How big a flood does it take to inspire homeowners to plant signs protesting insurance-company policies, to generate arguments over whether the Stampede should proceed, and to produce such outpourings of community spirit that Herald columnist Kevin Brooker is almost euphoric about living in the flood zone?

In the case of the Elbow River, a pretty big one. In the case of the Bow River, a not-so-big one.

Peak flow on the Bow River upstream of the Elbow was about 1,740 cubic metres per second (cms). That makes the Bow flood somewhat bigger than the 1932 flood, which peaked at 1,520 cms. But since the founding of Fort Calgary in 1875, there have been two floods significantly greater than our recent flood, in 1879 and 1897. High-water marks allowed estimates of 2,265 cms for those floods. In addition, there was a 1902 flood the same size as our recent flood, and a handful of other floods in the early 20th century larger than the 2005 flood peak of 790 cms.

What to make of these data? The lack of floods between 1932 and 2005 (and even that was a piker) apparently was not due to construction of the small power-generation dams upstream in the Bow basin, but rather to meteorological luck: large flood-producing storms went north or south, but did not stall over the Bow basin. Now it looks like they’re back.

Meanwhile, statistical processing of the Bow River record establishes that the recent flow of 1,740 cms is not a particularly rare event. The original flood studies in Calgary, by Montreal Engineering, settled on an average recurrence interval of 70 years for a Bow River flow of 2,270 cms. That would make our recent Bow flood smaller than a 50-year flood.

The subsequent 1983 study by Alberta Environment, using different methods, yielded a 100-year flood of 1,970 cms and a 50-year flood of 1,630. According to this scheme, the recent Bow flood would be something intermediate, perhaps a 70ish-year flood.

This was not a huge, “unprecedented” flood except in terms of damage, and that’s not the river’s doing. For huge, consider the “probable maximum flood” in Calgary, generated virtually by Montreal Engineering, which placed the worst meteorologically likely storm over the basin in the way that would generate the most runoff. The result: 6,145 cms — three-and-a-half times bigger than our recent flood and eight times bigger than the 2005 flood.

As for the Elbow River, the 1983 study calculated 100-year floods of 883 cms above Glenmore dam and 758 cms below the dam. The recent flood numbers were a little above 600 above the dam and around 700 (available data are still incomplete) below the dam. The Elbow flood was something less than the 100-year variety.

In the long lull after 1932, the city grew up with an increasingly dim memory of a flood hazard, gradually losing track of the definition of floodplain: a plain that floods. Houses were built in the most hazardous places, such as on point bars on the insides of meander bends (think Bow Crescent) and adjacent to abandoned meander loops (think Roxboro).