Trends in Timing, Magnitude, and Duration of Summer and 

Fall/Winter Streamflows for Unregulated Coastal River 

Basins in Maine During the 20th Century

Scientific Investigations Report 2005-5021

In cooperation with the

Maine Atlantic Salmon Commission

U.S. Department of the Interior

U.S. Geological Survey
Cover Photograph: West Branch Sheepscot River, Maine, June 2003. (Photography by Tim Sargent, U.S. Geological Survey)
Trends in Timing, Magnitude, and Duration of 

Summer and Fall/Winter Streamflows for 

Unregulated Coastal River Basins in Maine During 

the 20th Century

By Robert W. Dudley and Glenn A. Hodgkins

In cooperation with the

Maine Atlantic Salmon Commission 

Scientific Investigations Report 2005-5021

U.S. Department of the Interior

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

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2005

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 iii

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

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

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

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

Streamflow, Precipitation, and Surface Air Temperature Data................................................................................................ 5

Streamflow Data ............................................................................................................................................................ 5

Precipitation and Surface Air Temperature Data........................................................................................................... 6

Analysis of Streamflow, Precipitation, and Surface Air Temperature Data............................................................................. 6

Streamflow..................................................................................................................................................................... 7

Precipitation and Surface Air Temperature ................................................................................................................... 9

Trends in Timing, Magnitude, and Duration of Summer and Fall/Winter Streamflows for Unregulated

Coastal River Basins in Maine During the 20th Century..........................................................................................................10

Trends in Summer Streamflows....................................................................................................................................10

Trends in Fall/Winter Streamflows...............................................................................................................................13

Correlations Among Seasonal Streamflows.................................................................................................................16

Correlations of Seasonal Streamflow with Precipitation and Surface Air Temperature .............................................16

Conclusions.............................................................................................................................................................................. 22

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

Acknowledgements ................................................................................................................................................................. 25

References Cited ..................................................................................................................................................................... 25

Figures

1. Map showing locations of gaged coastal river basins, Maine .................................................................................. 3

2. Graph showing an average annual hydrograph for six rivers in coastal Maine......................................................... 5

3. Graph showing the Maintenance of Variance Extension, type 1 (MOVE.1) relation between the 

streamflow at Machias and Narraguagus Rivers, Maine .................................................................................... 8

4. Graph showing trends in total runoff volume (August 1 to September 30) for six coastal rivers

in Maine and the Narraguagus Extended Record (NER) ......................................................................................11

5. Graph showing trends in the magnitude of 31-day mean peak streamflow (September 1 to 

January 31) for six coastal rivers in Maine and the Narraguagus Extended Record (NER).................................14

Tables

1. Coastal streamflow-gaging stations in Maine used in this investigation.................................................................. 4

2. U.S. Historical Climatology Network station in Maine used in this investigation..................................................... 7

3. Pairing of U.S. Geological Survey coastal streamflow-gaging stations and U.S. Historical

Climatology Network stations for correlation testing.......................................................................................... 9

4. Statistical significance (p-values) and trend direction for summer (June 1 to October 31)

streamflow timing statistics for coastal river basins in Maine ...........................................................................10

5. Statistical significance (p-values) and trend direction for summer (June 1 to October 31)

streamflow threshold statistics for coastal river basins in Maine ......................................................................11

6. Statistical significance (p-values) and trend direction for summer (June 1 to October 31)

streamflow magnitude statistics for coastal river basins in Maine ....................................................................12

7. Statistical significance (p-values) and trend direction for the October median threshold

timing statistic for coastal river basins in Maine (September 1 to January 31)..................................................13

CONTENTS
iv 

Tables continued

8. Statistical significance (p-values) and trend direction for fall/winter (September 1 to January 31)

streamflow timing statistics for coastal river basins in Maine .......................................................................... 15

9. Statistical significance (p-values) and trend direction for fall/winter (September 1 to January 31)

streamflow threshold statistics for coastal river basins in Maine ..................................................................... 15

10. Statistical significance (p-values) and trend direction for fall/winter (September 1 to January 31)

streamflow magnitude statistics for coastal river basins in Maine ................................................................... 16

11. Interseasonal correlations of summer (June 1 to September 30) low-streamflow timing (ICVD) to 

spring (January1 to May 31) and to fall/winter (September 1 to January 31) high-streamflow

timings (CVD) and to the October median threshold date for coastal river basins in Maine ............................. 17

12. Interseasonal correlations of the magnitude of summer (June 1 to September 30) 

31-day mean low-streamflow (Low31Q) to fall/winter (September 1 to January 31) 

and to spring (January 1 to May 31) high-streamflow timings (CVD) and to the October median

threshold date for coastal river basins in Maine ................................................................................................ 17

13. Correlations of summer low-streamflow timing (ICVD) and fall/winter high-streamflow timing (CVD)

with monthly mean precipitation for coastal river basins in Maine .................................................................... 18

14. Correlations of summer low-streamflow timing (ICVD) and fall/winter high-streamflow timing (CVD)

with monthly mean surface air temperature for coastal river basins in Maine .................................................. 19

15. Correlations of summer 31-day mean low-streamflow magnitude and fall/winter 31-day mean

high-streamflow magnitude with monthly mean precipitation for coastal river basins in Maine ...................... 20

16. Correlations of summer 31-day mean low-streamflow magnitude and fall/winter 31-day mean highstreamflow magnitude with monthly mean surface air temperature for coastal river basins in Maine............. 21

17. Correlations of cumulative number of days below the long-term August median

streamflow threshold with monthly mean precipitation for coastal river basins in Maine ................................ 23

18. Correlations of cumulative number of days above the long-term December median

streamflow threshold with monthly mean precipitation for coastal river basins in Maine ................................ 23

19. General summary of correlation results...................................................................................................................... 25
Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

mile (mi) 1.609 kilometer (km)

Area

square foot (ft2) 0.009290 square meter (m2)

square mile (mi2) 2.590 square kilometer (km2)

Volume

cubic foot (ft3) 0.02832 cubic meter (m3)

Flow rate

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

cubic foot per second per square mile 

[(ft3

/s)/mi2] 0.01093 cubic meter per second per square kilometer 

[(m3

/s)/km2]

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) unless otherwise noted.

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

Trends in Timing, Magnitude, and Duration of Summer and 

Fall/Winter Streamflows for Unregulated Coastal River 

Basins in Maine During the 20th Century

by Robert W. Dudley and Glenn A. Hodgkins

Abstract

The U.S. Geological Survey (USGS), in cooperation 

with the Maine Atlantic Salmon Commission (ASC), began a 

study in 2003 to examine the timing, magnitude, and duration 

of summer (June through October) and fall/early winter 

(September through January) seasonal streamflows of 

unregulated coastal river basins in Maine and to correlate them 

to meteorological variables and winter/spring (January through 

May) seasonal streamflows. This study overlapped the summer 

seasonal window with the fall/early winter seasonal window to 

completely bracket the low-streamflow period during July, 

August, and September between periods of high streamflows in 

June and October. The ASC is concerned with the impacts of 

potentially changing meteorological and hydrologic conditions 

on Atlantic salmon survival. Because winter/spring high 

streamflows appear to have trended toward earlier dates over 

the 20th century in coastal Maine, it was hypothesized that the 

spring/summer recession to low streamflows could have a 

similar trend toward earlier, and possibly lower, longer lasting, 

late summer/early fall low streamflows during the 20th 

century. 

There were few statistically significant trends in the 

timing, magnitude, or duration of summer low streamflows for 

coastal river basins in Maine during the 20th century. The 

hypothesis that earlier winter/spring high streamflows may 

result in earlier or lower low streamflows is not supported by 

the data. No statistically significant trends in the magnitude of 

total runoff volume during the low-streamflow months of 

August and September were observed. The magnitude and 

timing of summer low streamflows correlated with the timing 

of fall/winter high streamflows and the amount of summer 

precipitation. The magnitude and timing of summer low 

streamflows did not correlate with the timing of spring 

snowmelt runoff. There were few correlations between the 

magnitude and timing of summer low streamflows and 

monthly mean surface air temperatures.

There were few statistically significant trends in the 

timing or duration of fall/winter high streamflows for coastal 

river basins in Maine during the 20th century. The timing of 

the bulk of fall/winter high streamflows correlated with 

seasonal precipitation. Earlier fall/winter center-of-volume 

dates correlated with higher September and October 

precipitation. In general, little evidence was observed of trends 

in the magnitude of seasonal runoff volume during fall/winter. 

The magnitude of fall/winter high streamflows positively 

correlated with November and December precipitation 

amounts. There were few correlations between the magnitude 

and timing of fall/winter high streamflows and monthly mean 

surface air temperatures.

Introduction

Combined land-surface air and sea-surface temperature 

data, presented in the Third Assessment Report of the 

Intergovernmental Panel on Climate Change (IPCC) (2001), 

show an overall increasing trend in annual surface air 

temperature for New England for the period 1901-2000. 

Greatest seasonal warming rates for New England during 

1976-2000 were during the winter months of December, 

January, and February (Intergovernmental Panel on Climate 

Change, 2001). Given these changes in air temperature and the 

demonstrated sensitivity of snowmelt to surface air 

temperature (Hartley and Dingman, 1993), the U.S. Geological 

Survey (USGS) and the Maine Atlantic Salmon Commission 

(ASC) did a cooperative study from 2001 to 2002 to determine 

whether there were statistically detectable changes in 

streamflow for unregulated coastal basins in Maine. The 

investigation revealed historical trends in streamflow, river ice, 

and snowpack consistent with a trend toward an earlier onset of 

spring conditions during the 20th century in coastal Maine 

(Dudley and Hodgkins, 2002). Dudley and Hodgkins (2002) 

documented a significant change in the timing of spring runoff 

toward earlier dates for an extended coastal streamflow record 

spanning 1906-1921 and 1929-2000. Coastal river streamflowgaging stations showed, in general, significantly earlier last 

ice-off dates in the spring and a statistically significant 

decrease over time in the total number of days of ice-affected 

streamflow at most stations on coastal rivers in Maine. The 

longest, most complete snow records indicated an increase in 

snowpack density for the March 1 snow-survey date during 

1938-2000. 

