U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2009–1115

Framework for a U.S. Geological Survey

Hydrologic Climate-Response Program in

Maine

MAINE

NEW HAMPSHIRE

County boundary

State boundary

Hydrologic climate-response region

Southern coastal, Saco, Presumpscot, mid-coastal

Eastern coastal (Downeast)

Lower Androscoggin, lower Kennebec

Lower Penobscot, St. Croix

Upper Androscoggin, upper Kennebec, upper Penobscot

Upper St. John, Allagash

Lower St. John, Fish, Aroostook, Meduxnekeag

EXPLANATION

Basin for which model exists or is being developed

44°

46°

70°

68°

46°

44°

68°

70°

0 25 50 100 MILES

0 50 100 KILOMETERS 25

Base from U.S. Geological Survey digital files,

Universal Transverse Mercator projection, Zone 19

MAINE

NEW HAMPSHIRE

Hydrologic climate-response region

Southern coastal, Saco, Presumpscot, mid-coastal

Eastern coastal (Downeast)

Lower Androscoggin, lower Kennebec

Lower Penobscot, St. Croix

Upper Androscoggin, upper Kennebec, upper Penobscot

Upper St. John, Allagash

Lower St. John, Fish, Aroostook, Meduxnekeag

70°

44°

68°

46°

68°

70°

46°

44° #

#

#

#

#

#

#

#

#

#

#

#

#

01022500

01030500

01013500

01014000

01010500

01031500

01038000

01010000

01047000

01060000

01057000

01055000

01052500

01064500

EXPLANATION

# Streamflow-gaging station and number

County boundary

01031500

#

State boundary

Framework for a U.S. Geological Survey 

Hydrologic Climate-Response Program in 

Maine

By Glenn A. Hodgkins, Robert M. Lent, Robert W. Dudley, and Charles W. Schalk 

Open-File Report 2009–1115

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

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

For product and ordering information:

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its natural and living resources, natural hazards, and the environment:

World Wide Web: http://www.usgs.gov

Telephone: 1-888-ASK-USGS

Suggested citation:

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 Open-File Report 2009–1115, 29 p.

Any use of trade, product, or firm names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government. Use of company names is for identification purposes only and does not imply 

responsibility.

Although this report is in the public domain, permission must be secured from the individual 

copyright owners to reproduce any copyrighted material contained within this report.
iii

Contents

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

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

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

Historical Hydrologic Changes in Maine ..........................................................................................2

Winter.............................................................................................................................................2

Spring.............................................................................................................................................3

Summer and Fall...........................................................................................................................4

Future Hydrologic Changes in Maine................................................................................................4

Potential Effects of Hydrologic Changes ..........................................................................................4

Framework for a Hydrologic Climate-Response Program ......................................................................5

Climate-Response Regions .................................................................................................................5

Hydrologic Climate-Response Data Network ..................................................................................5

Primary Components and Associated Key Variables ............................................................7

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

Lake Ice ................................................................................................................................9

River Ice................................................................................................................................9

Occurrence.................................................................................................................9

Thickness ..................................................................................................................13

Snowpack ..........................................................................................................................13

Groundwater......................................................................................................................14

Secondary Components ...........................................................................................................14

Existing Primary and Secondary Data-Collection Sites ......................................................14

New Data-Collection Sites.......................................................................................................14

Trend Updates for Key Variables......................................................................................................19

Process-Based Studies .....................................................................................................................19

Communication Plan ..........................................................................................................................22

References Cited..........................................................................................................................................22
iv

Figures

1–3. Graphs showing—

1. Annual average snowpack density at four sites in western Maine/northern 

New Hampshire for the March 15 sampling window (the 15-day period 

centered on March 15), 1914–2004 ...................................................................................3

2. Annual winter-spring center-of-volume dates (a measure of streamflow 

timing) for the Piscataquis River in central Maine, 1903–2000 ....................................3

3. Annual lake ice-out dates for Damariscotta Lake, midcoastal Maine, 

1837–2008 ..............................................................................................................................3

4–13. Maps showing—

4. Location of hydrologic climate-response regions in Maine ........................................6

5. Location of long-term streamflow-gaging stations that drain relatively 

natural basins in and near Maine .....................................................................................8

6. Location of lakes with long-term ice-out data in Maine.............................................10

7. Location of rivers with long-term ice-occurrence data in and near Maine............12

8. Location of long-term snowpack sites in and near Maine.........................................15

9. Location of Native American headquarters in Maine.................................................16

10. Location of U.S. Department of Interior and Agriculture lands in and 

near Maine..........................................................................................................................17

11. Location of major surface water suppliers in Maine ..................................................18

12. Location of drainage basins in Maine with rainfall-runoff models ...........................20

13. Rainfall-runoff model subareas for two drainage basins in eastern 

coastal Maine.....................................................................................................................21

Tables

1. Hydrologic climate-response regions in Maine......................................................................5

2. Long-term streamflow-gaging stations on Maine rivers that drain relatively 

natural basins................................................................................................................................7

3. Maine lakes with long-term ice-out data .................................................................................9

4. Maine rivers with long-term ice occurrence data................................................................11

5. Long-term snowpack sites in and near Maine......................................................................14

6. Examples of annual cycles of trend updates for key variables of hydrologic 

components .................................................................................................................................19
v

Conversion Factors

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)

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

°C=(°F-32)/1.8
-- THIS PAGE INTENTIONALLY LEFT BLANK --
Abstract

This report presents a framework for a U.S. Geological Survey (USGS) hydrologic climate-response program 

designed to provide early warning of changes in the seasonal 

water cycle of Maine. Climate-related hydrologic changes on 

Maine’s rivers and lakes in the winter and spring during the 

last century are well documented, and several river and lake 

variables have been shown to be sensitive to air-temperature 

changes. Monitoring of relevant hydrologic data would 

provide important baseline information against which future 

climate change can be measured.

The framework of the hydrologic climate-response program presented here consists of four major parts: (1) identifying homogeneous climate-response regions; (2) identifying 

hydrologic components and key variables of those components 

that would be included in a hydrologic climate-response data 

network—as an example, streamflow has been identified as a 

primary component, with a key variable of streamflow being 

winter-spring streamflow timing; the data network would be 

created by maintaining existing USGS data-collection stations 

and establishing new ones to fill data gaps; (3) regularly updating historical trends of hydrologic data network variables; and 

(4) establishing basins for process-based studies. 

Components proposed for inclusion in the hydrologic 

climate-response data network have at least one key variable 

for which substantial historical data are available. The proposed components are streamflow, lake ice, river ice, snowpack, and groundwater. The proposed key variables of each 

component have extensive historical data at multiple sites and 

are expected to be responsive to climate change in the next 

few decades. These variables are also important for human 

water use and (or) ecosystem function. 

Maine would be divided into seven climate-response 

regions that follow major river-basin boundaries (basins 

subdivided to hydrologic units with 8-digit codes or larger) 

and have relatively homogeneous climates. Key hydrologic 

variables within each climate-response region would be analyzed regularly to maintain up-to-date analyses of year-to-year 

variability, decadal variability, and longer term trends. Finally, 

one basin in each climate-response region would be identified 

for process-based hydrologic and ecological studies.

Introduction

It is important to monitor hydrologic systems in the 

United States that could change dramatically over the short 

term as a result of climate change. Many ecological effects of 

climate change can be well understood only if hydrologic data 

networks are in place. Because of its humid, temperate climate 

and its substantial annual snowpack, Maine’s seasonal water 

cycle is sensitive to small changes in air temperature (Hodgkins and others, 2003b). Monitoring of relevant hydrologic 

data provides important baseline information against which 

future climate change can be measured. 

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

Survey has documented changes in several components of the 

water cycle in Maine during the last 30 to 40 years (discussed 

in the Historical Hydrologic Changes in Maine section). 

Results from climate projections input to hydrologic models 

indicate that changes will continue to occur in the future (discussed in the Future Hydrologic Changes in Maine section). 

The effects of these past and future changes on habitats and 

ecosystems are largely unknown but may include a change 

in the timing of freshwater discharge to estuaries, decreased 

summer lake hypolimnion oxygen levels, and increased stress 

to river ecosystems, leading to possible endangerment or loss 

of species. The trends could affect species at the edge of their 

range in Maine, such as Atlantic salmon and Canada lynx. 

