U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2010–5094

Prepared in cooperation with the Maine Department of Transportation

Historical Changes in Annual Peak Flows in Maine

and Implications for Flood-Frequency Analyses
Cover. St. John River at Ft. Kent, Maine, April 29, 2009. Photo credit: Andrew Cloutier, USGS
Historical Changes in Annual Peak Flows 

in Maine and Implications for FloodFrequency Analyses

By Glenn A. Hodgkins

Prepared in cooperation with the Maine Department of Transportation

Scientific Investigations Report 2010–5094

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

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Suggested citation:

Hodgkins, G.A., 2010, Historical changes in annual peak flows in Maine and implications for flood-frequency analyses: 

U.S. Geological Survey Scientific Investigations Report 2010–5094, 38 p., at http://pubs.usgs.gov/sir/2010/5094.
iii

Figures

1. Graphs showing examples of annual peak flows and Sen slopes at A, St. John

River, Ft. Kent, Maine and B, Little Androscoggin River, South Paris, Maine.....................7

2. Map showing geographic distribution of the magnitude of annual peak-flow linear

changes over time as computed by use of the Sen slope. River and location for

map numbers are in table 1.........................................................................................................9

3. Map showing geographic distribution of the magnitude of annual peak-flow step

changes over time as computed by use of the Hodges-Lehmann estimator...................11

4. Graph showing distribution of differences for various rivers between 100-year

peak flows based on selected sub-periods and 100-year peak flows based on full

periods of record ........................................................................................................................25

5. Graph showing distribution of differences for various rivers between 5-year peak 

flows based on selected sub-periods and 5-year peak flows based on full periods 

of record.......................................................................................................................................25

6. Map showing geographic distribution of 100-year peak flows based on the 1967 to

1996 sub-period that are outside the 90-percent confidence intervals of 100-year

peak flows based on full periods .............................................................................................27

7. Map showing geographic distribution of 100-year peak flows based on the 1937 to

1966 sub-period that are outside the 90-percent confidence intervals of 100-year

peak flows based on full periods .............................................................................................28

8. Map showing geographic distribution of 100-year peak flows based on the 1971 to

2006 sub-period that are outside the 90-percent confidence intervals of 100-year

peak flows based on full periods .............................................................................................29

9. Map showing geographic distribution of 100-year peak flows based on the

start-of-record to 1970 sub-period that are outside the 90-percent confidence

intervals of 100-year peak flows based on full periods........................................................30

10. Map showing geographic distribution of 5-year peak flows based on the 1967 to

1996 sub-period that are outside the 90-percent confidence intervals of 5-year

peak flows based on full periods .............................................................................................32

Contents

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

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

Climate and Hydrology of Maine........................................................................................................2

Historical Streamflow Changes in Maine.........................................................................................3

Data and Methods for Trend Tests and Flood-Frequency Analyses .....................................................3

Historical Changes in Annual Peak Flows.................................................................................................6

Flood-Frequency Analyses for Full Periods of Record and Selected Sub-Periods ............................7

Detailed Results for 100-Year Peak Flows ......................................................................................26

Detailed Results for 5-Year Peak Flows ..........................................................................................31

Implications for Flood-Frequency Analyses............................................................................................31

Summary and Conclusions.........................................................................................................................36

References Cited..........................................................................................................................................37
iv

11. Map showing geographic distribution of 5-year peak flows based on the 1937 to

1966 sub-period that are outside the 90-percent confidence intervals of 5-year

peak flows based on full periods .............................................................................................33

12. Map showing geographic distribution of 5-year peak flows based on the 1971 to

2006 sub-period that are outside the 90-percent confidence intervals of 5-year

peak flows based on full periods .............................................................................................34

13. Map showing geographic distribution of 5-year peak flows based on the

start-of-record to 1970 sub-period that are outside the 90-percent confidence

intervals of 5-year peak flows based on full periods............................................................35

Tables

1. Peak flows for 5- and 100-year recurrence intervals computed from the full period

of record for 28 selected streamgages in or near Maine.....................................................4

2. Magnitude of linear changes over time for annual peak flows as computed by use

of the Sen slope ............................................................................................................................8

3. Magnitude of step changes over time in annual peak flows as computed by use of

the Hodges-Lehmann estimator...............................................................................................10

4. Peak flows with 100-year recurrence intervals based on multiple 30-year

sub-periods, and comparisons to 100-year peak flows based on full periods.................12

5. Peak flows with 100-year recurrence intervals based on the sub-periods 1971 to

2006 and start-of-record to 1970, and comparisons to 100-year peak flows based

on full periods..............................................................................................................................17

6. Peak flows with 5-year recurrence intervals based on multiple 30-year

sub-periods, and comparisons to 5-year peak flows based on full periods.....................18

7. Peak flows with 5-year recurrence intervals based on the sub-periods 1971 to 

2006 and start-of-record to 1970, and comparisons to 5-year peak flows based on 

full periods ...................................................................................................................................23

8. Median differences between peak flows for selected recurrence intervals based

on sub-periods and peak flows based on full periods of record........................................24

9. Number of streamgages with 100-year peak flows based on sub-periods that are

outside of 90-percent confidence intervals of 100-year peak flows based on full

periods of record ........................................................................................................................24

10. Number of streamgages with 5-year peak flows based on sub-periods that are

outside of 90-percent confidence intervals of 5-year peak flows based on 

full records...................................................................................................................................25
v

Conversion Factors and Datum

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 inch (in2) 6.452 square centimeter (cm2)

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) 

Flow rate

foot per second (ft/s) 0.3048 meter per second (m/s)

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

cubic foot per second per square 

mile [(ft3/s)/mi2]

0.01093 cubic meter per second per 

square kilometer [(m3/s)/km2]

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

°F=(1.8×°C)+32

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 1983 (NAD 83).

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

List of Acronyms

HCDN Hydro-Climatic Data Network

MaineDOT Maine Department of Transportation

NAO North Atlantic Oscillation

USGS U.S. Geological Survey

VIC variable infiltration capacity
This page has been left blank intentionally.
Historical Changes in Annual Peak Flows in Maine and 

Implications for Flood-Frequency Analyses

By Glenn A. Hodgkins

Abstract

Flood-frequency analyses use statistical methods to 

compute peak streamflows for selected recurrence intervals—

the average number of years between peak flows that are 

equal to or greater than a specified peak flow. Analyses are 

based on annual peak flows at a stream. It has long been 

assumed that the annual peak streamflows used in these 

computations were stationary (non-changing) over very long 

periods of time, except in river basins subject to direct effects 

of human activities, such as urbanization and regulation. 

Because of the potential effects of global warming on peak 

flows, the assumption of peak-flow stationarity has recently 

been questioned. Maine has many streamgages with 50 to 

105 years of recorded annual peak streamflows. In this study, 

this long-term record has been tested for historical floodfrequency stationarity, to provide some insight into future 

flood frequency.

Changes over time in annual instantaneous peak 

streamflows at 28 U.S. Geological Survey streamgages with 

long-term data (50 or more years) and relatively complete 

records were investigated by examining linear trends for each 

streamgage’s period of record. None of the 28 streamgages 

had more than 5 years of missing data. Eight streamgages have 

substantial streamflow regulation. Because previous studies 

have suggested that changes over time may have occurred 

as a step change around 1970, step changes between each 

streamgage’s older record (start year to 1970) and newer 

record (1971 to 2006) also were computed. The median 

change over time for all 28 streamgages is an increase of 

15.9 percent based on a linear change and an increase of 

12.4 percent based on a step change. The median change for 

the 20 unregulated streamgages is slightly higher than for all 

28 streamgages; it is 18.4 percent based on a linear change and 

15.0 percent based on a step change. 

Peak flows with 100- and 5-year recurrence intervals 

were computed for the 28 streamgages using the full annual 

peak-flow record and multiple sub-periods of that record using 

the guidelines (Bulletin 17B) of the Interagency Advisory 

Committee on Water Data. Magnitudes of 100- and 5-year 

peak flows computed from sub-periods then were compared to 

those computed from the full period. Sub-periods of 30 years 

with starting years staggered by 10 years were evaluated 

(1907–36, 1917–46, 1927–56, 1937–66, 1947–76, 1957–86, 

1967–96, and 1977–2006). Two other sub-periods were 

evaluated using older data (start-of-record to 1970) and newer 

data (1971 to 2006). The 5-year peak flow is used to represent 

small and relatively frequent flood flows in Maine, whereas 

the 100-year peak flow is used to represent large flood flows. 

The 1967–96 sub-period generated the highest 

100- and 5-year peak flows overall when compared to peak 

flows based on the full period of record; the median difference 

for all 28 streamgages is 8 percent for 100- and 5-year peak 

flows. The 1977–2006 and 1971–2006 sub-periods also 

generated 100- and 5-year peak flows higher than peak flows 

based on the full period of record, but not as high as the 

peak flows based on the 1967–96 sub-period. The 1937–66 

sub-period generated the lowest 100- and 5-year peak flows 

overall. The median difference from full-period peak flows 

is -11 percent for 100-year peak flows and -8 percent for 

5-year peak flows. Overall, differences between peak flows 

based on the sub-periods and those based on the full periods, 

generated using the 20 unregulated streamgages, are similar to 

differences using all 28 streamgages.

Increases in the 5- and 100-year peak flows based on 

recent years of record are, in general, modest when compared 

to peak flows based on complete periods of record. The 

highest peak flows are based on the 1967–96 sub-period rather 

than the most recent sub-period (1977-2006). Peak flows for 

selected recurrence intervals are sensitive to very high peak 

flows that may occur once in a century or even less frequently. 

It is difficult, therefore, to determine which approach will 

produce the most reliable future estimates of peak flows for 

selected recurrence intervals, using only recent years of record 

or the traditional method using the entire historical period. 

One possible conservative approach to computing peak flows 

of selected recurrence intervals would be to compute peak 

flows using recent annual peak flows and the entire period 

of record, then choose the higher computed value. Whether 

recent or entire periods of record are used to compute peak 

flows of selected recurrence intervals, the results of this 

study highlight the importance of using recent data in the 

computation of the peak flows. The use of older records alone 

could result in underestimation of peak flows, particularly 

peak flows with short recurrence intervals, such as the 5-year 

peak flows.
2 Historical Changes in Annual Peak Flows in Maine and Implications for Flood-Frequency Analyses

Introduction

Flood-frequency analyses use statistical methods to 

compute peak streamflows for selected recurrence intervals—

the average number of years between peak flows that are 

equal to or greater than a specified peak flow. For example, 

the 100-year peak flow is the flow that would be equaled 

or exceeded, on long-term average, once in 100 years. This 

does not imply, however, that flooding will happen at regular 

intervals. Two 100-year peak flows could occur within a few 

years of each other. In contrast, a 100-year peak flow might 

not occur in 200 years. The reciprocal of the recurrence 

interval is called the annual exceedance probability; that is, the 

probability that a given peak flow will be equaled or exceeded 

in any given year. For example, the annual exceedance 

probability of the 100-year peak flow would be 0.01. In other 

words, there is a 1 percent chance that the 100-year peak flow 

will be equaled or exceeded in any given year.

Determination of the magnitude of peak streamflows 

with specified recurrence intervals is necessary to safely and 

economically design bridges, culverts, and other structures 

that are in, or near, streams. These peak flows are also needed 

by Federal, State, regional, and local officials for effective 

floodplain management.

It has been known for decades that peak flows of selected 

recurrence intervals computed from shorter periods of annual 

peak-flow record can differ substantially from those computed 

from longer periods of record (Benson, 1960). It has long been 

assumed, however, that annual peak streamflows are stationary 

over very long periods of time, except in river basins subject 

to urbanization, regulation, and other human activities. 

Stationarity is the concept that natural systems fluctuate within 

an envelope of variability that does not change over time 

(Milly and others, 2008). Because of the potential effects of 

global warming on peak flows, the assumption of peak-flow 

stationarity has recently been questioned (Milly and others, 

2008). In New England, Hayhoe and others (2007) used 

climate projections as input to the variable infiltration capacity 

(VIC) hydrologic model. On the basis of this modeling, spring 

streamflow timing is expected to become earlier during the 

next 90 years, and high streamflows (50th to 95th percentile 

annual flows) are expected to increase.

If peak flows increase in the next century due to global 

warming, peak flows of selected recurrence intervals may 

also increase. Because bridges and culverts in Maine were 

designed with the assumption of peak-flow stationarity, 

bridges could be under-designed for future large floods. Maine 

has many streamgages with 50 to 105 years of recorded 

annual peak streamflows. In this study, this long-term record 

has been tested for historical flood-frequency stationarity 

to provide some insight into future flood frequency. Collins 

(2009) computed peak flows for a range of recurrence 

intervals for seven streamgages in New England that showed 

the strongest evidence of upward peak-flow trends and step 

(abrupt) increases around 1970 (out of 28 streamgages tested 

for trends). The peak flows for selected recurrence intervals 

were based on the entire periods of recorded peak flows at 

the streamgages, pre-1970 peak flows, and post-1970 peak 

flows. A majority of streamgages had higher magnitudes 

of peak flows for all recurrence intervals for computations 

based on post-1970 annual peak flows as compared to those 

based on pre-1970 peak flows. All seven streamgages had 

higher magnitudes of peak flows for 1- to 10-year recurrence 

intervals for computations based on post-1970 peak flows as 

compared to those based on pre-1970 peak flows.