Many studies of snowmelt-dominated river basins in 

western North America show seasonal shifts to greater winter 

streamflow and reduced summer streamflow (Cohen and 
2 Trends in Timing, Magnitude, and Duration of Summer and Fall/Winter Streamflows for Unregulated Coastal River Basins 

in Maine During the 20th Century

Miller, 2001). Similar changes in streamflow regime have been 

documented in historical river flows in southern British 

Columbia (Leith and Whitfield, 1998), and were modeled for 

four rivers in California (Lettenmaier and Gan, 1990) and for a 

river in Switzerland (Brubaker and Rango, 1996). These last 

three studies found lower summer low streamflows associated 

with increasing air temperatures.

The basins studied in British Columbia, California, and 

Switzerland have substantial summer snowmelt runoff, 

whereas snowmelt runoff in New England does not directly 

contribute appreciable quantities of water to summer 

streamflows. Spring snowmelt in northern New England, 

however, is important to ground-water recharge. Following 

spring recharge, ground water discharges into New England 

streams (as base flow) throughout the summer. Dudley (2004) 

found that the percentage of sand and gravel aquifers 

underlying Maine drainage basins is a significant explanatory 

variable for monthly mean streamflows for July through 

October (basins with more sand and gravel aquifers are 

associated with higher streamflows). This indicates the 

importance of base flow to summer and early fall streamflows. 

A base-flow recession from spring to late summer is typical in 

New England. With snowmelt occurring earlier (inferred from 

earlier high spring streamflows) (Dudley and Hodgkins, 2002), 

the base-flow recession may start earlier. This could lead to a 

longer period of low summer streamflow recession and lower 

minimum streamflows. Querner and others (1997), using a 

physically based model, found increased deficit volumes (the 

amount of water below a threshold streamflow) with increased 

air temperatures for a river that drains parts of southwestern 

Norway and for a river that drains parts of the Netherlands and 

Belgium.

The ASC is concerned with the impacts of potentially 

changing meteorological and hydrologic conditions on Atlantic 

salmon survival (Joan Trial, Maine Atlantic Salmon 

Commission, oral commun., 2002). To understand the 

potential effects of streamflow timing and quantity on survival 

of Atlantic salmon and other fish species in coastal Maine, it is 

important to characterize seasonal hydrologic conditions and 

whether they are (or not) changing over time. Low streamflows 

in the summer can result in high water temperatures and low 

concentrations of dissolved oxygen, stressing resident biota 

(Bradbury and others, 2002). Likewise, cold temperatures and 

ice formation during winter can be stressful to biota who reside 

in streams over winter. Surface ice can provide protective 

insulation between the stream and the air, but, in the absence of 

surface-ice cover, anchor ice can form. Anchor ice is an 

accumulation of ice crystals or slush adhering to riverbed 

material and can have serious effects on fish eggs developing 

within gravel beds (Prowse, 1994). 

The U.S. Geological Survey (USGS), in cooperation 

with the Maine Atlantic Salmon Commission (ASC), began a 

study in 2003 to examine the timing, magnitude, and duration 

of summer (June through October) and fall/early winter 

(September through January) seasonal streamflows of 

unregulated coastal river basins in Maine and to correlate them 

to meteorological variables and winter/spring (January through 

May) seasonal streamflow. This study overlapped the summer 

seasonal window with the fall/early winter seasonal window to 

completely bracket the low-streamflow period during July, 

August, and September between periods of high streamflows in 

June and October. 

Purpose and Scope

The purpose of this report is to specifically address the 

hypothesis that because winter/spring high streamflows appear 

to be occurring earlier in coastal Maine, base flow recessions 

could also be occurring earlier, resulting in a trend toward 

earlier, and possibly lower, longer lasting, summer low 

streamflows. This report presents the data and methods used to 

determine whether there are trends in the timing and (or) 

amount of streamflow during the summer and fall/winter for 

coastal river basins in Maine and whether these streamflow 

conditions are interseasonally correlated. The trend and 

correlation results presented in this report quantify whether 

seasonal changes observed in winter-spring hydrology relate to 

changes in summer low streamflows. The report also presents 

the data, methods, and results of correlation tests between 

seasonal streamflow statistics and meteorological data.

Description of the Study Area

The study area covers all unregulated coastal river basins 

in Maine that either currently are gaged or have been gaged by 

the USGS for more than 10 years (table 1, fig. 1). Six basins 

met these criteria; the basins are heavily forested and are 

characterized by low-relief rolling topography with little 

development. The coastal river basins lie in a 

hydrophysiographic region of broad lowlands that were 

inundated by the ocean during deglaciation (Randall, 2000). 

Consequently, the surficial geologic materials in the basins are 

predominantly glacial till, fine- and coarse-grained 

glaciomarine deposits, ice-contact glaciofluvial deposits, 

eskers, and bedrock (Thompson and Borns, 1985).

The greatest changes in land use in Maine have been the 

replacement of agriculture and pasture lands by forest during 

the 20th century. The overall forest cover in Maine, estimated 

at 67 percent in 1900, increased to 90 percent by 1995 (Irland, 

1998). Four of the six basins are in close proximity to each 

other in eastern coastal Maine (West Branch Union, 

Narraguagus, Machias, and East Machias, fig. 1)—a sparsely 

populated part of Maine. During the period 1880-1995, 

forested area in eastern coastal Maine increased from about 85- 

percent to 90-percent forested area (Irland, 1998). The 

remaining two river basins, Sheepscot and Royal, are in a 
Introduction 3

Figure 1. Locations of gaged coastal river basins, Maine.
U.S. Geological Survey streamflow-gaging station Latitude 

(north)

Longitude 

(west)

Drainage 

area 

(square 

miles)

Period of 

streamflow record 

used in this 

investigation1 Number Name

01021500 Machias River at Whitneyville 44°43’ 23” 67°31’ 15” 458 1906-21, 1930-77

01022000 East Machias River near East Machias 44°46’ 05” 67°24’ 30” 251 1927-58

01022500 Narraguagus River at Cherryfield 44°36’ 29” 67°56’ 10” 227 1948-2001

01023000 West Branch Union River at Amherst 44°50’ 25” 68°22’ 22” 148 1910-19, 1929-79

01038000 Sheepscot River at North Whitefield 44°13’ 23” 69°35’ 38” 145 1939-2001

01060000 Royal River at Yarmouth 43°47’ 57” 70°10’ 45” 141 1950-2001

4 Trends in Timing, Magnitude, and Duration of Summer and Fall/Winter Streamflows for Unregulated Coastal River Basins 

in Maine During the 20th Century

historically more populated part of Maine that was more 

heavily deforested during the late 19th and early 20th 

centuries. Since that time, forested area in the vicinity of the 

Sheepscot and Royal River Basins has increased from 

approximately 50-percent to 75-percent forested area (Irland, 

1998).

The population of Maine in 2000 was about 1.3 million 

people—a 72-percent increase since 1910 (U.S. Census 

Bureau, 2002; University of Maine, 2004). Population in 

counties in eastern coastal Maine (vicinity of West Branch 

Union, Narraguagus, Machias, and East Machias River Basins, 

fig. 1) decreased 18 percent from 1910 to 1970, and increased 

33 percent from 1970 to 2000, with a net population growth of 

9 percent from 1910 to 2000. Population growth in the vicinity 

of the Sheepscot River Basin from 1910 to 2000 was about 68 

percent; population growth during the period that streamflow 

gaging was done on the Sheepscot River (from 1940 to 2000) 

was about 87 percent. Population growth in the vicinity of the 

Royal River Basin from 1910 to 2000 was about 137 percent; 

population growth during the period that streamflow gaging 

was done on the Royal River (from 1950 to 2000) was about 57 

percent (U.S. Census Bureau, 2002; University of Maine, 

2004).

The climate of Maine’s coast is typified by mild 

summers and cold winters. Records from National Weather 

Service (NWS) stations indicate mean annual surface air 

temperatures from 1971 to 2000 of 45.7 °F in Portland and 

44.1 °F in Eastport (fig. 1). Mean monthly surface air 

temperatures range from 21.7 °F in January to 68.7 °F in July 

in Portland, and from 22.1 °F in January to 64.0 °F in July in 

Eastport. The mean annual precipitation is 45.8 in. (inches) at 

Portland and 44.8 in. at Eastport and is fairly evenly distributed 

throughout the year (National Oceanic and Atmospheric 

Administration, 2002). 

Proximity to the Gulf of Maine provides a moderating 

effect on surface air temperatures throughout the year. In 

general, coastal locations are warmer in the winter than inland 

locations. For comparison to the coastal locations of Portland 

and Eastport, mean annual surface air temperatures (for 1971-

2000) at the inland towns of Dover-Foxcroft and Farmington 

(fig. 1) are 40.3 °F and 41.7 °F, respectively (fig. 1). Mean 

January surface air temperatures are 12.1 °F in Dover-Foxcroft 

and 14.4 °F in Farmington (National Oceanic and Atmospheric 

Administration, 2002). The distance of a river basin from the 

coast is a significant explanatory variable in regression 

equations used to estimate monthly streamflows in Maine 

(Dudley, 2004). During December, January, February, and 

March, the regression equations indicate that higher monthly 

streamflows, on a per unit-area basis, correspond with shorter 

distances from the coast. The relation reverses in May when 

higher streamflows occur in basins farther from the coast; this 

is consistent with colder, inland river basins storing more water 

in snowpack during the winter and releasing it later in the 

spring (Dudley, 2004).

The largest streamflows in coastal Maine typically occur 

in the late winter (March and April) and spring (May and June) 

when rain falls on a dense (ripe) snowpack or on saturated soils 

(fig. 2). Streamflows then recede as snowmelt ends and 

evapotranspiration increases. The recession typically persists 

into late summer (August-September) because of high 

evapotranspiration. Streamflow in late summer is dominated 

by ground-water discharge and is frequently augmented by 

runoff from rainfall events. As evapotranspiration decreases in 

the fall (October-November), streamflow increases. Repeated 

rainfall events and the occasional contribution of hurricanerelated precipitation can result in high streamflows. Low 

streamflows can occur during the winter (December-February) 

if precipitation and surface water is frozen for extended periods 

of time.

