U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2008–5206

Prepared in cooperation with the 

New Hampshire Department of Transportation

Estimation of Flood Discharges at

Selected Recurrence Intervals for

Streams in New Hampshire
Cover. Photograph of Suncook River at Main Street in the Village of Suncook, New Hampshire, during the Flood of May 2006.
Estimation of Flood Discharges at 

Selected Recurrence Intervals for 

Streams in New Hampshire

By Scott A. Olson

Prepared in cooperation with the

New Hampshire Department of Transportation

Scientific Investigations Report 2008–5206

U.S. Department of the Interior

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

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

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

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Olson, S.A., 2009, Estimation of flood discharges at selected recurrence intervals for streams in New Hampshire: 

U.S. Geological Survey Scientific Investigations Report 2008–5206, 57 p.
iii

Contents

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

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

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

Description of Study Area ...................................................................................................................2

Flood Discharges at Selected Recurrence Intervals for Streamgages................................................4

Peak Discharge Data Used in this Study ..........................................................................................4

Determination of the Magnitude and Frequency of Flood Discharges for Streamgages.........7

Generalized Skew........................................................................................................................7

Record Extension.........................................................................................................................7

Combined Records of Nearby Streamgages...........................................................................7

Magnitude and Frequency of Flood Discharges for Streamgages ............................................10

Maximum Recorded Floods and Envelope Curves........................................................................10

Characteristics of Streamgage Drainage Basins...................................................................................10

Regression Equations for Estimation of Flood Discharges at Selected Recurrence Intervals 

for Ungaged Stream Sites.............................................................................................................13

Regression Analysis and Final Regression Equations..................................................................13

Limitations and Sensitivity.................................................................................................................14

Accuracy of the Regression Equations...........................................................................................16

Accuracy Analysis of Individual Estimates from the Regression Equations ............................17

Standard Error of Prediction....................................................................................................17

Equivalent Years of Record ......................................................................................................19

Prediction Intervals ...................................................................................................................19

Use of Regression Equations at or Near Streamgages................................................................19

Drainage-Area-Only Regression Equations ...................................................................................20

New Hampshire StreamStats ....................................................................................................................22

Summary........................................................................................................................................................22

Selected References...................................................................................................................................22

Appendix 1. Mean April Precipitation...................................................................................................48

Appendix 2. Wetland Areas ....................................................................................................................49

Appendix 3. Example Application ..........................................................................................................50
iv

Figures

1–3. Maps showing—

1. Location of major rivers and waterbodies in New Hampshire ....................................3

2. Locations of the 117 streamgages and upstream drainage basins used to 

develop regression equations for estimating the magnitude of flood flows for 

selected recurrence intervals in New Hampshire.........................................................5

3. Generalized skew coefficients of logarithms of annual peak discharge in 

New Hampshire ...................................................................................................................8

4–5. Graphs showing—

4. Flood-discharge frequency curves for selected streamgages in 

New Hampshire .................................................................................................................11

5. Maximum recorded flood discharges at streamgages in New Hampshire in and 

adjacent to New Hampshire in relation to drainage area, with envelope lines 

and a regression line relating the 100-year discharge (Q100) to drainage area.......12

6. Scatterplots showing residuals of the regression equation for estimating the 

magnitude of (A) a 10-year flood discharge, and (B) a 100-year flood discharge 

in relation to basin characteristics for selected streamgages in and adjacent to 

New Hampshire ..........................................................................................................................15

Tables

1. Descriptions of streamgages on selected streams in and adjacent to 

New Hampshire ..........................................................................................................................25

2. Differences between the 100-year flood discharge computed from the period of 

record and from the period 1988–2007 for selected streamgages in and adjacent to 

New Hampshire ............................................................................................................................6

3. Streamgages for which record extension was applied, the associated long-term 

record streamgage, and correlation coefficients for streams in and adjacent to 

New Hampshire ............................................................................................................................9

4. Discontinued streamgages, associated active downstream streamgage, and 

drainage area ratio for selected streams in and adjacent to New Hampshire................10

5. Flood discharges at selected recurrence intervals for streamgages on selected 

streams in and adjacent to New Hampshire..........................................................................34

6. Basin characteristics determined for use in regression analysis .....................................39

7. Basin characteristics of selected streamgages in and adjacent to New Hampshire 

and vicinity used in the development of the final regression equations...........................43

8. Ranges of explanatory variables used in the development of the regression 

equations for estimating flood discharges at selected recurrence intervals for 

ungaged, unregulated streams in New Hampshire..............................................................16

9. Results of sensitivity analysis presented as percent change in computed flood 

discharge as a result of a 10-percent change of the input basin characteristic.............16

10. Measures of accuracy of the regression equations for estimating flood discharges 

at selected recurrence intervals for ungaged, unregulated streams in rural 

drainage basins in New Hampshire ........................................................................................17

11. Model error variance and the (XtrΛ-1X)

-1 matrices for the regression equations.............18

12. Measures of accuracy of the drainage-area-only regression equations used for 

estimating flood discharges at selected recurrence intervals for ungaged, 

unregulated streams in rural drainage basins in New Hampshire ....................................21
v

Conversion Factors, Datum, and Abbreviations

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 foot (ft2) 929.0 square centimeter (cm2)

square foot (ft2) 0.09290 square meter (m2)

square mile (mi2) 259.0 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2) 

Volume

cubic foot (ft3) 28.32 cubic decimeter (dm3) 

cubic foot (ft3) 0.02832 cubic meter (m3) 

Discharge

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

cubic foot per second per square 

mile [(ft3/s)/mi2]

0.01093 cubic meter per second per square 

kilometer [(m3/s)/km2]

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

 °C=(°F–32)/1.8

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

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1927 (NAD 27).

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

OTHER ABBREVIATIONS

NHDOT New Hampshire Department of Transportation

GIS Geographic Information System

NED National Elevation Dataset

PRISM Parameter-elevation Regression on Independent Slopes Model

USGS U.S. Geological Survey

VIF Variance Inflation Factor
THIS PAGE INTENTIONALLY LEFT BLANK
Abstract

This report provides estimates of flood discharges at 

selected recurrence intervals for streamgages in and adjacent 

to New Hampshire and equations for estimating flood 

discharges at recurrence intervals of 2-, 5-, 10-, 25-, 50-, 

100-, and 500-years for ungaged, unregulated, rural streams 

in New Hampshire. The equations were developed using 

generalized least-squares regression. Flood-frequency and 

drainage-basin characteristics from 117 streamgages were used 

in developing the equations. The drainage-basin characteristics 

used as explanatory variables in the regression equations 

include drainage area, mean April precipitation, percentage of 

wetland area, and main channel slope. The average standard 

error of prediction for estimating the 2-, 5-, 10-, 25-, 50-, 

100-, and 500-year recurrence interval flood discharges with 

these equations are 30.0, 30.8, 32.0, 34.2, 36.0, 38.1, and 

43.4 percent, respectively.

Flood discharges at selected recurrence intervals for 

selected streamgages were computed following the guidelines 

in Bulletin 17B of the U.S. Interagency Advisory Committee 

on Water Data. To determine the flood-discharge exceedence 

probabilities at streamgages in New Hampshire, a new 

generalized skew coefficient map covering the State was 

developed. The standard error of the data on new map is 

0.298. To improve estimates of flood discharges at selected 

recurrence intervals for 20 streamgages with short-term 

records (10 to 15 years), record extension using the twostation comparison technique was applied. The two-station 

comparison method uses data from a streamgage with 

long-term record to adjust the frequency characteristics at a 

streamgage with a short-term record.

A technique for adjusting a flood-discharge frequency curve computed from a streamgage record with 

results from the regression equations is described in this 

report. Also, a technique is described for estimating flood 

discharge at a selected recurrence interval for an ungaged site 

upstream or downstream from a streamgage using a drainagearea adjustment. 

The final regression equations and the flood-discharge 

frequency data used in this study will be available in 

StreamStats. StreamStats is a World Wide Web application 

providing automated regression-equation solutions for userselected sites on streams.

Introduction

Flooding is the most costly natural hazard experienced 

in New Hampshire. Intense precipitation, a series of closely 

spaced major storms, springtime storms combined with 

snowmelt, tropical cyclones, and ice jams have all caused 

flooding in New Hampshire. Rarely do floods in New 

Hampshire have the same severity statewide. Since systematic 

monitoring of New Hampshire streams and their floods began 

in the early 1900s, floods with a recurrence interval greater 

than 50 years have occurred in parts of the State in 1927, 

1936, 1938, 1943, 1953, 1959, 1973, 1987 (Hammond, 1991), 

1996, 2005 (Olson, 2006), 2006 (Olson, 2007), and 2007 

(Flynn, 2008).

In response to extensive damage caused by the closely 

spaced floods of 1927, 1936, and 1938, flood-control dams 

and reservoirs were built by the U.S. Army Corps of Engineers 

in the Merrimack and Connecticut River basins to decrease 

damages caused by flooding of major rivers. However, 

flooding continues to be a constant threat. In recent years 

(2005–07), another series of closely spaced, severe floods 

has occurred, resulting in the loss of life and property. These 

recent floods have been the greatest recorded at several 

streamgages on rivers and streams in southern New Hampshire 

(Olson, 2006 and 2007; Flynn, 2008).

The U.S. Geological Survey (USGS) and other agencies 

have been measuring and recording discharge at numerous 

streamgage sites throughout New Hampshire for the past 

100 years. Continuous monitoring of discharge occurs at these 

streamgages. One purpose of the data collected from these 

streamgages is the characterization of the magnitudes and 

frequencies of flood discharges for rivers in the State. In 2008, 

there were 48 continuously operating streamgages in New 

Hampshire. 

Estimates of the magnitude and frequency of flood 

discharges are needed to design safe and economical bridges, 

culverts, and other structures in or near streams; identify 

flood-hazard areas; and manage flood plains. Computation of 

flood-discharge magnitude and frequency estimates requires 

a statistical analysis of peak discharge data collected at 

streamgages. However, these estimates often are required 

for ungaged sites where no observed peak-discharge data are 

available. Investigations that provide methods for estimating 

flood-discharge frequency at ungaged sites in New Hampshire 

Estimation of Flood Discharges at Selected Recurrence 

Intervals for Streams in New Hampshire

By Scott A. Olson
2 Estimation of Flood Discharges at Selected Recurrence Intervals for Streams in New Hampshire

include Benson (1962), Potter (1957a, 1957b), LeBlanc 

(1978), and Dingman and Palaia (1999). Updated flooddischarge frequency estimates benefiting from additional years 

of peak-discharge data and enhanced statistical procedures can 

improve techniques for estimating flood-discharge frequency 

at ungaged sites. To address this, the USGS, in cooperation 

with the New Hampshire Department of Transportation 

(NHDOT), conducted this study to develop updated methods 

for estimating the flood discharges at selected recurrence 

intervals for unregulated and ungaged stream locations in 

New Hampshire.

Purpose and Scope

This report (1) provides estimates of flood discharges 

at recurrence intervals of 2-, 5-, 10-, 25-, 50-, 100-, and 

500-years for streamgages in and adjacent to New Hampshire 

and (2) describes methods, including the use of equations 

developed from regression analyses, for estimating flood 

discharges at selected frequencies on ungaged, unregulated 

New Hampshire streams. In addition, this report (3) presents a 

method for estimating the standard error of prediction for each 

estimate made using the regression equations and (4) describes 

methods for transferring a flood-discharge estimate for a 

selected recurrence interval at a streamgage to a site upstream 

or downstream, based on drainage area.

Description of Study Area

New Hampshire (fig. 1) comprises a land area of 

9,304 mi2 in the northeastern United States, nearly one-seventh 

of the total land area of New England. The State is approximately 175 mi long from north to south and ranges from about 

100 mi wide (east to west) at the southern end of the State to 

nearly 20 mi wide at its northern end. The southeast corner of 

the State borders the Atlantic Ocean for 18 mi.

The terrain is hilly to mountainous with some flatlands 

along the coastline and along some river valleys. Land 

elevations range from sea level to about 500 ft in the 

southeastern part of the State from the Atlantic coastline 

inland, perpendicular to the coastline for 30 to 40 mi. 

Elevations are less than 500 ft in the valley floors of much 

of the Merrimack and Connecticut Rivers. Elevations in the 

remainder of the State generally range from 500 to 1,500 ft 

with many mountains having summits reaching elevations 

of 2,000 to 4,000 ft. The exception is the northernmost parts 

of the State where elevations typically exceed 1,500 ft. The 

White Mountains in the north-central part of New Hampshire 

have numerous summits that exceed 4,000 ft; Mount 

Washington reaches 6,288 ft. In the National Land Cover 

Data (Multi-Resolution Land Characteristics Consortium, 

2003) database, land-cover classification in New Hampshire is 

approximately 78 percent forest, 8 percent developed land, 

7 percent wetland, and 5 percent agricultural.

New Hampshire has about 40 rivers with a stream 

network totaling 41,800 mi (New Hampshire State Library, 

2008). Four major rivers drain New Hampshire. The 

Connecticut River is the largest. The headwaters extend to 

the northernmost tip of the State and the river flows southerly 

making up most of the western border of New Hampshire. The 

headwaters of the Merrimack River are in the southern slopes 

of the White Mountains and the river flows southerly through 

central and southern New Hampshire. The Androscoggin and 

Saco Rivers drain eastern slopes of mountains in northern 

New Hampshire and flow southeasterly into Maine. New 

Hampshire has approximately 1,300 lakes or ponds that 

account for 277 total square miles (New Hampshire State 

Library, 2008). The largest lake is Lake Winnipesaukee, which 

covers an area of 71 mi2, in the central part of the State.

Although the climate of New Hampshire is temperate 

and humid with four distinct seasons, it is renowned for its 

dramatic climatic extremes and rapid changes in weather. The 

mean annual air temperature ranges from about 41°F in the 

north to 45°F in the south. However, the historic range in temperature recorded in Concord, NH, is -37°F to 102°F.

Precipitation in the State is distributed fairly evenly 

throughout the year and averages about 43 in. annually. Areas 

with a high elevation may receive an additional 10 or more 

inches of precipitation annually. For example, the rain gage 

at 6,262 ft on Mount Washington receives an annual average 

of 102 in. of precipitation, whereas a rain gage located 18 mi 

north at an elevation of 930 ft receives an annual average of 

40 in. (National Climatic Data Center, 2008).

Annual snowfall also varies across the State. Elevation 

has a strong effect on snowfall amounts. The coastal areas 

of New Hampshire receive about 50 in. of snowfall a year, 

and inland areas receive 60 to 70 in. (National Climatic 

Data Center, 2008). The average snowfall increases to more 

than 100 in. at higher elevations in the northern and western 

portions of the State; Mt. Washington receives more than 

300 in. annually (Mount Washington Observatory, 2008).
Introduction 3

Figure 1. Location of major rivers and waterbodies in New Hampshire.

73° 72° 71°

45°

44°

43°

Shaded relief: U.S. Geological Survey National Elevation Dataset, 2007, 1/3 arc-second resolution

Lakes/rivers: U.S. Geological Survey National Hydrography Dataset, 2007, 1:24,000

State Boundaries: U.S. Geological Survey Digital Line Graphs, 1990–94, 1:24,000

Mt. Washington

Lake

Winnipesaukee

Concord

WHITE MOUNTAINS

NEW HAMPSHIRE

VERMONT

CANADA

UNITED STATES

MASSACHUSETTS

MAINE

Connecticut River

Merr mi ack

River

Androscoggin River

Saco River

Atlantic

Ocean

EXPLANATION

Hill shaded

representation

of New Hampshire State boundaries

Lakes

Major rivers

0 5 10 15 20 MILES

0 10 20 KILOMETERS
4 Estimation of Flood Discharges at Selected Recurrence Intervals for Streams in New Hampshire

Flood Discharges at Selected 

Recurrence Intervals for Streamgages

To develop techniques for estimating flood-discharge 

magnitude and frequencies for ungaged stream locations, 

flood-discharge magnitude and frequency are first established 

at streamgages. The magnitude and frequency of floods at 

streamgages then can be statistically related to the physical 

and climatic characteristics of the contributing drainage basin 

upstream from the streamgage. The statistical relations that are 

established at the streamgages then can be used to estimate the 

magnitudes and frequencies of floods at an ungaged site using 

drainage-basin characteristics.

Peak Discharge Data Used in this Study

All available peak discharge data in New Hampshire 

and adjacent, physiographically similar areas in Maine, 

Massachusetts, and Vermont, and Quebec, Canada, collected 

by the USGS, the U.S. Forest Service, the New Hampshire 

Department of Environmental Services, and Environment 

Canada were considered for this study. These data include 

records from continuously recording streamgages and creststage streamgages (streamgages that record only the annual 

peak discharge), both current (2008) and discontinued.

Of the sites considered, 117 streamgages were selected 

for use in this study (fig. 2; table 1 in back of report). The 

selection criteria required the streamgage to have a minimum 

of 10 years of annual peak-discharge data that were free 

of trends and unaffected by regulation or urbanization. 

Regulation was assumed to have a negligible effect on peak 

discharges if the usable storage in the basin was less than 

4.5 million cubic feet per square mile of drainage area 

(Benson, 1962). Peak-discharge data from sites that had 

greater than 4.5 million cubic feet of usable storage per square 

mile of drainage area were not used. 

None of the streamgages included in this investigation 

have drainage basins considered to be urbanized. The maximum percentage of land area in a streamgage drainage basin 

classified as having high intensity developed land in the 2001 

National Land Cover Data (Multi-Resolution Land Characteristics Consortium, 2003) is 2.8 percent; the average is 0.1 percent. The maximum percentage of land area in a streamgage 

drainage basin classified as having medium or high intensity 

developed land is 11 percent; the average is 0.9 percent.

The 117 streamgages selected are spatially well distributed in and adjacent to New Hampshire (fig. 2). However, a 

review of the sites reveals that the hundreds of small drainage 

basins in New Hampshire with drainage areas of less than 

15 mi2 are poorly represented. Only 12 small drainage basins 

in New Hampshire had sufficient discharge data for the study, 

and currently (2008) only one of the streamgages is active. 

Therefore, streamgages of small drainage basins in States 

adjacent to New Hampshire were included to compensate for 

this shortage.

To ensure that trends in the annual peak-discharge 

data did not exist, a two-sided Kendall Tau trend test 

(Helsel and Hirsch, 1992) was completed. The trend test 

was done with a program developed by the USGS called 

SWSTAT, a Computer Program for Interactive Computation 

of Surface-Water Statistics (A.M. Lumb, W.O. Thomas, Jr., 

and K.M. Flynn, U.S. Geological Survey, written commun., 

1994). Streamgage records with less than 15 years of annual 

peak-discharge data were not tested for trends because trends 

over a period of record this short cannot be distinguished from 

serial correlation.

No substantial trends were found by the Kendall Tau test 

for the peak-discharge data used in this study. The Kendall 

Tau statistics indicated that an upward trend may exist for 21 

of the 117 streamgages. However, for streamgages indicating 

a possible trend, the trend could be explained as the result 

of extreme climatic anomalies near the beginning or end 

of the peak-discharge record, such as the 2006 and 2007 

floods (Olson, 2007; Flynn, 2008) or the drought of 1960–69 

(Hammond, 1991). The evidence of trends did not exist or was 

statistically insignificant when extreme events, such as those, 

were eliminated from the Kendall Tau trend tests.

In recent years there has been much speculation regarding the impact of climatic changes on annual peak discharges 

(Milly and others, 2008). To evaluate if increased annual peak 

discharges have occurred in recent decades, flood-discharge 

frequency curves were computed for 14 active streamgages in 

and adjacent to New Hampshire having 60 years or more of 

unregulated annual peak discharge data. These flood-discharge 

frequency curves were compared to curves computed using 

only the most recent 20 years (1988–2007) of record. At 4 

of the 14 streamgages, the flood-frequency curves (10- to 

500-year recurrence interval) computed from the 1988–2007 

data had greater discharges than the flood-frequency curves 

computed from the period of record; at 7 of the streamgages, 

frequency curves had smaller discharges; and at 2 streamgages, the frequency curves had little (less than 3 percent) to 

no change. Nearly identical results were found using the most 

recent 25 years of record (1983 to 2007). Table 2 displays 

the flood discharges for the 14 streamgages at the 100-year 

recurrence interval computed using the period of record for the 

streamgages and using the 1988 to 2007 period.

The results shown in table 2 indicate no definitive patterns in annual peak discharges that would suggest limiting the 

use of the entire period of record available for a streamgage. 

The annual peak-discharge data used in this study are therefore 

regarded as random, independent events that are homogeneous 

for a streamgage throughout the period of record.
Flood Discharges at Selected Recurrence Intervals for Streamgages 5

Figure 2. Locations of the 117 streamgages and upstream drainage basins used to develop regression equations for 

estimating the magnitude of flood flows for selected recurrence intervals in New Hampshire. (Leading 0 has been dropped 

from the streamgage station number)

1174565

1173900

1170900

1170200

1170100

1169900

1169000 1165500

1163200 1163100

1162500

1162000

1161300 1161500

1101000

1100900

1100800

1100700

1100100

1097300

1097200 1096910

1096000

1095800

1095000

1094500

1069700

1066500

1066100

1065500 1066000

1063310

1062700

1057000

1055300

1055000

1054300

1054200

1050900

1167800

1160000

1158500

1157000

1156450

1156300

1155350

1155300

1155200

1154000 1155000

1153550

1153300

1152500

1151200 1150900

1150800 1145000

1144000

1142500

1142400

1142000

1141800

1140800

1139800

1139700 1139000

1138800

1138000

1137500

1135700

1135500

1135300

1135150

1135000

1134800

1134500 1133300

1133200

1133000 1130000

1129700

1129440

1129400

1129300

1127880

1094000 1093800

1091000

1089500

1089100

1086000 1085800

1084500

1084000

1082000

1078000

1076500 1076000

1075800

1075000

1074520

1073750

1073600

1073587

1073500

1073000

1072850

1072800

1065000

1064801

1064800

1064500

1064400

1064380

1064300

1052500

10965852

73° 72° 71° 70°

45°

44°

43°

CANADA

UNITED STATES

MASSACHUSETTS

MAINE

VERMONT

NEW HAMPSHIRE

EXPLANATION

Drainage basin of streamgage State boundary

Major river Streamgage and 

identifying number

1157000

Rivers: U.S. Geological Survey National Hydrography Dataset, 2007, 1:24,000

State Boundaries: U.S. Geological Survey Digital Line Graphs, 1990–94, 1:24,000

0 10 20 30 MILES

0 10 20 30 KILOMETERS

Atlantic

Ocean
6 Estimation of Flood Discharges at Selected Recurrence Intervals for Streams in New Hampshire

Table 2. Differences between the 100-year flood discharge computed from the period of record and from the period 1988–2007 for 

selected streamgages in and adjacent to New Hampshire.