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

(USGS) in cooperation with the Maine Department of 

Transportation (MaineDOT), will help MaineDOT and 

others better understand the effects of changing historical 

peak flows on flood-frequency computations for streams in 

Maine. To accomplish this, changes over time in annual peakflow magnitudes were evaluated for 28 streamgages in and 

near Maine with long-term records. Then, 5- and 100-year 

recurrence interval peak flows were computed for these 

streamgages using the full available annual peak-flow record 

and multiple sub-periods of that record. The 5-year peak flow 

is used to represent small and relatively frequent flood flows 

in Maine, whereas the 100-year peak flow is used to represent 

large flood flows. Magnitudes of 5- and 100-year peak flows 

computed from sub-periods are then compared to peak flows 

based on the full period and are also compared to 90-percent 

confidence intervals of the peak flows based on the full period. 

Climate and Hydrology of Maine

The climate of Maine is complex and variable across both 

space and time. Latitude, proximity to the Atlantic Coast, and 

variations in land-surface elevation have major effects on the 

climate (New England Regional Assessment Group, 2001). 

Maine is about halfway between the equator and the North 

Pole and is affected by warm, moist air from the south and 

cold, dry air from the north. The Atlantic Ocean moderates air 

temperatures in winter and summer, but the effects decrease 

with distance from the coast. In winter, the ocean variably 

affects the location of snow/rain boundaries. Despite the 

effects of the ocean, the prevailing air flow is not from the 

Atlantic Ocean, but from the drier North American continent. 

The mountainous topography of western and northwestern 

Maine affects precipitation and air temperatures. Precipitation 

is higher on the windward side of mountains and lower on 

the leeward side; however, storm-track directions through the 

mountains are highly variable so the windward and leeward 

areas differ for different storms. Air temperature decreases 

with higher elevation.

Maine generally has a temperate climate with mild 

summers and cold winters. From 1971 to 2000, the mean 

annual temperature statewide was about 42°F, but it ranged 

from 36°F in northern Maine to 47°F in southern Maine 

(National Oceanic and Atmospheric Administration, 2002). 

During the same period, statewide mean monthly temperatures 
Data and Methods for Trend Tests and Flood-Frequency Analyses 3

ranged from 15°F in January to 67°F in July. Precipitation in 

Maine is fairly evenly distributed throughout the year. The 

statewide mean annual precipitation for 1971 to 2000 was 

43 in., ranging from 35 in. in northern Maine to 57 in. in 

eastern coastal Maine. 

The snowpack in Maine typically accumulates 

throughout the winter and reaches its maximum depth and 

water equivalent (the depth of water that would result if the 

snowpack melted) in March or April. The median seasonal 

maximum depth of the snowpack for 1955 to 1992 varied from 

about 20 in. along the coast to more than 32 in. in the western 

mountains and in northern Maine (Cember and Wilks, 1993). 

The average water equivalent on or near March 1 ranged from 

3 to 5 in. along the coast to 7 to 9 in. at the western mountains 

and in northern Maine (Loiselle and Hodgkins, 2002). The 

109 snowpack data collection sites evaluated by Loiselle and 

Hodgkins (2002) had an average of 43 years of record through 

2000. Almost all of the data-collection sites were lower than 

2,000 ft in elevation and, therefore, do not represent the 

full range of average water equivalent in Maine, as many 

mountains have substantial areas higher than 2,000 ft.

Streamflows in Maine typically are highest in the 

spring, when rain falls on a ripe (dense, ready to melt) 

snowpack or on saturated soils. Streamflows recede as 

snowmelt ends and as evapotranspiration increases. This 

recession is frequently interrupted by runoff from rainstorms. 

Warm-season streamflows are usually lowest in August and 

September. In the fall, after evapotranspiration decreases 

substantially, repeated rains often saturate the soil, leading to 

high streamflows. Also in the fall, large amounts of rain can 

fall as a result of hurricanes, tropical storms, or their remnants. 

Winter streamflows are generally low in northern areas of 

Maine where winter precipitation typically falls as snow. 

Winter streamflows in southern areas of Maine can be more 

variable than streamflows in the northern areas because of 

more winter rain.

Historical Streamflow Changes in Maine

A series of recent investigations by the U.S. Geological 

Survey has documented changes in several components of the 

water cycle, including streamflows, in Maine during the last 

30 to 40 years. These changes are summarized in Hodgkins 

and others (2009). Winter-spring streamflows became earlier 

in northern and mountainous sections of Maine during the 

20th century, with most of the 1- to 2-week change occurring 

in the last 30 years (Dudley and Hodgkins, 2002; Hodgkins 

and others, 2003; Hodgkins and Dudley, 2006). Winter-spring 

streamflow timing is based on the center-of-volume date—

the date each year that half of the winter-spring streamflow 

volume passes a streamgage. Annual peak flows have 

increased significantly (p < 0.1) during the last 50 to 100 years 

at about one-third of streamgages in Maine (Hodgkins and 

Dudley, 2005; Collins, 2009); no streamgages had significantly 

decreasing peak flows. Historical peak-flow changes may have 

occurred in a step change around 1970 (Collins, 2009), as has 

been observed at streamgages in Maine for changes in winterspring streamflow timing (Hodgkins and Dudley, 2006). 

Weak, yet in some cases statistically significant, 

correlations have been found between various historical 

streamflows in New England and measures of the North 

Atlantic Oscillation (NAO) (Collins, 2009). The NAO refers 

to a redistribution of atmospheric mass between the Arctic and 

the subtropical Atlantic. It swings from one phase to another 

to produce large changes in wind speed and direction over 

the Atlantic, heat and moisture transport between the Atlantic 

and neighboring continents, and the intensity and number of 

storms (Hurrell and others, 2003). The NAO tended to stay 

in a strong negative phase in the 1960s and stay in a strong 

positive phase in the 1990s.

Data and Methods for Trend Tests and 

Flood-Frequency Analyses

The USGS has been collecting and publishing streamflow 

data for streamgages in and near Maine since 1901. The data 

are available online at http://waterdata.usgs.gov/me/nwis/

sw. Streamgage data were examined to locate streamgages, 

both regulated and unregulated, with at least 50 years of 

annual peak-flow data up to the present (2006) and with no 

more than 5 years of missing data from 1957 through 2006. 

Regulated streamgages were included to increase the number 

of streamgages in the study, particularly streamgages with 90 

or more years of record. Twenty-eight streamgages met the 

above criteria (table 1); annual instantaneous peak flows at 

these streamgages were used for all analyses in this report. 

Peak flows for 5- and 100-year recurrence intervals at the 

streamgages computed from full periods of record are listed in 

table 1, thus showing the magnitudes of small and large floods 

on various streams and rivers in and near Maine during the 

last century.

Eight of 28 streamgages that met minimum data 

requirements are considered to be substantially regulated 

(table 1) in terms of the effect of reservoir regulation on peak 

streamflows. Benson (1962) determined that usable reservoir 

storage of less than 4.5 million cubic feet per square mile 

would, in general, affect peak flows by less than 10 percent. 

The eight substantially regulated streamgages in this study 

have reservoir storage that exceeds this amount.

Most of the remaining 20 unregulated streamgages are 

part of the USGS Hydro-Climatic Data Network (HCDN), 

which includes data from 1,659 streamgages across the United 

States (Slack and Landwehr, 1992). This network includes 

streamgages with data from basins lacking overt effects of 

human activities, such as regulation, diversion, land-use 

change, or extreme groundwater withdrawals. The HCDN 

was developed for the purpose of studying the variation in 

streamflow over time that results from climatic variation. 

Three of 20 streamgages (streamgages 7, 17, and 28 in table 1) 
4 Historical Changes in Annual Peak Flows in Maine and Implications for Flood-Frequency Analyses

Table 1. Peak flows for 5- and 100-year recurrence intervals computed from the full period of record for 28 selected streamgages 

in or near Maine.

[USGS, U.S. Geological Survey; mi2, square miles; ft3/s, cubic feet per second; NB, New Brunswick; NH, New Hampshire; R, regulated]

Map 

numberA

USGS 

streamgage 

number

River 

and 

locationB

Drainage 

basin 

area 

(mi2)

Start 

of 

record

End 

of 

record

Recurrence 

interval 

of peak 

flows 

(years)

Peak 

flows 

(ft3/s)

Lower 

90-percent 

confidence 

interval 

(ft3/s)

Upper 

90-percent 

confidence 

interval 

(ft3/s)

1 01010000 St. John, Ninemile 1,341 1951 2006 5 31,490 29,690 33,670

100 47,970 43,830 53,590

2 01010500 St. John, Dickey 2,680 1947 2006 5 65,930 62,060 70,590

100 105,200 95,750 118,100

3 01011000 Allagash, Allagash 1,229 1932 2006 5 20,390 19,180 21,830

100 34,580 31,410 38,790

4 01011500 St. Francis, 524 1952 2006 5 9,279 8,692 9,993

Connors, NB 100 15,610 14,060 17,780

5 01013500 Fish, Ft. Kent 873 1904C 2006 5 10,350 9,909 10,870

100 15,450 14,420 16,770

6 01014000 St. John, Ft. Kent 5,665 1927 2006 5 107,000 102,000 112,900

100 164,800 152,800 180,300

7 01017000 Aroostook, 1,654 1931 2006 5 30,840 29,270 32,680

Washburn 100 48,580 44,750 53,590

8R 01019000 Grand Lake Stream, 227 1929 2006 5 1,942 1,815 2,093

Grand Lake Stream 100 3,218 2,914 3,621

9 01022500 Narraguagus, 227 1948 2006 5 5,595 5,196 6,083

Cherryfield 100 10,350 9,187 11,990

10 01030500 Mattawamkeag, 1,418 1903 2006 5 21,130 20,250 22,130

Mattawamkeag 100 34,420 32,070 37,380

11 01031500 Piscataquis, Dover- 298 1903 2006 5 12,630 11,790 13,610

Foxcroft 100 26,650 23,830 30,350

12R 01034500 Penobscot, West 6,671 1902 2006 5 85,810 81,310 91,030

Enfield 100 152,300 139,600 168,500

13 01038000 Sheepscot, North 145 1939 2006 5 3,008 2,781 3,284

Whitefield 100 6,612 5,780 7,784

14R 01042500 Kennebec, The Forks 1,590 1951 2006 5 17,130 16,090 18,380

100 34,960 31,020 40,580

15R 01046500 Kennebec, Bingham 2,715 1951 2006 5 36,900 33,850 40,700

100 66,500 58,420 77,980

16 01047000 Carrabassett, North 353 1926 2006 5 19,040 17,520 20,890

Anson 100 40,870 35,810 47,830
Data and Methods for Trend Tests and Flood-Frequency Analyses 5

Table 1. Peak flows for 5- and 100-year recurrence intervals computed from the full period of record for 28 selected streamgages 

in or near Maine.

[USGS, U.S. Geological Survey; mi2, square miles; ft3/s, cubic feet per second; NB, New Brunswick; NH, New Hampshire; R, regulated]

Map 

numberA

USGS 

streamgage 

number

River 

and 

locationB

Drainage 

basin 

area 

(mi2)

Start 

of 

record

End 

of 

record

Recurrence 

interval 

of peak 

flows 

(years)

Peak 

flows 

(ft3/s)

Lower 

90-percent 

confidence 

interval 

(ft3/s)

Upper 

90-percent 

confidence 

interval 

(ft3/s)

17 01049000 Sebasticook, 572 1929 2006 5 8,722 8,252 9,278

Pittsfield 100 14,410 13,200 16,010

18 01052500 Diamond, Wentworth 152 1942 2006 5 6,371 6,051 6,749

Location, NH 100 10,500 9,615 11,680

19R 01053500 Androscoggin, 1,046 1912 2006 5 10,210 9,582 10,950

Errol, NH 100 17,400 15,840 19,430

20R 01054000 Androscoggin, 1,361 1913 2006 5 14,530 13,810 15,370

Gorham, NH 100 22,260 20,630 24,320

21R 01054500 Androscoggin, 2,068 1912 2006 5 36,820 34,870 39,080

Rumford 100 61,560 56,560 67,980

22 01055000 Swift, Roxbury 96.9 1930 2006 5 9,830 8,960 10,900

100 21,700 18,790 25,770

23 01055500 Nezinscot, Turner 169 1942D 2006 5 5,170 4,710 5,742

Center 100 12,080 10,330 14,640

24 01057000 Little Androscoggin, 73.5 1914E 2006 5 3,388 3,124 3,706

South Paris 100 7,516 6,591 8,784

25R 01059000 Androscoggin, 3,263 1929 2006 5 53,590 50,630 57,070

Auburn 100 98,930 89,710 111,200

26 01060000 Royal, Yarmouth 141 1950 2004 5 5,405 4,946 5,977

100 10,960 9,505 13,080

27 01064500 Saco, Conway, NH 385 1904F 2006 5 26,170 24,120 28,660

100 54,560 48,000 63,500

28 01066000 Saco, Cornish 1,293 1917 2006 5 18,960 17,850 20,280

100 35,210 31,900 39,570

A “R” beside a number indicates regulated streamgage (streamgage has drainage basin with more than 4.5 million cubic feet of usable storage per mile 

(Benson, 1962)).

B All locations in Maine unless noted.

C No peak-flow record from 1909–1929.

D No peak-flow record from 1997–2001.

E No peak-flow record from 1924–1931.

F

 No peak-flow record from 1910–1929.
6 Historical Changes in Annual Peak Flows in Maine and Implications for Flood-Frequency Analyses

without substantial peak-flow regulation, as defined by Benson 

(1962), are not included in the HCDN network because they 

have regulation that is expected to substantially affect monthly 

mean flows.