Table 1. Coastal streamflow-gaging stations in Maine used in this investigation.

1 Period of streamflow record defined in water years; the term “water year” denotes the 12-month period from October 1 to September 30 and is 

designated by the calendar year in which it ends and which includes 9 of the 12 months.
Streamflow, Precipitation, and Surface Air Temperature Data 5

Streamflow, Precipitation, and Surface Air 

Temperature Data

Streamflow, precipitation, and air temperature data sets 

were assembled for historical trend and correlation analyses as 

described below.

Streamflow Data

Streamflow data from six coastal streamflow-gaging 

stations in the study area (table 1, fig. 1) were retrieved from 

the National Water Information System (NWIS) (U.S. 

Geological Survey, 1998). Periods of record of streamflow 

data for the six stations range from 32 to 65 years in length, 

with a mean of 56 years. The earliest and latest dates for 

streamflow data used in this study are October 1, 1905, and 

September 30, 2001, respectively. 

Because particular focus is placed on the analysis of low 

streamflows in this investigation, a careful evaluation of 

regulation during low-streamflow periods at each streamflowgaging station was done. In order for trend analyses of 

continuous streamflows records to represent changes over time 

of natural conditions, it is important that the records represent 

unregulated streamflow. Unregulated streamflow is defined as 

Figure 2. Average annual hydrograph for six rivers in coastal Maine. The rivers include Machias at Whitneyville, East Machias near East 

Machias, Narraguagus at Cherryfield, West Branch Union at Amherst, Sheepscot at North Whitefield, and Royal at Yarmouth.
6 Trends in Timing, Magnitude, and Duration of Summer and Fall/Winter Streamflows for Unregulated Coastal River Basins 

in Maine During the 20th Century

streamflow substantially unaffected by diversions (such as 

pumping) and (or) regulation by dams or reservoirs, and is 

described in greater detail below.

The presence of regulation at each station is qualitatively 

documented in the annual USGS state Water-Data Reports and 

their predecessor Water-Supply Papers. Two stations, Machias 

River at Whitneyville (U.S. Geological Survey streamflowgaging station number 01021500) and East Machias River near 

East Machias (01022000), have or historically have had some 

minor low-streamflow regulation. The low-streamflow 

regulation at these sites has been large enough as to have a 

transient effect on the magnitude of daily mean streamflows; 

however, the regulation at these sites is thought to be small 

enough as to have no effect on the computation of statistics (on 

the basis of daily mean streamflow) for monthly or longer 

timescales (such as the monthly mean) (Slack and Landwehr, 

1992). On the basis of the above criteria, submonthly statistics 

for the Machias River (01023000) and East Machias River 

(01038000) were not computed. The remaining four stations, 

Narraguagus River at Cherryfield (01022500), West Branch 

Union River at Amherst (01023000), Sheepscot River at North 

Whitefield (01038000), and the Royal River at Yarmouth 

(01060000) have little or no regulation, and, in general, daily 

and longer averages of streamflow are thought to be 

representative of natural conditions (Slack and Landwehr, 

1992).

Hydrograph plots of summer/fall and fall/winter 

streamflows during the periods of records at all the stations 

were examined to further evaluate the occurrence of substantial 

regulation. Substantial regulation was defined as regulation 

that caused the minimum or maximum daily streamflow for a 

season. If more than 10 percent of the period of record (3 to 6 

years) for a station was suspected to be affected by regulation, 

the statistics based on timescales less than 1 month were not 

computed for that site (for example, the minimum daily 

streamflow, or the 7-day mean low streamflow). On the basis 

of the above criteria, submonthly statistics for the West Branch 

Union River (01023000) and Sheepscot River (01038000) 

were not computed. All statistics were computed for the 

Narraguagus River (01022500) and Royal River (01060000). 

Summer streamflow records in 1978 and 1985 were censored 

for the Narraguagus River (01022500) due to documented 

unusual regulation. Summer streamflow record in 1980 and 

winter streamflow record in 1956 were censored for the Royal 

River (01060000) due to suspected unusual regulation. 

All six river basins are forested and rural, with little 

change in land use over the 20th century other than 

reforestation in the vicinity of the Sheepscot and Royal River 

Basins. The effects of reforestation on the timing and 

magnitude of streamflows for these basins are unknown.

Precipitation and Surface Air Temperature Data

Precipitation and surface air temperature time series 

used in this investigation were obtained from the U.S. 

Historical Climatology Network (USHCN) data set developed 

and maintained at the National Climatic Data Center (NCDC) 

(table 2, fig. 1) (Karl and others, 1990). The USHCN data have 

been subjected to quality control and homogeneity testing. 

Precipitation data have been adjusted for bias due to changes in 

station location and other station changes over time (Karl and 

Williams, 1987). Temperature data have been adjusted for bias 

originating from changes in observation time (Karl and others, 

1986), instrumentation (Quayle and others, 1991), station 

location and other changes (Karl and Williams, 1987), and 

urban heat island effects (Karl and others, 1988). Only data 

from USHCN stations within 30 mi (miles) of any study basin 

centroid were used.

Analysis of Streamflow, Precipitation, and 

Surface Air Temperature Data

Temporal trends were analyzed for all data sets 

discussed in this section using the non-parametric MannKendall test (Helsel and Hirsch, 1992). The data were 

smoothed for graphical presentation and serial correlation 

testing by use of locally weighted regression (LOESS) 

(Cleveland and Devlin, 1988) with local linear fitting and a 

robustness feature. There must be no serial correlation for the 

p-values from the Mann-Kendall test to be correct. Serial 

correlations in the trend tests were analyzed by computing the 

Durbin-Watson statistic on the residuals of the LOESS 

regressions for each data set with a significant temporal trend 

(p-value less than 0.10) in any category. In this report, a pvalue less than or equal to 0.10 is referred to as significant, and 

a p-value less than or equal to 0.01 is referred to as highly 

significant. 

Correlation tests were done using Pearson's productmoment correlation, which is a measure of the linear 

association between two variables (Helsel and Hirsch, 1992). 

The value of Pearson’s “r” statistic lies between -1 and +1. An 

r-value greater than 0 indicates a positive linear relation 

between variables, and an r-value less than 0 indicates a 

negative linear relation between variables. The statistical 

significance of any Pearson's product-moment correlation is 

indicated by an associated p-value.

A modified Julian date system was used for all timing 

analyses in this investigation. The date-numbering system used 

in this investigation begins date numbering on April 1. This 

numbering system was used in order to capture the entire 

summer period (beginning in June) and subsequent fall/winter 

period (ending in January) in a single, sequentially numbered 

date system. 
Analysis of Streamflow, Precipitation, and Surface Air Temperature Data 7

Streamflow

Daily mean streamflows for individual USGS gaging 

stations are the original data used for this study. Each daily 

mean streamflow data set was used to construct seasonal data 

sets described below. In addition to these data sets, an 

extended-record data set was constructed using streamflow 

records from the Narraguagus and Machias Rivers to examine 

trends in a single streamflow record spanning the 96-year 

period 1906-2001. The method of streamflow data extension, 

called MOVE.1 (Maintenance Of Variance Extension, type 1), 

uses the relation between the common data period of two 

streamflow records and produces streamflow estimates with a 

statistical distribution similar to that expected had the 

streamflow actually been measured (Helsel and Hirsch, 1992). 

In the case of the Narraguagus and Machias River data sets, 

their common 30-year period of 1948-77 was used to develop 

the relation between the stations (fig. 3). Using this relation, 

the Narraguagus record was extended backwards to cover the 

additional years of 1906-21 and 1929-47, thus generating a 

streamflow data set that spans from 1906 to 1921 and from 

1929 to 2001. The Narraguagus Extended Record is referred to 

as NER in the remainder of this report.

The linear relation between the streamflow at the 

Machias and Narraguagus Rivers has an overall Pearson’s r 

correlation coefficient of 0.93 (fig. 3). It is suspected that the 

relation of low streamflows between the two rivers exhibits 

some non-linearity and greater variability than the relation at 

the higher range of streamflows (fig. 3) due to the minor 

regulation of low streamflows on the Machias River and the 

absence of regulation at Narraguagus. Statistics based on 

timescales less than 1 month were not computed for the NER 

because of the variability in the relation between the Machias 

and Narraguagus Rivers for low streamflows.

To examine trends over time in the timing, magnitude, 

and duration of summer and fall/winter streamflows, daily 

mean streamflow data were partitioned into two temporal 

windows. The summer window was defined as June 1 to 

October 31, and the fall/winter window was defined as 

September 1 to January 31. The dates June 1 and October 31 

bracket the summer low-streamflow period between two 

periods of time that have characteristically high streamflows 

(fig. 2). The long-term daily mean streamflows for each station 

were normalized to their respective drainage areas (to yield 

daily mean cubic feet per second per square mile) and then 

arithmetically averaged across all stations to produce the graph 

shown in figure 2.

The fall/winter period was defined as September 1 to 

January 31. These dates bracket the fall/winter streamflow 

period between the period of lowest streamflow during the year 

(late August - early September) and the winter low-streamflow 

period that occurs roughly during January and February as 

precipitation and surface water freezes and substantial amounts 

of water are stored in snow pack (fig. 2). Although this is 

usually the period of lowest streamflow during the winter, 

Dudley and Hodgkins (2002) noted that, in general, low 

streamflows during December, January, and February have 

increased over the 20th century for coastal rivers in Maine.

Streamflow timing and magnitude statistics for the 

remainder of the year (February to May), during which the 

bulk of snowmelt runoff occurs, were not derived for this 

investigation. However, the winter/spring seasonal center-ofvolume statistics for the timing of snowmelt runoff derived by 

Dudley and Hodgkins (2002) in an earlier study of coastal river 

basins in Maine were used in this investigation in correlation 

tests with summer low-streamflow statistics.

