U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2008–3044

May 2008

Impacts of Low-Flow and Stream-Temperature Changes on 

Endangered Atlantic Salmon—Current Research

Printed on recycled paper

Introduction

Coastal rivers draining into the 

Gulf of Maine in the northeastern United 

States are home to Atlantic salmon 

(fig. 1). On November 17, 2000, The 

Federal government listed the Gulf of 

Maine Distinct Population Segment of 

Atlantic salmon as endangered under 

the Endangered Species Act. State and 

Federal fisheries biologists are concerned 

with the effects of climate-related streamRecent climate studies in New England and the northeastern United States have shown evidence of physical changes over 

time, including trends toward earlier snowmelt runoff, decreasing river ice, and increasing spring water temperatures. A U.S. 

Geological Survey (USGS) study funded by the National Global Warming and Wildlife Science Center will be investigating 

changes in summer low streamflows and stream temperatures and the potential effects of those changes on endangered Atlantic 

salmon populations. The study also will evaluate management options that would be most likely to mitigate the effects of any 

changes in streamflow and temperature.

flow changes on Atlantic salmon survival 

in these rivers. The summer low-flow 

period in northeastern United States 

rivers and climate-related changes during 

this period are of particular concern 

because this period is critical in the 

Atlantic salmon life cycle.

The USGS has documented many 

seasonal climate-related changes in the 

northeastern United States during the last 

30 to 150 years; these include changes 

toward earlier winter/spring snowmelt 

runoff (fig. 2), decreasing duration of 

ice on rivers and lakes, decreasing ratio 

of snowfall to total precipitation, and 

denser and thinner late-winter snowpack. 

All of these changes are consistent with 

warming winter/spring air temperatures 

(Hodgkins and Dudley, 2006a; Hodgkins 

and Dudley, 2006b; Hodgkins and others, 2005; Dudley and Hodgkins, 2002; 

Hodgkins and others, 2002; Huntington 

Spawning Atlantic salmon

Paul Nicklen/National Geographic Image Collection, rights managed
and others, 2004). Climate-model projections for the Northeast indicate airtemperature warming, earlier snowmelt 

runoff, increases in annual evaporation, 

and decreased low streamflows (Hayhoe 

and others, 2007).

Spring snowmelt in the northeastern 

United States is important to the recharge 

of ground-water supplies. Following 

spring recharge, ground water discharges 

into streams throughout the summer. 

This discharge is a source of cool water 

during the summer and composes a large 

proportion of the streamflow during 

summer low-flow periods. A recession in 

streamflow from spring to late summer 

is typical in the Northeast. With snowmelt occurring earlier, the recession may 

Figure 1. Northeastern coastal rivers are at the southern extent of the Atlantic salmon’s 

geographical range; as a result, Atlantic salmon in these rivers are expected to be 

particularly sensitive to changes in climate.

Shaded relief from USGS STRM 3 arc-second 

digital elevation model. Basemap from ESRI 

base layers, various scales. Map projection 

Universe Transverse Mercator, Zone 19N

0 20 40 60 80 MILES

0 20 KILOMETERS 40 60 80

EXPLANATION

Major rivers

Ocean

Major lakes

Shaded Relief, in feet

Water Bodies

Mid: 3,100

High: 6,200

Low: 0

MASSACHUSETTS

ATLANTIC OCEAN

QUEBEC

MAINE

NEW BRUNSWICK

VERMONT

United States of America

Canada

NEW 

YORK 

NEW 

HAMPSHIRE

CONNECTICUT

NOVA 

SCOTIA

RHODE 

ISLAND

GULF OF MAINE

LONG ISLAND SOUND

72°00'0"W 68°00'0"W

46°00'0"N

42°00'0"N

start earlier, resulting in longer periods of 

recession and lower streamflow during 

summer months. These changes could 

affect the survival of Atlantic salmon.

USGS researchers have made 

substantial progress toward completion of 

a salmon-survival model which describes 

the entire riverine life cycle as a function 

of streamflows, stream temperatures, 

and other variables. The model uses 

a unique detailed data set describing 

Atlantic salmon demographics in several 

northeastern streams (Letcher and others, 

2002; Letcher and others, 2004; Letcher 

and others, 2007). This model will 

provide the ability to forecast population dynamics and probability of survival 

under different climate-change scenarios.

 Study Approach

The USGS will be assessing 

historical changes in ground-water 

discharge to rivers in the Gulf of Maine 

area and estimating future groundwater discharge by using climate-model 

output and detailed watershed models of 

selected rivers in the study area. USGS 

researchers also will be completing 

development of a salmon population 

dynamics model to estimate Atlantic 

salmon survival as a function of potential 

future changes in ground-water discharge 

and in-stream temperature.

To assess historical changes in 

ground-water discharge, USGS researchers will be analyzing records of flow 

for rural, unregulated streams with at 

least 50 years of continuous record 

in the northeastern United States and 

selected areas in the Canadian provinces 

of Quebec and New Brunswick. The 

stormflow component of the total streamflow will be separated from the groundwater-discharge component. Timing, 

magnitude, and duration statistics will be 

derived from the resulting ground-water 

discharge records and will be analyzed to 

quantify any changes that have occurred 

over the periods of record.

In an effort to project future changes 

for summer low-flow conditions in the 

Northeast, watershed models will be 

constructed by using the USGS Precipitation Runoff Modeling System. The 

models will be calibrated to the historical 

record with special emphasis on the 

summer low-flow periods, when the 

ratio of ground-water discharge to total 

streamflow is greatest. Future estimates 

of streamflows, stream temperatures, and 

ground-water discharge to streamflow 

during the critical low-flow periods for 

Atlantic salmon will be simulated by 

using climate-model output as described 

in the Intergovernmental Panel on 

Climate Change Fourth Assessment 

Special Report on Emission Scenarios 

(Nakicenovic and Swart, 2000).

