As southern Pakistan grapples with deadly flooding along the Indus River, residents of another country with a lengthy history of floods may be wondering if it could happen here.
They’re also Canada’s most expensive and most common natural hazards, according to Public Safety Canada, affecting hundreds of thousands of Canadians.
“There are urban areas across the country that all deal with flooding,” Carleton University professor Jennifer Drake told CTVNews.ca in a phone interview on Friday. “But the causes of flooding can vary with the climate and the region.”
Here are the types of regions in Canada most likely to experience flooding events.
While many people associate the word “delta” with the Mississippi River, John Richardson, who teaches in the University of British Columbia’s department of forest conservation sciences, says Canada has many deltas, and they’re typically prone to flooding.
According to Richardson, river deltas form where the flow of a river slows as it reaches the body of water it drains into, causing that flow to spread out over a larger area and deposit sediment that eventually becomes a landmass.
“Many of the places in Canada where we see big floods are largely in those delta-type areas,” Richardson told CTVNews.ca in a phone interview Friday. “The thing that’s important is to think about river deltas as not just going into the ocean. River deltas also into other rivers, into lakes and wetlands.”
Deltas are especially prone to flooding when water levels rise, either in the river or the body of water it’s flowing into. This flooding, he said, has caused problems for communities situated in Canadian deltas, like those along the Fraser delta in British Columbia.
Flat, shallow areas of land adjacent to rivers, known as floodplains, are especially prone to flooding during storms, spring ice melts or any other event that causes water levels to overtop the riverbank.
“The name says it all,” Richardson said. “Floodplains were established over thousands of years from streams carrying sediment down from where it’s being eroded from.”
Richardson said they’ve historically been attractive places to settle, since the soil in floodplains is good for farming.
“So we have had a history as humans of building into those areas.”
Richardson said communities situated on floodplains are threatened by water level changes caused by storms and spring ice melt, not only due to the risk of a river overtopping its banks, but because these water level changes prevent storm-water drains from properly draining, leading to urban flooding.
Several communities in Toronto were devastated by historic flooding during Hurricane Hazel in October 1954. That event killed more than 80 people, left thousands homeless and destroyed bridges and roads in the west end of the city, near the Humber River floodplain.
According to Drake, spring brings additional risks for communities along rivers in the form of the freshets, or spring thaws, and accompanying ice jams. While freshets can raise water levels, ice jams create natural dams that impede the flow of water through the river.
“Ice jams are when you have that surface ice on a river that breaks up and gets stuck like a log jam, and causes the water behind it to back up,” said Drake, who teaches in Carleton University’s department of civil and environmental engineering. “These are hard to predict.”
Water dammed by ice jams can flow over the banks of a river, impacting nearby communities.
While ice jams can occur on any river that freezes during the winter, north-flowing rivers such as the Mackenzie River are especially prone to them. This is because upstream water to the south thaws faster than downstream water to the north, contributing to the build-up of ice jams and increasing the risk of local flooding.
Notable examples of major flooding in north-flowing rivers include the 1950 Red River flood in Winnipeg, and the Red River Valley; the 2020 Athabasca River flood in Fort McMurray, Alta.; and annual flooding along the Mackenzie, Hay and Peace rivers in Alberta and the Northwest Territories.
Along Canada’s coasts, storms bring the risk of atmospheric surges that can push seawater up onto the land.
“The water level rise that happens when you have a hurricane or a large storm creates flooding or exacerbates flooding for coastal cities,” Drake said.
In January 2000, a record storm surge event in New Brunswick caused more than $1.7 million in damage in communities from Shediac to Bathurst.
Richardson expects to see storm records broken with increasing regularity throughout the country as climate change intensifies.
“We all know with climate change we’re anticipating stronger storms,” Richardson said. “Even if you look at hurricane intensities, those have been increasing in average intensity for most of the last 40 years.”
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