A water main break flooded Lake Shore Boulevard, closing a part of the main thoroughfare to morning rush-hour traffic in Toronto Monday and causing damage that could take days to repair.

The boulevard was closed in both directions between Rees to Lower Simcoe streets for most of the day.

The westbound lanes reopened around 2 p.m. ET, but police said it could be days before traffic is flowing on the eastbound lanes because of the extensive road damage.

The cause of the water main break is believed to be an 80-year-old cast iron pipe that gave out from old age, said Lou DiGeronimo, the City of Toronto's general manager of water.

He said the water main broke during the night, causing damage to the roadway and spreading debris across east and westbound sections.

DiGeronimo said the water main may be the same one that's broken three times in the past year, although that won't be confirmed until workers locate the break.

The city is struggling to maintain and replace an aging infrastructure.

DiGeronimo said about 15 to 20 per cent of the city's water pipes are 80 to 100 years old and more than 50 per cent are more than 50 years old.

About 1,300 water mains broke last year, he said.

City council has increased the amount of money set aside to fix aging sections of water pipes over the next few years, said DiGeronimo, but it could take a decade to change all the pipes needing replacement.

The Lake Shore Boulevard section where the break occurred Monday was scheduled to be replaced this year.