Manitoba declared a state of emergency across several municipalities Monday afternoon as the flood threat rose in the southern areas of the province.

In a major development Monday night, Emergency Measures Minister Steve Ashton said Manitoba will be forced to divert water as soon as Wednesday, a diversion that will threaten an area of 225 square kilometres west and southwest of Winnipeg, placing 150 rural properties in the LaSalle River watershed at risk of flooding.

This "controlled release" of water will be done to prevent uncontrolled flooding of 850 properties in a wider 500-square-kilometre area, said Ashton.

Winnipeg is not considered at risk, officials said, as water ends up being diverted from the Assiniboine River into the LaSalle River watershed.

However, residents in the rural municipalities of Portage la Prairie, St. Francois Xavier and Headingley "should move key assets out of their homes or to higher levels in their homes," the province said Monday night. In the Rural Municipality of Cartier, evacuation notices went out to 1,500 residents Monday evening.

Ashton said the Assiniboine River is carrying an unprecedented volume of water and that dikes between Portage la Prairie and Headingley could be breached unless "controlled releases" of water along the Assiniboine between Portage la Prairie and Headingley are undertaken. Efforts also were underway to divert more water north into Lake Manitoba.

Frantic efforts were underway Monday night to top up existing dikes along the Assiniboine River, and to fortify previously unprotected properties along the LaSalle River system. The province plans to move mobile flood protection equipment into threatened areas as quickly as possible, Ashton said.

Canadian Forces at work

Earlier Monday Canadian Forces members were dispatched to Manitoba's second-largest city, Brandon, and were topping dikes on the Assiniboine River between Winnipeg and Portage la Prairie, 80 kilometres west of the capital.

The province called on engineers to find ways to increase capacity of a key flood-control structure, the Portage Diversion. That structure diverts some flow from the Assiniboine River and diverts it to Lake Manitoba. But water flows on the Assiniboine River are now well beyond diversion capacity and dikes from Portage la Prairie to Headingley need relief, officials said.

The provincial emergency declaration will allow the military and other flood fighters to access private property as needed, and for several municipalities including Portage la Prairie, Woodlands, Rosser, St. Francois Xavier, Headingley, Cartier, Macdonald and Grey to activate their emergency plans.

CBC News has learned that there may be another faulty flood gauge that has underreported the amount of water coming into the Assiniboine River from the west.

Premier Greg Selinger said despite the spending of $20 million on flood protection along the river, if the Assiniboine continues to rise "there's a very good chance it will overwhelm dikes [between Portage and Headingley]. The objective is to minimize damage to families, people and property."

The last time the province enacted a similar state of emergency was during the massive flood of 1997, when 30,000 people were evacuated from their homes in the Red River Valley south of Winnipeg.

Earlier Monday, a precautionary evacuation for about 350 homes and businesses began in Brandon. A second stage of the evacuation was postponed as officials there expressed some relief Monday afternoon that water levels had not risen too much and the dikes were holding.

Officials described the situation behind dikes in that city of 40,000 as "guarded." Additional rain was expected in the area Tuesday.