© Reuters/Fred GreensladeMembers of the Princess Patricia's Canadian Light Infantry (PPCLI) work at shoring up a dike along the Assiniboine River near Poplar Point, Manitoba, May 13, 2011.
Opening the dike will, over days, flood at least 225 square kilometres (55,600 acres) of land that includes 150 homes while taking the pressure off strained dikes.
After the deliberate dike breach, Manitoba Premier Greg Selinger said the gradual, controlled flood was going well and that he knew of no homes in its path that had been damaged.
Water was spreading rapidly across fields, however, swamping land when farmers are usually planting crops of wheat, canola and vegetables.
"This was a necessity because a catastrophic overflow would have taken all the people in this area, and around it they would have had up to five times more damage if the river opened up," Selinger told reporters near the breach site. "...The dikes are very stressed with the amount of water going through, which is why we had to do this opening here."
The controlled flood looked to continue for as long as a week with flows speeding up because the river was still rising, Selinger said.
Warm, clear weather was helping volunteers and about 1,500 soldiers reinforce dikes.
Levels on the Assiniboine, which flows eastward out of Saskatchewan into Manitoba, are the highest on record after a winter of heavy snowfall on saturated land.
Along with fortifying dikes, Manitoba officials hastily expanded this week an engineered channel that diverts water off the Assiniboine into Lake Manitoba to prevent widespread flooding caused by heavy winter snowfall and spring rains.
U.S. authorities fighting a swollen Mississippi River opened a key spillway on Saturday, a move that could flood homes and crops but spare Louisiana's two largest cities.
Across Manitoba, 3,300 people had left their homes as of Friday because of the threat of flooding, including 1,400 in Brandon, the province's second-largest city. About 100 homes are flooded, mostly on Indian reserves.
Residents in the area that will be flooded east of Portage la Prairie, Manitoba, shored up flood defenses this week as provincial officials repeatedly delayed the dike breach to give them more time.
It wasn't enough, said Doug Connery, whose family runs a large vegetable farm in the area.
"It all depends now on how fast this (water) is going to move," he said. "There's stuff out there that wasn't protected last night."
Manitoba officials have aimed to pump about one-third more water than design capacity through the Portage Diversion, out of the Assiniboine and into Lake Manitoba. While that has taken some strain off dikes, it has added to flooding in a key cattle-producing region.
Farmers have moved thousands of cattle to higher ground and worry they might not be able to feed them this year.
(Editing by Laura MacInnis and Paul Simao)
The floods would create new fresh topsoil for the lands by the river, but now all the pesticides etc make that more of a 'bad' thing.