U.S. Department of the Interior 

U.S. Geological Survey

Fact Sheet FS 2005-3001

January 2005

Trends in Streamflow, River Ice, and Snowpack for Coastal River Basins in 

Maine During the 20th Century

Introduction

 Air temperatures in New England 

have risen during the 20th century. The 

largest seasonal warming rates observed 

from 1976-2000 were during December, 

January, and February (Intergovernmental Panel on Climate Change, 2001). The 

response of streamflow timing and volumes to warmer winters may be significant because a major component of the 

hydrology of river basins in New 

England involves snowpack and the timing of snowmelt. Scientists with the 

U.S. Geological Survey, in cooperation 

with the Maine Atlantic Salmon Commission, analyzed trends in streamflow, 

river ice, and snowpack for coastal river 

basins in Maine to determine if any significant changes have taken place in 

response to observed changes in climate. 

 Of particular concern to the Maine 

Atlantic Salmon Commission are seasonal changes in water temperatures and 

streamflows that may affect the behavior 

and survival of Atlantic salmon in 

Maine. Atlantic salmon in Maine may be 

particularly sensitive to climatic changes 

that affect the prevailing patterns of 

streamflow and water temperatures 

because Maine’s coastal rivers are at the 

southern extent of the Atlantic salmon’s 

habitat range.

Key Findings

 Streamflow data from six coastal 

streamflow gages in Maine were analyzed for this study (Dudley and 

Hodgkins, 2002). The coastal gages have 

an average period of streamflow record 

of 54 years. Trend test results for combined streamflow data spanning 1906-

2000 (93-percent complete) indicate a 

significant trend in the timing of spring 

runoff toward earlier dates (fig. 1). The 

streamflow record also shows that 

streamflows have increased in winter 

and early spring and decreased in late 

spring and early summer; however, in 

general, no significant trends in annual 

total streamflow were observed. The 

most significant trends for increasing 

streamflows over time were in February, 

and the most significant trends for 

decreasing streamflows over time were 

in May and June. These months have 

highly significant trends for low, 

medium, and high streamflows. 

 River ice can affect the computation of streamflow at a gaging station if it 

is present in sufficient quantities. 

Because of this phenomenon, the USGS 

carefully records days of ice effect in its 

efforts to gage streamflows. The occurrence of ice can be discontinuous 

throughout the winter season. In general, 

results of trend tests indicate: the first 

day of ice-affected streamflow in the fall 

is later, the last day of ice-affected flow 

in the spring is earlier, and there are 

fewer total days of ice-affected flow (fig. 

2). The trends in later fall ice formation 

are significant at two of the five rivers 

with available ice data, whereas trends 

for earlier spring ice melting and 

breakup are significant at four of five of 

Changes in water resources over time may affect public and private water 

supplies and the health of aquatic ecosystems. The U.S. Geological Survey 

is studying the relation between climate and water-resource variables in 

Maine and New England to provide resource managers, the scientific community, and the public with better information about the sensitivity of water 

resources to climate change and variability. The long-term collection and 

availability of high-quality water-resources data are crucial to these efforts.

Ice conditions on the Sheepscot River in coastal Maine, late January 2001.
U.S. Department of the Interior 

U.S. Geological Survey

Fact Sheet FS 2005-3001

January 2005

1900 1920 1940 1960 1980 2000 60

80

120

140

March 1

March 21

April 10

April 30

May 20

DATE

YEAR

1930 1940 1950 1960 1970 1980 1990 2000

20

40

60

80

100

120

W. Branch Union

Machias

Narraguagus

Royal

Sheepscot

EXPLANATION

NUMBER OF DAYS OF ICE EFFECT

YEAR

the rivers. Later freezing, earlier melting/

breakup, and more intermittent ice during the winter contribute to a significant 

decrease in total winter days of iceaffected flow over time at four of five of 

the coastal rivers in Maine.

 Snowpack data have been collected 

through the Maine Cooperative Snow 

Survey program from the early 1900’s to 

the present. Two of the three stations in 

coastal Maine with periods of record 

beginning before 1945 have significant 

trends towards increasing snowpack density on or around March 1. These two 

sites had net increases of 12 and 10 percent in snow density during their periods 

of record. 

 The trends over time in streamflow 

statistics, timing and magnitude of runoff, ice occurrence, and snowpack data 

are all indicative of a shift towards earlier 

spring snowmelt runoff in coastal Maine. 

The findings from this study may be used 

to help determine if the changes in the 

timing and amount of streamflow and 

river ice are related to the timing of 

Atlantic salmon runs, numbers and (or) 

adult return rates, and other biological 

and population data.

References

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

Trends in streamflow, river ice, and 

snowpack for coastal river basins in 

Maine during the 20th century: U.S. 

Geological Survey, Water-Resources 

Investigations Report 02-4245, 26 p.

Intergovernmental Panel on Climate 

Change, 2001, Climate Change 2001, 

The Scientific Basis: Cambridge University Press, Cambridge, 881 p.

—By Robert W. Dudley and Glenn A. 

Hodgkins

For more information please contact:

District Chief

U.S. Geological Survey

196 Whitten Road

Augusta, Maine 04330

Telephone: (207) 622-8201

Email: dc_me@usgs.gov

Website: http://me.water.usgs.gov

Figure 1. Trend toward earlier spring center-volume date—the date by which half of the 

total spring runoff has occurred—for combined streamflow data in coastal Maine.

Figure 2. Trend toward decreasing number of days of ice for coastal rivers in Maine.