U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2006–3141

November 2006

A Century of Retreat at Portage Glacier,

South-Central Alaska

Printed on recycled paper

Introduction

The Portage Glacier, in south-central 

Alaska, is viewed by thousands of visitors annually who come to the U.S. Forest Service Begich, Boggs Visitor Center 

located on the road system between 

Anchorage and Whittier, Alaska. During the past century, the terminus of the 

glacier has retreated nearly 5 kilometers 

to its present location (fig. 1). Like other 

glaciers that terminate in water, such as 

Columbia Glacier near Valdez or Mendenhall Glacier near Juneau, Portage Glacier has experienced accelerated retreats 

in recent decades that likely were initially 

triggered by climate change begun at the 

end of the Little Ice Age in the mid1800s and subsequently controlled in 

recent history primarily by calving of the 

glacier terminus. Photographic records 

of the terminus covering 1914 until 

present day track the patterns of retreat. 

These data, coupled with USGS climate 

information collected from the southern 

end of the ice field, provide insight to the 

patterns of retreat that might be observed 

in the future.

History

During the late 1800s and early 1900s, 

Portage Glacier terminated on land at 

the western end of Portage Lake, filling Portage Lake with ice (Photo Plate 

1914). Since the early 1900s, the glacier 

has receded, leaving Portage Lake in the 

scoured basin. The initial retreat of the 

glacier coincides with known climate 

warming associated with the end of the 

Little Ice Age (circa mid-19th century). 

As the glacier receded, its land-based 

terminus retreated into proglacial Portage 

Lake and changed from its relatively stable land-based environment to an unstable calving environment. The most rapid 

recession of some 140 to 160 meters per 

year occurred between 1939 (Photo Plate, 

1939) and 1950, when water depth at the 

terminus was at its maximum—roughly 

200 meters. Recession continued through 

the 1970s and 1980s (Photo Plate, 1972, 

1984) until by late 1999, Portage Glacier 

had receded almost 5 kilometers, to a 

more stable position at the eastern end of 

Portage Lake (Photo Plate, 1999). The 

retreat was driven primarily by calving 

of unstable ice at the glacier terminus 

into Portage Lake. Ice loss resulting from 

increased melting of the glacier surface 

during the past century-long general 

warming trend contributed to glacier 

retreat, but to a lesser extent. Today, the 

terminus of Portage Glacier remains close 

to its 1999 location (Photo Plate, 2006).

0 1 1.5 3 KILOMETERS

0

0.5

0.5 1 1.5 2 MILES

2 2.5

1914

1939

1951

1972

1984

1999

Portage Pass

PORTAGE LAKE

PORTAGE

GLACIER

60o45'

148o45'

Placer Creek

Visitor 

Center

1984

1972

1951

Turnagain

Shoulder

0

0 25 50 MILES

25 50 75 KILOMETERS

FAIRBANKS

ANCHORAGE

C H U G A C H M O U N T A I N S

Prince William Sound

Valdez

G U L F O F A L A S K A

Seward

 K E N A I 

P E N I N S U L A

C O O K I N L E T

Anchorage

Soldotna

Kenai

Homer

Portage 

Glacier

ALASKA

KENAI MOUNTAINS

USGS Glacier

Research Site

147o

61o

60o

59o

150o

Figure 1.  Approximate terminus positions

of Portage Glacier 1914–1999 (Modified

from Mayo and others 1977 with additional

USGS imagery). The 2006 location is

very similar to the marked 1999 location.

U.S. Forest Service Begich, Boggs Visitor

Center location is marked. Portage Glacier

is located in south-central Alaska.
Future Status

Future changes of the terminus of Portage Glacier will depend on a variety 

of factors. However, present scientific 

theory (Meier and Post, 1987, Trabant 

and others, 2003) suggests that calving glaciers cycle between advance 

and retreat patterns; with rapid retreats, 

followed by stable retracted positions, 

slow advances, and then stable extended 

positions that are not directly related to 

climate change. Thus, based on its history 

to date, and if such a pattern holds for 

Portage Glacier, the glacier may now be 

in its stable retracted position and could 

eventually begin a slow advance. However, for a glacier to advance, annual net 

balance—the amount of new snow and 

ice added to the glacier minus the amount 

of glacier melt—must be positive. As 

a rule of thumb for valley glaciers, net 

balance is positive when annual snow and 

ice accumulation occurs over more than 

60–70 percent of the entire glacier area. 

Such accumulation occurs on the glacier 

above the end-of-summer snow-line 

altitude, recently measured by USGS at 

1,000 meters in altitude. Only 50 percent (not the rule-of-thumb 60–70 percent) of Portage Glacier is more than 

1,000 meters in altitude, thus, Portage 

Glacier may slowly thin and recede rather 

than experience a slow advance.

Regardless of predictions, the position 

of the Portage Glacier terminus has 

remained relatively unchanged at the 

eastern end of Portage Lake for the last 

7 years and continues to provide visitors 

with a spectacular view of glacier ice.

Selected References 

Barnes, F.F., 1943, Geology of the Portage Pass area, Alaska: U.S. Geological 

Survey Bulletin 926–D, p. 211–235.

Crossen, K.J., 1992, Guide to the Little 

Ice Age landforms and glacial dynamics in Portage Valley and Portage Pass: 

Alaska Geological Society, 46 p.

Mayo, L.R., Zenone, C., and Trabant, 

D.C., 1977, Reconnaissance hydrology 

of Portage Glacier basin, Alaska: U.S. 

Geological Survey Hydrologic Investigations Atlas HA–583, 2 sheets.

Meier, M.F., and Post, Austin, 1987, Fast 

tidewater glaciers: Journal of Geophysical Research, v. 92, No. B9, p. 

9051-9058. 

Schmidt, R.A.M., 1961, Recession of 

Portage Glacier, Alaska, in Geological 

Survey research 1961: U.S. Geological 

Survey Professional Paper 424–D, p. 

D202–D203.

Trabant, D.C., Krimmel, R.M., and Post, 

Austin, 1990, A preliminary forecast 

of the advance of Hubbard Glacier 

and its influence on Russell Fjord, 

Alaska: U.S. Geological Survey, 

Water-Resources Investigations Report 

90-4172, 34 pp.

Trabant, D.C., Krimmel, R.M., Echelmeyer, K.A., Zirnheld, S.L., and 

Elsberg, D.H., 2003, The slow advance 

of a calving glacier: Hubbard Glacier, 

Alaska, U.S.A.: Annals of Glaciology, 

Volume 36, Number 1, pp. 45-50(6).

Ben W. Kennedy, Dennis C. Trabant, and 

Lawrence R. Mayo

1999 Terminus 1999 Terminus

1999

Terminus

1999

Terminus

1972 1984

1999 2006

1914 1939

Photo Plate.  Series of photographs of the terminus of Portage Glacier. 1914 and 1939 taken

from Turnagain Shoulder NW to SE view; 1972 and 1984 oblique aerial view NW to SE; 1999

and 2006 oblique aerial view NE to SW toward Portage Glacier. Photo credits: 1914, Strough

http://www.photolib.noaa.gov/historic/c&gs/images/theb1390.jpg; 1939, Barnes 1943, Plate

17B; 1972, USGS photograph; 1984, Aeromap U.S.; 1999 and 2006, USGS photographs.

For further information contact:

U.S. Geological Survey

Alaska Science Center

4230 University Drive

Suite 201

Anchorage, AK 99508-4664

Or visit the USGS Alaska Science

Center homepage on the World

Wide Web at: http://alaska.usgs.gov/