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Left side of
Harriman Glacier in 1994. This is one of the few
tidewater glaciers currently advancing. This view
of the extreme right side at low tide, shows the
shoal that provides the footing for the glacier to
advance (Photo by Kristine Crossen). |
Glaciers form when snow
compresses and recrystallizes into ice, and then flows down
slope picking up debris and eroding the underlying bedrock.
Climate change affects glaciers when additional
precipitation or cooling temperatures add mass to the
glacier, allowing it to advance down valley. Climate
strongly controls the dynamics of glaciers that terminate on
land and lose ice (ablate) by melting mechanisms.
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In 1899,
Serpentine Glacier flows in a sinuous pattern down
the valley, exhibiting white ice all the way to the
terminus at the shoreline (Photo by Edward S.
Curtis). |
Principles of glacier formation, ice flow, and glacial landforms were well understood at the time of the original Harriman Expedition, and Alaskan examples were well documented by G.K. Gilbert and John Muir. Sharp-peaked horns and arêtes, high cirque basins, and steep-sided U-shaped valleys sculpted by glacial ice were documented in southern coastal Alaska. Many examples of morainal ridges and large outwash streams pouring from the glaciers were also seen.
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Serpentine
Glacier in 1998, showing a black surface covered
with debris, indicating a stagnant glacier front
(Photo by Kristine Crossen). |
In comparison, recent studies of glaciers have lead to a better understanding of both climate change during the Little Ice Age and the dynamics of tidewater glaciers.
The Little Ice Age is a period
of cold climate from approximately 1350 to 1850 AD. Glacial
activity, historic records, tree rings, and Native oral
history all give evidence of ice advance that overran
forests. During the 1800's many glaciers began spectacular
retreats in response to climatic warming at the end of the
Little Ice Age. This continues today with most Alaskan
glaciers in retreat. This results in stranded moraines,
abrupt trimlines, reduced ice volumes, stagnant ice masses,
and vegetation recolonizing deglaciated areas. In addition,
ice retreat has also uncovered forests around the Gulf of
Alaska previously overrun by the ice.
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Terminus of
Barry Glacier in 1899 near Point Doran, showing the
ice caves where englacial streams exited the ice
(Photo by C. Hart Merriam). |
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Terminus of
Barry Glacier, 1998. Note the three individual
glaciers separated by bedrock walls. From the left
are Cascade, Barry, and Coxe Glaciers respectively.
These glaciers retreated over 3 miles in 15 years,
and have occupied their current positions since
1914 (Lethcoe, 1987). (Photo by Kristine
Crossen). |
Recent research into tidewater
glacier dynamics was lead by studies at Columbia Glacier;
important because of the icebergs it calves into the
shipping lanes in Valdez Arm. This glacier acted as a
natural laboratory when it began its catastrophic retreat in
the 1970's, and taught glaciologists that the depth of water
at the ice terminus was the strongest forcing factor
affecting the dynamics of tidewater glaciers. Iceberg
calving rate and thus retreat rate were directly tied to the
water depth, as deep water caused more ice to come afloat
and calve away., thus causing increased retreat . Tidewater
glaciers commonly terminate in a vertical face with ice that
extends 200-300 feet above the water line and as much as
1000 feet below water to the fiord floor. Within the last
200 years, catastrophic retreat has occurred in many fiords
including Glacier Bay, Disenchantment Bay, Columbia Bay,
College Fiord and Harriman Arm. Catastrophic retreat of the
glacier continues until the glacier leaves deep water,
commonly at the head of the fiord. Most recently, Columbia
Glacier has retreated nearly 10 miles in the past 25
years.
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Aerial view
of Barry Arm showing Cascade Glacier on the left
and Barry Glacier on the right (Photo by Kristine
Crossen). |
The Harriman Expedition of 1899, at the end of the Little Ice Age, saw a mixture of tidewater glaciers, some having undergone prior retreat (in Glacier Bay) and others still filling their respective fiords (like Columbia Glacier). In Prince William Sound, they were able to "sneak" the G.W. Elder past the extended terminus of Barry Glacier to discover Harriman Fiord.
During the Harriman Expedition
Retraced 2001, most of the glaciers terminating on land and
many of the tidewater glaciers have been in retreat for at
least 100 years. Barren trimlines, debris covered stagnant
ice, and decreased ice masses show the glacier's response to
100 years of warming. However, a few tidewater glaciers are
advancing, in response to forcing factors related to water
depth and fiord sedimentation. Harriman, Hubbard, and Yale
Glaciers are all advancing by building morainal shoals that
they are then able to advance onto and bulldoze or redeposit
ahead of them as they move down the fiord.
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1899 map of
Barry Arm showing Barry Glacier extending to within
1000 feet of Point Doran, where the G. W.
Elder squeezed along the ice margin into
Harriman Fiord. Note the 3 major tributaries to
this glacier. |
References:
Lethcoe, N.R., 1987, Glaciers of Prince William Sound, Prince William Sound Books, Valdez, 151 p.
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For information on the Harriman Retraced Expedition e-mail: harriman2001@science.smith.edu |