Water levels at the hydroelectric dam at Bell Falls on the Rouge River in Quebec's western Laurentians reached heights never before seen Thursday, triggering a mandatory evacuation of houses downstream.

Provincial police were going door-to-door to ensure residents left the area, Quebec Public Security Minister Geneviève Guilbault said Thursday evening. Some people had to be evacuated by helicopter.

The area around the dam is mainly forest dotted with small farms. It's sparsely populated, but there are around 23 principal residences and 38 cottages — many of which may not be occupied — so the exact number of people affected is unknown, Guilbault said.

The Public Security Ministry sent an evacuation alert Thursday afternoon after Hydro-Québec notified the ministry that the Rouge River had reached levels never before seen.

Dam built to withstand once-in-1,000-years flood

"A dam like Bell Falls is designed to withstand a flood that happens once in a thousand years," said Simon Racicot, head of production at Hydro-Québec. "Today at noon we reached that level, and at that point there's a protocol in which we notify civil security and residents of imminent danger."

A "once in a thousand years" scenario means there is a 0.1 per cent chance of a flood of this magnitude happening in any given year.

The average flow at Bell Falls is 103 cubic metres per second, Hydro-Québec data shows. On Thursday, it increased ninefold to reach 980 cubic metres per second.

The Bell Falls dam, on the Rouge River, 16 kilometres north of the Ottawa River, is in danger of rupturing as the water level surpassed the dam's capacity by noon Thursday. The dam was built 'to withstand a flood that happens once in a thousand years,' said Hydro-Québec's head of production, Simon Racicot. (CBC)

The utility is confident that the dam will not break, Racicot said, but for the safety of citizens, it could take no risks.

"We're entering an unknown zone, really," he said. "So we have to wait and see what happens over the next few days."

If the dam ruptures, it would take around 45 minutes for floodwaters to reach those buildings under the evacuation order, police said.

Police said they are using "all the means at their disposal to ensure people's safety."

"We are asking residents of areas affected by forced evacuations to co-operate with the authorities," the Sûreté du Québec said on Twitter.

Watch helicopter footage of the dam at risk of breaking:

Quebec's Public Security Ministry is warning that the hydroelectric dam at Bell Falls on the Rouge River in the western Laurentians region has reached its capacity. 2:15

Helicopter ride under sunny skies

Brenda Finn, who lives on Rivière Rouge Road, was among those evacuated from her property by helicopter Thursday.

She said her house is high and dry, well above the road that skirts the river, so when soldiers arrived at her door last Sunday to say she should think about leaving, she told them she wasn't worried.

Brenda Finn, who lives downstream from the Bell Falls dam on the Rouge River, was taken out of the area by helicopter on Thursday, April 25 after water levels at the dam reached unprecedented heights. (Elias Abboud/CBC)

"I said, 'I'm not going.' But it's a different story when you have a dam breaking," she told CBC News.

When a municipal official called Thursday to tell her to call her neighbours and let them know the dam may not hold, she said she "knew it was all over. I knew they were coming."

By the time the helicopter arrived, Finn said, she had a bag packed and climbed aboard.

"It was really beautiful," she said of her helicopter ride. "The sun was out; you're looking at the River Rouge — the nice farmland and the trees."

However, rain in the forecast for the next few days means water levels at the dam could rise a further 70 centimetres, so Finn is well aware the worst might be yet to come.

"I'd be a fool not to worry — but it is what it is."

Highway 50 reopened at Grenville

On Thursday evening Transports Québec reopened Highway 50 in both directions along the stretch that passes through Grenville-sur-la-Rouge, after closing it in the afternoon.

That highway, which links Gatineau to Highway 15 at Mirabel, crosses the Rouge River about 16 kilometres downstream from the dam, where the Rouge empties into the Ottawa River.

The evacuation order for the area downstream from Bell Falls comes as large swaths of Quebec and neighbouring eastern Ontario are dealing with spring floods.

Ottawa declared a state of emergency on Thursday, while Montreal warned water levels could rise even higher this weekend.

Bell Falls, or Chute-Bell, is about 23 kilometres northwest of Grenville-sur-la-Rouge, Que., which is on the north side of the Ottawa river, across from Hawkesbury, Ont.