The magnitudes of annual peak-flow changes over 

time were computed using the Sen slope (also known as the 

Kendall-Theil robust line). This slope is computed as the 

median of all possible pairwise slopes in each temporal data 

set (Helsel and Hirsch, 1992). The slope was multiplied by the 

appropriate number of years of data to obtain changes over 

time for the period of record. Peak-flow step changes between 

old data (start-of-record to 1970) and new data (1971–2006) 

at streamgages were computed using the Hodges-Lehmann 

estimator, the median of all pairwise changes between two 

groups of data (Helsel and Hirsch, 1992). These methods 

are more robust to outliers than linear regression or the 

computation of the mean difference between two groups.

The significance of peak-flow changes over time is 

not reported for this study. The magnitude and direction of 

hydroclimatological trends over time can be determined 

with little ambiguity; however, the concept of statistical 

significance is meaningless when discussing systems with 

poorly understood serial structure (Cohn and Lins, 2005; 

Koutsoyiannis and Montanari, 2007). Trend tests that fail to 

consider long-term persistence in the data (such as the MannKendall test) greatly overstate the statistical significance of 

observed trends when long-term persistence is present (Cohn 

and Lins, 2005; Khaliq and others, 2009).

The 5- and 100-year peak flows for full periods of 

record and sub-periods at streamgages were computed using 

the guidelines (Bulletin 17B) of the Interagency Advisory 

Committee on Water Data (1982). The computations involved 

fitting the Pearson Type III probability distribution to the 

logarithms (base 10) of the observed annual peak flows at 

a streamgage. This required computation of the mean, the 

standard deviation, and the skew of the logarithms of the 

annual peak-flow data. Peak flows for selected recurrence 

intervals were determined from the fitted curve. 

There are several streamgages on regulated streams in 

Maine where substantial regulation was added (sometimes in 

addition to substantial regulation already in place) during the 

period for which annual peak flows are available. The older, 

less regulated annual peak flows were not used in the analyses 

based on the Bulletin 17B guidelines if regulation in the 

drainage basin, at the time of the older peaks, differed by more 

than approximately 4.5 million cubic feet of usable storage per 

square mile (Benson, 1962) from the regulation at the time of 

newer peaks. For example, the creation of Aziscohos Lake in 

1911 resulted in a major change in reservoir storage, affecting 

the Androscoggin River; therefore, recorded peak flows prior 

to 1912 were not used. In addition, older peaks in Maine 

were not used if the annual peak-flow data at a streamgage 

clearly indicated that the regulation of peak flows had changed 

substantially over time. 

Bulletin 17B guidelines were followed for the treatment 

of high and low outliers, for the conditional probability 

adjustment, and for weighting the streamgage skew coefficient 

with a generalized skew coefficient. In some cases, multiple 

low outliers that were near, but not below, the Bulletin 17B 

low outlier threshold were censored (dropped from the data 

set) if doing so improved the fit between the logs of the 

observed annual peaks and the Pearson Type III distribution. 

All high outliers were retained in the analyses. Historical 

information (information on peak flows outside the period 

of systematic data collection) was not used for this study. 

The historical information would not have been known for 

some of the sub-periods of continuous records that were 

used in this study, and for consistency historical information 

was not used for any of the full periods or sub-periods. The 

streamgage skews for the flood-frequency analyses were not 

weighted with the generalized skew if the annual peak flows 

at a streamgage were substantially affected by regulation. 

A streamgage was considered substantially regulated if its 

drainage basin had more than 4.5 million cubic feet of usable 

storage per mile (Benson, 1962). The generalized skew 

coefficient developed for Maine by Hodgkins (1999) was used 

for the current study. This skew coefficient is 0.029, with a 

mean square error of prediction of 0.088 (or a standard error 

of prediction of 0.297). This generalized skew was the most 

accurate of four tested methods of estimating the generalized 

skew coefficient. Average streamgage skews for the current 

study from all available streamgages without substantial 

regulation from older sub-periods (1937–66 and 1947–76) 

did not differ substantially from average streamgage skews 

from more recent sub-periods (1967–96 and 1977–2006); 

average streamgage skews were -0.01, 0.03, -0.05, and -0.04, 

respectively. The annual peak flows at the streamgages in this 

study did not show obvious evidence that they were caused 

by multiple generating mechanisms. The procedures used to 

handle such a situation, therefore, were not used. Expected 

probability adjustments, which are explained in Bulletin 17B, 

were not made. 

Peak flows for 5- and 100-year recurrence intervals at 

individual streamgages, computed as described above, were 

not combined with peak flows from regional regression 

equations for an individual streamgage. Peak flows for 

selected recurrence intervals at individual unregulated 

streamgages typically are combined with regional regressionequation peak flows to improve the peak-flow computations. 

For this report, however, the main interest is in the relative 

differences in peak flows for selected recurrence intervals 

that were computed using different periods of historical 

peak flows. 

Historical Changes in Annual 

Peak Flows

Changes over time in annual instantaneous peak 

streamflows at 28 USGS streamgages with long-term data 

(50 or more years) and relatively complete records (no more 
Flood-Frequency Analyses for Full Periods of Record and Selected Sub-Periods 7

than 5 years of missing data from 1957 through 2006) were 

investigated in two different ways. Linear trends for the 

period of record at each streamgage were computed using 

the Sen slope (see “Data and Methods for Trend Tests and 

Flood-frequency Analyses” section for more details). Because 

changes over time may have occurred as a step change around 

1970 (see “Historical Streamflow Changes in Maine” section), 

step changes between each streamgage’s older record (from 

starting year to 1970) and newer record (1971 to 2006) were 

computed using the Hodges-Lehmann estimator. Examples 

of annual peak flows and Sen slopes are shown in figure 1 

for two streamgages that had typical changes over time, St. 

John River (streamgage 6) and Little Androscoggin River 

(streamgage 24).

The Sen slopes for the 28 streamgages varied 

considerably, but 22 of the streamgages have positive slopes, 

indicating increasing annual peak flows over time (table 2, 

fig. 2). There is no obvious geographic pattern to the slopes 

(fig. 2). At many streamgages, increases are greater than 

10 percent, and at seven streamgages, increases are greater 

than 25 percent. In contrast, at one streamgage peak flows 

decreased more than 10 percent. Results for streamgages 

with substantial streamflow regulation (table 2) do not 

show a markedly different pattern (fig. 2) than those for the 

unregulated streamgages.

Step changes for the 28 streamgages also varied 

considerably, but at 23 streamgages, peak flows increased after 

1970 (table 3, fig. 3). Twenty-two of these 23 streamgages 

have positive linear slopes, as computed by use of the Sen 

slope. As with the linear trends, no geographic patterns for 

the step changes were obvious, and many increased annual 

peak flows of greater than 10 percent were noted when older 

data (1970 and earlier) were compared to newer data (1971 

to 2006). At three streamgages, increases were greater than 

25 percent, and at one streamgage, peak flows decreased more 

than 10 percent. Regulated streamgages (table 3) do not show 

a substantially different pattern of results (fig. 3) than those for 

the unregulated streamgages.

The median change over time for all 28 streamgages 

is an increase of 15.9 percent based on a linear change and 

12.4 percent based on a step change. The median change for 

the 20 unregulated streamgages is slightly higher than for all 

28 streamgages; it is 18.4 percent based on a linear change and 

15.0 percent based on a step change. The median record length 

is 77.0 years for all 28 streamgages and 75.5 years for the 

unregulated streamgages.

Flood-Frequency Analyses for Full 

Periods of Record and Selected 

Sub-Periods

Peak flows with 100- and 5-year recurrence intervals 

were computed for all 28 streamgages using the full available 

annual peak-flow record (table 1) and multiple sub-periods of 

that record (tables 4–7). Magnitudes of 100- and 5-year peak 

flows computed from sub-periods were then compared to those 

from the full period and to the 90-percent confidence intervals 

of the peak flows computed from the full period (tables 4–7). 

Thirty-year sub-periods with starting years staggered by 10 

years were used (1907–36, 1917–46, 1927–56, 1937–66, 

1947–76, 1957–86, 1967–96, and 1977–2006). The number 

of sub-periods used depended on the length of record at each 

streamgage. Because a change in peak flows over time may 

have occurred as a step change around 1970 (see “Historical 

Streamflow Changes in Maine” section), two other sub-periods 

were analyzed: older data (start-of-record to 1970) and newer 

data (1971 to 2006). All results related to 100- and 5-year peak 

flows are summarized in tables 8, 9, and 10.

Figure 1. Examples of annual peak flows and Sen slopes at A, St. 

John River, Ft. Kent, Maine and B, Little Androscoggin River, South 

Paris, Maine.

0

50,000

100,000

150,000

STREAMFLOW, IN CUBIC FEET PER SECOND

A

1920 1940 1960 1980 2000

YEARS

0

2,000

4,000

6,000

8,000

B

10,000

1930 1950 1970 1990 2010

Annual peak flows

Sen slope

Sen slope

Annual peak flows
8 Historical Changes in Annual Peak Flows in Maine and Implications for Flood-Frequency Analyses

Table 2. Magnitude of linear changes over time for annual 

peak flows as computed by use of the Sen slope.

[USGS, U.S. Geological Survey; R, regulated]

Map 

number

USGS 

streamgage 

number

Years of 

record

Magnitude 

of change 

(percent)

RegulationA

1 01010000 56 28.6

2 01010500 60 28.7

3 01011000 75 12.6

4 01011500 55 -4.0

5 01013500 77 4.8

6 01014000 80 17.0

7 01017000 76 13.9

8 01019000 78 -5.1 R

9 01022500 59 -24.0

10 01030500 104 0.0

11 01031500 104 40.3

12 01034500 105 17.9 R

13 01038000 68 22.8

14 01042500 56 4.9 R

15 01046500 56 3.3 R

16 01047000 81 23.2

17 01049000 78 -2.3

18 01052500 65 30.9

19 01053500 95 -1.2 R

20 01054000 94 14.7 R

21 01054500 95 31.2 R

22 01055000 77 45.7

23 01055500 60 24.7

24 01057000 75 19.9

25 01059000 78 20.8 R

26 01060000 55 0.8

27 01064500 77 63.4

28 01066000 90 5.2

A Unregulated unless noted (streamgage has drainage basin with less than 

4.5 million cubic feet of usable storage per mile (Benson, 1962)).
Flood-Frequency Analyses for Full Periods of Record and Selected Sub-Periods 9

9

7

6

5 4

3

2

1

28

27

26

24 23

22

18

17

16

13

11

10

8R

25R

21R

20R

19R

15R

14R

12R

22

13

9

EXPLANATION

Lake or pond

River

County boundary

Station and reference number.

Symbol indicates percent change

10 to 25 percent decrease

10 percent decrease to 10 percent increase

10 to 25 percent increase

Greater than 25 percent increase

0 15 30 60 MILES

Base from U.S. Geological Survey digital files,

Universal Transverse Mercator projection, zone 19

68°

70°

46°

44°

70°

44°

68°

46°

0 30 60 KILOMETERS

5

"R" in the reference number indicates station 

streamflow is regulated

NEW HAMPSHIRE

CANADA

CANADA

MAINE

C A N A D A

MAINE

VERMONT

NEW HAMPSHIRE

MASSACHUSETTS

RHODE ISLAND

CONNECTICUT

Atlantic Ocean

Atlantic Ocean

C A N A D A

Figure 2. Geographic distribution of the magnitude of annual peak-flow linear changes over time as computed by use of 

the Sen slope. River and location for map numbers are in table 1.
10 Historical Changes in Annual Peak Flows in Maine and Implications for Flood-Frequency Analyses

Table 3. Magnitude of step changes over time in annual peak 

flows as computed by use of the Hodges-Lehmann estimator.

[USGS, U.S. Geological Survey; R, regulated]

Map 

number

USGS 

streamgage 

number

Years of 

record

Magnitude 

of change 

(percent)

RegulationA

1 01010000 56 23.5

2 01010500 60 28.1

3 01011000 75 18.9

4 01011500 55 11.0

5 01013500 77 8.0

6 01014000 80 17.7

7 01017000 76 15.8

8 01019000 78 -4.2 R

9 01022500 59 -10.4

10 01030500 104 4.7

11 01031500 104 26.5

12 01034500 105 15.0 R

13 01038000 68 21.1

14 01042500 56 4.9 R

15 01046500 56 -0.6 R

16 01047000 81 15.6

17 01049000 78 -2.5

18 01052500 65 11.5

19 01053500 95 -0.9 R

20 01054000 94 7.6 R

21 01054500 95 17.2 R

22 01055000 77 19.3

23 01055500 60 11.8

24 01057000 75 11.7

25 01059000 78 13.1 R

26 01060000 55 14.3

27 01064500 77 34.4

28 01066000 90 9.2

A Unregulated unless noted (streamgage has drainage basin with less than 

4.5 million cubic feet of usable storage per mile (Benson, 1962)).
Flood-Frequency Analyses for Full Periods of Record and Selected Sub-Periods 11

9

7

6

5 4

3

2

1

28

27

26

24 23

22

18

17

16

13

11

10

8R

25R

21R

20R

19R

15R

14R 12R

27

13

9

68°

70°

46°

44°

70°

44°

68°

46°

NEW HAMPSHIRE

CANADA

CANADA

0 15 30 60 MILES

Base from U.S. Geological Survey digital files,

Universal Transverse Mercator projection, zone 19

0 30 60 KILOMETERS

EXPLANATION

Lake or pond

River

County boundary

Station and reference number.