These effects are described in more detail in the following 

sections.

This report presents a framework for a USGS hydrologic climate-response program. This program is designed to 

provide early warning of changes in the seasonal water cycle 

of Maine. The program would be created by maintaining a network of existing USGS data-collection stations and establishing new ones to fill data gaps. In this report, proposed hydrologic variables and data-collection stations to be included 

within homogeneous climate-response regions are discussed. 

Hydrologic data from the network would be analyzed periodically to maintain up-to-date analyses of trends and variability 

and an appropriate basin in each region in which to begin 

process-based studies would be identified.

The USGS has been collecting hydrologic data in Maine 

for more than a century. Since 2000, historical data for many 

Framework for a U.S. Geological Survey Hydrologic 

Climate-Response Program in Maine

By Glenn A. Hodgkins, Robert M. Lent, Charles W. Schalk, and Robert W. Dudley
2 Framework for a U.S. Geological Survey Hydrologic Climate-Response Program in Maine

variables have been analyzed to determine their appropriateness for use in computing climate-related trends and then have 

been used to compute trends. This data collection and analysis 

experience was used to develop the framework for a USGS 

hydrologic climate-response program in Maine. Not all hydrologic variables have been systematically analyzed for their 

appropriateness for computing climate-related trends, however, and therefore the level of detail presented in this report 

varies among hydrologic components. It is anticipated that a 

USGS hydrologic climate-response program in Maine would 

be modified as understanding of the effects of climate change 

on hydrology in Maine improves.

Climate and Hydrology of Maine

The climate of Maine is complex and variable in both 

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

variations in land-surface elevation have a major effect 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 both winter and summer. In winter, the 

ocean variably influences the location of snow/rain boundaries. Despite the influence 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 influences both precipitation and air 

temperatures. Precipitation is higher on the windward side of 

mountains and lower on the leeward side, however, because 

storm-track directions through the mountains are highly 

variable; the windward and leeward areas differ for different 

storms. Air temperature decreases with elevation.

Maine generally has a temperate climate with mild 

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

annual temperature in Maine was about 42 °F, with a range 

from 36 °F in Allagash in northern Maine to 47 °F in Sanford in southern Maine (National Oceanic and Atmospheric 

Administration, 2002). During the same period, statewide 

mean monthly temperatures ranged from 15 °F in January to 

67 °F in July. Precipitation in Maine is fairly evenly distributed throughout the year. The mean annual precipitation from 

1971 to 2000 was 43 in., ranging from 35 in. at Presque Isle 

in northern Maine to 57 in. at Acadia National Park 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 were 

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. for the western 

mountains and 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. for the western mountains and 

northern Maine (Loiselle and Hodgkins, 2002). The 109 data 

collection sites 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 elevations higher than 2,000 ft.

Streamflows in Maine typically are highest in the spring, 

when rain falls on a ripe 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 flows are usually lowest 

in August and September. In the fall, after evapotranspiration decreases substantially, repeated rains often saturate the 

soil, leading to high flows. Also in the fall, large amounts of 

rain can fall as a result of hurricanes, tropical storms, or their 

remnants. Winter flows are generally low in northern parts of 

Maine where winter precipitation typically falls as snow. Winter flows in southern parts of Maine can be more variable than 

flows in northern parts because of more winter rain.

Historical Hydrologic Changes in Maine

Climate-related seasonal hydrologic changes have 

been documented in Maine during the last 200 years. Many 

changes have been documented in winter and spring, whereas 

few consistent changes have been found in summer or fall. 

Overall, there is strong and consistent evidence of changes, 

all consistent with increasing temperatures, in late winter and 

spring in the last 30 to 40 years. There is some evidence of 

recent changes in mid-winter that are consistent with increasing temperatures (Hodgkins and others, 2005a; Huntington 

and others, 2003).

Winter

March mean streamflows increased significantly over 

time (p < 0.1) at 12 of 14 streamflow-gaging stations in or near 

Maine; flows increased 76 to 185 percent at the seven stations 

with the longest continuous records (1920s or 1930s through 

2002) (Hodgkins and Dudley, 2005). There were no stations 

with significant decreases. Eighteen of 23 snow-measurement 

sites in and near Maine with at least 50 years of records had 

a significant decrease (p < 0.1) in late-winter snowpack depth 

or an increase in snowpack density (Hodgkins and Dudley, 

2006a). Increased snowpack density (fig. 1) for a set late-winter date indicates that the snow is more ready to melt.

The ratio of December through March snowfall to total 

precipitation decreased significantly (p = 0.043) for the average of four U.S. Historical Climatology Network (USHCN) 

stations in northern New England from 1949 to 2000 (Huntington and others, 2004). The year-to-year ratio of snowfall 

to total precipitation in northern New England was correlated 

with air temperature (r = -0.45, p = 0.008) and total snowfall (r = 0.48, p = 0.0003) but not with total precipitation 

(r = -0.078, p = 0.59).
Introduction 3

Spring

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, 

2003b; Hodgkins and Dudley, 2006b). 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 streamflow-gaging station. Historical winter-spring 

streamflow timing for the Piscataquis River in central Maine 

is shown in figure 2. Year-to-year streamflow timing was 

highly correlated with March through April air temperature 

(r = -0.72, p < 0.0001) (Hodgkins and others, 2003b). Higher 

air temperatures were associated with earlier streamflows. The 

highest correlation coefficient with precipitation was -0.37 

(p = 0.0018) with January precipitation. May streamflows 

decreased significantly (p < 0.1) at 9 of 14 stations in or near 

Maine; flows decreased 9 to 46 percent at the seven stations 

with the longest continuous records (Hodgkins and Dudley, 

2005). There were no stations with significant increases. 

Annual peak flows often occur in the spring. Annual peak 

1900 1920 1940 1960 1980 2000

YEAR

0.15

0.20

0.25

0.30

0.35

0.40

DENSITY

FOUR-SITE AVERAGE

Locally weighted regression line

Figure 1. Annual average snowpack density at four sites in 

western Maine/northern New Hampshire for the March 15 

sampling window (the 15-day period centered on March 15), 

1914–2004.

Locally weighted regression line

1900 1920 1940 1960 1980 2000

YEAR

DATEApr

 10

Apr

 20

Apr

 30

May

 10

Mar

 31

Mar

 21

Mar

 11

Figure 2. Annual winter-spring center-of-volume dates 

(a measure of streamflow timing) for the Piscataquis River in 

central Maine, 1903–2000.

1820 1845 1870 1895 1920 1945 1970 1995 2020

YEAR

70

80

90

100

110

120

JULIAN DATE OF ICE OUT

130

Locally weighted regression line

Figure 3. Annual lake ice-out dates for Damariscotta Lake, 

midcoastal Maine, 1837–2008. The Julian date is the sequential 

day of each year, starting on January 1.

Huntington and others (2003) found a significant decrease 

over time (p = 0.0021) in average ice thickness around 

February 28 on the Piscataquis River in central Maine. The 

ice thinned about 9 in. (45 percent) from 1912 to 2001. On 

average, for the nine rivers in northern New England (Maine, 

New Hampshire, and Vermont) with the longest records, riverice occurrence (total number of winter days with ice-affected 

flow) decreased significantly (p = 0.0013) from 1936 to 2000 

(Hodgkins and others, 2005a). Most of the 20-day change in 

the total days of ice occurred from the 1960s to 2000. Year-toyear river-ice occurrence was highly correlated with winter air 

temperature (r = -0.70, p < 0.0001) and less highly correlated 

with winter precipitation (r = -0.52, p < 0.0001). More days of 

river ice were associated with colder temperatures and lower 

precipitation.
4 Framework for a U.S. Geological Survey Hydrologic Climate-Response Program in Maine

flows have increased significantly (p < 0.1) during the last 

50 to 100 years at some streamflow-gaging stations in Maine 

(Hodgkins and Dudley, 2005; Collins, 2009).

