Emergency services were called to the Gayton Marina in Blisworth yesterday to deal with a fuel leak.
Lock 17 was closed on the Northampton side and cordons put in place at the marina after reports of a strong smell of fuel and sightings of fuel on the water.
People in vehicles and on foot were advised not to attend the marina, with boats moored prohibited from leaving.
The road over the marina was also closed.
Northamptonshire Police and Northamptonshire Fire and Rescue services worked with the owners of the marina, ABC Boat Hire, to identify the source of the leak.
In June 2015 the Environment Agency was called to Gayton Marina to clean up a massive fuel spill after a pipeline was cut, in what police described as a criminal act.