The winter/spring seasonal center-of-volume timing 

statistic, used by Dudley and Hodgkins (2002) and also applied 

by Hodgkins and others (2003), was computed by summing the 

daily streamflow volumes from the start to the end of the 

snowmelt runoff season. The seasonal center-of-volume date 

(CVD) is the first date, from the beginning of the season, on 

which at least half the total seasonal volume of water flows by 

a streamflow-gaging station. The winter/spring CVD is a 

useful variable for examining streamflow trends over time 

because it is more resistant to noise in the data than the peakstreamflow date and is thought to correspond more closely to 

the bulk of the spring snowmelt. The CVD also is more 

sensitive to changes in the timing of streamflow than is the 

percentage of streamflow occurring in one or more fixed 

months (Court, 1962). The winter/spring seasonal window 

used by Dudley and Hodgkins (2002) was January 1 to May 

31. Because of the interest in describing the timing of the bulk 

of peak streamflows of the fall/winter time period for coastal 

Table 2. U.S. Historical Climatology Network stations in Maine used in this 

investigation.

U.S. Historical Climatology Network station Latitude 

(north)

Longitude 

Number Location (west)

170100 Acadia National Park 44°21' 00” 68°16' 12”

172426 Eastport 44°55' 12” 67°00' 00”

173046 Gardiner 44°13' 12” 69°46' 48”

174566 Lewiston 44°06' 00” 70°13' 12”

176430 Orono 44°54' 00” 68°40' 12”

176905 Portland 43°39' 00” 70°18' 00”

179891 Woodland 45°09' 00” 67°24' 00”
8 Trends in Timing, Magnitude, and Duration of Summer and Fall/Winter Streamflows for Unregulated Coastal River Basins 

in Maine During the 20th Century

river basins in Maine (fig. 2), the CVD was computed as a 

timing statistic for the fall/winter period, September 1 to 

January 31.

As part of this investigation into the timing of summer 

low streamflows, a timing statistic similar to the CVD was 

derived. In the case of characterizing the timing of the bulk of 

low streamflows, the inverse of all daily mean streamflow 

volumes from June 1 to October 31 was computed. By 

transforming the summer streamflow data in this manner, the 

summer hydrograph was effectively inverted and the timing of 

the new inverted peak streamflows corresponded to the timing 

of the bulk of low streamflows. The inverse volumes were 

summed, and the date by which half of this total inverse 

volume occurred was computed for each year at each 

streamflow-gaging station. This date is referred to as the 

inverse center-of-volume date (ICVD).

Other timing and streamflow-magnitude statistics 

derived for this investigation involved the averaging of daily 

mean streamflows over time scales of various lengths. For 

example, the 7-day-low streamflow is the lowest mean 

streamflow arithmetically averaged over a consecutive 7-day 

period within a given seasonal window. The date of the 7-daylow streamflow is the date at the center of the 7-day 

period—that is, day 4 of 7. The dates and mean streamflow 

magnitudes of the 1-day, 7-day, 15-day, 31-day, and 45-day 

low streamflows were computed for each summer period, for 

each year, for each station. Due to the mean shape of the fall/

winter hydrograph for coastal river basins in Maine (fig. 2), the 

dates and streamflow magnitudes of the 1-day, 7-day, 15-day, 

Figure 3. Maintenance of Variance Extension, type 1 (MOVE.1) relation between the streamflow at Machias and Narraguagus Rivers, 

Maine.

10 100 1,000 10,000 100,000

STREAMFLOW, IN CUBIC FEET PER SECOND, MACHIAS RIVER AT WHITNEYVILLE, MAINE

1

10

100

1,000

10,000

Correlation coefficient = 0.93

STREAMFLOW, IN CUBIC FEET PER SECOND

NARRAGUAGUS RIVER AT CHERRYFIELD, MAINE

Concurrent daily-mean streamflow

February 1948 to September 1977

MOVE.1 line

10,000

1,000

100

1

10

10 100 1,000 10,000 100,000

STREAMFLOW, IN CUBIC FEET PER SECOND

NARRAGUAGUS RIVER AT CHERRYFIELD, MAINE

STREAMFLOW, IN CUBIC FEET PER SECOND, MACHIAS RIVER AT WHITNEYVILLE, MAINE

Correlation coefficient = 0.93

Concurrent daily-mean streamflow

February 1948 to September 1977

MOVE.1 line
Analysis of Streamflow, Precipitation, and Surface Air Temperature Data 9

31-day, and 45-day peak streamflows were computed for each 

fall/winter period, for each year, for each station. As mentioned 

previously, statistics for time periods less than 31 days were 

not computed for stations with more than 10-percent of their 

record containing unusual regulation during low- or highstreamflow periods. Results of trend tests on streamflow 

statistics computed over timescales of 1, 7, and 15 days were 

intended to provide mutual corroboration; it was also 

hypothesized that longer timescales (7 and 15 days) may be 

more resistant to variability in the data. Streamflow statistics 

computed over timescales of 31 and 45 days were intended to 

provide common magnitude and timing statistics for all sites, 

providing a basis for comparison for sites for which 

submonthly statistics were censored.

Threshold statistics also were used as a measure of 

seasonal timing of streamflow. For the summer seasonal 

window, the long-term August median streamflow was used as 

a threshold statistic. The following threshold timing statistics 

were derived: the first date streamflow was equal to or less 

than the threshold; the last date streamflow was equal to or less 

than the threshold; the cumulative number of days that 

streamflow was equal to or less than the threshold; and the 

number of days between the first and last threshold dates, 

inclusive of those dates. For the purposes of trend testing, if 

streamflow was never equal to or less than the threshold 

streamflow during a given season in a given year, the first date 

was given a value of one day later than the latest first date for 

that station; the last date was given a value of one day earlier 

than the earliest last date for that station; the number of days 

between the first and last threshold dates was computed as 

usual; and the cumulative number of days that streamflow was 

equal to or less than the threshold was zero.

Similarly, the long-term December median was used as a 

threshold statistic for the fall-winter period because 

streamflows generally peak in December within this seasonal 

window (fig. 2). Fall/winter threshold timing statistics included 

the first date streamflow was equal to or greater than the 

threshold; the last date streamflow was equal to or greater than 

the threshold; the cumulative number of days that streamflow 

was equal to or greater than the threshold; and the number of 

days between the first and last threshold dates, inclusive of 

those dates. For years when streamflow was never equal to or 

greater than the threshold streamflow during a given year, 

statistics were handled similarly to those for the summer 

season described above. 

A single fall-season October threshold statistic was 

computed to investigate a possible lengthening of the latesummer/early-fall low-streamflow period. The first date within 

the fall/winter seasonal window on which the streamflow 

exceeded the long-term October median was used as an 

indicator for the end of the late-summer/early-fall lowstreamflow period.

Finally, total seasonal runoff volumes were computed to 

investigate trends in total seasonal streamflow. The fall/winter 

seasonal volume was computed by summing the daily mean 

streamflow volumes from the start to the end of the fall/winter 

seasonal window (September 1 to January 31). The summer 

seasonal volume was computed by summing the daily mean 

streamflow volumes from August 1 to September 30 only. The 

abbreviated seasonal window for summer total volume was 

used because high streamflows at the beginning and end of the 

summer seasonal window (fig. 2) would otherwise dominate 

the magnitude of the runoff volume, masking variability in low 

streamflows.

The Mann-Kendall trend test was used to test for 

significant changes over time in all of the aforementioned 

variables. Correlation tests between timing and total runoff 

volumes between seasons were done to investigate 

interseasonal relations of streamflow.

Precipitation and Surface Air Temperature

Monthly precipitation and surface air temperature data 

from the U.S. Historical Climatology Network (USHCN) 

stations were tested for correlation with streamflow timing and 

magnitudes for all streamflow-gaging stations. Only 

meteorological data from USHCN stations within 30 mi of a 

coastal river drainage-basin centroid were correlated with 

streamflow data for the corresponding USGS streamflowgaging station (table 3). If more than one USHCN station was 

Table 3. Pairing of U.S. Geological Survey coastal streamflow-gaging stations and U.S. Historical Climatology Network 

stations for correlation testing.

U.S. Geological Survey streamflow-gaging station U.S. Historical Climatology Network station(s)

Number Name Number(s) Location(s)

01021500 Machias River at Whitneyville 179891 Woodland

01022000 East Machias River near East Machias 179891, 172426 Woodland, Eastport

01022500 Narraguagus River at Cherryfield 176430, 170100 Orono, Acadia National Park

01023000 West Branch Union River at Amherst 176430 Orono

01038000 Sheepscot River at North Whitefield 173046 Gardiner

01060000 Royal River at Yarmouth 174566, 176905 Lewiston, Portland -- Narraguagus Extended Record (NER) 176430, 170100 Orono, Acadia National Park
10 Trends in Timing, Magnitude, and Duration of Summer and Fall/Winter Streamflows for Unregulated Coastal River Basins 

in Maine During the 20th Century

within 30 mi of the drainage-basin centroid, the meteorological 

data were aggregated using a distance-weighted algorithm with 

the nearby stations having more weight than more distant 

stations (equation 1, fig. 1). 

The algorithm for aggregating meteorological data 

weighted the data such that the sum of the weights for all of the 

aggregated stations was equal to one:

W1 + W2 + ... + Wn = 1 (1)

Where,

Wn is the weight for station n, and is a function of the 

inverse square of the station distance from the basin centroid.

Trends in Timing, Magnitude, and Duration 

of Summer and Fall/Winter Streamflows for 

Unregulated Coastal River Basins in Maine 

During the 20th Century

Trend test results and correlation results are presented 

and discussed in the following sections for each data set in this 

investigation. Trend test results calculated on the basis of 

individual streamflow records are referenced on a site-by-site 

basis. Generalizations about overall trends or hydrologic 

mechanisms rely primarily on the Narraguagus Extended 

Record (NER), with individual stations providing supporting 

information for the time period(s) they cover.

