Viking River Cruises and TUI have both warned customers of potential changes to itineraries, with recent reports of two ships running aground. The Viking Einar ran aground on the Rhine River in Cologne, Germany, on June 29 with around 190 passengers on board. River cruise holidays in Europe face disruptions and itinerary changesViking River Cruises, in a statement on its website, said: "Sections of the Danube and Rhine Rivers are currently experiencing exceptionally low water levels. "As a result, select Viking river itineraries may be modified. Do you have a river cruise holiday in Europe booked?