Symbol indicates percent change

10 to 25 percent decrease

10 percent decrease to 10 percent increase

10 to 25 percent increase

Greater than 25 percent increase

17

"R" in the reference number indicates station 

streamflow is regulated

MAINE

Atlantic Ocean

Figure 3. Geographic distribution of the magnitude of annual peak-flow step changes over time as computed by use of 

the Hodges-Lehmann estimator.
12 Historical Changes in Annual Peak Flows in Maine and Implications for Flood-Frequency Analyses

Table 4. Peak flows with 100-year recurrence intervals based on multiple 30-year sub-periods, and comparisons to 100-year peak 

flows based on full periods.

[USGS, U.S. Geological Survey; ft3/s, cubic feet per second; %, percent]

Map 

number

USGS 

streamgage 

number

Sub-period 

start year

Sub-period 

end year

100-year 

peak flow 

(ft3/s)

Comparison 

of peak flow 

to peak flow 

based on 

full record 

(percent higher 

or lower)

Comparison 

of peak flow 

to 90-percent 

confidence 

intervals of 

peak flow 

based on 

full recordA

RegulationB

1 01010000 1977 2006 49,260 2.7

2 01010500 1977 2006 109,500 4.1

3 01011000 1977 2006 37,790 9.3

4 01011500 1977 2006 15,070 -3.5

5 01013500 1977 2006 15,690 1.6

6 01014000 1977 2006 174,800 6.1

7 01017000 1977 2006 47,580 -2.1

8 01019000 1977 2006 4,361 35.5 Greater R

9 01022500 1977 2006 9,215 -11.0

10 01030500 1977 2006 29,910 -13.1 Less

11 01031500 1977 2006 31,620 18.6 Greater

12 01034500 1977 2006 162,400 6.6 R

13 01038000 1977 2006 6,880 4.1

14 01042500 1977 2006 39,110 11.9 R

15 01046500 1977 2006 79,300 19.2 Greater R

16 01047000 1977 2006 44,060 7.8

17 01049000 1977 2006 15,950 10.7

18 01052500 1977 2006 11,360 8.2

19 01053500 1977 2006 16,610 -4.5 R

20 01054000 1977 2006 22,610 1.6 R

21 01054500 1977 2006 60,260 -2.1 R

22 01055000 1977 2006 22,130 2.0

23 01055500 1977 2006 14,680 21.5 Greater

24 01057000 1977 2006 9,419 25.3 Greater

25 01059000 1977 2006 84,520 -14.6 Less R

26 01060000 1977 2004 13,610 24.2 Greater

27 01064500 1977 2006 50,240 -7.9

28 01066000 1977 2006 32,770 -6.9

1 01010000 1967 1996 50,860 6.0

2 01010500 1967 1996 112,600 7.0

3 01011000 1967 1996 39,570 14.4 Greater

4 01011500 1967 1996 16,960 8.6
Flood-Frequency Analyses for Full Periods of Record and Selected Sub-Periods 13

Table 4. Peak flows with 100-year recurrence intervals based on multiple 30-year sub-periods, and comparisons to 100-year peak 

flows based on full periods.

[USGS, U.S. Geological Survey; ft3/s, cubic feet per second; %, percent]

Map 

number

USGS 

streamgage 

number

Sub-period 

start year

Sub-period 

end year

100-year 

peak flow 

(ft3/s)

Comparison 

of peak flow 

to peak flow 

based on 

full record 

(percent higher 

or lower)

Comparison 

of peak flow 

to 90-percent 

confidence 

intervals of 

peak flow 

based on 

full recordA

RegulationB

5 01013500 1967 1996 16,900 9.4 Greater

6 01014000 1967 1996 183,200 11.2 Greater

7 01017000 1967 1996 53,630 10.4 Greater

8 01019000 1967 1996 3,097 -3.8 R

9 01022500 1967 1996 9,593 -7.3

10 01030500 1967 1996 33,350 -3.1

11 01031500 1967 1996 33,510 25.7 Greater

12 01034500 1967 1996 173,800 14.1 Greater R

13 01038000 1967 1996 7,591 14.8

14 01042500 1967 1996 41,800 19.6 Greater R

15 01046500 1967 1996 80,150 20.5 Greater R

16 01047000 1967 1996 44,410 8.7

17 01049000 1967 1996 17,040 18.3 Greater

18 01052500 1967 1996 8,740 -16.8 Less

19 01053500 1967 1996 17,350 -0.3 R

20 01054000 1967 1996 20,390 -8.4 Less R

21 01054500 1967 1996 61,080 -0.8 R

22 01055000 1967 1996 21,530 -0.8

23 01055500 1967 1996 12,150 0.6

24 01057000 1967 1996 8,900 18.4 Greater

25 01059000 1967 1996 84,130 -15.0 Less R

26 01060000 1967 1996 12,330 12.5

27 01064500 1967 1996 52,840 -3.2

28 01066000 1967 1996 33,910 -3.7

1 01010000 1957 1986 50,020 4.3

2 01010500 1957 1986 114,700 9.0

3 01011000 1957 1986 42,920 24.1 Greater

4 01011500 1957 1986 18,470 18.3 Greater

5 01013500 1957 1986 18,660 20.8 Greater

6 01014000 1957 1986 190,900 15.8 Greater

7 01017000 1957 1986 56,310 15.9 Greater

8 01019000 1957 1986 2,986 -7.2 R
14 Historical Changes in Annual Peak Flows in Maine and Implications for Flood-Frequency Analyses

Table 4. Peak flows with 100-year recurrence intervals based on multiple 30-year sub-periods, and comparisons to 100-year peak 

flows based on full periods.

[USGS, U.S. Geological Survey; ft3/s, cubic feet per second; %, percent]

Map 

number

USGS 

streamgage 

number

Sub-period 

start year

Sub-period 

end year

100-year 

peak flow 

(ft3/s)

Comparison 

of peak flow 

to peak flow 

based on 

full record 

(percent higher 

or lower)

Comparison 

of peak flow 

to 90-percent 

confidence 

intervals of 

peak flow 

based on 

full recordA

RegulationB

9 01022500 1957 1986 10,610 2.5

10 01030500 1957 1986 31,740 -7.8 Less

11 01031500 1957 1986 28,760 7.9

12 01034500 1957 1986 164,000 7.7 R

13 01038000 1957 1986 6,548 -1.0

14 01042500 1957 1986 41,780 19.5 Greater R

15 01046500 1957 1986 71,470 7.5 R

16 01047000 1957 1986 38,260 -6.4

17 01049000 1957 1986 13,920 -3.4

18 01052500 1957 1986 7,772 -26.0 Less

19 01053500 1957 1986 16,760 -3.7 R

20 01054000 1957 1986 17,840 -19.9 Less R

21 01054500 1957 1986 57,170 -7.1 R

22 01055000 1957 1986 21,200 -2.3

23 01055500 1957 1986 9,156 -24.2 Less

24 01057000 1957 1986 6,272 -16.6 Less

25 01059000 1957 1986 68,320 -30.9 Less R

26 01060000 1957 1986 9,743 -11.1

27 01064500 1957 1986 63,140 15.7

28 01066000 1957 1986 33,420 -5.1

2 01010500 1947 1976 101,600 -3.4

3 01011000 1947 1976 34,850 0.8

5 01013500 1947 1976 17,650 14.2 Greater

6 01014000 1947 1976 174,900 6.1

7 01017000 1947 1976 53,290 9.7

8 01019000 1947 1976 2,563 -20.4 Less R

10 01030500 1947 1976 32,870 -4.5

11 01031500 1947 1976 27,940 4.8

12 01034500 1947 1976 147,100 -3.4 R

13 01038000 1947 1976 5,817 -12.0

16 01047000 1947 1976 43,210 5.7
Flood-Frequency Analyses for Full Periods of Record and Selected Sub-Periods 15

Table 4. Peak flows with 100-year recurrence intervals based on multiple 30-year sub-periods, and comparisons to 100-year peak 

flows based on full periods.

[USGS, U.S. Geological Survey; ft3/s, cubic feet per second; %, percent]

Map 

number

USGS 

streamgage 

number

Sub-period 

start year

Sub-period 

end year

100-year 

peak flow 

(ft3/s)

Comparison 

of peak flow 

to peak flow 

based on 

full record 

(percent higher 

or lower)

Comparison 

of peak flow 

to 90-percent 

confidence 

intervals of 

peak flow 

based on 

full recordA

RegulationB

17 01049000 1947 1976 12,860 -10.8 Less

18 01052500 1947 1976 8,447 -19.6 Less

19 01053500 1947 1976 16,660 -4.3 R

20 01054000 1947 1976 19,770 -11.2 Less R

21 01054500 1947 1976 60,540 -1.7 R

22 01055000 1947 1976 23,260 7.2

23 01055500 1947 1976 10,360 -14.2

24 01057000 1947 1976 6,325 -15.8 Less

25 01059000 1947 1976 87,700 -11.4 Less R

27 01064500 1947 1976 65,340 19.8 Greater

28 01066000 1947 1976 38,360 8.9

3 01011000 1937 1966 30,280 -12.4 Less

5 01013500 1937 1966 15,290 -1.0

6 01014000 1937 1966 153,300 -7.0

7 01017000 1937 1966 46,370 -4.5

8 01019000 1937 1966 2,757 -14.3 Less R

10 01030500 1937 1966 30,690 -10.8 Less

11 01031500 1937 1966 23,190 -13.0 Less

12 01034500 1937 1966 132,800 -12.8 Less R

16 01047000 1937 1966 36,440 -10.8

17 01049000 1937 1966 12,410 -13.9 Less

19 01053500 1937 1966 16,530 -5.0 R

20 01054000 1937 1966 19,060 -14.4 Less R

21 01054500 1937 1966 59,810 -2.8 R

22 01055000 1937 1966 22,940 5.7

24 01057000 1937 1966 6,713 -10.7

25 01059000 1937 1966 87,100 -12.0 Less R

27 01064500 1937 1966 57,740 5.8

28 01066000 1937 1966 36,060 2.4

6 01014000 1927 1956 140,600 -14.7 Less

10 01030500 1927 1956 33,070 -3.9
16 Historical Changes in Annual Peak Flows in Maine and Implications for Flood-Frequency Analyses

Table 4. Peak flows with 100-year recurrence intervals based on multiple 30-year sub-periods, and comparisons to 100-year peak 

flows based on full periods.

[USGS, U.S. Geological Survey; ft3/s, cubic feet per second; %, percent]

Map 

number

USGS 

streamgage 

number

Sub-period 

start year

Sub-period 

end year

100-year 

peak flow 

(ft3/s)

Comparison 

of peak flow 

to peak flow 

based on 

full record 

(percent higher 

or lower)

Comparison 

of peak flow 

to 90-percent 

confidence 

intervals of 

peak flow 

based on 

full recordA

RegulationB

11 01031500 1927 1956 23,130 -13.2 Less

12 01034500 1927 1956 139,000 -8.7 Less R

16 01047000 1927 1956 41,000 0.3

19 01053500 1927 1956 19,220 10.5 R

20 01054000 1927 1956 21,330 -4.2 R

21 01054500 1927 1956 74,650 21.3 Greater R

28 01066000 1927 1956 40,700 15.6 Greater

10 01030500 1917 1946 40,530 17.8 Greater

11 01031500 1917 1946 22,430 -15.8 Less

12 01034500 1917 1946 171,200 12.4 Greater R

19 01053500 1917 1946 19,650 12.9 Greater R

20 01054000 1917 1946 25,730 15.6 Greater R

21 01054500 1917 1946 67,750 10.1 R

28 01066000 1917 1946 34,790 -1.2

10 01030500 1907 1936 42,130 22.4 Greater

11 01031500 1907 1936 22,780 -14.5 Less

12 01034500 1907 1936 159,900 5.0 R

A Blank indicates within 90-percent confidence intervals.

B Unregulated unless noted (streamgage has drainage basin with less than 4.5 million cubic feet of usable storage per mile (Benson, 1962)).
Flood-Frequency Analyses for Full Periods of Record and Selected Sub-Periods 17

Table 5. Peak flows with 100-year recurrence intervals based on the sub-periods 1971 to 2006 and start-of-record to 1970, and 

comparisons to 100-year peak flows based on full periods.

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

Map 

number

USGS 

streamgage 

number

Sub-period 

start year

Sub-period 

end year

100-year 

peak flow 

(ft3/s)

Comparison of peak 

flow to peak flow based 

on full record 

(percent higher or lower)

Comparison of peak flow 

to 90-percent confidence 

intervals of peak flow 

based on full recordA

RegulationB

3 01011000 1971 2006 38,060 10.1

5 01013500 1971 2006 16,600 7.4

6 01014000 1971 2006 177,300 7.6

7 01017000 1971 2006 50,630 4.2

8 01019000 1971 2006 3,984 23.8 Greater R 

10 01030500 1971 2006 31,250 -9.2 Less

11 01031500 1971 2006 30,470 14.3 Greater

12 01034500 1971 2006 165,800 8.9 R

13 01038000 1971 2006 7,327 10.8

16 01047000 1971 2006 42,410 3.8

17 01049000 1971 2006 15,550 7.9

19 01053500 1971 2006 16,760 -3.7 R

20 01054000 1971 2006 22,110 -0.7 R

21 01054500 1971 2006 58,400 -5.1 R

22 01055000 1971 2006 21,470 -1.1

24 01057000 1971 2006 8,643 15.0

25 01059000 1971 2006 85,080 -14.0 Less R

27 01064500 1971 2006 51,080 -6.4

28 01066000 1971 2006 31,000 -12.0 Less

3 01011000 1932 1970 30,290 -12.4 Less

5 01013500 1904 1970 15,020 -2.8

6 01014000 1927 1970 151,700 -7.9 Less

7 01017000 1931 1970 46,590 -4.1

8 01019000 1929 1970 2,559 -20.5 Less R

10 01030500 1903 1970 35,800 4.0

11 01031500 1903 1970 23,980 -10.0

12 01034500 1902 1970 140,500 -7.7 R

13 01038000 1939 1970 5,375 -18.7 Less

16 01047000 1926 1970 40,260 -1.5

17 01049000 1929 1970 13,740 -4.6

19 01053500 1912 1970 18,050 3.7 R

20 01054000 1913 1970 23,640 6.2 R

21 01054500 1912 1970 62,260 1.1 R

22 01055000 1930 1970 22,530 3.8

24 01057000 1914 1970 6,725 -10.5

25 01059000 1929 1970 113,600 14.8 Greater R

27 01064500 1904 1970 54,660 0.2

28 01066000 1917 1970 37,600 6.8

A Blank indicates within 90-percent confidence intervals.

