Another record-setting Ottawa River flood

by Environment & Climate Change Canada

Nature was all primed for another catastrophic spring flood along the Ottawa and St. Lawrence Rivers—the second record discharge in three years. Flooding on the Ottawa River is often a threat in the spring. In the past century, flooding has exceeded a flow of 8,000 cubic metres per second at Hawkesbury, Ontario, on eight occasions. But only twice, in 2017 and 2019, has the flow peaked above 9,000 cubic metres per second. This year’s flood was bigger than the 2017 event that was then considered the flood of the century. On April 5, Hydro-Québec reported that the dam at Chute Bell, Quebec, on the Rouge River (which feeds into the Ottawa River), was exceeding the dam’s specifications. The dam, built to withstand a once-in-one-thousand-year flood, saw water gushing over the top and around its sides, at eleven times its normal flow.

Everything about this year’s flood, including its size and duration, was unprecedented. All the weather ingredients were in place for a catastrophic event. Seven straight months of below-normal temperatures from October to April ensured the ground froze deeply and thawed late, preventing the infiltration of rain and snowmelt runoff. With little melting by mid-spring, the deep and icy snowpack stayed. In heavily forested areas upstream of the Ottawa River, snowfall accumulation was 50% greater than normal. Adding to this scenario were multiple rounds of heavy spring rains persisting over a five-week period from mid-April to mid-May. This included two storms from the Gulf of Mexico that brought an equivalent of a month’s worth of rain and triggered an immediate and lengthy spring flood. In Ottawa, twice the normal accumulation of rain, 150 mm to be exact, fell between April 10 and May 10.

It promised to be a long, drawn-out flood season along the rain-soaked Ottawa and Rideau Rivers and in Quebec along rivers such as the Rigaud, Mille-Îles and des Prairies. In mid-April, 20 to 40 cm of snow still lay on the ground in the northern reaches of the Ottawa River watershed. Incessant rains and warm air from the southwest assaulted the snow pack, causing rapid melting. On May 1, the bloated Ottawa River crested 30 cm above 2017’s peak flood levels. Water inundated several riverside communities, including Pembroke, Constance Bay, Fitzroy Harbour, Arnprior and Britannia in Ontario, and Pontiac, Gatineau, Rigaud and Laval in Quebec. Dozens of smaller rivers flowing into the Ottawa River also broke all-time flow records. For the second time in three years, homeowners, municipal workers, volunteers and armed forces personnel worked frantically to fill sandbags, build makeshift walls, pump water from homes, and assist first responders in evacuations. Hundreds of residents from Pembroke, Ontario, to Sherbrooke, Quebec, and on to the Beauce, Quebec, region were forced to leave their homes despite sandbagging efforts. In Ottawa and Gatineau, more than 6,000 dwellings were flooded or at risk. Roads and streets in flooded areas closed for long periods. Several bridges, including the Chaudière between Ottawa and Gatineau, also closed, and many of the ferry services were suspended. Farmland was flooded, delaying fieldwork and planting. Downstream in Montréal, the flood emergency remained until May 8. It took Ottawa more than a month longer to lift their state of emergency