A wooden footbridge has collapsed under the weight of a 25-tonne lorry.
The tanker, which was filled with water, had been driven to Willen Lake in Milton Keynes to support a Race for Life event on Saturday, the council said.
The lorry driver and a passenger were unhurt, the fire service confirmed.
The area has been secured by Buckinghamshire Fire and Rescue Service and the tanker has been removed from the scene.
A safety inspection of the bridge will take place on Monday.
The bridge collapsed at 07:30 BST on Saturday, with the lorry eventually removed from the scene by a crane at about 13:00.
Milton Keynes Council tweeted: "A 25-tonne tanker has driven over the footbridge on the Willen Lake South redway this morning and damaged it.
"The redway and bridge must be avoided pending inspections and repairs."
It is not known why the tanker was using the wooden bridge for access to the site.