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A New "Normal" in the Red River Valley

Norm Brandson  |   February 2020

Photo: Government of Manitoba

In the spring of 1950, the Red River over topped its banks onto the tabletop flat floodplain of the Red River valley – it drops a mere 69 metres over its 800-kilometre length – in Minnesota, North Dakota, and Manitoba. It was the worst flood in the basin since 1861. It submerged much of the city of Winnipeg and triggered the greatest mass evacuation in Canadian history. At the time this was considered a one-in-a-hundred-year flood event.

In response to this catastrophe, the provincial and federal governments commissioned the Greater Winnipeg Floodway, a large open channel diverting water around the city, which was completed in 1968. Designed to protect against a flood 50% greater than 1950, it did its job with absolutely no margin for error when the “flood of the century” struck in 1997. An 1852 flood was about the same magnitude and only in 1826 was there a recorded flood that we can say for certain exceeded that of ’97. Eight of the twelve worst Red River floods have occurred since 1974, two greater than and two virtually equal to the 1950 event. The probability curves have been revised and the ’97 flood is now deemed the hundred-year flood. Floodway capacity has been increased and Winnipeg is considered to be protected from a one-in-750-year Red River flood, although it is worth noting that the Assiniboine River enters Winnipeg downstream of the floodway inlet. In 2011 the Assiniboine basin experienced a one-in-300-year inundation.

The design of the floodway was predicated on protecting the city from spring flooding only. However, the operating rules drawn had to be revised on the fly during the summer of 2002, when a series of storms in the valley required floodway operation to prevent flooding in Winnipeg. Similar circumstances forced summer operation again in 2004, 2005, and 2010.

Fall operation was first considered in 2003 when high flows during the fall freeze-up prevented the formation of solid ice cover. Instead, frazil ice – essentially slush – formed, slowing the flow and backing up water into the city. With only a small margin for error, river flow diminished and use of the floodway proved unnecessary.

Fast-forward to October 2019. After September rains three times “normal” and more rain and a snowstorm early the following month, flow in the Red River was at its highest-ever recorded level for that time of year. Water was diverted around Winnipeg through the floodway to prevent flooding. Now, it seems, only in the dead of winter would the floodway not be used.

Something’s happening here. The Prairie Climate Centre at the University of Winnipeg says that this dramatic increase in the frequency and severity of flood events throughout the open water season is consistent with the destabilizing effect of climate change. The “normal” that we used to design our flood protection works, our agricultural drainage systems, and other critical infrastructure is no longer a reliable guide.