Emergency Measures Minister Steve Ashton says the region near Portage La Prairie will see a "very significant crest" by midnight Tuesday.
He says the water coming from Saskatchewan has crested in Brandon, Man., without incident.
Ashton says the military has been part of an "incredible effort" to protect about 350 homes that could be threatened by flooding.
Some 150 of those homes could be flooded if the province decides to deliberately breach a dike at the Hoop and Holler Bend.
Ashton says that is a last resort but would be preferable to an uncontrolled breach which could cause millions of dollars in damage.
Just over 770 people in Manitoba have been evacuated from their homes, many as a precaution.
"I don't want to underestimate the degree to which this is a huge challenge," Ashton said at a flood briefing Monday. "We are mobilizing every resource available to manage a challenging situation."
This summer flood was caused by torrential rain at the end of June and is expected to topple records set in 2011, which was one of Manitoba's worst floods.
Premier Greg Selinger declared a state of emergency Friday and asked for military assistance to protect rural homes from a surge of water coming from the west. About 500 troops are now on the ground, making sandbags and putting up temporary dikes around vulnerable homes.
Prime Minister Stephen Harper visited Brandon on Sunday and took a helicopter tour of the region's flooded farm fields, washed out roads and dikes holding back a swollen Assiniboine River.