ANCHORAGE - Southcentral's Bering Glacier, the continent's largest, has advanced to within 1 1/2 miles of the Gulf of Alaska, raising concerns about ice-covered habitat and icebergs in oil-tanker-shipping lanes.
The glacier has calved chunks up to one-half mile across. It was being watched by the Valdez-based Coast Guard.
"We don't know what's going to happen," said Edward Bovy with the Bureau of Land Management in Anchorage.
Scientists say galloping glaciers are a result of "bad plumbing," when a cushion of water builds beneath the ice and slicks the way for glaciers to flow faster.
In glacial terms, that can mean 100 yards a day.
The Bering Glacier, which is about 75 miles southeast of Cordova, advanced about six miles last year before coming to rest. This year, it has moved as much as four miles in some places since early May.
The 145-mile-long glacier has nearly reached the point of its farthest advance in historical times, the bureau said.
That last big move came in 1902; other advances were recorded at intervals of 20 or 25 years. Nobody knows what causes cyclical advances.
An underwater trough shows that in earlier geologic eras, the Bering was a tidewater glacier. It could reach the ocean again.
Glaciers move faster over water than over land, and the advance is on Vitus Lake, once a body of water 15 miles wide and about 7 miles long.
The lake, once a popular recreation destination, is now nearly
covered by glacier.
"When it hits land, it will ride up on those forelands, and it may pause," Bovy said. Or maybe not.
The Bering is roughly 30 miles wide and its largest lobe is 126 miles long.
The flow extends from the Bagley Ice Field in Canada and runs through Wrangell-St. Elias National Park on the way to its terminus.
The Bering made a major advance last summer, but it paused in September when a major blowout released tons of muddy water from beneath the glacier, allowing it to settle on solid ground.
"The mechanism is fairly well known - it's bad plumbing," said University of Alaska Fairbanks physics professor Will Harrison.
"Lots of water goes into the glacier, especially at this time of year," said Harrison, who studies glaciers at UAF's Geophysical Institute.
If the water is trapped under, ice will float and move downhill much more freely.
Harrison is among scientists studying surges at Variegated Glacier, a small but active river of ice near the coastal town of Yakutat. The Variegated recently advanced more than one-half mile - a major move for a glacier only about 10 miles long, Harrison said.
Surges in glaciers cause tremendous stress far from the face. Last year, scientists found evidence that the Bering Glacier had pulled away from its headwalls 100 miles "upstream."
In some areas, the glacier dropped 4 to 6 feet against the headwall. Fractures and crevasses in the glacier have made it impossible to land on the Bering, scientists say.
Bruce Molnia, who directs a U.S. Geological Survey study of the Bering, said the glacier was moving 2 feet a day or less before it started surging last year.
It accelerated to an average of 100 feet a day. During its peak, an estimated 30 tons of ice calved from the glacier each day.
Molnia set up a USGS camp near the glacier last year and plans to be there again next month. The Bering also is being monitored by satellite.
Effects of the glacier's surge on wildlife aren't clear. Ice dams could back up water and cause flooding that could damage salmon streams in the Yakataga State Game Refuge, just east of the glacier's face.
Similar flooding occurred last year when nearby Berg Lake broke through an ice dam formed by the Bering and water roared down to the ocean, leaving house-size icebergs where a 270-foot-deep lake had been.
Experts are concerned about the region's flourishing bird populations. Areas now covered by ice once attracted roughly 8 percent of the world's population of dusky Canada geese, about 7,000 birds.
The geese have moved to forelands, but scientists say the area could be overrun by moving ice.