B Unregulated unless noted (streamgage has drainage basin with less than 4.5 million cubic feet of usable storage per mile (Benson, 1962)).
18 Historical Changes in Annual Peak Flows in Maine and Implications for Flood-Frequency Analyses

Table 6. Peak flows with 5-year recurrence intervals based on multiple 30-year sub-periods, and comparisons to 5-year peak flows 

based on full periods.

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

Map 

number

USGS 

streamgage 

number

Sub-period 

start year

Sub-period 

end year

5-year 

peak flow 

(ft3/s)

Comparison 

of peak flow 

to peak flow 

based on 

full record 

(percent higher 

or lower)

Comparison 

of peak flow 

to 90-percent 

confidence 

intervals of 

peak flow 

based on 

full recordA

RegulationB

1 01010000 1977 2006 33,010 4.8

2 01010500 1977 2006 70,210 6.5

3 01011000 1977 2006 21,760 6.7

4 01011500 1977 2006 9,006 -2.9

5 01013500 1977 2006 10,310 -0.4

6 01014000 1977 2006 113,500 6.1 Greater

7 01017000 1977 2006 31,310 1.5

8 01019000 1977 2006 2,034 4.7 R 

9 01022500 1977 2006 5,197 -7.1

10 01030500 1977 2006 20,430 -3.3

11 01031500 1977 2006 14,520 15.0 Greater

12 01034500 1977 2006 93,110 8.5 Greater R

13 01038000 1977 2006 3,245 7.9

14 01042500 1977 2006 17,540 2.4 R

15 01046500 1977 2006 38,210 3.6 R

16 01047000 1977 2006 20,790 9.2

17 01049000 1977 2006 8,891 1.9

18 01052500 1977 2006 6,940 8.9 Greater

19 01053500 1977 2006 9,896 -3.1 R

20 01054000 1977 2006 14,840 2.1 R

21 01054500 1977 2006 40,160 9.1 Greater R

22 01055000 1977 2006 10,770 9.6

23 01055500 1977 2006 5,944 15.0 Greater

24 01057000 1977 2006 3,837 13.3 Greater

25 01059000 1977 2006 57,020 6.4 R

26 01060000 1977 2004 6,008 11.2 Greater

27 01064500 1977 2006 28,690 9.6 Greater

28 01066000 1977 2006 19,050 0.5

1 01010000 1967 1996 34,070 8.2 Greater

2 01010500 1967 1996 73,470 11.4 Greater

3 01011000 1967 1996 22,390 9.8 Greater
Flood-Frequency Analyses for Full Periods of Record and Selected Sub-Periods 19

Table 6. Peak flows with 5-year recurrence intervals based on multiple 30-year sub-periods, and comparisons to 5-year peak flows 

based on full periods.

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

Map 

number

USGS 

streamgage 

number

Sub-period 

start year

Sub-period 

end year

5-year 

peak flow 

(ft3/s)

Comparison 

of peak flow 

to peak flow 

based on 

full record 

(percent higher 

or lower)

Comparison 

of peak flow 

to 90-percent 

confidence 

intervals of 

peak flow 

based on 

full recordA

RegulationB

4 01011500 1967 1996 10,190 9.8 Greater

5 01013500 1967 1996 11,090 7.1 Greater

6 01014000 1967 1996 119,700 11.9 Greater

7 01017000 1967 1996 33,030 7.1 Greater

8 01019000 1967 1996 1,895 -2.4 R

9 01022500 1967 1996 5,463 -2.4

10 01030500 1967 1996 21,570 2.1

11 01031500 1967 1996 15,210 20.4 Greater

12 01034500 1967 1996 102,200 19.1 Greater R

13 01038000 1967 1996 3,417 13.6 Greater

14 01042500 1967 1996 18,300 6.8 R

15 01046500 1967 1996 40,530 9.8 R

16 01047000 1967 1996 20,920 9.9 Greater

17 01049000 1967 1996 9,002 3.2

18 01052500 1967 1996 5,992 -5.9 Less

19 01053500 1967 1996 9,345 -8.5 Less R

20 01054000 1967 1996 13,360 -8.1 Less R

21 01054500 1967 1996 40,280 9.4 Greater R

22 01055000 1967 1996 10,390 5.7

23 01055500 1967 1996 5,605 8.4

24 01057000 1967 1996 3,743 10.5 Greater

25 01059000 1967 1996 57,180 6.7 Greater R

26 01060000 1967 1996 5,994 10.9 Greater

27 01064500 1967 1996 28,250 7.9

28 01066000 1967 1996 19,680 3.8

1 01010000 1957 1986 32,350 2.7

2 01010500 1957 1986 70,110 6.3

3 01011000 1957 1986 22,280 9.3 Greater

4 01011500 1957 1986 10,320 11.2 Greater

5 01013500 1957 1986 11,440 10.5 Greater

6 01014000 1957 1986 115,800 8.2 Greater
20 Historical Changes in Annual Peak Flows in Maine and Implications for Flood-Frequency Analyses

Table 6. Peak flows with 5-year recurrence intervals based on multiple 30-year sub-periods, and comparisons to 5-year peak flows 

based on full periods.

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

Map 

number

USGS 

streamgage 

number

Sub-period 

start year

Sub-period 

end year

5-year 

peak flow 

(ft3/s)

Comparison 

of peak flow 

to peak flow 

based on 

full record 

(percent higher 

or lower)

Comparison 

of peak flow 

to 90-percent 

confidence 

intervals of 

peak flow 

based on 

full recordA

RegulationB

7 01017000 1957 1986 32,210 4.4

8 01019000 1957 1986 1,960 0.9 R

9 01022500 1957 1986 5,698 1.8

10 01030500 1957 1986 20,790 -1.6

11 01031500 1957 1986 13,410 6.2

12 01034500 1957 1986 95,660 11.5 Greater R

13 01038000 1957 1986 3,081 2.4

14 01042500 1957 1986 18,530 8.2 Greater R

15 01046500 1957 1986 38,740 5.0 R

16 01047000 1957 1986 18,420 -3.3

17 01049000 1957 1986 8,435 -3.3

18 01052500 1957 1986 5,408 -15.1 Less

19 01053500 1957 1986 8,829 -13.5 Less R

20 01054000 1957 1986 12,800 -11.9 Less R

21 01054500 1957 1986 38,000 3.2 R

22 01055000 1957 1986 9,200 -6.4

23 01055500 1957 1986 4,870 -5.8

24 01057000 1957 1986 3,091 -8.8 Less

25 01059000 1957 1986 52,140 -2.7 R

26 01060000 1957 1986 5,244 -3.0

27 01064500 1957 1986 26,730 2.1

28 01066000 1957 1986 18,830 -0.7

2 01010500 1947 1976 61,360 -6.9 Less

3 01011000 1947 1976 19,020 -6.7 Less

5 01013500 1947 1976 10,940 5.7 Greater

6 01014000 1947 1976 104,700 -2.1

7 01017000 1947 1976 30,930 0.3

8 01019000 1947 1976 1,977 1.8 R

10 01030500 1947 1976 21,180 0.2

11 01031500 1947 1976 13,420 6.3

12 01034500 1947 1976 92,070 7.3 Greater R
Flood-Frequency Analyses for Full Periods of Record and Selected Sub-Periods 21

Table 6. Peak flows with 5-year recurrence intervals based on multiple 30-year sub-periods, and comparisons to 5-year peak flows 

based on full periods.

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

Map 

number

USGS 

streamgage 

number

Sub-period 

start year

Sub-period 

end year

5-year 

peak flow 

(ft3/s)

Comparison 

of peak flow 

to peak flow 

based on 

full record 

(percent higher 

or lower)

Comparison 

of peak flow 

to 90-percent 

confidence 

intervals of 

peak flow 

based on 

full recordA

RegulationB

13 01038000 1947 1976 2,814 -6.4

16 01047000 1947 1976 19,200 0.8

17 01049000 1947 1976 8,375 -4.0

18 01052500 1947 1976 5,594 -12.2 Less

19 01053500 1947 1976 9,183 -10.1 Less R

20 01054000 1947 1976 13,040 -10.3 Less R

21 01054500 1947 1976 37,760 2.6 R

22 01055000 1947 1976 9,481 -3.6

23 01055500 1947 1976 4,917 -4.9

24 01057000 1947 1976 3,017 -11.0 Less

25 01059000 1947 1976 51,980 -3.0 R

27 01064500 1947 1976 26,510 1.3

28 01066000 1947 1976 19,670 3.7

3 01011000 1937 1966 17,780 -12.8 Less

5 01013500 1937 1966 9,858 -4.8 Less

6 01014000 1937 1966 96,460 -9.9 Less

7 01017000 1937 1966 28,410 -7.9 Less

8 01019000 1937 1966 1,963 1.1 R

10 01030500 1937 1966 19,800 -6.3 Less

11 01031500 1937 1966 11,290 -10.6 Less

12 01034500 1937 1966 79,070 -7.9 Less R

16 01047000 1937 1966 16,590 -12.9 Less

17 01049000 1937 1966 8,059 -7.6 Less

19 01053500 1937 1966 10,090 -1.2 R

20 01054000 1937 1966 13,880 -4.5 R

21 01054500 1937 1966 35,500 -3.6 R

22 01055000 1937 1966 8,906 -9.4 Less

24 01057000 1937 1966 2,919 -13.8 Less

25 01059000 1937 1966 47,810 -10.8 Less R

27 01064500 1937 1966 23,070 -11.8 Less

28 01066000 1937 1966 18,280 -3.6
22 Historical Changes in Annual Peak Flows in Maine and Implications for Flood-Frequency Analyses

Table 6. Peak flows with 5-year recurrence intervals based on multiple 30-year sub-periods, and comparisons to 5-year peak flows 

based on full periods.

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

Map 

number

USGS 

streamgage 

number

Sub-period 

start year

Sub-period 

end year

5-year 

peak flow 

(ft3/s)

Comparison 

of peak flow 

to peak flow 

based on 

full record 

(percent higher 

or lower)

Comparison 

of peak flow 

to 90-percent 

confidence 

intervals of 

peak flow 

based on 

full recordA

RegulationB

6 01014000 1927 1956 97,320 -9.0 Less

10 01030500 1927 1956 21,170 0.2

11 01031500 1927 1956 12,190 -3.5

12 01034500 1927 1956 79,490 -7.4 Less R

16 01047000 1927 1956 18,430 -3.2

19 01053500 1927 1956 11,660 14.2 Greater R

20 01054000 1927 1956 14,790 1.8 R

21 01054500 1927 1956 36,600 -0.6 R

28 01066000 1927 1956 19,670 3.7

10 01030500 1917 1946 22,500 6.5 Greater

11 01031500 1917 1946 11,340 -10.2 Less

12 01034500 1917 1946 77,100 -10.2 Less

19 01053500 1917 1946 11,680 14.4 Greater

20 01054000 1917 1946 15,270 5.1

21 01054500 1917 1946 34,030 -7.6 Less

28 01066000 1917 1946 18,340 -3.3

10 01030500 1907 1936 23,430 10.9 Greater

11 01031500 1907 1936 11,400 -9.7 Less

12 01034500 1907 1936 80,530 -6.2 Less

A Blank indicates within 90-percent confidence intervals.

B Unregulated unless noted (streamgage has drainage basin with less than 4.5 million cubic feet of usable storage per mile (Benson, 1962)).
Flood-Frequency Analyses for Full Periods of Record and Selected Sub-Periods 23

Table 7. Peak flows with 5-year recurrence intervals based on the sub-periods 1971 to 2006 and start-of-record to 1970, and 

comparisons to 5-year peak flows based on full periods.

