Spring breakup: what factors affect flood risk?

Those lucky enough to see the breakup of the North's major rivers first-hand know that it is an incredible sight to behold, with large chunks of ice tumbling downstream.

But for folks living on the riverside, spring breakup also brings the risk of flooding. 

How does the breakup happen and what determines the chance of flooding in shoreside communities?

Breakup occurs in stages that usually begin in April. As the snow melts, water levels in the rivers rise. This rising water adds pressure to ice above, which causes cracks parallel to the shorelines.

Ice that covers more turbulent stretches of a river will then begin to move with the rapids, breaking into smaller pieces. As these fragments flow into calm, still frozen stretches of the river, it will cause ice jams.

Ice jams

When upstream portions of a river thaw first these ice jams are more common, as thawed ice flows into still frozen sections of a river.

Ice jams act as a natural dam, trapping water behind them. When these jams finally clear, there is a sudden surge of water, which is responsible for downstream flooding.

Faye Hicks, a retired professor from the University of Alberta who studied river ice breakup and flood forecasting in Mackenzie River, says "the biggest factor [in the breakup] would be how much snow is in the basin upstream."

More snow will mean a more dramatic breakup in the river. That will also mean a greater risk for flooding downstream.

Forecasting breakup

Another factor would be the spring weather leading up to the breakup. A sudden warming in temperatures (or rainfall) will mean a faster melting of the snowpack, which in turn leads to an intense breakup. A slow warm up will mean a more gradual breakup. Late season snowfall can also help insulate the river and keep the breakup gradual.

Hicks did work on a couple of models to help predict the breakup of the river. Her first forecast is a longer range forecast that identifies, from the snowpack, if it will be a low, medium, of high break up.

The rest of the forecasting for the severity of the breakup and flood risk has to wait until the break up begins, because the weather can change so rapidly. Once the breakup starts, you can predict where ice jams may occur, and how big those jams can get which in turn predicts the possible flooding.

The models will need more research to become more accurate but the longer range forecast does give a good idea of the what the season will look like.