[Editor's note: Ben Gadd is a resident of a Canmore subdivision that Cougar Creek passes on its way to Calgary's Bow River. On June 19, his neighbourhood was flooded. The flood reached its height the following day. Gadd, who has an Earth science degree, documented it all for his friends and family, and we republish it here. All photos by Ben Gadd.]

Hello, all. Now that the Great Cougar Creek Flash Flood of 2013 is over -- we hope -- we can report that we are okay. Our house was threatened, so we spent a night away. But our digs were not damaged, unlike those of many other people in southern Alberta. This could very well go down as the province's worst-ever flood.

During the Cougar Creek event a big culvert just upstream from us was nearly overtopped. If it had collapsed, a very nasty debris flow would have come right through our neighbourhood. Close call!

Some friends and neighbours didn't fare as well. No one's house went down the creek, but some homes have been badly damaged.

This is because Cougar Creek's artificial channel failed to do its job. It was 40 metres wide and up to seven metres deep before the flood, pretty impressive. But now it is gone, filled with gravel and boulders. The stream course has widened to 90-110 m.

People living on the both sides of the creek watched the banks caving steadily toward them as the water advanced on their backyards. For those on the southeast side of the creek (river left), the water was soon rushing right against their concrete basement walls and undermining their foundations.

My rooftop weather station recorded 147.2 millimetres of rain from this storm, which became serious on June 19 and lasted until June 21. On the June 20 we got the most rain: 91.2 mm at a maximum rate of 17.4 mm per hour. Compare this with the mean annual precipitation around here, which is about 450 mm. We received a lot of that in just three days.

The flood began Wednesday evening, June 19. In the picture below, we are looking up Cougar Creek from the concrete culvert under the TransCanada Highway, at the lower end of the subdivision. Our house is in the neighbourhood to the left.

The water is rushing through the culvert. A few minutes after I took the photo below, the splashing was reaching to the top.

Below is another culvert, the big one that carries Cougar Creek under a busy street named Elk Run, just upstream from our neighbourhood. (The one that might have collapsed.) It's Thursday morning, June 20. The rain has been heavy all night. I'm out shooting video as Cia holds an umbrella over the camera.

In the video it's easy to hear the roar of the flood and the bumping of big boulders rolling along the bottom. Wish I could have captured the strange, sulphurous odor coming from the water as the grinding, shattering limestone released gases lying dormant in the stone for 350 million years.

Just down the street from our place, a pedestrian bridge has been built across the creek. Before the flood the top of the arch was about 10 m above the bed of the artificial channel. The channel was usually dry. But now the channel has filled with gravel and boulders.

One might think that the channel would deepen as the floodwater rushed through it. However, this is an alluvial fan situation. The slope of the fan is steep, as stream courses go, but it is not as steep as the stream bed in the mountains upstream from the fan. There, huge quantities of sediment are being picked up and carried away down the creek. When the creek reaches the fan, the water slows down. It cannot carry as heavy a load. It must start depositing it. Thus the channel has filled. The flood is spilling sideward…

…and eating away at the banks. Between Wednesday evening and Thursday morning, the width of the engineered stream course doubled to about 80 m, the width in this picture, looking across the creek from near our house. The flow is from left to right.

We evacuated shortly after I took this photo. All Gadds, including the grandkids, spent the night on the opposite side of the valley, safe and dry, far from Cougar Creek and well above the Bow River, which was also rising fast. (Downtown Canmore experienced some flooding, but the dykes along the Bow River held.)

In the end, the stream course was widened another 30 m, more at the lower end of the fan. The picture above was taken on Saturday, June 22, just after the flood abated. It shows the pedestrian bridge still in place but the riverbed much closer to its deck. A large digger, one of several at work that day, is building a temporary dyke to try to keep the water away from the houses shown. They are located on the southeast side of the creek, river left as we look in the direction of flow.

Those houses took a beating by the water.

The backyards are gone. On many of the houses, the foundations and concrete basements have been exposed. Some have been breached and even washed away. Sadly, homeowners' insurance typically does not cover flood-caused damage.

On the other side of the creek, in our neighbourhood, the bank erosion did not go quite as far. The two brown houses have basement walls exposed -- note the shed upside down in the creek -- but these were the only two that the water actually touched, except for some gentler flooding down by the highway, where the water backed up.

Here is the view from the yard of the grey house, the leftmost one clearly seen in the previous photo.

It's the home of some friends, just five doors away down our street. You can see those same brown houses and shed.

The lawn slopes down toward the house. If the water had cut only two or three metres farther into the bank, it would have broken through and gone rushing into their place.

Here is the TransCanada Highway after the flood. It's not merely buried beneath gravel; it will have to be rebuilt here. So will the Canadian Pacific Railway's main line a half-kilometre away downstream, and Highway 1A beside it.

On Sunday the public was allowed across the Elk Run culvert. From there, I took a panorama of the damage upstream. In the distance you can see the mountainsides between which Cougar Creek emerges onto its alluvial fan. Some houses in the right side of the photo have been damaged, along with the community hockey rink. A street on the left side has been cut through. This is Benchlands Trail, which provided the only road access not only to the houses seen here, but also to the rest of a sizable subdivision, now isolated. Ironically, Benchlands Trail was constructed right along the path that Cougar Creek took toward the Bow River before all this was built.

Lessons learned from this event:

1. Like a river floodplain, an alluvial fan is a lousy place upon which to build houses. Cia and I live in such a location. This is ironic, considering my background in geology.

2. The artificial channel of Cougar Creek has proved vulnerable twice before, when flash floods smaller than this one damaged it. But it was repaired rather than upgraded enough to function as a true floodway. Bad mistake. This time it failed completely.

3. If the Cougar Creek subdivision is to be better protected from flash floods -- I doubt that it can ever be completely protected -- the replacement artificial channel will have to be designed differently. I'm not an engineer, but a friend who specializes in this kind of thing visited us last summer, after Mother Nature's most recent practice flood had gouged the sides of the channel significantly. He said that we need a proper floodway here, wider and better-armoured, with attention to the problem of alluvial fan deposition. Boy, was he right!

4. Given the ongoing rainy weather this summer, another high-water event is likely. And a hastily thrown up emergency dyke along Cougar Creek isn't going to withstand much.

Yours for better civil engineering, and in memory of Will Durant, to whom is attributed the famous line, "Civilization exists by geological consent, subject to change without notice."

Ben Gadd, 67, is one of Canada's better-known naturalists and Rockies writers. Author of the ground-breaking Handbook of the Canadian Rockies, Ben has written nine other books and contributed to several more. His novel Raven's End has become a prize-winning Canadian bestseller.