Part of a towpath on the Kennet and Avon Canal has collapsed after a lock gate was left open next to the Caen Hill locks.
The weight of water released washed away part of the embankment.
The Canal and River Trust say the damage caused could cost hundreds of thousands of pounds and it does not know if the damage was caused by "carelessness or deliberate vandalism".
The canal was reopened this lunchtime. Local boat owners were the first to use the repaired stretch of canal.
The Grand Western Canal and towpath will officially re-open today more than a year after its banks were breached during a severe winter storm.
A group of residents have been told the risk to their homes has been reduced after firefighters managed to pump water away from their properties in Halberton. It follows the collapse of part of the Great Western Canal.
Jon Miller from Devon & Somerset Fire & Rescue explains what they plan to do next:
Fire crews in Devon are working flat out to pump-off water after part of the Grand Western Canal collapsed.
Hundreds of thousands of gallons of water poured into fields when the embankment collapsed on Wednesday. Emergency dams are keeping water levels high further up the canal.
Bob Cruwys reports:
Little if any of the water which flooded out of the Grand Western Canal into surrounding fields when an embankment collapsed seems to have drained away this morning.
Several gulls have taken up residence, perhaps finding easy pickings in this new lake with all the fish which were washed out of the canal in the flood.
Work to shore up emergency dams is continuing today to try to stop any more water leaking out of this canal. It will be some time before any attempt to rebuild the embankment and repair this stretch of canal.
It is likely to take years but officials hope to have it reopened in time for the 200th anniversary of the canal in 2014.