[Locations of streamgages are shown in figure 2. ft3/s, cubic feet per second]

USGS 

streamgage 

station 

number

Station name Years

Flood discharge 

at a 100-year 

recurrence interval 

(ft3/s)

Change in the 100-year 

flood discharge computed 

from the period of record 

to the period 1988–2007

01052500 Diamond River near Wentworth Location, NH 1942–2007 10,400 Increase by 15 percent

1988–2007 12,000

01064500 Saco River near Conway, NH 1904–09, 1930–2007 52,800 Decrease by 26 percent

1988–2007 38,900

01073000 Oyster River near Durham, NH 1935–2007 1,200 Increase by 56 percent

1988–2007 1,870

01073500 Lamprey River near Newmarket, NH 1935–2007 9,270 Increase by 53 percent

1988–2007 14,200

01076500 Pemigewassett River at Plymouth, NH 1904–2007 57,700 Decrease by 29 percent

1988–2007 40,800

01078000 Smith River near Bristol, NH 1919–2007 5,740 Decrease by 8 percent

1988–2007 5,290

01094000 Souhegan River at Merrimack, NH 1910–76, 1982–2007 12,200 No change1

1988–2007 12,100

01134500 Moose River at Victory, VT 1947–2007 4,740 Decrease by 6 percent

1988–2007 4,440

01135500 Passumpsic River at Passumpsic, VT 1929–2007 17,000 No change1

1988–2007 17,000

01137500 Ammoonoosuc River at Bethlehem Junction, NH 1940–2007 12,200 Decrease by 11 percent

1988–2007 10,800

01139000 Wells River at Wells River, VT 1941–2007 5,170 Decrease by 5 percent

1988–2007 4,900

01142500 Ayers Brook at Randolph, VT 1940–2007 2,820 Increase by 29 percent

1988–2007 3,640

01144000 White River at West Hartford, VT 1916–2007 61,800 Decrease by 31 percent

1988–2007 42,800

01152500 Sugar River at West Claremont, NH 1929–2007 12,800 Decrease by 10 percent

1988–2007 11,500

1

Less than 3-percent change in discharge is reported as no change.
Flood Discharges at Selected Recurrence Intervals for Streamgages 7

Determination of the Magnitude and Frequency 

of Flood Discharges for Streamgages

The 2-, 5-, 10-, 25-, 50-, 100-, and 500-year flood discharges for the 117 streamgages (table 1 in back of report) 

were computed using the guidelines in Bulletin 17B 

(U.S. Interagency Advisory Committee on Water Data, 1982). 

Bulletin 17B recommends fitting the systematic annual peakdischarge data to a log-Pearson Type III probability distribution for estimating flood-discharge magnitude and frequency 

and provides procedures for weighting station skews, historical peaks, and the detection and treatment of outliers. Software 

developed by the USGS to analyze flood-discharge frequency, 

PEAKFQ, was used for these computations (W.O. Thomas, 

Jr., A.M. Lumb, K.M. Flynn, and W.H. Kirby, U.S. Geological 

Survey, written commun., 1997). Peak discharges affected by 

dam failure, ice-jam breach, or a similar event are not included 

in the frequency analyses.

Generalized Skew

 Estimates of the magnitude and frequency of flood 

discharges are sensitive to skew—the measure of the lack 

of symmetry in the probability distribution of annual peakflow data. Extreme flood events often affect skews computed 

from a streamgage peak-discharge record, and the impact of 

an extreme flood on skew is greater the shorter the length of 

streamgage record. To compensate for this effect, the skew 

used in estimating flood discharges for selected recurrence 

intervals at a streamgage is weighted with a generalized skew 

estimated by pooling the skews from nearby streamgages. 

The generalized skew can be taken from the generalized skew 

map in Bulletin 17B (U.S. Interagency Advisory Committee 

on Water Data, 1982). However, the map, prepared in 1976, 

is outdated and lacks the resolution desired for the study area. 

For these reasons, a new method for obtaining generalized 

skews for New Hampshire was developed.

Several methods for estimating skews for streamgages 

were tested, including a statewide average of skew, a data 

interpolation technique, and a multiple-regression analysis 

using skew and basin characteristics. Skews with the smallest error were obtained from a new generalized skew map 

developed using a geographical information system (GIS) 

tool, ArcGIS Spatial Analyst Kriging (Environmental Systems Research Institute, Inc., 2006). The procedure generates 

an estimated surface from a point dataset, which then can be 

converted to a contour map. The point dataset was comprised 

of computed skews from streamgages used in this study with 

25 or more years of peak-discharge record. 

Because a computed skew coefficient is a biased 

estimator of skew, a bias correction factor, Cb, was developed 

by Tasker and Stedinger (1986), where Cb = 1+6/N and N is 

the number of years of streamgage peak-discharge record. 

All skew coefficients were adjusted using this bias correction 

factor prior to generating the contour map. 

Each computed skew was located at the centroid of the 

respective basin and was given the same weight regardless 

of the number of years of record for the streamgage or the 

drainage area of the basin the skew was representing. The 

new generalized skew map for New Hampshire (fig. 3) has 

a standard error of estimate of 0.298, an improvement over 

the standard error of estimate (0.461) for the State from 

the National isoline skew map (U.S. Interagency Advisory 

Committee on Water Data, 1982).

Record Extension

Record extension was used to improve the estimates 

of flood discharges at selected recurrence intervals for 

streamgages with relatively short peak-discharge records 

(10 to 15 years). The record-extension method used in this 

study was the two-station comparison method described 

in Guidelines for Determining Flood Flow Frequency 

(U.S. Interagency Advisory Committee on Water Data, 1982). 

The two-station comparison technique of record extension 

involves adjusting the logarithmic mean and standard 

deviation of a short-term streamgage record with flooddischarge statistics from a long-term streamgage record. Using 

this two-station comparison method, records were extended 

for 20 streamgages with short-term records.

For the application of the record-extension method, 

the streamgage with the long-term record had to be on an 

unregulated stream. In addition, the short- and long-term 

record streamgages had to have correlated peak-discharge 

data during their concurrent period of record. Of the 

20 streamgages for which record extension was done, the 

minimum correlation coefficient determined using data from 

the long- and short-term record pairs was 0.631, which is 

within the guidelines for the two-station comparison technique 

documented in Guidelines for Determining Flood Flow 

Frequency (U.S. Interagency Advisory Committee on Water 

Data, 1982); the average was 0.817. The 20 streamgages for 

which this technique was used are listed in table 3, along with 

the long-term record streamgage and correlation coefficients.

Combined Records of Nearby Streamgages

Three discontinued streamgages included in this 

investigation were located a short distance upstream from 

new, active streamgages; the period of record of the older 

streamgages falls within 1929 to 1987. In each case, the 

location of the discontinued streamgage and the newer, 

active streamgage are considered to be proximate enough to 

have similar drainage-basin characteristics and represent a 

single streamgage. For this reason, table 1 (in back of report) 

contains 120 streamgages although 117 streamgages are 

used in the regression analysis. Before combining the peakdischarge record of the older, upstream streamgage and peakdischarge record of the newer, downstream streamgage, the 

peak discharges from the upstream streamgage were adjusted 
8 Estimation of Flood Discharges at Selected Recurrence Intervals for Streams in New Hampshire

Figure 3. Generalized skew coefficients of logarithms of annual peak discharge in New Hampshire.

0

0.1

0.2

0.3

-0.1

0.4

-0.2

0.5

0.3

0.1

45°

71°

72°

44°

43°

NEW HAMPSHIRE

VERMONT

MAINE

CANADA

0 10 20 30 MILES

0 10 20 30 KILOMETERS

EXPLANATION

County boundary

State boundary

Generalized skew coefficient contour

MASSACHUSETTS

Coos

Strafford

Merrimack

Hillsborough

Grafton

Carroll

Rockingham

Sullivan

Cheshire

Belknap

County Boundaries: U.S. Geological Survey Digital Line Graphs, 1990–94, 1:24,000
Flood Discharges at Selected Recurrence Intervals for Streamgages 9

Table 3. Streamgages for which record extension was applied, the associated long-term record streamgage, and correlation 

coefficients for streams in and adjacent to New Hampshire.

[Locations of streamgages are shown on figure 2]

Short-term record Long-term record

Correlation coefficient

USGS 

streamgage 

station 

number

Station name

Years 

of 

record

USGS

streamgage 

station 

number

Station name

Years 

of 

record

01050900 Four Ponds Brook near Houghton, ME 11 01055000 Swift River near Roxbury, ME 78 0.789

01055300 Bog Brook near Buckfield, ME 11 01057000 Little Androscoggin River near 

South Paris, ME

86 0.799

01062700 Patte Brook near Bethel, ME 10 01064500 Saco River near Conway, NH 84 0.933

01064380 East Branch Saco River at Town Hall 

Road, near Lower Bartlett, NH

10 01064500 Saco River near Conway, NH 84 0.776

01064800 Cold Brook at South Tamworth, NH 10 01064500 Saco River near Conway, NH 84 0.777

01066100 Pease Brook near Cornish, ME 10 01066500 Little Ossipee River near 

South Limington, ME

43 0.736

01069700 Branch Brook near Kennebunk, ME 10 01073000 Oyster River near Durham, NH 73 0.703

01072800 Cocheco River near Rochester, NH 12 01073500 Lamprey River near Newmarket, NH 73 0.843

01073587 Exeter River at Haigh Road, near 

Brentwood, NH

11 01073500 Lamprey River near Newmarket, NH 73 0.939

01073750 Mill Brook near State Route 108, 

at Stratham, NH

11 01073000 Oyster River near Durham, NH 73 0.762

01095800 Easter Brook near North Leominster, MA 11 01094500 North Nashua River near 

Leominster, MA

72 0.859

01097200 Heath Hen Meadow Brook at Stow, MA 11 01097300 Nashoba Brook near Acton, MA 44 0.844

01100700 East Meadow River near Haverhill, MA 12 01073000 Oyster River near Durham, NH 73 0.939

01138800 Keenan Brook near Groton, VT 11 01139800 East Orange Branch at East 

Orange, VT

49 0.916

01155200 Sackets Brook near Putney, VT 11 01154000 Saxtons River at Saxtons River, VT 48 0.927

01155300 Flood Brook near Londonderry, VT 11 01154000 Saxtons River at Saxtons River, VT 48 0.631

01163100 Wilder Brook near Gardner, MA 11 01162000 Millers River near Winchendon, MA 90 0.818

01170200 Allen Brook near Shelburne Falls, MA 11 01162000 Millers River near Winchendon, MA 90 0.796

01170900 Mill River near South Deerfield, MA 12 01165500 Moss Brook at Wendell Depot, MA 66 0.743

01173900 Middle Branch Swift River at North 

New Salem, MA

11 01162500 Priest Brook near Winchendon, MA 90 0.810
10 Estimation of Flood Discharges at Selected Recurrence Intervals for Streams in New Hampshire

by the drainage area ratio of the two sites. Discontinued 

streamgages for which peak discharge records were adjusted 

for drainage area and combined with the record from a newer, 

downstream streamgage are shown in table 4.

Magnitude and Frequency of Flood Discharges 

for Streamgages

The magnitudes of flood discharges at selected recurrence 

intervals for streamgages used in this study are shown in 

table 5 (in back of report). The flood discharge at a given 

recurrence interval, in years, represents the flood-discharge 

frequency curve of each streamgage used in this investigation. 

Flood-discharge frequency curves, determined using logPearson Type III analysis, are shown in figure 4.

Maximum Recorded Floods and 

Envelope Curves

The maximum recorded flood discharges (table 5 in back 

of report) plotted against drainage area for each streamgage in 

this study are displayed in figure 5. Flood discharges affected 

by dam failure, ice-jam breach, or a similar event are not 

included. An envelope curve encompassing the maximum 

recorded floods is included in the figure, along with a line 

developed using generalized least-squares regression analysis 

showing the relation between drainage area and the 100-year 

discharge for each streamgage. A regional envelope developed 

by Crippen and Bue (1977) is also shown. Figure 5 can be 

used to evaluate the reasonableness of flood estimates made 

using techniques described in this report.

Characteristics of Streamgage 

Drainage Basins

In flood-frequency regression analysis, the variations 

in the magnitude of flood discharges at a selected recurrence 

interval for streamgages used in the study are related to 

variations in basin characteristics. The flood discharges are 

the dependent variables, and the basin characteristics are the 

independent or explanatory variables. For this study, 110 basin 

characteristics were determined for each streamgage, including 

physical properties such as drainage area, channel slope, 

elevation, forest cover, lake area, and soil permeability; and 

climatic characteristics such as precipitation and temperature.

Boundaries for the drainage basins of the streamgages 

were obtained from the Watershed Boundary Dataset 

(Natural Resources Conservation Service, 2001) or were 

digitized manually on 1:24,000 digital raster graphs 

(U.S. Geological Survey, 2001) when the basin boundary was 

not defined by the Watershed Boundary Dataset. Additional 

dimensional properties of the drainage basins and waterways 

were computed with the ArcHydro software (Environmental 

Systems Research Institute, Inc., 2008) using a digital 

elevation dataset derived from the National Elevation Dataset 

(NED) (U.S. Geological Survey, 2007a). Prior to being used 

for basin delineation, the NED was hydrologically corrected 

(U.S. Geological Survey, 2004, and P.A. Steeves and 

A.H. Rea, U.S. Geological Survey, written commun., 2007) 

using the National Hydrography Dataset (U.S. Geological 

Survey, 2007b) and the Watershed Boundary Dataset (National 

Resources Conservation Service, 2001).

Table 4. Discontinued streamgages, associated active downstream streamgage, and drainage area ratio for selected streams in and 

adjacent to New Hampshire.

[mi2, square miles]

Discontinued streamgage Active downstream streamgage

Drainage area 

ratio

USGS 

streamgage 

station 

number

Station name

Drainage area 

(mi2)

Period of 

record

USGS 

streamgage 

station 

number

Station name

Drainage area 

(mi2)

Period of 

record

01074500 East Branch Pemigewassett River near 

Lincoln, NH

106 1929–52, 

1960, 

1968–70

01074520 East Branch Pemigewasset River at 

Lincoln, NH

115 1993–2007 1.085

01089000 Soucook River near 

Concord, NH

77.7 1952–87 01089100 Soucook River, at 

Pembroke Road, 

near Concord, NH

82.7 1989–2007 1.064

01153500 Williams River at 

Brockways Mills, VT

102 1941–84 01153550 Williams River near 

Rockingham, VT

112 1987–2007 1.098
Characteristics of Streamgage Drainage Basins 11 Figure 4. Flood-discharge frequency curves for selected streamgages in New Hampshire.

RECURRENCE INTERVAL, IN YEARS

ANNUAL NONEXCEEDANCE PROBABILITY, IN PERCENT

DISCHARGE, IN CUBIC FEET PER SECOND

2 5 10 25 50 100 250 500

99.8 99.5 99 98 95 90 85 80 70 60 50

500

5,0002,0001,000

50,00020,00010,000

100,000

01157000 Ashuelot River near Gilsum, NH01089500 Suncook River at North Chichester, NH01076500 Pemigewasset River at Plymouth, NH

010965852 Beaver Brook at North Pelham, NH

01074520 East Branch Pemigewasset River at Lincoln, NH

EXPLANATION
12 Estimation of Flood Discharges at Selected Recurrence Intervals for Streams in New Hampshire

Figure 5. Maximum recorded flood discharges at streamgages in New Hampshire in and adjacent to 

New Hampshire in relation to drainage area, with envelope lines and a regression line relating the 100-year 

discharge (Q100) to drainage area.

0.1 1 10 100 1,000 10,000

DRAINAGE AREA, IN SQUARE MILES

10

100

1,000

10,000

100,000

1,000,000

DISCHARGE, IN CUBIC FEET PER SECOND

Maximum recorded flood discharge

at streamgages in and adjacent 

to New Hampshire

Envelope line for maximum recorded

flood discharges at streamgages in 

and adjacent to New Hampshire

Regional envelope line for 

maximum flood discharges

(Crippen and Bue, 1977)

Q100 = 219A0.800

EXPLANATION
Regression Equations for Estimation of Flood Discharges at Selected Recurrence Intervals for Ungaged Stream Sites 13

With appropriate GIS datasets, other basin characteristics also were delineated with the ArcHydro software. The 

National Hydrography Dataset, the 2001 National Land Cover 

Data (Multi-Resolution Land Characteristics Consortium, 

2003), the State soil geographic (STATSGO) database (U.S. 

Geological Survey, 1995), and the National Wetlands Inventory (U.S. Fish and Wildlife Service, 2007) were the source 

GIS datasets for land-surface properties. The sources for 

climatic data were PRISM (Parameter-elevation Regressions 

on Independent Slopes Model) (PRISM Group, Oregon State 

University, 2006a–c) and Climatesource (Daly and others, 

2000). The PRISM climatic data were resampled to a 180-ftcell resolution. The source for runoff data was Randall (1996), 

and the source for population data was the U.S. Census Bureau 

(2006). The physiographic divisions were from Fenneman and 

Johnson (1946). Maps showing the 24-hr rainfall with 2- and 

100-year recurrence intervals (U.S. Department of Commerce, 

1961) were delineated manually for use in a GIS. A complete 

list of basin characteristics determined for potential use as 

explanatory variables in the regression analysis is presented in 

table 6 (in back of report).

Regression Equations for Estimation 

of Flood Discharges at Selected 

Recurrence Intervals for Ungaged 

Stream Sites

Multiple-regression techniques, employing generalized 

least-squares regression (Stedinger and Tasker, 1985), 

were used to define relations between the flood-discharges 

determined for the streamgages at the 2-, 5-, 10-, 25-, 50-, 

100-, and 500-year recurrence intervals (dependent variables) 

and the basin characteristics (independent variables) of those 

streamgages. The use of generalized least-squares regression 

allows for weighting of streamgage data to compensate for 

the differences in record length and the cross-correlation 

of concurrent records among streamgages. Furthermore, 

Stedinger and Tasker (1985) showed that generalized leastsquares regression equations are more accurate and provide 

better estimates of model error than ordinary least-squares 

regression equations, when working with flood frequency.

The regression results provide equations for estimating 

the values of dependent variables from one or more independent variables. The regression equations take the general form

YT = bo + b1X1 + b2X2 +...+ bjXj , (1)

where

YT is the magnitude of the flood discharge having 

a recurrence interval of T years,

bo to bj are coefficients developed from the regression 

analysis, and

X1 to Xj are the basin characteristics.

When transformation to the explanatory and response 

variable are logarithmic, equation 1 can be manipulated to 

take the form

YT = 10boX1

b1

X2

b2

... Xj

bj . (2)

The limitations, sensitivity, and accuracy of the regression equations are reported following the final regression 

equations. In addition, techniques are discussed for determining the accuracy, equivalent years of record, and confidence 

intervals for each individual estimate from the regression 

equations. Methods of weighting regression equation estimates 

with streamgage data when the regression equations are used 

for a site near or at a streamgage also are discussed.

Regression Analysis and Final Regression 

Equations

More than 100 basin characteristics were determined for 

each streamgage and used in the regression analysis. Mathematical transformations were applied to each basin characteristic and flood discharge statistic to obtain the most linear 

relations. The transformations used were logarithms, square 

roots, squares, and raising the values to the -0.125 power. 

Correlation data and stepwise linear regression (SAS Institute, 

Inc., 1990) were used to evaluate which basin characteristics, transformed or untransformed, were the most significant 

explanatory variables. 

Next, generalized least-squares regression techniques were used to determine the final significant basin 

characteristics and to compute the final regression equations. 

