The reluctant move to breach a large dike along the Assiniboine River near Portage la Prairie was made after several days of postponements, the Winnipeg Free Press reported.
Emergency Measures Minister Steve Ashton told reporters surveyors had determined the controlled flow would flood 70 square miles instead of the original estimate of nearly 90 miles.
Officials said the move would affect about 150 homes, but if nothing was done, at least 850 homes would have been flooded.
The seasonal spring thaw and rain flooding in southern Manitoba has been called the worst in about 300 years and property and agricultural losses are projected to surpass $1 billion, provincial leaders said last week.
Meanwhile in Winnipeg, the confluence of the Red and Assiniboine Rivers, city police said one man drowned Saturday and a second had to be rescued from the Assiniboine when their canoe capsized in fast-flowing water.
Neither was wearing a flotation vest, police said.