Most of the classic European cruise rivers have been canalised: a chain of weirs and locks holds the water at dependable levels, so navigation is largely independent of the weather. Long stretches of the Elbe were never engineered that way. The river runs closer to its natural state, and its depth follows rainfall and snowmelt.
That single difference shapes everything about cruising here. Vessels built for the Elbe are shallower in draught and lower in profile than the standard European river ship, because they have to work in less water and pass beneath fixed bridges when the level is high.
It also means an Elbe itinerary carries a genuine risk of change. In a dry period the river can fall too low for a section to be sailed, and the usual response is a coach transfer to rejoin the vessel or to reach the next town. This is a documented characteristic of the river rather than a failing of any operator, and it is the first thing to understand before comparing Elbe voyages.