What you need to know about the flooding in Quebec and across Canada

Four hundred soldiers were deployed in Quebec on Saturday as flood waters threatened many parts of the province.

So far, more than 130 communities have been hit by flooding, with some 700 people forced to abandon their homes.

Eastern Ontario is also enduring the same woes befalling Quebec, while on the other side of the country, British Columbia is suffering flooding and mudslides.

Here's how we got here – and what's likely to happen next:

Perhaps it's helpful to start with a little context: the 30-year average for April rainfall in Ontario and Quebec is in the neighbourhood of 80 millimetres. This year's accumulation of the wet stuff was almost exactly double that, flirting with a record set in 2005. It's a simple equation: more rain equals more floods, especially in spring.

Indeed, what we're witnessing this weekend is merely the zenith in a series of what began as more localized events last month; several towns in the Outaouais region of Quebec and in low-lying areas around Montreal began evacuating families two weeks ago.

This weekend is a different beast altogether. According to municipal officials in the Montreal region, the water levels of some of the major rivers jumped by as much as 10 centimetres overnight, thanks to pelting rain on Friday and Saturday morning. The offshoot is that flood waters have washed through areas that in some cases had not seen any for decades.

A record-setting snowstorm in late March probably added to the flooding. Combined with a heavier snow pack and mostly cool April temperatures, the spring melt in Eastern Ontario and Western Quebec has taken longer than in a typical year. With rivers and streams already surging with melted snow and overflowing water tables saturating the ground, it doesn't take much rain to cause mayhem. And there's been lots and lots of rain.

Rivers – spoiler alert – flow downstream, and while destructive spring surges are mostly a thing of the past on the St. Lawrence River (because of dredging), such is not the case for the Ottawa River and its tributaries. Some of the heaviest flooding has hit the Gatineau region. Civic officials in Gatineau have reportedly said that the Gatineau River could rise as much as another 25 to 35 centimetres in the coming days. The situation there and in points north and west cause run-on effects that can reverberate all the way to the Gulf of St. Lawrence.

The situation has prompted a response from the Canadian Armed Forces. On Saturday, 400 soldiers based in Valcartier, near Quebec City, were dispatched to aid in stemming the floods. One convoy of military trucks and personnel carriers was sent westward to the Gatineau area, while another trucked to the Mauricie region near Trois-Rivières. Contingents were also sent to Rigaud, just west of Montreal, and to Laval, just north of the city. Others could soon be sent to the Lanaudière region northeast of the city, and to the Laurentians, Quebec Public Security Minister Martin Coiteux said at a news conference in Gatineau. The provincial disaster preparedness department's war room, which is co-ordinating the relief effort, has been relocated from its usual headquarters in Quebec City to Montreal. East of Ottawa, the community of Clarence-Rockland declared a state of emergency on Friday in anticipation of continuing heavy rain.

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The bad news is it's going to get worse. A gigantic, slow-moving low-pressure system is stuck over most of Eastern Ontario, Quebec and western New Brunswick; forecasters indicate rainfall between Saturday and Sunday could range from 70 to 100 millimetres (ie., about a month's worth of rain in 24 hours). Environment Canada has issued rainfall warnings for 46 districts in Ontario, Quebec and New Brunswick, and flood alerts in a dozen more. By Tuesday, the edge of the front is expected to pour rain on Nova Scotia.

This isn't the first time that flooding has severely impacted Canadians in Ontario and Quebec. As awful as the current devastation is, the evacuations, casualties and damage don't really compare to historically destructive natural disasters in terms of scale. In 1954, Hurricane Hazel dumped 200 millimetres of rain on the Toronto region; 81 people died in the flooding. On July 14, 1987, a flash flood in Montreal, caused by heat-wave thunderstorms (a one-day record of 100 millimetres of rain), forced the evacuation of 40,000 homes and knocked out power to 350,000 more. Nine years later, a flood in the Saguenay towns of Chicoutimi and La Baie wiped out entire neighbourhoods, claiming 10 lives and causing $1.5-billion in property damage.

The combination of the spring melt and recent rainfall has also impacted parts of British Columbia, with flooding and mudslides throughout the province. The City of West Kelowna declared a local state of emergency Saturday to address the flooding, and residents whose safety was at risk have been evacuated. Central Okanagan Emergency officials said 90 properties in the Fintry Delta area north of Kelowna were under an evacuation order due to flooding, while neighbouring residents are warned to be prepared to leave their homes on short notice if conditions worsen. DriveBC said sections of the Trans-Canada Highway near Salmon Arm and Glacier National Park had been closed due to mudslides.

With files from The Canadian Press