Understanding the dynamics of snow cover in forests can help us predict flood risks

• For more than six months a year, Quebec’s boreal forest is covered in a thick blanket of snow.
• The major floods of spring 2023 in the Charlevoix region show why the snow cover poses a risk.
• Last winter, the Rivière du Gouffre watershed, of which nearly 75 per cent is covered by forests, accumulated a large amount of snow.

Rain as an energy carrier

• As we saw in the spring of 2023, rain events combined with snow cover can lead to a sudden rise in river water levels.
• A heat exchange occurs between rain and snow when their temperatures differ.
• Once the snow has reached a temperature of 0°C, any additional heat from the rain causes melting.

The snow cover, a complexly structured environment

• Rather, it is a stack of snow layers that represent the history of the winter’s meteorological events.
• Rainwater must percolate through all the snow layers to reach the ground, and eventually, the watercourse.

The role of the forest

• By intercepting part of the precipitation in its solid form (snow), trees limit the accumulation of snow on the ground.
• In addition, the discharge of snow intercepted by trees in solid or liquid form increases the heterogeneity of the snow cover.

The same everywhere?

• It’s more akin to sparse vegetation with treeless zones known as gaps.
• In these gaps, the structure of the snow cover is very different from that under the trees.
• The greater accumulation of snow in the gaps favours the compaction of snow layers and the formation of fine grains.

Snow melts faster in gaps

• Although more snow would accumulate in the gaps, it takes less time to melt and reaches the watercourse more quickly than the snow under the trees.
• The combination of thicker snow cover in the gaps and more permeable snow layers under the trees contributed to the Rivière du Gouffre flooding Baie-Saint-Paul during the extreme rainfall of spring 2023.
• However, increased knowledge of the interactions between snow cover and forest will help improve hydrological models and ensure better public protection against flooding.