Prepared in cooperation with the 

U.S. Department of Homeland Security Federal Emergency Management Agency

Characteristics of the April 2007 Flood 

at 10 Streamflow-Gaging Stations 

in Massachusetts

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2009–5068

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2009–5068

Prepared in cooperation with the 

U.S. Department of Homeland Security Federal Emergency Management Agency

Characteristics of the April 2007 Flood 

at 10 Streamflow-Gaging Stations 

in Massachusetts
Front cover. Background image showing “Powerful Spring Storm.” (From National Oceanic and Atmospheric Administration (NOAA) 

Geostationary Operational Environmental Satellite (GOES)-12 visible spectrum, April 16, 2007, at 13:15 p.m. Eastern Daylight Savings 

Time (EDT)). 

Photograph of Shawsheen River upstream from Haverhill Street bridge in Andover, Massachusetts, on April 18, 2007, at 10:31 a.m. (EDT); 

stage at the time of the photograph was 1.33 feet below the storm-peak stage. Ironically, the yellow sign attached to the bridge warns of 

a downstream dam. (Photograph by Joseph Zanca, U.S. Geological Survey)

Back cover. Photograph shows same site under normal flow conditions.
Characteristics of the April 2007 Flood 

at 10 Streamflow-Gaging Stations in 

Massachusetts

By Phillip J. Zarriello and Carl S. Carlson

Prepared in cooperation with the 

U.S. Department of Homeland Security Federal Emergency Management Agency

Scientific Investigations Report 2009–5068

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

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

Revised: 2009

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Zarriello, P.J., and Carlson, C.S., 2009, Characteristics of the April 2007 flood at 10 streamflow-gaging stations in 

Massachusetts: U.S. Geological Survey Scientific Investigations Report 2009–5068, 75 p., also available online at 

http://pubs.usgs.gov/sir/2009/5068.
iii

Contents

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

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

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

Study Area..............................................................................................................................................2

Previous Studies ...................................................................................................................................2

Antecedent and Storm Conditions..............................................................................................................5

Streamflow.............................................................................................................................................5

Precipitation...........................................................................................................................................5

Methods...........................................................................................................................................................5

Annual Peak Flows ...............................................................................................................................8

Magnitude and Frequency of Flood Flows .......................................................................................9

Flood-Insurance Studies and Flood-Warning Levels....................................................................10

Characteristics of the April 2007 Flood at Selected Streamflow-Gaging Stations ...........................11

North Nashua River at Fitchburg–01094400 ...................................................................................11

Stillwater River near Sterling–01095220 .........................................................................................16

Squannacook River near West Groton–01096000 .........................................................................20

Nissitissit River at Pepperell–01096503...........................................................................................25

Merrimack River at Lowell–01100000..............................................................................................30

Spicket River near Methuen–01100561...........................................................................................35

Shawsheen River at Andover–01100627.........................................................................................40

Mill River at Northampton–01171500...............................................................................................46

Sevenmile River near Spencer–01175670.......................................................................................50

West Branch Westfield River near Huntington–01181000 ...........................................................56

Magnitude of Peak-Flow in Relation to Drainage Area................................................................60

Summary........................................................................................................................................................62

References Cited..........................................................................................................................................65

Figures

1. Map showing location of selected streamflow-gaging stations in Massachusetts 

used to characterize the April 2007 flood .................................................................................3

2. Graphs showing daily mean period-of-record flow percentiles and 2007 flows at 

(A) Squannacook River near West Groton (01096000) and (B) West Branch Westfield 

River near Huntington (01181000), Massachusetts.................................................................6

3. Map showing total precipitation from April 15 through 18, 2007, in and around 

Massachusetts..............................................................................................................................7

4–7. Graphs related to North Nashua River at Fitchburg (01094400), Massachusetts 

showing:

4. Relation between observed annual peak discharges for water years 1973 

through 2007 to North Nashua River at Leominster (01094500) .................................12

5. Observed and estimated annual peak discharge for water years 1936 

through 2007 .......................................................................................................................12
iv

6. Peak-flow frequency computed from observed and estimated annual peaks 

for water years 1936 through 2007..................................................................................14

7. April 15–23, 2007 (A) stage and (B) discharge ..............................................................15

8–11. Graphs related to Stillwater River near Sterling (01095220), Massachusetts 

showing:

8. Relation between observed annual peak discharges for water years 1995 

through 2007 to North Nashua River at Leominster (01094500) .................................17

9. Observed and estimated annual peak discharge for water years 1936 

through 2007 .......................................................................................................................17

10. Peak-flow frequency computed from observed and estimated annual peaks 

for water years 1936 through 2007..................................................................................18

11. April 15–23, 2007 (A) stage and (B) discharge ..............................................................19

12–15. Graphs related to Squannacook River near West Groton (01096000), Massachusetts 

showing:

12. Relation between observed annual peak discharges for water years 1973 

through 2007 to North Nashua River at Leominster (01094500) .................................21

13. Observed and estimated annual peak discharge for water years 1936 

through 2007 .......................................................................................................................21

14. Peak-flow frequency computed from observed and estimated annual peaks 

for water years 1936 through 2007..................................................................................22

15. April 15–23, 2007 (A) stage and (B) discharge ..............................................................24

16–19. Graphs related to Nissitissit River at Pepperell (01096503), Massachusetts 

showing:

16. Relation between observed high flows for water years 2006 through 2007 to 

Squannacook River near West Groton (01096000).......................................................26

17. Observed and estimated annual peak discharge for water years 1936 

through 2007 .......................................................................................................................26

18. Peak-flow frequency computed from observed and estimated annual peaks 

for water years 1936 through 2007..................................................................................27

19. April 15–23, 2007 (A) stage and (B) discharge ..............................................................28

20–22. Graphs related to Merrimack River at Lowell (01100000), Massachusetts 

showing:

20. Annual peak discharge for water years 1924 through 2007 .......................................30

21. Peak-flow frequency computed from observed and estimated annual peaks 

for water years 1924 through 2007..................................................................................33

22. April 15–23, 2007 (A) stage and (B) discharge ..............................................................34

23–26. Graphs related to Spicket River near Methuen (01100561), Massachusetts 

showing:

23. Relation between observed annual peak discharges for water years 2001 

through 2007 to North Nashua River at Leominster (01094500) and to 

Ipswich River at South Middleton (01101500)...............................................................36

24. Observed and estimated annual peak discharge for water years 1936 

through 2007 .......................................................................................................................36

25. Peak-flow frequency computed from observed and estimated annual peaks 

for water years 1936 through 2007..................................................................................37

26. April 15–23, 2007 (A) stage and (B) discharge ..............................................................39
v

27–30. Graphs related to Shawsheen River at Andover (01100627), Massachusetts 

showing:

27. Relation between observed annual peak discharges for water years 1973 

through 2007 at Shawsheen River near Wilmington (01100600) to North Nashua 

River at Leominster (01094500) and to Ipswich River near Ipswich (01102000) ......41

28. Observed and estimated peak-annual discharge for water years 1936 

through 2007 .......................................................................................................................41

29. Peak-flow frequency computed from observed and estimated annual peaks 

discharges for water years 1936 through 2007.............................................................42

30. April 15–23, 2007 (A) stage and (B) discharge ..............................................................44

31–34. Graphs related to Mill River at Northampton (01171500), Massachusetts 

showing:

31. Relation between observed annual peak discharges for water years 1938 

through 2007 to Quaboag River at West Brimfield (01176000) and to 

West Branch Westfield River at Huntington (01181000)..............................................47

32. Observed and estimated annual peak discharge for water years 1936 

through 2007 .......................................................................................................................47

33. Peak-flow frequency computed from observed and estimated annual peaks 

for water years 1936 through 2007..................................................................................48

34. April 15–23, 2007 (A) stage and (B) discharge ..............................................................49

35–38. Graphs related to Sevenmile River near Spencer (01175670), Massachusetts 

showing:

35. Relation between observed annual peak discharges for water years 1961 

through 2007 to North Nashua River at Leominster (01094500) and to 

Quaboag River at West Brimfield (01176000) ................................................................51

36. Observed and estimated annual peak discharge for water years 1936 

through 2007 .......................................................................................................................51

37. Peak-flow frequency computed from observed and estimated annual peaks 

for water years 1936 through 2007..................................................................................52

38. April 15–23, 2007 (A) stage and (B) discharge ..............................................................55

39–41. Graphs related to West Branch Westfield River near Huntington (01181000), 

Massachusetts showing:

39. Observed annual peak discharge for water years 1936 through 2007......................57

40. Peak-flow frequency computed from observed annual peaks for water 

years 1936 through 2007 ...................................................................................................57

41. April 15–23, 2007 (A) stage and (B) discharge ..............................................................58

42. Graph showing magnitude of flood flows in relation to drainage area at selected 

streamflow-gaging stations in central Massachusetts .......................................................61
vi

Tables

1. Generalized characteristics of the contributing area to selected streamflowgaging stations in Massachusetts.............................................................................................4

2. Precipitation and antecedent conditions associated with the April 2007 flooding at 

three climate stations in Massachusetts and New Hampshire............................................8

3. Streamflow-gaging stations and support stations used to characterize the 

April 2007 flood in Massachusetts...........................................................................................10

4–5. North Nashua River at Fitchburg (01094400), Massachusetts:

4. Magnitude and confidence limits of flood flows at selected return intervals ........13

5. Comparison of flood stage and discharge at selected return intervals reported 

in community flood-insurance study to values determined in this study.................14

6–7. Stillwater River near Sterling (01095220), Massachusetts:

6. Magnitude and confidence limits of flood flows at selected return intervals ........18

7. Comparison of flood stage and discharge at selected return intervals reported 

in community flood-insurance study to values determined in this study.................20

8–9. Squannacook River near West Groton (01096000), Massachusetts:

8. Magnitude and confidence limits of flood flows at selected return intervals ........23

9. Comparison of flood stage and discharge at selected return intervals reported 

in community flood-insurance study to values determined in this study.................25

10–11. Nissitissit River at Pepperell (01096503), Massachusetts:

10. Magnitude and confidence limits of flood flows at selected return intervals ........27

11. Comparison of flood stage and discharge at selected return intervals reported 

in community flood-insurance study to values determined in this study.................29

12–13. Merrimack River at Lowell (01100000), Massachusetts:

12. Magnitude and confidence limits of flood flows at selected return intervals ........32

13. Comparison of flood stage and discharge at selected return intervals reported 

in community flood-insurance study to values determined in this study.................35

14–15. Spicket River near Methuen (01100561), Massachusetts:

14. Magnitude and confidence limits of flood flows at selected return intervals ........37

15. Comparison of flood stage and discharge at selected return intervals reported 

in community flood-insurance study to values determined in this study.................40

16–17. Shawsheen River near Andover (01106270), Massachusetts:

16. Magnitude and confidence limits of flood flows at selected return intervals ........43

17. Comparison of flood stage and discharge at selected return intervals reported 

in community flood-insurance study to values determined in this study.................45

18–19. Mill River at Northampton (01171500), Massachusetts:

18. Magnitude and confidence limits of flood flows at selected return intervals ........48

19. Comparison of flood stage and discharge at selected return intervals reported 

in community flood-insurance study to values determined in this study.................50

20–21. Sevenmile River near Spencer (01175670), Massachusetts:

20. Magnitude and confidence limits of flood flows at select return intervals.............53

21. Comparison of flood stage and discharge at selected return intervals reported 

in community flood-insurance study to values determined in this study.................56
vii

22–23. West Branch Westfield River near Huntington (01181000), Massachusetts:

22. Magnitude and confidence limits of flood flows at select return intervals.............59

23. Comparison of flood stage and discharge at selected return intervals reported 

in community flood-insurance study to values determined in this study.................59

24. Simple regression equations for estimating the magnitude of peak flows at 5-, 10-, 

25-, 50-, 100-, 200-, and 500-year return intervals from drainage area in unregulated 

streams in central Massachusetts ..........................................................................................62

25. Summary of April 2007 peak flows and stages, and the estimated return interval of 

the storm peak discharge and stage at ten selected streamflow-gaging stations 

in Massachusetts .......................................................................................................................64
viii

Conversion Factors, Datum, and Abbreviations

Inch/Pound to SI

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)

Area

square mile (mi2) 2.590 square kilometer (km2) 

Flow rate

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

cubic foot per second per square mile 

[(ft3/s)/mi2]

0.01093 cubic meter per second per square 

kilometer [(m3/s)/km2]

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

(NGVD 29).

Horizontal coordinate information is referenced to North American Datum of 1983 (NAD 83).

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

ABBREVIATIONS

FEMA Federal Emergency Management Agency

FIS Flood-Insurance Study

MOVE-1 Maintenance of Variance Extension (version 1)

NWS National Weather Service

RMSE Root Mean Square Error

USACE U.S. Army Corps of Engineers

USGS U.S. Geological Survey
Characteristics of the April 2007 Flood at 10 StreamflowGaging Stations in Massachusetts

By Phillip J. Zarriello and Carl S. Carlson

Abstract

A large “nor’easter” storm on April 15–18, 2007, brought 

heavy rains to the southern New England region that, coupled 

with normal seasonal high flows and associated wet soilmoisture conditions, caused extensive flooding in many parts 

of Massachusetts and neighboring states. To characterize the 

magnitude of the April 2007 flood, a peak-flow frequency 

analysis was undertaken at 10 selected streamflow-gaging 

stations in Massachusetts to determine the magnitude of 

flood flows at 5-, 10-, 25-, 50-, 100-, 200-, and 500-year 

return intervals. The magnitude of flood flows at various 

return intervals were determined from the logarithms of the 

annual peaks fit to a Pearson Type III probability distribution. 

Analysis included augmenting the station record with longerterm records from one or more nearby stations to provide a 

common period of comparison that includes notable floods in 

1936, 1938, and 1955. 

The April 2007 peak flow was among the highest 

recorded or estimated since 1936, often ranking between the 

3d and 5th highest peak for that period. In general, the peakflow frequency analysis indicates the April 2007 peak flow 

has an estimated return interval between 25 and 50 years; at 

stations in the northeastern and central areas of the state, the 

storm was less severe resulting in flows with return intervals 

of about 5 and 10 years, respectively. At Merrimack River 

at Lowell, the April 2007 peak flow approached a 100-year 

return interval that was computed from post-flood control 

records and the 1936 and 1938 peak flows adjusted for 

flood control. 

In general, the magnitude of flood flow for a given 

return interval computed from the streamflow-gaging station 

period-of-record was greater than those used to calculate flood 

profiles in various community flood-insurance studies. In 

addition, the magnitude of the updated flood flow and current 

(2008) stage-discharge relation at a given streamflow-gaging 

station often produced a flood stage that was considerably 

different than the flood stage indicated in the flood-insurance 

study flood profile at that station. 

Equations for estimating the flow magnitudes for 5-, 10-, 

25-, 50-, 100-, 200-, and 500-year floods were developed from 

the relation of the magnitude of flood flows to drainage area 

calculated from the six streamflow-gaging stations with the 

longest unaltered record. These equations produced a more 

conservative estimate of flood flows (higher discharges) than 

the existing regional equations for estimating flood flows at 

ungaged rivers in Massachusetts. Large differences in the 

magnitude of flood flows for various return intervals determined in this study compared to results from existing regional 

equations and flood insurance studies indicate a need for 

updating regional analyses and equations for estimating the 

expected magnitude of flood flows in Massachusetts.

Introduction

In mid-April 2007, a strong low-pressure system over 

southern New England produced heavy rainfall that, coupled 

with wet-antecedent conditions, produced extensive flooding 

in many streams and rivers in the region. This storm, known as 

the 2007 Patriots’ Day Nor’easter, is one of the largest springtime storms to hit New England in memory (FEMA, 2007). 

The flooding and the resulting flood damages were extensive 

enough to cause the Commonwealth of Massachusetts to 

declare a state of emergency and to prompt a Presidential 

disaster declaration on May 16, 2007.

The National Oceanic and Atmospheric Administration 

(NOAA) National Climatic Data Center (NCDC) provided 

the following description of the April 2007 storm for Essex 

County, Massachusetts (National Climatic Data Center, 

2008d): 

An unusually strong and slow moving coastal storm 

for mid April tracked to western Long Island Sound 

on April 16th before weakening slowly and drifting 

offshore. This storm brought a variety of impacts in 

southern New England, including heavy snow to the 

higher elevations of western Massachusetts, damaging winds in excess of 60 mph, widespread river and 

stream flooding, and significant coastal flooding 

through several high tide cycles. Rainfall totals of 

3 to 5 inches, combined with wet antecedent conditions, resulted in widespread river and stream flooding, as well as significant flooding of urban areas. 

The worst flooding affected the Merrimack Valley, 
2 Characteristics of the April 2007 Flood at 10 Streamflow-Gaging Stations in Massachusetts

where moderate to major flooding occurred on the 

Merrimack, Nashua, and North Nashua Rivers. 

For many locations, this may have been the worst 

flooding since the May, 2006 or April, 1987 floods, 

but along the North Nashua, the preliminary crests 

recorded may have been the highest since the floods 

of September, 1938. Many small streams throughout 

the region also rose out of their banks and flooded 

nearby areas, including roadways. Major flooding occurred along the Mill River in Northampton, 

which required the evacuation of nearby residents.

Data collected and analyzed to document the magnitude 

and extent of flooding from this large storm provide important 

information for flood management, bridge and culvert design, 

and risk assessment. Streamflow-gaging stations with long 

periods of record are the single best tool for evaluating the 

magnitude and severity of flooding. Up-to-date information 

on the magnitude and frequency of flood flows and associated 

river stage is crucial for the development and guidance of mitigation measures to minimize flood losses in future disasters. 

The Federal Emergency Management Agency (FEMA), part 

of the Department of Homeland Security, is responsible for 

protecting life and property from all hazards, including natural 

disasters, through comprehensive emergency management 

preparedness, protection, response, recovery, and mitigation. 

To assist FEMA in its mission, the U.S. Geological Survey 

(USGS) entered into an agreement with FEMA in 2008 to 

characterize flooding from the April 2007 storm at 10 selected 

streamflow-gaging stations that experienced some of the most 

extensive flooding in Massachusetts.

Purpose and Scope

This report documents streamflow and river-stage 

conditions at 10 selected streamflow-gaging stations in westcentral and northeastern Massachusetts during the flood of 

April 2007. The report describes the analysis of peak-flow 

magnitudes calculated for 5-, 10-, 25-, 50-, 100-, 200-, and 

500-year return intervals from currently available data to 

characterize the magnitude and return interval of the April 

2007 storm. The results of this analysis are also compared 

to flood stages and discharges reported in flood-insurance 

studies done in the vicinity of the streamflow-gaging stations 

examined. The streamflow-gaging stations examined 

include (in order of the USGS identification number): North 

Nashua River at Fitchburg (01094400), Stillwater River 

near Sterling (01095220), Squannacook River near West 

Groton (01096000), Nissitissit River at Pepperell (01096503), 

Merrimack River at Lowell (01100000), Spicket River 

near Methuen (01100561), Shawsheen River at Andover 

(01100627), Mill River at Northampton (01171500), 

Sevenmile River near Spencer (01175670), and West Branch 

Westfield River near Huntington (01181000).

Study Area

The 10 streamflow-gaging stations examined cover an 

area from west-central to northeastern Massachusetts (fig. 1). 

Drainage areas range from 8.81 to 4,635 mi2 with a median 

drainage area of 63 mi2 (fig. 1). Four of these streamflowgaging stations have drainage areas that extend into New 

Hampshire: the Squannacook River near West Groton, 

Nissitissit River at Pepperell, Merrimack River at Lowell, and 

Spicket River near Methuen. Most of the drainage areas of the 

latter three stations are in New Hampshire. In addition to these 

10 streamflow-gaging stations, records from 7 other stations 

were used to support the peak-flow analysis; these stations 

are referred to as support stations. Two of the support stations 

are in the northern part of the Merrimack River Basin and are 

beyond the map extent of figure 1. 

The drainage basin characteristics vary (table 1). In 

general, elevation and steepness increase, while storage (water 

and wetland) and urban area decrease from the coast inland. 

Storage, as measured by the combined percent of the basin 

area classified as wetlands or open water, ranged from 

1.9 percent at Mill River at Northampton to about 14 percent 

at Spicket River near Methuen. Urban areas (areas classified as 

high-density residential, multifamily, or commercial) ranged 

from 0.2 to 20 percent of the basin area (Sevenmile River near 

Spencer and Shawsheen River at Andover, respectively). Areas 

classified as “other” are mostly low-density residential lands 

and were largest in the Spicket and Shawsheen River Basins 

(about 22 and 40 percent of the basin area, respectively). 

The Spicket and Shawsheen River Basins have the highest 

percentage of combined urban land use. 

Previous Studies

Numerous flood investigations have been made throughout Massachusetts as part of the FEMA’s Flood Insurance 

Program to delineate the expected flood elevation for large, 

infrequent events such as the 100- and 500-year flood. Floodinsurance studies relevant to the streamflow-gaging stations 

examined in this study are included in the sections of this 

report that describe flood flows at the individual stations. The 

discussion below is limited to specific flood events previously 

documented and to regional flood-flow analyses prepared by 

the USGS.

The USGS has documented flooding following major 

events, including the precipitation-snowmelt flood of 1936 

(Grover, 1937), and hurricane floods of 1938 (Paulsen 

and others, 1940) and 1955 (Bogart, 1960) although these 

reports are broad in scope and cover an area extending far 

beyond Massachusetts. Jahns (1947) used the earlier flood 

information in the analysis of flooding to geologic features 

of the Connecticut River valley. Smaller flood events focused 

mainly in Massachusetts have been documented by Wood 

and others (1970), Swallow and others (1971), Swallow and 

Fogarty (1973), Swallow and Wood (1973), and Parker and 
Introduction 

3

Figure 1. Location of selected streamflow-gaging stations in Massachusetts used to characterize the April 2007 flood.

Atlantic

Ocean

43°00'

42°30'

42°00'

73°00' 72°00' 71°00' 70°00'

41°30'

Base from U.S. Geological Survey and MassGIS data sources, Massachusetts State Plane Coordinate System, Mainland Zone. 