Trends in Summer Streamflows

Only one statistically significant trend in the timing or 

duration of summer low streamflows for coastal rivers in 

Maine was observed in this investigation (tables 4 and 5). The 

date of the 31-day low streamflow for the West Branch Union 

River had a statistically significant trend toward earlier dates 

with a p-value of 0.046 (table 4). Trend test results indicate the 

duration of the summer low-streamflow period has not 

changed significantly for coastal river basins in Maine during 

the 20th century, and no statistically significant trends were 

observed in the first date, last date, and cumulative number of 

days streamflow has been less than or equal to the long-term 

August median streamflow (table 5).

No statistically significant trends in total summer runoff 

volume for the months of August and September were 

observed (table 6). LOESS smooth plots illustrate the general 

pattern over time in August-September runoff volumes and 

indicate runoff volumes reached their lowest during the period 

1945-65 (fig. 4). Only one statistically significant trend in the 

magnitude of summer low streamflows for coastal rivers in 

Maine was observed in this investigation (table 6). Although 

nearly all trend test results indicate negative trends (toward 

lower streamflows), only the trend in the magnitude of the 7-

Table 4. Statistical significance (p-values) and trend direction for summer (June 1 to October 31) streamflow 

timing statistics for coastal river basins in Maine.

[ICVD, inverse center-of-volume date; Low1D, date of the low 1-day mean streamflow; Low7D, date of the low 7-day 

mean streamflow; Low15D, date of the low 15-day mean streamflow; Low31D, date of the low 31-day mean 

streamflow; Low45D, date of the low 45-day mean streamflow; p-values in bold are significant at less than or equal to 

0.10; p-values in bold and underlined are highly significant at less than or equal to 0.01; +, trend over time toward later 

dates; -, trend over time toward earlier dates; *, statistic not tested]

U.S. Geological Survey 

streamflow-gaging station

Number Name ICVD Low1D Low7D Low15D Low31D Low45D

01021500 Machias River at 

Whitneyville

0.903 + * * * 0.893 - 0.566 +

01022000 East Machias River 

near East Machias

0.268 + * * * 0.404 + 0.496 +

01022500 Narraguagus River 

at Cherryfield

0.467 + 0.740 - 0.716 + 0.597 + 0.664 + 0.710 +

01023000 West Branch Union 

River at Amherst

0.311 - * * * 0.046 - 0.260 -

01038000 Sheepscot River at 

North Whitefield

0.156 - * * * 0.322 - 0.176 -

01060000 Royal River at 

Yarmouth

0.337 + 0.490 + 0.935 + 0.490 - 0.150 + 0.480 +

-- Narraguagus 

Extended Record 

(NER)

0.360 - * * * 0.370 - 0.253 -
Trends in Timing, Magnitude, and Duration of Summer and Fall/Winter Streamflows for Unregulated Coastal River Basins in 

Maine During the 20th Century 11

Figure 4. Trends in total runoff volume (August 1 to September 30) for six coastal rivers in Maine and the Narraguagus 

Extended Record (NER) (dashed line). LOESS regression lines are based on a 45-year weighting window.

Table 5. Statistical significance (p-values) and trend direction for summer (June 1 to 

October 31) streamflow threshold statistics for coastal river basins in Maine.

[Threshold streamflow for each station is the long-term August median; FD, first date 

streamflow is at or below the threshold; LD, last date streamflow is at or below threshold; 

DD, number of days between the FD and LD, inclusive; Cum, cumulative number days in the 

season streamflow is at or below the threshold; p-values in bold and underlined are highly 

significant at less than or equal to 0.01; +, trend over time toward later dates; -, trend over 

time toward earlier dates; *, statistic not tested]

U.S. Geological Survey streamflowgaging station

Number Name FD LD DD Cum

01022500 Narraguagus River at 

Cherryfield

0.613 + 0.692 - 0.312 - 0.630 -

01060000 Royal River at 

Yarmouth

0.295 - 0.974 - 0.764 + 0.431 +
12 Trends in Timing, Magnitude, and Duration of Summer and Fall/Winter Streamflows for Unregulated Coastal River Basins 

in Maine During the 20th Century

Table 6. Statistical significance (p-values) and trend direction for summer (June 1 to October 31) streamflow 

magnitude statistics for coastal river basins in Maine.

[TotVol, total runoff volume from August 1 to September 30; Low1Q, low 1-day mean streamflow; Low7Q, low 7-day 

mean streamflow; Low15Q, low 15-day mean streamflow; Low31Q, low 31-day mean streamflow; Low45Q, low 45-

day mean streamflow; p-values in bold are significant at less than or equal to 0.10; p-values in bold and underlined are 

highly significant at less than or equal to 0.01; +, trend over time toward higher flows; -, trend over time toward lower 

flows; *, statistic not tested]

U.S. Geological Survey 

streamflow-gaging station

Number Name TotVol Low1Q Low7Q Low15Q Low31Q Low45Q

01021500 Machias River at 

Whitneyville

0.948 - * * * 0.789 + 0.944 -

01022000 East Machias River 

near East Machias

0.721 - * * * 0.386 - 0.465 -

01022500 Narraguagus River 

at Cherryfield

0.747 + 0.794 - 0.956 - 0.758 - 0.636 - 0.813 -

01023000 West Branch Union 

River at Amherst

0.896 + * * * 0.702 - 0.697 -

01038000 Sheepscot River at 

North Whitefield

0.698 + * * * 0.831 - 0.776 -

01060000 Royal River at 

Yarmouth

0.937 - 0.145 - 0.096 - 0.155 - 0.139 - 0.151 -

-- Narraguagus 

Extended Record 

(NER)

0.288 - * * * 0.243 - 0.248 -
Trends in Timing, Magnitude, and Duration of Summer and Fall/Winter Streamflows for Unregulated Coastal River Basins in 

Maine During the 20th Century 13

day low streamflow for the Royal River had a statistically 

significant result (p-value of 0.096). 

Trends in Fall/Winter Streamflows 

The October median streamflow threshold date was used 

as a measure of the onset of fall high-streamflow conditions as 

streamflows transition from summer low streamflows to fall 

high streamflows. Later or earlier October threshold dates 

could indicate changes in the length of the summer lowstreamflow period. No statistically significant trends in the first 

date within the fall/winter seasonal window (September 1 to 

January 31) on which the streamflow exceeded the long-term 

October median were observed (table 7). Five out of the seven 

statistically insignificant trend test results indicate trends 

toward earlier dates, and two indicated later dates.

Little evidence of statistically significant trends in the 

timing or duration of fall/winter high streamflows for coastal 

river basins in Maine during the 20th century was observed, 

with only two statistically significant trend results (tables 8 and 

9). The date of the 1-day peak streamflow for the East Machias 

River (1927-1958) had a statistically significant trend toward 

later dates with a p-value of 0.038 (table 8). Of the statistically 

insignificant results, 77 percent indicate trends toward later 

dates, 23 percent toward earlier dates. The Machias River 

(1906-21, 1930-77) had a statistically significant trend toward 

increasing difference between the first and last dates of 

streamflow equal to or exceeding the long-term December 

median (p-value of 0.079); however, trends for both the first 

threshold data and last threshold date for this site were 

statistically insignificant (table 9). These results are consistent 

with those presented in Hodgkins and others (2003), who 

demonstrated few statistically significant changes in the timing 

of fall high streamflows for 27 rural rivers in New England.

In general, little evidence of trends in total seasonal 

runoff volume during fall/winter was observed (table 10). Only 

the West Branch Union River (1910-19, 1929-79) had 

statistically significant trends toward higher total fall/winter 

runoff volume (table 10, fig. 5). Nine of 12 statistically 

Table 7. Statistical significance (p-values) and trend direction for 

the October median threshold timing statistic for coastal river 

basins in Maine (September 1 to January 31).

[FD, first date that fall streamflow exceeds the long-term October 

median; p-values in bold are significant at less than or equal to 0.10; pvalues in bold and underlined are highly significant at less than or equal 

to 0.01; +, trend over time toward later dates; -, trend over time toward 

earlier dates; *, statistic not tested]

U.S. Geological Survey streamflow-gaging 

station

Number Name FD

01021500 Machias River at Whitneyville 0.740 -

01022000 East Machias River near East 

Machias

0.102 +

01022500 Narraguagus River at Cherryfield 0.722 +

01023000 West Branch Union River at 

Amherst

0.107 -

01038000 Sheepscot River at North 

Whitefield

0.124 -

01060000 Royal River at Yarmouth 0.961 -

-- Narraguagus Extended Record 

(NER)

0.147 -
14 Trends in Timing, Magnitude, and Duration of Summer and Fall/Winter Streamflows for Unregulated Coastal River Basins 

in Maine During the 20th Century

1900 1920 1940 1960 1980 2000

YEAR

0

500

1,000

1,500

2,000

31-DAY MEAN PEAK STREAMFLOW, IN CUBIC FEET PER SECOND

Machias

East Machias

Sheepscot

West Branch Union

Royal

Narraguagus

NER

Figure 5. Trends in the magnitude of 31-day mean peak streamflow (September 1 to January 31) for six coastal rivers in Maine 

and the Narraguagus Extended Record (NER) (dashed line). LOESS regression lines are based on a 45-year weighting window.
Trends in Timing, Magnitude, and Duration of Summer and Fall/Winter Streamflows for Unregulated Coastal River Basins in 

Maine During the 20th Century 15

Table 9. Statistical significance (p-values) and trend direction for fall/winter 

(September 1 to January 31) streamflow threshold statistics for coastal river basins 

in Maine.

