By Nia Williams

(Reuters) - Power outages in the Canadian oil capital of Calgary could last for weeks or even months, city authorities said on Sunday, as record breaking flood waters moved downstream to threaten smaller communities in southeastern Alberta.

Even as officials told 65,000 Calgary residents they could start returning to their water-damaged homes, 10,000 people in were evacuated in Medicine Hat, where the South Saskatchewan River is expected to burst its banks.

CF Industries said it was temporarily halting production at its Medicine Hat facility, Canada's largest nitrogen fertilizer complex, as a precautionary measure in view of the flooding.

The floods have closed key transit arteries, including some rail lines and the east-west TransCanada Highway, and displaced more than 100,000 people.

The floods already look significantly worse than those of 2005, which caused C$400 million ($383 million) in damage.

Three people have been confirmed dead.

But officials say it is too early to put a figure on the damage in the Western Canadian province and in Calgary, its largest city, with a population of 1.1 million.

"It will certainly be at least the middle of the week before people will be going back to work. There are some pockets of downtown where normality will not return for weeks," Calgary Mayor Naheed Nenshi told a news conference.

"We have turned a corner. We are still in a state of emergency, but our hearts and thoughts and prayers are now with our colleagues downstream."

Bruce Burrell, director of the Calgary Emergency Management Agency, said it could take days, weeks or perhaps even months to restore all power in the downtown core, where many of Canada's oil companies have their headquarters.

Many of the companies were making plans for employees to work from home.

Heavy rains were blamed for 750 barrels of synthetic oil spilling from a pipeline approximately 70 kilometers (40 miles) south of the oil town of Fort McMurray, in northern Alberta, early on Saturday.

Enbridge Inc, Canada's largest pipeline company, said the cause of the spill was still under investigation but unusually heavy rain may have resulted in ground movements that impacted the pipeline.

The oilfields were not threatened by the floods.

Enbridge also has shut down two other major oil pipelines serving Canada's oil sands region as a precaution. Canada is the biggest supplier of energy to the United States.

There was little, if any, trade in Canadian crude oil on Friday, after the evacuations started in Calgary. Net Energy Inc, one of the two main Calgary crude brokers, said it would be open for business this week with brokers working remotely.

INSURANCE GAPS

Jeff Burke, president and CEO of insurers Western Financial Group, said many Canadian homeowners would not be covered for damages resulting from groundwater flooding.

"There is no flood insurance in Canada, so people do not have coverage for this. Where there's some coverage in some cases is if they have sewer back-up cover," Burke said.

"Just looking at water levels, what we have experienced and the destruction is much more severe now than in 2005."

The company is headquartered in High River, one of the communities worst affected by the flooding, and the site of the three deaths recorded so far.

The floods followed 36 hours of unusually heavy rain that pushed the volume of water in rivers to record levels. Some communities received six months of their normal rainfall in fewer than two days.

Evacuations started on Thursday and utility Enmax switched off power to central Calgary on Friday afternoon to avoid water damage to its downtown facilities.

Troops have been helping evacuate residents and throw up sand or earth barriers to protect low-lying communities.

Soldiers on Sunday helped shore up the east bank of the Bow River in Calgary and ensure the stability of an substation belonging to utility Enmax.

City authorities said 24,000 Enmax customers were still without power.