The generalized least-squares regression analysis was done 

using GLSNET, a hydrologic regression and streamflownetwork analysis program that uses the generalized leastsquares regression procedure (G.D. Tasker, K.M. Flynn, 

A.M. Lumb, W.O. Thomas, Jr., U.S. Geological Survey, 

written commun., 1995). The basin characteristics used in 

the development of the final regression equations are listed 

in table 7 (in back of report), by streamgage. Logarithmic 

base-10 transformations were made on all final variables in the 

equations except for the percentage of the basin covered by 

wetlands. The final regression equations (equations 3–9) for 

estimating flood discharges on ungaged, unregulated streams 

in rural drainage basins in New Hampshire are as follows:

Q2 = 2.60A0.958P1.5010-0.0245(W)S0.205 , (3)

Q5 = 3.23A0.929P1.7310-0.0245(W)S0.211 , (4)

Q10 = 3.88A0.912P1.8310-0.0247(W)S0.211 , (5)

Q25 = 4.99A0.892P1.9010-0.0250(W)S0.207 , (6)
14 Estimation of Flood Discharges at Selected Recurrence Intervals for Streams in New Hampshire

Q50 = 5.96A0.879P1.9410-0.0252(W)S0.203 , (7)

 Q100 = 7.13A0.867P1.9810-0.0254(W)S0.198 , and (8)

Q500 = 10.6A0.841P2.0310-0.0259(W)S0.183 , (9)

where

QT is the estimated flood discharge, in cubic feet 

per second, at a T-year recurrence interval;

A is the drainage area of the basin, in square 

miles, computed using the ArcHydro 

software (Environmental Systems 

Research Institute, Inc., 2008) (boundaries 

were from the Watershed Boundary 

Dataset (Natural Resources Conservation 

Service, 2001) or digitized manually when 

a basin boundary was not defined by this 

GIS coverage);

P is the basinwide mean of the average April 

precipitation, in inches, determined with 

the PRISM 1971–2000 April precipitation 

dataset (PRISM Group, Oregon State 

University, 2006c) resampled with bilinear 

interpolation to a 180-ft-cell resolution;

W is the percentage of the basin with land cover 

categorized as wetland from the National 

Land Cover Data (Multi-Resolution Land 

Characteristics Consortium, 2003) using 

a GIS. Waterbody areas from the National 

Hydrography Dataset (U.S. Geological 

Survey, 2007b), which include lakes, 

ponds, and swamps, were used in areas 

north of the New Hampshire-Quebec 

border where the National Land Cover 

Data does not extend; and

S is the slope of the main channel, in feet 

per mile, determined between points 

10- and 85-percent up the main channel 

from the selected stream site extended to 

the drainage divide using the ArcHydro 

software (Environmental Systems Research 

Institute, Inc., 2008) and elevation datasets 

derived from the National Elevation 

Dataset (U.S. Geological Survey, 2007a).

The procedure for estimating the flood discharge at a selected 

recurrence interval for an ungaged stream site using the regression equations is described in appendix 3.

Attempts were made to group streamgages with similar 

geographic, peak-discharge, or drainage-basin characteristics 

into subregions to reduce the standard error of the regression equations. This grouping would have resulted in a set of 

regression equations for each subregion. To evaluate whether 

subregions should be generated, residuals, the difference 

between the flood discharges estimated from the frequency 

analysis and the flood discharges predicted from the regression 

equations, were determined for each streamgage and for each 

regression equation. These residuals were plotted spatially at 

the streamgage location and plotted in relation to drainagebasin characteristics. Residuals of the 10- and 100-year regression equations in relation to drainage-basin characteristics are 

shown as examples in figures 6A and 6B. No apparent trends 

or patterns were observed in these plots. Thus, the streamgages 

were not grouped into subregions, and the equations presented 

in this report are intended for statewide use.

The residual plots shown in figures 6A and 6B also were 

used as a diagnostic tool for the regression equations. The random scatter of the points above and below the zero reference 

line provides verification that the model is satisfactorily meeting the assumptions of multiple-linear-regression techniques. 

Other diagnostics tools included the evaluation of Cook’s D 

and the variance inflation factor (VIF) (Helsel and Hirsch, 

1992). Cook’s D is a value that is computed for each observation—the data used for developing the regression equations. 

It is a measure of the influence of each observation on the 

regression equations and can be used to assist in the identification of outliers. The magnitude of Cook’s D flagged some 

observations as potential outliers; however, it was concluded 

that the potential outliers were sound data and there was no 

justification for excluding them from the regression analysis.

The VIF is a diagnostic tool that may be used to evaluate collinearity of explanatory variables. There are no formal 

criteria for VIF, although some authors suggest that a VIF 

exceeding 10 may be cause for concern (Freund and Littell, 

2000), indicating that explanatory variables may be correlated. 

The greatest VIF computed for a variable used in the final 

regression equations was 3.9.

Limitations and Sensitivity

It is important to note that basin characteristics used to 

develop equations 3–9 were determined with the ArcHydro 

(Environmental Systems Research Institute, Inc., 2008) software using datasets described in the section entitled “Regression Analysis and Results.” Determining the basin characteristics for use in the regression equations with alternate data 

sources or using different computational methods than those of 

the ArcHydro software may produce statistics that are different than those reported here, and may introduce bias and yield 

discharge estimates that have unknown error.

The regression equations are applicable only to sites 

on ungaged, unregulated streams in rural New Hampshire 

basins. Use of the equations is appropriate to sites with 

drainage-basin characteristics that are within the range of 

drainage-basin characteristics used in the development of the 

equations. The ranges of drainage-basin characteristics used 

in the analysis are shown in table 8. If independent variables 

used in the regression equations are outside of these ranges, 

the results of the equations are considered extrapolations, 
Regression Equations for Estimation of Flood Discharges at Selected Recurrence Intervals for Ungaged Stream Sites 15

Figure 6. Residuals of the regression equation for estimating the magnitude of (A) a 10-year flood discharge, 

and (B) a 100-year flood discharge in relation to basin characteristics for selected streamgages in and adjacent to 

New Hampshire.

0.1 1.0 10.0 100.0 1,000.0 10,000.0

DRAINAGE AREA, IN SQUARE MILES

-0.5

0.0

0.5

-0.4

-0.3

-0.2

-0.1

0.1

0.2

0.3

0.4

RESIDUAL, IN LOGARITHMIC UNITS

5.0 6.0

APRIL PRECIPITATION, IN INCHES

-0.5

0.0

0.5

-0.4

-0.3

-0.2

-0.1

0.1

0.2

0.3

0.4

RESIDUAL, IN LOGARITHMIC UNITS

15 20 25

PERCENT OF BASIN COVERED BY WETLANDS

-0.5

0.0

0.5

-0.4

-0.3

-0.2

-0.1

0.1

0.2

0.3

0.4

RESIDUAL, IN LOGARITHMIC UNITS

0 10 100 1,000

CHANNEL SLOPE, IN FEET PER MILE

-0.5

0.0

0.5

-0.4

-0.3

-0.2

-0.1

0.1

0.2

0.3

0.4

RESIDUAL, IN LOGARITHMIC UNITS

0.1 1 10 100 1,000 10,000

DRAINAGE AREA, IN SQUARE MILES

-0.5

0.0

0.5

-0.4

-0.3

-0.2

-0.1

0.1

0.2

0.3

0.4

RESIDUAL, IN LOGARITHMIC UNITS

APRIL PRECIPITATION, IN INCHES

-0.5

0.0

0.5

-0.4

-0.3

-0.2

-0.1

0.1

0.2

0.3

0.4

RESIDUAL, IN LOGARITHMIC UNITS

0 5 10 15 20 25

PERCENT OF BASIN COVERED BY WETLANDS

-0.5

0.0

0.5

-0.4

-0.3

-0.2

-0.1

0.1

0.2

0.3

0.4

RESIDUAL, IN LOGARITHMIC UNITS

10 100 1,000

CHANNEL SLOPE, IN FEET PER MILE

-0.5

0.0

0.5

-0.4

-0.3

-0.2

-0.1

0.1

0.2

0.3

0.4

RESIDUAL, IN LOGARITHMIC UNITS

2 3 4

2.0 3.0 4.0

0 5 10

5 6 7

1

A 10-Year Flood Discharge

B 100-Year Flood Discharge
16 Estimation of Flood Discharges at Selected Recurrence Intervals for Streams in New Hampshire

and the accuracy of the predictions is unknown. For sites that 

have drainage-basin characteristics outside the acceptable 

ranges, a simplified equation that uses drainage area as the 

only explanatory variable is provided in the section entitled 

“Drainage-Area-Only Regression Equations.” For sites that 

are considered urban, Moglen and Shivers (2006) describe 

techniques for transforming rural flood-discharge-frequency 

estimates to estimates for urban watersheds.

The sensitivity of each regression equation to changes 

in the magnitude of the independent variables was tested to 

evaluate the amount of error that can be introduced if basin 

characteristics are incorrectly computed. The sensitivity 

analysis was conducted by adjusting a basin characteristic 

by plus or minus 10 percent while holding the other basin 

characteristics constant at their respective mean magnitudes. 

The results of the sensitivity analysis listed in table 9 indicate 

that the regression equations are most sensitive to changes in 

drainage area and the average April precipitation. However, a 

10-percent change in the April precipitation value accounts for 

a much greater percentage of its range (table 8) than does a 

10-percent change in drainage area.

Accuracy of the Regression Equations

There are several measures of the accuracy of a regression equation. The adjusted coefficient of determination, or 

adjusted R2, indicates the proportion of variability observed 

in the data that is accounted for by the regression model 

(SAS Institute, Inc., 2000). The closer the adjusted coefficient 

of determination is to 1, the better the regression explains the 

variation in the response variables. The adjusted coefficient 

of determination for each regression equation is presented 

in table 10.

Table 8. Ranges of explanatory variables used in the development of the regression equations for estimating flood 

discharges at selected recurrence intervals for ungaged, unregulated streams in New Hampshire.

Explanatory variable Minimum Maximum Mean

Drainage area, in square miles 0.70 1,290 83.6

Basinwide mean of average April precipitation, in inches 2.79 6.23 3.94

Percent of the basin covered by wetlands 0.0 21.8 4.86

Main channel slope, in feet per mile 5.43 543 114

Table 9. Results of sensitivity analysis presented as percent change in computed flood discharge as a result of a 10-percent 

change of the input basin characteristic.

Percent change in 

basin characteristic

Percent change in computed flood discharge by the regression equation for 

estimating floods having a recurrence interval of:

2-years 5-years 10-years 25-years 50-years 100-years 500-years

Drainage area

+10 9.6 9.3 9.1 8.9 8.7 8.6 8.3

-10 -9.6 -9.3 -9.2 -9.0 -8.8 -8.7 -8.5

Basinwide mean of average April precipitation

+10 15.4 18.0 19.0 20.0 20.4 20.7 21.4

-10 -14.6 -16.7 -17.5 -18.2 -18.5 -18.8 -19.3

Percentage of basin covered by wetlands

+10 -2.7 -2.7 -2.7 -2.8 -2.8 -2.8 -2.9

-10 2.8 2.8 2.8 2.8 2.9 2.9 2.9

Main channel slope

+10 1.8 2.0 2.0 2.0 2.0 1.9 1.8

-10 -2.1 -2.2 -2.2 -2.2 -2.1 -2.1 -1.9
Regression Equations for Estimation of Flood Discharges at Selected Recurrence Intervals for Ungaged Stream Sites 17

One of the most common measures of accuracy is the 

standard error of regression (table 10). This is a measure of 

how much the regression results deviate from the observed 

data. It is computed from the variance of the regression results 

with 5 degrees of freedom (Ott, 1988). 

Another measure of accuracy is the average standard 

error of prediction (table 10). This has a model error component—the error resulting from the model—and a sampling 

error component—the error that results from development 

of model parameters from samples of the population. Thus, 

the average standard error of prediction is a measure of 

the expected accuracy of a regression model applied at an 

ungaged location with basin characteristics similar to those 

used to develop the regression equation. This measure of 

accuracy is needed because regression equations typically are 

used for ungaged locations. About two-thirds of the estimates 

made using a regression equation for ungaged locations will 

have errors less than the average standard error of prediction 

for that equation. 

A fourth accuracy metric shown in table 10 is the average equivalent years of record. This is defined as the number 

of years of data collection at a stream location that would be 

required to achieve flood-discharge frequency results with 

accuracy equal to that of the regression equations. 

Accuracy Analysis of Individual Estimates from 

the Regression Equations

Because the standard error of regression, the average 

standard error of prediction, and the average equivalent years 

of record (table 10) are computed using all the streamgage 

data, they are approximations of the regression equation’s 

overall accuracy for ungaged sites. Techniques for computing 

the accuracy of individual regression equation estimates for 

ungaged sites are available and are discussed in this section. 

The measures of accuracy for an individual estimate include 

standard error of prediction, equivalent years of record, and 

prediction intervals.

Standard Error of Prediction

Hodge and Tasker (1995) describe the mathematical formulation for computing the standard error of prediction, SEpred, 

of a flood-discharge frequency estimate as

SEpred = [γ2+ xi(XtrΛ-1X)-1xi

tr]1/2 , (10)

where

SEpred is the standard error of prediction;

γ2 is the model error variance (table 11);

xi is a row vector containing 1, log10(A), 

log10(P), W, and log10(S) for the study 

site i;

tr is the matrix algebra symbol for transposing a 

matrix; and

(XtrΛ-1X)-1 is the (p × p) matrix with X being a (n × p) 

matrix that has rows of logarithmically 

transformed basin characteristics 

augmented by a 1 and Λ being the 

(n × n) covariance matrix used for 

weighting sample data in the generalized 

least-squares regression; n is the number 

of streamgages used in the regression 

analysis, and p is the number of basin 

characteristics plus 1; the (XtrΛ-1X)-1

matrices for selected recurrence intervals 

are shown in table 11.

Table 10. Measures of accuracy of the regression equations for estimating flood discharges at selected recurrence intervals for 

ungaged, unregulated streams in rural drainage basins in New Hampshire.

[R2, coefficient of determination]

Flood frequency with 

recurrence interval of: Adjusted R2 Standard error of regression Average standard error 

of prediction Average equivalent 

years of record in log units in percent in log units in percent

2 years 0.97 0.132 -26.2 to 35.5 0.128 -25.5 to 34.3 3.2

5 years 0.97 0.139 -27.4 to 37.7 0.132 -26.2 to 35.5 4.7

10 years 0.96 0.147 -28.7 to 40.3 0.137 -27.1 to 37.1 6.2

25 years 0.95 0.159 -30.7 to 44.2 0.145 -28.4 to 39.6 8.0

50 years 0.94 0.170 -32.4 to 47.9 0.153 -29.7 to 42.2 9.0

100 years 0.94 0.181 -34.1 to 51.7 0.162 -31.1 to 45.2 9.8

500 years 0.91 0.210 -38.3 to 62.2 0.183 -34.4 to 52.4 11.0
18 Estimation of Flood Discharges at Selected Recurrence Intervals for Streams in New Hampshire

Table 11. Model error variance and the (XtrΛ-1X)-1 matrices for the regression equations.

[Numbers in matrices are in scientific notation]

Flood-frequency 

characteristic

Model error 

variance,

γ2

(XtrΛ-1X)-1 matrix

Intercept Drainage 

area

April 

precipitation

Percent 

wetland

Channel 

slope

Flood discharge with 

a 2-year recurrence 

interval

0.0153 Intercept

Drainage area

April precipitation

Percent wetland

Channel slope

 0.29131E-01 -0.24237E-02 -0.23530E-01 -0.29390E-03 -0.55764E-02

 -0.24237E-02 0.61481E-03 -0.61196E-03 0.51072E-04 0.91224E-03

 -0.23530E-01 -0.61196E-03 0.47946E-01 -0.19204E-03 -0.17764E-02

 -0.29390E-03 0.51072E-04 -0.19204E-03 0.15059E-04 0.14686E-03

 -0.55764E-02 0.91224E-03 -0.17764E-02 0.14686E-03 0.26378E-02

Flood discharge with 

a 5-year recurrence 

interval

0.0160 Intercept

Drainage area

April precipitation

Percent wetland

Channel slope

 0.33144E-01 -0.27925E-02 -0.27017E-01 -0.33090E-03 -0.61190E-02

 -0.27925E-02 0.71282E-03 -0.71391E-03 0.58038E-04 0.10377E-02

 -0.27017E-01 -0.71391E-03 0.55423E-01 -0.21153E-03 -0.22804E-02

 -0.33090E-03 0.58038E-04 -0.21153E-03 0.16780E-04 0.16337E-03

 -0.61190E-02 0.10377E-02 -0.22804E-02 0.16337E-03 0.29618E-02

Flood discharge with a 

10-year recurrence 

interval

0.0171 Intercept

Drainage area

April precipitation

Percent wetland

Channel slope

 0.38041E-01 -0.32220E-02 -0.31238E-01 -0.37706E-03 -0.68710E-02

 -0.32220E-02 0.82466E-03 -0.83869E-03 0.66422E-04 0.11942E-02

 -0.31238E-01 -0.83869E-03 0.64408E-01 -0.23771E-03 -0.28151E-02

 -0.37706E-03 0.66422E-04 -0.23771E-03 0.18999E-04 0.18498E-03

 -0.68710E-02 0.11942E-02 -0.28151E-02 0.18498E-03 0.33809E-02

Flood discharge with a 

25-year recurrence 

interval

0.0192 Intercept

Drainage area

April precipitation

Percent wetland

Channel slope

 0.45989E-01 -0.39111E-02 -0.37992E-01 -0.45400E-03 -0.81537E-02

 -0.39111E-02 0.10022E-02 -0.10366E-02 0.80152E-04 0.14491E-02

 -0.37992E-01 -0.10366E-02 0.78706E-01 -0.28148E-03 -0.36210E-02

 -0.45400E-03 0.80152E-04 -0.28148E-03 0.22721E-04 0.22133E-03

 -0.81537E-02 0.14491E-02 -0.36210E-02 0.22133E-03 0.40725E-02

Flood discharge with a 

50-year recurrence 

interval

0.0212 Intercept

Drainage area

April precipitation

Percent wetland

Channel slope

 0.52917E-01 -0.45085E-02 -0.43828E-01 -0.52217E-03 -0.93013E-02

 -0.45085E-02 0.11551E-02 -0.12062E-02 0.92210E-04 0.16714E-02

 -0.43828E-01 -0.12062E-02 0.91015E-01 -0.32027E-03 -0.42923E-02

 -0.52217E-03 0.92210E-04 -0.32027E-03 0.26028E-04 0.25364E-03

 -0.93013E-02 0.16714E-02 -0.42923E-02 0.25364E-03 0.46798E-02

Flood discharge with a 

100-year recurrence 

interval

0.0235 Intercept

Drainage area

April precipitation

Percent wetland

Channel slope

 0.60575E-01 -0.51667E-02 -0.50248E-01 -0.59817E-03 -0.10587E-01

 -0.51667E-02 0.13231E-02 -0.13914E-02 0.10560E-03 0.19168E-02

 -0.50248E-01 -0.13914E-02 0.10452E-00 -0.36357E-03 -0.50145E-02

 -0.59817E-03 0.10560E-03 -0.36357E-03 0.29723E-04 0.28972E-03

 -0.10587E-01 0.19168E-02 -0.50145E-02 0.28972E-03 0.53530E-02

Flood discharge with a 

500-year recurrence 

interval

0.0301 Intercept

Drainage area

April precipitation

Percent wetland

Channel slope

 0.80866E-01 -0.69046E-02 -0.67187E-01 -0.80128E-03 -0.14036E-01

 -0.69046E-02 0.17649E-02 -0.18749E-02 0.14126E-03 0.25660E-02

 -0.67187E-01 -0.18749E-02 0.14008E-00 -0.47958E-03 -0.68759E-02

 -0.80128E-03 0.14126E-03 -0.47958E-03 0.39623E-04 0.38630E-03

 -0.14036E-01 0.25660E-02 -0.68759E-02 0.38630E-03 0.71400E-02
Regression Equations for Estimation of Flood Discharges at Selected Recurrence Intervals for Ungaged Stream Sites 19

The standard error of prediction of an estimate can be 

converted to positive and negative percent errors with the 

following formulas:

Spos

SEpred = − ( ) 100 10 1 , and (11)

Sneg

SEpred = − −

( ) 100 10 1 , (12)

where

Spos is the positive percent error of prediction,

SEpred is the standard error of prediction in 

logarithmic units, and

Sneg is the negative percent error of prediction.

The two formulas above apply not only to the standard 

error of prediction, but also to the standard error of regression 

by substituting the appropriate error term in logarithmic units. 

The probability that the true value of flood discharge at a 

given frequency is between the positive- and negative-percent 

standard error of prediction is approximately 68 percent. For 

example, there is a 68-percent chance that the true 10-year 

discharge at a site ranges from -27.1 to +37.1 percent of the 

estimated 10-year discharge.

Equivalent Years of Record

The equivalent years of record (Hardison, 1971) shown 

in table 10 is another measure of accuracy of the regression 

equations that can be computed for individual estimates. It can 

be interpreted as the number of years of data collection at an 

ungaged site that would be required to achieve flood-discharge 

frequency results with accuracy equal to that of the regression 

equations. It is computed with the formula

E

s k g k g

SE

T T

pred

=

 + + ( ) +  

 2 2 2

2

1 0. . 5 1 0 75 , (13)

where

E is the equivalent years of record;

s is the standard deviation of annual events 

estimated from a regression of the standard 

deviation and drainage area in square 

miles, A, at gages used in the regression 

analysis (s = e-(1.31 + 0.134log(A)));

kT is the log-Pearson type III frequency factor 

for the T-year event;

g is the skew used in the computation of the 

frequency curve (assumed to be zero when 

computing equivalent years of record for 

an estimate at an ungaged site); and

SEpred is the standard error of prediction.

Prediction Intervals

Prediction intervals indicate the uncertainty in the result 

of the equations. For example, one can be 90-percent confident that the true value of a flood-discharge estimate lies 

within the 90-percent prediction interval. Prediction intervals 

for selected percentages can be computed as follows:

Let

V t S n p Epred= ( − ) 10 / , 2 , (14)

then

(1/V)Qpred < Qtrue < (V)Qpred , (15)

where

t

α/2,n-p is the critical value from a Students-t

distribution at alpha level α (α = 0.10 

for a 90-percent confidence interval of a 

prediction) with n–p degrees of freedom; 

n = 117, the number of stations used in the 

regression analysis and p = 5, the number 

of basin characteristics in the regression 

equation, plus 1;

SEpred is the standard error of prediction of a flood’s 

discharge frequency estimate;

Qpred is the computed discharge at a selected 

frequency from the regression equation; 

and

Qtrue is the true value of discharge at a selected 

frequency.

