A multimillion-dollar bridge offering the sole east-west route across part of northern Ontario has been partially reopened after sustaining serious damage over the weekend, provincial officials said Monday.

A statement from Transportation Minister Steven Del Duca said one lane is available to cars and regular-weight transport trucks, but said engineers were still working to determine whether the Nipigon River Bridge can sustain the weight of oversized trucks.

Del Duca said staff worked through the night in order to reopen a route that the province's premier described as critically important.

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"As soon as I heard about the closure of the bridge I was concerned because it is the lifeline in terms of product and transportation in northwestern Ontario," Kathleen Wynne said. "It connects the east to the west, and there is only that one route."

The OPP closed the bridge indefinitely on Sunday afternoon when part of the steel decking on the western side separated about 60 centimetres from the rest of the structure. There were no injuries reported, and pedestrians were still able to make use of the bridge.

Word of the partial reopening came as a relief to local businesses, said Dan Bevilacqua, manager of the North of Superior Travel Association.

He said companies were bracing to function without bridge access for anywhere from two days to a full month, adding that the flow of supplies wasn't even the main concern.

"I think what people were worried about is just essentially the country being split in two," he said. "Here in Nipigon, we are at the middle of the country. Everything has to come through this area. ... It was more concern for travellers."

The news was equally welcome to 20-year-old Mariah Veneziano, who had travelled to Thunder Bay, Ont., for the weekend to help her sister catch a flight.

She avoided a night shivering in her car by staying with friends, but set out for her home in Schreiber, Ont., home as soon as she heard traffic was moving again.

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Veneziano said she initially felt nervous about crossing the structure, but found the scene less harrowing and more efficient than she expected.

"It was probably only a 10-minute wait because they were letting both sides of traffic go by," she said. "We were going about 25 kilometres an hour across."

No one has yet offered an estimate as to when the bridge, which is part of the Trans-Canada Highway and spans the Nipigon River, will be fully functional.

Nor is there any word as to what caused the separation, an issue Wynne said the government is determined to "get to the bottom of."

The Ontario government began building what it touts as the province's first cable-stayed bridge in 2013 and opened westbound lanes to two-way traffic in November. The project, which the government pegs at $106 million, is due to be completed in 2017.

Ontario Northern Development and Mines Minister Michael Gravelle was planning to visit the site on Monday to assess the extent of the damage, and Del Duca is planning to inspect the site on Wednesday.

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The nearby municipality of Greenstone declared a state of emergency on Sunday out of what deputy Mayor Eric Pietsch describes as an abundance of caution. He said the community itself was not threatened, but wanted to be in a better position to assist stranded travellers if necessary.

Pietsch said the state of emergency will likely remain in effect until Tuesday morning.

Ontario's New Democrats said the failure of the bridge "shows the Liberal government's mismanagement of northern Ontario's roads and highways."

"Many companies in northern Ontario depend on the bridge to transport product across the country," said Wayne Gates, the party's transportation critic. "This will hurt industry, and many small communities will be economically impacted with less motorists passing through."

Progressive Conservative transportation critic Michael Harris called for an explanation, noting the bridge failed only 42 days after opening the new westbound lanes.

"This is a route that sees between 5,000 and 10,000 vehicles per day — many that can't simply divert through the United States to get from east to west," said Harris. "Where was the backup plan to prevent the potential of a failure that effectively splits Canada in two?"