Open this photo in gallery Montreal Mayor Valérie Plante declared an emergency anticipating what may be coming. Paul Chiasson/The Canadian Press

Waters are rising quickly in flooded areas from Ottawa to Montreal and vast areas north and south, putting thousands more homes under threat, and triggering emergency evacuations and pleas for help from local officials.

A dry spell earlier this week offered hope that water levels in Eastern Ontario and western Quebec might stabilize below the record-breaking disaster of 2017. Those hopes were dashed on Friday as heavy rains fell with up to 50 mm forecast through Saturday, adding to already high water levels and melting snow.

Parts of the St. Lawrence River and Ottawa River were projected to reach 2017 levels early Saturday morning. The Ottawa River could hit 50 centimetres above the 2017 levels that damaged 5,371 homes and forced 4,000 people to seek high ground. The water is not expected to crest before Monday or Tuesday.

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Prime Minister Justin Trudeau picked up a shovel Saturday to lend a hand with sandbagging efforts, as he and his two sons visited a west-end Ottawa community hit hard by rising floodwaters.

Mr. Trudeau was briefed by officials in charge of the fight against the flood at a community centre before filling sandbags and thanking those who came to the aid of local residents.

“Thank you for doing what you are doing,” Mr. Trudeau said as he and sons Xavier and Hadrien spoke with volunteers working around a large sandpile.

A group of local leaders from 10 small bedroom communities just west of Montreal pleaded for help from the province on Friday afternoon, saying the slow-moving flood of the past 10 days is about to turn into a larger-scale disaster. “In coming days, we alone will have to evacuate 1,000 people. This is enormous,” said Patrick Bousez, head of the Vaudreuil-Soulange regional council. “We need help.”

The disaster is playing out in scores of small pockets in the 1,000 kilometres from the cottage country of Bracebridge, Ont. to the heavily populated St. Lawrence Valley and on to Fredericton and Saint John. However, it has already hit Quebec hardest and the province faces the biggest fresh threat.

Around 3,018 homes were already flooded in 42 Quebec municipalities and 1,110 people had left their homes on Friday. In New Brunswick, nearly 1,000 people have evacuated homes voluntarily and 400 are staying in temporary hotel accommodations provided by the Canadian Red Cross. However, about 1,000 people did return to their homes late in the week, along the Chaudière River in eastern Quebec where flooding hit earliest.

In Montreal, only 49 homes were damaged on Friday but Mayor Valérie Plante declared an emergency anticipating what may be coming.

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About 400 soldiers rolled into Ottawa to assist with its state of emergency, sandbagging and potential evacuations. On the Gatineau side of the capital, where 240 houses were already flooded on Friday, Mayor Maxime Pedneaud-Jobin prepared for the worst. “What we are expecting in coming days, we have never seen before,” the mayor said.

One factor complicating matters compared with 2017 is that dozens of smaller rivers flowing from the north are breaking all-time flood records. Among them is the Rouge River, where authorities were closely monitoring a small hydro dam in Bell Falls, Que. Rushing water overflowed the structure and provoked an automatic emergency evacuation of about 66 homes and cottages.

Montreal declares state of emergency, Canadian Forces arrive in Ottawa as flood waters set to rise

Water levels exceeded one-in-a-thousand-year projections for the dam, but Hydro-Quebec officials said they are confident it will hold. The structure, known as a run-of-the-river dam has no reservoir and holds back a relatively small amount of water compared with major dams. But a complete failure would still send about 2.5 metres of water downstream. Quebec Transport Minister François Bonnardel said the main risk is that rushing water could wash out highways.

The dam warning drove home the gravity of the situation, even if few people are directly threatened, according to Marc Olivier Labelle, mayor of the small town of Saint-André d’Argenteuil about 50 kilometres away. “It has really struck people’s imaginations. We are having a much easier time convincing them to leave,” he said. The water is also colder than it was in 2017, increasing the risk of mitigation efforts and sapping the morale of the workers, he said.

