Boaters are being urged to take care going through Higham Ferrers Lock due to major vandalism at the site.
Steps have been ripped out and thrown in the water, part of the landing stage damaged and signs obliterated.
Railings on the bridge have also been damaged.
The vandalism has all taken place during the past week.
A spokesman for the Environment Agency said: “We advise boaters to take extreme care and avoid walking on damaged areas of the landing stage and steps which we are marking.
“This has been an act of mindless vandalism and we are working to repair the site as soon as possible.”
Anyone with information about the vandalism is asked to contact the police.