CALGARY -- The Transportation Safety Board says unprecedented flood water was to blame for a derailment and partial bridge collapse in Calgary last year.

Investigator George Fowler says the water, at the height of southern Alberta's devastating floods, scoured away the foundation of the Bonnybrook bridge.

Six cars in a Canadian Pacific Railway (TSX:CP) freight that were loaded with petroleum dilutant derailed and hung over the Bow River, but were safely removed.

Fowler says CP's inspection of the bridge exceeded Transport Canada requirements.

He says the bridge, which was built in 1897 and expanded in 1912, simply failed because of the force of the water.

Fowler also says it wouldn't have been possible to anticipate the accident would happen.