On average, for the nine rivers in northern New England with the longest records, river-ice breakup (the annual 

last spring days of ice-affected flow) became significantly 

earlier (p = 0.0037) from 1936 to 2000. Most of the 11-day 

change in river-ice breakup occurred from the 1960s to 2000 

(Hodgkins and others, 2005a). River-ice breakup was highly 

correlated with March through April air temperature (r = -0.73, 

p < 0.0001) and less highly correlated with precipitation (highest r = -0.37, p = 0.0027). Lake ice-out dates have become significantly earlier in Maine since the early 1800s (p < 0.0001). 

Ice-out dates between 1850 and 2000 were 9 days earlier in 

northern/mountainous areas of Maine and 16 days earlier in 

southern areas (Hodgkins and others, 2002). Historical ice-out 

dates for Damariscotta Lake in midcoastal Maine are shown in 

figure 3.

Summer and Fall

There is only weak evidence of historical summer or 

fall hydrologic changes in Maine. In summer, few significant 

changes during the last century in the magnitude, timing, or 

duration of low streamflows in Maine have been found (Dudley and Hodgkins, 2005; Hodgkins and Dudley, 2005; Hodgkins and others, 2005b). The timing and magnitude of low 

flows were much more highly correlated with summer precipitation than with air temperature (Hodgkins and others, 2005b). 

One of 11 rivers in Maine had significantly later (p < 0.1) 

first fall days of river ice (Hodgkins and others, 2005a). Few 

significant changes in the timing or magnitude of fall high 

flows have been found (Hodgkins and others, 2003b; Dudley and Hodgkins, 2005; Hodgkins and Dudley, 2005). Fall 

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

date, each year, that half of the fall streamflow volume passes 

a streamflow-gaging station.

Future Hydrologic Changes in Maine

Output from recent modeling using multiple atmosphereocean general circulation models, regionally down-scaled, 

and forced with low (B1), mid-high (A2), and high (A1FI) 

greenhouse-gas emission scenarios (Nakicenovic and others, 

2000) provides a range of climate projections for the northeastern United States (Hayhoe and others, 2007). Hayhoe and 

others (2007) used climate projections as inputs to the variable 

infiltration capacity (VIC) hydrologic model to simulate future 

hydrologic responses. 

Spring streamflow timing is expected to become earlier by 5 to 8 days, and annual 7-day low flows are expected 

to decrease by 1 to 4 percent between the periods 1961–90 

and 2035–64 (Hayhoe and others, 2007). Projected low-flow 

changes by mid-century are very small; however, under the 

high-emissions scenario, larger decreases (11 percent) are 

projected by the period 2070–99. The number of short-term 

and long-term droughts (based on soil-moisture deficits) is 

expected to increase by 2035–64. Total annual snow-water 

equivalent and the number of days with snow per month are 

expected to decrease.

Potential Effects of Hydrologic Changes

Human water supply and transportation infrastructure 

in the northeastern United States may be affected by climate 

change. Earlier snowmelt runoff could cause sensitive water 

systems to be impacted because of less water availability 

in the summer. Managers of storage reservoirs may need to 

modify operation schedules or increase reservoir storage to 

allow successful operation in a potentially longer summer 

low-flow season. If summer low flows decrease, water-use 

conflicts will likely increase. Also, changes in streamflow in 

the northeastern United States may be important for other 

reasons; for example, higher winter flows can cause and have 

caused an increase in the frequency of mid-winter ice jams 

(Beltaos, 2002). River ice jams can cause major flooding and 

damage river infrastructure. Ice jams played a major role in 

Maine flooding in 1936 and 1991 (Grover, 1937; Wuebben 

and others, 1995).

Changes in the timing or magnitude of future streamflows 

will likely affect ecosystems. The ecological implications 

of changes in winter-spring streamflow timing in Maine are 

not well understood. One possible effect is a decrease in the 

survival rates of Atlantic salmon. If the peak spring migration 

of juvenile salmon from freshwater rivers (which is controlled 

by photoperiod, temperature, and flow) becomes out of phase 

by 2 weeks with optimal environmental conditions in rivers, estuaries, or the ocean, salmon survival could decrease 

substantially (McCormick and others, 1998). Changes in the 

timing of spring river flows may affect freshwater mixing in 

estuarine ecosystems. Earlier snowmelt and increases in summer evapotranspiration rates could shorten the annual periods 

of standing water used by amphibians for breeding in forested 

depressional wetlands (vernal pools). If summer low flows 

decrease, there could be higher water temperatures, reduced 

dissolved oxygen levels, and increased concentrations of 

contaminants (Bradbury and others, 2002). If river base flows 

(which are influenced by groundwater discharge) change, this 

could greatly affect cold-water fish because of changes in 

water temperature and dissolved-oxygen concentrations.

River ice is responsible for the creation of many erosional 

and depositional features in river channels and on channel 

floodplains. Physical disturbances associated with ice break-up 

scouring and flooding are primary to nutrient and organicmatter dynamics, water chemistry, and the abundance and 

diversity of river biota (Prowse and Beltaos, 2002). The succession of riparian vegetation is directly linked to the scouring 

effects of ice (Prowse and Beltaos, 2002). River-ice break-up 
Framework for a Hydrologic Climate-Response Program 5

is likely to have important effects on primary producers, 

consumers, and food-web dynamics of river biota, although 

detailed information describing the magnitude of their effects 

is scarce (Scrimgeour and others, 1994). Mortality, emigration, or displacement of fishes at all life stages is often the 

result of severe ice conditions, through the action of damming, 

scouring, associated flooding, or direct freezing (Power and 

others, 1993). Anchor ice, frazil (slush) ice that adheres to 

streambeds, can have serious effects on fish eggs and embryos 

developing within gravel beds (Prowse, 1994). Anchor ice 

does not form when surface-ice cover is present and, therefore 

could increase as surface-ice cover decreases.

Reduced late-summer hypolimnion oxygen levels are 

likely related to earlier lake ice-out dates (discussed by 

Stewart (1976) for some lakes in Madison, Wisconsin). Earlier 

ice-outs could affect the productivity and abundance of phytoplankton (Maeda and Ichimura, 1973) and organisms at higher 

trophic levels (Porter and others, 1996).

Framework for a Hydrologic ClimateResponse Program

The framework of the hydrologic climate-response program presented here consists of four major parts: (1) identifying homogeneous climate-response regions; (2) identifying 

hydrologic components, key variables of those components, 

and data-collection sites that would be included in a hydrologic climate-response data network; (3) regularly updating 

historical trends of hydrologic network variables; and (4) 

establishing basins for process-based studies.

Climate-Response Regions

Hydrologic climate-response regions should have 

relatively homogeneous climates. Maine’s climate, as 

described in the Introduction section, is complex, with a 

large north-to-south gradient, coastal-to-inland gradient, 

and sea-level-to-mountain-region gradient. Maine would be 

divided into seven regions (fig. 4, table 1) that follow major 

river-basin boundaries, have relatively homogeneous climates, and are broadly consistent with the U.S. Environmental 

Protection Agency ecoregions (U.S. Environmental Protection 

Agency, 2007) and biophysical regions of Maine as defined 

by Krohn and others (1999). Major river-basin boundaries 

would be followed by subdividing basins to hydrologic units 

with 8-digit codes (Seaber and others, 1987) or larger. Results 

of additional analyses of hydrologic variations may indicate a 

need to change the boundaries and (or) number of hydrologic 

climate-response regions in the future. It may also be necessary to divide regions on a scale finer than major river-basin 

boundaries, particularly in region 1 where coastal and mountainous areas are included in the same hydrologic unit.

Hydrologic Climate-Response Data Network

An important part of developing a USGS hydrologic 

climate-response program in Maine is the establishment of 

a hydrologic climate-response data network, including the 

identification of hydrologic components, key variables of 

those components, and data-collection sites. As an example, 

streamflow has been identified as a primary component, with 

a key variable of streamflow being winter-spring streamflow 

timing. The key variables of components discussed in this section have been demonstrated or are expected to be responsive 

to climate change in the next few decades and are important 

for human water use or ecosystem function. 

Two criteria were used to identify key variables to be 

included in a hydrologic climate-response data network: 

the existence of long-term historical data and the human or 

ecological importance of the component variables. Longterm historical data allow a much better perspective on future 

climate-related changes than short-term data because past 

decadal variability and longer term changes are better known. 

