Canadian authorities in the western city of Medicine Hat on Sunday ordered the evacuation of buildings in low-lying areas after flooding killed three people and forced 100,000 to flee.

Some 10,000 residents have started leaving their homes in Medicine Hat, the sixth largest city in Alberta, with a population of 60,000, after the South Saskatchewan River overflowed its banks, swamping several square miles.

In recent days heavy rains have caused several rivers in western Canada to overflow their banks, and at one point a state of emergency was declared in more than 20 towns, including parts of the provincial capital Calgary.

Some 1,000 troops as well as several military planes have been sent to Alberta to assist rescue teams, the Canadian military said in a statement.

Three bodies were found Saturday in the town of High River. A fourth person has been reported missing.

The mayor of Calgary, Naheed Nenshi, said the flooded downtown area of the provincial capital may not be accessible until the middle of the week, nor will there be electricity until then.

Some of the people forced out of Calgary have begun to return home. News footage showed them pumping water from flooded basements.

CBC television said 75,000 people are still waiting for authorities to allow them to go home.

The unusually heavy rains also appeared to have caused a pipeline leak some 70 kilometers (43 miles) southeast of Fort McMurray, according to the Enbridge oil company.

The company said in a statement that it had shut down the pipeline after an estimated 750 barrels of light synthetic crude spilled out of it.