Alaska's Hubbard Glacier, which has been astounding geologists by surging rapidly since winter, has cut across the mouth of a 30-mile-long Pacific fiord.

The glacier formed an ice dam that has isolated the fiord from the ocean and trapped saltwater fish, whales and other wildlife in what is now a lake that is slowly turning into a freshwater body.

"This is probably the largest natural alteration in ocean, glaciers, lakes and rivers to occur in North America within our lifetimes," said Larry Mayo, a glaciologist with the U.S. Geological Survey, based in Fairbanks, Alaska.

Mayo said the change may be part of a "thousand-year cycle" of glacial advances and retreats driven by unknown processes, and that Russell Lake, which was Russell Fiord until early June, may remain an isolated body for centuries.

Hubbard Glacier, a vast river of ice flowing slowly down mountain valleys to the ocean, is in southeastern Alaska where the panhandle meets the main part of the state. Like most glaciers that reach coastlines, the leading edge "calves," causing icebergs to break off and drift into the sea. Sometimes the calving rate matches the flow rate, keeping the edge roughly stationary. At other times, however, the rates vary and the edge retreats or advances.

Hubbard Glacier has been slowly advancing since around 1900, but last winter it began moving much faster, apparently fed by a tributary, Valerie Glacier, which has advanced in some parts by as much as 130 feet per day.

About 20 other Alaskan glaciers are also surging, although at slower rates, but still others are retreating. Columbia Glacier, about 200 miles down the Alaskan coast, is in rapid retreat.

Meltwater from Hubbard Glacier has raised Russell Lake by some 30 feet. Over the next two years, Mayo estimates, the lake level will rise another 200 feet and spill out over the ice dam or in the opposite direction into a nearby river that drains into the ocean.

It is not clear how the wildlife trapped in the lake will be affected, but many animals are probably doomed by the expected changes in salinity and in decreased oxygen levels. Mayo said the situation offered an unusually rare opportunity to study effects on local climate, wildlife, forests and on human habitation in the nearby town of Yakutat.