The Introduction section of this report lists known historical 

changes for variables of several hydrologic components and 

their known importance for humans and ecosystems. There 

are many variables to describe some of the components in 

a hydrologic climate-response data network. For example, 

streamflow variables could include annual peak flows, March 

mean flows, or many other measures. On the basis of extensive past work, some variables are more likely than others to 

be responsive to past and future climate changes (see Introduction section). Components proposed for inclusion in a 

hydrologic climate-response data network have at least one 

key variable with extensive historical data at multiple sites: 

streamflow, lake ice, river ice, snowpack, and groundwater. 

Other hydrologic components are important to humans and 

(or) ecosystems but lack variables with extensive historical 

data. Key variables of primary components are listed in the 

following sections; additional variables could be added as part 

of a larger program. 

Table 1. Hydrologic climate-response regions in Maine.

Region 

number River basins included

1 Southern coastal, Saco, Presumpscot, mid-coastal

2 Eastern coastal (Downeast)

3 Lower Androscoggin, lower Kennebec

4 Lower Penobscot, St. Croix

5 Upper Androscoggin, upper Kennebec, upper Penobscot

6 Upper St. John, Allagash

7 Lower St. John, Fish, Aroostook, Meduxnekeag
6 Framework for a U.S. Geological Survey Hydrologic Climate-Response Program in Maine MAINE NEW HAMPSHIRE

STATE BOUNDARY

COUNTY BOUNDARY

EXPLANATION 70o

44o

68o

46o

68o

70o

46o

44o

0 25 50 100 MILES

0 25 50 100 KILOMETERS

Base from U.S. Geological Survey digital files, Universal Transverse 

Mercator projection, Zone 19

HYDROLOGIC CLIMATE-RESPONSE REGION

Southern coastal, Saco, Presumpscot, mid-coastal

Eastern coastal (Downeast)

Lower Androscoggin, lower Kennebec

Lower Penobscot, St. Croix

Upper Androscoggin, upper Kennebec, upper Penobscot

Upper St. John, Allagash

Lower St. John, Fish, Aroostook, Meduxnekeag

Figure 4. Location of hydrologic climate-response regions in Maine.
Framework for a Hydrologic Climate-Response Program 7

Primary Components and Associated Key 

Variables

Streamflow

For the streamflow component, two key variables are 

proposed: winter-spring streamflow timing and the magnitude 

of the annual peak flow. Winter-spring streamflow timing is 

measured by the winter-spring center-of-volume (WSCV) date 

(Dudley and Hodgkins, 2002; Hodgkins and others, 2003b; 

Hodgkins and Dudley, 2006b). The WSCV is a robust measure 

of streamflow timing. To compute the WSCV date, daily flow 

volumes from the start to the end of the annual winter-spring 

season are summed. The WSCV date is then computed as the 

date, from the start of the season, by which half of the volume 

flows by a gaging station. Another key variable derived from 

streamflow data—the magnitude of summer base flows—is 

discussed in the Groundwater section.

The USGS has been measuring and recording daily 

streamflow at some sites in Maine for more than a century. 

More than 50 years of USGS streamflow data are available for 

many rural, unregulated streams (http://waterdata.usgs.gov/

usa/nwis/sw). The streamflow data at these sites have been 

collected by the USGS using consistent, well-documented, 

high-quality methods (Corbett and others, 1943; Rantz and 

others, 1982). Therefore, historical changes in the data can be 

determined with confidence.

A network of streamflow-gaging stations whose basins 

are relatively free of human influences such as regulation, 

diversion, land-use change, or extreme groundwater pumpage was defined by Slack and Landwehr (1992). This HydroClimatic Data Network (HCDN) includes data from more 

than 1,500 streamflow-gaging stations across the United 

States. Hodgkins and others (2003b) used all HCDN stations 

in Maine with daily streamflows free of substantial human 

influences and at least 50 continuous years of record through 

2000 to examine changes over time in the timing of winterspring and fall high streamflows. Fourteen streamflow-gaging 

stations in Maine met the criteria of that study (table 2, fig. 5). 

The two stations in New Hampshire are on rivers that flow 

into Maine. These 14 stations would be the starting point 

for stations to be included in the streamflow component of a 

hydrologic climate-response data network in Maine.

Regulation at each of these stations was analyzed further 

because low flows can be sensitive to even minor regulation (Hodgkins and others, 2005b). The amount of regulation 

upstream from each U.S. Geological Survey gaging station is 

listed in the annual USGS Water-Data Reports for Maine and 

in their predecessor USGS Water-Supply Papers. If a river 

Table 2. Long–term streamflow-gaging stations on Maine rivers that drain relatively natural basins.

[USGS, U.S. Geological Survey]

Sequential 

number

USGS streamflow-gaging 

station number USGS streamflow–gaging station name Period of streamflow record

1 01010000 St. John River at Ninemile Bridge, Maine 1950–2007

2 01010500 St. John River at Dickey, Maine 1910–1911, 1946–2007

3 01013500 Fish River near Fort Kent, Maine 1903–1908, 1911, 1929–2007

4 01014000 St. John River below Fish River at Fort Kent, Maine 1926–2007

5 01022500 Narraguagus River at Cherryfield, Maine1 1948–2007

6 01030500 Mattawamkeag River near Mattawamkeag, Maine 1934–2007

7 01031500 Piscataquis River near Dover-Foxcroft, Maine2 1902–2007

8 01038000 Sheepscot River at North Whitefield, Maine 1938–2007

9 01047000 Carrabassett River near North Anson, Maine3 1902–1907, 1925–2007

10 01052500 Diamond River near Wentworth Location, New Hampshire 1941–2007

11 01055000 Swift River near Roxbury, Maine 1929–2007

12 01057000 Little Androscoggin River near South Paris, Maine4 1913–1924, 1931–2007

13 01060000 Royal River at Yarmouth, Maine5 1949–2004

14 01064500 Saco River near Conway, New Hampshire 1903–1909, 1929–2007

1

 Unusual regulation affected flows September 2–6, 1978, and September 22–26, 1985.

2

 Data from this station inappropriate for analysis of historical low–flow changes because of historical regulation.

3

 Data from this station for years prior to 1931 inappropriate for analysis of historical low–flow changes because of historical regulation.

4

 Data from this station for years prior to 1932 inappropriate for analysis of historical low–flow changes because of historical regulation.

5

 Unusual regulation affected flows July 23–25, 1980.
8 Framework for a U.S. Geological Survey Hydrologic Climate-Response Program in Maine MAINE NEW HAMPSHIRE

STATE BOUNDARY

COUNTY BOUNDARY

EXPLANATION

HYDROLOGIC CLIMATE-RESPONSE REGION

Southern coastal, Saco, Presumpscot, mid-coastal

Eastern coastal (Downeast)

Lower Androscoggin, lower Kennebec

Lower Penobscot, St. Croix

Upper Androscoggin, upper Kennebec, upper Penobscot

Upper St. John, Allagash

Lower St. John, Fish, Aroostook, Meduxnekeag

70o

44o

68o

46o

68o

70o

46o

44o

0 25 50 100 MILES

Base from U.S. Geological Survey digital files, Universal Transverse 

Mercator projection, Zone 19

0 25 50 100 KILOMETERS

01022500

01030500

01013500

01014000

01010500

01031500

01038000

01010000

01047000

01060000

01057000

01055000

01052500

01064500

STREAMFLOW-GAGING STATION AND NUMBER 01031500

Figure 5. Location of long-term streamflow-gaging stations that drain relatively natural basins in and near Maine.
Framework for a Hydrologic Climate-Response Program 9

was noted in the USGS reports as having low-flow regulation 

or significant storage regulation, low flows from those years 

are not considered appropriate for inclusion in analyses of 

historical climate-related trends. If a river was noted as having 

some, infrequent, or diurnal low-flow regulation, the flows are 

considered appropriate. Plots of low flows over the period of 

record at all sites were used to verify the qualitative regulation 

criteria. Based on these regulation criteria, data from one station (Piscataquis River) should not be analyzed for low flows, 

and early years of data at two stations (Carrabassett and Little 

Androscoggin Rivers) are not appropriate for inclusion in low 

flow analyses, nor are data for a few days with unusual regulation at two stations (Narraguagus and Royal Rivers) (table 2). 