Vermont New Hampshire

New York

Rhode Island Connecticut

Massachusetts

01102000

01181000 01175670

01100600

01100000

01096000

01095220

01094500

01094400

01171500

01101500

01176000

01096503

01100627

01100561

Drainage area extends beyond map limits

Merrimack River Basin

EXPLANATION

Station and number

01181000

0 20 KILOMETERS

0 20 MILES

Station and number

01094500

Contributing

drainage area

Contributing

drainage area

Study streamflow-gaging station

Support streamflow-gaging station
4 Characteristics of the April 2007 Flood at 10 Streamflow-Gaging Stations in Massachusetts Table 1. Generalized characteristics of the contributing area to selected streamflow-gaging stations in Massachusetts. [Contributing area shown in figure 1; Regulation code: 0, none; 1, low flow only; 3, all flows. Region code: CU, Central Uplands; CL, Central Lowlands; WH, Western Highlands; --, not determined; basin characteristics from Zarriello and Socolow, 2003, unless otherwise noted; mi2, square miles] U.S. Geological Survey streamflow-gaging station Drainage area (mi2

)

Regulation 

code

Region 

code

Mean 

elevation 

(feet)

Mean 

slope 

(percent)

Basin characteristics 

(percent of basin area)

Name Number Water Wetland Forest Urban Agricul- ture Other Sand and gravel

North Nashua River at Fitchburg 01094400 64.2 1 CU 981 8.1 3.6 2.4 71 6.3 5.7 11 21Stillwater River near Sterling 01095220 31.6 0 CL 766 7.7 1.9 2.5 77 1.2 10 7.4 18Squannacook River near West Groton 01096000 63.7 0 CL 622 7.5 0.9 2.2 82 1.3 6.0 7.6 27Nissitissit River at Pepperell1 01096503 59.6 0 CU 447 8.1 1.8 4.8 79 1.2 6.8 6.2 28Merrimack River at Lowell 01100000 4,635 3 -- -- -- -- -- -- -- -- -- --Spicket River near Methuen1 01100561 62.1 3 CL 246 6.1 4.1 9.4 49 8.6 7.0 22 --Shawsheen River at Andover1 01100627 72.8 1 CL 146 4.3 0.8 3.8 32 20 3.0 40 64Mill River at Northampton 01171500 52.6 1 WH 841 11.1 1.0 0.9 81 3.1 6.8 7.2 16Sevenmile River near Spencer 01175670 8.81 1 CU 871 7.4 2.3 3.7 73 0.2 14 6.8 13West Branch Westfield River near 

Huntington

01181000 94.0 0 WH 1,422 13.2 1.3 1.3 91 0.4 2.3 3.7 4.0

1 Land use and land cover (LULC) from National Atlas cover grid, 2001, 30-square meter resolution, downloaded (http://nmviewogc.cr.usgs.gov/viewer.htm?BBOX=-88.23975,40.09448,-

88.22655,40.1014&CLASSIDSON=39|6&LAYERSON=3380|10464|11787&LAYERSOFF=1652), July 10, 2008; elevation data from National Elevation Data (NED), 10-square meter resolution, downloaded (http://ned.usgs.gov/), July 10, 2008.
Methods 5

others (1998). Other reports that cover floods in the New 

England region include Kinnison (1930), Thomson and others 

(1964), and Gadoury (1979). A summary report of floods in 

Massachusetts done by Wandle and Lautzenheiser (1991) is 

included in the 1988–89 National Water Summary.

Several studies provide a regional analysis for the 

purpose of developing equations for determining flood flows 

at ungaged sites in Massachusetts. These studies began with 

Knox and Johnson (1965) and were followed by Johnson and 

Tasker (1974a), Wandle (1977), and Wandle (1983). Wandle’s 

1983 report in which regional equations were developed for 

estimating flood flows at 2-, 5-, 10-, 25-, 50-, and 100-year 

return intervals from annual peak flows at 95 sites is the most 

comprehensive of these studies. Wandle’s analysis was made 

from systematic peak-flow records through the 1976 water 

year. These equations were used in a number of community 

flood-insurance studies to estimate flood flows at ungaged 

sites. Later work by Murphy (2001a, 2001b) provided 

equations for estimating flood flows from mixed probability 

distribution populations. The analysis was limited to peakflow records through 1993 at 30 streamflow-gaging stations. 

More recent regional equations for estimating flood flows at 

ungaged sites in neighboring states have been developed for 

Connecticut (Ahearn, 2003), New Hampshire (Olson, 

2009), New York (Lumia and others, 2006), and Vermont 

(Olson, 2002). 

Antecedent and Storm Conditions

Antecedent moisture conditions can have a large effect on 

the magnitude of flows generated by a storm–for example, the 

highest recorded streamflow in south-central Massachusetts 

occurred following back-to-back Hurricanes Connie and Diane 

in August 1955 (National Weather Service Northeast River 

Forecast Center, 2008). Although either storm would have produced flood flows, the fact that Hurricane Diane (rainfall totals 

of about 15 in.) followed Hurricane Connie (rainfall totals of 

about 8 in.) within a 2-week period produced exceptionally 

high flows in parts of the state. This storm is used as an example because of its historical significance and its influence on 

the magnitude of peak flow in some of the sites examined in 

this report. In general, a given storm will produce higher flows 

if existing conditions are wet than if they are dry. Therefore, 

antecedent climatic and hydrologic conditions as well as the 

storm characteristics themselves are helpful in characterizing 

the April 2007 storm.

Streamflow

Streamflow prior to the April 16–18 flooding was in the 

normal range of the seasonal high flow at each of the stations 

examined. This is illustrated by the daily mean flow for the 

2007 water year in relation to the 10th-, 25th-, 50th-, 75th-, 

and 90th-percentile flows for a given day calculated from 

the period-of-record daily mean flows at Squannacook River 

near West Groton and West Branch Westfield River near 

Huntington (fig. 2). These stations represent the geographic 

range of the stations examined in this study and the stations 

with the longest record. Antecedent flows at these two stations 

were between the median and 25th percentile for this time 

of year. Had flows been in the higher range of normal flows 

for this time of year, the 2007 Patriots’ Day storm would 

likely have resulted in even greater flooding. If the storm had 

occurred at most other times of the year, however, flooding 

likely would have been less extensive.

Precipitation

Hourly precipitation data were compiled from three 

NOAA climate stations—Groveland and Worcester in 

Massachusetts and Hopkinton Lake in New Hampshire 

(fig. 3). Prior to the April 15–18 storm, the last precipitation 

was from a storm on April 12, 2007, which totaled 0.87, 1.03, 

and 0.90 in. at Groveland, Worcester, and Hopkinton Lake, 

respectively (table 2). As a result, hydrographs were receding 

prior to the April 15–18 storm (fig. 2).

The April 2007 nor’easter began as snowfall—1 to 

3 in. were reported from northern Worcester County into 

northwestern Middlesex County, but changed to all rain as 

milder air was drawn into the interior (National Climatic 

Data Center, 2008d). Total precipitation from the nor’easter 

during April 15–18, 2007, was compiled from the three hourly 

climate stations and 290 daily climate stations in southern 

New England and contoured (fig. 3) using the ESRI spatial 

analyst inverse distance weighting technique. Precipitation 

in the region ranged from about 1.5 to 7 in. with the greatest 

amounts falling along a north-south line over the Berkshire 

Mountains in western Massachusetts and in an arc north 

from Worcester through Fitchburg, Massachusetts, into 

New Hampshire, and then northeastward (National Weather 

Service, 2008b). The streamflow-gaging stations examined in 

this study generally have drainage areas that lie in the areas of 

greatest precipitation from the April 2007 nor’easter. 

Methods

Characterization of the April 2007 flood flows consisted of three main parts. First, the April 2007 peak flow 

is described in relation to previously recorded annual peak 

flows and estimated annual peak flows dating back to 1936. 

Second, the April 2007 peak flow is put in the context of the 

magnitude and frequency of flood flows. Third, the April 2007 

peak stage and discharge is examined in relation to existing 

flood-insurance-study information and the NWS flood-stage 

warnings. The general approach to each of these components 

is described below, but some of the specific details as they 

apply to individual streamflow-gaging stations are described 

later in the report for that station.
6 Characteristics of the April 2007 Flood at 10 Streamflow-Gaging Stations in Massachusetts

Figure 2. Daily mean period-of-record flow percentiles and 2007 flows at (A) Squannacook River near West Groton (01096000) 

and (B) West Branch Westfield River near Huntington (01181000), Massachusetts. 

OCT NOV DEC JAN FEB MAR APR MAY JUNE JULY AUG SEP

1

10

100

1,000

10,000

A. Squannacook River

OCT NOV DEC JAN FEB MAR APR MAY JUNE JULY AUG SEP

1

10

100

1,000

10,000

DISCHARGE, IN CUBIC FEET PER SECOND

B. West Branch Westfield River

90

75

50

25

10

2007 daily mean flow Flow percentile th

th

th

th

th

EXPLANATION

Period of record October 1949 through September 2007

Period of record September 1935 through September 2007
Methods 

7

Figure 3. Total precipitation from April 15 through 18, 2007, in and around Massachusetts. 

Atlantic

Ocean

Vermont New Hampshire

Rhode Island Connecticut

New York

GROVELAND

HOPKINTON

LAKE

WORCESTER 

01100627

01100561

01181000 01175670

01171500

01100000

01096503

01096000

01094400

01095220

Total rainfall, in inches April 15–18, 2007

EXPLANATION

Hourly

USGS streamflow-gaging station and number

01181000

Daily

National Weather Servicerain gage

42°00'

42°30'

43°00'

73°00' 72°00' 71°00' 70°00'

41°00'

> 7 to 7.5> 6.5 to 7> 6 to 6.5> 5.5 to 6> 5 to 5.5

> 4.5 to 5> 4 to 4.5> 3.5 to 4> 3 to 3.5> 2.5 to 3> 1.5 to 2.5

Base from U.S. Geological Survey and MassGIS data sources, Massachusetts State Plane Coordinate System, Mainland Zone. 

0 20 KILOMETERS

0 20 MILES
8 Characteristics of the April 2007 Flood at 10 Streamflow-Gaging Stations in Massachusetts

Annual Peak Flows

Recorded and estimated annual peak flows at the 

streamflow-gaging stations provide a historical perspective 

on the magnitude of the April 2007 flood to prior flood flows. 

Systematic annual peak flows are recorded and maintained in 

the USGS National Water Information System (NWIS) Peak 

Flow File (PFF) available at http://nwis.waterdata.usgs.gov/

usa/nwis/peak. The PFF provides key information necessary 

for the computation of the magnitude and frequency of flood 

flows, which in turn provide the foundation for flood insurance 

studies and other purposes such as bridge design. 

Annual peak-flow data are generally limited to the period 

of streamflow-gaging station operation that varied from 1 to 

84 years for the stations examined in this study. Occasionally, peak flow data outside the systematic streamflow record 

are entered into the PFF. These peaks are tagged as historical 

in the database and have special meaning in the way they are 

treated in peak-flow frequency analysis. Only one station, 

Merrimack River at Lowell (01100000), had a peak in the 

PFF outside the systematic record that occurred on April 23, 

1852 (108,000 ft3/s). Because of a special circumstance at this 

site, an analysis was made of post-flood control peaks treating 

the two highest peaks of record (1936 and 1938) as historical peaks adjusted for the effects of flood control as described 

later in the report.

To provide a common period of comparison, the variablelength peak-flow records for the streamflow-gaging stations 

were augmented by extending records from the beginning of 

the period of record back to the 1936 water year. The 1936 

water year was chosen as a common starting base because it 

represents the highest known peak flow in some parts of the 

state, or includes the period of highest flow, notably the floods 

of 1938 and 1955. The 72 years of estimated and observed 

annual peak-flow record also provide a sufficient period of 

time to place the April 2007 peak discharge into historical context and to minimize the effects of the regional skew values 

used in the peak-flow frequency analysis.

Records that were extended were done so by a mathematical procedure developed by Hirsch (1982) known as 

Maintenance of Variance Extension (MOVE-1). MOVE-1 

preserves the statistical moments of the data, namely the 

means and standard deviations of the log-transformed annual 

peaks at the short-term station to the long-term station. At 

two stations, Nissitissit River at Pepperell (01096503) and 

Shawsheen River at Andover (01100627), the record length 

was insufficient (1 year) to apply this method to annual peaks; 

therefore, selected independent storm peak flows were used in 

the MOVE-1 analysis.

Scatterplots of log-transformed concurrent annual peakflows or independent storm-peak flows show the relation 

between the short-term and long-term streamflow-gaging 

station records. The long-term station, herein referred to as the 

index station, was selected on the basis of its proximity to the 

short-term station. In some instances, two or three index stations were used in the analysis because they spatially straddle 

the short-term station, and the results of these multiple station 

analyses reveal the range of potential discharges. In some 

cases, the estimated flows represent a weighted average of 

flows determined from each index station determined on the 

basis of the root mean square error (RMSE). The RMSE is 

determined by the square root of the average squared differences between observed and estimated peak flows for the 

period of concurrent record. 

Table 2. Precipitation and antecedent conditions associated with the April 2007 flooding at three climate stations in Massachusetts 

(MA) and New Hampshire (NH).

[EDT, Eastern Daylight Savings Time; hrs, hours; in, inches; in/hr, inches per hour; >, greater than; COOP-ID, National Weather Service station cooperative 

identification number; station location shown in figure 3]

Station COOP-ID

Begin Storm characteristics

Date Time a.m. 

(EDT)

Duration

(hrs)

Volume

(in)

Average 

intensity 

(in/hr)

Maximum 

intensity 

(in/hr)

Groveland, MA 193276 4/15/2007 9:00 99 5.11 0.05 0.39

Worcester, MA 199923 4/15/2007 7:00 90 4.63 0.05 0.44

Hopkinton Lake, NH 274218 4/15/2007 11:00 68 5.70 0.08 0.40

Antecedent conditions

Station

Time since total precipitation 

(hrs)

Antecedent rainfall 

(in)

>0 in >0.2 in > 0.5 in >1.0 in 24 hrs 48 hrs 72 hrs 168 hrs

Groveland, MA 50 50 50 238 0.00 0.00 0.87 0.87

Worcester, MA 60 60 60 60 0.00 0.00 1.03 1.03

Hopkinton Lake, NH 60 60 60 236 0.00 0.00 0.90 1.00
Methods 9

Magnitude and Frequency of Flood Flows

An important consideration in flood-risk management 

and in the development of local flood-insurance studies is 

the determination of the magnitude of peak flow for selected 

exceedance probabilities. The inverse of the exceedance 

probability is the expected return interval of a flood flow. 

For example, a flow with a 1-percent exceedance probability 

(0.01) has a 1-percent chance of being equaled or exceeded 

in a given year, or as more commonly expressed, an expected 

return interval of once every 100 years. For convenience, 

magnitude of flood flows are generally referred to in this 

report in terms of return interval and are given at 5, 10, 25, 50, 

100, 200, and 500 years, which correspond to annual exceedance probabilities of 0.20, 0.10, 0.04, 0.02, 0.01, 0.005, and 

0.002, respectively. However, as noted the flows associated 

with these return intervals can occur in any given year, but the 

likelihood of that flow occurring in a given year decreases as 

the return interval increases.

Although floods at low exceedance probabilities are 

rare and expected to occur infrequently, they can and often 

do occur with greater frequency than expected. This is partly 

because of the nature of probability theory, which represents 

the magnitude of a flood for a given exceedance probability 

over the long term defined around a stationary mean and 

variance. Milly and others (2008) have raised questions 

about the appropriateness of traditional probability methods 

in the midst of changing basin and climatic characteristics. 

Nevertheless, standard hydrologic methods were used to 

determine the magnitude and frequency of flood flows because 

they are still widely accepted and non-stationary methods are 

still in their infancy.

The magnitude and frequency of flood flows were 

determined at each of the streamflow-gaging stations by use 

of the computer program PeakFQ (Flynn and others, 2006). 

PeakFQ analyzes annual peak flows following guidelines in 

Bulletin 17B (Interagency Advisory Committee on Water 

Data, 1982). The period of systematic peak-flow data from 

six stations with records dating back to 1972 or earlier were 

analyzed with PeakFQ. The combined recorded and estimated 

annual peak flows estimated with MOVE-1 as described 

above were also analyzed with PeakFQ for all but one of the 

stations examined in this study. The exception is the West 

Branch of the Westfield River near Huntington (01181000), 

which has streamflow records dating back to 1935 and are not 

appreciably affected by regulation; hence, no record extension 

was necessary at this station.

In general, PeakFQ fits the logarithms of the annual peak 

flows to a Pearson Type III probability distribution function 

to calculate the discharge over a range of annual recurrence 

intervals from about 1 to 500 years (exceedance probabilities 0.995 to 0.002, respectively). PeakFQ also calculates the 

discharges at the 95-percent confidence limits over the range 

of annual recurrence intervals. Parameters of the Pearson 

Type III frequency curve are estimated from the logarithmic 

sample moments (mean, standard deviation, and coefficient 

of skewness). Adjustments to the Pearson Type III parameters are made for low outliers, high outliers, historic peaks, 

and generalized skew. No outliers were detected in the data 

sets examined, and therefore, no adjustment was needed for 

outliers. Historical peak adjustments were made only for the 

Merrimack River at Lowell analysis for post-flood control 

peak-flow analysis as described later in the report. 

The skew calculated from the systematic station annual 

peak flows can greatly affect the shape of the probability 

distribution function. This is particularly true of stations with 

short records that are heavily leveraged by extreme events. To 

compensate, Bulletin 17B (Interagency Advisory Committee 

on Water Data, 1982) specifies a weighting procedure 

calculated from the station skew and a generalized skew value 

to improve the accuracy of the probability distribution skew 

used in the peak-flow frequency analysis. Generalized skews 

have not been calculated for Massachusetts, and the program 

defaults to the skews determined from a national database 

of systematic peak-flow records up to 1973 (Interagency 

Advisory Committee on Water Data, 1982). Because these 

regional skew values are outdated, generalized skews for 

the stations used in this analysis were estimated from recent 

regionalized skew values determined for neighboring states 

(Ahearn, 2003; Lumia and Baevsky, 2000; Olson, 2002 and 

2009). The generalized skews determined for the neighboring 

states do not always align with each other, but in general were 

less than the national skews for this region. The generalized 

skew specified for each station is given in the analysis section 

for that station. The effect of the specified skews on the station 

peak-flow frequency analysis increased as the period of record 

decreased, but their effect on the extended peak-flow record 

analysis was minimal.

At some stations, the two-station comparison method 

(Bulletin 17B, Appendix 7; Interagency Advisory Committee 

on Water Data, 1982) was used to adjust the logarithmic mean 

and standard deviation of the short-term station peak-flow 

record to a nearby long-term station peak-flow record. The 

skew used in this analysis was determined from the weighted 

station skew described above. The two-station comparison 

method was employed in the peak-flow frequency analysis at 

five streamflow-gaging stations. At the other five stations, the 

systematic annual peak-flow record was either too short or too 

long to warrant use of this method. 

The Nissitissit River at Pepperell (01096503) and Spicket 

River near Methuen (01100561) streamflow-gaging stations 

began operation in March 2006 and March 2001, respectively. 

Their short records and the fact that most of these basins 

lie within southern New Hampshire made these sites suitable for estimating the magnitudes of floods using recently 

developed multiple linear-regression flood-flow equations 

for New Hampshire (Olson, 2009). The Shawsheen River 

at Andover station (01100627) began operation in October 

2006; therefore, the upstream station on the Shawsheen River 

near Wilmington (01100600), which has been in operation 

since November 1963, was used in the peak-flow frequency 

analysis. The results of these analyses were then adjusted by 
10 Characteristics of the April 2007 Flood at 10 Streamflow-Gaging Stations in Massachusetts

the ratio of the drainage areas of the Andover and Wilmington 

stations to estimate the magnitude of flood flows for various 

return intervals at Andover.

Flood-Insurance Studies and Flood-Warning 

Levels

Flood-insurance studies provide water-surface profiles for 

10-, 50-, 100-, and 500-year floods calculated from hydraulic 

analysis of the river’s conveyance and other hydrologic 

properties. This analysis entails simulating the water-surface 

elevation, typically with an early version of the HEC-2 stepbackwater program (Bonner, 1988) that was commonly in use 

at the time most of these studies were done. The peak flows at 

selected return intervals were used to simulate water-surface 

profiles. These flows were determined by one or more of 

the following methods—(1) log-Pearson Type III analysis 

of the available systematic peak-flows at streamflow-gaging 

stations at the time of the study, (2) interpolation on the basis 

of drainage area from calculated magnitudes of peak flows 

for given return frequencies at a nearby streamflow-gaging 

station, and (3) regional regression equations for estimating 

the magnitude of peak flows.

The April 2007 peak river stage was compared to the 

flood elevation reported in various flood-insurance studies 

and to the NWS flood-warning elevation, if available. Flood 

flows and flood elevations for various return intervals from the 

flood-insurance studies were compared to those determined as 

part of this study. These comparisons were made by determining the river elevation for flood flows at various return 

intervals on the basis of the current (2008) stage-discharge 

relation (rating curve) at the streamflow-gaging station. In 

some instances, flood flows were determined from the floodinsurance study flood-profile elevations at the streamflowgaging station and the rating curve in use at the time of the 

study. If reported, the flood flows used in the flood-insurance 

study step-backwater analysis were compared to flood flows at 

various return intervals determined in this study. 

Table 3. Streamflow-gaging stations and support stations used to characterize the April 2007 flood in Massachusetts.

[NH, New Hampshire; mi2, square miles; EOEEA, Massachusetts Executive Office of Energy and Environmental Affairs; --, not applicable]

U.S. Geological Survey streamflow-gaging station Record 

begins

Drainage area 

(mi2) EOEEA basin Support stations Name Number

North Nashua River at Fitchburg 01094400 Oct. 1972 64.2 Nashua 01094500

Stillwater River near Sterling 01095220 Apr. 1994 31.6 Nashua 01094500

Squannacook River near West Groton 01096000 Oct. 1949 63.7 Nashua 01094500

Nissitissit River at Pepperell 01096503 Mar. 2006 59.6 Nashua 01094500 01096000

Merrimack River at Lowell 01100000 Jun. 1923 4,635 Merrimack 01076500 01078000 01094500

Spicket River near Methuen 01100561 Mar. 2001 62.1 Merrimack 01094500 01101500

Shawsheen River at Andover 01100627 Oct. 2006 72.8 Shawsheen 01094500 01100600 01102000

Mill River at Northampton 01171500 Oct. 1938 52.6 Connecticut 01176000 01181000

Sevenmile River near Spencer 01175670 Feb. 1961 8.81 Chicopee 01094500 01176000

West Branch Westfield River near 

Huntington

01181000 Sep. 1935 94.0 Westfield

Support stations used in analysis

Pemigewasset River at Plymouth, NH 01076500 Oct. 1903 622 --

Smith River near Bristol, NH 01078000 May 1918 85.8 --

North Nashua River near Leominster 01094500 Sep. 1935 110 Nashua

Shawsheen River near Wilmington 01100600 Nov. 1963 36.5 Shawsheen

Ipswich River at South Middleton 01101500 Jun. 1938 44.5 Ipswich

Ipswich River near Ipswich 01102000 Jun. 1930 125 Ipswich

Quaboag River at West Brimfield 01176000 Aug. 1909 150 Chicopee
Characteristics of the April 2007 Flood at Selected Streamflow-Gaging Stations 11

Characteristics of the April 2007 

Flood at Selected Streamflow-Gaging 

Stations

The discharge characteristics at the selected streamflowgaging stations are presented in order of their USGS identification number. Seven of the streamflow-gaging stations are in 

the Merrimack River Basin and three are in the Connecticut 

River Basin. The data available for the streamflow-gaging stations range from 1 to 84 years of record (table 3) and, thus, the 

methods used to characterize the April 2007 flood flows varied 

accordingly. For most streamflow-gaging stations, a nearby 

station or stations with records dating back to 1936 were used 

to extend the annual peak-flow records for analysis; the support stations are also listed in table 3 and shown in figure 1.

