In late fall, when the snow is just beginning to build up and most of the winter still lies ahead, it's hard to think seriously about floods. But Richard Janowicz does.

Janowicz, a hydrologist with Indian and Northern Affairs Canada, prepares flood forecasts. He says flooding can happen in any season in the Yukon.

In fact, the Yukon has a history of destructive floods. Although the territory's population is sparse, most communities are located on flood plains, a by-product of our historical reliance on river transportation. That makes them particularly vulnerable to flooding.

"We have a cycle of flooding events through the year," says Janowicz.

The earliest floods in the spring are ice-jam floods, which usually occur in May. Dawson City and Old Crow are particularly susceptible. The floods happen during break-up, when moving ice piles up against a barrier or constriction in the river. The water rises behind the jam and spills over the riverbank.

Dawson City has had five major ice-jam floods in its century-long history.

"It's got a dyke to protect it now, but we still can't turn our backs on it because the big flood's going to come along one day," Janowicz says.

Old Crow has a major ice-jam flood about once every 10 years and, because of the community's location and the nature of the terrain, dyking isn't practical. Janowicz and his colleagues keep a close eye on the Porcupine River every May in order to give residents as much warning as possible.

Old Crow's problem, he explains, is an icing formation that traditionally develops at the mouth of the Bluefish River, 40 kilometres downstream from the community.

"In certain years, this formation gets really thick. It blocks any ice coming downstream, and the water backs up all the way to Old Crow."

As soon as the danger of ice-jam floods passes, the season for snow-melt flooding begins. Ross River and Mayo are the communities most likely to be affected by rivers swollen with melted snow, mainly in early June. Other communities, such as Upper Liard and Carmacks, also have problems in some years, says Janowicz.

Flooding as a result of heavy rain storms can happen any time through the summer, the hydrologist says, and can cause considerable damage.

"They generate debris torrents and mudflows as well as high water," he explains.

Small, isolated stream systems draining mountainous areas are usually the hardest hit. Rainstorm flooding has played havoc with stream crossings on the Canol Road, the Alaska Highway, and the Dempster Highway in recent years.

And just when you thought it was safe to go back in the water -- along come the glacier and snowfield flooding events. They hit in late summer, when there's a good snow-pack high in the mountains and warm temperatures to melt it. Glacier-fed systems like the Yukon's southern lakes can reach their high-water mark as late as mid-September, causing problems for places like Carcross and the cottage communities along Marsh Lake.

The last event in the annual flood cycle is the winter ice-jam flood, which usually hits in December or January. Winter ice-jam floods are triggered during freeze-up or sudden cold spells, Janowicz says, when ice blocks a channel that is not yet fully frozen, and the upstream water backs up and overflows its banks.

The Marwell area of Whitehorse suffers a winter ice-jam flood every couple of years, he says, because of a constriction in the Yukon River just downstream. If ice blocks the constriction before the upper river has frozen, houses and businesses in Marwell are soon awash. In late 2000, Marwell experienced the biggest ice-jam flood in 25 years.

In next week's column, we'll look at the science of flood prediction in the Yukon. For more information about Yukon flooding, contact the Water Resources Division of Indian and Northern Affairs Canada, Whitehorse, Yukon.