Lake Ice

The key (and only commonly available) historical 

measure of lake ice is lake ice-out date—the annual date in 

spring when winter ice cover leaves a lake (Hodgkins and 

others, 2002). In Northern Hemisphere mid-latitude areas such 

as Maine, ice-out dates can serve as useful indicators of late 

winter and early spring climate change. A remarkable amount 

of lake ice-out data has been recorded and saved in Maine 

over the past two centuries. Lake ice-out dates for 23 lakes 

in Maine were compiled (Hodgkins and James, 2002) and 

analyzed (Hodgkins and others, 2002) (table 3, fig. 6). As of 

2008, 5 lakes have more than 160 years of data and 15 have 

more than 100 years of data. Long-term lake ice-out data for 

many lakes in Maine represent a unique hydroclimatic data 

set that is very useful for continued monitoring of hydrologic 

response to regional climate change. These lakes would be the 

starting point for lakes used in a hydrologic climate-response 

data network.

Most of the lakes in this study are in rural areas (Hodgkins and others, 2002). Lakes in northern Maine generally 

drain remote, undeveloped forests. Lakes in southern Maine 

generally drain rural areas with forests, some low-density 

residential development, and small towns. Lake Auburn is near 

a relatively small urban area. The ice-out dates for this lake 

could be affected by urban heat-island temperature effects. It is 

unlikely that any activities that would affect the trends in iceout dates over time, such as warm-water discharges, occurred 

on the rural lakes in this study or on their inlet streams. 

Ice-out definitions for individual lakes can vary over time 

and among observers. (Hodgkins and others, 2002). Twenty 

years or more of overlapping data from independent observers were available for six lakes. Based on this limited data set, 

Hodgkins and others (2002) concluded that observer biases 

(different people on the same lake recording different ice-out 

dates for the same year) of more than 1 day are unlikely on 

relatively round lakes. In contrast, 3- to 4-day biases appear 

likely on long, narrow lakes if the observation locations are 

on opposite ends of the lake. On two long, narrow lakes with 

north-south axes, the observer with later ice-out dates was 

located at the northern end of the lakes. These observational 

biases could result in large biases in ice-out trends over time 

at individual lakes. At a large number of lakes, however, it is 

unlikely that observer-location biases would tend to bias trend 

tests in any one direction. For example, for a large number of 

lakes there is no reason to expect that people observed older 

ice-outs at the southern end of lakes and observed more recent 

ice-outs at the northern end of lakes.

River Ice

Occurrence

For river-ice occurrence, key variables are the annual 

first day of ice-affected flow in the winter, the last day of iceaffected flow in the spring, and the total number of days of 

ice-affected flow. The primary product of USGS streamflowgaging efforts has been the computation and publication of 

daily mean river flows. Flows, however, are not typically 

Table 3. Maine lakes with long-term ice-out data. 

Lake 

number Lake name Period of record

Years of 

record

1 Eagle 1922–2008 75

2 Portage 1925–2008 83

3 Squa Pan 1930–2008 70

4 Moosehead 1848–2008 161

5 Aziscohos 1913–2008 94

6 Rangeley 1880–2008 129

7 Mooselookmeguntic 1884–2008 107

8 Richardson 1880–2008 127

9 Umbagog 1880–2008 118

10 West Grand 1878–2008 131

11 Sebec 1879–2008 128

12 Embden 1925–2005 80

13 Pennesseewassee 1874–2008 133

14 Kezar 1901–2008 107

15 Swan 1891–2008 118

16 Messalonskee 1909–2008 75

17 China 1874–2008 82

18 Maranacook 1925–2008 84

19 Cobbosseecontee 1840–2008 167

20 Damariscotta 1837–2008 171

21 Auburn 1836–2008 166

22 Thompson 1902–2008 102

23 Sebago 1807–2008 168
10 Framework for a U.S. Geological Survey Hydrologic Climate-Response Program in Maine MAINE NEW HAMPSHIRE

STATE BOUNDARY

COUNTY BOUNDARY

EXPLANATION

70o

68o

46o

44o

68o

70o

44o

46o

0 25 50 100 MILES

0 25 50 100 KILOMETERS

Base from U.S. Geological Survey digital files, Universal Transverse 

Mercator projection, Zone 19

Sebec Lake

HYDROLOGIC CLIMATE-RESPONSE REGION

Southern coastal, Saco, Presumpscot, mid-coastal

Eastern coastal (Downeast)

Lower Androscoggin, lower Kennebec

Lower Penobscot, St. Croix

Upper Androscoggin, upper Kennebec, upper Penobscot

Upper St. John, Allagash

Lower St. John, Fish, Aroostook, Meduxnekeag

LAKE-ICE SITE AND NAME

Swan 

Lake

Sebec Lake

Kezar Lake

Eagle Lake

China 

Lake

Sebago 

Lake

Embden Pond

Lake Auburn

Umbagog 

Lake

Portage Lake

Thompson 

Lake

Squa Pan Lake

Rangeley Lake

Moosehead Lake

Aziscohos Lake

West Grand 

Lake

Richardson Lake

Maranacook Lake

Messalonskee Lake

Damariscotta 

Lake

Mooselookmeguntic Lake

Pennesseewassee Lake Cobbosseecontee Lake

Figure 6. Location of lakes with long-term ice-out data in Maine.
Framework for a Hydrologic Climate-Response Program 11

measured continuously. Instead, the river height is measured 

continuously (currently once every 15 minutes) and the flows 

computed with a river height/flow relation, called a rating 

curve. The presence of ice in a river channel affects the relation between river height and flow (Rantz and others, 1982); 

therefore, the presence of ice in rivers has been historically 

determined and recorded. 

Records from all gaging stations on rivers in Maine were 

searched for long-term, usable records of the occurrence of 

ice-affected flow. Long-term records were defined as those 

from a gaging station that was operated for at least 50 years 

with records through 2000 (Hodgkins and others, 2005a). 

Usable records were defined as those from a gaging station 

whose hydraulic-control-section location (typically, the riffles 

downstream from a gaging station that control the river height 

at the station at low to medium flows) did not change during the period of record and whose flows were not subject 

to substantial regulation or urbanization. Ice conditions can 

differ substantially with location, even over short distances 

upstream or downstream from any given location. For this 

reason, rivers on which the control-section location changed 

over the period of record were not used. Substantial regulation 

was defined as regulation that could affect the formation and 

(or) stability of ice at the station. This required that a river at a 

gaging station had either no known flow regulation over time 

or only low-flow regulation, of an amount that was judged to 

be insignificant. 

The formation of ice in river channels, in particular on 

hydraulic-control sections, affects the river height/flow relation by causing backwater (a higher than normal river height 

for a given flow) (Rantz and others, 1982). This backwater 

varies with the quantity and nature of the ice, as well as 

with the flow. Backwater at a gaging station can be caused 

by anchor ice or by surface ice. Anchor ice is an accumulation of frazil (slush) ice that adheres to the rocks of a river 

bed. It may build up on the river bed and (or) the hydraulic 

control section to the extent that backwater occurs. Surface ice 

normally forms first along the edges of a river and then builds 

out from the shore, eventually forming an ice sheet across the 

entire river. Surface ice, where it is in contact with the river, 

increases the frictional resistance to the river flow and the river 

height will increase for a given flow. 

Rises in river height due to anchor ice are clearly recognizable from continuous river-height records (Hodgkins 

and others, 2005a). Rises in river height due to surface ice 

are often recognizable from continuous river-height records. 

River-height records are supplemented by visual observations 

of river-ice conditions, river-flow measurements, daily temperature and precipitation records, and flows from nearby gages 

to determine days of ice-affected flow. River-height rises, 

particularly rises that lack the smoothness of those caused by 

an increase in flow, combined with very cold temperatures and 

a lack of precipitation are indicative of ice-affected river flow.