North Nashua River at Fitchburg–01094400

Annual peak flows recorded at North Nashua River at 

Fitchburg span a 35-year period, water years 1973 through 

2007. The April 2007 peak discharge (3,930 ft3/s) was the 

highest recorded and exceeded the previously recorded peak 

flow in the 1987 water year by about 12 percent. Annual peak 

flows at North Nashua River at Fitchburg were estimated from 

1936 through 1972 by MOVE-1 using the 35 years of concurrent record with the downstream gaging station at North 

Nashua River at Leominster (01094500). The drainage area 

above the Leominster station is about twice that above the 

Fitchburg station (110 and 64.2 mi2, respectively). MOVE-1 

analysis indicates a strong relation between peak flows at the 

Fitchburg and Leominster stations (fig. 4), RMSE of 258 ft3/s. 

Estimated peak flows from 1936–72 exceeded the April 2007 

peak in water years 1936, 1938, 1944, and 1956 by about 98, 

32, 7, and 16 percent, respectively (fig. 5). 

The return period of the April 2007 peak discharge was 

determined from the period of record (1973–2007), the twostation comparison method, and the MOVE-1 extended record 

analyses (1936–2007). The peak-flow database indicates that 

discharge is affected to unknown degree by regulation. There 

are 11 dams with small to moderate-size impoundments on 

the river and its tributaries. None are known to be operated for 

flood control, but storage behind these impoundments could 

affect peak flows, particularly for small, frequent floods. The 

two-station comparison method utilized the 72-year peakflow record for the North Nashua River at Leominster station 

(01094500). A generalized skew of 0.32 was specified in the 

PeakFQ analysis on the basis of region skew coefficients 

for southern New Hampshire (Scott Olson, USGS, written 

commun., 2008). A weighted skew of 0.38 computed by 

PeakFQ from the regional skew and the 1936–2007 systematic 

peak-flow record skew was specified in the two-station peakflow analysis.

The return interval of the April 2007 peak discharge 

(3,930 ft3/s) at North Nashua River at Fitchburg determined 

from the 1936–2007 record is about 25 years and is within 

the 95-percent confidence limits for discharges with a 25-year 

return interval (table 4; fig. 6). The two-station comparison 

method produced results similar to those produced by 

the extended record analysis. The period-of-record peakflow analysis (1973–2007) indicates the April 2007 peak 

discharge has nearly a 50-year return interval and is within the 

95-percent confidence limits for discharges with 25- to 100-

year return intervals (table 4).

The flood-insurance study revised in 1991 for the City 

of Fitchburg area (FEMA, 1991) indicates that the April 

2007 peak stage (403.48 ft) was between a 10- and 50-year 

flood stage (fig. 7A). During the April 2007 storm, the river 

remained above the NWS flood stage (401.39 ft) for about 

1 day (fig. 7A) and exceeded the NWS flood stage by about 

2.1 ft at its maximum stage (403.48 ft). The return interval of 

the April 2007 peak discharge determined from the extendedrecord peak-flow analysis made in this study was about 

25 years and greatly exceeded the flood-insurance study 

10-year flood (fig. 7B).

Although the flood-insurance-study stage and the peakflow analysis return period of the April 2007 storm were in 

general agreement, there were appreciable differences between 

the studies. In general, the flood-insurance-study discharge 

and stage became increasingly greater relative to the findings in this study as the return interval increased (table 5). 

Discharges used in the flood-insurance study for the North 

Nashua River interpolated between the downstream corporate 

limits and the confluence with Baker Brook (FEMA, 1991, 

table 1) to estimate flow at the Fitchburg streamflow-gaging 

station were about 54, 130, 144, and 189 percent greater for 

return intervals of 10, 50, 100, and 500 years, respectively, 

than the discharges determined for the same return intervals 

in this study. Converting the flood-insurance study flood stage 

at various return intervals to discharge from the streamflowgaging station stage-discharge rating in place at the time of 

the study (rating 9) also yielded discharges greater than those 

determined in this study except for the 10-year return interval, 

which was smaller. Likewise, converting discharges for various return intervals determined in this study into stage from 

the current stage-discharge rating (12.1) yielded flood stages 

that generally were lower than those shown in the flood profile 

at the streamflow-gaging station (FEMA, 1991, plate 03P, at 

17,360 ft from corporate limit). Flood stage in this study is 

about 1.9 ft higher for a 10-year return interval and about 

2.6, 3.9, and 9.2 ft lower for 50-, 100-, and 500-year return 

intervals, respectively, than the stage reported in the floodinsurance study. Ratings 9 and 12.1 are identical at high flows 

and are estimated above a stage of 401 ft (gage datum of 6 ft).
12 Characteristics of the April 2007 Flood at 10 Streamflow-Gaging Stations in Massachusetts

Figure 5. Observed and estimated annual peak discharge for water years 1936 through 2007 at North Nashua River at 

Fitchburg (01094400), Massachusetts.

500

200

100

5

50

25

10

April 2007 peak

(3,930)

Observed

Annual peak discharge

50

Peak-flow frequency computed from observed 

and estimated annual peak flows

Estimated magnitude of peak flow; 

number is return interval in years 

Estimated 

DISCHARGE, IN CUBIC FEET PER SECOND 

0

2,000

4,000

6,000

8,000

10,000

1930 1940 1950 1960 1970 1980 1990 2000 2010

WATER YEAR

2.5

2.8

3.0

3.3

3.5

3.8

4.0

4.3

4.5

2.5 2.8 3.0 3.3 3.5 3.8 4.0 4.3 4.5

DISCHARGE AT 01094500,

IN LOG CUBIC FEET PER SECOND

Concurrent peaks

MOVE-1 estimated peaks (1936–72)

DISCHARGE AT 01094400, 

IN LOG CUBIC FEET PER SECOND

Figure 4. Relation between observed annual 

peak discharges for water years 1973 through 

2007 at North Nashua River at Fitchburg 

(01094400) and North Nashua River at Leominster 

(01094500), Massachusetts. 
Characteristics of the April 2007 Flood at Selected Streamflow-Gaging Stations 13

Table 4. Magnitude and confidence limits of flood flows at selected return intervals at North Nashua River at Fitchburg (01094400), Massachusetts.

Return 

interval 

(years)

Exceedance 

probability

Estimated magnitude of flood flow, 

in cubic feet per second

Period-of-record analysis 

(1973–2007)

Extended-record analysis (1936–2007) with North Nashua River at Leominster (01094500)

Two-station comparison1 MOVE-1 extended record2

Expected 

peak

95-percent confidence limit Expected 

peak

95-percent confidence limit Expected 

peak

95-percent confidence limit

Lower Upper Lower Upper Lower Upper

5 0.2 2,460 2,170 2,870 2,420 2,150 2,780 2,430 2,160 2,780

10 0.1 2,940 2,560 3,540 3,160 2,750 3,730 3,200 2,790 3,76025 0.04 3,560 3,030 4,450 4,240 3,600 5,220 4,340 3,700 5,31050 0.02 4,030 3,370 5,170 5,180 4,310 6,560 5,340 4,450 6,720

100 0.01 4,500 3,710 5,920 6,240 5,090 8,130 6,470 5,290 8,370200 0.005 4,980 4,050 6,700 7,430 5,940 9,940 7,750 6,220 10,300500 0.002 5,620 4,490 7,790 9,240 7,210 12,800 9,720 7,600 13,400

1 Computed following Bulletin 17B, Appendix 7 guidelines (Interagency Advisory Committee on Water Data, 1982).2 Estimated annual-peak flows 1936–72 using MOVE-1 method (Hirsch, 1982).
14 Characteristics of the April 2007 Flood at 10 Streamflow-Gaging Stations in Massachusetts

Table 5. Comparison of flood stage and discharge at selected return intervals reported in community flood-insurance study to values 

determined in this study for North Nashua River at Fitchburg (01094400), Massachusetts.

[ft, feet NGVD 29; ft3/s, cubic feet per second; --, not determined]

Return 

interval 

(years)

1

Flood-insurance study Present study Difference between present and 

flood-insurance studies

2

Stage 

(ft)

3

Discharge 

table 1 

(ft3/s)

4

Discharge 

rating 12.1 

(ft3/s)

5

Discharge 

(ft3/s)

4

Stage rating 12.1 

(ft)

Stage 

(ft)

Discharge

Table 1 

(ft3/s)

Rating 

(ft3/s)

5 -- -- -- 2,430 402.1 -- -- --

10 400.9 4,920 1,330 3,200 402.8 1.9 -1,720 1,870

25 -- -- -- 4,340 403.8 -- -- --

50 407.1 12,260 8,200 5,340 404.5 -2.6 -6,920 -2,860

100 409.4 15,780 12,400 6,470 405.5 -3.9 -9,310 -5,930

200 -- -- -- 7,750 406.2 -- -- --

500 416.9 28,080 23,000 9,720 407.7 -9.2 -18,360 -13,280

1

 Town of Fitchburg flood-insurance study (FEMA, 1991).

2

 Flood stages at streamflow-gaging station; 17,360 ft from corporate limit on flood profile (FEMA, 1991, pl. 03P).

3

 Flood-insurance-study summary of discharges used in determining flood elevations (FEMA, 1991; table 1, weighted by drainage area ratio between values 

at corporate limit and confluence with Baker Brook).

4

 Stage determined from rating number 12.1; above 400 ft rating is estimated.

5

 Determined from flood-frequency analysis of MOVE-1 extended record.

Peak-flow frequency

Observed peaks (1973–2007) 

95-percent confidence limits

Line color varies according to point data above

Observed and estimated peaks (1936–2007)

10 2

10 3

10 4

10 5

ANNUAL PEAK DISCHARGE, IN CUBIC FEET PER SECOND

99.5 0.5 0.2

ANNUAL EXCEEDANCE PROBABILITY, IN PERCENT

April 2007 peak (3,930)

98 95 90 80 70 50 30 20 10 5 2 1

RETURN INTERVAL, IN YEARS 

1.00 1.02 1.05 1.11 1.25 1.43 2 3.33 5 10 20 50 100 200 500

Figure 6. Peak-flow 

frequency computed from 

observed and estimated 

annual peaks at North Nashua 

River at Fitchburg (01094400), 

Massachusetts.
Characteristics of the April 2007 Flood at Selected Streamflow-Gaging Stations 15

Figure 7. April 15–23, 2007, (A) stage and (B) discharge at North Nashua River at Fitchburg (01094400), Massachusetts.

[NWS, National Weather Service; FIS, flood-insurance study by FEMA (1991)]

0

1,000

2,000

3,000

4,000

5,000

6,000

Computed from extended-record 

peak-flow frequency analysis and 

stage-discharge rating 12.1

Stage reported in FIS

FIS 10-year 

50-year 

25-year 

10-year 

Computed from extended-record 

peak-flow frequency analysis

Peak-flow frequency reported in FIS

DISCHARGE, IN CUBIC FEET PER SECOND 

15 16 17 18 19 20 21 22 23 24

APRIL 2007

396

398

400

404

406

408

410

402 NWS-designated flood stage

FIS 50-year 

FIS 10-year 

FIS 100-year 

 10-year 

50-year

25-year

15 16 17 18 19 20 21 22 23 24

STAGE, IN FEET (NGVD 29)

A. River stage

B. River discharge

100-year
16 Characteristics of the April 2007 Flood at 10 Streamflow-Gaging Stations in Massachusetts

Stillwater River near Sterling–01095220

Annual peak flows recorded at Stillwater River near 

Sterling span a 13-year period, water years 1995 through 

2007. The April 2007 peak discharge (1,850 ft3/s) was the 

highest recorded and exceeded the previously recorded peak 

flow in the 2004 water year by about 35 percent. Annual peak 

flows at Stillwater River near Sterling from 1936 through 

1994 were estimated by MOVE-1 using the 13 years of 

concurrent record with North Nashua River at Leominster 

(01094500); the basin centroid lies about 7.7 mi north of the 

Stillwater River Basin centroid (in this report, “basin centroid” 

refers to the geographic center of the streamflow-gaging 

station drainage basin depicted in figure 1). The MOVE-1 

analysis indicates peak flows at Stillwater River and North 

Nashua River at Leominster are closely related (fig. 8, RMSE 

770 ft3/s). Estimated peak flows for 1936–94 exceeded the 

April 2007 peak in water years 1936, 1938, 1944, 1956, and 

1987 by about 104, 35, 9, 18, and 1 percent, respectively 

(fig. 9).

The return period of the April 2007 peak flow was 

determined by the two-station comparison method and 

analysis of the extended record (1936–2007). The peak-flow 

analysis from the period of record (1995–2007) was not made 

because the record was considered to be of insufficient length. 

Streamflow at Stillwater River near Sterling is considered 

unregulated. The two-station comparison method utilized the 

72-year peak-flow record at North Nashua River at Leominster 

(01094500). A generalized skew of 0.30 was specified in the 

PeakFQ analysis on the basis of the regional skews reported 

for southern New Hampshire and Connecticut. A weighted 

skew of 0.42 computed by PeakFQ from the regional skew 

and the systematic record for the 1936–2007 period was specified in the two-station peak-flow analysis.

The April 2007 peak discharge at Stillwater River near 

Sterling (1,850 ft3/s) has a return interval of 10 to 25 years as 

determined from the 1936–2007 record analysis (fig. 10), but 

is within the 95-percent confidence limits for discharges with 

a 25-year return interval (table 6). Peak-flow analysis determined by the two-station comparison method produced similar 

results, with the exception that the April 2007 peak discharge 

is within the 95-percent confidence limits for discharges with a 

return interval 10 to 25 years (table 6).

The flood-insurance study completed in 1981 for the 

Town of Sterling (FEMA, 1981) indicates that the April 2007 

peak stage (406.15 ft) was between a 50- and 100-year flood 

stage (fig. 11A). Stillwater River at Sterling is not part of the 

NWS Advanced Hydrologic Prediction network; therefore, 

a flood-warning stage is not available. The April 2007 peak 

discharge exceeded the discharge for a 50-year return interval 

flood as determined in the flood-insurance study (fig. 11B), 

whereas it was between a 10- and 25-year flood as determined from the extended record peak-flow analysis made in 

this study.

The flood-insurance-study stage and the peak-flow for 

various return intervals was lower than those determined from 

the extended-record peak-flow analysis made during this study 

(table 7). The extended-record peak-flow analysis values are 

similar to the results of the two-station analysis (table 6). In 

general, discharges for various return intervals used in the 

flood-insurance study at the location of the streamflow-gaging 

station (FEMA, 1981; table 1–Stillwater River at Muddy Pond 

Road) were about 25 percent less than the discharges for the 

same return intervals determined in this study. Converting the 

flood-insurance-study flood stage at various return intervals 

to discharge with the streamflow-gaging station rating 4 also 

yielded discharges that were lower than those determined in 

this study; as a percentage, the differences decreased as the 

return interval increased. Likewise, converting discharges for 

various return intervals determined in this study into stage 

from the stage-discharge rating (4) yielded flood stages that 

were higher than those shown in the flood profile at Muddy 

Pond Road (FEMA, 1981; plate 03P at the corporate limit). 

Flood stages as determined by the extended record analysis 

in this study relative to those reported in the flood-insurance 

study were about 1.4, 1.2, 1.0, and 0.7 ft higher for floods with 

return periods of 10-, 50-, 100-, and 500-years, respectively. 

The flood profile indicates the stage changes rapidly near the 

gage location, particularly for the 500-year return-interval 

flood profile, which may explain some of the differences. 

Rating 4 was used exclusively because it was best defined by 

high-flow measurements, but the rating was still estimated 

above 406 ft (gage datum of 9 ft). 
Characteristics of the April 2007 Flood at Selected Streamflow-Gaging Stations 17

Figure 8. Relation between observed annual peak discharges for water 

years 1995 through 2007 at Stillwater River near Sterling (01095220) and 

North Nashua River at Leominster (01094500), Massachusetts.

Figure 9. Observed and estimated annual peak discharge for water years 1936 through 2007 at Stillwater River near Sterling 

(01095220), Massachusetts.

2.5 2.8 3.0 3.3 3.5 3.8 4.0 4.3 4.5

DISCHARGE AT 01094500,

IN LOG CUBIC FEET PER SECOND

Concurrent peaks

MOVE-1 estimated peaks (1936–94)

DISCHARGE AT 01095220, 

IN LOG CUBIC FEET PER SECOND

2.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

4,500

5,000

1930 1940 1950 1960 1970 1980 1990 2000 2010

WATER YEAR

DISCHARGE, IN CUBIC FEET PER SECOND

500

200

100

5

50

25

10

April 2007 peak

(1,850)

Estimated 

Observed

Annual peak discharge

50 Estimated magnitude of peak flow; 

number is return interval in years

Peak-flow frequency computed from estimated and 

observed annual peak flows.
18 Characteristics of the April 2007 Flood at 10 Streamflow-Gaging Stations in Massachusetts

Figure 10. Peak-flow 

frequency computed from 

observed and estimated 

annual peaks for water years 

1936 through 2007 at Stillwater 

River near Sterling (01095220), 

Massachusetts.

10 2

10 3

10 4

ANNUAL PEAK DISCHARGE, IN CUBIC FEET PER SECOND

ANNUAL EXCEEDANCE PROBABILITY, IN PERCENT

99.5 98 95 90 80 70 50 30 20 10 5 2 1 0.5 0.2

RETURN INTERVAL, IN YEARS 

1.00 1.02 1.05 1.11 1.25 1.43 2 3.33 5 10 20 50 100 200 500

April 2007 peak

(1,850)

Peak-flow frequency

95-percent confidence limits

Observed and estimated peaks

Table 6. Magnitude and confidence limits of flood flows at selected return intervals at Stillwater River near Sterling (01095220), 

Massachusetts.

Return interval 

(years)

Exceedance 

probability

Estimated magnitude of flood flow, 

in cubic feet per second

Extended-record analysis (1936–2007) with North Nashua River at Leominster (01094500)

Two-station comparison1 MOVE-1 extended record2

Expected peak 95-percent confidence limit Expected peak 95-percent confidence limit

Lower Upper Lower Upper

5 0.2 1,160 1,010 1,370 1,150 1,030 1,320

10 0.1 1,520 1,300 1,860 1,520 1,330 1,800

25 0.04 2,070 1,710 2,670 2,090 1,780 2,560

50 0.02 2,550 2,050 3,410 2,590 2,160 3,270

100 0.01 3,090 2,430 4,290 3,160 2,580 4,100

200 0.005 3,710 2,850 5,330 3,820 3,050 5,090

500 0.002 4,670 3,480 7,000 4,830 3,760 6,670

1

 Computed following Bulletin 17B, Appendix 7 guidelines (Interagency Advisory Committee on Water Data, 1982).

2

 Estimated annual-peak flows 1936–94 using MOVE-1 method (Hirsch, 1982).
Characteristics of the April 2007 Flood at Selected Streamflow-Gaging Stations 19

Figure 11. April 15–23, 2007, (A) stage and (B) discharge at Stillwater River near Sterling (01095220), Massachusetts. 

[FIS, flood-insurance study by FEMA (1981)]

FIS 10-year

FIS 50-year

DISCHARGE, IN CUBIC FEET PER SECOND 

15 16 17 18 19 20 21 22 23

APRIL 2007

0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

2,000

2,200

25-year 

10-year 

A. River stage

B. River discharge

Determined from extended-record 

peak-flow frequency analysis

Peak-flow frequency reported in FIS

15 16 17 18 19 20 21 22 23

STAGE, IN FEET (NGVD 29)

402

404

406

408

FIS 10-year

25-year 

10-year FIS 50-year

FIS 100-year

50-year 

100-year 

200-year 

5-year 

FIS 500-year

Determined from extended-record 

peak-flow frequency analysis and 

stage-discharge rating number 4

Stage reported in FIS
20 Characteristics of the April 2007 Flood at 10 Streamflow-Gaging Stations in Massachusetts

Squannacook River near West Groton–01096000

Annual peak flows recorded at Squannacook River near 

West Groton span a 58-year period, water years 1950 through 

2007. The April 2007 peak discharge (4,820 ft3/s) was the 

highest recorded and exceeded the highest previously 

recorded peak flow that occurred in the 1987 water year by 

about 14 percent. Annual peak flows from 1936 through 1949 

were estimated by MOVE-1 using the 58 years of concurrent 

record with North Nashua River at Leominster (01094500); 

the basin centroid lies about 9.2 mi southwest of the 

Squannacook River Basin centroid. The MOVE-1 analysis 

indicates peak flows at Squannacook River and North Nashua 

River at Leominster are closely related (fig. 12; RMSE 

1,513 ft3/s). Estimated peak flows during 1936–49 exceeded 

the April 2007 peak in water years 1936, 1938, and 1944 by 

about 118, 32, and 1 percent, respectively (fig. 13).

The return period of the April 2007 peak flow was 

determined from the period of record (1950–2007) by the 

two-station comparison method and by the extended-record 

(1936–2007) analyses. Streamflow at Squannacook River 

near West Groton is considered unregulated. The two-station 

comparison method utilized the 72-year peak-flow record at 

North Nashua River at Leominster (01094500). A generalized 

skew of 0.30 was specified in the PeakFQ analysis on the basis 

of regional skews reported for southern New Hampshire and 

Connecticut. The computed weighted skew of 0.38 was specified in the two-station peak-flow analysis.

The April 2007 peak discharge (4,820 ft3/s) at 

Squannacook River near West Groton determined from the 

1936–2007 record has a return interval of about 25 years 

(fig. 14) and is within the 95-percent confidence limits for 

discharges with a 25-year return interval (table 8). Peak-flow 

analysis by the two-station comparison method produced 

results similar to those of the extended-record analysis. The 

58-year period-of-record analysis places the April 2007 storm 

close to a 50-year return-interval discharge and within a much 

broader 95-percent confidence range of 25- to 100-year return 

interval (table 8). 

The flood insurance study completed in 1982 for the 

Town of Groton (FEMA, 1982) indicates the April 2007 peak 

stage (252.77 ft) approached a 10-year flood stage (fig. 15A). 