Use of Regression Equations at or 

Near Streamgages

An estimate of flood discharge at a selected recurrence 

interval made at a streamgage can be adjusted by combining 

regression equation results with the frequency curve computed 

from the streamgage record. This technique may be particularly useful when a streamgage has a limited number of years 

available for frequency analysis. The procedure recommended 

in Bulletin 17B of the U.S. Interagency Advisory Committee on Water Data (1982) is to compute a weighted average 

of flood discharge using the regression equation estimate and 

the result of a log-Pearson type III analysis of the streamgage 

record using the following equation:

log Q N Q E Q

N E T w

T s T r g

10

10 10

,

, , log log = ( ) + ( )

+

( ) , (16)
20 Estimation of Flood Discharges at Selected Recurrence Intervals for Streams in New Hampshire

where

QT,w is the weighted flood discharge for the T-year 

recurrence interval,

QT,s is the flood-discharge for the T-year 

recurrence interval computed from the 

streamgage record,

QT,r(g) is the flood-discharge estimate for the T-year 

recurrence interval from the regression 

equation at the streamgage,

N is the number of years of streamgage record 

used to compute QT,s, and

E is the equivalent years of record for QT,r(g).

Magnitude and frequency of flood discharges for ungaged 

sites that are not at, but are relatively near, a streamgage 

and are on the same unregulated stream can be calculated 

by combined use of the regression equations and the nearby 

streamgage data. It is assumed for the procedure that drainage 

area and flood discharge are linearly related in logarithmic 

coordinates. The procedure should not be applied to sites 

that have a drainage area less than 50 percent, or greater than 

150 percent, of the drainage area of the streamgage. The 

method requires that estimates from the regression equations 

be determined at both the gaged and ungaged locations. The 

logarithmic slope, m, between the regression estimate for the 

streamgage and the regression estimate for the ungaged location of interest is computed as follows:

m

Q Q

A A

T r u T r g

u g

= ( ( ) ( ) )

( )

log /

log /

10 , ,

10

, (17)

where

QT,r(u) is the flood-discharge estimate at the T-year 

recurrence interval generated using the 

regression equation for the ungaged site,

QT,r(g) is the flood-discharge estimate at the T-year 

recurrence interval generated using the 

regression equation for the streamgage,

Au is the drainage area of the ungaged site, and 

Ag is the drainage area at the streamgage.

The next step is to determine the slope, c, of a line in 

logarithmic coordinates that goes through the weighted estimate of flood discharge at the T-year recurrence interval, QT,w, 

determined using equation 16 and intersects the line having 

slope equal to m at a point where full weight will be given 

to the regression equation. This slope can be computed as 

follows:

c m Q Q

a

T r g T w

= + ( )

( )

( ) log /

log

10 , ,

10

, (18)

where

a is the percentage of the drainage area, in 

decimal units, where full weight is given to 

the regression equation results. Typically

a = 0.5 for Au < Ag and a = 1.5 for Au > Ag.

The value of a determines where the lines having slope m

and c intersect. At this intersection, full weight is given to the 

regression equation. Modification of the magnitude of a will 

change how far upstream or downstream from the streamgage, 

in terms of percent of drainage area, an estimate can extend 

before full weight is given to the regression equations. In 

the definition of a above, the drainage area limits are 50 to 

150 percent of the streamgage drainage area, matching the 

rule of thumb often applied to these adjustments (Wandle, 

1983). Because of the log-linear relation of drainage area to 

discharge, the user may consider applying a = 0.667 when 

Au< Ag and limiting the weighting of a streamgage estimate 

to 66.7 percent of the drainage area. The reason for this is 

using drainage area limits of 66.7 to 150 percent will result 

in drainage areas that are symmetrical in difference from the 

streamgage drainage area in logarithmic units. Using 50 to 

150 percent of the streamgage drainage area will not result in 

a symmetrical change about the streamgage drainage area in 

logarithmic units.

The final step is to compute the weighted flood-frequency 

estimate for the ungaged site, QT,u, using

Q Q A

A T u T w

u

g

c

, , = 















. (19)

As with any technique used to compute a weighted flooddischarge estimate, unexpected results could occur if there is a 

substantial difference between the discharges being weighted. 

If the difference is substantial, c could become negative, indicating discharge and drainage area are inversely related. This 

procedure is not valid if c is negative.

Drainage-Area-Only Regression Equations

Some ungaged sites may have basin characteristics 

outside the acceptable ranges required by the full regression 

equations (equations 3–9). The acceptable ranges of the 

basin characteristics are described in the section titled 

“Limitations and Sensitivity.” Because of this, a set of 

simplified regression equations that incorporate drainage area 

as the only independent variable was developed. Generalized 

least-squares regression techniques were used to compute 

the coefficients in the equations. The simplified regression 

equations (equations 20–26) for estimating flood discharges 

on ungaged, unregulated streams in rural drainage basins in 

New Hampshire are as follows:
Regression Equations for Estimation of Flood Discharges at Selected Recurrence Intervals for Ungaged Stream Sites 21

Q2 = 43.5A0.885, (20)

Q5 = 75.5A0.858, (21)

Q10 = 102A0.842, (22)

Q25 = 143A0.824, (23)

Q50 = 179A0.811, (24)

 Q100 = 219A0.800, and (25)

Q500 = 331A0.777, (26)

where

QT is the estimated flood discharge, in cubic 

feet per second, at the T-year recurrence 

interval; and

A is the drainage area of the basin, in 

square miles.

The same 117 streamgages used to develop the previously presented regression equations were used to develop the 

simplified equations; hence, the equations would be applicable 

to sites with drainage areas from 0.70 to 1,290 mi2. Having only one explanatory variable, the simplified regression 

equations are less accurate than the full regression equations 

presented in this report. The standard error of regression and 

the average standard error of prediction of the simplified equations are presented in table 12.

Although the accuracy of the drainage-area-only regression equations is relatively poor, these simplified equations 

are valuable. The exponent in each of the drainage-area-only 

regression equations is the slope of the average linear logarithmic relation between drainage area and flood discharge for a 

selected recurrence interval. Hence, the exponent can be used 

in an alternate method for adjusting flood-frequency data from 

a streamgage to locations upstream and downstream. This 

use of the method should be limited to sites within 50- to 

150-percent of the streamgage drainage area (Wandle, 1983). 

Using this approach, one would use equation 19 with the 

exponent from the simplified regression equation at a selected 

recurrence interval substituted for c. 

Table 12. Measures of accuracy of the drainage-area-only regression equations used for estimating flood discharges at 

selected recurrence intervals for ungaged, unregulated streams in rural drainage basins in New Hampshire.

Flood frequency 

with recurrence 

interval of:

Standard error of regression Average standard error of prediction Average equivalent 

in log units in percent in log units in percent years of record

2 years 0.231 -41.3 to 70.2 0.228 -40.8 to 69.0 1.0

5 years 0.238 -42.2 to 73.0 0.233 -41.5 to 71.0 1.5

10 years 0.243 -42.9 to 75.0 0.237 -42.1 to 72.6 2.1

25 years 0.251 -43.9 to 78.2 0.241 -42.6 to 74.2 2.9

50 years 0.258 -44.8 to 81.1 0.245 -43.1 to 75.8 3.5

100 years 0.265 -45.7 to 84.1 0.250 -43.8 to 77.8 4.1

500 years 0.283 -47.9 to 91.9 0.262 -45.3 to 82.8 5.4
22 Estimation of Flood Discharges at Selected Recurrence Intervals for Streams in New Hampshire

New Hampshire StreamStats

StreamStats, a World Wide Web application (http://water.

usgs.gov/osw/streamstats/), allows users to obtain discharge 

statistics, drainage-basin characteristics, and other information for user-selected sites on streams. StreamStats users 

choose stream sites of interest from an interactive map. If 

a user selects the location of a USGS streamgage, the user 

will get previously published information for the site from a 

database. If a user selects an ungaged site, a GIS program will 

determine the boundary of the drainage basin upstream from 

the site and measure the basin characteristics required by the 

regression equations to estimate discharge statistics for the 

site. The application then solves the equations. The results 

are presented in a table along with a map showing the basin 

outline. Historically, determining the basin characteristics and 

solving the regression equations for an ungaged site could take 

an experienced person hours. StreamStats reduces the effort to 

only a few minutes.

Furthermore, the application ensures that the basin 

characteristics input to the regression equations are determined using the same data and methodologies as the basin 

characteristics used to develop the equations. This avoids 

bias that could be introduced by improperly estimating 

basin characteristics. 

New Hampshire StreamStats will become available 

online immediately following the publication of this report. 

The web application will provide flood-discharge frequency 

data for streamgages used in this study and compute flooddischarge frequency estimates for ungaged locations using the 

final regression equations (equations 3–9).

Summary

This report, prepared by the U.S. Geological Survey in 

cooperation with the New Hampshire Department of Transportation, documents the development of regression equations for 

estimating flood-discharge magnitudes for rural, unregulated 

New Hampshire streams at recurrence intervals of 2, 5, 10, 25, 

50, 100, and 500 years. Regression techniques were used to 

determine relations between the flood discharge magnitudes 

and selected basin characteristics at 117 streamgages in and 

adjacent to New Hampshire. 

The flood discharge magnitudes at selected recurrence 

intervals for the 117 streamgages were determined following 

guidelines in Bulletin 17B of the U.S. Interagency Advisory 

Committee on Water Data. Record-extension techniques 

were applied to improve the estimates of flood discharges for 

selected recurrence intervals at 20 streamgages with shortterm records (10–15 years). A generalized skew coefficient 

map, having a standard error of 0.298, was developed for the 

frequency analysis.

A total of 110 basin characteristics for each streamgage 

was determined using a Geographic Information System. 

Using correlation data, stepwise linear regression techniques, 

and generalized least-squares regression techniques, the 

110 basin characteristics were narrowed down to the four 

variables that best explained the magnitude and variability 

of flood discharges: drainage area, mean April precipitation, 

percentage of basin in wetlands, and slope of the main 

channel. The final regression equations were developed using 

generalized least-squares regression techniques. The average 

standard error of prediction for estimating the 2-, 5-, 10-, 25-, 

50-, 100-, and 500-year recurrence interval flood discharges 

with these equations are 30.0, 30.8, 32.0, 34.2, 36.0, 38.1, and 

43.4 percent, respectively. 

The regression equations developed from these relations 

can be used as a method for estimating flood discharges at 

selected recurrence intervals for ungaged, unregulated, rural 

streams. This report also presents methods for adjusting a 

flood-discharge frequency curve computed from a streamgage 

record with results from the regression equations. In 

addition, a technique is described for estimating flood 

discharge at a selected recurrence interval for an ungaged 

site upstream or downstream from a streamgage using a 

drainage-area adjustment. 

The equations and flood-discharge frequency data used in 

this study will be available in StreamStats, a World Wide Web 

application (http://water.usgs.gov/osw/streamstats/) providing 

statistics, drainage-basin characteristics, and other information 

for user-selected sites on streams.

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2000, 30 arc-second normals: Corvallis, OR, Oregon State 

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24 Estimation of Flood Discharges at Selected Recurrence Intervals for Streams in New Hampshire

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Data Coordination, 183 p.

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rural streams in Massachusetts: U.S. Geological Survey 

Water-Supply Paper 2214, 26 p.
Table 1 25

Table 1. Descriptions of streamgages on selected streams in and adjacent to New Hampshire.—Continued

[All streamgages are shown in figure 2; USGS, U.S. Geological Survey; lat, latitude; long, longitude; mi, mile; ft, feet]

USGS 

streamgage 

station 

number

Station name Station location

Period of 

record 

(water years)

01050900 Four Ponds Brook near 

Houghton, ME

Lat 44°49'55", long 70°42'10", Franklin County, at culvert on State Route 17, 

1.1 mi upstream of mouth at Bemis Stream, and 1.2 mi downstream of 

Long Pond outlet.

1964–74

01052500 Diamond River near 

Wentworth Location, NH

Lat 44°52'40", long 71°03'25", Coos County, on left bank, 0.8 mi downstream of 

the confluence of the Swift Diamond River and Dead Diamond River, 0.8 mi 

upstream of mouth, 1.3 mi north of Wentworth Location, and 7.7 northeast 

of Errol.

1942–2007

01054200 Wild River at Gilead, ME Lat 44°23'25", long 70°58'53", Oxford County, on right bank 200 ft upstream of 

U.S. Route 2 highway bridge, 2,000 ft upstream of mouth, and 0.4 mi west 

of Gilead.

1960, 

1965–2007

01054300 Ellis River at South 

Andover, ME

Lat 44°35'37", long 70°44'01", Oxford County, on left bank 100 ft upstream of 

covered bridge at South Andover.

1964–82, 

2001–2007

01055000 Swift River near 

Roxbury, ME

Lat 44°38'34", long 70°35'20", Oxford County, on left bank 0.2 mi downstream 

of Philbrick Brook, 2.1 mi downstream of Roxbury, and 7.2 mi upstream 

of mouth.

1930–2007

01055300 Bog Brook near 

Buckfield, ME

Lat 44°15'57", long 70°19'00", Oxford County, at culvert on State Route 117, 

0.2 mi upstream of mouth at Nezinscot River, and 3.0 mi southeast of State 

Routes 117 and 140 intersection in Buckfield.

1964–74

01057000 Little Androscoggin River 

near South Paris, ME

Lat 44°17'08", long 70°32'16", Oxford County, on island 50 ft upstream of 

Snow Falls, 5.9 mi north of the intersection of State Routes 26 and 117 in 

South Paris, and 6 mi upstream of South Paris.

1914–23, 

1932–2007

01062700 Patte Brook near 

Bethel, ME

Lat 44°20'41", long 70°47'34", Oxford County, at culvert on Old West Bethel 

Road 0.3 mi upstream of confluence with Crooked River, and 0.6 mi northwest 

of Old West Bethel Road and State Route 5 intersection.

1965–74

01063310 Stony Brook at East 

Sebago, ME

Lat 43°51'20", long 70°38'25", Cumberland County, on left bank at upstream side 

of culvert under State Route 11/114, 0.1 mi upstream of the confluence with 

Northwest River, and 0.6 mi upstream of mouth of Northwest River at 

Sebago Lake.

1996–2007

01064300 Ellis River near 

Jackson, NH

Lat 44°13'08", long 71°14'59", Carroll County, in White Mountain National 

Forest, on right bank, 0.2 mi downstream of small right-bank tributary, 0.4 mi 

upstream of small left-bank tributary, 1.3 mi upstream of bridge on NH 16, and 

6.1 mi northwest of the intersection of State Routes 16A and 16B in Jackson.

1964–2004

01064380 East Branch Saco River at 

Town Hall Road, near 

Lower Bartlett, NH

Lat 44°07'18", long 71°07'50", Carroll County, at bridge on Town Hall Road, 

725 ft downstream of Gardiner Brook confluence, 1.6 mi northeast of the intersection of State Routes 16 and 16A at Lower Bartlett, 2.5 mi west of the intersection of State Route 16 and U.S. Route 302 at Glen, and 3.0 mi upstream of 

mouth at Saco River. 

1967–76

01064400 Lucy Brook near 

North Conway, NH

Lat 44°04'13", long 71°10'24", Carroll County, on left bank, 0.6 mi west of 

Dianas Bath Road and River Road intersection, 1.8 mi west of Hurricane 

Mountain Road and U.S. Route 302 intersection at Intervale, 1.6 mi upstream 

of mouth at Saco River, and 2.3 mi northwest of River Road and U.S. Route 

302 intersection at North Conway.

1965–92

01064500 Saco River near 

Conway, NH

Lat 43°59'27", long 71°05'29", Carroll County, on left bank, at Odell Falls, 

0.4 mi upstream of U.S. Route 302, 1.5 mi northeast of the intersection of 

State Routes 16 and 113 in Conway, 1.8 mi downstream of the Swift River 

confluence.

1904–09, 

1930–2007
26 Estimation of Flood Discharges at Selected Recurrence Intervals for Streams in New Hampshire

Table 1. Descriptions of streamgages on selected streams in and adjacent to New Hampshire.—Continued

[All streamgages are shown in figure 2; USGS, U.S. Geological Survey; lat, latitude; long, longitude; mi, mile; ft, feet]

USGS 

streamgage 

station 

number

Station name Station location

Period of 

record 

(water years)

01064800 Cold Brook at South 

Tamworth, NH

Lat 43°48'57", long 71°17'50", Carroll County, on right bank 100 ft upstream 

of Bemis Mountain Road bridge, 0.7 mi south of Bemis Mountain Road and 

State Route 25 intersection at South Tamworth, 0.9 mi upstream of mouth at 

Bearcamp River.

1964–73

01064801 Bearcamp River at South 

Tamworth, NH

Lat 43°49'48", long 71°17'18", Carroll County, on right bank, 0.7 mi upstream 

of Sanger Brook confluence, 0.8 mi east of Bemis Mountain Road and 

State Route 25 intersection at South Tamworth, and 1.0 mi downstream of 

Cold Brook confluence.

1993–2007

01065000 Ossipee River at Effingham 

Falls, NH

Lat 43°47'42", long 71°03'35", Carroll County, on left bank, 0.2 mi upstream 

from State Route 153, 0.2 mi west of the intersection of State Routes 25 and 

153 at Effingham Falls, and 0.2 mi downstream of Ossipee Lake outlet.

1943–90, 

1998–2007

01065500 Ossipee River at 

Cornish, ME

Lat 43°48'26", long 70°47'55", Oxford County, on left bank 100 ft downstream of 

Bridge Street bridge in Cornish, and 1.3 mi upstream of mouth.

1917–96

01066000 Saco River at Cornish, ME Lat 43°48'35", long 70°46'53", Cumberland County, on left bank 300 ft upstream 

of State Route 117 highway bridge at Cornish and 0.4 mi downstream of 

Ossipee River confluence.

1917–2007

01066100 Pease Brook near 

Cornish, ME

Lat 43°47'19", long 70°46'00", York County, at culvert on State Route 25, 0.3 mi 

upstream of mouth at Saco River, and 0.7 mi northwest of the intersection of 

State Routes 117 and 25 in Limington.

1965–74, 1997

01066500 Little Ossipee River near 

South Limington, ME

Lat 43°41'22", long 70°40'15", York County, on right bank 25 ft upstream of 

Sand Pond Road bridge, 2.0 mi southeast of South Limington, and 5.8 mi 

upstream of mouth.

1936, 

1941–82, 

1997, 2007

01069700 Branch Brook near 

Kennebunk, ME

Lat 43°22'44", long 70°34'59", York County, at culvert on State Route 9A, 1.8 mi 

west of the intersection of State Routes 9A and 99 in Kennebunk.

1965–74, 1996

01072800 Cocheco River near 

Rochester, NH

Lat 43°16'06", long 70°58'27", Strafford County, on right bank, directly behind 

Rochester Country Club, 0.5 mi southeast of Main Street and Church Street intersection in Gonic, 2.5 mi south of Rochester City Hall, and 4.0 mi upstream 

of mouth of Isinglass River.

1996–2007

01072850 Mohawk Brook near 

Center Strafford, NH

Lat 43°15'47", long 71°05'50", Strafford County, on left bank, 0.5 mi downstream of State Route 202A bridge, and 1.5 mi east of the intersection of 

State Routes 202A and 126 in Center Strafford. 

1965–77, 2006

01073000 Oyster River near 

Durham, NH

Lat 43°08'55", long 70°57'56", Strafford County, on left bank, 200 ft upstream 

of Old Concord Road, 0.6 mi east of the intersection of State Route 155 and 

U.S. Route 4, and 7 mi upstream from mouth on Little Bay.

1935–2007

01073500 Lamprey River near 

Newmarket, NH

Lat 43°06'09", long 70°57'11", Strafford County, on right bank, 200 ft upstream 

of Packers Falls and Packers Falls Road, 1.8 mi northwest of Newmarket Town 

Hall, 2.6 mi southwest of the intersection of State Routes 108 and 155A in 

Durham, and 4.6 mi upstream from mouth on Great Bay.

1935–2007

01073587 Exeter River at Haigh Road, 

near Brentwood, NH

Lat 42°59'04", long 71°02'20", Rockingham County, on right bank, 10 ft downstream of Haigh Road bridge, 0.4 mi south of State Route 111A and Haigh 

Road intersection, and 0.8 mi upstream of the Little River confluence.

1997–2007

01073600 Dudley Brook near 

Exeter, NH

Lat 42°59'35", long 71°01'20", Rockingham County, on right bank, upstream side 

of breached dam, 100 ft upstream from State Route 111A, 2.2 mi upstream of 

mouth at Little River, 2.8 mi east of the intersection of State Routes 111A and 

125, Middle Road (State Route 111A) and State Route 125 in Brentwood.

1963–85, 

2006, 2007
Table 1 27

Table 1. Descriptions of streamgages on selected streams in and adjacent to New Hampshire.—Continued

[All streamgages are shown in figure 2; USGS, U.S. Geological Survey; lat, latitude; long, longitude; mi, mile; ft, feet]

USGS 

streamgage 

station 

number

Station name Station location

Period of 

record 

(water years)

01073750 Mill Brook near State Route 

108, at Stratham, NH

Lat 43°01'24", long 70°55'04", Rockingham County, at downstream most culvert 

of the Stratham traffic circle, 100 ft northeast of junction of State Route 108 

and unnamed road, 1,000 ft north of the junction of State Routes 33 and 108, 

and 1.5 mi upstream of mouth at Squamscott River.

1973–79, 

2003–04, 

2006–07

1

01074500 East Branch Pemigewasset 

River near Lincoln, NH

Lat 44°03'41", long 71°37'00", Grafton County, on right bank, 0.3 mi upstream of 

Clear Brook confluence, 0.3 mi east of Clearbrook Road and State Route 112 

intersection, and 1.1 mi downstream of Hancock Branch confluence.

