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b.c. flooding

People look on as a house comes off its foundation in Two Mile just outside of Sicamous, B.C. on Monday, June 25, 2012.JONATHAN HAYWARD/The Canadian Press

The big, brawling, mud-brown Fraser River has topped its natural banks and is approaching historic high-water levels.

By the end of the week it will be known whether the flood threat has passed. But for the next few days everyone will be watching to see if water levels drop after peaking just below the lip of the dikes that are holding back the churning waters.

With heavy rainstorms and sudden snow melt causing a flurry of flooding around the province over the weekend, officials were worried the Fraser could rise to levels it has reached only four times before in the Lower Mainland, when the mighty river swept across the flood plain in the valley, where most of British Columbia's population is concentrated.

Not since 1972 has the Fraser crested the dikes in the Fraser Valley, but a pulse of water pouring into the main stem from swollen tributaries throughout the Interior, and slowly making its way downstream, promises to push the river close to the limit. (Follow the Fraser River streamflow with this interactive tool.)

Federal and provincial experts say it should begin to drop before it breaches any dikes.

"I'll be shocked if we flood," Mission Mayor Ted Adlem said after he had gone down to the river's edge to watch the powerful current surging toward the Pacific.

"Unless a disaster happens and a dike fails, we should be fine … but it will be a week on edge for folks that are marginal and are not behind the dike system," he said.

Mr. Adlem said daily dike patrols have been started but there are no signs of trouble, although water has come up through riverbank trees and is pooling in low-lying areas.

At a daily flood briefing on Monday, Environment Canada meteorologist Doug Lundquist said the worst of the rain is over, and Dave Campbell, of the B.C. River Forecast Centre, predicted the Fraser would peak at about 6.6 metres at Mission – a high-water mark that will be below the top of the dikes.

"We may go to seven metres," Mr. Adlem said, "but I think we'll be fine."

That projection would put the river slightly lower than the 7.1-metre level it hit in 1972, the previous time flooding occurred, causing $10-million in damages.

In 1948, the Fraser rose to 7.6 metres at Mission, breaching several dikes, flooding 220,000 hectares and severing two transcontinental rail lines and the Trans-Canada Highway. An estimated $20-million in damages occurred.

The dike system has been upgraded since then, and flooding is not expected unless a sudden rainstorm pushes up the river level.

"It's a day-by-day thing and we've got people out patrolling the dikes 24/7," Mr. Adlem said.

The highly developed dike system, which parallels much of the lower river, stands a little more than eight metres above sea level.

Only once, in 1894, has the Fraser reached that height. It has swelled above seven metres only three times, in 1948, 1950 and 1972.

Mr. Campbell said the Fraser dropped slightly Monday, but will start rising again Wednesday and it should peak Friday or Saturday.

Provincial and federal authorities track the flow of the Fraser at several points along the river and the gauge at Mission, just upstream of Metro Vancouver, is a key measuring point.

Mr. Adlem said people in Mission are keeping a wary eye on the river, but there isn't any sense of alarm.

"We are used to this. We get a spring freshet every year about this time," he said. "This week, things look actually very good [with the projections]. Of course, everything is kind of rain-dependent."

Ian Fitzpatrick, fire chief and emergency preparedness co-ordinator for Mission, said a sandbagging machine is on stand-by, but so far there hasn't been any need to mobilize emergency crews.

"Wednesday is a key day for us," he said, noting that is when the river forecast centre will update the situation with the latest data.

Heavy rainfall and rapid snow melt occurred in the Thompson River basin over the weekend, including an intense rainstorm that caused flash-flooding in the small community of Sicamous, near Salmon Arm.

It typically takes three days for water to reach the Lower Fraser from that region.

Chris Duffy, executive director of emergency co-ordination for Emergency Management B.C., said 672 people were on evacuation orders around the province on Monday and 1,007 had been given alerts.

He said emergency officials will wait for safe water levels before lifting the evacuation orders.

Although the worst of the flood threat seems to have passed for the Fraser River system, there are still concerns along the Skeena River, in northwest B.C., where the river is running high and more rain is expected.