The river stage was above the NWS-designated flood stage 

for nearly a day and peaked about 1.5 ft above the NWS flood 

stage. The return interval of the April 2007 peak discharge 

determined from the extended-record peak-flow analysis made 

in this study was between 10 and 25 years; the 10-year flood 

Table 7. Comparison of flood stage and discharge at selected return intervals reported in community flood-insurance study to values 

determined in this study for Stillwater River near Sterling (01095220), Massachusetts.

[ft, feet NGVD 29; ft3/s, cubic feet per second; --, not determined]

Return interval 

(years)

1

Flood-insurance study Present study Difference between present and 

flood-insurance studies

2

Stage 

(ft)

3

Discharge 

table 1 

(ft3/s)

4

Discharge 

rating 4 

(ft3/s)

5

Discharge 

(ft3/s)

4

Stage rating 4 

(ft)

Stage 

(ft)

Discharge

Table 1 

(ft3/s)

Rating 

(ft3/s)

5 -- -- -- 1,150 405.3 -- -- --

10 404.4 1,100 610 1,520 405.8 1.4 870 420

25 -- -- -- 2,090 406.4 -- -- --

50 405.7 1,960 1,410 2,590 406.9 1.2 1,150 630

100 406.4 2,430 2,060 3,160 407.4 1.0 1,100 730

200 -- -- -- 3,820 407.9 -- -- --

500 407.9 3,620 3,820 4,830 408.6 0.7 1,010 1,210

1

 Town of Sterling flood-insurance study (FEMA, 1981).

2

 Flood stages at streamflow-gaging station; at corporate limit on flood profile (FEMA, 1981, pl. 01P).

3

 Flood-insurance-study summary of discharges used in determining flood elevations (FEMA, 1981; table 1, Stillwater River at Muddy Pond Road).

4

 Stage determined from stage-discharge rating number 4; above 406 ft rating is estimated.

5

 Determined from flood-frequency analysis of MOVE-1 extended record.
Characteristics of the April 2007 Flood at Selected Streamflow-Gaging Stations 21

Figure 12. Relation between observed 

annual peak discharges for water years 1950 

through 2007 at Squannacook River near West 

Groton (01096000) and North Nashua River at 

Leominster (01094500), Massachusetts.

2.5 2.8 3.0 3.3 3.5 3.8 4.0 4.3 4.5

DISCHARGE AT 01094500,

IN LOG CUBIC FEET PER SECOND

Concurrent peaks

MOVE-1 estimated peaks (1936–49)

DISCHARGE AT 01096000, 

IN LOG CUBIC FEET PER SECOND

2.0

2.5

3.0

3.5

4.0

4.5

Figure 13. Observed and estimated annual peak discharge for water years 1936 through 2007 at Squannacook River near 

West Groton (01096000), Massachusetts.

500

200

100

5

50

25

10

April 2007 peak (4,820)

1930 1940 1950 1960 1970 1980 1990 2000 2010

WATER YEAR

DISCHARGE, IN CUBIC FEET PER SECOND

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

Peak-flow frequency computed from observed 

and estimated annual peak flows.

Observed

Annual peak discharge

50

Estimated 

Estimated magnitude of peak flow; 

number is return interval in years 
22 Characteristics of the April 2007 Flood at 10 Streamflow-Gaging Stations in Massachusetts

10

2

10 3

10 4

April 2007 peak (4,820)

Peak-flow frequency

Observed peaks (1950–2007) 

95-percent confidence limits

Line color varies according to point data above

Observed and estimated peaks (1936–2007)

99.5 98 95 90 80 70 50 30 20 10 5 2 1 0.5 0.2

10 5

ANNUAL PEAK DISCHARGE, IN CUBIC FEET PER SECOND

ANNUAL EXCEEDANCE PROBABILITY, IN PERCENT

RETURN INTERVAL, IN YEARS 

1.00 1.02 1.05 1.11 1.25 1.43 2 3.33 5 10 20 50 100 200 500

Figure 14. Peak-flow frequency computed from observed and estimated annual peaks for water 

years 1936 through 2007, Squannacook River near West Groton (01096000), Massachusetts.

discharge reported in the flood-insurance study is close to 

10-year flood calculated in this study (fig. 11B).

The location of flood flows near the streamflow-gaging 

station reported in the flood-insurance study is uncertain 

(FEMA, 1982; table 1). The data row in the table indicates 

discharge in the Squannacook River was calculated at the 

confluence with the Nashua River, but the drainage area given 

at that location (62.8 mi2) is about equal to the drainage area 

at the streamflow-gaging station (63.7 mi2). The drainage 

area 0.2 mi upstream of the confluence is 73.0 mi2, or about 

16 percent more than stated; therefore, no adjustments were 

made to the discharge values reported in table 1. The floodinsurance-study stages and the peak flows at this location 

for various return intervals were slightly greater than those 

determined from the peak-flow analysis made during this 

study. The extended-record peak-flow analysis values are 

similar to the two-station-analysis values (table 8). The 

discharge values determined from the extended-record peakflow analysis in this study are about 2 to 11 percent lower 

than the values given in the flood-insurance study for 10- and 

500-year return intervals, respectively (table 9); the difference 

between discharges increases as the return interval increases. 

Converting the flood-insurance-study flood stage at various return intervals to discharge with the streamflow-gaging 

station rating 22.1 yields discharges that were two or more 

times larger than those determined in this study. Conversely, 

converting discharges for various return intervals determined 

in this study into stage with the same rating yields lower flood 

stages for equivalent return intervals than those shown in the 

flood profile at the gage location (FEMA, 1982; plate 07P 

at 34,600 ft upstream from the confluence with the Nashua 

River). Flood stages in this study for 10-, 50-, 100-, and 

500-year return intervals are about 1.8, 3.2, 3.7, and 5.1 ft 

lower, respectively, than in the flood-insurance study. Rating 

22.1 was used exclusively because it was best defined by highflow measurements, but the rating is still estimated above 

252 ft (gage datum of 8.2 ft).
Characteristics of the April 2007 Flood at Selected Streamflow-Gaging Stations 23

Table 8. Magnitude and confidence limits of flood flows at selected return intervals at Squannacook River near West Groton (01096000), Massachusetts.

Return 

interval 

(years)

Exceedance 

probability

Estimated magnitude of flood flow, 

in cubic feet per second

Period-of-record analysis 

(1950–2007)

Extended-record analysis (1936–2007) with North Nashua River at Leominster (01094500)

Two-station comparison1 MOVE-1 extended record2

Expected 

peak

95-percent confidence limit Expected 

peak

95-percent confidence limit Expected 

peak

95-percent confidence limit

Lower Upper Lower Upper Lower Upper

5 0.2 2,380 2,070 2,790 2,440 2,110 2,880 2,460 2,130 2,910

10 0.1 3,060 2,620 3,710 3,390 2,880 4,150 3,460 2,930 4,22025 0.04 4,020 3,360 5,060 4,920 4,040 6,310 5,050 4,140 6,48050 0.02 4,800 3,930 6,200 6,320 5,060 8,390 6,530 5,220 8,670

100 0.01 5,620 4,530 7,440 7,970 6,230 11,000 8,280 6,460 11,400200 0.005 6,490 5,150 8,790 9,920 7,570 14,100 10,400 7,900 14,700500 0.002 7,730 6,000 10,800 13,000 9,650 19,300 13,700 10,100 20,200

1 Computed following Bulletin 17B, Appendix 7 guidelines (Interagency Advisory Committee on Water Data, 1982).2 Estimated annual-peak flows 1936–49 using MOVE-1 method (Hirsch, 1982).
24 Characteristics of the April 2007 Flood at 10 Streamflow-Gaging Stations in Massachusetts

Figure 15. April 15–23, 2007, (A) stage and (B) discharge at Squannacook River near West Groton (01096000), Massachusetts. 

[NWS, National Weather Service; FIS, flood-insurance study by FEMA (1982)] DISCHARGE, IN CUBIC FEET PER SECOND 

15 16 17 18 19 20 21 22

APRIL 2007

1,000

2,000

3,000

4,000

5,000

6,000

0

25-year 

A. River stage

B. River discharge

5-year 

Computed from extended-record 

peak-flow frequency analysis

Peak-flow frequency reported in FIS

Computed from extended-record 

peak-flow frequency analysis and 

stage-discharge rating number 4

Stage reported in FIS

NWS-designated flood stage

15 16 17 18 19 20 21 22

STAGE, IN FEET (NGVD 29)

247

249

251

253

255

245

23

10-year 

FIS 10-year 25-year

10-year 

50-year

5-year 

23

FIS 10-year 
Characteristics of the April 2007 Flood at Selected Streamflow-Gaging Stations 25

Nissitissit River at Pepperell–01096503

The streamflow-gaging station on the Nissitissit River at 

Pepperell has been in operation since March 2006. To characterize the April 2007 storm, annual peak flows for the 1936 

through 2006 water years were estimated using MOVE-1 and 

the Squannacook River near West Groton (01096000) streamflow-gaging station. The Squannacook River station was 

considered the most appropriate for estimating the Nissitissit 

River peak flows because the basins are nearly equal in size at 

their gages (63.7 and 59.6 mi2, respectively) and the basin centroids are about 6.4 mi from each other (fig. 1). Note that part 

of the Squannacook River peak-flow record (1936–49) was 

estimated from the North Nashua River at Leominster station 

(01094500) record. The basin centroid of North Nashua River 

at Leominster lies about 14.5 mi southwest of the centroid of 

the Nissitissit River Basin. The available record at the 

Nissitissit River station limited the MOVE-1 analysis to five 

independent high-flow events in water years 2006 through 

2008, which included the April 2007 peak (fig. 16). Three of 

the high flow events used in this analysis were from the 2008 

water year, and although these data are provisional at the 

time of the analysis, a preliminary review of the data indicated that they were appropriate for use. Annual peak flows at 

Squannacook River near West Groton span a 58-year period; 

water years 1950 through 2007. Peak flows for water years 

1936 through 1949 were estimated from a MOVE-1 analysis 

with the North Nashua River at Leominster station (01094500) 

(see Squannacook River peak-flow analysis for details). 

Estimated annual peak flows from 1936–2006 exceeded 

the April 2007 peak (3,700 ft3/s) in water years 1936, 1938, 

and 1944, by about 143, 44, and 10 percent, respectively 

(fig. 17). The annual peak discharge in water years 1956, 

1987, and 2004 approached the April 2007 peak discharge. 

The return interval of the April 2007 peak discharge was 

computed by (1) peak-flow frequency analysis of observed 

and estimated annual peak flows, and (2) regional regression 

equations developed for New Hampshire. Peak-flow frequency 

analysis of the observed and estimated annual peaks indicates 

the April 2007 peak has a return interval between 10 and 

25 years and is within the 95-percent confidence limits for 

these return intervals (table 10, fig. 18). The New Hampshire 

regional regression equations place the April 2007 peak 

discharge near a 25-year return-interval discharge. Although 

these two methods produced comparable results for the 

April 2007 storm, the differences increase as the discharge 

increases (less frequent events); for example, storm discharges 

for 100- and 500-year return intervals calculated using the 

New Hampshire equations are about 27 and 41 percent less, 

respectively, than flows calculated by the peak-flow frequency 

analysis of mostly estimated annual peak-flow records. 

Table 9. Comparison of flood stage and discharge at selected return intervals reported in community flood-insurance study to 

values determined in this study for Squannacook River near West Groton (0109600), Massachusetts.

[ft, feet in NVGD 29; ft3/s, cubic feet per second; --, not determined]

Return 

interval 

(years)

1

Flood-insurance study Present study Difference between present and 

flood-insurance studies

2

Stage 

(ft)

3

Discharge 

table 1 

(ft3/s)

4

Discharge 

rating 4 

(ft3/s)

5

Discharge 

(ft3/s)

3

Stage rating 4

(ft)

Stage 

(ft)

Discharge

Table 1 

(ft3/s)

Rating 

(ft3/s)

5 -- -- -- 2,460 251.2 -- -- --

10 253.8 3,540 6,990 3,460 252.0 -1.8 -3,530 -80

25 -- -- -- 5,050 252.9 -- -- --

50 256.8 6,880 17,100 6,530 253.6 -3.2 -10,570 -350

100 258.0 8,840 21,800 8,280 254.3 -3.7 -13,520 -560

200 -- -- -- 10,400 255.0 -- -- --

500 261.1 15,160 30,000 13,700 256.0 -5.1 -16,300 -1,460

1

 Town of Groton flood-insurance study (FEMA, 1982).

2

 Flood stages at streamflow-gaging station; 34,600 ft above confluence with Nashua River on flood profile (FEMA, 1982; pl. 07P).

3

 Flood-insurance-study summary of discharges used in determining flood elevations (FEMA, 1982; table 1, at confluence with Nashua River).

4

 Stage determined from stage-discharge rating number 22.1; above 252 ft rating is estimated.

5

 Determined from flood-frequency analysis of MOVE-1 extended record.
26 Characteristics of the April 2007 Flood at 10 Streamflow-Gaging Stations in Massachusetts

Figure 16. Relation between observed 

high flows for water years 2006 through 2007 

at Nissitissit River at Pepperell (01096503) 

and Squannacook River near West Groton 

(01096000), Massachusetts.

DISCHARGE AT 01096000,

IN LOG CUBIC FEET PER SECOND

Concurrent high flows

MOVE-1 estimated peaks (1936–06)

DISCHARGE AT 01096503, 

IN LOG CUBIC FEET PER SECOND

2.0

2.5

3.0

3.5

4.0

4.5

2.0 2.5 3.0 3.5 4.0 4.5

500

200

100

5

50

25

10

April 2007 peak

(3,700)

1930 1940 1950 1960 1970 1980 1990 2000 2010

WATER YEAR

DISCHARGE, IN CUBIC FEET PER SECOND

Observed

Annual peak discharge

Estimated

50

Peak-flow frequency computed from observed 

and estimated annual peak flows

Estimated magnitude of peak flow; 

number is return interval in years 

0

2,000

4,000

6,000

8,000

10,000

12,000

Figure 17. Observed and estimated annual peak discharge for water years 1936 through 2007 at Nissitissit River at Pepperell 

(01096503), Massachusetts.
Characteristics of the April 2007 Flood at Selected Streamflow-Gaging Stations 27

Table 10. Magnitude and confidence limits of flood flows at selected return intervals at 

Nissitissit River at Pepperell (01096503), Massachusetts.

[--, not determined]

Return 

interval 

(years) 

Exceedance 

probability

Estimated magnitude of flood flow, 

in cubic feet per second

NH regional 

regression 

equation1

MOVE-1 extended-record analysis (1936–2007) with 

Squannacook River near West Groton (01096000)2

Expected peak 95-percent confidence limit

Lower Upper

5 0.2 2,200 2,000 1,730 2,380

10 0.1 2,830 2,840 2,390 3,490

25 0.04 3,640 4,200 3,420 5,420

50 0.02 4,140 5,460 4,340 7,310

100 0.01 5,070 6,980 5,400 9,670

200 0.005 -- 8,780 6,640 12,600

500 0.002 6,880 11,700 8,580 17,500

1

 Olson, 2009.

2

 Estimated annual-peak flows 1936–2006 using MOVE-1 method (Hirsch, 1982).

ANNUAL PEAK DISCHARGE, IN CUBIC FEET PER SECOND

ANNUAL EXCEEDANCE PROBABILITY, IN PERCENT

April 2007 peak

(3,700)

10

2

10 3

10 4

10 5

RETURN INTERVAL, IN YEARS 

1.00 1.02 1.05 1.11 1.25 1.43 2 3.33 5 10 20 50 100 200 500

99.5 98 95 90 80 70 50 30 20 10 5 2 1 0.5 0.2

Calculated by the New Hampshire regional regression equation (Olson, 2009)

Peak-flow frequency

95-percent confidence limits

Observed and estimated peaks (1936–2007)

Figure 18. Peak-flow frequency computed from observed and estimated annual peaks at Nissitissit 

River at Pepperell (01096503), Massachusetts.
28 Characteristics of the April 2007 Flood at 10 Streamflow-Gaging Stations in Massachusetts

Figure 19. April 15–23, 2007, (A) stage and (B) discharge at Nissitissit River at Pepperell (01096503), Massachusetts. 

[FIS, flood-insurance study by FEMA (1993)] DISCHARGE, IN CUBIC FEET PER SECOND 

APRIL 2007

0

1,000

2,000

3,000

4,000

5,000

15 16 17 18 19 20 21 22 23 24

STAGE, IN FEET (NGVD 29)

175

177

179

181

183

185

15 16 17 18 19 20 21 22 23 24

A. River stage

B. River discharge

FIS 10-year 

25-year

10-year 

5-year 

FIS 10-year 

FIS 50-year 

FIS 100-year 

25-year 

5-year 

10-year 

FIS 50-year 

FIS 100-year 

Computed from extended-record 

peak-flow frequency analysis

Peak-flow frequency reported in FIS

Computed from extended-record 

peak-flow frequency analysis and 

stage-discharge rating number 2

Stage reported in FIS
Characteristics of the April 2007 Flood at Selected Streamflow-Gaging Stations 29

The flood-insurance study revised in 1993 for the Town 

of Pepperell (FEMA, 1993) indicates the April 2007 peak 

stage (183.26 ft) approached a 50-year flood stage (fig. 19A). 

The Nissitissit River streamflow-gaging station is not part of 

the NWS Advanced Hydrologic Prediction network; therefore, 

a flood-warning stage is not available. The return interval of 

the April 2007 peak discharge was about equal to a 100-year 

return interval discharge reported in the flood-insurance study 

(fig. 19B), whereas the extended-record peak-flow analysis made in this study placed the April 2007 peak discharge 

between 10- and 25-year event.

The Town of Pepperell flood-insurance study (FEMA, 

1993) reports the discharge frequency for the Nissitissit River 

was determined from regional equations developed by 

Johnson and Tasker (1974b) for Vermont streams; discharges 

of 1,500, 2,640, 3,640, and 5,000 ft3/s were reported for 

10-, 50-, 100- and 500-year return intervals, respectively 

(FEMA, 1993; table 1). These discharges are between 27 and 

47 percent less than discharges determined from the peak-flow 

equations recently developed for New Hampshire (Olson, 

2009) for similar return intervals (the difference decreased as 

discharge increased). The flood-insurance-study discharges 

for the Nissitissit River are 47 to 62 percent less than the 

discharges for similar return intervals determined from peakflow frequency analysis of mostly estimated annual peaks 

(table 11). 

Converting the flood-insurance-study flood stage at 

various return intervals to discharge from the station stagedischarge rating 2 also yielded discharges that were 17 to 

39 percent less than those determined for similar return 

intervals in this study. Conversely, converting discharges for 

various return intervals determined in this study into stage by 

using rating 2 yielded higher flood stages for equivalent return 

intervals than those shown in the flood profile at the gage location (FEMA, 1993; plate 05P at 4,800 ft upstream from the 

confluence with the Nashua River). Flood stages in this study 

for 10-, 50-, 100-, and 500-year return intervals are about 0.6, 

1.6, 1.9, and 1.3 ft higher, respectively, than in the flood-insurance study. The rating 2 stage-discharge relation is estimated 

above 181 ft (gage datum of 7 ft).

Table 11. Comparison of flood stage and discharge at selected return intervals reported in community flood-insurance study to 

values determined in this study for Nissitissit River near Pepperell (01096503), Massachusetts.

[ft, feet NGVD 29; ft3/s, cubic feet per second; --, not determined]

Return 

interval 

(years)

1

Flood-insurance study Present study Difference between present and 

flood-insurance studies

2

Stage 

(ft)

3

Discharge 

table 1 

(ft3/s)

4

Discharge 

rating 2 

(ft3/s)

5

Discharge 

(ft3/s)

4

Stage rating 2

(ft)

Stage 

(ft)

Discharge

Table 1 

(ft3/s)

Rating 

(ft3/s)

5 -- -- -- 2,000 181.5 -- -- --

10 181.7 1,497 2,350 2,840 182.3 0.6 1,343 490

25 -- -- -- 4,200 183.8 -- -- --

50 183.4 2,642 3,800 5,460 185.0 1.6 2,818 1,660

100 184.3 3,642 4,700 6,980 186.2 1.9 3,338 2,280

200 -- -- -- 8,780 187.6 -- -- --

500 187.3 5,000 7,100 11,700 188.6 1.3 6,700 4,600

1

 Town of Pepperell flood-insurance study (FEMA, 1993).

2

 Flood stages at streamflow-gaging station; 4,800,000 ft above confluence with Nashua River on flood profile (FEMA, 1993; pl. 05P).

3

 Flood-insurance-study summary of discharges used in determining flood elevations (FEMA, 1993; table 1, at confluence with Nashoba River).

4

 Stage determined from stage-discharge rating number 2; above 181 ft rating is estimated.

5

 Determined from flood-frequency analysis of MOVE-1 extended record.
30 Characteristics of the April 2007 Flood at 10 Streamflow-Gaging Stations in Massachusetts

Merrimack River at Lowell–01100000

The streamflow-gaging station on the Merrimack River 

below the confluence with the Concord River at Lowell has 

continuous flow records dating back to 1924. The flow records 

represent periods of various regulation, which complicate the 

peak-flow frequency analysis. Following major floods in 1936 

and 1938, the U.S. Army Corps of Engineers (USACE) constructed five flood-control reservoirs in the Merrimack River 

Basin that were completed between 1941 and 1963–Hopkinton 

Dam, Everett Dam, Blackwater Dam, Edward MacDowell 

Dam, and Franklin Falls Dam. There are also a number of 

water-supply reservoirs, particularly in the Massachusetts 

portion of the basin, that have been in operation since the early 

1900s or earlier. Although these supply reservoirs are not operated for flood-control purposes, they can still affect high flows 

in the lower Merrimack River, particularly for small, more 

frequent floods.

The USACE flood-control reservoirs were constructed on 

large tributaries in New Hampshire and are capable of storing 

from 2.8 to 6.8 in. of runoff upstream from the reservoir. 

Combined drainage area to these reservoirs affects about 

36 percent of the total basin area above the Lowell streamflow-gaging station. The total available storage in these 

reservoirs represents about 1.9 in. of runoff over the entire 

basin. Storage in the flood-control reservoirs several days 

prior to the April 17, 2007, peak flow was fully available and 

was between 25 and 70 percent utilized with about 57 percent 

of the combined total storage utilized at the time of the peak 

(Paul Marinelli, USACE, written commun., 2008). 

The April 17, 2007, peak discharge (89,900 ft3/s, stage 

63.27 ft) was the second highest recorded since the five 

USACE flood-control reservoirs became fully operational in 

1964 (fig. 20). Peak discharges after flood-control reservoirs 

were built exceeded the April 2007 peak on May 15, 2006, 

(96,400 ft3/s, stage 64.02 ft) by about 7 percent and was about 

equalled in April 1987 (84,700 ft3/s, stage 62.34 ft). Prior to 

Figure 20. Observed and estimated annual peak discharge for water years 1924 through 2007 at Merrimack River at Lowell 

(01100000), Massachusetts.

