Ten people were evacuated and assessed by paramedics after thousands of litres of fuel were leaked into Grand Union Canal at Gayton Marina.
Northamptonshire Police have launched an investigatiion and have described it as a “criminal act”.
A spokesman said firefighters were called shortly before 4.15am on Saturday to discover a highly inflammable fuel from a pipeline near the marina was leaking directly into the water.
“Assisted by the Environment Agency, the Canal and River Trust and specialist contractors, an operation was started immediately to try to stop the leak and to remove the fuel from the surface of the water.
“The area affected was contained to about a 600-metre stretch from the Blisworth Arm, running south towards the M1 at Rothershorpe,” he added.
“The vapour created can cause breathing difficulties and as a result 10 people were evacuated from the immediate area and have been individually assessed by triage staff from the East Midlands Ambulance Service,” he said.
The affected people were placed in temporary accommodation by staff from Northamptonshire County Council’s emergency planning service.
Police also searched all 150 boats in the immediate area to check if anyone else was on board.
Chief Supt Paul Fell, who is leading the operation for Northamptonshire Police, said: “I’d like to reassure people this leak will have no detrimental effect on fuel supplies in the local area. However, this was a criminal act which has had very significant environmental consequences in that stretch of the canal.
“We are appealing for any witnesses who may have seen anything suspicious in that area during the early hours of today to contact us as soon as possible on 101 or Crimestoppers anonymously on 0800 111 222.