Floods in British Columbia | When nature takes back its rights

The men wanted to tame the waters, but they managed, once again, to come back. A huge lake stood a hundred years ago on the Sumas Agricultural Plain east of Vancouver, which is now heavily affected by the flooding in British Columbia.

Cows up to their necks in water, submerged stables, a stretch of water as far as the eye can see: the Sumas agricultural plain in Abbotsford, east of Vancouver, looks like a huge lake .

And that’s what it was barely a century ago.

A lake whose area varied enormously from season to season, from year to year, ranging from some 80 km^⁠2, during low water, 134 km away^⁠2, during floods, extending beyond the Canada-United States border.

These huge discrepancies were giving headaches to nearby ranchers, said Terence Day, a professor in the geography, earth and environmental sciences department at Okanagan College in Kelowna.

“The breeders had a hard time juggling the level going up, down, then up and down,” he says. They grazed the cattle and then had to move them when the lake rose, which is why the decision to drain it was made. ”

A diversion channel, dikes and a pumping station were built in the early 1920s, turning this vast body of water into a fertile plain, which today feeds much of British Columbia with its many farms. dairy and market gardening as well as its production of eggs and poultry.

Even the Trans-Canada Highway now crosses what was once Sumas Lake.

However, this work did not prevent sporadic floods from occurring, including a very significant one in November 1990.

Was it a good idea to drain the lake?

Asking the question with a contemporary perspective is irrelevant, believes Professor Day, who doubts that such a project would be realized today.

“The challenge is to manage the legacy of this decision,” he said.

Known risk … which is evolving

Even though the lake has faded into oblivion over time, living on drained land has always been a known risk, says Professor Day.

“This is why the pumping capacities are so high,” he says.

When operating at full speed, the four pumps can move the equivalent of the contents of an Olympic swimming pool per minute, the local newspaper illustrated on Friday. Abbostford News.

But the problem is that the calculations of the probabilities of flood recurrence are based on historical data that does not take into account new factors, such as global warming, notes Professor Day.

“It is clear that the risks [d’inondation] increase with global warming, but there are also more people living [dans la plaine de Sumas] than 100 years ago, ”he says, which at the same time increases the importance of the consequences of a flood.

A study carried out for the municipality of Abbotsford in 2019 also recalled the risk that a major flood would flood the plain and recreate Sumas Lake.

“The risks have always been there, but they get bigger over time,” says Professor Day, believing that we need to intervene by combining long-term and short-term solutions.

In the short term, of course, we have to help people who have lost their homes, their source of income, but in the long term, “we have to recognize that the climate is changing”, and act accordingly, he said, citing the idea. applied elsewhere in the world to expropriate certain properties to direct heavy flooding in these sectors, in order to protect others.

Floodplain

The large variability in the area of ​​ancient Sumas Lake is explained by the fact that it was located in the alluvial plain of the Fraser River, which drains a large watershed.

“An alluvial plain is the area in which a watercourse can migrate,” explains river behavior specialist Maxime Boivin, professor-researcher in geography and hydrogeomorphology at the University of Quebec at Chicoutimi (UQAC).

Since it is a place by definition with very low variations in altitude, an overflow of the river could result in the submersion of vast expanses of land, exactly what is currently happening in the area.

Another well-known lake had at one time a very variable area: Lac Saint-Jean, before it was transformed into a reservoir by the construction of dams.

But the topography of the region being very different from that of the Fraser Valley, the flooding of Lake Saint-Jean was reflected above all in a phenomenal rise in its level, recalls Maxime Boivin.

Thus, for the period from 1913 to 1925, the lowest level of the lake recorded during the low water period had reached 86 meters, against 101 meters during the flood period, a difference of 15 meters.

This situation was moreover “quite unique on a global scale”, underlines Professor Boivin.