0

20,000

40,000

60,000

80,000

100,000

120,000

140,000

160,000

180,000

200,000

1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

DISCHARGE, IN CUBIC FEET PER SECOND 

Blackwater Dam46,000 acre-feetFranklin Falls Dam150,600 acre-feetEdward MacDowell Dam12,650 acre-feetEverett Dam161,600 acre-feetHopkinton Dam91,500 acre-feet

Historic peak

of 1852

Annual peak discharge

WATER YEAR

April 2007 peak

(89,800)

500

200

100

5

50

25

10

50

U.S. Army Corps of Engineers 

flood-control reservoir completion date

Peak-flow frequency computed from the annual peak flows 

following completion of flood-control reservoirs, 1964–07 and 

adjusted 1936 and 1938 peak flows with flood control.

Estimated magnitude of peak flow; 

number is return interval in years 

U.S. Army Corps of 

Engineers estimated 

peak with flood control
Characteristics of the April 2007 Flood at Selected Streamflow-Gaging Stations 31

the completion of the flood-control reservoirs, the recorded 

peak discharges in 1936 (173,000 ft3/s, stage 73.58 ft) and in 

1938 (121,100 ft3/s, stage 72.75 ft) exceeded the April 2007 

peak discharge by about 92 and 35 percent, respectively. If the 

flood-control reservoirs had been in operation, the USACE 

estimates that the peak flows in 1936 and 1938 would have 

been 113,000 and 66,000 ft3/s, respectively (Paul Marinelli, 

USACE, written commun., 2008). The adjusted 1936 peak discharge still would have exceeded the April 2007 peak by about 

26 percent, but the adjusted 1938 peak discharge would have 

been about 27 percent less than the April 2007 peak discharge.

Peak-flow analysis of the station record is complicated by 

three distinct periods of record with differing characteristics 

that affect peak flows—record from 1924 through 1940 represents pre-flood control flows, the record from 1940 through 

1963 represents a transition as reservoirs were completed and 

brought into operation, and the record from 1964 to present 

represents present flood-controlled flows. As such, estimating the magnitude of floods from the period of record is not 

appropriate because of the changes in flood control over that 

time. Further, following completion of the flood-control reservoirs (1964–2007) peak flows have been influenced by their 

operation. For example, the available storage may be more or 

less utilized in anticipation of forecast predictions. Because 

of these factors, the estimates of peak flow magnitudes for 

various return periods are subject to a high degree of uncertainty. To quantify the possible range in flow magnitude for 

various return intervals, the peak-flow frequency analysis was 

made using the period-of-record data and data representing 

estimated and observed annual peaks with flood control for the 

period 1936–2007.

The period-of-record data (1924–2007) are heavily influenced by the unregulated peak discharges in 1936 and 1938, 

which are markedly higher than other peaks over the period 

of record. Data for the period with flood control (1964–2007) 

were combined with the peaks of 1936 and 1938 adjusted by 

the USACE as if the present flood-control reservoir system 

had been in operation. These two peaks were treated as historical peaks in the peak-flow frequency analysis. The peak-flow 

frequency analysis of historical peaks generally improves the 

frequency distribution of the systematic record by accounting for major floods and the intervening period of no data. 

The result of this analysis is dependent on the accuracy of 

the adjusted 1936 peak discharge and, to a lesser extent, the 

1938 peak because it was exceeded by three other peaks in the 

record analyzed. Data representing the 17-year period of preflood control (1924–1940) also were examined, but the short 

record and the occurrence of the two largest peaks in the past 

century within this record greatly leverage the frequency distribution and cause misleading results. Therefore, these results 

were not reported. The magnitudes of peak flows for various 

return intervals were determined using skews calculated from 

the systematic record. Regional skew values were not considered appropriate because of the size of the basin, the period of 

record available for the analysis, and the extent of regulation 

in the basin.

The Merrimack River peak flow for the April 2007 storm 

(89,800 ft3/s) has a return interval of about 25 years, as determined from the period-of-record analysis, and was within the 

95-percent confidence limits for this return interval (table 12; 

fig. 21). The analysis representing post-flood-control data and 

the adjusted 1936 and 1938 peak discharges treated as historical peaks indicates the April 2007 peak flow was between a 

50- and 100-year return interval and was within the 95-percent 

confidence limits for a 50-to 200-year return interval (table 12; 

fig. 21). 

Additional peak-flow frequency analysis was made using 

a continuous record of peak flow from 1936 through 2007 by 

estimating the pre-flood-control period peaks (1936–1964) 

with flood control. The pre-flood-control peaks were adjusted 

by a MOVE-1 analysis of the post-flood-control data (1964–

2007) to concurrent peak-flow records at three streamflowgaging stations distributed throughout the Merrimack River 

Basin—Nashua River near Leominster, Mass. (01094500), 

Pemigewasset River at Plymouth, N.H. (01076500), and 

Smith River near Bristol, N.H. (01078000). These basins 

are in the southwestern, northern, and northwestern part 

of the Merrimack River Basin, respectively. The estimated 

adjusted peaks varied by station, but a weighted estimate 

using the records from the North Nashua and Smith River 

stations yielded the smallest RMSE of estimated peak flows 

to observed peak flows for the 1964–2007 period. The 1936 

and 1938 peaks estimated by this technique were about 6 and 

34 percent greater than the adjusted peaks estimated by the 

USACE, respectively, and about 31 and 27 percent less than 

the recorded peaks, respectively. The estimated 1938 peak was 

generally in agreement with comparable peaks in 1987, 2006, 

and 2007; however, the agreement depended on the station 

used in the analysis. This result suggests that the flood-control 

reservoir storage was fully utilized in the USACE adjusted 

1938 peak, a condition that may not be realized in practice 

because reservoir operators may reserve some storage in 

anticipation of additional precipitation. Peak-flow analysis of 

the extended record (1936–1963) with flood control is subject 

to a large degree of uncertainty. The return interval of the April 

2007 peak discharge, as determined from the extended- and 

observed-record data with flood control (1936–2007), is close 

to a 50-year flood and is within the 95-percent confidence 

limits for a 25- to 50-year flood. 

Review of the peak-flow database and station historical file revealed that the stage-discharge rating in place at the 

time of the 1936 flood may have underrepresented the peak 

discharge. Correspondence between the USGS and the then 

Proprietors of Locks and Canals in Lowell, Mass., and the 

USACE indicates that a jetty in the river at the time of the 

flood may have been washed out or partially washed out during the flood. As a result, the stage-discharge rating may have 

shifted by as much as 2 ft on the basis of the rating developed 

after the jetty was removed; the peak flow determined from 

this rating would be about 190,000 ft3/s, or about 11 percent greater than the recorded peak of 1936. No adjustments 

were made to the peak-flow database, however, because the 
32 Characteristics of the April 2007 Flood at 10 Streamflow-Gaging Stations in Massachusetts Table 12. Magnitude and confidence limits of flood flows at selected return intervals at Merrimack River at Lowell (01100000), Massachusetts. Return interval (years) Exceedance probability Estimated magnitude of flood flow, in cubic feet per second

1Period-of-record analysis 

(1924–2007)

Analysis of data representing flood control

1936, 21938, 1964–2007 3MOVE-1 extended record (1936–2007)

Expected 

peak

95-percent confidence limit Expected 

peak

95-percent confidence limit Expected 

peak

95-percent confidence limit

Lower Upper Lower Upper Lower Upper

5 0.2 59,100 54,700 64,400 53,300 49,000 59,100 53,600 49,700 58,500

10 0.1 73,000 66,700 81,300 62,500 56,600 70,900 64,400 59,000 71,60025 0.04 93,400 83,600 107,000 74,600 66,400 87,200 79,300 71,300 90,60050 0.02 111,000 97,600 129,000 84,000 73,700 100,000 91,400 81,100 106,000

100 0.01 130,000 113,000 155,000 93,800 81,100 114,000 104,000 91,300 124,000200 0.005 151,000 130,000 184,000 104,000 88,800 130,000 118,000 102,000 143,000500 0.002 184,000 154,000 230,000 118,000 99,200 151,000 138,000 117,000 171,000

1 Represents periods of no flood control (1924–40), transitional as flood-control reservoirs were built (1941–63), and post-flood control (1964–2007).2 Estimated 1936 and 1938 peak flows adjusted for flood control by the U.S. Army Corps of Engineers treated as historical peaks flood-frequency analysis.3 Weighted estimate of 1936–63 peak flows with flood control using Nashua River near Leominster, Mass. (01094500), Pemigewasset River at Plymouth, N.H. (01076500), and Smith River near Bristol, 

N.H. (01078000).
Characteristics of the April 2007 Flood at Selected Streamflow-Gaging Stations 33

Figure 21. Peak-flow frequency computed from observed and estimated annual peaks for water 

years 1924 through 2007 at Merrimack River at Lowell (01100000), Massachusetts.

information was not sufficient to make this change permanent. 

Assuming an 11-percent increase in the USACE adjusted peak 

flow would cause the peak flows to increase from about 1 to 

6 percent for 10- and 500-year return-interval peaks, respectively. Accounting for this change would still place the April 

2007 peak discharge at about a 50-year return interval and 

within the 95-percent confidence limits of peak discharges 

for 25- and 100-year return intervals as determined by postflood-control peaks (1964–2007) and historical 1936 and 1938 

adjusted peaks.

The flood-insurance study revised in 1992 for the City 

of Lowell (FEMA, 1992) indicates the April 2007 peak stage 

(63.27 ft) was about 6 ft lower than the 50-year flood stage 

(fig. 22A). The river stage was above the NWS-designated 

flood stage (57.18 ft) for nearly 5 days and peaked about 6.1 ft 

above the NWS flood stage on April 17. The return interval of 

the April 2007 peak discharge determined from the peak-flow 

frequency analysis made from the post-flood-control annual 

peak-flow record and the 1936 and 1938 peaks flows adjusted 

for flood control was between a 50- and 100-year flood and 

was about equal to a 50-year discharge reported in the floodinsurance study (fig. 22B).

The City of Lowell flood-insurance study (FEMA, 1992) 

reports the discharge frequency for the Merrimack River about 

8,300 ft downstream from the streamflow-gaging station of 

58,000; 90,000; 111,000; and 156,000 ft3/s for the 10-, 50-, 

100- and 500-year return intervals, respectively (table 13). 

These discharges are about 7 percent less at a 10-year return 

interval to about 32 percent greater at a 500-year return interval than discharges computed from the peak-flow frequency 

analysis using post-flood-control annual peaks and adjusted 

1936 and 1938 peaks treated as historical peaks.

Discharges and stage used in the flood-insurance study 

are comparable to the discharges and stage (as determined 

from the present rating, 25) determined in this study 

(table 13). Stages at the streamflow-gaging stations under the 

present stage-discharge rating are about 1.4 and 0.2 ft higher 

than the flood-insurance flood stages for flood flows with a 

10- and 50-year return interval and about 1.2 and 3.2 ft lower 

for flood flows with a 100- and 500-year return interval 

(table 13). The stage-discharge relation is estimated above 

65 ft (gage datum of about 60 ft).

April 2007 peak 

(89,800)

104

105

106

ANNUAL PEAK DISCHARGE, IN CUBIC FEET PER SECOND

99.5 0.5 0.2

ANNUAL EXCEEDANCE PROBABILITY, IN PERCENT

98 95 90 80 70 50 30 20 10 5 2 1

RETURN INTERVAL, IN YEARS 

1.00 1.02 1.05 1.11 1.25 1.43 2 3.33 5 10 20 50 100 200 500

Peak-flow frequency

Observed (1924–2007) peaks

95-percent confidence limits

Line color varies according to point data above

Observed peaks with flood control (1964–2007) and 

adjusted 1936 and 1938 peaks treated as historical peaks
34 Characteristics of the April 2007 Flood at 10 Streamflow-Gaging Stations in Massachusetts DISCHARGE, IN CUBIC FEET PER SECOND 

A. River stage

B. River discharge

Computed from extended-record 

peak-flow frequency analysis and 

stage-discharge rating number 25

Stage reported in FIS

Computed from extended-record post-flood 

control peak-flow frequency analysis

Peak-flow frequency reported in FIS

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

90,000

100,000

15 16 17 18 19 20 21 22 23 24

APRIL 2007

5-year 

10-year 

25-year 

50-year 

100-year 

FIS 10-year

FIS 50-year

15 16 17 18 19 20 21 22 23 24

STAGE, IN FEET (NGVD 29)

50

55

60

65

70

FIS 500-year

FIS 50-year

FIS 10-year

FIS 100-year

NWS-designated flood stage

5-year 

10-year 

25-year 

50-year 

100-year 

500-year 

Figure 22. April 15–23, 2007, (A) stage and (B) discharge at Merrimack River at Lowell (01100000), Massachusetts. 

[NWS, National Weather Service; FIS, flood insurance study by FEMA (1992)]
Characteristics of the April 2007 Flood at Selected Streamflow-Gaging Stations 35

Spicket River near Methuen–01100561

The streamflow-gaging station on the Spicket River near 

Methuen has been in operation since March 2001, limiting the 

annual peak flow record to 7 years. In addition, peak flows 

were estimated in 2 of the years (2001–02), and all peaks are 

subject to an unknown degree of regulation from a reservoir in 

the basin making estimates of flood magnitudes difficult and 

uncertain. Two methods were used in the peak-flow frequency 

analysis—(1) estimation of annual series of peak flows from 

two nearby index sites and (2) computation by regionalized 

regression equations developed for New Hampshire.

In the first method, peak flows for 1936 through 2000 

were estimated by MOVE-1 using concurrent peak-flow 

records at Spicket River with index stations at North Nashua 

River near Leominster (01094500) and Ipswich River at South 

Middleton (01101500). The centroids of the North Nashua 

River and Ipswich River Basins are about 35 mi southwest and 

20 mi south of the Spicket River Basin centroid, respectively. 

Estimated peak discharges could vary considerably from the 

observed (fig. 23); the RMSEs of the estimated discharge 

determined from the North Nashua River and Ipswich River 

station records were 691 and 334 ft3/s, respectively. Although 

the RMSE of estimated peaks from Ipswich River was about 

half that determined from North Nashua River the Ipswich 

River is not necessarily a more representative index station. 

For example, although the meterologic conditions during the 

period of concurrent record may be similar, this condition may 

not hold true in the long-term. The uncertainties associated 

with the estimated peaks are large because of an unknown 

degree of regulation and the differences between estimated 

peaks determined from different index stations are large 

(reflecting that the meteorologic conditions differ between 

basins). Therefore, the results of this analysis should be 

viewed with the uncertainties of these estimates in mind.

The recorded April 2007 peak discharge (1,360 ft3/s) 

was the second highest annual peak recorded during the 

7-year period of record; the 2006 water year peak (2,480 ft3/s) 

exceeded the 2007 peak (fig. 24) by about 82 percent. The 

estimated peak flows (1936–2000) determined from the North 

Nashua River record exceeded the April 2007 peak seven 

times, most notably in 1936, 1938, 1944, and 1956 by about 

310, 130, 76, and 96 percent, respectively. The estimated peak 

flows (1936–2000) determined from the Ipswich River record 

exceeded the April 2007 peak twice (1936 and 1987), but 

only by small amounts (about 3 and 12 percent, respectively). 

These differences underscore the uncertainty in the estimated 

peaks and the possible range of discharges that might have 

occurred during this period.

The Spicket River near Methuen April 2007 peak discharge was slightly greater than the discharge for a 5-year 

flood flow determined by the regional equations for New 

Hampshire (table 14; fig. 25). The April 2007 flood flow was 

about midway between a 5- and 10-year return interval for 

discharges computed from records extended with the North 

Nashua River data and midway between a 10- and 25-year 

Table 13. Comparison of flood stage and discharge at selected return intervals reported in community flood-insurance study to 

values determined in this study for Merrimack River at Lowell (01100000), Massachusetts.

[ft, feet in NVGD 29; ft3/s, cubic feet per second; --, not determined]

Return 

interval 

(years)

1

Flood-insurance study Present study Difference between present and 

flood-insurance studies

2

Stage 

(ft)

3

Discharge 

table 1 

(ft3/s)

4

Discharge 

rating 25 

(ft3/s)

5

Discharge 

(ft3/s)

3

Stage 

rating 25

(ft)

Stage 

(ft)

Discharge

Table 1 

(ft3/s)

Rating 

(ft3/s)

5 -- -- -- 53,300 58.4 -- -- --

10 58.4 58,000 53,300 62,500 59.8 1.4 4,500 9,200

25 -- -- -- 74,600 61.5 -- -- --

50 62.5 90,000 83,700 84,000 62.7 0.2 -6,000 300

100 65.0 111,000 104,000 93,800 63.8 -1.2 -17,200 -10,200

200 -- -- -- 104,000 65.0 -- -- --

500 69.6 156,000 152,000 118,000 66.4 -3.2 -38,000 -34,000

1

 City of Lowell flood-insurance study (FEMA, 1992).

2

 Flood stages at streamflow-gaging station; 8,300 ft from corporate limit on flood profile (FEMA, 1992; pl. 01P).

3

 Flood-insurance-study summary of discharges used in determining flood elevations (FEMA, 1992; table 1, at corporate limit).

4

 Stage determined from stage-discharge rating number 25; above 65 ft rating is estimated.

5

 Determined from post-flood-contol flood-frequency analysis (1964–2007) and 1936 and 1938 adjusted peaks.
36 Characteristics of the April 2007 Flood at 10 Streamflow-Gaging Stations in Massachusetts

DISCHARGE AT 01094500 AND 01101500,

IN LOG CUBIC FEET PER SECOND

Concurrent peaks (2001–2007)

MOVE-1 estimated peaks (1936–2000)

DISCHARGE AT 01100561, 

IN LOG CUBIC FEET PER SECOND

01094500

01101500 

01094500

01101500 

2.0

2.5

3.0

3.5

4.0

2.0 2.5 3.0 3.5 4.0 4.5

Figure 23. Relation between observed annual 

peak discharges for water years 2001 through 

2007 at Spicket River near Methuen (0110561) 

and Nashua River at Leominster (01094500) and 

to Ipswich River at South Middleton (01101500), 

Massachusetts.

500

200

100

5

50

25

10

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

April 2007 peak

(1,360)

1930 1940 1950 1960 1970 1980 1990 2000 2010

WATER YEAR

DISCHARGE, IN CUBIC FEET PER SECOND

Annual peak discharge

50

Observed

Estimated by MOVE-1 analysis with North 

Nashua River near Leominster (01094500)

Estimated by MOVE-1 analysis with Ipswich 

River at Middleton (01101500)

Peak-flow frequency computed from 

observed and estimated peak flows 

determined from North Nashua River 

near Leominster; number is return 

interval in years

Figure 24. Observed and estimated annual peak discharge for water years 1936 through 2007 at Spicket River near 

Methuen (01100561), Massachusetts.
Characteristics of the April 2007 Flood at Selected Streamflow-Gaging Stations 37

Table 14. Magnitude and confidence limits of flood flows at selected return intervals at Spicket River near Methuen (01100561), 

Massachusetts.

[NH, New Hampshire; --, not determined]

Return 

interval 

(years)

Exceedance 

probability

Estimated magnitude of flood flow, 

in cubic feet per second

NH regional 

regression 

equation1

Extended-record analysis 

(1936–2007)2

with North Nashua River at 

Leominster (01094500)

with Ipswich River at 

South Middleton (01101500)

Expected 

peak

95-percent confidence limit Expected 

peak

95-percent confidence limit

Lower Upper Lower Upper

5 0.2 1,270 1,120 953 1,340 939 846 1,060

10 0.1 1,630 1,630 1,360 2,040 1,190 1,050 1,370

25 0.04 2,090 2,510 2,010 3,310 1,540 1,330 1,830

50 0.02 2,380 3,360 2,620 4,610 1,820 1,560 2,220

100 0.01 2,920 4,400 3,340 6,280 2,130 1,790 2,660

200 0.005 -- 5,690 4,190 8,420 2,470 2,040 3,150

500 0.002 3,980 7,830 5,580 12,200 2,960 2,400 3,880

1

 Olson, 2009.

2

 Estimated annual-peak flows 1936–2000 using MOVE-1 method (Hirsch, 1982).

ANNUAL PEAK DISCHARGE, IN CUBIC FEET PER SECOND

ANNUAL EXCEEDANCE PROBABILITY, IN PERCENT

April 2007 peak

(1,360)

10

102

103

104

105

RETURN INTERVAL, IN YEARS 

1.00 1.02 1.05 1.11 1.25 1.43 2 3.33 5 10 20 50 100 200 500

99.5 98 95 90 80 70 50 30 20 10 5 2 1 0.5 0.2

Calculated by the New Hampshire regional regression equation (Olson, 2009)

Peak-flow frequency

95-percent confidence limits

Line color varies according to point data above

Observed (2001–07) and estimated peaks (1936–2000) from 

North Nashua River near Leominster (01101500)

Observed (2001–07) and estimated peaks (1936–2000) from 

Ipswich River at Middleton (01094500)

Figure 25. Peak-flow frequency 

computed from observed and 

estimated annual peaks for 

water years 1936 through 2007 

at Spicket River near Methuen 

(01100561), Massachusetts.
38 Characteristics of the April 2007 Flood at 10 Streamflow-Gaging Stations in Massachusetts

return interval for discharges computed from records extended 

with the Ipswich River data (table 14). At the 95-percent 

confidence limit the April 2007 flood flow was between these 

return intervals for the corresponding extended record used in 

the analysis. Although the difference in the return period of 

the April 2007 peak flow determined by the various methods 

was not large, the differences between methods increase as 

discharge increases. The peak discharge for a 100-year return 

interval determined from records extended with the North 

Nashua River data was about twice that determined from 

records extended with the Ipswich River data. The 100-year 

peak discharge determined from the New Hampshire regional 

regression equation was about midway between the discharges 

computed from the records extended with the North Nashua 

River and Ipswich River data for a 100-year peak discharge 

(fig. 25). 

The flood-insurance study completed in 1987 for the 

City of Methuen (FEMA, 1987) indicates the April 2007 peak 

stage (63.27 ft) was between a 10- and 50-year flood stage 

(fig. 26A). The river stage was above the NWS-designated 

flood stage (108.91 ft) for nearly 3.5 days and peaked about 

1 ft above the NWS flood stage on April 18. The return 

interval of the April 2007 peak discharge determined from the 

extended-record peak-flow analysis made in this study was 

between 5 and 10 years and between a 10- and 50-year return 

interval as reported in the flood-insurance study (fig. 26B).