[Threshold streamflow for each station is the long-term December median; FD, first date 

streamflow is at or above threshold; LD, last date streamflow is at or above threshold; DD, 

number of days between the FD and LD, inclusive; Cum, cumulative number days in the 

season streamflow is at or above the threshold; p-values in bold are significant at less than or 

equal to 0.10; p-values in bold and underlined are highly significant at less than or equal to 

0.01; +, trend over time toward later dates; -, trend over time toward earlier dates; *, statistic 

not tested]

U.S. Geological Survey 

streamflow-gaging station

Number Name FD LD DD Cum

01021500 Machias River at 

Whitneyville

0.230 - 0.191 + 0.079 + 0.960 -

01022000 East Machias River 

near East Machias

0.323 + 0.810 + 0.452 - 0.984 -

01022500 Narraguagus River at 

Cherryfield

0.483 + 0.518 - 0.660 - 0.152 -

01023000 West Branch Union 

River at Amherst

0.130 - 0.340 + 0.152 + 0.318 +

01038000 Sheepscot River at 

North Whitefield

0.852 - 0.884 - 0.945 - 0.891 +

01060000 Royal River at 

Yarmouth

0.864 + 0.824 - 0.948 - 0.727 -

-- Narraguagus 

Extended Record 

(NER)

0.731 + 0.102 + 0.698 + 0.644 +

Table 8. Statistical significance (p-values) and trend direction for fall/winter (September 1 to January 31) 

streamflow timing statistics for coastal river basins in Maine.

[CVD, center-of-volume date; High1D, date of the high 1-day mean streamflow; High7D, date of the high 7-day mean 

streamflow; High15D, date of the high 15-day mean streamflow; High31D, date of the high 31-day mean streamflow; 

High45D, date of the high 45-day mean streamflow; p-values in bold are significant at less than or equal to 0.10; p-values 

in bold and underlined are highly significant at less than or equal to 0.01; +, trend over time toward later dates;

 -, trend over time toward earlier dates; *, statistic not tested]

U.S. Geological Survey 

streamflow-gaging station

Number Name CVD High1D High7D High15D High31D High45D

01021500 Machias River at 

Whitneyville

0.229 + * * * 0.429 + 0.145 +

01022000 East Machias River 

near East Machias

0.205 + 0.038 + 0.149 + 0.138 + 0.192 + 0.148 +

01022500 Narraguagus River 

at Cherryfield

0.530 - 0.777 + 0.464 + 0.952 + 0.823 - 0.595 -

01023000 West Branch Union 

River at Amherst

0.581 + 0.615 + 0.762 - 0.491 + 0.432 + 0.127 +

01038000 Sheepscot River at 

North Whitefield

0.798 + 0.129 + 0.571 + 0.709 + 0.667 + 0.449 +

01060000 Royal River at 

Yarmouth

0.826 - 0.807 - 0.445 - 0.974 - 0.884 + 0.832 +

-- Narraguagus 

Extended Record 

(NER)

0.131 + * * * 0.265 + 0.258 +
16 Trends in Timing, Magnitude, and Duration of Summer and Fall/Winter Streamflows for Unregulated Coastal River Basins 

in Maine During the 20th Century

significant trends in the magnitude of high fall/winter 

streamflows were toward higher streamflows, for the Machias 

(1906-21, 1929-77) and West Branch Union (1909-19, 1929-

79) Rivers, and the NER (1906-21, 1929-2002). The 

Narraguagus River (1948-2002) had three statistically 

significant trends toward lower peak fall/winter streamflows 

(table 10). The mixed statistically significant trends in the 

magnitude of high fall/winter streamflows might be a result of 

the differences in time periods tested with a trend toward lower 

streamflows only appearing in the contemporary record (fig. 

5). The mixed statistically significant trends also might be 

explained by correlations with seasonal precipitation and 

surface air temperature discussed later.

Correlations Among Seasonal Streamflows

The timing of summer low streamflows was not 

significantly correlated with the timing of spring snowmelt 

runoff (table 11), but later summer low streamflows correlated 

with later fall/winter high streamflows, with statistically 

significant correlations at all six stations. In general, later 

summer low streamflows correlated with later October median 

streamflow threshold dates, with statistically significant 

correlations at three of six stations. 

The magnitude of summer low streamflows was not 

significantly correlated to the timing of spring snowmelt runoff 

(table 12), but lower summer 31-day mean low streamflows 

correlated with later fall/winter high streamflows, with 

statistically significant correlations at four of six stations. In 

general, lower summer low streamflows correlated with later 

October median streamflow threshold dates, with statistically 

significant correlations at two of six stations.

Correlations of Seasonal Streamflow with 

Precipitation and Surface Air Temperature

Overall, seasonal streamflows correlated more strongly 

with monthly mean precipitation than with monthly mean air 

temperatures (tables 13-16). Later summer low streamflows 

correlated with lower mean monthly precipitation in August 

(four of six stations had statistically significant correlations) 

and September (all six) (table 13). The correlation between 

later summer low streamflows, later fall/winter high 

streamflows, and later October threshold dates is supported by 

correlation results between the timing of summer low 

streamflows and late summer/early fall precipitation. Later fall/

winter high streamflows correlated with lower mean monthly 

precipitation in September (4 of 6) and October (3 of 6) (table 

13). There were few significant correlations between 

streamflow timing statistics and monthly mean surface air 

temperatures (table 14); later summer low streamflows 

correlated with higher mean July air temperatures (2 of 6).

Streamflow magnitude statistics correlate with monthly 

mean precipitation. The magnitude of summer low 

streamflows was correlated with mid-summer precipitation; 

lower summer low streamflows correlated with lower July (4

Table 10. Statistical significance (p-values) and trend direction for fall/winter (September 1 to January 31) 

streamflow magnitude statistics for coastal river basins in Maine.

[TotVol, total runoff volume from September 1 to January 31; High1Q, high 1-day mean streamflow; High7Q, high 7-

day mean streamflow; High15Q, high 15-day mean streamflow; High31Q, high 31-day mean streamflow; High45Q, high 

45-day mean streamflow; p-values in bold are significant at less than or equal to 0.10; p-values in bold and underlined are 

highly significant at less than or equal to 0.01; +, upward trend over time; -, downward trend over time; *, statistic not 

tested]

U.S. Geological Survey 

streamflow-gaging station

Number Name TotVol High1Q High7Q High15Q High31Q High45Q

01021500 Machias River at 

Whitneyville

0.370 + * * * 0.071 + 0.084 +

01022000 East Machias River 

near East Machias

0.693 + 0.220 + 0.167 + 0.179 + 0.186 + 0.155 +

01022500 Narraguagus River 

at Cherryfield

0.266 - 0.072 - 0.093 - 0.075 - 0.159 - 0.134 -

01023000 West Branch Union 

River at Amherst

0.083 + 0.042 + 0.027 + 0.046 + 0.051 + 0.035 +

01038000 Sheepscot River at 

North Whitefield

0.654 + 0.360 + 0.494 + 0.575 + 0.486 + 0.654 +

01060000 Royal River at 

Yarmouth

0.564 - 0.655 - 0.350 - 0.252 - 0.287 - 0.232 -

-- Narraguagus 

Extended Record 

(NER)

0.228 + * * * 0.037 + 0.039 +
Trends in Timing, Magnitude, and Duration of Summer and Fall/Winter Streamflows for Unregulated Coastal River Basins in 

Maine During the 20th Century 17

Table 11. Interseasonal correlations of summer (June 1 to September 30) low-streamflow timing (ICVD) to 

spring (January 1 to May 31) and to fall/winter (September 1 to January 31) high-streamflow timings (CVD) 

and to the October median threshold date for coastal river basins in Maine.

[ICVD, inverse center-of-volume date; CVD, center-of-volume date; October median threshold date, the first date 

within the fall/winter seasonal window on which the streamflow exceeds the long-term October median; Pearson rvalues in bold are statistically significant with p-values less than 0.01; Pearson r-values in bold and underlined are 

statistically significant with p-values less than 0.0001]

U.S. Geological Survey streamflow-gaging station Spring

CVD

Fall/Winter 

CVD

October 

median 

threshold date Number Name

01021500 Machias River at Whitneyville -0.025 0.347 0.376

01022000 East Machias River near East Machias -0.108 0.731 0.499

01022500 Narraguagus River at Cherryfield 0.022 0.547 0.290

01023000 West Branch Union River at Amherst -0.092 0.499 0.331

01038000 Sheepscot River at North Whitefield 0.055 0.556 0.580

01060000 Royal River at Yarmouth 0.169 0.478 0.318

Table 12. Interseasonal correlations of the magnitude of summer (June 1 to September 30) 31-day mean 

low-streamflow (Low31Q) to fall/winter (September 1 to January 31) and to spring (January 1 to May 31) 

high-streamflow timings (CVD) and to the October median threshold date for coastal river basins in Maine.

[CVD, center-of-volume date; October median threshold date, the first date within the fall/winter seasonal window on 

which the streamflow exceeds the long-term October median; Pearson r-values in bold are statistically significant with 

p-values less than 0.01; Pearson r-values in bold and underlined are statistically significant with p-values less than 

0.0001]

U.S. Geological Survey streamflow-gaging station Spring

CVD

Fall/Winter 

CVD

October 

median 

Number Name threshold date

01021500 Machias River at Whitneyville 0.085 -0.401 -0.318

01022000 East Machias River near East Machias 0.013 -0.532 -0.376

01022500 Narraguagus River at Cherryfield 0.011 -0.414 -0.312

01023000 West Branch Union River at Amherst -0.038 -0.324 -0.366

01038000 Sheepscot River at North Whitefield 0.050 -0.430 -0.593

01060000 Royal River at Yarmouth 0.108 -0.286 -0.341
Table 13. Correlations of summer low-streamflow timing (ICVD) and fall/winter high-streamflow timing (CVD) with monthly mean precipitation 

Maine.