1929–52, 

1960, 

1968–70

01074520 East Branch Pemigewasset 

River at Lincoln, NH

Lat 44°02'51", long 71°39'37", Grafton County, on right bank, at old crib dam, 

locally known as "the old hole," 800 ft upstream of bridge, 0.4 mi downstream 

of Pollard Brook confluence, 0.8 mi east of the intersection of Connector Road 

and State Route 112 in Lincoln, and 1.8 mi above mouth.

1993–2007

01075000 Pemigewasset River at 

Woodstock, NH

Lat 43°58'34", long 71°40'48", Grafton County, on right bank, 300 ft upstream of 

southernmost State Route 175 bridge over Pemigewasset River, 300 ft east of 

North Station Road and State Route 175 intersection in Woodstock, and 0.7 mi 

upstream of Eastman Brook confluence.

1940–80, 

1985–2007

01075800 Stevens Brook near 

Wentworth, NH

Lat 43°50'10", long 71°53'09", Grafton County, on left bank, 150 ft upstream of 

Bufflo Road bridge, 0.3 mi upstream of mouth, 1.7 mi northwest of Sand Hill 

Road and State Route 25 intersection in West Rumney.

1964–98, 2006

01076000 Baker River near 

Rumney, NH

Lat 43°47'44", long 71°50'45", Grafton County, on right bank, 200 ft upstream 

of a small right bank tributary, 0.3 mi upstream of Halls Brook confluence, 

and 1.7 mi southeast of Sand Hill Road and State Route 25 intersection in 

West Rumney.

1928–77, 

1982, 

1985–93, 

1995–2007

01076500 Pemigewasset River at 

Plymouth, NH

Lat 43°45'33", long 71°41'10", Grafton County, on right bank, 150 ft downstream 

of State Route 175A bridge in Plymouth, 0.1 mi northeast of Plymouth Town 

Hall, 0.3 mi downstream of Baker River confluence, and 0.8 mi south of the 

intersection of State Route 3A and U.S. Route 3.

1904–2007

01078000 Smith River near 

Bristol, NH

Lat 43°33'59", long 71°44'54", Merrimack County, on right bank, 0.4 mi north of 

Borough Road and Axtell Road intersection, 0.6 mi upstream of Borough Road 

bridge, 1.5 mi upstream of the mouth, and 1.7 mi southwest of the intersection 

of State Routes 3A and 104 in Bristol.

1919–2007

01082000 Contoocook River at 

Peterborough, NH

Lat 42°51'45", long 71°57'35", Hillsborough County, on left bank, 0.2 mi 

downstream of a mill dam, 0.3 mi northwest of Powersbridge Road and Old 

Sharon Road intersection in Noone, 0.6 mi southwest of the intersection of 

Grove Street, U.S. Route 202, and State Route 101 in Peterborough, and 1.2 mi 

upstream of Nubanusit Brook confluence.

1946–77, 

1980, 

1982–2007

01084000 North Branch River near 

Antrim, NH

Lat 43°04'54", long 71°58'44", Hillsborough County, on right bank, 0.1 mi 

upstream of Old North Branch Road bridge, 0.5 mi northeast of the intersection of State Routes 9 and 31, 4.0 mi northwest of the intersection of 

State Route 31 and U.S. Route 202 in Antrim, and 5.2 mi upstream of Beards 

Brook confluence.

1925–70

01084500 Beards Brook near 

Hillsborough, NH

Lat 43°06'51", long 71°55'36", Hillsborough County, on right bank, 300 ft 

upstream from West Main Street bridge, 560 ft upstream of mouth at North 

Branch, 0.5 mi west of West Main Street and U.S. Route 202 intersection, and 

1.6 mi west of the intersection of State Route 149, School Street, Henniker 

Street, and West Main Street intersections in Hillsborough.

1946–76, 

2006–07

01085800 West Branch Warner River 

near Bradford, NH

Lat 43°15'33", long 72°01'35", Merrimack County, on left bank, 75 ft downstream of a small right-bank tributary, 200 ft upstream of Fairground Road 

bridge, 750 ft east of the intersection of Fairground Road and West Road, 

3.5 mi west of Main Street and State Route 103 intersection in Bradford.

1963–2004, 

2006
28 Estimation of Flood Discharges at Selected Recurrence Intervals for Streams in New Hampshire

Table 1. Descriptions of streamgages on selected streams in and adjacent to New Hampshire.—Continued

[All streamgages are shown in figure 2; USGS, U.S. Geological Survey; lat, latitude; long, longitude; mi, mile; ft, feet]

USGS 

streamgage 

station 

number

Station name Station location

Period of 

record 

(water years)

01086000 Warner River at 

Davisville, NH

Lat 43°15'03", long 71°43'58", Merrimack County, on left bank, 60 ft downstream of State Route 127 bridge at Davisville, 0.9 mi east of Interstate 89 

and State Route 103 interchange, 2.2 mi northwest of the intersection of State 

Routes 103 and 127 in Contoocook, and 2.3 mi upstream of mouth at 

Contoocook River.

1940–78, 

1999–2007

2

01089000 Soucook River near 

Concord, NH

Lat 43°14'19", long 71°27'45", Merrimack County, on left bank, 500 ft upstream 

of State Route 9 bridge, 0.8 mi upstream of Cemetery Brook confluence, 

0.4 mi northeast of the intersection of State Routes 9 and 106.

1952–87

01089100 Soucook River at Pembroke 

Road, near Concord, NH

Lat 43°12'49", long 71°28'51", Merrimack County, on left bank, 100 ft upstream 

of Pembroke Road bridge, 550 ft upstream of Frenchs Brook confluence, and 

770 ft east of the intersection of Pembroke Road and State Route 106.

1989–2007

01089500 Suncook River at North 

Chichester, NH

Lat 43°15'24", long 71°22'12", Merrimack County, on left bank, 100 ft 

downstream from Depot Road bridge, 0.1 mi east of the intersection of 

Depot Road, Main Street, and State Route 28 at North Chichester, 0.4 mi 

upstream of Sanders Brook confluence, and 3.1 mi upstream of Little Suncook 

River confluence.

1919–20, 

1922–27, 

1929–77, 

2006–07

01091000 South Branch Piscataquog 

River near 

Goffstown, NH

Lat 43°00'53", long 71°38'31", Hillsborough County, on right bank, 20 ft 

upstream of Parker Road bridge, 50 ft north of the intersection of Parker Road 

and State Route 13, 1.7 mi upstream of mouth at Piscataquog River, 2.1 mi 

west of the intersection of State Routes 13 and 114, and Elm Street in 

Goffstown.

1941–78, 

2006–07

01093800 Stony Brook Tributary near 

Temple, NH

Lat 42°51'36", long 71°50'00", Hillsborough County, on left bank, 450 ft downstream of Putnam Road bridge, 0.3 mi northwest of Putnam Road and Webster 

Highway intersection, 1.6 mi northeast of the intersection of Webster Highway 

and State Routes 101 and 45, and 5.2 mi upstream of mouth at Stony Brook.

1964–2004

01094000 Souhegan River at 

Merrimack, NH

Lat 42°51'27", long 71°30'24", Hillsborough County, on left bank, at head of 

Wildcat Falls, 0.6 mi upstream of south bound bridge on Everett Turnpike, 

0.9 mi southwest of Baboosic Lake Road and U.S. Route 3 intersection in 

Merrimack, and 1.3 mi upstream from mouth at Merrimack River.

1910–76, 

1980, 

1982–2007

01094500 North Nashua River near 

Leominster, MA

Lat 42°30'06", long 71°43'23", Worcester County, on right bank 1.3 mi upstream 

of Wekepeke Brook confluence, 2.5 mi southeast of Leominster, and 6.1 mi 

upstream of confluence with Nashua River.

1936–2007

01095000 Rocky Brook near 

Sterling, MA

Lat 42°26'57", long 71°48'10", Worcester County, on right bank 150 ft downstream of Beaman Road bridge, 0.7 mi upstream of mouth, and 2.2 mi west 

of Sterling.

1947–67

01095800 Easter Brook near North 

Leominster, MA

Lat 42°32'46", long 71°42'45", Worcester County, at culvert on Lancaster 

Avenue, and 1.5 mi east of North Leominster.

1964–74

01096000 Squannacook River near 

West Groton, MA

Lat 42°38'03", long 71°39'30", Middlesex County, on left bank 0.7 mi downstream of Trout Brook confluence, and 2.7 mi northwest of West Groton.1950–2007

010965852 Beaver Brook at 

North Pelham, NH

Lat 42°46'58", long 71°21'15", Rockingham County, on right bank, 10 ft downstream from State Route 128 bridge at the Windham-Pelham town line, 0.7 mi 

north of State Route 128 and Castle Hill Road intersection at North Pelham, 

and 1.3 mi south of the intersection of State Routes 128 and 111 at 

West Windham.

1987–2007

01096910 Boulder Brook at 

East Bolton, MA

Lat 42°27'04", long 71°34'39", Worcester County, on right bank 900 ft downstream from Interstate 495, 0.9 mi west of East Bolton, and 1.3 mi upstream 

of mouth.

1972–83
Table 1 29

Table 1. Descriptions of streamgages on selected streams in and adjacent to New Hampshire.—Continued

[All streamgages are shown in figure 2; USGS, U.S. Geological Survey; lat, latitude; long, longitude; mi, mile; ft, feet]

USGS 

streamgage 

station 

number

Station name Station location

Period of 

record 

(water years)

01097200 Heath Hen Meadow Brook 

at Stow, MA

Lat 42°26'44", long 71°30'02", Middlesex County, at culvert on West Acton 

Road, 0.7 mi northeast of Stow.

1964–74

01097300 Nashoba Brook near 

Acton, MA

Lat 42°30'45", long 71°24'17", Middlesex County, on right bank 500 ft downstream of dam at North Acton, 2.2 mi northeast of Acton, and 5 mi upstream of 

mouth. Prior to January 8, 1997, lat 42°30'39", long 71°24'25", on right bank 

1,500 ft downstream of dam at North Acton.

1964–2007

01100100 Richardson Brook near 

Lowell, MA

Lat 42°39'48", long 71°16'02", Middlesex County, at culvert on Methuen Street, 

and 2.0 mi northeast of Lowell.

1963–83

01100700 East Meadow River near 

Haverhill, MA

Lat 42°48'41", long 71°01'59", Essex County, on left bank 10 ft downstream of 

culvert on State Route 110, and 3.5 mi northeast of Haverhill.

1963–74

01100800 Cobbler Brook near 

Merrimack, MA

Lat 42°50'55", long 71°01'10", Essex County, at culvert on Highland Street, 

1.3 mi northwest of Merrimack.

1963–83

01100900 Parker River Tributary near 

Georgetown, MA

Lat 42°44'03", long 70°58'22", Essex County, at culvert on North Street, 1.2 mi 

northeast of Georgetown.

1964–74

01101000 Parker River at Byfield, MA Lat 42°45'10", long 70°56'46", Essex County, on left bank 1,400 ft downstream 

of dam, 0.5 mi south of Byfield, 0.7 mi upstream of Wheeler Brook confluence, and 5.5 mi southwest of Newburyport.

1946–2007

01127880 Big Brook near 

Pittsburg, NH

Lat 45°08'06", long 71°12'23", Coos County, on left bank, 10 ft downstream of 

culvert on U.S. Route 3, 0.3 mi upstream of mouth, 8.2 mi south of the U.S. 

Route 3 border crossing at the U.S. and Canada Border, and 10.7 mi northeast 

of U.S. Route 3 and State Route 145 intersection in Pittsburg.

1964–84

01129300 Halls Stream near East 

Hereford, Quebec

Lat 45°02'41", long 71°29'54", Compton County, on right bank, opposite Alain’s 

farm, 2.3 mi south of East Hereford, Quebec, Canada, 2.5 mi north of Post 

Office in Beecher Falls, 3.7 mi upstream of mouth, and 5.2 mi west of 

U.S. Route 3 and State Highway 145 intersection in Pittsburg. 

1943, 

1963–86, 

1988–94

01129400 Black Brook at Averill, VT Lat 45°00'14", long 71°41'34", Essex County, at culvert on State Highway 114, at 

Averill-Canaan town line, 0.6 mi south of the U.S.-Canada Border Monument 

#530, 1.1 mi northeast of Averill, 1.3 mi upstream of mouth on Leach Creek, 

and 3.3 mi west of Wallace Pond.

1964–78

01129440 Mohawk River near 

Colebrook, NH

Lat 44°52'28", long 71°24'38", Coos County, on right bank, upstream of Bungy 

Road bridge, south of the intersection of State Highway 26 and Bungy Road, 

0.8 mi upstream of Read Brook confluence, 1.7 mi downstream of Roaring 

Brook confluence, and 5 mi east of Colebrook.

1987–2004

01129700 Paul Stream Tributary near 

Brunswick, VT

Lat 44°41'06", long 71°37'18", Essex County, at culvert on Maidstone Lake 

Road, 400 ft upstream of mouth at Paul Stream, 1.7 mi west of Mason, NH, 

1.9 mi northeast of Maidstone Lake outlet, 3.5 mi south of Brunswick Springs, 

and 4.6 mi south of North Stratford, NH.

1966–78, 

1999–2006

01130000 Upper Ammonoosuc River 

near Groveton, NH

Lat 44°37'30", long 71°28'10", Coos County, on left bank, 75 ft upstream of 

Emerson Road bridge, 0.2 mi downstream of Nash Stream confluence, and 

2.8 mi northeast of Groveton.

1941–80, 

1983–2004

01133000 East Branch Passumpsic 

River near East 

Haven, VT

Lat 44°38'02", long 71°53'53", Caledonia County, on right bank, in Town of 

Burke, downstream of Watkins Road, 0.5 mi upstream of Flower Brook 

confluence, 0.9 mi south of Hartwellville, 4.2 mi east of Post Office in West 

Burke, and 8.4 mi upstream of mouth.

1940–45, 

1949–79, 

1998–2007

01133200 Quimby Brook near 

Lyndonville, VT

Lat 44°34'52", long 71°59'11", Caledonia County, at culvert on Sutton Road, 

0.1 mi north of Sutton Road and U.S. Route 5 intersection, 2.0 mi west of Post 

Office in East Burke, and 3.3 mi north of Lyndon Town Hall in Lyndonville.

1964–74, 

1999–2000, 

2002–07
30 Estimation of Flood Discharges at Selected Recurrence Intervals for Streams in New Hampshire

Table 1. Descriptions of streamgages on selected streams in and adjacent to New Hampshire.—Continued

[All streamgages are shown in figure 2; USGS, U.S. Geological Survey; lat, latitude; long, longitude; mi, mile; ft, feet]

USGS 

streamgage 

station 

number

Station name Station location

Period of 

record 

(water years)

01133300 Cold Hill Brook near 

Lyndon, VT

Lat 44°31'45", long 72°02'57", Caledonia County, at culvert on Brown Brady 

Road that runs along Cold Hill Brook, 100 ft east of Brown Bradley Road and 

Penton Chester Road intersection, 0.3 mi upstream of confluence with South 

Wheelock Branch, 2.1 mi northwest of Interstate 91 and U.S. Route 5 intersection in Lyndon.

1964–78

01134500 Moose River at Victory, VT Lat 44°30'42", long 71°50'16", Essex County, on right bank, 0.5 mi northeast 

of Victory, 0.8 mi downstream of Cold Brook confluence, 1.1 mi upstream 

of Stanley Brook confluence, and 5.0 mi southwest of Burke Road and River 

Road intersection in Gallup Mills.

1947–2007

01134800 Kirby Brook at 

Concord, VT

Lat 44°26'30", long 71°52'44", Essex County, at culvert on U.S. Route 2, 

600 ft southwest of Kirby Road and U.S. Route 2 intersection, 700 ft upstream 

of mouth, 1.1 mi northeast of High Street and U.S. Route 2 intersection in 

Concord.

1964–74, 

1999–2007

01135000 Moose River at 

St. Johnsbury, VT

Lat 44°25'22", long 72°00'02", Caledonia County, on left bank, 750 ft downstream of U.S. Route 2 bridge, 0.5 mi upstream from mouth, and 1.1 mi east of 

Town Hall in St. Johnsbury.

1929–83

01135150 Pope Brook near 

N. Danville, VT

Lat 44°28'34", long 72°07'30", Caledonia County, on left bank, 200 ft upstream 

of Morrill Flat Road, 0.3 mi north of Pope Cemetery, 1.1 mi upstream of North 

Brook confluence, and 1.7 mi northwest of North Danville.

1991–2007

01135300 Sleepers River near 

St. Johnsbury, VT

Lat 44°26'07", long 72°02'20", Caledonia County, on left bank, just upstream of 

Emerson Falls, 0.6 mi upstream of U.S. Route 2 bridge, 1.5 mi northwest of 

Post Office in St. Johnsbury, and 2.7 mi above mouth.

1991–2007

01135500 Passumpsic River at 

Passumpsic, VT

Lat 44°21'56", long 72°02'23", Caledonia County, on right bank, 0.7 mi 

upstream of Water Andric, 1.1 mi downstream of dam, bridge, and village of 

Passumpsic, 3.8 mi south of Town Hall in St. Johnsbury, 4.0 mi upstream from 

mouth, and 4.8 mi north of Post Office in Barnet.

1929–2007

01135700 Joes Brook Tributary near 

East Barnet, VT

Lat 44°20'39", long 72°03'53", Caledonia County, at culvert on Joes Brook Road, 

just southeast of Warden Pond Road and Joes Brook Road intersection, 100 ft 

upstream of mouth, 1.8 mi northwest of East Barnet, 2.9 mi southwest of 

Passumpsic, and 3.4 mi north of Post Office in Barnet.

1964–74, 

1999, 

2001–07

01137500 Ammonoosuc River at 

Bethlehem Junction, NH

Lat 44°16'07", long 71°37'51", Grafton County, on left bank, 0.2 mi upstream of 

Pierce Bridge and Bethlehem Junction, 0.8 mi upstream of unnamed tributary, 

3.0 mi east of U.S. Route 302 and State Route 142 intersection in Bethlehem, 

and 3.4 mi downstream of Little River confluence.

1940–2007

01138000 Ammonoosuc River near 

Bath, NH

Lat 44°09'14", long 71°59'10", Grafton County, on left bank, 0.4 mi downstream of Wild Ammonoosuc River, 1.4 mi southwest of Bath, 2.5 mi east of 

U.S. Route 302 and State Route 135 intersection in Woodsville, and 3.1 mi 

upstream of mouth.

1936–80

01138800 Keenan Brook at 

Groton, VT

Lat 44°12'08", long 72°12'03", Caledonia County, at downstream culvert on 

Topsham Road, 0.6 mi south of U.S. Route 302 and Topsham Road intersection in Groton, 1.1 mi upstream of mouth on Wells River, and 3.0 mi west of 

South Ryegate, VT.

1964–74

01139000 Wells River at Wells 

River, VT

Lat 44°09'01", long 72°03'56", Orange County, on right bank, 0.8 mi west of 

village of Wells River, 1.3 mi southeast of Interstate 91 and U.S. Route 302 

intersection in Four Corners, and 1.5 mi upstream from mouth.

1941–2007

01139700 Waits River Tributary near 

West Topsham, VT

Lat 44°08'29", long 72°18'52", Orange County, at culvert on U.S. Route 302, 

800 ft upstream of mouth at Waits River, 0.3 mi east of U.S. Route 302 and 

State Route 25 intersection, and 2.0 mi north of West Topsham.

1964–74, 

1999–2000, 

2002–06
Table 1 31

Table 1. Descriptions of streamgages on selected streams in and adjacent to New Hampshire.—Continued

[All streamgages are shown in figure 2; USGS, U.S. Geological Survey; lat, latitude; long, longitude; mi, mile; ft, feet]

USGS 

streamgage 

station 

number

Station name Station location

Period of 

record 

(water years)

01139800 East Orange Branch at 

East Orange, VT

Lat 44°05'34", long 72°20'10", Orange County, on left bank, 0.3 mi east of East 

Orange Road and Fish Pond Road intersection in East Orange, 1.7 mi upstream 

of mouth, 2.0 mi southwest of West Topsham, and 5.0 mi southwest of Orange.

1959–2007

01140800 West Branch Ompompanoosuc River Tributary at 

South Strafford, VT

Lat 43°49'56", long 72°22'20", Orange County, at culvert on Prestonville Road, 

500 ft north of Prestonville Road intersection with State Route 132, 0.4 mi 

southwest of Tunbridge Road and State Route 132 intersection in South 

Strafford, 0.6 mi upstream of mouth at West Branch Ompompanoosuc River, 

and 5.3 mi northeast of State Routes 14 and 132 intersection in Sharon.

1964–77

01141800 Mink Brook near Etna, NH Lat 43°42'08", long 72°11'l5", Grafton County, on left bank, 0.1 mi west of Three 

Mile Road and Ruddsboro Road intersection, 1.6 mi northeast of Etna Road 

and King Road intersection in Etna, 4.8 mi northwest of City Hall in Enfield, 

and 5.1 mi east of Post Office in Hanover.

1963–98, 2006

01142000 White River near 

Bethel, VT

Lat 43°48'41", long 72°39'24", Windsor County, on right bank, 0.3 mi upstream 

of Locust Creek confluence, 0.3 mi northwest of the State Routes 12 and 107 

intersection, and 1.8 mi southwest of State Routes 12 and 107 intersection 

in Bethel.

1932–55

01142400 Third Branch White River 

Tributary at 

Randolph, VT

Lat 43°55'54", long 72°40'54", Orange County, at culvert on State Route 12A, 

0.3 mi upstream of mouth, 0.8 mi west of junction of State Highways 12 and 

12A in Randolph, and 0.8 mi northwest of Town Hall in Randolph.

1964–74, 

1998–2007

01142500 Ayers Brook at 

Randolph, VT

Lat 43°56'04", long 72°39'30", Orange County, on right bank, 135 ft upstream of 

bridge on State Highway 12, 0.4 mi upstream of Adams Brook confluence, 

0.7 mi upstream of mouth, and 0.9 mi northeast of Town Hall in Randolph.

