Landscapes are constantly changing, but major changes usually take years, even millennia.

Glaciers, however, can break all the rules. If the landscape is partly ice, change can happen with startling speed.

Lakes can appear or disappear in a matter of days or weeks. Dry river beds can fill with a torrent of ice, mud, and water a minute or two and be dry again a few hours later.

Even among glaciers, the Donjek Glacier in the St. Elias Range along the Yukon's western edge is notable for the dramatic changes it has made to the local landscape.

University of Ottawa geographer Peter Johnson first worked at the terminus of the Donjek Glacier in 1970. In that year, the glacier was surging, its terminus moving down the Donjek Valley.

"It came close to blocking the Donjek River, which flowed against the east wall of the Donjek Valley past the glacier terminus," Johnson says. Shorelines of long-vanished lakes south of the terminus are evidence that blockage has happened in the past.

The glacier stopped short of blocking the valley in 1970 and has shown no further sign of advancing that far. Nevertheless, it hasn't left the surrounding landscape in peace, says Johnson, who has observed the glacier on the ground or from the air frequently in the past 30 years.

"Four years ago, in the spring of 1998, there were reports from pilots that something strange had happened to the glacier terminus, and there were also some reports of high water levels in the Donjek River at the Alaska Highway."

Johnson flew over the glacier to check it out and found the Donjek River no longer flowed along the eastern side of the valley below the terminus of the glacier. The river had changed its course to flow under the ice where it was dammed until the pressure of the water broke the dam and sent a flood wave down the valley.

"Vivien Lougheed, who has written a book on hiking trails in the park, was in the valley just after this event and took some photographs of ice blocks the size of couches spread across the valley floor."

For a while after that event the river ran through a tunnel underneath the glacier, but gradually the tunnel collapsed, creating a new ice-walled channel for the river. Johnson says there are signs of several blockages and smaller floods during the time the channel was forming.

The channel has continued to change as the active ice of the glacier shifts with the seasons and with the forces of the glacier itself. Some of the changes can be dramatic, and even dangerous, warns Johnson, relating the experience of some students from the United States who had been hiking in the Donjek Valley in 2000.

"They told a story of a very low water period on the river which tempted them to cross to the west side. Just as they had almost decided to do this a flood wave came racing down the valley and forced them to run up a side channel to escape. Obviously there had been another small blockage and outburst."

The changes in the Donjek glacier and river are likely related to relatively rapid back-wasting and down-wasting of the glacier, which is happening to most glaciers in the region, Johnson says.

That means that changes are likely to continue and that there is an increasing potential for more outbreak floods that could threaten hikers in the valley. The river might even shift back to its old course along the east side of the valley.

Larger changes could also occur, the geographer says, as the warming and melting trend continues.

"There's the possibility of new ice-dammed sites forming higher up the glaciers as melting continues. There could also be changes in the drainage at other glaciers -- for example, the Kaskawulsh, where historically the discharge has occasionally switched to the Alsek River system from the Slims River/Kluane Lake system."

For more information about glaciers and glacier-related landscape changes, contact Peter Johnson at peterj@aix1.uottawa.ca.