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

Map 

number

USGS 

streamgage 

number

Sub-period 

start year

Sub-period 

end year

5-year peak 

flow 

(ft3/s)

Comparison of peak flow 

to peak flow based 

on full record 

(percent higher or lower)

Comparison of peak flow 

to 90-percent confidence 

intervals of peak flow 

based on full recordA

RegulationB

3 01011000 1971 2006 22,340 9.6 Greater

5 01013500 1971 2006 10,850 4.8

6 01014000 1971 2006 116,200 8.6 Greater

7 01017000 1971 2006 32,730 6.1 Greater

8 01019000 1971 2006 2,027 4.4 R 

10 01030500 1971 2006 21,150 0.1

11 01031500 1971 2006 14,510 14.9 Greater

12 01034500 1971 2006 97,810 14.0 Greater R

13 01038000 1971 2006 3,333 10.8 Greater

16 01047000 1971 2006 20,400 7.1

17 01049000 1971 2006 8,843 1.4

19 01053500 1971 2006 9,812 -3.9 R

20 01054000 1971 2006 14,500 -0.2 R

21 01054500 1971 2006 39,980 8.6 Greater R

22 01055000 1971 2006 10,420 6.0

24 01057000 1971 2006 3,647 7.6

25 01059000 1971 2006 55,440 3.5 R

27 01064500 1971 2006 28,330 8.3

28 01066000 1971 2006 18,930 -0.2

3 01011000 1932 1970 18,270 -10.4 Less

5 01013500 1904 1970 10,060 -2.8

6 01014000 1927 1970 98,890 -7.6 Less

7 01017000 1931 1970 28,900 -6.3 Less

8 01019000 1929 1970 1,860 -4.2 R

10 01030500 1903 1970 21,120 -0.0

11 01031500 1903 1970 11,630 -7.9 Less

12 01034500 1902 1970 79,680 -7.1 Less R

13 01038000 1939 1970 2,664 -11.4 Less

16 01047000 1926 1970 17,970 -5.6

17 01049000 1929 1970 8,610 -1.3

19 01053500 1912 1970 10,460 2.4 R

20 01054000 1913 1970 14,340 -1.3 R

21 01054500 1912 1970 34,710 -5.7 Less R

22 01055000 1930 1970 9,250 -5.9

24 01057000 1914 1970 3,219 -5.0

25 01059000 1929 1970 50,850 -5.1 R

27 01064500 1904 1970 23,540 -10.0 Less

28 01066000 1917 1970 18,830 -0.7

A Blank indicates within 90-percent confidence intervals.

B Unregulated unless noted (streamgage has drainage basin with less than 4.5 million cubic feet of usable storage per mile (Benson, 1962)).
24 Historical Changes in Annual Peak Flows in Maine and Implications for Flood-Frequency Analyses

Peak flows for selected recurrence intervals are subject 

to uncertainty because the annual peak flows used in the 

computations are only a sample of a larger population 

(Interagency Advisory Committee on Water Data, 1982). 

Confidence intervals are a measure of the uncertainty of these 

computations. With a 90-percent confidence interval, one is 

90-percent confident that the true peak flow is between the 

lower and upper 90-percent confidence limits. The 100-year 

confidence intervals are larger than the 5-year confidence 

intervals because more uncertainty is associated with them.

The 1967 to 1996 sub-period yielded the highest 100- and 

5-year peak flows overall when compared with peak flows 

based on the full period of record (table 8, figs. 4 and 5). The 

Table 8. Median differences between peak flows for selected recurrence intervals based on sub-periods and peak flows based on 

full periods of record.

[percent represents percent higher or lower than full-period peak flows]

Sub-period 

start year

Sub-period 

end year

Total 

number 

of 

streamgages

Number 

of 

unregulated

streamgages

Median differences

100-year 

peak flows, 

all 

streamgages 

(percent)

100-year 

peak flows, 

unregulated 

streamgages 

(percent)

5-year 

peak flows, 

all 

streamgages 

(percent)

5-year 

peak flows, 

unregulated 

streamgages 

(percent)

1977 2006 28 20 4.1 4.1 6.2 6.6

1967 1996 28 20 7.8 8.7 8.1 8.3

1957 1986 28 20 -1.6 0.8 2.0 2.0

1947 1976 22 16 -3.4 2.8 -2.6 -2.8

1937 1966 18 12 -10.8 -8.8 -7.9 -9.6

1927 1956 9 5 -3.9 -3.9 -0.6 -3.2

1971 2006 19 13 4.2 7.4 6.1 7.1

Start of record 1970 19 13 -2.8 -4.1 -5.6 -5.9

Table 9. Number of streamgages with 100-year peak flows based on sub-periods that are outside of 90-percent confidence intervals 

of 100-year peak flows based on full periods of record.

Sub-period 

start year

Sub-period 

end year

Total 

number 

of 

streamgages

Number 

of 

unregulated 

streamgages

Greater than 90-percent 

confidence interval

Less than 90-percent 

confidence interval

All 

streamgages

Unregulated 

streamgages

All 

streamgages

Unregulated 

streamgages

1977 2006 28 20 6 4 2 1

1967 1996 28 20 10 7 3 1

1957 1986 28 20 6 5 6 4

1947 1976 22 16 2 2 6 3

1937 1966 18 12 0 0 8 4

1927 1956 9 5 2 1 3 2

1971 2006 19 13 2 1 3 2

Start of record 1970 19 13 1 0 4 3

median difference for all 28 streamgages is 8 percent for both 

100- and 5-year peak flows. The 1977–2006 and 1971–2006 

sub-periods overall yielded higher flows but not as high 

as the 1967–96 sub-period; the median differences for all 

streamgages for both periods (1977–2006 and 1971–2006) 

are 4 percent for 100-year peak flows and 6 percent for 5-year 

peak flows. The 1937–66 sub-period had the lowest 100- and 

5-year peak flows overall. The median differences are 

-11 percent for 100-year peak flows and -8 percent for 5-year 

peak flows. Differences between the sub-periods and the full 

periods based on the 20 unregulated streamgages are similar to 

differences based on all 28 streamgages (table 8). Magnitudes 

of peak-flow differences (between peak flows based on the 
Flood-Frequency Analyses for Full Periods of Record and Selected Sub-Periods 25

Table 10. Number of streamgages with 5-year peak flows based on sub-periods that are outside of 90-percent confidence intervals of 

5-year peak flows based on full records.

Sub-period 

start year

Sub-period 

end year

Total 

number of 

streamgages

Number of 

unregulated 

streamgages

Greater than 90-percent 

confidence interval

Less than 90-percent confidence 

interval

All 

streamgages

Unregulated 

streamgages

All 

streamgages

Unregulated 

streamgages

1977 2006 28 20 9 7 0 0

1967 1996 28 20 15 12 3 1

1957 1986 28 20 6 4 4 2

1947 1976 22 16 2 1 6 4

1937 1966 18 12 0 0 13 11

1927 1956 9 5 1 0 2 1

1971 2006 19 13 7 5 0 0

Start of record 1970 19 13 0 0 8 6

1937–19661947–19761957–19861967–19961977–2006

Start–1970

1971–2006

-30

-20

-10

0

10

20

30

DIFFERENCE BETWEEN 100-YEAR PEAK FLOWS,

IN PERCENT

EXPLANATION 

90th percentile

75th percentile

Median

25th percentile

10th percentile

EXPLANATION 

90th percentile

75th percentile

Median

25th percentile

10th percentile

1937–19661947–19761957–19861967–19961977–2006

Start–1970

1971–2006

-20

-10

0

10

20

30

DIFFERENCE BETWEEN 5-YEAR PEAK FLOWS,

IN PERCENT

Figure 4. Distribution of differences for various rivers between 

100-year peak flows based on selected sub-periods and 100-year 

peak flows based on full periods of record.

Figure 5. Distribution of differences for various rivers between 

5-year peak flows based on selected sub-periods and 5-year peak 

flows based on full periods of record.
26 Historical Changes in Annual Peak Flows in Maine and Implications for Flood-Frequency Analyses

sub-periods and those based on the full periods) vary more 

among streamgages for 100-year peak flows than for 5-year 

peak flows (figs. 4 and 5). The boxplots in figures 4 and 5 

show the 10th, 25th, 50th (median), 75th, and 90th percentiles 

of differences for all available rivers, for each sub-period.

The 1967–1996 sub-period produced the most 

streamgages (10 of 28) with 100-year peak flows higher 

than the 90-percent confidence intervals of the peak flows 

computed from full periods of record (table 9). The 1967–96 

sub-period also produced the most streamgages (15 of 28) 

with 5-year peak flows higher than the full-period 90-percent 

confidence intervals (table 10). The 1937–66 sub-period 

yielded the most streamgages (8 of 18) with 100-year peak 

flows lower than the 90-percent confidence intervals of the 

full-period peak flows. The 1937–66 sub-period also yielded 

the most streamgages (13 of 18) with 5-year peak flows less 

than the 90-percent confidence intervals. More streamgages 

had 5-year peak flows outside the confidence intervals for 

these two sub-periods (1967–96 and 1937–66) than had 

100-year peak flows outside the confidence intervals. This is 

likely due to the lower uncertainty for the 5-year peak flows 

than for the 100-year peak flows (table 1). The 1971–2006 

sub-period also produced more streamgages (7 of 19) with 

5-year peak flows greater than the full-record confidence 

intervals than streamgages with 100-year peak flows greater 

than full-record confidence intervals (2 of 19) (tables 9 and 

10). The older sub-period (start-of-record to 1970) produced 

more streamgages (8 of 19) with 5-year peak flows less 

than the confidence intervals than with 100-year peak flows 

(4 of 19) less than the confidence intervals. Overall, for the 

various time periods considered, the relative number of total 

(regulated and unregulated) streamgages with peak flows 

outside the confidence intervals is consistent with the relative 

number of unregulated streamgages with peak flows outside 

the confidence intervals (tables 9 and 10).

Detailed Results for 100-Year Peak Flows

For 6 of 28 streamgages, the 100-year peak flows 

based on the 1977–2006 sub-period were greater than the 

90-percent confidence intervals based on full periods, and for 

2 streamgages, the peak flows were less than the confidence 

intervals (table 4). The two streamgages with peak flows less 

than the confidence intervals had high-outlier peak flows early 

in their records. One of six streamgages with peaks greater 

than the confidence intervals had a high-outlier peak flow 

during the 1977–2006 sub-period. High-outlier peak flows 

for the full period of record, identified as outliers by use of 

the guidelines (Bulletin 17B) of the Interagency Advisory 

Committee on Water Data (1982), were present for five of the 

streamgages. The high-outlier peaks are approximately three 

standard deviations (depending on the length of record) above 

the mean of all the annual peaks. Peak flows with 100-year 

recurrence intervals based on the 1977–2006 sub-period 

ranged from 13 percent lower to 25 percent higher than flows 

based on full records at the 20 unregulated streamgages.

For 10 of 28 streamgages, 100-year peak flows based 

on the 1967–96 sub-period were greater than the 90-percent 

confidence intervals based on full periods of record, and for 

3 streamgages, flows were less than the confidence intervals 

(table 4). The streamgages with peak flows greater than the 

confidence intervals are spread throughout the State (fig. 6); 

all three streamgages (streamgages 18, 20, and 25) with peak 

flows lower than the confidence intervals drain the upper 

Androscoggin River Basin. Two of the three streamgages 

with peak flows less than the confidence intervals had highoutlier peak flows outside of the 1967–96 sub-period. One 

of 10 streamgages with peaks greater than the confidence 

interval had a high-outlier peak within the sub-period. Peak 

flows based on the sub-period 1967-96 ranged from 17 percent 

lower to 26 percent higher than flows based on full records at 

the 20 unregulated streamgages. 

For 6 of 28 streamgages, the 100-year peak flows based 

on the 1957–1986 sub-period were greater than the 90-percent 

confidence intervals based on full periods of record, and for 

6 streamgages, flows were less than the confidence intervals 

(table 4). For two streamgages (of 22 total), the 100-year peak 

flows based on the 1947–1976 sub-period were greater than 

the confidence intervals, and for six streamgages, flows were 

less than the confidence intervals.

For 8 of 18 streamgages, the 100-year peak flows based 

on the 1937–66 sub-period were less than the full-period 

90-percent confidence intervals (table 4); no streamgages 

had flows greater than the confidence intervals. The eight 

streamgages with peak flows less than the confidence intervals 

are spread throughout the State (fig. 7). Three of the eight 

streamgages had high-outlier peak flows outside of the 

1937–66 sub-period. For the limited number of streamgages 

with records from the 1927–56, 1917–46, and 1907–36 

sub-periods, peak flows were both greater and less than the 

90-percent confidence intervals based on full periods (table 4).

For 2 of 19 streamgages, 100-year peak flows based on 

the 1971–2006 sub-period were greater than the full-period 

90-percent confidence intervals, and for 3 streamgages, flows 

were less than the confidence intervals (table 5, fig. 8). One of 

the two streamgages with peaks greater than the confidence 

interval had a high-outlier peak within the 1971–2006 

sub-period. All three of the streamgages with peak flows less 

than the confidence intervals had high-outlier peak flows 

prior to the 1971–2006 sub-period. Peak flows based on the 

sub-period ranged from 12 percent lower to 15 percent higher 

than flows based on the full period at the 13 unregulated 

streamgages. For four streamgages, the 100-year peak flows 

based on flows from start-of-record to 1970 were less than 

the full-period 90-percent confidence intervals, and for one 

streamgage, flows were greater than the confidence interval 

(table 5, fig. 9). This latter streamgage had a high outlier in the 

sub-period start-of-record to 1970.
Flood-Frequency Analyses for Full Periods of Record and Selected Sub-Periods 27

9

7

6

5 4

3

2

1

28

27

26

24 23

22

18

17

16

13

11

10

8R

25R

21R

20R

19R

15R

14R

12R

24

18

EXPLANATION

Lake or pond

River

County boundary

Station and reference number—Symbol indicates how 100-year peak

flows based on the period 1967–96 compare to those based on the full record

Greater than 90-percent confidence interval of full record

Within 90-percent confidence interval of full record

Less than 90-percent confidence interval of full record

68°

70°

46°

44°

70°

44°

68°

46°

10

"R" in the reference number indicates station 

streamflow is regulated

NEW HAMPSHIRE

CANADA

CANADA

0 15 30 60 MILES

Base from U.S. Geological Survey digital files,

Universal Transverse Mercator projection, zone 19

0 30 60 KILOMETERS MAINE

Atlantic Ocean

Figure 6. Geographic distribution of 100-year peak flows based on the 1967 to 1996 sub-period that are outside the 

90-percent confidence intervals of 100-year peak flows based on full periods.
28 Historical Changes in Annual Peak Flows in Maine and Implications for Flood-Frequency Analyses

Figure 7. Geographic distribution of 100-year peak flows based on the 1937 to 1966 sub-period that are outside 

the 90-percent confidence intervals of 100-year peak flows based on full periods.

