Mother Nature (water levels and weather conditions), as well as man-made conditions (maintenance or repair of locks that all ships must pass through), as well as logistical obstacles due to labour strikes or natural elements can create unforeseen disruptions to river navigation.
In these cases, itinerary changes may become necessary and may include limited sailing, docking in alternative ports, hotel overnights and longer than normal motorcoach drives to some destinations.
Many companies employ sister ships on the same itinerary but sailing in opposite directions. In the event of high (or low) water, this tactic allows them to implement a ship swap that is typically seamless for guests – both guests and their luggage can be transferred to their exact, identical stateroom on a sister ship that was purposefully sailing on the other half of the river in preparation for such a disruption.