The City of Methuen flood-insurance study (FEMA, 

1987) reports the discharge of the Spicket River near the 

streamgage (about 16,300 ft upstream of the corporate limit) 

for 10-, 50-, 100- and 500-year return intervals of 900; 

1,600; 1,900; and 2,900 ft3/s, respectively (table 15). These 

discharges, obtained from an upstream flood-insurance study 

for Salem, New Hampshire (U.S. Department of Housing and 

Urban Development, 1978), appear to have been determined 

from a flow-frequency analysis of 30 years of record from 

the Parker River in northeastern Massachusetts adjusted by 

drainage-area ratio. As previously described, the discharges 

determined from the peak-flow analysis of extended record 

made with the Ipswich River at South Middleton station 

(basin just south of the Parker River Basin) results in a much 

lower discharge than that calculated from the North Nashua 

River data. Hence, the peak flows for various return intervals reported in the flood-insurance study were similar to 

those determined from the extended-record analysis with the 

Ipswich River at South Middleton data but were considerably 

lower than the peak flows for those return intervals determined 

from the extended-record analysis with the North Nashua 

River data. 

Peak flows calculated from extended records using the 

North Nashua River differed most from the flood flows for the 

Spicket River reported in the flood-insurance study (table 15). 

The magnitude of flood flows estimated from records extended 

with the North Nashua River were about 1.8 to 2.7 times 

greater than those in flood-insurance study; the differences 

increased as the return interval increased. Converted to stage 

(rating 2.1), the peak flows calculated from the extended 

record using North Nashua River were about 2.7, 3.8, 5.9, 

and 6.6 ft greater than the flood stages shown on the floodinsurance-study profile (FEMA, 1987; plate 04) for return 

intervals of 10, 50, 100, and 500 years, respectively. The 

rating is estimated above 113 ft (gage datum of about 11.3 ft). 
Characteristics of the April 2007 Flood at Selected Streamflow-Gaging Stations 39

DISCHARGE, IN CUBIC FEET PER SECOND 

APRIL 2007

0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

15 16 17 18 19 20 21 22 23 24

A. River stage

B. River discharge

STAGE, IN FEET (NGVD 29)

15 16 17 18 19 20 21 22 23 24

104

106

108

110

112

114

FIS 10-year 

FIS 50-year 

5-year 

10-year 

NWS-designated flood stage and FIS 10-year

FIS 50-year 

FIS 100-year 

FIS 500-year 

10-year 

5-year 

25-year 

Computed from extended-record 

peak-flow frequency analysis and 

stage-discharge rating number 2.1

Stage reported in FIS

Computed from extended-record 

peak-flow frequency analysis

Peak-flow frequency reported in FIS

Figure 26. April 15–23, 2007, (A) stage and (B) discharge at Spicket River near Methuen (01100561), Massachusetts. 

[NWS, National Weather Service; FIS, flood-insurance study by FEMA (1987)]
40 Characteristics of the April 2007 Flood at 10 Streamflow-Gaging Stations in Massachusetts

Shawsheen River at Andover–01100627

The streamflow-gaging station on the Shawsheen River 

at Andover has been in operation since October 2006 limiting annual peak-flow data to 1 year. Therefore, the upstream 

streamflow-gaging station on the Shawsheen River at 

Wilmington (01100600) was used as a surrogate to characterize high flows in relation to the April 2007 storm at Andover 

(01100627). Peak flows from 14 independent storms with 

concurrent record at Andover and Wilmington were used to 

evaluate the relation of peak flows from the long-term site 

to those at the recently installed streamflow-gaging station. 

Although a drainage-area ratio with an exponent of 0.688 

yielded the smallest RMSE between observed peaks for these 

14 storms, a drainage-area ratio with an exponent of 1.0 best 

fit the observed April 2007 peak. Therefore, a simple drainagearea ratio (exponent of 1.0) was used to estimate peak flows 

at Andover from peak flows at Wilmington. The drainage area 

of the Shawsheen River at Andover and at Wilmington is 72.8 

and 36.5 mi2, respectively. Additional data are needed to determine whether modification of the ratio exponent is warranted.

Annual peak flows at Shawsheen River at Wilmington for 

water years 1936 through 1963 were estimated by MOVE-1 

using the 44 years of concurrent record (1964–2007) with 

North Nashua River at Leominster (01094500) and Ipswich 

River near Ipswich (01102000). The centroids of the drainage 

basins to the North Nashua River at Leominster and Ipswich 

River near Ipswich are about 31 mi west and 14 mi east 

of the basin centroid of Shawsheen River at Wilmington, 

respectively. The relation of concurrent peak-flow data at 

Shawsheen River at Wilmington with North Nashua River at 

Leominster varied widely (fig. 27; RMSE of 368 ft3/s). The 

relation of concurrent peak-flow data at Shawsheen River at 

Wilmington and the Ipswich River near Ipswich was better 

(less scatter; RMSE 222 ft3/s); however, the relation of peak 

flows between sites is not as strong as it was at other sites 

where MOVE-1 was used to estimate peak flows. Therefore, 

the estimated peak-flow record from 1936–63 at Shawsheen 

River at Wilmington generally has a larger degree uncertainty 

than at other sites where peak-flows were estimated. This 

uncertainty is exemplified by the difference in estimated peaks 

at Andover derived from the North Nashua River relation as 

opposed to those derived from the relation with the Ipswich 

River, particularly in water years 1936, 1938, 1944, and 

1956; estimated peaks for these years derived from the North 

Nashua River data greatly exceeded the peaks derived from 

Table 15. Comparison of flood stage and discharge at selected return intervals reported in community flood-insurance study to 

values determined in this study for Spicket River near Methuen (01100561), Massachusetts.

[ft, feet in NVGD 29; ft3/s, cubic feet per second; --, not determined]

Return 

interval 

(years)

1

Flood-insurance study Present study Difference between present and 

flood-insurance studies

2

Stage 

(ft)

3

Discharge 

table 1 

(ft3/s)

4

Discharge 

rating 2.1 

(ft3/s)

5

Discharge 

(ft3/s)

3

Stage 

rating 2.1

(ft)

Stage 

(ft)

Discharge

Table 1 

(ft3/s)

Rating 

(ft3/s)

5 -- -- -- 1,120 109.9 -- -- --

10 108.9 900 850 1,630 111.6 2.7 780 780

25 -- -- -- 2,510 113.7 -- -- --

50 111.4 1,600 1,550 3,360 115.2 3.8 1,760 1,810

100 111.8 1,900 1,700 4,400 117.7 5.9 2,500 2,700

200 -- -- -- 5,690 118.2 -- -- --

500 113.8 2,900 2,500 7,830 120.4 6.6 4,930 5,330

1

 City of Methuen flood-insurance study (FEMA, 1987).

2

 Flood stages at streamflow-gaging station; 16,200 ft from corporate limit on flood profile (FEMA, 1987; pl. 04P).

3

 Flood-insurance-study summary of discharges used in determining flood elevations (FEMA, 1987; table 1, at state line).

4

 Stage determined from stage-discharge rating number 2.0 and 2.1; above 113 ft rating is estimated.

5

 Determined from flood-frequency analysis of MOVE-1 extended record with North Nashua River at Leominster (01094500).
Characteristics of the April 2007 Flood at Selected Streamflow-Gaging Stations 41

DISCHARGE AT 01094500 AND 01102000,

IN LOG CUBIC FEET PER SECOND

Concurrent peaks (1964–2007)

MOVE-1 estimated peaks (1936–63)

DISCHARGE AT 01100600, 

IN LOG CUBIC FEET PER SECOND

01094500

01102000 

01094500

01102000 

2.0

2.5

3.0

3.5

4.0

2.5 3.0 3.5 4.0 4.5

Figure 27. Relation between observed 

annual peak discharges for water years 

1964 through 2007 at Shawsheen River near 

Wilmington (0110600) and North Nashua River 

at Leominster (01094500) and to Ipswich River 

near Ipswich (01102000), Massachusetts.

500

200

100

5

50

25

10

April 2007

(1,670)

Annual peak discharge Peak-flow frequency calculated from annual peak flows at 

Shawsheen River near Wilmington (1964–07) adjusted by 

Determined by drainage-area ratio with drainage-area ratio to Shawsheen River at Andover 

Shawsheen River near Wilmington (0100600)

Estimated at Shawsheen River near Wilmington 

from Ipswich River near Ipswich (01102000) then 

adjusted by drainage-area ratio

Estimated at Shawsheen River near Wilmington 

from North Nashua River near Leominster 

(01094500) then adjusted by drainage-area ratio

50

1930 1940 1950 1960 1970 1980 1990 2000 2010

WATER YEAR

DISCHARGE, IN CUBIC FEET PER SECOND

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

Figure 28. Observed and estimated annual peak discharge for water years 1936 through 2007 at Shawsheen River at 

Andover (0110627), Massachusetts.
42 Characteristics of the April 2007 Flood at 10 Streamflow-Gaging Stations in Massachusetts

the Ipswich River data. The 1936 estimated peak derived 

from the North Nashua River data was about three times 

greater than the peak derived from the Ipswich River data. 

These differences greatly influence the peak-flow analysis and 

particularly affect the magnitude of flood flows with a low 

exceedance probability (long return interval). 

The estimated peak flows at Andover determined directly 

from the peak-flow record at Wilmington exceeded the April 

2007 peak 12 times, most notably in 1979, 1997, and 2001 

by about 98, 121, and 89 percent, respectively (fig. 28). The 

recorded April 2007 peak discharge (1,470 ft3/s at Andover) 

was ranked 12th out of 44 annual peaks over the 1964–2007 

period. Estimated annual peak flows derived from the Ipswich 

River record (1936–63) exceeded the April 2007 peak four 

times, but generally only by a small amount (1 to 38 percent). 

From these annual peak data the recorded April 2007 peak discharge was ranked 16th out of 72 over the 1936–2007 period. 

Estimated annual peak flows derived from the North Nashua 

River data (1936–63) exceeded the April 2007 peak six times 

with peaks in 1936, 1938, 1944, and 1956 greatly exceeding 

the April 2007 peak (123 to 358 percent). These four peaks 

also exceeded the peak discharge estimated directly from the 

Wilmington record (fig. 28) by as much as twice. From these 

annual peak data the recorded April 2007 peak discharge was 

ranked 18th out of 72 over the 1936–2007 period.

The Shawsheen River at Andover April 2007 peak discharge (1,470 ft3

/s) has about a 5-year return interval or less 

as determined by the different methods described above 

(table 16; fig. 29). The April 2007 peak discharge does not 

exceed the upper 95-percent confidence limit for a 5-year 

return interval as determined by the different methods 

(table 16). Although the relative magnitude of the April 2007 

peak as determined by the various methods did not yield 

appreciably different results, the methods did diverge as the 

discharge increased. The peak discharge for a 100-year return 

interval determined from records extended with the North 

Nashua River data was about 65 percent greater than that 

determined from records extended with the Ipswich River 

data. Discharges for various return intervals determined from 

the period of record are between discharges determined from 

the extended record, but the magnitude of peak flows determined from the period of record were closer those determined 

from records extended with the Ipswich River data, reflecting 

the tighter relation of peak flows at Shawsheen River at 

Wilmington with those at Ipswich River near Ipswich than 

those at North Nashua River near Leominster (fig. 27). 

ANNUAL PEAK DISCHARGE, IN CUBIC FEET PER SECOND

ANNUAL EXCEEDANCE PROBABILITY, IN PERCENT

April 2007 peak (1,670)

10 2

10 3

10 4

99.5 98 95 90 80 70 50 30 20 10 5 2 1 0.5 0.2

RETURN INTERVAL, IN YEARS 

1.00 1.02 1.05 1.11 1.25 1.43 2 3.33 5 10 20 50 100 200 500

Peak-flow frequency

95-percent confidence limits

Line color varies according to point data above

Estimated peaks (1936–2007) from Shawsheen River near 

Wilmington (01100600) adjusted by drainage-area ratio 

Estimated peaks (1936–2007) from Ipswich River at Ipswich 

(01102000) to 01100600 adjusted by drainage-area ratio 

Estimated peaks (1936–2007) from Nashua River near Wilmington 

(01094500) to 01100600 adjusted by drainage-area ratio 

Figure 29. Peak-flow frequency computed from observed and estimated annual peak discharges for 

water years 1936 through 2007 for Shawsheen River at Andover (01100627), Massachusetts.
Characteristics of the April 2007 Flood at Selected Streamflow-Gaging Stations 43

Table 16. Magnitude and confidence limits of flood flows at selected return intervals at Shawsheen River near Andover (01106270), Massachusetts.

Return 

interval 

(years)

Exceedance 

probability

Estimated magnitude of flood flow, 

in cubic feet per second

Period-of-record analysis 

(1950–2007)

Extended-record analysis 

(1936–2007)

Two-station comparison1 MOVE-1 extended record2

Expected 

peak

95-percent confidence limit Expected 

peak

95-percent confidence limit Expected 

peak

95-percent confidence limit

Lower Upper Lower Upper Lower Upper

with Ipswich River near Ipswich (01102000)

5 0.2 1,850 1,600 2,200 1,700 1,520 1,920 1,680 1,510 1,890

10 0.1 2,390 2,030 2,960 2,150 1,900 2,510 2,130 1,890 2,47025 0.04 3,190 2,620 4,140 2,810 2,420 3,390 2,780 2,410 3,32050 0.02 3,860 3,100 5,190 3,360 2,840 4,160 3,320 2,830 4,070

100 0.01 4,600 3,610 6,390 3,960 3,290 5,020 3,910 3,280 4,900200 0.005 5,410 4,160 7,760 4,620 3,770 5,990 4,560 3,760 5,840500 0.002 6,620 4,960 9,890 5,580 4,470 7,470 5,520 4,460 7,270

with North Nashua River at Leominster (01094500)

5 0.2 1,880 1,630 2,230 1,950 1,710 2,280

10 0.1 2,540 2,150 3,130 2,710 2,320 3,28025 0.04 3,580 2,930 4,660 3,940 3,270 4,99050 0.02 4,530 3,610 6,130 5,090 4,110 6,680

100 0.01 5,650 4,380 7,930 6,470 5,100 8,780200 0.005 6,960 5,260 10,100 8,120 6,240 11,400500 0.002 9,040 6,620 13,800 10,800 8,050 15,800

1 Computed following Bulletin 17B, Appendix 7 guidelines (Interagency Advisory Committee on Water Data, 1982).2 Estimated annual-peak flows 1936–49 using MOVE-1 method (Hirsch, 1982).
44 Characteristics of the April 2007 Flood at 10 Streamflow-Gaging Stations in Massachusetts DISCHARGE, IN CUBIC FEET PER SECOND 

APRIL 2007

STAGE, IN FEET (NGVD 29)

25

30

35

40

0

400

800

1,200

1,600

2,000

15 16 17 18 19 20 21 22 23 24

FIS 10-year 

5-year 

FIS 50-year 

FIS 100-year 

15 16 17 18 19 20 21 22 23 24

A. River stage

B. River discharge

FIS 10-year 

FIS 50-year 

FIS 100-year 

5-year 

Computed from extended-record 

peak-flow frequency analysis and 

stage-discharge rating number 1

Stage reported in FIS

Computed from extended-record 

peak-flow frequency analysis

Peak-flow frequency reported in FIS

Figure 30. April 15–23, 2007, (A) stage and (B) discharge at Shawsheen River at Andover (01100627), Massachusetts. 

[FIS, flood-insurance study by FEMA (1989)]
Characteristics of the April 2007 Flood at Selected Streamflow-Gaging Stations 45

The FEMA flood-insurance study revised in 1989 for the 

Town of Andover (FEMA, 1989) indicates the April 2007 

peak stage (34.15 ft) was between a 10- and 50-year stage 

(fig. 30A). Shawsheen River at Andover is not part of the 

NWS Advanced Hydrologic Prediction network; therefore, a flood-warning stage is not available. The upstream 

streamflow-gaging station at Wilmington, is part of the NWS 

Advanced Hydrologic Prediction network; stage at this location was above the NWS flood stage (87.44 ft) for about 

1.5 days and peaked on April 17 about 0.8 ft above flood 

stage. The return interval of the April 2007 peak discharge 

determined from the peak-flow frequency analysis varied 

depending on the data used, but was less than a 5-year return 

interval (fig. 30B). The flood-insurance study indicates the 

April 2007 peak discharge has about a 10-year return interval.

The Town of Andover flood insurance study (FEMA, 

1989) reports the discharge for 10-, 50-, 100-, and 500-year 

return intervals for the Shawsheen River near the streamgage 

(about 3,800,000 ft upstream of the corporate limit) of 1,450; 

2,170; 2,525; and 3,550 ft3/s, respectively (table 17). These 

discharges were about two to three times lower than the estimated flood magnitudes determined by the various methods 

(table 16). 

Peak flows calculated from extended records using the 

North Nashua River differed most from the flood flows for 

the Shawsheen River reported in the flood-insurance study 

(table 17). The magnitude of flood flows estimated from 

records extended with the North Nashua River were about 

two to three times greater than those in flood-insurance study; 

the differences increased as the return interval increased. The 

rating differs substantially from the stage-discharge relation 

defined by the flows and flood profile determined in the floodinsurance study. The estimated stage (rating 1) for a 10-year 

flood flow calculated from the extended record using the North 

Nashua River data at the 10-year return interval (2,710 ft3/s) 

is about 22 ft greater than the 10-year flood stage shown on 

the flood insurance study profile (FEMA, 1989; plate 03). 

The estimated stage for a 10-year flood flow calculated from 

the extended record using the Ipswich River data (2,130 ft3/s) 

is about 41 ft, or about 9 ft greater than that reported in the 

flood-insurance study. The stage-discharge relation at this 

site is defined by only a few measurements and the rating is 

estimated above 113 ft (gage datum of about 11.3 ft). 

Table 17. Comparison of flood stage and discharge at selected return intervals reported in community flood-insurance study to 

values determined in this study for Shawsheen River at Andover (01100627), Massachusetts.

[ft, feet in NVGD 29; ft3/s, cubic feet per second; --, not determined]

Return 

interval 

(years)

1

Flood-insurance study Present study Difference between present and 

flood-insurance studies

2

Stage 

(ft)

3

Discharge 

table 1 

(ft3/s)

4

Discharge 

rating 1 

(ft3/s)

5

Discharge 

(ft3/s)

4

Stage 

rating 1

(ft)

Stage 

(ft)

Discharge

Table 1 

(ft3/s)

Rating 

(ft3/s)

5 -- -- -- 1,950 39.2 -- -- --

10 32.4 1,450 1,240 2,710 53.0 21.6 1,470 1,260

25 -- -- -- 3,940 -- -- -- --

50 35.4 2,170 1,590 5,090 -- -- 3,500 2,920

100 36.7 2,525 1,710 6,470 -- -- 4,760 3,945

200 -- -- -- 8,120 -- -- -- --

500 44.0 3,550 2,310 10,800 -- -- 8,490 7,250

1

 Town of Andover flood-insurance study (FEMA, 1989).

2

 Flood stages at streamflow-gaging station; 3,800 ft above corporate limit on flood profile (FEMA, 1989; pl. 03P).

3

 Flood-insurance study summary of discharges used in determining flood elevations (FEMA, 1989; table 1, at Andover-Lawrence corporate limits).

4

 Stage determined from stage-discharge rating number 1; above 34 ft rating is estimated; stage for discharges greater than 3,600 ft3/s not determined 

because they extend far beyond the rating.

5

 Determined from flood-frequency analysis of MOVE-1 extended record with North Nashua River at Leominster (01094500).
46 Characteristics of the April 2007 Flood at 10 Streamflow-Gaging Stations in Massachusetts

Mill River at Northampton–01171500

Annual peak-flow records for the Mill River at 

Northampton span a 70-year period from water years 1938 

through 2007. The 1938 water year peak predates the start of 

continuous-streamgaging operation in October 1938 (beginning of the 1939 water year), but was determined following a 

hurricane in September of that year by indirect measurements 

conducted near the streamflow-gaging station (6,680 ft3/s) and 

6 mi upstream of the station (7,330 ft3/s). The average of these 

two measurements (7,000 ft3/s) was entered in the peak-flow 

file and used in the peak-flow frequency analysis. For consistency with the other stations examined in this study, the peak 

flows for 1936 and 1937 were estimated using MOVE-1 and 

concurrent peak-flow records at the Quaboag River at West 

Brimfield (01176000) and the West Branch Westfield River 

at Huntington (01181000). The centroids of the Quaboag and 

West Branch Westfield River Basins are about 34 and 17 mi to 

the southeast and southwest of the centroid of the Mill River 

Basin, respectively. Estimated peak discharges varied from the 

observed (fig. 31) and, in general, the peaks calculated from 

the West Branch Westfield River data were in better agreement 

with the observed peaks (RMSE 1,008 ft3/s) than the peaks 

calculated from the Quaboag River data (RMSE 2,524 ft3/s). 

Regardless of which index station was used, the estimated 

1936 peak discharge was near or below the three largest peaks 

of record and, therefore, did not exert a large influence in the 

peak-flow analysis. The estimated peaks shown in figure 32 

were determined from the two stations weighted on the basis 

of the RMSE. The peak in April 2007 was the third highest 

discharge recorded, exceeded only by the peak discharge in 

the 1938 and 1955 water years. The 1938 and 1955 peak discharges are relatively close in magnitude (within 14 percent) 

to the April 2007 peak discharge (fig. 32). 

Peak-flow-frequency analysis determined from the period 

of record indicates the April 2007 peak flow (6,150 ft3/s) has a 

return interval of about 50 years and is within the 95-percent 

confidence limits of a 25- to 100-year return interval 

(table 18, fig. 33). The extended-record analysis, unlike 

many of the other stations examined, yielded little change in 

the magnitudes of peaks for various return intervals mainly 

because the records were extended for only 2 years and the 

magnitudes of the estimated peaks were near other peaks in 

the period of record. A peak-flow analysis of the weighted 

estimates of the 1936 and 1937 peaks and the recorded 

peaks indicate the April 2007 peak discharge had about the 

same return interval, but the 95-percent confidence limit was 

between a 25- and 50-year return interval (table 18). Records 

extended on the basis of the relation with the Quaboag River 

data produced discharges that were only about 2 to 6 percent 

higher than the discharges determined from the periodof-record analysis for return intervals of 5 to 500 years, 

respectively. Records extended on the basis of the relation 

with the West Branch Westfield River produced discharges 

that were about 1 percent less than the discharges determined 

from the period-of-record analysis for return intervals of 5- to 

500-years. 

The FEMA flood-insurance study completed in 1976 for 

the City of Northampton indicates the April 2007 peak stage 

(188.26 ft) was between a 10- and 50-year flood (fig. 34A) at 

Clement Street (present location of streamflow-gaging station) 

on the flood profile (FEMA, 1976; plate-O5P). Mill River at 

Northampton exceeded the NWS flood stage (182.68 ft) for 

about 1 day and peaked on April 16 at about 5.6 ft above flood 

stage. The return interval of the April 2007 peak discharge 

determined from the peak-flow frequency analysis made in 

this study approached that of a 50-year flood (fig. 34B).

