Scientists have long studied Alaska’s fast-moving Columbia Glacier, a tidewater glacier that descends through the Chugach Mountains into Prince William Sound. Yet the river of ice continues to deliver new surprises.
When British explorers first surveyed the glacier in 1794, its nose extended to the northern edge of Heather Island, near the mouth of Columbia Bay. The glacier held that position until 1980, when it began a rapid retreat.
The image series begins in July 1986 (bottom image) with a false-color image captured by the Thematic Mapper (TM) sensor on the Landsat 5 satellite. The false-color image from July 2014 (top image), acquired by the Operational Land Imager on the Landsat 8 satellite, shows the extent of retreat after 28 years. Use the image comparison tool to better see the details.
Recent changes to the glacier include the unexpected retreat of the West Branch. Scientists such as Shad O’Neel, a U.S. Geological Survey glaciologist at the Alaska Science Center, thought the branch had stabilized by 2011. “We thought that one was pretty much done,” O’Neel said. “Obviously we were wrong.”
Columbia Glacier expert Tad Pfeffer of the University of Colorado, Boulder, was equally surprised. The ground below a glacier—the bed—plays an important role in determining how much a glacier will retreat. It turns out, however, that it’s not so easy to measure the elevation of the bed. The best information available to scientists had suggested that the bed of the West Branch rose above sea level immediately upstream from the nose, or terminus, which would have slowed the retreat of the branch.
“That’s clearly wrong given that the terminus is now 3 kilometers back from its previously ‘stable’ point,” Pfeffer said. “Why it hung in its advanced position for so long and why it started retreating is a mystery.”
To better characterize recent changes to the Columbia Glacier, Ryan Casotto (University of New Hampshire) and colleagues used ground-based radar to measure the glacier’s speed every three minutes for eight days in early October 2014. Preliminary results show that both the West Branch and the East Branch (which feeds into the Main Branch) are now moving between 5 and 10 meters (16 and 33 feet) per day. That’s slow for Columbia, but fast compared to other glaciers.
NASA Earth Observatory image by Jesse Allen, using Landsat data from the U.S. Geological Survey. Caption by Kathryn Hansen.
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