Part of the Grand Canal at Suir Road in Dublin has been drained completely dry.
More than 1,000 fish have been relocated after the collapse of a storm drain underneath the canal.
All boat and angling traffic has been suspended, following the breach which was discovered on Sunday.
It is affecting a 750 metre stretch of between Grand Canal Dock and the lock at Suir Road in Inchicore.
A temporary dam is being installed by Irish Water today, and partial re-watering will then begin.
It is unlikely to re-open to boats for several weeks.