People row a boat on a flooded residential street in Gatineau, Quebec May 4, 2017.
Hundreds of people have been forced to evacuate in Montreal's West Island area, along with other parts of southern Quebec.
Heavy rains and melting snowpack across Quebec have so far flooded 2,426 residences in the province, forcing the evacuation of 1,520 people in nearly 150 municipalities.
Federal Public Safety Minister Ralph Goodale says the army will work with local authorities to coordinate relief efforts.
Whatever hope Montreal-area flood zone residents had that Nature would throw them a break was extinguished in a haze of constant drizzle early Friday as Environment Canada updated its special weather statement to a full-fledged rain warning. The rain is expected to lessen by Saturday night, Environment Canada said on its website.
"The water is up to your knees on the bridge", said Nello Dicaprio, who has lived on the island for 11 years and made a decision to remain in the chalet he rents.
The New Brunswick Emergency Measures Organization said the St John River basin is at capacity, which means flooding is likely.
In Atlantic Canada, some parts of New Brunswick recorded more than 150 millimetres of rain after a almost 36-hour non-stop downpour.More news: Trump administration declares intent to renegotiate NAFTA
Toronto, Canada's biggest city, was forecast to receive 20 mm (1 inch) to 30 mm of rain on Friday, with more over the weekend, according to Environment Canada, which issued a rainfall warning.
In the nation's capital, Ottawa, 15 to 25 mm of rain was expected.
Many have blamed the flooding on the International Joint Commission's Plan 2014, which regulates water levels on the lake and river.
"We are very aware of the situation around the province and monitoring it very, very closely", she said.
Environment Canada has also posted severe thunderstorm watches for a large section of British Columbia's central and southern Interior, an area that is already contending with spring flooding.
Dozens of properties north of Kelowna were also under an evacuation order due to flooding, while sections of the Trans Canada Highway near Salmon Arm and Glacier National Park were closed due to mudslides.
The city has also warned that the heavily-travelled Don Valley Parkway through the heart of Toronto could be shut down on Friday afternoon, should the adjacent Don River overflow its banks.
Flooding has also caused washouts of roads and bridges, as well as a rock slide near Highway 16 just east of the Alberta boundary.