The New Junction Canal is closed to through navigation, because the flood gates at the Don Aqueduct (the "Don Doors") have been closed.
The River Don, which passes below the aqueduct, is contained within raised banks to prevent flooding of the surrounding flat land. These banks are higher than the canal level and, when the river is in flood, it can overtop the aqueduct. Therefore, when high river levels are expected, the guillotine gates at each end of the aqueduct are lowered, preventing the river from flooding the canal and, through the canal, the surrounding land.
There is very high rainfall today, with more expected in the next day or so. It is likely that the flood gates will remain closed for several days.
Update Monday 9th July:
The flood gates ("Don Doors") have been opened and the New Junction Canal is now fully navigable again.
Don Aqueduct, New Junction Canal