Watershed-model output describing 

future flow and in-stream temperature 

estimates will be used in conjunction 

with the USGS salmon-survival model 

to describe future scenarios of salmonpopulation dynamics. Specific results 

will include the number and size of 

emigrating salmon and the numbers of 

adult salmon returning to their home 

rivers to spawn, in addition to estimates 
Figure 2. Number of significant changes in monthly streamflow magnitudes over time (average of 71 years through 2002) for streamflowgaging stations in New England. Increasing streamflows in February, March, and April and decreasing streamflows in May are supporting 

evidence of a shift in timing toward earlier winter/spring runoff (from Hodgkins and Dudley, 2005).

TIME PERIOD

-10

-15

-5

0

5

10

15

20

N

U

M

B

E

R

O

F

SIG

NIFIC

A

N

T

T

R

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D

S (p

<

0.1)

Jan.

Feb.

Mar.

Apr.

May Jun. Jul.

Aug. Sept.

Oct

Nov.

Dec. Annual

NUMBER OF SIGNIFICANT TRENDS FOR MEAN FLOWS FOR ALL SITES

27 trend tests per month

Oct.

(Zero)

Three age classes of freshwater Atlantic salmon from the West Brook, Whately, 

Massachusetts (photo courtesy USGS Conte Anadromous Fish Laboratory)

of population growth and survival. The 

model output also will allow estimation 

of the effects of different management 

strategies (for example, stocking 

strategies and habitat manipulation), 

and evaluation of the consequences of 

choosing each strategy to determine 

which would be most likely to mitigate 

the effects of streamflow and streamtemperature changes.

Benefits

Much recent progress has been made 

in describing hydrologic changes in the 

Northeast during the last century and the 

life cycle and population dynamics of 

Atlantic salmon. Little progress, however, 

has been made in quantitatively determining the physical and biological effects 

of documented hydrologic changes in 

such variables as the timing of winter/

spring snowmelt runoff. This research 

will add substantially to the existing 

knowledge of the effects of changing 

climate on an endangered fish species 

and will contribute to the body of science 

necessary to allow managers to manage 

entire ecosystems and physical systems 

in the Northeast effectively in the face 

of short-term variability and long-term 

changes in climate.
Hodgkins, G.A., Dudley, R.W., and 

Huntington, T.G., 2005, Changes in 

the number and timing of days of 

ice-affected flow on northern New 

England rivers, 1930–2000: Climatic 

Change, v. 71, p. 319-340.

Hodgkins, G.A., James, I.C., II, and Huntington, T.G., 2002, Historical changes 

in lake ice-out dates as indicators 

of climate change in New England, 

1850–2000: International Journal of 

Climatology, v. 22, p. 1819–1827.

Huntington, T.G., Hodgkins, G.A., 

Keim, B.D., and Dudley, R.W., 2004, 

Changes in the proportion of precipitation occurring as snow in New England 

(1949 to 2000): Journal of Climate, 

v. 17, p. 2626–2636.

Letcher, B.H., Dubreuil, T.D., O’Donnell, 

M.J., Obedzinski, M., Griswold, K., 

and Nislow, K.H., 2004, Long-term 

consequences of variation in timing 

and manner of fry introduction on 

juvenile Atlantic salmon growth, survival and life history expression: Canadian Journal of Fisheries and Aquatic 

Sciences, v. 61, p. 2288–2301.

Letcher, B.H., Gries, G., and Juanes, F., 

2002, Survival of stream-dwelling 

Atlantic salmon—Effects of life 

history variation, season and age: 

Transactions of the American Fisheries 

Society, v. 131, p. 838–854.

Letcher, B.H., Nislow, K.H., Coombs, 

J., O’Donnell, M.J., and Dubreuil, 

T.L., 2007, Population response to 

habitat fragmentation in a brook trout 

population: PloS One, 2(11): e1139. 

doi:10.1371/journal.pone.0001139.

Nakicenovic, N., and Swart, R., eds., 

2000, IPCC special report on emissions scenarios: Cambridge, U.K., and 

New York, N.Y., Cambridge University 

Press, 570 p.

—By Robert W. Dudley, Glenn A. 

Hodgkins, and Benjamin H. Letcher

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.

Hayhoe, K., Wake, C.P., Huntington, 

T.G., Luo, L., Schwartz, M., Sheffield, 

J., Wood, E., Anderson, B., Bradbury 

J., DeGaetano, A., Troy, T., and Wolfe 

D., 2007, Past and future changes in 

climate and hydrological indicators in 

the U.S. Northeast: Climate Dynamics, 

v. 28, p. 381–407.

Hodgkins, G.A., and Dudley, R.W., 2005, 

Changes in the magnitude of annual 

and monthly streamflows in New 

England, 1902–2002: U.S. Geological 

Survey Scientific Investigations Report 

2005–5135, 37 p. 

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

2006a, Changes in late-winter snowpack depth, water equivalent, and 

density in Maine, 1926–2004: Hydrological Processes, v. 20, p. 741-751.

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

2006b, Changes in the timing of 

winter–spring streamflows in eastern 

North America, 1913–2002: Geophysical Research Letters, v. 33: L06402, 

doi 10.1029/2005GL025593.

For more information please contact:

Director, USGS Maine Water 

 Science Center

196 Whitten Road

Augusta, Maine 04330

Telephone: (207) 622-8201

Email: dc_me@usgs.gov

Web site: http://me.water.usgs.gov

Geographic coverage for two watershed models in coastal Maine

45o00'0"N

45o

05'0"N

67o

30'0"W 67o25'0"W 67o 67o20'0"W 15'0"W

44o

55'0"N

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