The City of Northampton flood-insurance study discharge 

for 10-, 50-, 100- and 500-year return intervals for the Mill 

River near the streamgage (about 4.66 mi upstream from the 

mouth) was estimated to be 4,700; 8,200; 10,500; and 

16,000 ft3/s, respectively (FEMA, 1976; interpolated from 

fig. 6). These discharges were obtained from a log-Pearson 

Type III distribution of 36 years of annual peak-flow record 

(1938–73) at the Mill River streamflow-gaging station. The 

flood-insurance study flood flows are about 7, 27, 42, and 

64 percent greater than discharges calculated in this study for 

10-, 50-, 100-, and 500-year return intervals as determined 

from the extended record analysis. This result demonstrates 

the leveraging of the large peaks in 1938 and 1955 over the 

relatively short record used in the flood-insurance study 

(1938–73) compared to the length of the record used in this 

analysis (1936–2007).

Correspondingly, the flood stages determined from the 

current stage-discharge relation (rating 46) for flood flows 

calculated in this study are somewhat less than the flood stages 

shown at Clement Street (FEMA, 1976). Differences in stage 

ranged from about 0.8 to 1.6 ft for flood flows with return 

intervals of 10 to 500 years (table 19).
Characteristics of the April 2007 Flood at Selected Streamflow-Gaging Stations 47

Figure 31. Relation between observed annual 

peak discharges for water years 1938 through 

2007 at Mill River at Northampton (01171500) and 

Quaboag River at West Brimfield (01176000) and 

to West Branch Westfield River at Huntington 

(01181000), Massachusetts.

DISCHARGE AT 01176000 AND 01181000,

IN LOG CUBIC FEET PER SECOND

DISCHARGE AT 01171500, 

IN LOG CUBIC FEET PER SECOND

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

4.0

2.5 3.0 3.5 4.0 4.5 5.0

Concurrent peaks

(1938–2007)

Estimated peaks

(1936–37)

01176000

01181000 

01176000

01181000 

WATER YEAR

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

9,000

10,000

DISCHARGE, IN CUBIC FEET PER SECOND

1930 1940 1950 1960 1970 1980 1990 2000 2010

500

200

100

5

50

25

10

April 2007 peak (6,150)

Estimated record from:

West Branch Westfield River (01181000)

Quaboag River at West Brimfield (01176000)

Observed

Annual peak discharge

50

Weighted from 01181000 and 01176000

Estimated magnitude of peak flow determined 

from observed and weighted estimated annual 

peak flows, number is return interval in years 

Figure 32. Observed and estimated peak annual discharge for water years 1936 through 2007, Mill River at Northampton 

(01171500), Massachusetts.
48 Characteristics of the April 2007 Flood at 10 Streamflow-Gaging Stations in Massachusetts

Figure 33. Peak-flow frequency computed from observed and estimated annual peaks for water 

years 1936 through 2007 at Mill River at Northampton (01171500), Massachusetts.

Table 18. Magnitude and confidence limits of flood flows at selected return intervals at Mill River at Northampton 

(01171500), Massachusetts.

Return 

interval 

(years)

Exceedance 

probability

Estimated magnitude of flood flow, 

in cubic feet per second

Period-of-record analysis 

(1938–2007)

Extended-record analysis 

(1936–2007)1

Expected 

peak

95-percent confidence limit Expected 

peak

95-percent confidence limit

Lower Upper Lower Upper

5 0.2 3,470 3,110 3,940 3,520 3,160 3,980

10 0.1 4,320 3,820 5,020 4,390 3,880 5,090

25 0.04 5,460 4,730 6,530 5,560 4,820 6,630

50 0.02 6,350 5,420 7,740 6,470 5,530 7,880

100 0.01 7,260 6,120 9,030 7,410 6,250 9,200

200 0.005 8,210 6,830 10,400 8,400 6,990 10,600

500 0.002 9,520 7,800 12,300 9,760 8,000 12,600

1

 Estimated annual-peak flows 1936–37 using MOVE-1 method (Hirsch, 1982); weighted average of flows determined from Quaboag River at 

West Brimfield (01176000) and West Branch Westfield River at Huntington (01181000).

102

103

104

10

ANNUAL PEAK DISCHARGE, IN CUBIC FEET PER SECOND

ANNUAL EXCEEDANCE PROBABILITY, IN PERCENT

5

April 2007 peak

(6,150)

RETURN INTERVAL, IN YEARS 

1.00 1.02 1.05 1.11 1.25 1.43 2 3.33 5 10 20 50 100 200 500

99.5 98 95 90 80 70 50 30 20 10 5 2 1 0.5 0.2

Observed (1938–2007) and estimated peaks (1936–37)

Peak-flow frequency

95-percent confidence limits
Characteristics of the April 2007 Flood at Selected Streamflow-Gaging Stations 49

DISCHARGE, IN CUBIC FEET PER SECOND 

APRIL 2007

15 16 17 18 19 20 21 22 23 24

0

2,000

4,000

6,000

8,000

NWS-designated flood stage

50-year

100-year 

200-year 

500-year 

25-year

10-year 

5-year 

STAGE, IN FEET (NGVD 29)

15 16 17 18 19 20 21 22 23 24

175

180

185

190

195

A. River stage

B. River discharge

FIS 10-year 

25-year 

5-year 

10-year 

50-year

100-year 

FIS 500-year 

FIS 10-year 

FIS 50-year 

FIS 100-year 

Computed from weighted extendedrecord peak-flow frequency analysis

Peak-flow frequency reported in FIS

Computed from weighted extendedrecord peak-flow frequency analysis 

and stage-discharge rating number 46

Stage reported in FIS

Figure 34. April 15–23, 2007, (A) stage and (B) discharge at Mill River at Northampton (01171500), Massachusetts. 

[NWS, National Weather Service; FIS, flood-insurance study by FEMA (1976)]
50 Characteristics of the April 2007 Flood at 10 Streamflow-Gaging Stations in Massachusetts

by about 206, 84, 40, and 56 percent, respectively; the 1936 

peak exceeded the 2006 peak by about 53 percent. The estimated peaks derived from the Quaboag River data exceeded 

the April 2007 peak in 1936, 1938, and 1955 by about 66, 

442, and 861 percent, respectively; the 1936 and 1955 peaks 

exceeded the 2006 peak by about 171 and 381 percent, 

respectively.

As in the MOVE-1 relation described for the Shawsheen 

River, the most appropriate index station is not necessarily 

determined by the lowest RMSE between estimated and 

observed flows. The RMSE resulting from the Quaboag 

River data was about half that resulting from North Nashua 

River data, but some of the estimated peaks during 1936–60 

reflect much different meterologic conditions at the two 

index stations, particularly in 1938 and 1955. If meterologic 

conditions that contributed to the peak discharge in one 

basin better reflect those in the Sevenmile River Basin in a 

particular year, then that index basin would provide a better 

estimate of the magnitude of flood flows. Determining which 

index station, or weighting of index stations, best matches 

the meteorologic and hydrologic conditions leading to the 

peak for a particular event requires a detailed analysis of the 

storms that leverage the distribution of peaks. Although such 

an analysis was not possible as part of this investigation, this 

type of analysis could improve estimates of the magnitude 

of peak flows for various return intervals. For example, 

Sevenmile River near Spencer–01175670

Annual peak-flow records for Sevenmile River near 

Spencer span a 47 year period, water years 1961 through 

2007. Peak flows for 1936 through 1960 were estimated 

using MOVE-1 and concurrent peak-flow records from North 

Nashua River near Leominster (01094500) and Quaboag River 

at West Brimfield (01176000). The centroids of the North 

Nashua River and Quaboag River Basins are about 20 and 

6 mi north and southwest of the centroid of the Sevenmile 

River Basin, respectively; the Sevenmile River is tributary to 

the Quaboag River. Estimated peak discharges varied from the 

observed, particularly with estimated peaks derived from the 

North Nashua River data (fig. 35); the RMSE of the estimated 

discharge determined from North Nashua River and Quaboag 

River data was 122 and 64 ft3/s, respectively. 

The April 2007 peak discharge (453 ft3/s) was the 

second highest recorded, exceeded only in the 2006 water 

year by about twice (905 ft3/s), but was nearly equaled in the 

1968 water year (fig. 36). Although some of the estimated 

peaks derived from either index station were higher than the 

observed peaks of record (fig. 36), the 1938 and 1955 peaks 

derived from the Quaboag River data were substantially 

greater than those derived from the Nashua River data. The 

estimated peaks derived from the North Nashua River data 

exceeded the April 2007 peak in 1936, 1938, 1944, and 1956 

Table 19. Comparison of flood stage and discharge at selected return intervals reported in community flood-insurance study to 

values determined in this study for Mill River at Northampton (01171500), Massachusetts.

[ft, feet in NVGD 29; ft3/s, cubic feet per second; --, not determined]

Return 

interval 

(years)

1

Flood-insurance study Present study Difference between present and 

flood-insurance studies

2

Stage 

(ft)

3

Discharge 

figure 6 

(ft3/s)

4

Discharge 

rating 46 

(ft3/s)

5

Discharge 

(ft3/s)

4

Stage 

rating 46

(ft)

Stage 

(ft)

Discharge

Table 1 

(ft3/s)

Rating 

(ft3/s)

5 -- -- -- 3,520 185.4 -- -- --

10 187.5 4,700 5,400 4,390 186.4 -1.1 -310 -1,010

25 -- -- -- 5,560 187.7 -- -- --

50 189.4 8,200 7,400 6,470 188.6 -0.8 -1,730 -930

100 190.9 10,500 9,050 7,410 189.5 -1.4 -3,090 -1,640

200 -- -- -- 8,400 190.4 -- -- --

500 193.3 16,000 12,000 9,760 191.7 -1.6 -6,240 -2,240

1

 City of Northampton flood-insurance study (FEMA, 1976).

2

 Flood stages at streamflow-gaging station; 4.66 mi above mouth (Clements Streeet) on flood profile (FEMA, 1976; pl. 05P).

3

 Flood-insurance study interpolated discharges (FEMA, 1976; fig. 6).

4

 Stage determined from stage-discharge rating number 46; above 188 ft rating is estimated.

5

 Determined from flood-frequency analysis of MOVE-1 extended record.
Characteristics of the April 2007 Flood at Selected Streamflow-Gaging Stations 51

Figure 35. Relation between observed annual 

peak discharges for water years 1961 through 

2007 at Sevenmile River near Spencer (01175670) 

and North Nashua River at Leominster (01094500) 

and to Quaboag River at West Brimfield 

(01176000), Massachusetts.

Figure 36. Observed and estimated annual peak discharge for water years 1936 through 2007 at Sevenmile River near Spencer 

(01175670), Massachusetts.

DISCHARGE AT 01094500 AND 01176000,

IN LOG CUBIC FEET PER SECOND

Concurrent peaks (1961–2007)

MOVE-1 estimated peaks (1936–1960)

DISCHARGE AT 01175670, 

IN LOG CUBIC FEET PER SECOND

01094500

01176000 

01094500

01176000 

1.0

1.5

2.0

2.5

3.0

3.5

4.0

2.5 3.0 3.5 4.0 4.5

500

200

100

5

50 25

10

500

200

100

5

50

25

10

0

DISCHARGE, IN CUBIC FEET PER SECOND

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

4,500

1930 1940 1950 1960 1970 1980 1990 2000 2010

WATER YEAR

Observed

Quaboag River at West Brimfield (01176000)

Annual peak discharge

North Nashua River at Leominster (01094500)

April 2007

(453)

50

50

Annual peaks estimated from:

Peak-flow frequency computed from observed 

(1961–2007) and estimated annual peak flows (1936–60) 

using the Quaboag River (01176000); number is return 

interval in years

Peak-flow frequency calculated from observed annual 

peaks (1961–2007); number is return interval in years
52 Characteristics of the April 2007 Flood at 10 Streamflow-Gaging Stations in Massachusetts

and was within the 95-percent confidence limits of a 10-year 

return interval determined by each of these methods. 

Discharges for various return intervals determined 

from the period-of-record analysis were less for all return 

intervals greater than 5 years than that those determined by 

the various extended-record methods. Discharges for various 

return intervals determined from the two-station analysis with 

the North Nashua River and the Quaboag River data and the 

extended-record analysis determined from the North Nashua 

River data were similar for a given return interval. Discharges 

determined from records extended with the Quaboag River 

data were always greater than those produced by the other 

methods for various return intervals; these differences 

increased as the exceedance probability decreased (return 

interval increased). Discharges for various return intervals 

determined from records extended with the Quaboag River 

data underscore the leveraging of the estimated 1938 and 1955 

peak discharges, which are not expressed to the same extent in 

the two-station comparison method that uses the ratios of the 

log mean discharges and standard deviations of the long- and 

short-term stations. 

back-to-back hurricanes Connie and Diane produced about 

20 in. of precipitation in a 2-week period in August 1955 

in south-central Massachusetts that resulted in the peak of 

record at Quaboag River, but did not even result in the 1955 

water-year peak discharge in the North Nashua River. Without 

further investigation, the question remains as to whether the 

precipitation in the Sevenmile River Basin in August 1955 was 

closer to that in the Quaboag River Basin, which produced 

the highest peak flow in a 94-year record by a wide margin, or 

closer to that of the North Nashua River Basin, which did not 

even result in the peak discharge for the water year. Estimation 

of other major peaks should consider similar questions about 

the meterologic and hydrologic conditions in the choice of an 

index station because of the influence these storms exert on the 

magnitude of flows for small exceedance probabilities (long 

return intervals).

Peak-flow frequency analysis for the period of record 

indicates the April 2007 peak flow (453 ft3/s) has a return 

interval of 10 to 25 years (table 20; fig. 37). The April peak 

flow was close to a 10-year discharge computed from the 

extended-record analysis and the two-station analysis with the 

North Nashua River and Quaboag River data (table 20; fig. 37) 

102

103

104

10

ANNUAL PEAK DISCHARGE, IN CUBIC FEET PER SECOND

99.5 0.5 0.2

ANNUAL EXCEEDANCE PROBABILITY, IN PERCENT

April 2007 peak (453)

98 95 90 80 70 50 30 20 10 5 2 1

RETURN INTERVAL, IN YEARS 

1.00 1.02 1.05 1.11 1.25 1.43 2 3.33 5 10 20 50 100 200 500

Peak-flow frequency

Observed peaks (1961–2007)

95-percent confidence limits

Line color changes according to point data above

Observed and estimated peaks (1936–60) from 

Quaboag River at West Brimfield (01176000)

Observed and estimated peaks (1936–60) from 

North Nashua River at Leominster (01094500)

Figure 37. Peak-flow frequency computed from observed and estimated annual peaks for water 

years 1936 through 2007 at Sevenmile River near Spencer (01175670), Massachusetts.
Characteristics of the April 2007 Flood at Selected Streamflow-Gaging Stations 53

Table 20. Magnitude and confidence limits of flood flows at select return intervals at Sevenmile River near Spencer (01175670), Massachusetts.

Return 

interval 

(years)

Exceedance 

probability

Estimated magnitude of flood flow, 

in cubic feet per second

Period-of-record analysis 

(1961–2007)

Extended-record analysis 

(1936–2007)

Two-station comparison1 MOVE-1 extended record2

Expected 

peak

95-percent confidence limit Expected 

peak

95-percent confidence limit Expected 

peak

95-percent confidence limit

Lower Upper Lower Upper Lower Upper

with North Nashua River at Leominster (01094500)

5 0.2 290 250 340 300 260 360 320 270 380

10 0.1 370 320 460 420 350 520 450 380 55025 0.04 500 410 640 600 490 790 660 540 86050 0.02 600 480 800 770 610 1,050 860 690 1,160

100 0.01 710 560 980 967 750 1,360 1,100 860 1,530200 0.005 840 650 1,190 1,200 900 1,740 1,390 1,050 2,000500 0.002 1,030 780 1,520 1,560 1,140 2,380 1,850 1,360 2,780

with Quaboag River at West Brimfield (01176000)

5 0.2 270 240 310 330 280 390

10 0.1 380 330 460 510 420 63025 0.04 580 480 740 850 670 1,13050 0.02 780 630 1,030 1,220 940 1,720

100 0.01 1,040 810 1,420 1,740 1,280 2,580200 0.005 1,370 1,040 1,950 2,450 1,740 3,820500 0.002 1,950 1,420 2,940 3,800 2,560 6,330

1 Computed following Bulletin 17B, Appendix 7 guidelines (Interagency Advisory Committee on Water Data, 1982).2 Estimated annual-peak flows 1936–60 using MOVE-1 method (Hirsch, 1982).
54 Characteristics of the April 2007 Flood at 10 Streamflow-Gaging Stations in Massachusetts

Although the expected return period of the April 2007 

peak determined by the various methods was not large, the 

magnitude of flood flows determined by the various methods 

can differ substantially as the discharge increases. This is particularly true of the analysis made with records extended with 

the Quaboag River data, which diverged most substantially 

from the other methods with increasing discharge (fig. 37). 

The peak discharge for a 100-year return interval determined 

from records extended with the Quaboag River data was about 

140 percent greater than that determined from the period of 

record and about 60 percent greater than that determined from 

records extended with the North Nashua River data. The discharges determined from the records extended with the North 

Nashua River data were about midway between the discharges 

computed from the records extended with the Quaboag River 

data and those determined from the period of record for 

exceedance probabilities less than about 20 percent (fig. 37). 

The FEMA flood-insurance study revised in 1990 for 

the Town of Spencer (FEMA, 1990) did not extend to the 

streamflow-gaging station location (gage is about 3,500 ft 

upstream from the study limits). Therefore, comparisons to the 

flood-insurance study were limited to discharges used in flow 

routing for the flood-insurance study. The streamflow-gaging 

station on Sevenmile River near Spencer is not part of the 

NWS Advanced Hydrologic Prediction network; therefore, a 

flood-warning stage is not available. The river peaked at the 

streamflow-gaging station on April 16 at a stage of 646.5 ft 

(fig. 38A).

Peak-flow analysis varied by the method and data used; 

for reference, peak flows determined from records extended 

with the North Nashua River data and the Quaboag River data 

are shown in figure 38B. The April 2007 peak flow approached 

or equalled the 10-year discharge on the basis of the record 

extended using the Quaboag River and North Nashua River 

data, respectively, and exceeded the flood-insurance study 

10-year discharge (fig. 38B).

The Town of Spencer flood insurance study reports 

discharge for 10-, 50-, 100- and 500-year return intervals 

for Sevenmile River near the confluence of Cranberry River 

(FEMA, 1990; table 1). These flows were determined by a 

precipitation-runoff model run with 48-hour design-storm 

hyetograph. The reported drainage area near the confluence 

with the Cranberry River is 31.6 mi2 and is about 3.6 times 

larger than the drainage area at the streamflow-gaging station 

(8.81 mi2). Using a simple drainage-area ratio, discharges 

reported near the confluence with the Cranberry River were 

adjusted to those at the streamflow-gaging station and are 

estimated at about 350, 620, 800, and 1,260 ft3/s for return 

intervals of 10, 50, 100, and 500 years, respectively (table 21). 

The flood flows calculated in this study from extended-record 

analysis methods are about 10 to 200 percent greater than the 

interpolated flood flows; differences increase as the return 

interval increases. Discharges computed by the two-station 

comparison method with the North Nashua River generally 

yielded the smallest differences with the flood-insurance-study 

discharges, ranging from about 20 to 25 percent. Discharge 

computed from records extended with the Quaboag River 

data yielded the greatest differences with the flood-insurancestudy discharges, ranging from about 45 to 200 percent for 

return intervals of 10 and 500 years, respectively. The periodof-record peak-flow analysis yielded the smallest overall 

differences with the flood-insurance-study discharges, ranging 

from about 7 to -18 percent for return intervals of 10 and 

500 years, respectively.
Characteristics of the April 2007 Flood at Selected Streamflow-Gaging Stations 55

APRIL 2007

A. River stage

B. River discharge

STAGE, IN FEET (NGVD 29)

15 16 17 18 19 20 21 22 23 24

644

645

646

647

648

DISCHARGE, IN CUBIC FEET PER SECOND 

15 16 17 18 19 20 21 22 23 24

0

100

200

300

400

500

600

FIS 10-year 

5-year 

10-year 

5-year 

10-year 

Quaboag River at West Brimfield (01176000)

North Nashua River at Leominster (01094500)

Calculated from extended records using annual peaks from:

Figure 38. April 15–23, 2007, (A) stage and (B) discharge at Sevenmile River near Spencer (01175670), Massachusetts.

[FIS, flood insurance study by FEMA (1990)]
56 Characteristics of the April 2007 Flood at 10 Streamflow-Gaging Stations in Massachusetts

West Branch Westfield River near 

Huntington–01181000

Annual peak-flow records for the West Branch Westfield 

River near Huntington span a 72 year period, water years 1936 

through 2007. Because the observed record spans the desired 

period of time used in the analysis and because there is no 

appreciable regulation that affects flows, no extension of 

record was necessary at this site. The peak in April 2007 

(19,700 ft3/s) was the third highest discharge recorded, 

exceeded only by the 1938, 1955, and 2006 water year peaks. 

The 1938, 1955, and 2006 peak discharges exceeded the April 

2007 peak by about 11, 32, and 43 percent, respectively 

(fig. 39). The April 2007 peak discharge was about 310 percent greater than the period-of-record median discharge.

The FEMA flood-insurance study completed in 1988 for 

the Town of Huntington (FEMA, 1988) indicates the April 

2007 peak stage (396.42 ft) was still about 2 ft below the 

10-year flood stage (fig. 41A). The river stage was above the 

NWS-designated flood stage (392.60 ft) for less than a day and 

peaked about 3.8 ft above the NWS flood stage on April 16. 

Peak-flow frequency analysis indicates the April 2007 peak 

flow had a return interval of about 25 years and was within 

the 95-percent confidence limits of the 25-year return interval 

(table 22, fig. 40B). The flood-insurance study indicates the 

April 2007 peak discharge was between the 10- and 50-year 

discharges. The peak discharge in October 2006 (28,100 ft3/s) 

was near a 100-year discharge.

The Town of Huntington flood-insurance study reports 

peak discharges determined from 43 years record at the West 

Branch Westfield River streamflow-gaging station of 12,780; 

23,360; 29,200; and 46,600 ft3/s for 10-, 50-, 100- and 500-

year return intervals, respectively (table 23). These discharges 

are about 3 to 7 percent less than those estimated for the 

same return intervals in this study; the difference increases as 

discharge increases. This difference is expected because the 

flood-insurance study was completed in April 1988, prior to 

two of the highest annual peaks recorded in 2006 and the 

April 2007. 