[ICVD, summer inverse center-of-volume date; CVD, fall/winter center-of-volume date; Pearson r-values in bold are statistically significant with p-values lesin bold and underlined are statistically significant with p-values less than 0.0001]

U.S. Geological Survey streamflowgaging station Timing Number Name statistic Jan Feb Mar Apr May June July Aug Sep O01021500 Machias River at 

Whitneyville

ICVD -0.063 0.020 0.029 0.060 -0.002 -0.048 -0.281 -0.151 -0.610 -0CVD 0.189 0.059 -0.091 0.081 0.055 -0.294 0.009 -0.053 -0.386 -0

01022000 East Machias River near 

East Machias

ICVD 0.018 0.089 0.009 -0.306 0.040 -0.251 0.028 -0.457 -0.577 -0CVD 0.187 0.132 -0.088 -0.151 0.157 -0.390 0.075 -0.252 -0.548 -0

01022500 Narraguagus River at 

Cherryfield

ICVD -0.046 0.099 -0.046 -0.039 0.067 0.104 -0.315 -0.494 -0.497 -0CVD 0.029 -0.072 0.090 -0.064 -0.119 0.014 -0.374 -0.339 -0.322 -0

01023000 West Branch Union 

River at Amherst

ICVD 0.151 -0.014 0.207 -0.073 0.131 -0.021 -0.049 -0.281 -0.388 -0CVD 0.168 -0.036 -0.032 -0.126 -0.027 -0.108 -0.386 -0.266 -0.322 -0

01038000 Sheepscot River at 

North Whitefield

ICVD -0.239 0.186 -0.115 -0.048 0.165 0.180 0.097 -0.342 -0.618 -0CVD -0.199 0.044 -0.067 -0.194 -0.112 0.195 -0.212 -0.168 -0.453 -0

01060000 Royal River at 

Yarmouth

ICVD -0.080 0.118 -0.046 0.141 0.251 0.410 -0.068 -0.576 -0.533 -0CVD -0.093 -0.114 0.063 0.062 -0.170 0.041 -0.234 -0.224 -0.461 -0
Table 14. Correlations of summer low-streamflow timing (ICVD) and fall/winter high-streamflow timing (CVD) with monthly mean surface air t

river basins in Maine.

[ICVD, inverse center-of-volume date; CVD, center-of-volume date; Pearson r-values in bold are statistically significant with p-values less than 0.01; Pearso

underlined are statistically significant with p-values less than 0.0001]

U.S. Geological Survey streamflowgaging station Timing statistic Number Name Jan Feb Mar Apr May June July Aug Sep 01021500 Machias River at 

Whitneyville

ICVD -0.168 0.001 -0.043 -0.071 0.144 0.099 0.418 0.101 0.135 CVD -0.057 -0.012 -0.045 0.151 0.021 0.121 0.143 0.033 0.035 -

01022000 East Machias River near 

East Machias

ICVD -0.151 -0.120 0.032 -0.194 -0.050 0.067 0.263 0.235 0.057 CVD -0.203 -0.059 0.017 0.029 -0.136 0.102 0.240 0.092 -0.129 

01022500 Narraguagus River at 

Cherryfield

ICVD 0.001 0.042 -0.272 -0.153 -0.100 0.136 0.149 0.282 0.183 CVD -0.016 0.064 -0.129 0.083 0.072 0.005 0.220 0.207 0.096 -

01023000 West Branch Union 

River at Amherst

ICVD 0.024 0.260 0.034 0.202 0.126 0.168 0.406 0.307 0.282 CVD 0.049 0.171 0.037 0.205 0.183 0.282 0.239 0.113 0.200 -

01038000 Sheepscot River at North 

Whitefield

ICVD -0.110 0.157 -0.193 0.010 -0.101 -0.089 0.050 0.106 0.174 CVD -0.056 0.010 -0.132 0.055 0.083 -0.153 0.118 0.171 0.136 

01060000 Royal River at 

Yarmouth

ICVD 0.101 -0.038 -0.281 -0.115 -0.223 -0.098 -0.037 0.119 0.113 CVD -0.089 -0.195 -0.184 -0.100 -0.030 -0.129 0.134 0.037 -0.080 
Table 15. Correlations of the summer 31-day mean low-streamflow magnitude and fall/winter 31-day mean high-streamflow magnitude with m

for coastal river basins in Maine.

[Pearson r-values in bold are statistically significant with p-values less than 0.01; Pearson r-values in bold and underlined are statistically significant with p-v

U.S. Geological Survey streamflowgaging station Magnitude statistic Number Name Jan Feb Mar Apr May June July Aug Sep 01021500 Machias River at 

Whitneyville

31-day low 0.122 0.227 0.169 0.268 0.189 0.226 0.224 0.442 0.298 31-day high 0.073 0.021 0.048 0.309 -0.188 0.191 0.210 0.623 0.067 

01022000 East Machias River near 

East Machias

31-day low 0.003 0.069 -0.180 0.221 0.141 0.139 0.310 0.740 0.100 31-day high 0.084 0.110 -0.081 0.420 -0.211 0.066 0.171 0.682 -0.030 

01022500 Narraguagus River at 

Cherryfield

31-day low 0.046 0.147 -0.049 0.419 0.187 0.238 0.626 0.419 0.270 31-day high 0.025 -0.091 0.080 0.209 -0.015 0.158 0.245 0.065 0.181 

01023000 West Branch Union 

River at Amherst

31-day low -0.019 0.213 -0.039 0.235 0.196 0.318 0.571 0.497 0.325 31-day high 0.017 -0.052 0.119 -0.020 -0.154 0.172 0.104 0.112 0.243 

01038000 Sheepscot River at 

North Whitefield

31-day low 0.182 0.143 -0.090 0.329 0.153 0.198 0.394 0.193 0.284 31-day high 0.151 -0.037 0.112 0.280 0.061 0.122 0.030 0.076 0.055 

01060000 Royal River at 

Yarmouth

31-day low 0.105 0.265 0.042 0.411 0.332 0.146 0.497 0.232 0.263 31-day high 0.043 -0.005 0.164 0.258 0.081 0.044 0.203 0.271 0.068 
Table 16. Correlations of summer 31-day mean low-streamflow magnitude and fall/winter 31-day mean high-streamflow magnitude with montemperature for coastal river basins in Maine.[Pearson r-values in bold are statistically significant with p-values less than 0.01; Pearson r-values in bold and underlined are statistically significant with p-v

U.S. Geological Survey streamflowgaging station Magnitude Number Name statistic Jan Feb Mar Apr May June July Aug Sep 01021500 Machias River at 

Whitneyville

31-day low 0.129 0.056 0.036 0.051 -0.091 -0.100 -0.170 -0.111 -0.071 31-day high 0.244 -0.086 -0.142 -0.051 0.089 -0.108 -0.022 -0.037 -0.180 -

01022000 East Machias River near 

East Machias

31-day low 0.186 0.189 0.052 0.185 -0.103 -0.244 -0.224 -0.313 -0.025 31-day high 0.144 0.036 -0.017 0.093 0.090 -0.043 -0.170 -0.149 -0.257 -

01022500 Narraguagus River at 

Cherryfield

31-day low -0.004 0.176 0.282 0.294 -0.004 -0.168 -0.014 -0.042 0.015 -31-day high 0.326 -0.138 0.146 0.101 0.053 -0.083 0.112 0.091 0.015 -

01023000 West Branch Union 

River at Amherst

31-day low 0.123 0.185 0.125 0.250 -0.174 -0.112 -0.161 0.027 -0.070 31-day high 0.215 0.050 -0.040 -0.075 0.092 -0.045 -0.044 -0.052 -0.197 -

01038000 Sheepscot River at 

North Whitefield

31-day low 0.139 0.052 0.102 0.116 -0.201 -0.209 -0.142 -0.069 0.056 -31-day high 0.310 -0.022 0.287 0.116 -0.006 -0.027 0.043 0.085 -0.072 

01060000 Royal River at 

Yarmouth

31-day low 0.033 0.184 0.143 0.236 -0.174 -0.258 -0.091 0.030 -0.023 31-day high 0.164 0.065 0.407 0.335 -0.027 -0.104 0.275 0.118 -0.003 
22 Trends in Timing, Magnitude, and Duration of Summer and Fall/Winter Streamflows for Unregulated Coastal River Basins 

in Maine During the 20th Century

of 6) and August (4 of 6) monthly mean precipitation (table 15). 

Lower summer low streamflows also correlated with lower 

spring/early summer precipitation during April (3 of 6) (table 

15) suggesting either spring recharge of ground water or seasonal weather patterns that produce covarying precipitation 

during spring and summer. Higher fall/winter high streamflows 

correlated with higher November and December mean monthly 

precipitation at all six stations for both months (table 15). The 

mixed statistically significant trends in the magnitude of high 

fall/winter streamflows might be explained by changes in 

November and December monthly mean precipitation over 

time. There were few significant correlations between streamflow magnitude statistics and monthly mean surface air temperatures (table 16).

The number of days that streamflow is equal to or below 

the long-term August median streamflow was negatively 

correlated with precipitation throughout late spring and 

summer (table 17); fewer days below the August median 

streamflow threshold correlated with greater amounts of 

precipitation in July (2 of 2) and August (1 of 2). Higher spring 

precipitation during April (2 of 2) and May (1 of 2) correlates 

to fewer days of streamflow equal to or below the long-term 

August median, suggesting either spring recharge of ground 

water or seasonal weather patterns that produce covarying 

precipitation during spring and summer. The number of days 

that streamflow is equal to or above the long-term December 

median streamflow was positively correlated with precipitation 

in late summer, fall, and early winter (table 18); more days 

above the December median streamflow threshold correlated 

with greater amounts of precipitation in August (3 of 6), 

September (5 of 6), October (6 of 6), and December (5 of 6). 

Conclusions

It was hypothesized that summer low streamflows could 

be occurring earlier and that a longer period of low-streamflow 

recession could result in a decrease in the magnitude of low 

streamflows for coastal river basins in Maine. As discussed 

earlier, these hypothesized changes have been documented in 

historical streamflow records in southern British Columbia 

(Leith and Whitfield, 1989) and modeled for rivers in 

California (Lettenmaier and Gan, 1990) and Switzerland 

(Brubaker and Rango, 1996). The basins studied in British 

Columbia, California, and Switzerland have substantial 

summer snowmelt runoff, whereas most snow in Maine has 

melted by mid-May; in the spring, much of the snowmelt in 

Maine recharges ground water, which is later discharged to 

streams over the summer. However, a change in the timing of 

snowmelt runoff could still be important to the timing and 

magnitude of summer low streamflows in Maine. Querner and 

others (1997) found lower streamflows associated with 

increased air temperatures for a river in southwestern Norway 

and a river in the Netherlands and Belgium.

