic 

feet per second (ft3/s).

streamgage A particular site on a stream 

where a record of streamflow is obtained.
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Or visit the Office of Surface Water Web site at:

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Suro and others—Flooding in the Northeastern United States, 2011—Professional Paper 1821

ISSN 2330-7102 (online)

http://dx.doi.org/10.3133/pp1821