Canada: Increased threat of springtime flood events nationwide through at least June 30

There is an elevated threat of areal flooding across Canada in the coming weeks as snowmelt runoff associated with the warmer spring season (March-May) increases water levels at local and regional watercourses. Heavy rainfall during this period could also exacerbate existing flood threats, particularly in rural and remote riverine communities.

Flooding and landslides could affect business, transport, and utility services in many areas. Localized disruptions could persist well after hazardous events have concluded; floodwaters may take days to subside, and debris from mudslides may take weeks to be cleared. Rural communities and informal settlements often sustain the greatest damage and casualties during such events due to poor infrastructure. Mandatory evacuations are possible where severe flooding is forecast.

The risk of flooding is highest in low-lying communities near watercourses in recognized floodplains. Canada has several major rivers and lakes, including the Athabasca, Hay, Mackenzie, Peace, and Red rivers and the Great Lakes, that could flood during periods of intense rainfall. Floods often threaten communities downstream of major rivers or dams in the event of structural failure. Urban flooding is possible in areas with poorly maintained or inadequate drainage systems. Landslides and mudslides are possible in hilly and mountainous areas.

Rivers generally rise slowly during snowmelt and spring runoff floods, allowing time for preparations or evacuations. Water level changes also occur in lakes but occur more slowly than in rivers. Runoff meltwater increases the threat of flooding in low-lying communities both downstream and upstream of riverbeds, with the latter exacerbated by the presence of ice jams (sheets of ice that restrict the flow of water in river channels during spring melting). Rapid snowmelt caused by rapid warmups and high temperatures can also increase streamflow - which poses a danger to those outside flooded areas - and trigger landslides in hilly or mountainous terrains. High soil moisture conditions prior to snowmelt, frozen soil, heavy snow cover, and widespread heavy rainfall during snowmelt can exacerbate flooding.

Floodwaters may take some time to recede in affected areas, and residual disruptions may persist. Stagnant pools of water during and after inundation often increase the threat of waterborne disease outbreaks.

In addition to posing a direct threat to personal health and safety, flood events can trigger localized business, transport, and utility disruptions in affected areas. Floodwaters and related debris may render some bridges or roadways impassable - impacting ground travel in and around affected areas - or damage critical infrastructure like ground transport networks and power lines. Flooding could also cause flight disruptions at smaller regional airports. Authorities may suspend riverine ferry and cargo services where significant flood events occur and/or river ports are damaged.