A half mile stretch of the Lancaster Canal - including the Lune Aqueduct - has been closed after it sprang a leak.
The waterway is closed between Dolphin Bridge 105 and Halton Road Bridge as engineers try to discover the cause.
The canal has been dammed up and the water has been drained from the 200-year-old Grade I Listed aqueduct.
Fish have been stunned and removed to another part of the canal by contractors.
The Lune Aqueduct recently underwent a £2m restoration, which was completed in 2012.
A spokeswoman for the Canal and Rivers Trust, which looks after the waterway, said: “The trust identified a leak out of the Lune Aqueduct.
“It was noticed last Friday and monitored over the weekend.
“They have closed the canal over a half mile stretch either side of the aqueduct and drained it and are now looking into what has happened.
“They carried out a fish rescue which involved stunning the fish and moving them to the other side of the dam.
“This does not harm the fish.”
Water could be seen pooling up underneath the aqueduct on Monday and Tuesday, blocking off the entry to a new outdoor education facility which links the aqueduct to Caton Road.