Correspondingly, the flood stages determined from the 

current stage-discharge relation (rating 12) in this study are 

lower than the flood-insurance-study flood stages, but the differences are too large to be attributed to differences in discharge alone. Differences ranged from about 3.3 to 5.8 ft for 

flood flows with return intervals of 10 to 500 years, respectively (table 23). Some of these differences are attributed to a 

change in the gage pool in October 1989 (moved 200 ft downstream), which resulted in a different stage-discharge relation 

than that in place at the time of the flood-insurance study and 

a flood profile that changes rapidly near the gage location. 

Because of the major change in the rating, stage or converted 

stage to discharge between the flood-insurance study and this 

study were not compared.

Table 21. Comparison of flood stage and discharge at selected return intervals reported in 

community flood-insurance study to values determined in this study for Sevenmile River near 

Spencer (01175670), Massachusetts.

[ft3/s, cubic feet per second; Delta, difference between peak flows from flood-insurance study and extended-record 

peak-flow analysis; --, not determined]

Return 

interval 

(years)

Floodinsurance study 

discharge 

(ft3/s)1

Discharge determined by flood-frequency analysis 

made with MOVE-1 extended record from

North Nashua River at 

Leominster (01094500)

Quaboag River at 

West Brimfield (01176000)

(ft3/s) Delta 

(ft3/s) (ft3

/s) Delta 

(ft3/s)

5 -- 320 -- 330 --

10 350 450 100 510 160

25 -- 660 -- 850 --

50 620 860 240 1,220 600

100 800 1,100 300 1,740 940

200 -- 1,390 -- 2,450 --

500 1,260 1,850 590 3,800 2,540

1

 Town of Spencer flood-insurance study adjusted by drainage-area ratio 0.28 (FEMA, 1990; table 1, upstream 

from Cranberry River).
Characteristics of the April 2007 Flood at Selected Streamflow-Gaging Stations 57

Figure 39. Observed annual peak discharge for water years 1936 through 2007 at West Branch Westfield River near 

Huntington (01181000), Massachusetts.

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

45,000

50,000

55,000

1930 1940 1950 1960 1970 1980 1990 2000 2010

WATER YEAR

DISCHARGE, IN CUBIC FEET PER SECOND

Annual peak discharge

50

500

200

100

5

50

25

10

April 2007 peak (19,700)

Estimated magnitude of peak flow; 

number is return interval in years

10

2

10

3

10

4

10

5

Peak-flow frequency

Observed peaks (1936–2007)

95-percent confidence limits

ANNUAL PEAK DISCHARGE, IN CUBIC FEET PER SECOND

ANNUAL EXCEEDANCE PROBABILITY, IN PERCENT

April 2007 peak

(19,700)

99.5 98 95 90 80 70 50 30 20 10 5 2 1 0.5 0.2

RETURN INTERVAL, IN YEARS 

1.00 1.02 1.05 1.11 1.25 1.43 2 3.33 5 10 20 50 100 200 500

Figure 40. Peak-flow 

frequency computed from 

observed annual peaks for 

water years 1936 through 2007 

at West Branch Westfield River 

near Huntington (01181000), 

Massachusetts.
58 Characteristics of the April 2007 Flood at 10 Streamflow-Gaging Stations in Massachusetts STAGE, IN FEET (NGVD 29)

15 16 17 18 19 20 21 22 23 24

385

390

395

400

405

NWS-designated flood stage

500-year 

100-year 

200-year 

5-year 

25-year 

10-year 

50-year 

DISCHARGE, IN CUBIC FEET PER SECOND 

APRIL 2007

15 16 17 18 19 20 21 22 23 24

0

5,000

10,000

15,000

20,000

25,000

A. River stage

B. River discharge

FIS 10-year 

FIS 50-year 

FIS 10-year 

FIS 50-year 

FIS100-year 

25-year 

10-year 

5-year 

50-year 

Determined from peak-flow frequency analysis

Peak-flow frequency reported in FIS

Determined from peak-flow frequency analysis 

and stage-discharge rating number 12

Stage reported in FIS

Figure 41. April 15–23, 2007, (A) stage and (B) discharge at West Branch Westfield River near Huntington (01181000), 

Massachusetts. [NWS, National Weather Service; FIS, flood-insurance study by FEMA (1988)]
Characteristics of the April 2007 Flood at Selected Streamflow-Gaging Stations 59

Table 22. Magnitude and confidence limits of flood flows at select 

return intervals at West Branch Westfield River near Huntington 

(01181000), Massachusetts.

Return 

interval 

(years)

Exceedance 

probability

Estimated magnitude of flood flow, 

in cubic feet per second

Period-of-record analysis 

(1936–2007)

Expected 

peak

95-percent confidence limit

Lower Upper

5 0.2 9,490 8,240 11,200

10 0.1 13,200 11,200 16,100

25 0.04 19,100 15,700 24,300

50 0.02 24,500 19,700 32,300

100 0.01 30,800 24,200 42,000

200 0.005 38,200 29,300 53,800

500 0.002 50,100 37,300 73,300

Table 23. Comparison of flood stage and discharge at selected return intervals reported in 

community flood-insurance study to values determined in this study for West Branch Westfield 

River near Huntington (01181000), Massachusetts.

[ft, feet in NVGD 29; ft3/s, cubic feet per second; --, not determined]

Return 

interval 

(years)

1

Flood-insurance study Present study

Difference between 

present and floodinsurance studies

2

Stage 

(ft)

3

Discharge 

table 1 

(ft3/s)

Discharge 

(ft3/s)

4

Stage 

rating 12 

(ft)

Stage 

(ft)

Discharge 

(ft3/s)

5 -- -- 9,490 393.9 -- --

10 398.2 12,780 13,200 394.9 -3.3 420

25 -- -- 19,100 396.3 -- --

50 401.8 23,360 24,500 397.4 -4.4 1,140

100 403.6 29,200 30,800 398.4 -5.2 1,600

200 -- -- 38,200 399.7 -- --

500 407.4 46,600 50,100 401.6 -5.8 3,500

1

 Town of Huntington flood-insurance study (FEMA, 1988).

2

 Flood stages at streamflow-gaging station; 7,831,000 ft above mouth on flood profile (FEMA, 1988; pl. 07P).

3

 Flood-insurance study (FEMA, 1988; table 1, at streamflow-gaging station).

4

 Stage determined from stage-discharge rating number 12; above 398 ft rating is estimated.
60 Characteristics of the April 2007 Flood at 10 Streamflow-Gaging Stations in Massachusetts

Magnitude of Peak-Flow in Relation to 

Drainage Area

A general relation exists between the size of the drainage basin and the magnitude of flood flows at selected return 

intervals. Basin area typically dominates multivariate regression equations for estimating the magnitude of flood flows. In 

Connecticut (Ahearn, 2003) and New York (Lumia and others, 

2006), single-variable explanatory equations using drainage area were developed along with multivariate equations 

because drainage area explained the majority of the variance. 

The six streamflow-gaging stations in this study with the 

longest unaltered peak-flow records (North Nashua River at 

Fitchburg, Stillwater River near Sterling, Squannacook River 

near West Groton, Mill River at Northampton, Sevenmile 

River near Spencer, and West Branch Westfield River near 

Huntington) also indicate a strong relation between drainage 

area and the magnitude of flows at selected return intervals 

(fig. 42). 

Lines shown in figure 42 are drawn from simple exponential equations developed from the point data at the six 

streamflow-gaging stations. Equations for estimating the flow 

magnitudes for 5-, 10-, 25-, 50-, 100-, 200-, and 500-year 

floods and coefficients of determination (r2) are provided in 

table 24. The coefficients of determination from these equations range from 0.89 to 0.95; the coefficients are smallest at 

the 5- and 200-year return intervals (0.93) and at the 500-year 

return interval (0.89). These equations were developed from 

peak-flow frequency analysis made from extended records 

(except at West Branch Westfield River); the points used 

to develop the equations in table 24 are from the extended 

records that produced the highest magnitude flows. 

Generally, these equations produced a better estimate of 

flood flows at ungaged rivers for drainage areas between 8 and 

100 mi2 for central Massachusetts than the existing state-wide 

equations developed by Wandle (1983). Compared to the 

flood-flow magnitudes determined for various return intervals 

from this study, Wandle’s equations consistently underestimate 

flood flows for return periods of 10 years or more. Wandle’s 

equations underestimated flood flows by a median of -50 

percent for a 5-year return flood to -61 percent for a 100-year 

return flood (table 24) at 9 of the 10 sites examined in this 

study; the Merrimack River at Lowell is not included in this 

analysis because of its drainage-area size. Wandle did not 

develop equations for 200- and 500-year flood flows. 

The drainage-area equation results (table 24) generally 

overestimated the magnitude of flood flows determined 

from the peak-flow analysis at the nine streamflow-gaging 

stations in this study. The median difference between the 

station peak-flow analysis and the drainage-area equation 

flood flows ranged from 4.7 to 22 percent for 200- and 

5-year return intervals, respectively (table 24). However, the 

drainage-area equations overestimated flood flows in some 

basins and underestimated flood flows in others (fig. 42; 

table 24). Generally, the differences increased as the return 

interval decreased. This analysis indicates that, until robust 

regional equations can be developed from all appropriate 

streamflow-gaging stations in Massachusetts and nearby 

states, the drainage-area equations in table 24 provide a more 

conservative estimate (higher discharges) of flood flows at 

ungaged basins in central Massachusetts than those presently 

available from Wandle’s equations. In basins in or near 

surrounding states, flood-flow equations developed for that 

state provide the most reasonable estimate of flood flows at 

an ungaged site.
Characteristics of the April 2007 Flood at Selected Streamflow-Gaging Stations 61

0 20 40 60 80 100

DRAINAGE AREA, IN SQUARE MILES

Sevenmile River near Spencer

Stillwater River near Sterling

Mill River at Northampton

Squannacook River 

near West Groton 

North Nashua River at Fitchburg 

West Branch Westfield River near Huntington

500-year

200-year

100-year

50-year

25-year

10-year

5-year

100

1,000

10,000

100,000

DISCHARGE, IN CUBIC FEET PER SECOND

Extended record with Quaboag River

Nissitissit River at Pepperell

Spicket River near Methuen

Shawsheen River at Andover

500

200

100

50

25

10

5

April 2007 peak discharge

EXPLANATION

Return interval, in years, determined from peakflow analysis at the streamflow-gaging station

Symbols greyed back indicate stations 

not used to develop lines of equal 

return intervals

Figure 42. Magnitude of flood flows in relation to drainage area at selected streamflow-gaging stations in central 

Massachusetts.
62 Characteristics of the April 2007 Flood at 10 Streamflow-Gaging Stations in Massachusetts

Summary

In 2008 the USGS, in cooperation with FEMA, 

conducted a study to characterize flooding at 10 selected 

streamflow-gaging stations in Massachusetts from the large 

nor’easter storm of April 15–18, 2007. The nor’easter brought 

heavy rains to the region with the highest quantities (about 

7 in.) falling along a north-south line over the Berkshire 

Mountains in western Massachusetts and in an arc north 

from Worcester through Fitchburg, Massachusetts, into 

New Hampshire. Coupled with normal high seasonal flows, 

this storm caused extensive flooding in parts of the state 

that prompted a disaster declaration by the President and a 

response by FEMA.

To assist FEMA in its mission of natural-hazard 

preparedness and mitigation, the USGS undertook an analysis 

of peak-flow magnitudes for 5-, 10-, 25-, 50-, 100-, 200-, 

and 500-year return intervals from currently available data 

(peak-flow records up to 2007) from 10 streamflow-gaging 

stations that experienced the heaviest flooding from the April 

2007 storm. The analysis helped characterize the magnitude 

of the April 2007 flood relative to historical data and to results 

of previous flood-insurance studies conducted in various 

communities. The streamflow-gaging stations examined 

include (in order of USGS identification number)—North 

Nashua River at Fitchburg (01094400), Stillwater River 

near Sterling (01095220), Squannacook River near West 

Groton (01096000), Nissitissit River at Pepperell (01096503), 

Merrimack River at Lowell (01100000), Spicket River 

near Methuen (01100561), Shawsheen River at Andover 

(01100627), Mill River at Northampton (01171500), 

Sevenmile River near Spencer (01175670), and West Branch 

Westfield River near Huntington (01181000).

Systematic annual peak-flow data from the USGS NWIS 

Peak-Flow File were analyzed using the program PeakFQ, 

which fits the logarithms of the annual peaks to a Pearson 

Type III probability distribution following the guidelines 

of flood-flow frequency analysis in Bulletin 17B. For most 

stations, the analysis included augmenting the station record 

with longer-term records from one or more nearby stations 

using the two-station comparison method in Bulletin 17B and 

record extension using the Maintenance of Variance Extension 

(MOVE-1) technique. The latter method was used to provide a common period of comparison to the 1936 water year 

because it dates back to the year with the highest known peak 

flow in some parts of the state or includes the highest flows in 

the other parts of the state, notably the floods of 1936, 1938, 

and 1955. In addition, recently developed regional floodflow equations for New Hampshire were used to calculate 

Table 24. Simple regression equations for estimating the magnitude of peak flow at 5-, 10-, 25-, 50-, 100-, 200-, and 

500-year return intervals from drainage area in unregulated streams in central Massachusetts.

[EXP(n) represents e (approximately equal to 2.718) raised to the nth power; n is the drainage area multiplied by a coefficient that depends on 

the return interval. Qx, flood flows for selected return intervals in cubic feet per second; DA, drainage area in square miles; r2, coefficient of determination; --, equation not available]

Exponential equation r

2

Percent difference1

Drainage area equation Wandle equations

Median Range Median Range

Low High Low High

Q5 = 266.3*EXP(0.038*DA) 0.93 22 -44 152 -50 10 -64

Q10 = 393.1*EXP(0.037*DA) 0.94 20 -37 140 -55 -3.4 -68

Q25 = 625.1*EXP(0.035*DA) 0.95 15 -29 119 -58 -16 -71

Q50 = 865.5*EXP(0.034*DA) 0.95 16 -20 113 -60 -10 -74

Q100 = 1189*EXP(0.032*DA) 0.94 10 -22 97 -61 -3.6 -76

Q200 = 1614*EXP(0.031*DA) 0.93 4.7 -22 94 -- -- --

Q500 = 2382*EXP(0.028*DA) 0.89 8.0 -34 73 -- -- --

1

 Calculated from flow magnitudes determined from peak-flow frequency analysis in this study at nine streamflow-gaging stations (excludes 

Merrimack River at Lowell, Mass.) compared to drainage area equations developed in this study and to Wandle’s regional equations (Wandle, 

1983). Stations used to develop the equations are indicated in figure 42.
Summary 63

the magnitude of floods at Nissitissit River at Pepperell and 

Spicket River near Methuen because these stations have been 

in operation a short time, the contributing area lies mostly in 

New Hampshire, and no surrogate station could be used as an 

alternative way of estimating floods. At Shawsheen River at 

Andover, which also has a short record, the upstream station 

at Wilmington was used to estimate the magnitude of flood 

flows. The return interval of the April 2007 flood was determined at each of the 10 stations by the various peak-flow 

frequency analyses. In addition, flood magnitudes for various return intervals determined in this study were compared 

to flood discharge and stage information available in various 

flood-insurance-study reports. 

The April 2007 peak discharge was among the highest 

discharges recorded or estimated since 1936, often ranking 

between the 3d- and 5th-highest discharges for that period. 

The return interval of the April 2007 peak discharge differed 

among sites and the methods used to compute flood flows. 

The peak discharge and stage from the April 2007 storm, 

along with the return interval of the storm on the basis of 

discharge and stage, are summarized in table 25. If the April 

2007 peak discharge was within 10 percent of the computed 

discharge from the peak-flow frequency analysis for a given 

return interval, then that return interval was assigned to the 

storm. If the April 2007 peak discharge differed by more 

than 10 percent from the computed discharge for any given 

return interval, then the storm return interval was assigned to 

the return interval for the discharge closest to the storm peak 

with a “less than” symbol (<) to indicate that the storm peak 

was between the return interval and the next lowest return 

interval, but closest to the return interval indicated. The April 

2007 peak discharge was never closer to a return interval with 

a discharge greater than the storm peak; therefore, a “greater 

than” symbol (>) was never used. The return intervals for the 

95-percent confidence limits of the April 2007 peak discharge 

are also included in table 25.

In general, the April 2007 storm peak discharge has a 

return interval between 25 and 50 years, although at the northeasternmost stations the storm has an expected return interval of about 5 years. At Sevenmile River near Spencer, the 

April 2007 peak discharge return interval computed from the 

extended records was less than at most other stations (about a 

10-year return interval). The return interval computed from the 

extended-record analysis generally was the same at the 95-percent confidence limits, except at stations where the April 2007 

peak discharge differed from the computed return interval by 

more than 10 percent. The return period of the April 2007 peak 

discharge computed from the period of record, although long 

in many cases, had a much larger band of uncertainty–often 

encompassing the next-lowest and next-highest return interval bands. In general, the April 2007 peak-discharge return 

interval computed from the extended-record analysis was 

less than the return interval computed from the period-ofrecord analysis, indicating that for a given return interval, the 

period-of-record discharge was less than the discharge computed from the extended-record analysis. This finding underscores the leveraging effect of the large pre-record storms in 

the computation of flood-flow magnitude.

The magnitude of flood flows for the Merrimack River at 

Lowell is complicated by a historical record with a period of 

no flood control, a transition period as flood-control reservoirs 

were built, and the period of current flood-control operation. 

Various methods for analyzing the peak-flows record for the 

Merrimack River at Lowell produced appreciably different results. Flood flows computed from post-flood-control 

peak-flow records with the adjusted 1936 and 1938 peak 

flow treated as historical events indicate the April 2007 storm 

approached a 100-year return flow. Flood flows computed 

from an estimated continuous post-flood-control peak record 

back to 1936 indicate the April 2007 peak discharge is near 

a 50-year return flow. The uncertainty of the return interval 

of the post-flood-control floods is large, spanning from the 

next-lowest to the next-highest return interval regardless of the 

analysis method used.

The return intervals of the April 2007 nor’easter peak 

stage in relation to various flood-insurance-study flood profiles 

differed considerably from the results of peak-flow frequency 

analysis at some sites, but were comparable at other sites. The 

magnitude of flood flows used in the flood-insurance studies used to compute flood stage often differed considerably 

(typically much lower) from the magnitude of flood flows 

determined by peak-flow frequency analysis in this study. 

The flood stage determined from the current (2008) stagedischarge rating at a given streamflow-gaging station and the 

computed magnitude of flood flows for various return intervals 

also resulted in a much different stage at the station than that 

shown in the flood-insurance-study flood profiles.

Equations for estimating the flow magnitudes for 5-, 10-, 

25-, 50-, 100-, 200-, and 500- year floods were developed 

from the relation of drainage area to the magnitude of flood 

flows calculated at six streamflow-gaging stations with the 

longest unaltered peak-flow record (North Nashua River at 

Fitchburg, Stillwater River near Sterling, Squannacook River 

near West Groton, Mill River at Northampton, Sevenmile 

River near Spencer, and West Branch Westfield River near 

Huntington). These equations produced a more conservative 

estimate of flood flows (higher discharges) than the existing 

regional equations for estimating flood flows at ungaged rivers 

in Massachusetts. Large differences in the magnitude of flood 

flows for various return intervals between the current peakflow frequency analysis results and those of existing regional 

equations and discharges used in flood-insurance studies 

indicate a need to develop up-to-date, robust regional equations from all appropriate streamflow-gaging station data in 

Massachusetts and nearby states.
64 Characteristics of the April 2007 Flood at 10 Streamflow-Gaging Stations in Massachusetts Table 25. Summary of April 2007 peak flows and stages, and the estimated return interval of the storm peak discharge and stage at ten selected streamflow-gaging stations in Massachusetts. [USACE, U.S. Army Corps of Engineers; mi2, square miles; ft3/s, cubic feet per second; ft, feet in NVGD 29; FEMA, Federal Emergency Management Agency; --, not determined; <, less than] U.S. Geological Survey streamflow-gaging station Drain- age area (mi2

)

Start of 

record

Years of 

record

April 2007 peak

Expected return interval of April 2007 peak discharge or stage

Period-of-record 

discharge analysis

Extended-record 

discharge analysis

5FEMA 

stage

Discharge 

(ft3/s)

Stage

(gage 

datum)

Stage

(ft)

Return 

interval

95-percent 

confidence limit Return 

interval

95-percent 

confidence limit Return 

interval

Name Number Lower Upper Lower Upper

North Nashua River at Fitchburg 01094400 64.2 Oct. 1972 35 3,930 8.59 403.48 50 25 100 25 25 25 10–25Stillwater River near Sterling 01095220 31.6 Apr. 1994 13 1,850 9.10 406.15 -- -- -- < 25 10 25 50–100Squannacook River near 

West Groton

01096000 63.7 Oct. 1949 58 4,820 8.50 252.77 50 25 100 25 25 25 10

Nissitissit River at Pepperell 01096503 59.6 Mar. 2006 1 3,700 8.29 183.26 325 -- -- < 50 25 50 50Merrimack River at Lowell 01100000 4,635 Jun. 1923 84 89,800 58.09 63.27 25 25 25 -- -- -- 50with USACE flood-control dams 1964 144–272 1<100 50 200 250 25 100 --Spicket River near Methuen 01100561 62.1 Mar. 2001 6 1,360 9.01 109.91 3

5 -- -- 5 5 10 10–50

Shawsheen River at Andover 01100627 72.8 Oct. 2006 1 1,470 34.15 34.15 -- -- -- 4

5 5 5 10–50

Mill River at Northampton 01171500 52.6 Oct. 1938 70 6,150 15.58 188.26 50 25 100 50 25 50 10–50Sevenmile River near Spencer 01175670 8.81 Feb. 1961 47 453 12.12 646.55 25 10 25 10 10 10 --West Branch Westfield River near 

Huntington

01181000 94.0 Sep. 1935 72 19,700 12.82 396.42 25 25 50 -- -- -- 10

1 Record included USACE estimates of 1936 and 1938 peaks as if flood-control reservoirs were in place.2 Record estimated for 1936–64 period using MOVE-1 as if USACE flood-control reservoirs were in place.3 Estimated using New Hampshire regional equations (Olson, 2009).4 Estimated from Shawsheen River near Wilmington record adjusted by drainage-area ratio.5 FEMA community flood-insurance studies.
References Cited 65

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Prepared by the Pembroke Publishing Service Center.

For more information concerning this report, contact:

Director

U.S. Geological Survey

Massachusetts-Rhode Island Water Science Center

10 Bearfoot Road

Northborough, MA 01532

dc_ma@usgs.gov

or visit our Web site at:

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Zarriello—Characteristics of the April 2007 Flood at 10 Streamflow-Gaging Stations in Massachusetts—Scientific Investigations Report 2009–5068