Heavy rain is not the only reason the Assiniboine River is flooding in the height of summer and threatening a swath of communities across southern Manitoba this week, a new report suggests.

The elimination of vast numbers of small ponds and wetlands across the Canadian prairie has removed a crucial buffer that can temporarily store water on the landscape during periods of excessive precipitation.

So profound is the effect, the report’s authors find, that had the same amount of rain fallen in the 1950s as has this past year, the swollen Assiniboine might have reached only half of its current peak level.

“It’s one of the strongest land-use impacts [on water movement] I’ve seen anywhere in the world,” said John Pomeroy, a hydrologist at the University of Saskatchewan, lead author of the report.

Some of the lost “wetlands” would hardly qualify as wet. They are no more than slight dips in the flat prairie terrain where water pools up after abundant rainfall. But farmers have plenty of incentive to create drainage systems that efficiently clear these temporary reservoirs and keep water flowing downstream, while increasing the fraction of land that can be cultivated. Over time, this has led to a steady growth in drainage networks across the prairie.

To better understand the impact of these networks, Dr. Pomeroy and his colleagues made a detailed study of the Smith Creek basin near the Saskatchewan-Manitoba border.

Using airborne lasers, they created a precise elevation map to show where water should be collecting in the basin and how it drains.

The team also looked back more than half a century to a time when 24 per cent of the region was covered by wetlands, as compared with 11 per cent at the start of 2009, and developed a computer model to compare water flow then and now.

The analysis shows peak water flow during the last major flooding event, in 2011, would have been 32 per cent lower had a similar flood occurred in 1958. The researchers also found the same flood would be 78 per cent higher with no wetlands at all, a likely scenario if drainage networks continue to proliferate unchecked.

As climate change brings more extreme weather to the region, the report suggests that flooding along the Assiniboine as well as Lake Manitoba, where flood waters are diverted, will be more frequent.

And flood damage is only part of the problem, experts say, as the fast-draining water carries fertilizer downriver toward Manitoba’s lakes. This sets the stage for massive algal blooms that can suck up oxygen and kill off aquatic habitats.

“My great fear is that we will see the world’s largest Jell-O bowl,” in Lake Manitoba, said Scott Forbes, an ecologist with the University of Winnipeg.

Dr. Pomeroy said the issue of drainage regulation should be a priority for the Prairie Province Water Board, which oversees water use across provincial boundaries.

Manitoba has recently moved toward improving its own regulations and has requested the board to “follow up on the issue of Saskatchewan drainage impacts to Manitoba,” said Steve Topping, who represents Manitoba on the board.

The Globe and Mail
Published Wednesday, Jul. 09 2014, 8:36 PM EDT
Last updated Wednesday, Jul. 09 2014, 8:38 PM EDT