The first ice-affected river flows in Maine each fall are 

caused by the first substantial presence of anchor ice and (or) 

surface ice (Hodgkins and others, 2005a). The breakup of river 

ice in the spring, at rivers with complete or nearly complete 

ice cover, typically is a dramatic event in which the ice is 

picked up and transported by medium or high river flows. 

Dynamic river-ice breakup can also occur in the winter. Days 

of ice-affected flow each winter, in general, are continuous or 

nearly continuous in northern Maine and can be less continuous in southern and coastal Maine. It is unlikely that any 

activities, such as warm-water discharges, that would affect 

the trends in ice-affected river flows over time occurred on or 

near the rivers or on their tributaries. 

Eleven gaging stations on rivers in Maine met the criteria 

in Hodgkins and others (2005a) (table 4, fig. 7). These stations 

would be the starting point for river-ice data collection for a 

hydrologic climate-response data network.

Table 4. Maine rivers with long-term ice occurrence data.

[USGS, U.S. Geological Survey]

USGS streamflow-gaging 

station number USGS streamflow-gaging station name Period of record

01011000 Allagash River near Allagash, Maine 1932–2007

01013500 Fish River near Fort Kent, Maine 1930–2007

01014000 St. John River below Fish River at Fort Kent, Maine 1934–2007

01022500 Narraguagus River at Cherryfield, Maine 1949–2007

01031500 Piscataquis River near Dover-Foxcroft, Maine 1931–2007

01038000 Sheepscot River at North Whitefield, Maine 1939–2007

01048000 Sandy River near Mercer, Maine 1929–1979, 1988–2007

01052500 Diamond River near Wentworth Location, New Hampshire 1942–2007

01055000 Swift River near Roxbury, Maine 1931–2007

01060000 Royal River at Yarmouth, Maine 1950–2004

01064500 Saco River near Conway, New Hampshire 1930–2007
12 Framework for a U.S. Geological Survey Hydrologic Climate-Response Program in Maine MAINE NEW HAMPSHIRE

STATE BOUNDARY

COUNTY BOUNDARY

EXPLANATION 70o

44o

68o

46o

68o

70 o

46 o

44o

01022500

01013500

01014000 01011000

01031500

01038000

01048000

01060000

01055000

01052500

01064500

0 25 50 100 MILES

0 25 50 100 KILOMETERS

Base from U.S. Geological Survey digital files, Universal Transverse 

Mercator projection, Zone 19

HYDROLOGIC CLIMATE-RESPONSE REGION

Southern coastal, Saco, Presumpscot, mid-coastal

Eastern coastal (Downeast)

Lower Androscoggin, lower Kennebec

Lower Penobscot, St. Croix

Upper Androscoggin, upper Kennebec, upper Penobscot

Upper St. John, Allagash

Lower St. John, Fish, Aroostook, Meduxnekeag

01052500 RIVER-ICE SITE AND NUMBER

Figure 7. Location of rivers with long-term ice-occurrence data in and near Maine.
Framework for a Hydrologic Climate-Response Program 13

The presence of ice in rivers has been determined by the 

USGS using the same methods since 1913 (and probably for 

several years previously) to the present (Hoyt, 1913; Rantz 

and others, 1982), with one known exception. The earliest 

river-height records in New England typically were collected 

daily by an observer, rather than by a continuous recorder, 

until the 1930s (Hodgkins and others, 2005a). The presence of 

ice, especially anchor ice, that affected the river height/flow 

relation may have been interpreted differently using these two 

methods of data collection. Therefore, only years in which 

continuous recorders were used at each gaging station are considered appropriate for use in river-ice analyses (table 4).

The determination of days of ice-affected flow that occur 

during periods of missing river-height record at a gaging 

station can be less accurate than that for periods when these 

records are available (Hodgkins and others, 2005a). This 

is because estimates during periods of missing river-height 

records are based on flows from nearby rivers and on daily 

temperature and precipitation records. Individual years of 

record at gaging stations with missing winter river-height 

records were censored if there was any question about the 

accuracy of the ice-affected-flow records (Hodgkins and others, 2003a). On average, 4 years of data were censored at each 

station.

Thickness

Annual river-ice thickness for set winter dates (such as 

February 28th) is a key variable of river ice. USGS winter 

streamflow measurements allow for the historical estimation 

of river-ice thickness at streamflow-gaging stations, as has 

been done for the Piscataquis River in Maine (Huntington 

and others, 2003). Rivers should be free of substantial human 

influence, as described in the preceding Occurrence section, 

and streamflow-measurement locations should be constant 

over time, as ice conditions can vary dramatically upstream 

or downstream from a location. Another consideration in 

river selection is that frazil (slush) ice should not be present 

underneath solid ice because the two types of ice historically 

have not been differentiated when making winter streamflow 

measurements.

To measure streamflow under ice in winter, holes are 

drilled through the ice to reach the underlying water (Rantz 

and others, 1982). A standard part of USGS winter measurements has been to measure the distance from the water surface 

to the bottom of the ice (WSBI), which has been done with 

consistent methods over time. WSBI is different from ice 

thickness because the water surface can be above or below 

the top of the ice; however, it is highly correlated with ice 

thickness (Huntington and others, 2003). Actual ice-thickness 

measurements at sites can therefore be estimated by regression 

methods if the relation between WSBI and ice thickness is 

strong and linear, as it was for the Piscataquis River (r = 0.95). 

USGS streamflow-gaging stations in Maine with long-term 

records would be analyzed for the criteria described above, 

and appropriate stations would become part of a hydrologic 

climate-response data network.

Snowpack 

Key snowpack variables are the magnitude of late-winter 

water equivalent, depth, and density for selected dates. Emergency management and response agencies, flood-forecasting 

agencies, people who live near streams, and many waterdependent industries need to know how much water to expect 

each year from snowmelt. Primarily for this reason, the Maine 

Cooperative Snow Survey Program, run jointly by the Maine 

Geological Survey (MGS) and the USGS, compiles snowpack 

data collected by the USGS, the MGS, the National Weather 

Service, electric-power utilities, water-power companies, pulp 

and paper companies, and others on a regular basis (currently 

weekly, historically biweekly) in late-winter and spring (Hodgkins and Dudley, 2006a).

The depth and water equivalent (the depth of water that 

would result if the snowpack were melted) of the snowpack 

have been measured at selected sites in Maine since the early 

part of the 20th century (Hodgkins and others, 2005c). Most of 

the sites are in flat or gently sloping areas of mixed hardwood 

and conifer forest. Measurements are not made near conifers. 

Most data are collected at locations with an elevation of less 

than 2,000 ft. Historical site information for the sites in this 

report is extremely limited. Some sites have been moved away 

from the local effects of development, extensive logging, or 

unacceptable amounts of conifer growth.

Snowpack depth and water-equivalent data were analyzed 

for those sites with data spanning at least 50 years through 

2004 (Hodgkins and Dudley, 2006a). The exact date of sampling at a site varies from year to year. Data for a site commonly are not available for every year for a given sampling 

window. Sampling windows were defined as 15-day windows 

centered on February 15, March 1, March 15, April 1, April 15, 

and May 1. To be included for analysis, the sites were required 

to have at least 50-percent complete data for the first and 

second halves of their record for at least one sampling window. 

Thirty-seven sites in and near Maine met the described criteria. 

Historical snow depth and water-equivalent data for these sites 

are reported in Hodgkins and others (2005c).

Because sampling windows were used, it is possible to 

have biased sampling over time at sites (Hodgkins and Dudley, 

2006a). If sampling tends to be earlier or later over time, any 

significant trends in the snowpack data could be the result of 

sampling bias. All data sets (individual sites for each applicable sampling window) that met the criteria for inclusion 

were tested for significant changes over time in the date of 

sampling. Data from many sites sampled during the March 1 

sampling window were found to be biased, with many sampling dates prior to March 1 early in the record and many dates 

after March 1 later in the record. Fourteen sites were eliminated because of significant bias in the date of sampling. Data 

from 23 sites with data for at least one sampling window did 

not show significant sampling bias and were considered appropriate for use in computing climate-related trends over time 

(table 5, fig. 8). These sites would be the starting point for sites 

to be included in a hydrologic climate-response data network.
14 Framework for a U.S. Geological Survey Hydrologic Climate-Response Program in Maine

Groundwater

Key variables for the groundwater component are the 

amount of winter recharge and the amount of spring recharge, 

as measured by groundwater levels, and the base-flow component of summer streamflows. The USGS has collected 

groundwater-level data in Maine for several decades but 

no long-term trend analyses of groundwater data in Maine 

have been completed. The groundwater network would be 

examined for relatively complete long-term data at wells that 

have not been moved or have other changes that may make 

comparisons between older and newer data inappropriate. The 

most appropriate sites would be identified for inclusion in a 

hydrologic climate-response data network. Given the relatively low density of groundwater-monitoring wells in Maine, 

it may be necessary to add wells to the network. 