1940–2007

01144000 White River at 

West Hartford, VT

Lat 43°42'51", long 72°25'07", Windsor County, on left bank, 700 ft upstream 

of Quechee-West Hartford Road bridge at West Hartford, 0.2 mi south of the 

State Route 14 and Tigertown Road intersection in West Hartford, 5.1 mi south 

of State Routes 14 and 132 intersection in Sharon, 5.5 mi west of Post Office 

in Norwich, and 7.4 mi upstream from mouth.

1916–2007

01145000 Mascoma River at 

West Canaan, NH

Lat 43°39'04", long 72°05'07", Grafton County, on right bank, 45 ft downstream 

from abandoned railroad bridge, 0.6 mi east of U.S. Route 4 and South Road 

intersection in West Canaan, 1.4 mi downstream of Indian River confluence, 

and 3.0 mi east of City Hall in Enfield.

1938, 

1940–78, 

1985–2006

01150800 Kent Brook near 

Killington, VT

Lat 43°40'24", long 72°48'33", Rutland County, at culvert on State Highway 100, 

0.4 mi north of junction of State Route 100 and U.S. Route 4, 1.6 mi upstream 

of mouth, and 2.0 mi northwest of River Road and U.S. Route 4 intersection 

in Killington.

1964–74, 

1999–2007

01150900 Ottauquechee River near 

West Bridgewater, VT

Lat 43°37'20", long 72°45'34", Rutland County, on right bank, 50 ft upstream of 

Mission Chapel Road bridge, 1.6 mi north of State Route 100 and U.S. Route 4 

intersection in West Bridgewater, and 2.6 mi south of River Road and 

U.S. Route 4 intersection in Sherburne Center.

1985–2007

01151200 Ottauquechee River Tributary near Quechee, VTLat 43°39'37", long 72°25'55", Windsor County, at culvert on West HartfordQuechee Road, 0.2 mi upstream of mouth, 1.2 mi northwest of Quechee Main 

Street, Deweys Mills Road and Waterman Hill Road intersection in Quechee, 

and 2.8 mi northeast of Happy Valley Road and U.S. Route 4 intersection 

in Taftsville.

1964–74, 

1999–2004, 

2007

01152500 Sugar River at West 

Claremont, NH

Lat 43°23'15", long 72°21'45", Sullivan County, on right bank, 0.2 mi downstream of Redwater Brook confluence, 0.7 mi southeast of Clay Hill Road and 

Paddy Hollow Road intersection in West Claremont, 1.6 mi northwest of City 

Hall in Claremont, and 2.4 mi upstream of mouth.

1929–2007
32 Estimation of Flood Discharges at Selected Recurrence Intervals for Streams in New Hampshire

Table 1. Descriptions of streamgages on selected streams in and adjacent to New Hampshire.—Continued

[All streamgages are shown in figure 2; USGS, U.S. Geological Survey; lat, latitude; long, longitude; mi, mile; ft, feet]

USGS 

streamgage 

station 

number

Station name Station location

Period of 

record 

(water years)

01153300 Middle Branch Williams 

River Tributary at 

Chester, VT

Lat 43°16'13", long 72°36'32", Windsor County, at culvert on Lover Lane Road, 

0.2 mi north of Lover Lane Road and State Route 11 intersection, 0.8 mi northeast of intersection of State Routes 11 and 35 in Chester, and 1.5 mi upstream 

of mouth.

1964–78, 

1999–2004, 

2007

3

01153500 Williams River at 

Brockways Mills, VT

Lat 43°12'31", long 72°31'05", Windham County, on left bank, 25 ft upstream of 

road bridge at Brockways Mills, 1.0 mi downstream of Stream Brook confluence, 2.2 mi upstream of Station 01153550, “Williams River near Rockingham,” 3.9 mi downstream of Hall Brook confluence, and 4.4 mi upstream 

from mouth.

1941–84

01153550 Williams River near 

Rockingham, VT

Lat 43°11'30", long 72°29'08", Windham County, on left bank, 50 ft downstream 

of Parker Hill Road bridge, 0.2 mi downstream of Divoll Brook confluence, 

0.3 mi northeast of Rockingham, 2.2 mi upstream from mouth, 2.2 mi downstream of Station 01153500, “Williams River at Brockways Mills.”

1987–2007

01154000 Saxtons River at 

Saxtons River, VT

Lat 43°08'15", long 72°29'19", Windham County, on right bank, 130 ft upstream 

of Hall Bridge Road bridge, 1.1 mi east of Saxtons River, 1.3 mi upstream of 

Bundy Brook confluence, and 3.9 mi upstream of mouth.

1941–82, 

2002–07

01155000 Cold River at 

Drewsville, NH

Lat 43°07'54", long 72°23'27", Cheshire County, on left bank, 50 ft upstream of 

State Route 123 bridge at Drewsville, 0.9 mi upstream of Great Brook confluence, 1.9 mi southwest of Alstead, and 3.2 mi upstream from mouth.

1941–78, 2006

01155200 Sacketts Brook near 

Putney, VT

Lat 42°59'57", long 72°31'59", Windham County, on left bank, 50 ft upstream of 

Westminster West Road bridge, 1.8 mi north of Westminster West Road and 

U.S. Route 5 intersection in Putney, and 7.5 mi southeast of the intersection of 

State Routes 30 and 35 in Townshend.

1964–74

01155300 Flood Brook near 

Londonderry, VT

Lat 43°14'11", long 72°51'23", Windham County, on left bank, 20 ft downstream 

of State Route 11 bridge, 0.9 mi upstream of Burnt Meadow Brook confluence, 

2.5 mi west of State Highway 11 and 100 intersection in Londonderry, 

and 3.6 mi northwest of Main Street and State Route 100 intersection in 

South Londonderry.

1964–74

01155350 Tributary to West River 

Tributary near 

Jamaica, VT

Lat 43°07'33", long 72°48'46", Windham County, at culvert on State Route 100, 

800 ft north of Stratton Gate Road and State Route 100 intersection, 0.5 mi upstream of mouth, 1.9 mi west of Ball Mountain Dam, 2.0 mi southeast of State 

Routes 30 and 100 intersection in Rawsonville, and 2.5 mi northwest of Depot 

Street and State Route 30/100 intersection in Jamaica.

1964–78, 

1999–2007

01156300 Whetstone Brook Tributary 

near Marlboro, VT

Lat 42°52'42", long 72°42'32", Windham County, at culvert on State Route 9, 

600 ft southwest of Sunset Lake Road and State Route 9 intersection, 800 ft 

upstream of mouth, 0.5 mi southeast of mouth of Hidden Lake, and 1.5 mi 

northeast of Marlboro.

1963, 

1965–74, 

1999–2002, 

2004–07

01156450 Connecticut River Tributary 

near Vernon, VT

Lat 42°47'01", long 72°31'57", Windham County, at downstream culvert on Tyler 

Hill Road, 0.3 mi west of Tyler Hill Road and State Route 142 intersection, 

0.6 mi upstream of mouth, 1.3 mi northwest of Vernon Dam, and 1.8 mi northwest of West Road and State Route 142 intersection in Vernon.

1964–74, 

1999–2001, 

2003–07

01157000 Ashuelot River near 

Gilsum, NH

Lat 43°02'21", long 72°16'14", Cheshire County, on right bank, 50 ft downstream 

of White Brook confluence, 60 ft upstream of stone-arch bridge on Surry 

Road, 200 ft west of Surry Road and State Route 10 intersection, and 0.8 mi 

southwest of Post Office in Gilsum.

1923–80, 2006
Table 1 33

Table 1. Descriptions of streamgages on selected streams in and adjacent to New Hampshire.—Continued

[All streamgages are shown in figure 2; USGS, U.S. Geological Survey; lat, latitude; long, longitude; mi, mile; ft, feet]

USGS 

streamgage 

station 

number

Station name Station location

Period of 

record 

(water years)

01158500 Otter Brook near 

Keene, NH

Lat 42°57'55", long 72°14'05", Cheshire County, on left bank, 10 ft downstream 

from unnamed road bridge, 0.2 mi south of unnamed road and State Route 9 

intersection, 1.3 mi north of Otter Brook Flood Control Dam, 1.4 mi east of 

the intersection of State Routes 9 and 10, 1.5 mi upstream of station 01158600, 

Otter Brook below Otter Brook Dam, near Keene, and 3.8 mi upstream of 

confluence with Minnewawa Brook to form The Branch.

1924–57

01160000 South Branch Ashuelot 

River at Webb, near 

Marlborough, NH

Lat 42°52'19", long 72°12'51", Cheshire County, on right bank, 15 ft downstream 

of bridge (destroyed) at Webb, 400 ft upstream of State Route 12 bridge, and 

2.3 mi south of Town Hall in Marlborough.

1921–78

01161300 Millers Brook at 

Northfield, MA

Lat 42°41'07", long 72°27'11", Franklin County, at culvert on Beers Plain Road, 

0.8 mi south of Northfield.

1964, 1966–83

01161500 Tarbell Brook near 

Winchendon, MA

Lat 42°42'45", long 72°05'09", Worcester County, on left bank 0.1 mi downstream of Spud Brook confluence, 0.3 mi downstream of MassachusettsNew Hampshire State line, and 2.8 mi northwest of Winchendon.1917–82

01162000 Millers River near 

Winchendon, MA

Lat 42°41'03", long 72°05'02", Worcester County, on right bank 10 ft downstream of Nolan Bridge, 0.3 mi downstream of Tarbell Brook confluence, and 

2.0 mi west of Winchendon.

1918–2007

01162500 Priest Brook near 

Winchendon, MA

Lat 42°40'57", long 72°06'56", Worcester County, on right bank 100 ft downstream of bridge, 3.0 mi upstream of mouth, and 3.5 mi west of Winchendon.1917, 

1918–2007

01163100 Wilder Brook near 

Gardner, MA

Lat 42°35'42", long 72°00'53", Worcester County, at culvert on Clark Street, and 

1.5 mi northwest of Gardner.

1964–74

01163200 Otter River at 

Otter River, MA

Lat 42°35'18", long 72°02'29", Worcester County, on right bank at upstream side 

of Turner Street bridge, 0.2 mi upstream of Bailey Brook confluence, 0.8 mi 

southeast of Otter River, and 2.0 mi northwest of Gardner.

1965–2007

01165500 Moss Brook at 

Wendell Depot, MA

Lat 42°36'10", long 72°21'36", Franklin County, on left bank 0.2 mi upstream of 

mouth, 0.2 mi north of Wendell Depot, and 2.5 mi west of Orange.

1917–82

01167800 Beaver Brook at 

Wilmington, VT

Lat 42°51'38", long 72°51'06", Windham County, on right bank 20 ft downstream 

of bridge on State Route 9, 0.1 mi east of the eastern intersection of State 

Routes 9 and 100, 1.2 mi southeast of the intersection of State Routes 9 and 

100 intersection in Wilmington, and 1.6 mi upstream of mouth.

1963–77

01169000 North River at 

Shattuckville, MA

Lat 42°38'18", long 72°43'32", Franklin County, on right bank in Shattuckville, 

1.2 mi south of Griswoldville, and 1.3 mi upstream of mouth.

1940–2007

01169900 South River near 

Conway, MA

Lat 42°32'31", long 72°41'39", Franklin County, on left bank at upstream side 

of Reeds Bridge just off Bardwell Road, 2.2 mi north of Conway, and 2.6 mi 

upstream of mouth.

1967–2007

01170100 Green River near 

Colrain, MA

Lat 42°42'12", long 72°40'16", Franklin County, on right bank 0.5 mi upstream of 

bridge on West Leyden Road and 2.5 mi northeast of Colrain.

1968–2007

01170200 Allen Brook near 

Shelburne Falls, MA

Lat 42°36'46", long 72°40'02", Franklin County, at culvert on Peckville Road, 

and 3.5 mi east of Shelburne Falls.

1964–74

01170900 Mill River near 

South Deerfield, MA

Lat 42°28'09", long 72°38'31", Franklin County, at culvert on North Street, and 

2.0 mi southwest of South Deerfield.

1963–74

01173900 Middle Branch Swift River 

at North New Salem, MA

Lat 42°32'45", long 72°19'10", Franklin County, at culvert on Elm Street at 

North New Salem.

1964–74

01174565 West Branch Swift River 

near Shutesbury, MA

Lat 42°27'18", long 72°22'56", Franklin County, on left bank 800 ft downstream 

of State Route 202, and 1.4 mi east of Shutesbury.

1984–85, 

1996–2007

1

Record combined with record from streamgage 01074520, East Branch Pemigewasset River at Lincoln, NH, for frequency analysis.

2

Record combined with record from streamgage 01089100, Soucook River at Pembroke Road, near Concord, NH, for frequency analysis.

3

Record combined with record from streamgage 01153550, Williams River near Rockingham, VT, for frequency analysis.
34 Estimation of Flood Discharges at Selected Recurrence Intervals for Streams in New Hampshire Table 5. Flood discharges at selected recurrence intervals for streamgages on selected streams in and adjacent to New Hampshire.—Continued [All streamgages are shown in figure 2; USGS, U.S. Geological Survey; mi2, square miles; ft3/s, cubic feet per second] USGS stream- gage station number Station name Drainage area (mi2

)

Flood discharge at given recurrence intervals (ft3/s) Maximum recorded flood

2-year 5-year 10-year 25-year 50-year 100-year 500-year Discharge (ft3/s) Date

01050900 Four Ponds Brook near Houghton, ME 3.26 95.4 159 205 268 317 369 495 349 7-30-196901052500 Diamond River near Wentworth Location, NH 153 4,950 6,450 7,420 8,630 9,520 10,400 12,500 12,800 3-31-199801054200 Wild River at Gilead, ME 69.9 9,110 14,600 18,200 22,600 25,700 28,800 35,400 28,300 10-24-195901054300 Ellis River at South Andover, ME 131 4,280 5,970 7,010 8,220 9,070 9,860 11,600 7,830 12-18-200301055000 Swift River near Roxbury, ME 96.8 6,260 9,890 12,400 15,600 18,000 20,400 26,000 16,800 10-24-195901055300 Bog Brook near Buckfield, ME 10.5 189 262 308 364 404 443 530 289 2-11-197001057000 Little Androscoggin River near South Paris, ME 76.5 2,200 3,400 4,260 5,400 6,290 7,210 9,480 9,340 4-1-198701062700 Patte Brook near Bethel, ME 5.67 241 388 493 630 735 841 1,100 664 7-1-197301063310 Stony Brook at East Sebago, ME 1.51 44.6 85.8 121 176 223 277 432 130 9-17-199901064300 Ellis River near Jackson, NH 10.5 1,260 2,030 2,570 3,250 3,770 4,290 5,490 4,500 11-3-196601064380 East Branch Saco River at Town Hall Road, near 

Lower Bartlett, NH

33.9 1,950 3,010 3,750 4,700 5,430 6,160 7,920 4,610 4-25-1968

01064400 Lucy Brook near North Conway, NH 4.69 576 1,050 1,420 1,920 2,320 2,730 3,750 2,660 8-11-199001064500 Saco River near Conway, NH 385 17,200 26,300 32,500 40,600 46,700 52,800 67,400 47,200 3-27-195301064800 Cold Brook at South Tamworth, NH 5.46 464 1,020 1,570 2,510 3,430 4,580 8,310 2,800 7-4-197301064801 Bearcamp River at South Tamworth, NH 67.4 3,600 5,070 6,050 7,300 8,240 9,180 11,400 6,150 6-14-199801065000 Ossipee River at Effingham Falls, NH 329 3,550 4,900 5,870 7,160 8,170 9,240 11,900 11,700 3-28-195301065500 Ossipee River at Cornish, ME 452 4,450 6,350 7,710 9,520 10,900 12,400 16,200 17,200 3-21-193601066000 Saco River at Cornish, ME 1,290 13,500 19,000 22,700 27,500 31,100 34,800 43,600 46,600 3-21-193601066100 Pease Brook near Cornish, ME 4.63 180 324 447 636 802 992 1,540 486 4-23-196901066500 Little Ossipee River near South Limington, ME 163 2,040 3,220 4,150 5,510 6,660 7,920 11,400 8,530 3-19-193601069700 Branch Brook near Kennebunk, ME 10.5 321 558 755 1,050 1,310 1,610 2,450 1,020 10-22-199601072800 Cocheco River near Rochester, NH 79.9 1,640 2,650 3,460 4,660 5,680 6,820 10,000 7,240 4-16-200701072850 Mohawk Brook near Center Strafford, NH 7.30 293 686 1,100 1,870 2,670 3,700 7,360 2,370 5-14-200601073000 Oyster River near Durham, NH 12.2 305 488 632 839 1,010 1,200 1,730 1,320 4-16-200701073500 Lamprey River near Newmarket, NH 185 2,200 3,550 4,650 6,280 7,690 9,270 13,700 8,970 5-16-2006
Table 5 35

Table 5. Flood discharges at selected recurrence intervals for streamgages on selected streams in and adjacent to New Hampshire.—Continued[All streamgages are shown in figure 2; USGS, U.S. Geological Survey; mi2, square miles; ft3/s, cubic feet per second]

USGS 

streamgage 

station 

number

Station name

Drainage 

area

(mi2)

Flood discharge at given recurrence intervals (ft3/s) Maximum recorded flood

2-year 5-year 10-year 25-year 50-year 100-year 500-year Discharge (ft3/s) Date

01073587 Exeter River at Haigh Road, near Brentwood, NH 63.5 882 1,420 1,840 2,450 2,960 3,530 5,060 3,520 5-15-200601073600 Dudley Brook near Exeter, NH 5.79 168 257 325 422 502 590 825 660 5-14-200601073750 Mill Brook near State Route 108, at Stratham, NH 2.31 114 214 302 440 564 706 1,130 390 4-2-197301074520 East Branch Pemigewasset River at Lincoln, NH 115 8,210 12,400 15,500 19,700 23,000 26,500 35,500 16,900 4-14-200201075000 Pemigewasset River at Woodstock, NH 195 11,800 19,000 24,500 32,200 38,400 45,100 62,400 47,000 10-24-195901075800 Stevens Brook near Wentworth, NH 3.29 206 395 570 857 1,130 1,460 2,490 1,120 6-30-197301076000 Baker River near Rumney, NH 143 5,180 8,650 11,600 16,000 20,000 24,500 37,900 25,900 11-3-192701076500 Pemigewasset River at Plymouth, NH 623 21,300 30,200 36,400 44,600 51,000 57,700 74,100 65,400 3-19-193601078000 Smith River near Bristol, NH 86.1 1,760 2,620 3,260 4,170 4,920 5,740 7,910 8,100 3-19-193601082000 Contoocook River at Peterborough, NH 67.4 1,300 1,920 2,390 3,050 3,600 4,190 5,770 4,110 4-16-200701084000 North Branch River near Antrim, NH 54.2 910 1,460 1,920 2,610 3,230 3,930 6,010 5,000 3-19-193601084500 Beards Brook near Hillsborough, NH 55.4 1,210 1,750 2,150 2,710 3,160 3,640 4,920 4,800 10-9-200501085800 West Branch Warner River near Bradford, NH 5.88 354 582 752 987 1,180 1,380 1,890 1,010 5-14-200601086000 Warner River at Davisville, NH 145 2,210 3,380 4,260 5,490 6,500 7,580 10,400 8,640 5-15-200601089100 Soucook River at Pembroke Road, near Concord, NH 82.7 1,360 2,130 2,730 3,570 4,280 5,040 7,110 5,110 5-14-200601089500 Suncook River at North Chichester, NH 154 2,270 3,610 4,720 6,390 7,860 9,540 14,400 10,600 4-17-200701091000 South Branch Piscataquog River near Goffstown, NH 103 2,070 3,230 4,110 5,340 6,340 7,410 10,200 8,880 4-17-200701093800 Stony Brook Tributary near Temple, NH 3.60 188 308 407 556 685 831 1,250 648 10-21-199601094000 Souhegan River at Merrimack, NH 171 3,200 4,950 6,350 8,430 10,200 12,200 17,900 16,900 3-19-193601094500 North Nashua River near Leominster, MA 108 2,480 4,200 5,660 7,920 9,920 12,200 19,100 16,300 3-18-193601095000 Rocky Brook near Sterling, MA 2.23 59.4 120 181 287 393 527 987 470 7-20-195901095800 Easter Brook near North Leominster, MA 0.95 52.3 89.4 120 167 208 254 389 85.0 3-18-196801096000 Squannacook River near West Groton, MA 65.6 1,440 2,370 3,090 4,130 4,990 5,930 8,460 4,820 4-16-2007010965852 Beaver Brook at North Pelham, NH 47.6 809 1,400 1,910 2,680 3,360 4,140 6,400 2,940 5-15-200601096910 Boulder Brook at East Bolton, MA 1.60 44.7 78.5 108 155 197 246 397 208 1-25-1979
36 Estimation of Flood Discharges at Selected Recurrence Intervals for Streams in New Hampshire Table 5. Flood discharges at selected recurrence intervals for streamgages on selected streams in and adjacent to New Hampshire.—Continued [All streamgages are shown in figure 2; USGS, U.S. Geological Survey; mi2, square miles; ft3/s, cubic feet per second] USGS stream- gage station number Station name Drainage area (mi2

)