Marie-Pierre Chalifoux, who built a temporary retaining wall with 4,000 sandbags to protect her waterfront home in the town, is not going anywhere. She expected the water to break the 2017 level early Saturday morning but says she believes her fortification will hold. “I had 40 friends working two days to build it,” she said. “I’m fortunate I have a lot of good friends.”

In New Brunswick, water in the Fredericton area remained relatively stable on Friday, buoying hopes that it will soon recede. But Ian Hubbard, a meteorologist for Environment and Climate Change Canada, said a downpour combined with winds gusting up to 60 km an hour and thunderstorms are expected to worsen conditions in the flood zones around Fredericton and Saint John.

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Up to 44 millimetres of rain could fall by midnight Saturday and emergency-response officials warned residents not to expect flood waters to drop until early next week. “Now is not the time to relax,” said Stacey Cooling, director of operations for the New Brunswick Emergency Measures Organization.

TOTAL EXPECTED PRECIPITATION FOR THIS WEEKEND Flood warnings are in effect for much of Central and Eastern Canada, as of Friday at 3:22 p.m. Rainfall, in mm Snowfall, in cm up to 30 25 to 40 25 to 50 30 to 60 10 to 15 15 to 20 20 to 30 may exceed 50 mm in local areas Freezing rain * QUE. NFLD. * ONT. PEI N.S. N.B. CARRIE COCKBURN / THE GLOBE AND MAIL, SOURCE: GOVERNMENT OF CANADA TOTAL EXPECTED PRECIPITATION FOR THIS WEEKEND Flood warnings are in effect for much of Central and Eastern Canada, as of Friday at 3:22 p.m. Rainfall, in mm may exceed 50 mm in local areas Snowfall, in cm Rainfall, in mm up to 30 25 to 40 25 to 50 30 to 60 10 to 15 15 to 20 20 to 30 Freezing rain * QUE. NFLD. * ONT. PEI N.S. N.B. CARRIE COCKBURN / THE GLOBE AND MAIL, SOURCE: GOVERNMENT OF CANADA TOTAL EXPECTED PRECIPITATION FOR THIS WEEKEND Flood warnings are in effect for much of Central and Eastern Canada, as of Friday at 3:22 p.m. Rainfall, in mm Rainfall, in mm may exceed 50 mm in local areas Snowfall, in cm 10 to 15 15 to 20 20 to 30 up to 30 25 to 40 25 to 50 30 to 60 Freezing rain * QUE. NFLD. * ONT. PEI N.S. N.B. CARRIE COCKBURN / THE GLOBE AND MAIL, SOURCE: GOVERNMENT OF CANADA

A section of the TransCanada Highway between Fredericton and Moncton that was closed on Wednesday evening remains under water. Trucks and travellers trying to make their way between the two cities are being forced onto a 90-kilometre detour. A total of 88 roads in the province have been closed owing to the floods.

In Saint John, four communities have been “totally isolated” from the mainland due to flooding, said Kevin Clifford, fire chief and director of the Saint John Emergency Management Organization. More than 25,000 sand bags have been distributed in and around that city.

Federal and provincial leaders toured hit and threatened areas and offered support.

Ontario Premier Doug Ford dropped into a rural area west of Ottawa and called on volunteers to help out with sandbags. The situation “just rips your heart out,” he said. Governor-General Julie Payette toured the Saint John River basin along with Premier Blaine Higgs.

Federal Public Security and Emergency Preparedness Minister Ralph Goodale, who briefed reporters at his riding in Regina, said federal assistance is available and his officials were co-ordinating closely with the provinces and local authorities. More military, Fisheries and Coast Guard resources were available to help, he said.

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Mr. Goodale ended his briefing on a sliver of positive news, saying that while Manitoba is experiencing some localized flooding, the “very significant risk for serious flooding" south of Winnipeg has not happened.

WIth a file from The Canadian Press