Main & the Main–Danube Canal

The Main is a different kind of river from the Rhine or the free-flowing Danube reaches: it's canalized behind a long chain of weirs and locks all the way from Frankfurt through Würzburg and Bamberg to the Main-Danube Canal near Nuremberg. That engineering removes low water as a real concern here — the maintained channel keeps enough depth almost regardless of rainfall, which is genuinely unusual on these rivers.

What does interrupt the Main is high water. A rise here can close the river earlier than you might expect for a canalized waterway, because the constraint is bridge clearance and lock operation rather than depth — a section closes to protect the infrastructure, not because ships would run aground. There's also a second, water-level-independent constraint worth knowing: the canal's summit stretch near Hilpoltstein depends on lock availability, so a maintenance closure there can pause transits even when every gauge reads normal.

The Main and the Main-Danube Canal are the connector between the Rhine system and the Danube — and because low water essentially never touches this reach, it's the segment operators lean on to bus-bridge guests around trouble elsewhere. When a cruise update mentions a coach transfer along the Main, this reliable reach is doing the work.