The Canal and River Trust says it may have to drain three million litres of water from the Kennet and Avon after a narrowboat capsized.
The way the boat has fallen on to its side means it is difficult to crane out of Lock Eleven.
So the pound beneath the lock must be drained - which is the longest pound in the Bath flight of locks.
And draining the canal means the wildlife needs to be rescued first too. A specialist will be brought in to stun fish and save them before the water is pumped out.