How weather, geography made southern Alberta a prime target for flooding

This image from Canmore, AB, on Friday, June 21, shows the damage to homes that once stood 40 metres from the edge …The rains have all but stopped in Alberta and the weather warnings have passed, however the flooding threat not only continues for many of the communities already affected, but many more are under flooding and flash flood warnings as the incredible amount of water winds its way through the province's streams and rivers.

As the people displaced by the flooding wait it out until they can return home and assess the damage, and others brace for what's to come, questions turn to how the flooding got so bad and how it happened so quickly.

Flooding has happened before in Alberta, of course. In June of 2005, the province saw the worst flooding they'd experienced up to that point, as three major storms passed over, about a week apart, adding more and more water to streams, rivers and the ground until the 'system' became so saturated that the water had nowhere to go but to spill over land.

[ More Geekquinox: Severe flooding prompts evacuations in S. Alberta ]

This 'omega block' over western North America happened on May 15, 2006.In this case, it wasn't three storms, but just one persistent weather pattern that set up over western Canada, that meteorologists call an 'omega block'. This is because when they trace height lines on a weather map (to show the 'ridges' and 'valleys' of high and low pressure that drive winds and weather), the pattern ends up looking a bit like the greek letter for omega — Ω — and these patterns can sit in place for days or weeks, blocking the normal 'flow' of weather from west to east.

As this omega block developed, a weather system over the B.C. mountains started pulling a 'river' of moisture through the atmosphere from over the Pacific Coast, which ended up flowing over the mountains and then straight into the 'valley' created between the Rockies and the 'atmospheric mountain' created by the block. With this 'atmospheric river' flowing straight into the foothills of the Rockies, and unable to move on due to the block, it created a nearly constant downpour of rain over the mountains. Further adding to the problem was the amount of snow that was still on the mountaintops, which melted as the rains fell and joined in with the rest of the water.

Normally, even with persistent heavy rains, a region can still escape from this kind of flooding if the ground is dry enough to soak up most of the rain. However, having just gone through several days of thunderstorms passing through, the already saturated ground in southern Alberta just couldn't soak up any more water. The massive deluge that resulted from this situation had nowhere else to go but directly into the flow of streams and rivers.

The Cougar Creek gathers the flow from an immense granite 'bowl'Canmore was the first community hit by the flooding, purely due to location. The Bow River flows past the town, alongside the Trans-Canada Highway from Banff towards Calgary, and it was likely seeing a fair increase in its flow from the rain, but the real problem for the town came from the Cougar Creek.

Just to the northeast of the town is what amounts to a massive granite 'bowl' rimmed by several mountains, with another mountain smack-dab in the middle of it. Any rain that falls inside that bowl runs down the sides of those mountains and gathers at the base to form the Cougar Creek, which flows out of the the only convenient exit, straight through Canmore.

From pictures of the creek from before, and from the repairs to the creek that were started earlier this year, it's clear that it has seen some stronger flows of water, but with the incredible amount of rain that this weather pattern delivered, and the destruction it caused in the town, it's also clear that situation this was completely unprecedented.

[ More Geekquinox: Devastating Alberta floods force thousands from their homes ]

As this rain continued to fall in the mountains, running downhill and downstream, it was simply a cascade effect from there, as streams dumped their flow into rivers that were already swollen, adding to the overall speed and momentum of the flow until the rivers overflowed, washing out roads, tearing away at their banks, and flooding any lowlands along their path.

The immense volume of water that has already caused devastation throughout parts of the province so far is still working its way downstream, and communities in southeastern Alberta, mainly along the South Saskatchewan River, are those next in the flooding's path. Flood warnings are still in effect for the Bow River, the Elbow River, the Highwood River, the Red Deer River and Little Red Deer River, the Oldman River downstream from Oldman Dam and Willow Creek, and most recently the South Saskatchewan River, including the City of Medicine Hat.

(Images courtesy: The Canadian Press, Wikimedia Commons, Google)

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