7

6

5

3

28

27

24

22

17

16

11

10

8R

25R

21R

20R

19R

12R

17

16

68°

70°

46°

44°

70°

44°

68°

46°

NEW HAMPSHIRE

CANADA

CANADA

0 15 30 60 MILES

Base from U.S. Geological Survey digital files,

Universal Transverse Mercator projection, zone 19

0 30 60 KILOMETERS

EXPLANATION

Lake or pond

River

County boundary

Station and reference number—Symbol indicates how 100-year peak

flows based on the period 1937–66 compare to those based on the full record

Within 90-percent confidence interval of full record

Less than 90-percent confidence interval of full record

"R" in the reference number indicates station 

streamflow is regulated

MAINE

Atlantic Ocean
Flood-Frequency Analyses for Full Periods of Record and Selected Sub-Periods 29

7

6

5

3

28

27

24

22

17

16

13

11

10

8R

25R

21R

20R

19R

12R

11

10

68°

70°

46°

44°

70°

44°

68°

46°

NEW HAMPSHIRE

CANADA

CANADA

EXPLANATION

Lake or pond

River

County boundary

Station and reference number—Symbol indicates how 100-year peak

flows based on the period 1970–2006 compare to those based on the full record

Greater than 90-percent confidence interval of full record

Within 90-percent confidence interval of full record

Less than 90-percent confidence interval of full record

3

"R" in the reference number indicates station 

streamflow is regulated

0 15 30 60 MILES

Base from U.S. Geological Survey digital files,

Universal Transverse Mercator projection, zone 19

0 30 60 KILOMETERS MAINE

Atlantic Ocean

Figure 8. Geographic distribution of 100-year peak flows based on the 1971 to 2006 sub-period that are outside 

the 90-percent confidence intervals of 100-year peak flows based on full periods.
30 Historical Changes in Annual Peak Flows in Maine and Implications for Flood-Frequency Analyses

Figure 9. Geographic distribution of 100-year peak flows based on the start-of-record to 1970 sub-period that are 

outside the 90-percent confidence intervals of 100-year peak flows based on full periods.

7

6

5

3

28

27

24

22

17

16

13

11

10

8R

25R

21R

20R

19R

12R

13

25R

68°

70°

46°

44°

70°

44°

68°

46°

NEW HAMPSHIRE

CANADA

CANADA

EXPLANATION

Lake or pond

River

County boundary

Station and reference number—Symbol indicates how 100-year peak flows

based on the period before 1970 compare to those based on the full record

Greater than 90-percent confidence interval of full record

Within 90-percent confidence interval of full record

Less than 90-percent confidence interval of full record

17

"R" in the reference number indicates station 

streamflow is regulated

0 15 30 60 MILES

Base from U.S. Geological Survey digital files,

Universal Transverse Mercator projection, zone 19

0 30 60 KILOMETERS MAINE

Atlantic Ocean
Implications for Flood-Frequency Analyses 31

Detailed Results for 5-Year Peak Flows

For 9 of 28 streamgages, 5-year peak flows based on 

the 1977–2006 sub-period were greater than the 90-percent 

confidence intervals based on full periods, and no streamgages 

had flows less than the confidence intervals (table 6). Two of 

the nine streamgages with peaks greater than the confidence 

interval had a high-outlier peak within the 1977–2006 

sub-period. Peak flows with 5-year recurrence intervals based 

on the 1977–2006 sub-period ranged from 7 percent less to 

15 percent greater than flows based on full records at the 

20 unregulated streamgages.

For 15 of 28 streamgages, 5-year peak flows based on 

the 1967–96 sub-period were greater than the 90-percent 

confidence intervals based on full periods of record, and for 

3 streamgages, flows were less than the confidence intervals 

(table 6). The streamgages with peak flows greater than the 

confidence intervals are spread throughout the State (fig. 10); 

all three streamgages with peak flows less than the confidence 

intervals (streamgages 18, 19, and 20) drain the upper 

Androscoggin River Basin. One of the three streamgages had 

a high-outlier peak flow outside of the 1967–96 sub-period. 

One of 15 streamgages with peaks greater than the confidence 

interval had a high-outlier peak within the sub-period. 

One streamgage had a high outlier outside the 1967–96 

sub-period but still had a 5-year peak flow greater than the 

full-period 90-percent confidence interval. Peak flows based 

on the 1967–1996 sub-period ranged from 6 percent lower 

to 20 percent higher than flows based on full records at the 

20 unregulated streamgages. 

For 6 of 28 streamgages, 5-year peak flows based on 

the 1957–1986 sub-period were greater than the 90-percent 

confidence intervals based on full periods of record, and for 

4 streamgages, flows were less than the confidence intervals 

(table 6). For two streamgages (of 22 total), peak flows based 

on the 1947–1976 sub-period were greater than the confidence 

intervals, and for six streamgages, flows were less than the 

confidence intervals.

For 13 streamgages (of 18 total), 5-year peak flows 

based on the 1937–66 sub-period were less than the 

full-record 90-percent confidence intervals (table 6); no 

streamgages had flows greater than the confidence intervals. 

The 13 streamgages with peak flows less than the confidence 

intervals are spread throughout the State (fig. 11). Three of 

the 13 streamgages had high-outlier peak flows outside of the 

1937–1966 sub-period. For the limited number of streamgages 

with records from the 1927–56, 1917–46, and 1907–36 

sub-periods, peak flows were both higher and lower than the 

90-percent confidence intervals based on full periods of record 

(table 6).

 For 7 of 19 streamgages, 5-year peak flows based on 

the 1971–2006 sub-period were greater than the 90-percent 

full-period confidence intervals, and no streamgages had flows 

less than the confidence intervals (table 7, fig. 12). One of 

the seven streamgages with peaks greater than the confidence 

intervals had a high-outlier peak within the 1971–2006 

sub-period. Peak flows based on the sub-period ranged from 

0.2 percent lower to 15 percent higher than flows based on the 

full period at the 13 unregulated streamgages. Peak flows for 

this time period had a consistent signal of higher 5-year peak 

flows compared to full-period peak flows though not as strong 

a signal as peak flows based on 1967–1996 flows. For eight 

streamgages, 5-year peak flows based on flows from the startof-record to 1970 were less than the 90-percent full-period 

confidence intervals, and no streamgages had flows greater 

than the confidence intervals (table 7, fig. 13). One of the eight 

streamgages with peaks less than the confidence interval had a 

high-outlier peak outside of this sub-period.

Implications for FloodFrequency Analyses

The increases in 5-year peak flows based on data from 

recent decades are consistent with the observed increasing 

annual peak flows in Maine. The increases in 100-year 

peak flows based on data from recent decades are generally 

consistent with observed increasing annual peak flows. 

In limited cases, 100-year peak flows based on data from 

recent decades have decreased substantially. Many of these 

streamgages have high outlier peak flows early in their 

records; the outliers affected 100-year peak flows more 

than they affected long-term changes in annual peak flows. 

Collins (2009) also found flood-frequency computations to 

be sensitive to the very high peak flows at some streamgages 

in New England, as did Lumia and others (2006, fig. 8) in 

New York.

Annual peak flows have, in general, increased in recent 

years in Maine as have peak flows with 5- and 100-year 

recurrence intervals that are based on recent records. Increases 

in the 5- and 100-year peak flows are, in general, modest when 

compared to those based on complete periods of record. The 

highest peak flows are based on the 1967–1996 sub-period 

rather than the most recent time period. Because large floods 

in Maine typically result from a combination of snowmelt and 

rainfall, and in the future may be affected by global warming 

and by climatic variability (related to sea surface temperature 

variability or large scale atmospheric patterns), future patterns 

of change are likely to be complex over both space and time. 

Peak flows of selected recurrence intervals are sensitive to 

very high peak flows that may occur once in a century or 

even less frequently. Therefore, it is difficult to determine 

whether peak flows for selected recurrence intervals based 

on recent years of record are more accurate than peak flows 

of selected recurrence intervals based on the entire historical 

period (the traditional method). Although peak flows in Maine 

have, in general, increased during the last 50–100 years, it 

is difficult to determine whether peak flows are stationary 

because multi-decadal changes can turn out to be oscillations 

in longer streamflow records (Koutsoyiannis, 2006; Hamed, 

2008). Because peak flows of selected recurrence intervals 
32 Historical Changes in Annual Peak Flows in Maine and Implications for Flood-Frequency Analyses

Figure 10. Geographic distribution of 5-year peak flows based on the 1967 to 1996 sub-period that are outside the 

90-percent confidence intervals of 5-year peak flows based on full periods.

9

7

6

5 4

3

2

1

28

27

26

24 23

22

18

17

16

13

11

10

8R

25R

21R

20R

19R

15R

14R

12R

18

11

68°

70°

46°

44°

70°

44°

68°

46°

NEW HAMPSHIRE

CANADA

CANADA

EXPLANATION

Lake or pond

River

County boundary

Station and reference number—Symbol indicates how 5-year peak flows

based on the period before 1967−96 compare to those based on the full record

Greater than 90-percent confidence interval of full record

Within 90-percent confidence interval of full record

Less than 90-percent confidence interval of full record

22

"R" in the reference number indicates station 

streamflow is regulated

0 15 30 60 MILES

Base from U.S. Geological Survey digital files,

Universal Transverse Mercator projection, zone 19

0 30 60 KILOMETERS MAINE

Atlantic Ocean
Implications for Flood-Frequency Analyses 33

Figure 11. Geographic distribution of 5-year peak flows based on the 1937 to 1966 sub-period that are outside the 

90-percent confidence intervals of 5-year peak flows based on full periods.

7

6

5

3

28

27

24

22

17

16

11

10

8R

25R

21R

20R

19R

12R

11

8R

68°

70°

46°

44°

70°

44°

68°

46°

NEW HAMPSHIRE

CANADA

CANADA

0 15 30 60 MILES

Base from U.S. Geological Survey digital files,

Universal Transverse Mercator projection, zone 19

0 30 60 KILOMETERS

EXPLANATION

Lake or pond

River

County boundary

Station and reference number—Symbol indicates how 5-year peak flows

based on the period 1937–66 compare to those based on the full record

Within 90-percent confidence interval of full record

Less than 90-percent confidence interval of full record

"R" in the reference number indicates station 

streamflow is regulated

MAINE

Atlantic Ocean
34 Historical Changes in Annual Peak Flows in Maine and Implications for Flood-Frequency Analyses

Figure 12. Geographic distribution of 5-year peak flows based on the 1971 to 2006 sub-period that are outside the 

90-percent confidence intervals of 5-year peak flows based on full periods.

7

6

5

3

28

27

24

22

17

16

13

11

10

8R

25R

21R 20R

19R

12R

3

10

68°

70°

46°

44°

70°

44°

68°

46°

NEW HAMPSHIRE

CANADA

CANADA

EXPLANATION

Lake or pond

River

County boundary

Station and reference number—Symbol indicates how 5-year peak flows

based on the period 1970–2006 compare to those based on the full record

Greater than 90-percent confidence interval of full record

Within 90-percent confidence interval of full record

"R" in the reference number indicates station 

streamflow is regulated

0 15 30 60 MILES

Base from U.S. Geological Survey digital files,

Universal Transverse Mercator projection, zone 19

0 30 60 KILOMETERS MAINE

Atlantic Ocean
Implications for Flood-Frequency Analyses 35

Figure 13. Geographic distribution of 5-year peak flows based on the start-of-record to 1970 sub-period that are outside 

the 90-percent confidence intervals of 5-year peak flows based on full periods.

7

6

5

3

28

27

24

22

17

16

13

11

10

8R

25R

21R

20R

19R

12R

11

10

68°

70°

46°

44°

70°

44°

68°

46°

NEW HAMPSHIRE

CANADA

CANADA

0 15 30 60 MILES

Base from U.S. Geological Survey digital files,

Universal Transverse Mercator projection, zone 19

0 30 60 KILOMETERS

EXPLANATION

Lake or pond

River

County boundary

Station and reference number—Symbol indicates how 5-year peak flows

based on the period before 1970 compare to those based on the full record

Within 90-percent confidence interval of full record

Less than 90-percent confidence interval of full record

"R" in the reference number indicates station 

streamflow is regulated

MAINE

Atlantic Ocean
36 Historical Changes in Annual Peak Flows in Maine and Implications for Flood-Frequency Analyses

in Maine may or may not be stationary, one possible 

conservative approach to computing peak flows of selected 

recurrence intervals would be to compute these peak flows 

from both recent annual peak flows and from the entire period 

of peak flows and then choose the higher computed value 

(Collins, 2009). 

Whether recent or entire periods of record are used to 

compute peak flows of selected recurrence intervals, the 

results of this study highlight the importance of using recent 

data in the computation of the peak flows. The use of only 

older records could result in the underestimation of peak 

flows, particularly for peak flows with shorter recurrence 

intervals such as the 5-year peak flow.

Summary and Conclusions

For flood-frequency analyses, statistical methods are 

used to compute peak streamflows for selected recurrence 

intervals—the average number of years between peak flows 

that are equal to or greater than a specified peak flow. It is 

necessary to determine the magnitude of peak streamflows 

with specified recurrence intervals to safely and economically 

design bridges, culverts, and other structures that are in, or 

near, streams. It has long been assumed that the annual peak 

streamflows used in these computations were stationary over 

very long periods of time, except in river basins subject to 

urbanization, regulation, and other direct human activities. 

