A forklift truck was used to smash up a bridge and dump a work cabin in a canal, causing about £70,000 of damage, a waterways body has said.
The Canal and River Trust said the "malicious" incident at Coxhead Bridge near Ormskirk happened between Tuesday afternoon and Wednesday morning.
Lancashire Police said the truck was later found abandoned nearby.
A trust spokeswoman said it had caused "significant damage" to the Leeds and Liverpool Canal swing bridge.
The body has been carrying out improvement work to the structure, which is also known as Rimmer's Bridge, on Green's Lane, including installing a new landing stage for boaters.
The spokeswoman said "significant damage" had been done to the "mechanics box which operates the swing bridge", meaning it would have to opened and closed by hand twice a week until it could be fixed.
Contractors would supply that service at a weekly cost of £1,000, she added.
Lancashire Police have appealed for information about what happened.