Though it sounds like a contradiction, warmer winters are leading to thicker river ice and more risk of ice jam floods in some parts of the Yukon.

The severe flooding along the Klondike River near Dawson this spring was an example, says Ric Janowicz, a hydrologist with Yukon Environment.

"It was really quite unusual."

This spring's flood was the second-biggest in almost 40 years of flood records on the Klondike, he says. And it was closely related a highly unusual midwinter break-up, months earlier.

Janowicz had predicted the odd combination of events that struck the Dawson area in a recent study of the possible impacts of climate change on Yukon streamflow and flooding.

"Increased temperatures have generally resulted in thinner ice cover, and a tendency towards earlier break-up dates," he wrote.

Those two factors should reduce the potential scale of ice jam flooding, which is a chronic risk for some Yukon communities, particularly Dawson and Old Crow. However, Janowicz noted, a third temperature-related factor complicates the picture.

Higher winter temperatures have led to midwinter break-ups, a phenomenon that is common in places like Atlantic Canada but extremely rare in the North. In December 2002, warm weather resulted in an unseasonable break-up on the Klondike River.

"As far as I know, that's never happened before," says Janowicz.

Ice pans floated down the Klondike until they grounded in a shallow part of the river about half a kilometre upstream from the confluence of the Klondike and Yukon rivers.

More broken ice piled up behind the leading edge of the jam, until the ice jam was about three kilometres long, reaching about a kilometre upstream of the Klondike Highway bridge.

The December jam briefly raised the level of the Klondike River by about two metres, flooding a few low-lying areas but causing no serious damage. However, officials from the Emergency Measures Organization and Water Resources were already concerned about what might happen in a few months, when spring break-up hit.

Once it was locked into place, the broken ice that formed the December ice jam refroze. As temperatures dropped again, more ice formed on it, creating a three-kilometre stretch of very solid "jumble" ice.

Janowicz and his colleague, Glenn Ford, surveyed the jam early in April and found that the jumble ice was as much as three metres thick in places -- much thicker and stronger than the ice upstream. The best hope of avoiding a serious spring flood, they wrote at the time, was a slow spring melt that would weaken the jammed ice before the upstream ice reached it.

Unfortunately, spring temperatures didn't co-operate. In the last few days of April, the upstream river ice began to break up and float down toward the December jam. It piled up against the upstream end of the jumble ice, about 300 metres above the Klondike Highway bridge, and blocked most of the river channel.

Behind the ice, the river began to rise and overflow its banks. In the space of about 24 hours, the water level rose three metres.

Two houses and a mechanical shop upstream of the bridge suffered severe flooding. One house was inundated by about two metres of water, enough to force it off its foundations. In addition, the Guggieville campground, a stretch of highway, and several nearby areas were covered in about 10 centimetres of water, several vehicles were swamped, and a boat was swept away and sunk.

The flood lasted for several days, affecting different parts of the river as the plug of ice weakened, shifted, and reformed, slowing making its way toward the river's mouth. At one point, the water was less than a metre below the deck of the Klondike Highway bridge.

Eventually, the ice broke free, floated away, and the river settled back within its banks. But, more than two months later, the clean-up is still going on, and so are preparations for next year.

Janowicz attributes the Klondike River's double break-up to the global phenomenon of climate change. Winters have been warming steadily in much of the Yukon over the past few years and show every sign of continuing that trend. That could mean more midwinter break-ups in the future, and more patches of thick jumble ice.

"We're not used to this situation, this type of event here," he says. "It hasn't been a problem in the past."

However, he expects it to be a problem in the future. Janowicz has already contacted ice-jam experts in the Maritimes, where winter break-ups are common, and plans a full analysis of the winter break-up and resulting flood over the winter.

For more information about floods and flood prediction in the Yukon, contact Yukon Environment's Water Resources Branch at (867) 667-3171.