The base-flow component of streamflow comes from 

groundwater and other delayed sources. The USGS streamflow-gaging stations identified for potential inclusion in a 

hydrologic climate-response data network and appropriate for 

the analysis of low flows (table 2) would be appropriate to use 

as input to an automated base-flow-separation technique, such 

as HYSEP (Sloto and Crouse, 1996), to separate base flow 

from total streamflow.

Secondary Components

To design a more comprehensive hydrologic climateresponse data network, secondary components would be 

added. In general, these would be hydrologic components for 

which historical data are limited. Components could include 

streamflow temperature, dissolved-oxygen concentration, 

and other water-quality parameters. They could also include 

components representing parts of the water cycle, such as 

evapotranspiration, that historically have not been measured at 

many sites.

Existing Primary and Secondary Data-Collection 

Sites

Within each climate-response region, for each key 

variable, the most appropriate data-collection sites would be 

identified. The starting point for sites would be those with 

long-term, high-quality data that were identified earlier in this 

report. Primary sites would then be identified that are representative of the region and are important for other reasons—

such as relevance to endangered species like Atlantic salmon, 

proximity to Native American interests (fig. 9) and U.S. 

Department of Interior and Agriculture lands (fig. 10), and 

(or) importance for water supply. The surface-water sources 

in Maine with the largest withdrawals are shown in figure 11. 

Secondary sites within each hydrologic region would be identified as the long-term, high-quality sites that are in excess of 

the minimum number of sites needed. 

New Data-Collection Sites

Some regions do not contain sites with long-term historical data for some key hydrologic variables. In these cases, 

sites with less historical data would be considered. If no 

appropriate sites are found, data collection at new sites would 

be started. 

Table 5. Long-term snowpack sites in and near Maine.

[Abbreviations in site name are part of the official name. All sites are in 

Maine, unless otherwise noted. A sampling window is defined as a 15-day 

period centered on the date shown.]

Site 

number Site name Sampling window

1002 The Forks March 1

1004 Grindstone March 1

1015 Mercer March 1

1020 Dover-Foxcroft (B) March 1

1024 North Anson March 1

1044 Pinkham Notch (UWP), N.H. March 1

1050 Pittsfield (B) March 1

1053 Gorham, N.H. March 1

1066 Mayfield (Bingham Upper) March 1

1292 Kokadjo (KWP) March 1

1002 The Forks March 15

1018 Amherst (BH) March 15

1022 Fort Kent March 15

1024 North Anson March 15

1027 Errol/Errol Dam (UWP), N.H. March 15

1028 Aziscohos/Aziscohos Dam (UWP) March 15

1030 Middle Dam (UWP) March 15

1031 Upper Dam (UWP) March 15

1046 South Paris March 15

1061 Guerrette March 15

1100 Eustis (KWP) March 15

1246 Stratton (KWP) March 15

1288 Carrabassett (KWP) March 15

1289 Dallas (KWP) March 15

1290 Chain of Ponds (KWP) March 15

1027 Errol/Errol Dam (UWP), N.H. April 1

1028 Aziscohos/Aziscohos Dam (UWP) April 1

1030 Middle Dam (UWP) April 1

1031 Upper Dam (UWP) April 1
Framework for a Hydrologic Climate-Response Program 15

Gorham

Mercer

Guerette

Fort Kent

The Forks

Grindstone

South Paris

North Anson

Dallas (KWP)

Eustis (KWP)

Amherst (BH)

Kokadjo (KWP)

Stratton (KWP)

Pittsfield (B)

Upper Dam (UWP)

Errol Dam (UWP)

Middle Dam (UWP)

Carrabassett (KWP) Dover-Foxcroft (B)

Pinkham Notch (UWP)

Aziscohos Dam (UWP)

Chain of Ponds (KWP)

Mayfield 

(Bingham Upper)

HYDROLOGIC CLIMATE-RESPONSE REGION

Southern coastal, Saco, Presumpscot, mid-coastal

Eastern coastal (Downeast)

Lower Androscoggin, lower Kennebec

Lower Penobscot, St. Croix

Upper Androscoggin, upper Kennebec, upper Penobscot

Upper St. John, Allagash

Lower St. John, Fish, Aroostook, Meduxnekeag

SNOWPACK SITE AND NAME Gorham

MAINE

NEW HAMPSHIRE

STATE BOUNDARY

COUNTY BOUNDARY

EXPLANATION

0 25 50 100 MILES

0 25 50 100 KILOMETERS

68o

70o

46o

44o

70o

44o

68o

46o

Base from U.S. Geological Survey digital files, Universal Transverse 

Mercator projection, Zone 19

Figure 8. Location of long-term snowpack sites in and near Maine.
16 Framework for a U.S. Geological Survey Hydrologic Climate-Response Program in Maine

HYDROLOGIC CLIMATE-RESPONSE REGION

Southern coastal, Saco, Presumpscot, mid-coastal

Eastern coastal (Downeast)

Lower Androscoggin, lower Kennebec

Lower Penobscot, St. Croix

Upper Androscoggin, upper Kennebec, upper Penobscot

Upper St. John, Allagash

Lower St. John, Fish, Aroostook, Meduxnekeag

NATIVE AMERICAN HEADQUARTERS

MAINE

NEW HAMPSHIRE

STATE BOUNDARY

COUNTY BOUNDARY

EXPLANATION

Passamaquoddy Tribe

Pleasant Point

Passamaquoddy Tribe

Indian Township

Houlton Band of Maliseet Indians

Littleton

Aroostook Band of 

Micmac Indians

Presque Isle

Penobscot Nation

Indian Island

44o

46o

70o

68o

46o

44o

68o

70o

0 25 50 100 MILES

0 25 50 100 KILOMETERS

Base from U.S. Geological Survey digital files, Universal Transverse 

Mercator projection, Zone 19

Figure 9. Location of Native American headquarters in Maine.
Framework for a Hydrologic Climate-Response Program 17

HYDROLOGIC CLIMATE-RESPONSE REGION

Southern coastal, Saco, Presumpscot, mid-coastal

Eastern coastal (Downeast)

Lower Androscoggin, lower Kennebec

Lower Penobscot, St. Croix

Upper Androscoggin, upper Kennebec, upper Penobscot

Upper St. John, Allagash

Lower St. John, Fish, Aroostook, Meduxnekeag

SELECTED FEDERAL CONSERVATION 

LAND AND NAME

Aroostook 

NWR

MAINE

NEW HAMPSHIRE

STATE BOUNDARY

COUNTY BOUNDARY

EXPLANATION

Moosehorn NWR

o

70o

44o

68

46o

68o

70o

46o

44o

Aroostook NWR

Appalachian Trail Corridor

Lake Umbagog NWR

White Mountain NF

Acadia NP

Sunkhaze NWR

Moosehorn NWR

Cross Island NWR

Petit Manan NWR

Rachel Carson 

NWR

Massabesic Experimental Forest

Carlton Pond WPA

0 25 50 100 MILES

0 25 50 100 KILOMETERS

Base from U.S. Geological Survey digital files, Universal Transverse 

Mercator projection, Zone 19

Figure 10. Location of U.S. Department of Interior and Agriculture lands in and near Maine.
18 Framework for a U.S. Geological Survey Hydrologic Climate-Response Program in Maine

SURFACE-WATER SUPPLIES AND NAME

HYDROLOGIC CLIMATE-RESPONSE REGION

Southern coastal, Saco, Presumpscot, mid-coastal

Eastern coastal (Downeast)

