When the ice breaks up on the Yukon River, it is generally a time for celebrating in Dawson City. Since the days of the riverboats, this event has signaled the end of winter in the Klondike.

Since 1896 people have held a lottery to guess the exact minute that the ice will go out in front of Dawson. A tripod holding a bell is placed on the river ice in front of the town. When shifting ice moves the tripod and rings the bell, breakup is official as far as most Dawsonites are concerned.

Staff from the Water Resources Division of the Department of Indian Affairs and Northern Development are also on hand for the occasion. Their job is to predict whether ice jams could cause flooding in the town. And when the bell on the tripod rings, their job is not yet over.

Dawson is built on a floodplain at the confluence of the Yukon and Klondike Rivers, and on five occasions in the last 100 years ice jams have caused major floods in Dawson City. The worst event occurred in 1979 when most of the town was covered by two metres of water, resulting in 8 million dollars worth of damage.

In 1987, a dyke was completed along the riverfront to protect the town from flooding. The dyke is designed to withstand a 200-year flood, an event that would raise water levels even higher than in 1979. But as Rick Janowicz, the head of hydrology with Water Resources, points out, "This is a statistical concept and in reality a 200-year flood could happen any year, even in two consecutive years."

In the late 1980s Water Resources developed a model to help forecast when break-up will occur and whether flooding is a concern. They consider factors such as the thickness of the ice and the timing of the spring thaw and daily changes in water level.

"We're pretty good at it so we can usually pin down the day of breakup a week in advance," says Janowicz.

Janowicz says that the ringing of the bell on the tripod just signals the start of breakup. "I've seen the tripod move 50 metres and then just sit there for two more days," he says.

Water flowing out of the Klondike River often causes an initial shift in the ice. But it is water from the Stewart and White Rivers that finishes off the job, and sometimes causes flooding.

Once breakup has started in Dawson, staff from Water Resources keep a close eye on conditions both upstream and downstream of Dawson, keeping track of where the ice is running and how snowmelt is progressing. They also use satellite images to monitor conditions in the area.

Every day they fly over the section of river between the Chandindu and White Rivers. The White, which drains the western St. Elias Mountains, is warmed by coastal flows of milder air from the Pacific, and can open up as much as two weeks before the Yukon River.

The hydrologists estimate the thickness of the ice by tracking the mean daily temperatures in winter; basically a cold winter means thick ice. Thicker ice is more likely to form ice jams, which in turn can cause flooding.

Around Dawson ice jams come in two different forms. The most common ones form downstream of Dawson, causing the water to back up for a time. But ice jams also form upstream when water from the White and Stewart Rivers flows into the Yukon, causing the ice there to decay.

These upstream jams can cause surges in the water flowing down the Yukon River. "One jam will break and release a surge of water that causes another ice jam to break, forming a bigger surge of water," says Janowicz.

Eventually a wave of water a metre high can form. If there are no ice jams or solid ice downstream of Dawson, the surge just rolls past the town. But if ice does block the way, floods can occur.

Janowicz says that he has seen water on the river rise two to three metres within less than an hour. During the 1979 flood, witnesses said that the water rose at a rate of a third of a metre per minute at the peak of the flood.

The potential for a flood in 2000 is predicted to be lower than normal. The ice is thin, and temperatures were warmer than normal early in the season. But in the end it is a matter of timing.

During years when the ice is thick, a sudden burst of warm weather in the spring can lead to flooding. Rapid snowmelt can raise upstream water levels while the ice remains locked in place.

Over the 104 years that records on breakup have been kept in Dawson, the average date has been May 9. But during the last decade breakup regularly has occurred earlier than normal, a phenomenon that Janowicz attributes to climate change.

Historically breakup has almost always occurred sometime in May. Only seven times in the last century has the ice broken up in April, and five of those events were in the last decade.

Whatever the date, breakup is usually a sight worth seeing. "Every once in awhile the ice just mushes out, but usually it's pretty dramatic, with lots of crunching and grinding going on in the ice," says Janowicz.

For more information on flood forecasting in the Yukon, contact Water Resources at 667-3145.