Stationarity is the concept that natural systems fluctuate 

within an envelope of variability that does not change over 

time. Because of the potential effects of global warming on 

peak flows, the assumption of peak-flow stationarity has 

recently been questioned. This study was conducted by the 

U.S. Geological Survey (USGS), in cooperation with the 

Maine Department of Transportation (MaineDOT). It will 

help MaineDOT and others better understand the effects of 

changing historical peak flows on flood-frequency analyses 

for streams in Maine. Maine has many streamgages with 

50–105 years of recorded annual peak streamflows. In this 

study, this long-term record has been tested for historical 

flood-frequency stationarity, to provide some insight into 

future flood frequency.

Changes over time in annual instantaneous peak 

streamflows at 28 USGS streamgages with long-term data 

(50 or more years) and relatively complete records (no more 

than 5 years of missing data from 1957 through 2006) were 

investigated in two different ways. Linear trends for the period 

of record for each streamgage were computed with the Sen 

slope. Because previous studies have suggested that changes 

over time may have occurred as a step change around 1970, 

the step change between each streamgage’s older record (startyear to 1970) and newer record (1971 to 2006) was computed 

using the Hodges-Lehmann estimator.

The median change in peak flows over time for all 

28 streamgages (including 8 streamgages with substantial 

streamflow regulation) was an increase of 15.9 percent based 

on a linear change and an increase of 12.4 percent based on 

a step change. The median change for the 20 unregulated 

streamgages is slightly higher than for all 28 streamgages; 

it is 18.4 percent based on a linear change and 15.0 percent 

based on a step change. There were many linear increases of 

greater than 10 percent, and peak flows at seven streamgages 

increased by more than 25 percent. In contrast, peak flows 

at one streamgage decreased by more than 10 percent. Many 

step changes of greater than 10 percent were noted when 

comparing older data (1970 and earlier) to newer data (1971 to 

2006). Peak flows at three streamgages had step increases of 

greater than 25 percent, and one streamgage had a decrease of 

more than 10 percent.

Peak flows for 100- and 5-year recurrence intervals were 

computed for the 28 streamgages using the entire available 

annual peak-flow record and multiple sub-periods of that 

record, using the guidelines (Bulletin 17B) of the Interagency 

Advisory Committee on Water Data. Magnitudes of 100- and 

5-year peak flows computed from the sub-periods were then 

compared to those for the full period and to the 90-percent 

confidence intervals of the peak flows computed from the full 

period. Sub-periods consisting of 30 years were staggered by 

10 years (1907–36, 1917–46, 1927–56, 1937–66, 1947–76, 

1957–86, 1967–96, and 1977–2006). Because changes over 

time in peak flows may have occurred as a step change around 

1970, two other sub-periods were analyzed: older data (startof-record to 1970) and newer data (1971 to 2006). The 5-year 

peak flow is used to represent small and relatively frequent 

flood flows in Maine, whereas the 100-year peak flow is used 

to represent large flood flows. 

The 1967–1996 sub-period had the highest 100- and 

5-year peak flows overall when compared to 100- and 5-year 

peak flows based on the full period of record; the median 

difference for all 28 streamgages is 8 percent for both the 

100- and 5-year peak flows. Peak flows with 100-year 

recurrence intervals based on the sub-period are from 

17 percent lower to 26 percent higher than 100-year peak 

flows based on full records at the 20 unregulated streamgages, 

and peak flows with 5-year recurrence intervals are from 

6 percent lower to 20 percent higher than 5-year peak flows 

based on full records. The 1977–2006 and 1971–2006 

sub-periods also showed overall higher peak flows than the 

full-period peak flows, but these sub-period peak flows are 

not as high as those based on the 1967–1996 sub-period; the 

median difference for all streamgages for both sub-periods 

(1977–2006 and 1971–2006) is 4 percent for 100-year peak 

flows and 6 percent for 5-year peak flows. The 1937–1966 

sub-period showed the lowest 100- and 5-year peak flows 

overall. The median differences from full-period peak flows 

are -11 percent for 100-year peak flows and -8 percent 

for 5-year peak flows. Overall, differences between the 
References Cited 37

sub-periods and the full periods for the 20 unregulated 

streamgages are similar to differences for all 28 streamgages.

The 1967–96 sub-period had the most streamgages 

(10 of 28) with 100-year peak flows higher than the 90-percent 

confidence intervals of the peak flows computed from full 

periods of record. This sub-period also produced the most 

streamgages (15 of 28) with 5-year peak flows higher than 

the full-period 90-percent confidence intervals. The 1937–66 

sub-period had the most streamgages (8 of 18) with 100-year 

peak flows lower than the 90-percent confidence intervals of 

the full-period peak flows. This sub-period also had the most 

streamgages (13 of 28) with 5-year peak flows less than the 

90-percent confidence intervals. The streamgages with peak 

flows outside the confidence intervals are spread throughout 

the State. More streamgages have 5-year peak flows outside 

the confidence intervals for the 1967–96 and 1937–66 

sub-periods than have 100-year peak flows outside the 

confidence intervals; however, the magnitude of percentage 

differences between the sub-periods and the full periods are 

similar for the 5- and 100-year peak flows. This discrepancy 

is likely due to the lower uncertainty of the 5-year peak flows 

(narrower 90-percent confidence intervals) compared to the 

100-year peak flows.

The increases in 5-year peak flows based on data from 

recent decades are consistent with the observed increasing 

annual peak flows in Maine. The increases in 100-year 

peak flows based on data from recent decades are generally 

consistent with observed increasing annual peak flows. 

In limited cases, 100-year peak flows based on data from 

recent decades have decreased substantially. Many of these 

streamgages have high outlier peak flows early in their 

records; the outliers affected 100-year peak flows more than 

they affected long-term changes in annual peak flows. 

Although peak flows in Maine have, in general, increased 

during the last 50–100 years, it is difficult to determine 

whether peak flows are stationary because multi-decadal 

changes can turn out to be oscillations in longer streamflow 

records. Increases in the 5- and 100-year peak flows based 

on data from recent decades are, in general, modest when 

compared to peak flows based on complete periods of record. 

The highest peak flows are based on the 1967–96 sub-period 

rather than the most recent sub-period (1977-2006). Peak 

flows of selected recurrence intervals are sensitive to very 

high peak flows that may occur once in a century or even 

less frequently.

Because large floods in Maine typically result from a 

combination of snowmelt and rainfall, and in the future may 

be affected by global warming and by climatic variability 

(related to sea surface temperature variability or large scale 

atmospheric patterns), future patterns of change are likely to 

be complex over both space and time. Because peak flows 

of selected recurrence intervals in Maine may or may not be 

stationary, it is difficult to determine whether peak flows for 

selected recurrence intervals based on recent years of record 

are more accurate than peak flows of selected recurrence 

intervals based on the entire historical period (the traditional 

method). One possible conservative approach to computing 

peak flows of selected recurrence intervals would be to 

compute these peak flows from both recent annual peak flows 

and from the entire period of peak flows and then choose the 

higher computed value. 

Whether recent or entire periods of record are used to 

compute peak flows of selected recurrence intervals, the 

results of this study highlight the importance of using recent 

data in the computation of the peak flows. The use of only 

older records could result in the underestimation of peak 

flows, particularly for peak flows with shorter recurrence 

intervals such as the 5-year peak flow.

References Cited

Benson, M.A., 1960, Characteristics of frequency curves 

based on a theoretical 1,000-year record, in Dalrymple, 

T., Flood frequency analysis, manual of hydrology, part 

3, flood-flow techniques: U.S. Geological Survey WaterSupply Paper 1543–A, 80 p. 

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

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

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

Cember, R.P., and Wilks, D.S., 1993, Climatological atlas of 

snowfall and snow depth for the northeastern United States 

and southeastern Canada: Northeast Regional Climate 

Center Publication No. RR 93-1, Ithaca, N.Y., 213 p.

Cohn, T.A., and Lins, H.F., 2005, Nature’s style—

Naturally trendy: Geophysical Research Letters, v. 32, 

doi:10.1029/2005GL024476.

Collins, M.J., 2009, Evidence for changing flood risk in 

New England since the late 20th century: Journal of the 

American Water Resources Association, v. 45, p. 279–290. 

DOI: 10.1111/j.1752-1688.2008.00277.x.

Dudley, R.W., and Hodgkins, G.A., 2002, Trends in 

streamflow, river ice, and snowpack for coastal river basins 

in Maine during the 20th century: U.S. Geological Survey 

Water-Resources Investigations Report 02–4245, 26 p.

Hamed, H.H., 2008, Trend detection in hydrologic data: The 

Mann-Kendall trend test under the scaling hypothesis: 

Journal of Hydrology, v. 349, p. 350–363.

Hayhoe, K., Wake, C.P., Huntington, T.G., Luo, L., Schwartz, 

M.D., Sheffield, J., Wood, E., Anderson, B., Bradbury, J., 

DeGaetano, A., Troy, T.J., and Wolfe, D., 2007, Past and 

future changes in climate and hydrological indicators in the 

U.S. Northeast: Climate Dynamics, v. 28, p. 381–407.
38 Historical Changes in Annual Peak Flows in Maine and Implications for Flood-Frequency Analyses

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

water resources: Elsevier, N.Y., Elsevier, 522 p.

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

flows for streams in Maine for selected recurrence intervals: 

U.S. Geological Survey Water-Resources Investigations 

Report 99–4008, 45 p.

Hodgkins, G.A., and Dudley, R.W., 2005, Changes in the 

magnitude of annual and monthly streamflows in New 

England, 1902–2002: U.S. Geological Survey Scientific 

Investigations Report 2005–5135, 37 p.

Hodgkins, G.A., and Dudley, R.W., 2006, Changes in the 

timing of winter–spring streamflows in eastern North 

America, 1913–2002: Geophysical Research Letters, v. 33, 

L06402, doi:10.1029/2005GL025593.

Hodgkins, G.A., Dudley, R.W., and Huntington, T.G., 2003, 

Changes in the timing of high river flows in New England 

over the 20th century: Journal of Hydrology, v. 278, p. 

242–250.

Hodgkins, G.A., Lent, R.M., Dudley, R.W., and Schalk, C.W., 

2009, Framework for a U.S. Geological Survey Hydrologic 

Climate-Response Program in Maine: U.S. Geological 

Survey Scientific Investigations Report 2009–1115, 24 p.

Hurrell, J.W., Kushnir, Y., Ottersen, G., and Visbeck, M., 

2003, An overview of the North Atlantic Oscillation, in 

The North Atlantic Oscillation—Climatic significance 

and environmental impact: American Geophysical Union 

Geophysical Monograph 134.

Interagency Advisory Committee on Water Data, 1982, 

Guidelines for determining flood flow frequency–Bulletin 

17B of the Hydrology Subcommittee: U.S. Geological 

Survey, Office of Water-Data Coordination, 183 p.

Khaliq, M.N., Ouarda, T.B.M.J., and Gachon, P., 2009, 

Identification of temporal trends in annual and seasonal low 

flows occurring in Canadian rivers: The effect of short- and 

long-term persistence: Journal of Hydrology, v. 369, p. 

183–197.

Koutsoyiannis, D., 2006, Nonstationarity versus scaling in 

hydrology: Journal of Hydrology, v. 324, p. 239–254.

Koutsoyiannis, D., and Montanari, A., 2007, Statistical 

analysis of hydroclimatic time series: Uncertainty 

and insights: Water Resources Research, v. 43, 

doi:10.1029/2006WR005592.

Loiselle, M.C., and Hodgkins, G.A., 2002, Snowpack in 

Maine—Maximum observed and March 1 mean equivalent 

water content: U.S. Geological Survey Water-Resources 

Investigations Report 01–4258, 19 p.

Lumia, R., Freehafer, D.A., and Smith, M.J., 2006, Magnitude 

and frequency of floods in New York: U.S. Geological 

Survey Scientific Investigations Report 2006–5112, 152 p.

Milly, P.C.D., Betancourt, J., Falkenmark, M., Hirsch, R.M., 

Kundzewicz, Z.W., Lettenmaier, D.P., and Stouffer, R.J., 

2008, Stationarity is dead: Whither water management?: 

Science, v. 319, p. 573–574.

National Oceanic and Atmospheric Administration, 2002, 

Climatography of the United States, no. 81: Monthly 

station normals of temperature, precipitation, and heating 

and cooling degree days, 1971–2000, Region 17, Maine, 

16 p.

New England Regional Assessment Group, 2001, Preparing 

for a changing climate—The potential consequences of 

climate variability and change, New England regional 

overview: Durham, New Hampshire, University of New 

Hampshire, U.S. Global Change Research Program, 96 p.

Slack, J.R., and Landwehr, J.M., 1992, Hydro-climatic data 

network (HCDN)—A U.S. Geological Survey streamflow 

data set for the United States for the study of climate 

variations, 1874–1988: U.S. Geological Survey Open-File 

Report 92–129, 193 p.
Prepared by the Pembroke Publishing Service Center.

For more information concerning this report, contact:

Director

U.S. Geological Survey

Maine Water Science Center

196 Whitten Road

Augusta, ME 04330

dc_me@usgs.gov

or visit our Web site at:

http://me.water.usgs.gov
Hodgkins—Historical Changes in Annual Peak Flows in Maine and Implications for Flood-Frequency Analyses—Scientific Investigations Report 2010–5094