River sailings are occasionally replaced in part by coach travel or a transfer to a second vessel. This is a planned contingency triggered by water depth, and it follows a fixed logic.
Draft is the governing number
A loaded river ship needs a certain depth of water beneath it, and that requirement is close to the shallowest depths the river reaches in a dry season.
Authorities publish gauge readings at fixed points along the waterway. Operators watch those readings the way an airline watches weather, because they determine whether a stretch is passable.
When a gauge falls below the working threshold, the ship cannot proceed regardless of how calm conditions are. Grounding risk is absolute rather than a matter of judgement.
The first response is to lighten the ship
Draft depends on load, so operators reduce weight before abandoning a leg. Water tanks are carried at minimum levels, and swimming pools where fitted are drained.
This buys a few centimetres, which can be the difference between passing a shallow section and stopping. It is invisible to passengers apart from small facility closures.
Beyond that point the options narrow quickly, because passenger numbers and fuel cannot be reduced and the hull cannot be made shallower.
Ship swaps preserve the itinerary
Large operators run several vessels on the same river, so a common solution is to move passengers by coach past the impassable section onto a sister ship waiting downstream.
The two groups effectively trade vessels. Cabins are cleaned in between, luggage is transferred by the crew, and the published itinerary continues from the new position.
This is why swaps are more likely with the biggest fleets. A single-ship operator has no partner vessel to exchange with and must fall back on hotels and coaches.
What the contract usually says
Booking conditions generally reserve the right to alter the route, substitute the vessel or replace sailing with land transport when water levels or ice make navigation unsafe.
Because the change is contractually anticipated, it is normally not treated as a cancellation, and refunds are usually partial and discretionary rather than automatic.
Reading that clause before booking is more informative than any assurance about the river, since it states exactly what the operator has promised to deliver.
Seasonal patterns are predictable in shape
Low water is most likely in late summer and early autumn on rain-fed rivers, and in high summer where a river depends on snowmelt that has already passed through.
High water causes the opposite problem: bridges become impassable and sailings stop for clearance rather than depth, which affects the same routes at different times of year.
Neither risk can be eliminated by booking a particular operator, but choosing a season and a fleet size with these mechanisms in mind changes the odds meaningfully.