Lower Androscoggin, lower Kennebec

Lower Penobscot, St. Croix

Upper Androscoggin, upper Kennebec, upper Penobscot

Upper St. John, Allagash

Lower St. John, Fish, Aroostook, Meduxnekeag

WATERSHED USED FOR SURFACE-WATER 

SUPPLY

Bangor Water 

District

MAINE

NEW HAMPSHIRE

STATE BOUNDARY

COUNTY BOUNDARY

EXPLANATION

York Water District

Bath Water 

District

Bangor Water 

District

Kittery Water 

District

Kennebec Water District

Portland Water 

District

Presque Isle Water District

Biddeford & Saco 

Water Company

Town of Bar Harbor, Water Division Lewiston Public Works, 

Water Division

Aqua Maine, Camden 

& Rockland Division

Kennebunk, Kennebunkport, 

and Wells Water District

Base from U.S. Geological Survey digital files, Universal Transverse 

Mercator projection, Zone 19

44o

46o

70o

68o

46o

68o

44o

70o

0 25 50 100 MILES

0 25 50 100 KILOMETERS

Figure 11. Location of major surface water suppliers in Maine.
Framework for a Hydrologic Climate-Response Program 19

Trend Updates for Key Variables

It is important to not only collect data in a hydrologic 

climate-response program, but also to analyze the data on a 

regular basis. Many analyses of historical data for the key 

hydrologic variables included in this program have been completed (see Historical Hydrologic Changes in Maine section). 

As additional data are collected, existing historical analyses 

would be updated. New data would increase our understanding of year-to-year variability, decadal variability, and longer 

term changes. Trends in key variables would be updated on a 

rotating cycle.

Different cycles for updates would represent different 

levels of effort. For example, in a lower intensity effort, trend 

updates would be completed for five key variables at a rate of 

one variable per year (table 6), resulting in a 5-year cycle of 

updates for each variable. In a higher intensity effort, data for 

three variables per year would be analyzed, and the historical trends and variability for all variables (table 6) would be 

updated on a 3-year cycle.

Process-Based Studies

An important part of a hydrologic climate-response program in Maine would be to identify basins that are appropriate 

for use in process-based studies designed to understand and 

predict hydrologic and ecosystem change. The combination 

of extensive historical hydrologic data, ecological data, and 

deterministic watershed modeling would greatly increase our 

understanding of the effects of climate change on hydrologic 

components and ecosystem health.

For each region in the proposed hydrologic climateresponse program, a representative basin (for example, in 

geology and land cover) that has extensive historical data for 

multiple hydrologic components would be identified. Specific 

effort would be made to select candidate basins that contain 

important resources such as Atlantic salmon habitat and (or) 

Native American and U.S. Department of Interior or Agriculture lands. A deterministic watershed model would then be 

developed for each of these basins to improve understanding 

of basin hydrologic processes. The watershed models would 

use numerical methods to describe the physical processes 

that affect the movement of water throughout a basin. Runoff 

processes simulated by the models would include overland 

flow, shallow subsurface flow, and groundwater flow. Watershed models for multiple basins in Maine have already been 

completed or are being assembled (fig. 12).

The watershed models would be constructed using 

the USGS Precipitation-Runoff Modeling System (PRMS) 

(Leavesley and others, 1983). PRMS is well suited for simulating runoff from rural basins and has been applied to many 

basins in the United States. PRMS is a deterministic, distributed-parameter modeling system. The model is deterministic 

in that it computationally incorporates multiple components 

of the hydrologic cycle as understood through known physical 

laws or empirical relations in hydrologic science. The modeled 

hydrologic relations are typically governed by quantifiable 

physical characteristics of the basin. Parameters describing 

the physical basin characteristics are assigned in a distributed 

fashion, representing the spatial variation (heterogeneity) in 

basin characteristics. In this manner, the model is designed to 

simulate the hydrologic system as realistically as possible. 

The models would be calibrated to the historical record. 

Calibrated watershed models provide simulated daily streamflow time series for many locations throughout a basin. The 

models leverage limited streamflow-gaging-station data and 

provide a method for characterizing sub-basin hydrology. An 

example of rainfall-runoff-model subareas for two basins in 

eastern coastal Maine is shown in figure 13. Model output 

provides information regarding surface runoff, subsurface 

flow, and groundwater flow because the model explicitly 

Table 6. Examples of annual cycles of trend updates for key variables of hydrologic components.

[-- indicates no planned trend updates]

Primary component and key variables to be included in trend updates Lower intensity effort 

(one variable per year)

Higher intensity effort 

(three variables per year)

Streamflow—timing of winter-spring flows Year 1 Year 1

Streamflow—magnitude of annual peak flow Year 2 Year 1

Groundwater—magnitude of annual summer base flows Year 3 Year 1

Lake ice—date of spring ice-out Year 4 Year 2

River ice—winter days of ice-affected flow Year 5 Year 2

River ice—ice thickness for set winter date -- Year 2

Snowpack—magnitude of late-winter water equivalent, depth, and density 

for set date

-- Year 3

Groundwater—winter recharge -- Year 3

Groundwater—spring recharge -- Year 3
20 Framework for a U.S. Geological Survey Hydrologic Climate-Response Program in Maine

HYDROLOGIC CLIMATE-RESPONSE REGION

Southern coastal, Saco, Presumpscot, mid-coastal

Eastern coastal (Downeast)

Lower Androscoggin, lower Kennebec

Lower Penobscot, St. Croix

Upper Androscoggin, upper Kennebec, upper Penobscot

Upper St. John, Allagash

Lower St. John, Fish, Aroostook, Meduxnekeag

BASIN FOR WHICH MODEL EXISTS OR IS BEING 

DEVELOPED

MAINE

NEW HAMPSHIRE

STATE BOUNDARY

COUNTY BOUNDARY

EXPLANATION

44o

46o

70o

68o

46o

44o

68o

70o

0 25 50 100 MILES

0 25 50 100 KILOMETERS

Base from U.S. Geological Survey digital files, Universal Transverse 

Mercator projection, Zone 19

Figure 12. Location of drainage basins in Maine with rainfall-runoff models.
Framework for a Hydrologic Climate-Response Program 21

45o

00'

0"

45o

05'

0"

67o30'0" 67o25'0" 67o 67 15'0" o20'0"

44o

55'

0"

Dennys Basin

Cathance Basin

EXPLANATION

Shaded polygons

show model

areas—variously

colored

Basin boundary

0 1 3 4 MILES

0 3 6 KILOMETERS

2 Base from USGS and Maine Office of GIS digital files,

NAD 83, Universal Transverse Mercator projection, Zone 19

Figure 13. Rainfall-runoff model subareas for two drainage basins in eastern coastal Maine.
22 Framework for a U.S. Geological Survey Hydrologic Climate-Response Program in Maine

simulates these physical processes. This information would 

enable natural-resource managers to characterize the timing 

and quantity of water moving through the basin to support 

many endeavors, including geochemical calculations, wateruse assessment, river biota population dynamics modeling, 

and habitat modeling and assessment. It also allows scenario 

testing for interactions between climate change and changes in 

water-use and land-use regulations. 

Additional research would be conducted to improve 

understanding of important processes such as groundwater/

surface-water interactions, ice dynamics, and hydrologic/

ecological interactions for important habitats and species. For 

example, with improved availability of groundwater data, 

PRMS can be integrated with the USGS groundwater model, 

MODFLOW, in an application called GSFLOW (Markstrom 

and others, 2008) to support basin-scale groundwater/surfacewater resource investigations. Using models to characterize 

the relative contributions of surface water and groundwater to 

streamflow throughout the basin would lead to an improved 

understanding of water quantity and quality and the effects of 

future hydrologic changes on ecosystems in the basin.

Communication Plan

In order to be useful, the data and analyses from a USGS 

hydrologic climate-response program in Maine must be available to resource managers and to the public. A web site would 

be created to disseminate information regarding the design 

of the program, the data collected as part of the program, 

and interpreted historical-trend results. The trend results, as 

discussed in the Trend Updates for Key Variables section, 

would also be published on a regular basis, as USGS reports 

or journal articles.

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