Flood discharge at given recurrence intervals (ft3/s) Maximum recorded flood

2-year 5-year 10-year 25-year 50-year 100-year 500-year Discharge (ft3/s) Date

01097200 Heath Hen Meadow Brook at Stow, MA 3.76 59.1 98.9 131 179 219 265 392 156 3-18-196801097300 Nashoba Brook near Acton, MA 12.3 198 326 425 566 683 811 1,150 679 1-26-197901100100 Richardson Brook near Lowell, MA 4.04 111 166 208 268 318 373 525 424 1-25-197901100700 East Meadow River near Haverhill, MA 4.81 109 185 246 337 415 502 743 211 3-19-196801100800 Cobbler Brook near Merrimack, MA 0.76 57.9 80.9 97.2 119 136 154 199 117 3-18-196801100900 Parker River Tributary near Georgetown, MA 0.70 18.2 34.6 49.1 72.4 93.6 119 194 76.0 3-18-196801101000 Parker River at Byfield, MA 21.4 218 357 472 646 797 969 1,460 883 10-22-199601127880 Big Brook near Pittsburg, NH 6.47 264 341 389 446 487 527 616 441 5-3-196701129300 Halls Stream near East Hereford, Quebec 84.8 3,330 4,560 5,460 6,680 7,650 8,690 11,400 21,000 6-194301129400 Black Brook at Averill, VT 0.88 30.8 40.8 47.7 56.5 63.2 70.1 86.9 54.0 12-21-197301129440 Mohawk River near Colebrook, NH 35.2 2,140 3,160 3,880 4,840 5,580 6,340 8,230 4,880 3-31-199801129700 Paul Stream Tributary near Brunswick, VT 1.48 50.8 74.3 91.4 115 133 153 203 126 6-12-200201130000 Upper Ammonoosuc River near Groveton, NH 230 4,910 6,660 7,770 9,110 10,100 11,000 13,100 124,100 5-20-196901133000 East Branch Passumpsic River near East Haven, VT 51.3 1,370 1,970 2,410 3,030 3,540 4,080 5,500 4,450 6-30-197301133200 Quimby Brook near Lyndonville, VT 2.16 80.6 131 172 232 285 344 512 290 6-12-200201133300 Cold Hill Brook near Lyndon, VT 1.64 55.9 93.4 124 171 211 257 387 195 6-30-197301134500 Moose River at Victory, VT 75.2 2,090 2,760 3,220 3,810 4,270 4,740 5,880 4,940 7-1-197301134800 Kirby Brook at Concord, VT 8.13 394 628 814 1,090 1,320 1,580 2,320 1,600 6-30-197301135000 Moose River at St. Johnsbury, VT 129 2,660 3,790 4,600 5,690 6,550 7,440 9,700 5,820 5-5-197201135150 Pope Brook near North Danville, VT 3.27 152 190 214 245 267 290 342 249 7-15-199701135300 Sleepers River near St. Johnsbury, VT 42.5 1,870 2,930 3,780 5,030 6,110 7,310 10,700 7,570 8-12-199801135500 Passumpsic River at Passumpsic, VT 434 7,630 10,100 11,700 13,800 15,400 17,000 20,800 18,200 7-1-197301135700 Joes Brook Tributary near East Barnet, VT 0.70 34.0 53.8 69.6 92.8 112 134 195 103 12-17-200001137500 Ammonoosuc River at Bethlehem Junction, NH 88.2 4,400 6,320 7,660 9,440 10,800 12,200 15,800 11,300 11-12-199501138000 Ammonoosuc River near Bath, NH 396 10,700 15,800 19,600 24,800 29,100 33,600 45,600 27,900 3-18-1936
Table 5 37

Table 5. Flood discharges at selected recurrence intervals for streamgages on selected streams in and adjacent to New Hampshire.—Continued[All streamgages are shown in figure 2; USGS, U.S. Geological Survey; mi2, square miles; ft3/s, cubic feet per second]

USGS 

streamgage 

station 

number

Station name

Drainage 

area

(mi2)

Flood discharge at given recurrence intervals (ft3/s) Maximum recorded flood

2-year 5-year 10-year 25-year 50-year 100-year 500-year Discharge (ft3/s) Date

01138800 Keenan Brook at Groton, VT 4.72 96.9 165 220 302 372 451 672 275 6-30-197301139000 Wells River at Wells River, VT 98.8 1,720 2,500 3,080 3,870 4,500 5,170 6,900 5,970 6-30-197301139700 Waits River Tributary near West Topsham, VT 1.21 44.2 69.7 88.9 116 138 162 224 94.0 12-21-197301139800 East Orange Branch at East Orange, VT 8.79 255 397 507 661 789 927 1,300 800 7-23-199001140800 West Branch Ompompanoosuc River Tributary at 

South Strafford, VT

1.35 78.8 105 123 148 168 189 242 168 6-30-1973

01141800 Mink Brook near Etna, NH 4.76 221 385 521 724 900 1,100 1,660 870 5-14-200601142000 White River near Bethel, VT 239 10,100 15,400 19,600 25,700 30,900 36,800 53,000 32,200 9-21-193801142400 Third Branch White River Tributary at Randolph, VT 0.80 47.9 86.3 122 183 241 313 550 327 6-27-199801142500 Ayers Brook at Randolph, VT 30.5 737 1,130 1,450 1,920 2,340 2,820 4,180 3,480 6-27-199801144000 White River at West Hartford, VT 687 17,700 26,100 32,900 43,100 51,900 61,800 90,500 120,000 11-4-192701145000 Mascoma River at West Canaan, NH 80.4 1,630 2,340 2,810 3,430 3,900 4,370 5,510 4,310 9-193801150800 Kent Brook near Killington, VT 3.25 212 368 500 701 878 1,080 1,660 792 4-14-200201150900 Ottauquechee River near West Bridgewater, VT 23.3 990 1,350 1,610 1,950 2,230 2,510 3,230 1,960 10-22-199501151200 Ottauquechee River Tributary near Quechee, VT 0.78 17.1 27.8 36.7 50.4 62.5 76.3 117 93.0 6-30-197301152500 Sugar River at West Claremont, NH 270 4,720 6,650 8,020 9,860 11,300 12,800 16,700 14,000 3-19-193601153300 Middle Branch Williams River Tributary at 

Chester, VT

3.19 140 198 239 294 338 384 499 367 8-10-1976

01153550 Williams River near Rockingham, VT 112 4,930 7,180 8,760 10,800 12,400 14,100 18,200 11,500 3-31-198701154000 Saxtons River at Saxtons River, VT 72.1 2,690 4,070 5,100 6,540 7,720 8,970 12,300 8,460 8-10-197601155000 Cold River at Drewsville, NH 83.3 1,990 3,150 4,080 5,470 6,680 8,040 11,900 221,800 10-9-200501155200 Sacketts Brook near Putney, VT 10.2 321 527 691 933 1,140 1,370 2,000 903 6-30-197301155300 Flood Brook near Londonderry, VT 9.28 561 918 1,210 1,630 2,000 2,400 3,540 1,800 6-30-197301155350 Tributary to West River Tributary at State Route 30 

near Jamaica, VT

0.93 57.4 99.4 135 191 242 300 473 320 6-30-1973

01156300 Whetstone Brook Tributary near Marlboro, VT 1.08 112 191 255 350 431 521 770 284 10-8-200501156450 Connecticut River Tributary near Vernon, VT 1.10 52.2 86.3 115 161 201 249 391 250 10-8-200501157000 Ashuelot River near Gilsum, NH 71.9 1,600 2,470 3,180 4,220 5,130 6,150 9,070 10,200 10-9-2005
38 Estimation of Flood Discharges at Selected Recurrence Intervals for Streams in New Hampshire Table 5. Flood discharges at selected recurrence intervals for streamgages on selected streams in and adjacent to New Hampshire.—Continued [All streamgages are shown in figure 2; USGS, U.S. Geological Survey; mi2, square miles; ft3/s, cubic feet per second] USGS stream- gage station number Station name Drainage area (mi2

)

Flood discharge at given recurrence intervals (ft3/s) Maximum recorded flood

2-year 5-year 10-year 25-year 50-year 100-year 500-year Discharge (ft3/s) Date

01158500 Otter Brook near Keene, NH 42.2 1,150 2,010 2,770 3,980 5,090 6,410 10,500 6,130 9-21-193801160000 South Branch Ashuelot River at Webb, near 

Marlborough, NH

35.9 958 1,690 2,350 3,430 4,460 5,710 9,700 5,960 9-21-1938

01161300 Millers Brook at Northfield, MA 2.32 111 196 274 403 525 673 1,150 680 9-26-197501161500 Tarbell Brook near Winchendon, MA 18.8 248 408 552 787 1,010 1,280 2,130 2,630 9-21-193801162000 Millers River near Winchendon, MA 82.3 1,030 1,640 2,160 3,000 3,770 4,680 7,510 8,500 9-22-193801162500 Priest Brook near Winchendon, MA 18.9 347 570 763 1,070 1,340 1,670 2,650 3,000 9-21-193801163100 Wilder Brook near Gardner, MA 2.65 46.2 61.7 72.5 86.8 97.8 109 138 56.0 3-23-1972,

3-17-1973

01163200 Otter River at Otter River, MA 34.0 456 636 764 937 1,070 1,220 1,580 948 3-7-197901165500 Moss Brook at Wendell Depot, MA 12.2 258 431 575 796 991 1,220 1,870 1,540 3-19-193601167800 Beaver Brook at Wilmington, VT 6.36 436 745 1,010 1,410 1,770 2,180 3,400 1,170 8-10-197601169000 North River at Shattuckville, MA 89.0 4,850 7,840 10,200 13,700 16,700 20,000 29,200 18,800 10-9-200501169900 South River near Conway, MA 24.2 1,940 3,210 4,300 5,990 7,510 9,280 14,600 8,770 10-9-200501170100 Green River near Colrain, MA 41.2 2,510 3,470 4,150 5,060 5,770 6,520 8,400 6,540 10-9-200501170200 Allen Brook near Shelburne Falls, MA 0.72 33.3 72.8 112 181 248 333 616 89.0 6-18-197601170900 Mill River near South Deerfield, MA 6.41 156 247 318 419 504 597 849 300 4-3-197001173900 Middle Branch Swift River at North New Salem, MA 4.72 114 189 253 352 442 546 861 320 12-21-197301174565 West Branch Swift River near Shutesbury, MA 12.8 557 1,020 1,430 2,080 2,680 3,380 5,530 1,520 10-9-2005

 1Peak discharge affected by dam failure. 2Peak discharge affected by failure of embankment temporarily impounding flood waters.
Table 6 39

Table 6. Basin characteristics determined for use in regression analysis.—Continued[PRISM, parameter-elevation regressions on independent slopes model; USGS, U.S. Geological Survey; °F, degrees Fahrenheit; %, percent; /, divided by; ×, multiplied by; --, none]

Characteristic description Units Source Minimum Mean Maximum

Drainage area Square miles National Elevation Dataset1 0.70 83.7 1,290Maximum elevation Feet National Elevation Dataset1 149 2,419 6,286Minimum elevation Feet National Elevation Dataset1 24 636 2,059Relief (maximum minus minimum elevation) Feet National Elevation Dataset1 81 1,783 6,026Mean elevation Feet National Elevation Dataset1 94.8 1,220 3,360Mean basin slope Percent National Elevation Dataset1 3.38 13.9 38.2Percentage of basin having a slope greater than 30 percent Percent National Elevation Dataset1 0 8.83 65.1Percentage of basin facing north having a slope greater than 30 percent Percent National Elevation Dataset1 0 2.29 16.8Mean annual precipitation Inches PRISM2 39.7 48.0 74.3Mean January precipitation Inches PRISM2 2.87 3.87 6.34Mean February precipitation Inches PRISM2 2.10 3.00 4.78Mean March precipitation Inches PRISM2 2.72 3.88 6.65Mean April precipitation Inches PRISM2 2.79 3.94 6.23Mean May precipitation Inches PRISM2 3.25 4.10 5.89Mean June precipitation Inches PRISM2 3.52 4.22 6.33Mean July precipitation Inches PRISM2 3.38 4.20 5.93Mean August precipitation Inches PRISM2 3.40 4.36 6.20Mean September precipitation Inches PRISM2 3.26 3.99 6.15Mean October precipitation Inches PRISM2 3.50 4.24 6.38Mean November precipitation Inches PRISM2 3.39 4.39 7.31Mean December precipitation Inches PRISM2 2.91 3.83 6.16Mean fall (October through November) precipitation Inches PRISM2 6.89 8.58 13.69Mean winter (December through February) precipitation Inches PRISM2 7.88 10.70 17.28Mean spring (March through May) precipitation Inches PRISM2 8.98 11.92 18.76Mean maximum annual temperature °F PRISM3 7.0 12.1 15.1Mean maximum December temperature °F PRISM3 -4.4 0.1 3.8Mean maximum January temperature °F PRISM3 -9.4 -2.1 1.6Mean maximum February temperature °F PRISM3 -5.3 -0.7 2.8Mean maximum March temperature °F PRISM3 -1.2 4.2 7.5Mean maximum April temperature °F PRISM3 5.2 11.1 14.1Mean maximum May temperature °F PRISM3 12.7 18.5 20.7Mean minimum annual temperature °F PRISM4 -3.7 0.1 3.9Mean minimum December temperature °F PRISM4 -15.0 -10.0 -5.6Mean minimum January temperature °F PRISM4 -19.1 -13.4 -8.3Mean minimum February temperature °F PRISM4 -19.2 -12.7 -7.6
40 Estimation of Flood Discharges at Selected Recurrence Intervals for Streams in New Hampshire Table 6. Basin characteristics determined for use in regression analysis.—Continued [PRISM, parameter-elevation regressions on independent slopes model; USGS, U.S. Geological Survey; °F, degrees Fahrenheit; %, percent; /, divided by; ×, multiplied by; --, none] Characteristic description Units Source Minimum Mean Maximum Mean minimum March temperature °F PRISM4 -12.7 -7.2 -3.0 Mean minimum April temperature °F PRISM4 -4.9 -1.0 2.3 Mean minimum May temperature °F PRISM4 2.0 4.9 7.7 Percentage of basin that is wetland Percent National Wetlands Inventory5 0 5.68 20.6 Percentage of basin that is lacustrine or palustrine wetland Percent National Wetlands Inventory5 0 5.65 20.6 Percentage of basin that is lacustrine wetland Percent National Wetlands Inventory5 0 0.89 17.8 Percentage of basin that is palustrine wetland Percent National Wetlands Inventory5 0 4.75 19.0 Percentage of basin that is riverine wetland Percent National Wetlands Inventory5 0 0.04 0.35 Percentage of basin covered by forest Percent National Land Cover Data6 40.7 80.8 99.5 Percentage of basin covered by shrubland Percent National Land Cover Data6 0 1.54 8.62 Percentage of basin covered by water Percent National Land Cover Data6 0 0.97 15.8 Percentage of basin covered by wetlands Percent National Land Cover Data6 0 4.86 21.8 Percentage of basin covered by developed land Percent National Land Cover Data6 0 6.20 35.4 Percentage of basin covered by barren land Percent National Land Cover Data6 0 0.32 10.5 Percentage of basin covered by agricultural land Percent National Land Cover Data6 0 6.25 25.7 Percentage of basin with imperviousness greater than 0% Percent National Land Cover Data6 0 5.98 33.5 Percentage of basin with imperviousness greater than or equal to 20% Percent National Land Cover Data6 0 2.58 23.9 Percentage of basin with imperviousness greater than or equal to 40% Percent National Land Cover Data6 0 1.43 16.5 Percentage of basin with imperviousness greater than or equal to 60% Percent National Land Cover Data6 0 0.52 8.02 Percentage of basin with imperviousness greater than or equal to 80% Percent National Land Cover Data6 0 0.14 3.15 Average imperviousness Percent National Land Cover Data6 0 1.40 12.5 Percentage of basin with tree canopy greater than or equal to 20% Percent National Land Cover Data6 55.6 87.0 99.9 Percentage of basin with tree canopy greater than or equal to 40% Percent National Land Cover Data6 52.5 85.0 99.9 Percentage of basin with tree canopy greater than or equal to 60% Percent National Land Cover Data6 47.7 81.0 99.5 Percentage of basin with tree canopy greater than or equal to 80% Percent National Land Cover Data6 31.6 69.1 96.6 Average tree canopy percentage Percent National Land Cover Data6 43.5 74.5 93.1 Northing of basin centroid Feet National Elevation Dataset1 -14,476 384,493 969,502

Easting of basin centroid Feet National Elevation Dataset1 654,632 951,250Percentage of basin covered by lakes and ponds Percent National Hydrography Dataset7 0 1.22 18.2Percentage of basin covered by swamps Percent National Hydrography Dataset7 0 1.41 11.1Percentage of basin covered by lakes, ponds, and swamps Percent National Hydrography Dataset7 0 2.63 18.2Mean permeability of soils Inches per hour State Soil Geographic (STATSGO) Database8 1.2 4.0 14.1Mean liquid limit of soils Percent State Soil Geographic (STATSGO) Database8 14.7 21.4 28.6Mean available water content Inches per inch State Soil Geographic (STATSGO) Database8 0.1 0.1 0.2Mean quality of soil drainage -- State Soil Geographic (STATSGO) Database8 2.6 3.4 4.4
Table 6 41

Table 6. Basin characteristics determined for use in regression analysis.—Continued[PRISM, parameter-elevation regressions on independent slopes model; USGS, U.S. Geological Survey; °F, degrees Fahrenheit; %, percent; /, divided by; ×, multiplied by; --, none]

Characteristic description Units Source Minimum Mean Maximum

Mean of hydrologic grouping of soils -- State Soil Geographic (STATSGO) Database8 2.0 2.8 3.5Percentage of basin with hydric soils Percent State Soil Geographic (STATSGO) Database8 0 0.1 0.4Percentage of the basin above 500 feet Percent National Elevation Dataset1 0 82.1 100Percentage of the basin above 750 feet Percent National Elevation Dataset1 0 72.9 100Percentage of the basin above 1,000 feet Percent National Elevation Dataset1 0 60.4 100Percentage of the basin above 1,200 feet Percent National Elevation Dataset1 0 47.0 100Percentage of the basin above 1,400 feet Percent National Elevation Dataset1 0 36.1 100Population in basin Persons 2000 Census9 0 5,161 88,426Population density in basin Persons per 

square mile

2000 Census9 0 104 912

Number of housing units in basin Units 2000 Census9 3 2,563 35,894Housing unit density in basin Units per 

square mile

2000 Census9 0.58 42.9 334

Longest flow path length Miles National Elevation Dataset1 1.47 16.4 96.1Maximum elevation change along longest flow path Feet National Elevation Dataset1 76 1,526 5,628Channel slope of longest flow path Feet per mile National Elevation Dataset1 12.8 158 691Outlet elevation Feet National Elevation Dataset1 25 636 2,049Elevation at upstream end of longest flow path Feet National Elevation Dataset1 143 2,162 6,273Elevation 10% of the distance up the longest flow path Feet National Elevation Dataset1 38 681 2,135Elevation 85% of the distance up the longest flow path Feet National Elevation Dataset1 97 1,337 4,337Channel slope between points 10- and 85-percent up longest flow path Feet per mile National Elevation Dataset1 5.43 114 543Basin perimeter Miles National Elevation Dataset1 3.71 39.8 223Relative relief (relief/basin perimeter) -- National Elevation Dataset1 9.5 73.8 321Compactness ratio10 -- National Elevation Dataset1 1.18 1.57 2.13Basin length11 Miles National Elevation Dataset1 1.00 11.0 44.6Sinuosity (longest flow path length/basin length) -- National Elevation Dataset1 1.07 1.41 2.55Basin width (drainage area/basin length) Miles National Elevation Dataset1 0.39 4.28 29.1Shape factor (basin length/basin width) -- National Elevation Dataset1 0.98 2.99 6.74Elongation ratio12 -- National Elevation Dataset1 0.43 0.70 1.14Rotundity ratio13 -- National Elevation Dataset1 1.11 1.91 2.93Slope ratio (channel slope/basin slope) -- National Elevation Dataset1 0.04 0.20 0.55Total length of streams Miles National Hydrography Dataset7 0.33 158 2,258
42 Estimation of Flood Discharges at Selected Recurrence Intervals for Streams in New Hampshire Table 6. Basin characteristics determined for use in regression analysis.—Continued [PRISM, parameter-elevation regressions on independent slopes model; USGS, U.S. Geological Survey; °F, degrees Fahrenheit; %, percent; /, divided by; ×, multiplied by; --, none] Characteristic description Units Source Minimum Mean Maximum Stream density Miles per square mile National Elevation Dataset1 and National Hydrography Dataset7 0.37 2.01 4.46 Constant of channel maintenance (drainage area/total length of streams) Miles National Elevation Dataset1 and National Hydrography Dataset7 0.22 0.56 2.67 Ruggedness number (relative relief × total length of stream/ drainage area) -- National Elevation Dataset1 and National Hydrography Dataset7 202 3,460 14,600 Basinwide mean of the 2-year, 24-hour rainfall Inches Weather Bureau Technical Paper No. 4014 2.36 2.82 3.59 Basinwide mean of the 100-year, 24-hour rainfall Inches Weather Bureau Technical Paper No. 4014 4.91 6.11 7.16 Basinwide mean of mean annual runoff Inches Mean annual runoff, precipitation, and evapo- transpiration in the glaciated northeastern United States, 1951–8015 18.4 25.4 39.5 Basinwide mean of the mean annual snowfall Inches Climatesource16 51.3 92.2 218 Percentage of basin in the Seaboard Lowland section of the New England Physiographic Province Percent Physiographic divisions of the conterminous United States17