The results in this report indicate little evidence of trends 

over the 20th century in the timing, magnitude, or duration of 

summer low streamflows in coastal Maine. Correlation results 

indicate that summer low streamflows are more sensitive to 

precipitation than temperature. Statistically significant results 

indicate, in general, later timing of the bulk of summer low 

streamflows is correlated with decreasing August and 

September precipitation and later timing of the bulk of fall/

winter high streamflows; lower magnitude summer low 

streamflows are correlated with lower April, July and August 

precipitation and later timing of the bulk of fall/winter high 

streamflows; and longer duration summer low-streamflows are 

correlated with lower amounts of April and July precipitation 

(table 19). 

There is little evidence of trends over the 20th century in 

the timing or duration of fall/winter high streamflows in 

coastal Maine. There were statistically significant trends in the 

magnitude of high fall/winter streamflows at four of the six 

rivers tested. Seventy-five percent of the statistically 

significant results were toward higher peak fall/winter 

streamflows; the remaining trends toward lower peak fall/

winter streamflows. The mixed trends in the magnitude of high 

fall/winter streamflows might be a result of the differences in 

time periods tested with a trend toward lower streamflows only 

appearing in the contemporary record. The mixed statistically 

significant trends also might be explained by correlations with 

seasonal precipitation and surface air temperature. Correlation 

results indicate that fall/winter high streamflows are more 

sensitive to precipitation than temperature. Statistically 

significant results indicate, in general, later timing of the bulk 

of fall/winter high streamflows is correlated with decreasing 

September and October precipitation; lower magnitude fall/

winter high streamflows are correlated with lower November 

and December precipitation; and longer duration fall/winter 

high streamflows are correlated with higher amounts of 

August, September, October, and December precipitation 

(table 19).

There were few statistically significant correlations of 

mean monthly air temperature with the timing, magnitude, or 

duration of summer or fall/winter streamflows. The relatively 

abundant precipitation in coastal Maine may mask any strong 

temperature-dependent signal in streamflows examined in this 

report. Future changes in summer low streamflows and fall/

winter high streamflows in coastal Maine may depend more on 

future changes in seasonal precipitation than changes in air 

temperature.

Summary

The U.S. Geological Survey (USGS), in cooperation 

with the Maine Atlantic Salmon Commission (ASC), began a 

study in 2003 to examine the timing, magnitude, and duration 

of summer (June through October) and fall/early winter 

(September through January) seasonal streamflows of 
Table 17. Correlations of cumulative number of days below the long-term August median streamflow threshold with monthly mean precipitati

basins in Maine.

[Pearson r-values in bold are significant with p-values less than 0.01; Pearson r-values in bold and underlined are statistically significant with p-values less th

U.S. Geological Survey streamflowgaging stationNumber Name Jan Feb Mar Apr May June July Aug Sep Oct N01022500 Narraguagus River at 

Cherryfield

-0.135 -0.096 -0.076 -0.401 -0.221 -0.310 -0.650 -0.355 -0.319 -0.314 0

01060000 Royal River at Yarmouth -0.205 -0.238 -0.028 -0.377 -0.428 -0.261 -0.476 -0.270 -0.195 -0.240 0

Table 18. Correlations of cumulative number of days above the long-term December median streamflow threshold with monthly mean precipit

basins in Maine.

[Pearson r-values in bold are significant with p-values less than 0.01; Pearson r-values in bold and underlined are statistically significant with p-values less th

U.S. Geological Survey streamflowgaging stationNumber Name Jan Feb Mar Apr May June July Aug Sep Oct N01021500 Machias River at 

Whitneyville

0.098 0.060 0.130 0.346 -0.037 0.329 0.082 0.447 0.459 0.443 0

01022000 East Machias River near East 

Machias

-0.016 0.028 -0.210 0.281 -0.123 0.201 0.027 0.559 0.391 0.516 0

01022500 Narraguagus River at 

Cherryfield

0.058 -0.030 0.082 0.214 0.127 0.228 0.351 0.216 0.516 0.435 0

01023000 West Branch Union River at 

Amherst

0.022 0.018 0.050 0.183 -0.003 0.296 0.326 0.176 0.482 0.359 0

01038000 Sheepscot River at North 

Whitefield

0.232 -0.097 0.163 0.167 0.009 -0.006 0.102 0.232 0.473 0.427 0

01060000 Royal River at Yarmouth 0.098 0.050 0.140 0.117 0.073 0.016 0.245 0.425 0.427 0.426 0
24 Trends in Timing, Magnitude, and Duration of Summer and Fall/Winter Streamflows for Unregulated Coastal River Basins 

in Maine During the 20th Century

unregulated coastal river basins in Maine and to correlate them 

to meteorological variables and winter/spring (January through 

May) seasonal streamflow. This study overlapped the summer 

seasonal window with the fall/early winter seasonal window to 

completely bracket the low-streamflow period during July, 

August, and September between periods of high streamflows in 

June and October. The ASC is concerned with the impacts of 

potentially changing meteorological and hydrologic conditions 

on Atlantic salmon survival. Because winter/spring high 

streamflows appear to have trended toward earlier dates over 

the 20th century in coastal Maine, it was hypothesized that the 

spring/summer recession to low streamflows could have a 

similar trend toward earlier, and possibly lower, longer lasting, 

late summer/early fall low streamflows during the 20th 

century. 

Streamflow data from six streamflow-gaging stations in 

coastal Maine were retrieved from the USGS National Water 

Information System (NWIS). Periods of record of streamflow 

data for the six stations range from 32 to 65 years, with a mean 

of 56 years. In addition to these data sets, an extended-record 

data set was constructed using streamflow records from the 

Narraguagus and Machias Rivers to examine trends in a single 

streamflow record spanning the 95-year period 1906-2002. To 

examine trends over time in the timing, magnitude, and 

duration of summer and fall/winter streamflows, daily mean 

streamflow data were partitioned into two seasonal windows. 

The summer window was defined as June 1 to October 31, and 

the fall/winter window was defined as September 1 to January 

31. Streamflow timing and magnitude statistics for the 

remainder of the year (February to May), during which most 

snowmelt runoff occurs, were not derived for this 

investigation. The winter/spring seasonal center-of-volume 

statistics for the timing of snowmelt runoff derived in an earlier 

study of coastal river basins in Maine were used in this 

investigation in correlation tests with summer low-streamflow 

statistics.

Surface air temperature and precipitation time series 

used in this investigation were obtained from the U.S. 

Historical Climatology Network (USHCN) data set developed 

and maintained at the National Climatic Data Center. Only 

meteorological data from USHCN stations within 30 miles of 

coastal river drainage-basin centroids were correlated with 

streamflow data for the corresponding USGS streamflowgaging station.

Little evidence of statistically significant trends in the 

timing, duration, or magnitude of summer low streamflows for 

coastal river basins in Maine during the 20th century was 

observed; the hypothesis that earlier winter/spring high 

streamflows may result in earlier and lower summer low 

streamflows is not supported by the data. Trend test results 

indicate the duration of the summer low-streamflow period has 

not changed significantly for coastal river basins in Maine over 

the 20th century. There also were no statistically significant 

trends in total runoff volume during the combined lowstreamflow months of August and September. 

Although little evidence of trends in the timing and 

magnitude of summer low streamflows for coastal river basins 

in Maine during the 20th century was observed, the magnitude 

and timing of summer low streamflows correlated with the 

timing of fall/winter high streamflows and with seasonal 

precipitation. Notably, the magnitude and timing of summer 

low streamflows were not correlated with the spring snowmelt 

runoff; however, later summer low streamflows did correlate 

with later fall/winter high streamflows. This is supported by 

correlation results between the timing of summer low 

streamflows and late summer/early fall precipitation—in 

particular, earlier summer low streamflows correlated with 

higher September precipitation. Lower summer low 

streamflows were also correlated with later fall/winter high 

streamflows. The magnitude of summer low streamflows 

correlated with mid-summer precipitation; higher precipitation 

amounts correlated with higher low streamflows. The 

correlation with precipitation is much stronger than that with 

monthly mean surface air temperatures. In general, summer 

low streamflow was positively correlated most strongly with 

July and August monthly mean precipitation. The cumulative 

number of days of streamflow at or below the long-term 

August median streamflow, at the two streamflow-gaging 

stations tested, was negatively correlated with summer 

precipitation (particularly July), indicating higher summer 

precipitation resulted in fewer days of streamflow below the 

long-term August median.

The October median streamflow threshold date was used 

as a measure of the onset of fall high-streamflow conditions as 

streamflows transition from summer low streamflows to fall 

high streamflows. Later or earlier October threshold dates 

could indicate changes in the length of the summer lowstreamflow period. No statistically significant trends were 

observed in the occurrence of the first date during late summer/

early fall on which the streamflow exceeded the long-term 

October median. 

Little evidence of statistically significant trends in the 

timing or duration of fall/winter high streamflows for coastal 

river basins in Maine during the 20th century was observed. In 

general, there was little evidence of trends in the magnitude of 

total seasonal runoff volume during fall/winter. The timing of 

fall/winter high streamflows was correlated with seasonal 

precipitation. Earlier fall/winter high streamflows correlated 

with higher September and October precipitation. The 

magnitude of fall/winter high streamflows was positively 

correlated with November and December precipitation. 

Correlations with monthly mean precipitation amounts were 

stronger than those with monthly mean surface air temperature. 
Acknowledgements 25

Acknowledgements

The authors thank the Maine Atlantic Salmon 

Commission for providing support for this study. We also 

thank the many dedicated hydrologic technicians that collected 

the field data over nearly a century following U.S. Geological 

Survey protocols often during dangerous conditions. Reviews 

by Joan Trial, Thomas Huntington, Gene Parker, and Barbara 

Korzendorfer resulted in scientific and editorial improvements 

in this report. This study was cooperatively funded by the U.S. 

Geological Survey and the Maine Atlantic Salmon 

Commission.

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