0 12.0 100

Percentage of basin in the White Mountain section of the New England 

Physiographic Province

Percent Physiographic divisions of the conterminous 

United States17

0 22.0 100

Percentage of basin in the New England Upland section of the 

New England Physiographic Province

Percent Physiographic divisions of the conterminous 

United States17

0 58.3 100

1U.S. Geological Survey (2007a).2PRISM Group, Oregon State University (2006c).3PRISM Group, Oregon State University (2006a).4PRISM Group, Oregon State University (2006b).5U.S. Fish and Wildlife Service (2007).6Multi-Resolution Land Characteristics Consortium (2003).7U.S. Geological Survey (2007b).8U.S. Geological Survey (1995).9U.S. Census Bureau (2006).10Ratio of basin perimeter to the perimeter of a circle have an area equal to the drainage area of the basin. 11Length of a line areally centered through the basin from the endpoint of the basin’s longest flow path.12Ratio of the diameter of a circle have an area equal to the drainage area of the basin to the basin length.13Ratio of basin width to the diameter of a circle have an area equal to the drainage area of the basin.14U.S. Department of Commerce (1961).15Randall (1996).16Daly and others (2000).17Fenneman and Johnson (1946).
Table 7 43

Table 7. Basin characteristics of selected streamgages in and adjacent to New Hampshire and vicinity used in the development of the 

final regression equations.—Continued

[USGS, U.S. Geological Survey; all station locations are shown on figure 2; mi2, square miles]

USGS 

streamgage 

station 

number

Station name

Drainage 

area

(mi2)

Basinwide 

mean of April 

precipitation 

(inches)

Percent 

of basin 

covered by 

wetlands

Main channel 

slope 

(feet per mile)

01050900 Four Ponds Brook near Houghton, ME 3.26 4.35 17.0 83.1

01052500 Diamond River near Wentworth Location, NH 153 3.34 2.58 29.9

01054200 Wild River at Gilead, ME 69.9 4.54 0.397 123

01054300 Ellis River at South Andover, ME 131 4.20 5.15 32.7

01055000 Swift River near Roxbury, ME 96.8 4.59 0.827 74.0

01055300 Bog Brook near Buckfield, ME 10.5 3.92 11.2 22.2

01057000 Little Androscoggin River near South Paris, ME 76.5 3.71 5.47 42.1

01062700 Patte Brook near Bethel, ME 5.67 3.97 3.90 116

01063310 Stony Brook at East Sebago, ME 1.51 4.39 7.52 36.2

01064300 Ellis River near Jackson, NH 10.5 6.23 0 533

01064380 East Branch Saco River at Town Hall Road, near 

Lower Bartlett, NH

33.9 4.90 0.782 148

01064400 Lucy Brook near North Conway, NH 4.69 4.45 0.768 428

01064500 Saco River near Conway, NH 385 4.70 1.83 47.3

01064800 Cold Brook at South Tamworth, NH 5.46 4.28 0 421

01064801 Bearcamp River at South Tamworth, NH 67.4 4.16 4.89 131

01065000 Ossipee River at Effingham Falls, NH 329 4.14 8.99 22.5

01065500 Ossipee River at Cornish, ME 452 4.20 8.92 12.9

01066000 Saco River at Cornish, ME 1,290 4.36 8.18 7.33

01066100 Pease Brook near Cornish, ME 4.63 4.50 2.41 133

01066500 Little Ossipee River near South Limington, ME 163 4.48 11.6 10.1

01069700 Branch Brook near Kennebunk, ME 10.5 4.34 9.48 24.0

01072800 Cocheco River near Rochester, NH 79.9 4.28 6.12 16.7

01072850 Mohawk Brook near Center Strafford, NH 7.30 4.29 4.28 86.6

01073000 Oyster River near Durham, NH 12.2 4.18 9.60 17.9

01073500 Lamprey River near Newmarket, NH 185 4.07 7.79 9.36

01073587 Exeter River at Haigh Road, near Brentwood, NH 63.5 4.06 13.7 7.09

01073600 Dudley Brook near Exeter, NH 5.79 4.20 6.88 8.80

01073750 Mill Brook near State Route 108, at Stratham, NH 2.31 4.30 9.09 43.2

01074520 East Branch Pemigewasset River at Lincoln, NH 115 4.53 0.206 86.1

01075000 Pemigewasset River at Woodstock, NH 195 4.31 0.664 78.2

01075800 Stevens Brook near Wentworth, NH 3.29 3.75 0 472

01076000 Baker River near Rumney, NH 143 3.45 1.58 94.5

01076500 Pemigewasset River at Plymouth, NH 623 3.99 1.43 38.9

01078000 Smith River near Bristol, NH 86.1 3.54 4.30 23.6

01082000 Contoocook River at Peterborough, NH 67.4 3.97 10.2 21.8

01084000 North Branch River near Antrim, NH 54.2 4.34 9.92 22.5

01084500 Beards Brook near Hillsborough, NH 55.4 4.16 7.43 68.6

01085800 West Branch Warner River near Bradford, NH 5.88 4.35 1.38 388

01086000 Warner River at Davisville, NH 145 3.96 6.08 29.0

01089100 Soucook River at Pembroke Road, near Concord, NH 82.7 3.61 5.58 26.7
44 Estimation of Flood Discharges at Selected Recurrence Intervals for Streams in New Hampshire

Table 7. Basin characteristics of selected streamgages in and adjacent to New Hampshire and vicinity used in the development of the 

final regression equations.—Continued

[USGS, U.S. Geological Survey; all station locations are shown on figure 2; mi2, square miles]

USGS 

streamgage 

station 

number

Station name

Drainage 

area

(mi2)

Basinwide 

mean of April 

precipitation 

(inches)

Percent 

of basin 

covered by 

wetlands

Main channel 

slope 

(feet per mile)

01089500 Suncook River at North Chichester, NH 154 4.03 7.05 16.6

01091000 South Branch Piscataquog River near Goffstown, NH 103 4.03 5.48 28.0

01093800 Stony Brook Tributary near Temple, NH 3.60 4.23 0.363 258

01094000 Souhegan River at Merrimack, NH 171 4.17 4.73 25.5

01094500 North Nashua River near Leominster, MA 108 4.25 7.68 37.7

01095000 Rocky Brook near Sterling, MA 2.23 4.47 14.6 101

01095800 Easter Brook near North Leominster, MA 0.95 4.22 4.53 158

01096000 Squannacook River near West Groton, MA 65.6 4.15 6.80 47.0

010965852 Beaver Brook at North Pelham, NH 47.6 3.94 7.89 12.6

01096910 Boulder Brook at East Bolton, MA 1.60 4.10 0.128 75.2

01097200 Heath Hen Meadow Brook at Stow, MA 3.76 4.06 15.1 14.2

01097300 Nashoba Brook near Acton, MA 12.3 3.99 13.6 20.2

01100100 Richardson Brook near Lowell, MA 4.04 4.00 7.81 30.2

01100700 East Meadow River near Haverhill, MA 4.81 4.30 13.0 20.1

01100800 Cobbler Brook near Merrimack, MA 0.76 4.29 2.03 56.7

01100900 Parker River Tributary near Georgetown, MA 0.70 4.35 21.8 23.8

01101000 Parker River at Byfield, MA 21.4 4.32 20.5 5.43

01127880 Big Brook near Pittsburg, NH 6.47 3.30 3.39 234

01129300 Halls Stream near East Hereford, Quebec 84.8 3.04 0.615 32.6

01129400 Black Brook at Averill, VT 0.88 3.25 0.066 207

01129440 Mohawk River near Colebrook, NH 35.2 3.09 1.56 127

01129700 Paul Stream Tributary near Brunswick, VT 1.48 2.97 0.844 240

01130000 Upper Ammonoosuc River near Groveton, NH 230 3.52 3.44 23.0

01133000 East Branch Passumpsic River near East Haven, VT 51.3 3.15 3.05 84.7

01133200 Quimby Brook near Lyndonville, VT 2.16 2.83 0.873 229

01133300 Cold Hill Brook near Lyndon, VT 1.64 2.88 0.177 252

01134500 Moose River at Victory, VT 75.2 3.22 4.50 66.7

01134800 Kirby Brook at Concord, VT 8.13 2.98 0.636 125

01135000 Moose River at St. Johnsbury, VT 129 3.09 3.38 37.3

01135150 Pope Brook near North Danville, VT 3.27 3.05 0.062 369

01135300 Sleepers River near St. Johnsbury, VT 42.5 2.94 0.776 104

01135500 Passumpsic River at Passumpsic, VT 434 3.00 2.08 36.2

01135700 Joes Brook Tributary near East Barnet, VT 0.70 2.79 0 543

01137500 Ammonoosuc River at Bethlehem Junction, NH 88.2 4.31 0.574 71.5

01138000 Ammonoosuc River near Bath, NH 396 3.32 1.45 26.4

01138800 Keenan Brook at Groton, VT 4.72 3.05 3.49 138

01139000 Wells River at Wells River, VT 98.8 3.00 3.39 32.9

01139700 Waits River Tributary near West Topsham, VT 1.21 3.43 0.578 475

01139800 East Orange Branch at East Orange, VT 8.79 3.35 0.163 153

01140800 West Branch Ompompanoosuc River Tributary at 

South Strafford, VT

1.35 3.36 0 330
Table 7 45

Table 7. Basin characteristics of selected streamgages in and adjacent to New Hampshire and vicinity used in the development of the 

final regression equations.—Continued

[USGS, U.S. Geological Survey; all station locations are shown on figure 2; mi2, square miles]

USGS 

streamgage 

station 

number

Station name

Drainage 

area

(mi2)

Basinwide 

mean of April 

precipitation 

(inches)

Percent 

of basin 

covered by 

wetlands

Main channel 

slope 

(feet per mile)

01141800 Mink Brook near Etna, NH 4.76 3.31 0.622 172

01142000 White River near Bethel, VT 239 4.11 0.701 16.2

01142400 Third Branch White River Tributary at Randolph, VT 0.80 3.36 0 376

01142500 Ayers Brook at Randolph, VT 30.5 3.11 0.653 61.5

01144000 White River at West Hartford, VT 687 3.60 0.874 12.5

01145000 Mascoma River at West Canaan, NH 80.4 3.37 5.35 47.8

01150800 Kent Brook near Killington, VT 3.25 4.89 2.03 438

01150900 Ottauquechee River near West Bridgewater, VT 23.3 4.54 3.27 121

01151200 Ottauquechee River Tributary near Quechee, VT 0.78 3.42 1.04 249

01152500 Sugar River at West Claremont, NH 270 3.54 7.21 23.3

01153300 Middle Branch Williams River Tributary at Chester, VT 3.19 3.94 0.319 237

01153550 Williams River near Rockingham, VT 112 3.98 1.13 49.6

01154000 Saxtons River at Saxtons River, VT 72.1 4.05 1.60 79.1

01155000 Cold River at Drewsville, NH 83.3 3.54 2.83 41.1

01155200 Sacketts Brook near Putney, VT 10.2 3.89 1.03 163

01155300 Flood Brook near Londonderry, VT 9.28 4.51 1.77 164

01155350 Tributary to West River Tributary near Jamaica, VT 0.93 4.43 0.282 390

01156300 Whetstone Brook Tributary near Marlboro, VT 1.08 4.40 1.43 156

01156450 Connecticut River Tributary near Vernon, VT 1.10 4.01 1.29 153

01157000 Ashuelot River near Gilsum, NH 71.9 3.85 7.12 31.0

01158500 Otter Brook near Keene, NH 42.2 3.81 4.92 60.8

01160000 South Branch Ashuelot River at Webb, near Marlborough, NH 35.9 3.85 5.90 62.6

01161300 Millers Brook at Northfield, MA 2.32 3.89 0.376 324

01161500 Tarbell Brook near Winchendon, MA 18.8 3.92 14.6 28.6

01162000 Millers River near Winchendon, MA 82.3 3.97 15.2 17.7

01162500 Priest Brook near Winchendon, MA 18.9 3.89 13.4 22.0

01163100 Wilder Brook near Gardner, MA 2.65 4.05 17.9 49.3

01163200 Otter River at Otter River, MA 34.0 4.06 18.4 19.2

01165500 Moss Brook at Wendell Depot, MA 12.2 3.89 6.73 43.8

01167800 Beaver Brook at Wilmington, VT 6.36 4.54 3.12 117

01169000 North River at Shattuckville, MA 89.0 4.34 2.13 58.8

01169900 South River near Conway, MA 24.2 4.71 2.02 67.6

01170100 Green River near Colrain, MA 41.2 4.36 1.69 67.6

01170200 Allen Brook near Shelburne Falls, MA 0.72 4.18 6.92 224

01170900 Mill River near South Deerfield, MA 6.41 4.08 2.21 98.1

01173900 Middle Branch Swift River at North New Salem, MA 4.72 4.18 4.12 123

01174565 West Branch Swift River near Shutesbury, MA 12.8 4.30 1.70 68.2
THIS PAGE INTENTIONALLY LEFT BLANK
Appendixes 1–3
48 Estimation of Flood Discharges at Selected Recurrence Intervals for Streams in New Hampshire

Appendix 1. Mean April precipitation. (PRISM Group, Oregon State University, http://www.prismclimate.org, map 

created April 8, 2008)

0 5 10 15 20 MILES

0 10 20 KILOMETERS

73° 72° 71°

45°

44°

43°

VERMONT

MAINE

MASSACHUSETTS

NEW HAMPSHIRE

CANADA

Coos

Grafton

Sullivan

Cheshire Hillsborough

Merrimack

Belknap

Carroll

Strafford

Rockingham

EXPLANATION

County boundary

2.3 to 3.0 inches

greater than 3.0 to 4.0 inches greater than 5.0 to 6.0 inches

greater than 4.0 to 5.0 inches greater than 6.0 to 7.0 inches

greater than 7.0 to 8.0 inches

County Boundaries: U.S. Geological Survey Digital Line Graphs, 1990–94, 1:24,000

Precipitation: PRISM Group United States Average Monthly Precipitation, 2006, 30 arc-second resolution

Mean April precipitation, in inches

Atlantic

Ocean
Appendix 2 49

Appendix 2. Wetland areas. (Multi-Resolution Land Characteristics Consortium, 2003)

73° 72° 71°

45°

44°

43°

VERMONT

MAINE

MASSACHUSETTS

NEW HAMPSHIRE

CANADA

Coos

Grafton

Sullivan

Cheshire Hillsborough

Merrimack

Belknap

Carroll

Strafford

Rockingham

County boundary

EXPLANATION

Wetland area

County Boundaries: U.S. Geological Survey Digital Line Graphs, 1990–94, 1:24,000

Wetlands: Multi-Resolution Land Characteristics Consortium 2001 National Land Cover Data, 2003,

30 meter resolution, Albers conical equal area projection, 

Spheroid GRS 1980, Datum NAD 83

0 5 10 15 20 MILES

0 10 20 KILOMETERS

Atlantic

Ocean
50 Estimation of Flood Discharges at Selected Recurrence Intervals for Streams in New Hampshire

The general procedure for estimating the magnitude of 

a flood discharge at a selected recurrence interval using the 

methods described in the report is illustrated in the following 

example. The example does not apply to any specific location as the basin characteristics were selected randomly. The 

purpose of the example is to illustrate the methods described 

in the report.

An estimate of the 100-year flood discharge (Q100) is 

needed for a rural, ungaged stream in New Hampshire. The 

drainage area, basinwide mean of the average April precipitation, areal percent of wetlands in the basin, and main channel 

slope are determined to be 22.0 mi2, 4.05 in., 3.27 percent, and 

62.6 ft/mi, respectively, using the same basin-characteristics 

data sources described in this report. To determine the magnitude of the 100-year flood discharge in cubic feet per second, 

equation 8 would be solved:

Q100 = 7.13A0.867P1.9810-0.0254(W)S0.198, (8)

Q100 = 7.13(22.0)0.867(4.05)1.9810-0.0254(3.27)(62.6)0.198, and

Q100,r(g) = Q100 = 3,110 ft3/s.

The standard error of prediction for this estimate 

would be computed using equation 10. The row vector, xi, 

in equation 10 is based on the basin characteristics and is 

as follows:

xi = [1 log10(22.0) log10(4.05) 3.27 log10(62.6)] or

xi = [1 1.34 0.607 3.27 1.80].

The model error variance, γ2, obtained from table 11 is 

0.0235. The (XtrΛ-1X)

-1

 matrix is

(XtrΛ-1X)

-1

 = 

Inserting xi, γ2, and (XtΛ-1X)

-1

 into equation 10, the standard 

error of prediction is

SEpred = [γ2+ xi(XtrΛ-1X)-1xi

tr]1/2, (10)

SEpred = [0.0235 + 0.00117]1/2 or

SEpred = 0.157.

 0. - 060575 0. - 0051667 0. - 050248 0.00059817 0 010587

0 0051667 0 0013231 0 0013914

 

 

- .

- . . - . 

 

0 00010560 0 0019168

0 050248 0 001

. .

- . - . 3914 0 10452 0 00036357 0 0050145

0 0

 . - . - .

- . 0059817 0. - 00010560 0.00036357 0.000029723 

 

0 00028972

0 010587 0 0019168 0 0050145

.

- . . - . 0.00028972 0.0053530

































The standard error of prediction for the estimate in percent is 

then computed using equations 11 and 12

Spos = 100(10SEpred-1), and (11)

Sneg = 100(10-SEpred-1), (12)

or

Spos = 100(100.157-1), and

Sneg = 100(10-0.157-1) = -30.3.

Thus, Spos = 43.5 percent and Sneg = -30.3 percent.

The equivalent years of record for this estimate is computed using equation 13. Because the drainage area, A, used 

for this estimate is 22.0 mi2, the standard deviation of annual 

events as defined for equation 13 is

s = e-(1.31 + 0.134 log(22.0)) or

s = 0.225.

The log-Pearson type III frequency factor, kT, is 2.326 obtained 

from Bulletin 17B (U.S. Interagency Advisory Committee on 

Water Data, 1982), with the skew, g, equal to zero since this is 

an estimate at an ungaged site. Equation 13 is solved as 

follows:

E

s k g k g

SE

T T

pred

=

 + + ( ) +  

 2 2 2

2

1 0. . 5 1 0 75 , (13)

E =

( ) + ( ) 

 

( )

0 225 1 0 5 2 326

0 157

2 2

2

. . .

. or

E = 7.6 years.

The prediction interval for this estimate can be computed 

using equations 14 and 15. For example, if the 90-percent 

confidence interval were required, the critical value from a 

Student’s-t distribution for the appropriate confidence interval 

would be found using any standard statistics textbook. The 

value for t

0.05,112 is 1.66, and the standard error of prediction 

was found earlier to be 0.157. Equation 14 is solved as 

follows: 

Appendix 3. Example Application
Appendix 3 51

V t S n p Epred= ( − ) 10 / , 2 , (14)

V = 10(1.66*0.157), and

V = 1.82.

Therefore, the 90-percent prediction interval is

(1/V)Qpred < Qtrue < (V)Qpred ,

 (15)

(1/1.82)3,110 < Qtrue < (1.82)3,110 or

1,710 ft3/s < Qtrue < 5,660 ft3/s.

If the regression equation estimate of the 100-year 

discharge shown above was made for a streamgage location 

having 10 years of record, a weighted average, Q100,w, of the 

regression estimate and the 100-year discharge estimate from a 

frequency analysis of the 10 years of record can be computed. 

If the 100-year discharge determined from the streamgage 

data, Q100,s, is 3,700 ft3/s, then the weighted average can be 

computed using equation 16.

log Q N Q E Q

N E T w

T s T r g

10

10 10

,

, , log log = ( ) + ( )

+

( ) , (16)

log

log , . log ,

. 10 100,

10 10 10 3 700 7 6 3 100

10 7 6 Q w = +

+

( )( ) ( )( ) , and

Q100,w= 3,430 ft3/s.

If a flood discharge estimate is required at an ungaged 

site near a streamgage, a weighted estimate, Q100,u, of the 

regression equation result and the flood discharge for the 

streamgage can be made using equations 17, 18, and 19. 

Continuing with the same example data, assume that a 

100-year discharge estimate is required at a location 

downstream from a streamgage with a drainage area of 

30 mi2. At this ungaged site, assume that the mean annual 

April precipitation is 4.00 in., the percent of wetlands is 

3.40 percent, and the main channel slope is 59.0 ft per mile. 

Using the regression equation for estimating a 100-year 

discharge,

Q100 = 7.13A0.867P1.9810-0.0254(W)S0.198, (8)

Q100 = 7.13(30.0)0.867(4.00)1.9810-0.0254(3.40)(59.0)0.198, and

Q100,r(u) = Q100 = 3,890 ft3/s.

With the data obtained, the slope of the regression line, 

m, between streamgage and the ungaged site can be determined from equation 17.

m

Q Q

A A

T r u T r g

u g

= ( ( ) ( ) )

( )

log /

log /

10 , ,

10

, (17)

m = ( )

( )

log , / ,

log . / .

10

10

3 890 3 110

30 0 22 0 , and

m = 0.722.

Then the slope of the line that passes through the best 

estimate for the streamgage and intersects the regression line 

with slope m at a point with a drainage area of 150 percent of 

the streamgage drainage area is determined from equation 18.

c m Q Q

a

T r g T w

= + ( )

( )

( ) log /

log

10 , ,

10

, (18)

c = + ( )

( ) 0 722

3 110 3 430

1 5

10

10

. log , / ,

log .

, and

c = 0.480.

This slope (0.480) is then used to adjust the estimate of 

the 100-year discharge for the streamgage, Q100,w = 3,430 ft3/s 

in this example, to obtain the estimate for the ungaged site 

with a drainage area of 30 mi2 using equation 19.

Q Q A

A T u T w

u

g

c

, , = 















, (19)

Q100 u

0 480

3 430 30 0

22 0 ,

.

, .

. = 



 



 , and

Q100,u = 3,980 ft3/s.
Prepared by the Pembroke Publishing Service Center.

For more information concerning this report, contact:

Director

U.S. Geological Survey

New Hampshire-Vermont Water Science Center

361 Commerce Way

Pembroke, NH 03275

dc_nh@usgs.gov

or visit our Web site at:

http://nh.water.usgs.gov
Olson—Estimation of Flood Discharges at Selected Recurrence Intervals for Streams in New Hampshire—Scientific Investigations Report 2008–5206

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