Aerial view over river Elbe with Augustus Bridge

(2:25 p.m. EDT) -- Unusually hot weather in Europe is affecting river cruises, causing lines to swap ships between different port cities, change itineraries and use buses to take passengers to various sites.

The issues are primarily on Germany's Elbe River, which historically has had low water issues, and parts of the Danube River north of Passau. The Rhine River is also low, with daily fluctuating levels.

River cruise lines handle low water levels in a number of ways, doing their best to keep as close to the itinerary as possible. Ports often close with little advance notice, with the local officials making the determination, leading to last-minute decisions by the line.

It's also hard to predict which season water issues will arise, as it's entirely dependent on the weather. Sometimes high water strikes in the spring, when snow melts in the Alps, but that's not always a given (and it wasn't this year). Low water is generally more common in the fall, but a hot summer can cause issues earlier. Conversely, a particularly stormy summer can cause flooding.

The most common -- and least disruptive -- practice for cruise lines is to keep identical ships as floating hotels in various ports. That way, when passengers swap ships, they have the same cabin category and amenities.

On the Elbe, CroisiEurope's shallow-draft ships Elbe Princess I and Elbe Princess II -- both of which have paddlewheels to contend with low waters -- have been sailing modified schedules. Passengers embark in Prague and sail four nights on one ship and then transfer to a sister ship for four nights toward Berlin

Lines on the Danube have been creative with their solutions. On Cruise Critic's river forum, passengers are talking about their varying experiences.

Cruise Critic member senior lady said that her Avalon Waterways Danube sailing between required busing between Budapest and Vienna, making a stop in Bratislava with free tours and lunch. Instead of going to Passau, the ship went to Linz, Austria, where passengers were again given free tours, either to Salzburg or Cesky Krumlov.

Passengers who had already bought these optional tours were refunded; everyone onboard also received $200 refund. The passengers did still visit Passau by bus, as well as Regensburg where lunch vouches were given. People who were not taking the post tour to Prague were given a hotel night in Nuremberg at the Le Meridien Grand Hotel with meals included.

"I didn't hear a lot of complaining from other passengers," senior lady said. "All we really missed was the day of cruising but I think the $200 pp refund was more than enough compensation."

Cruise Critic editors have been among the passengers experiencing the changes.

On AmaSonata last week, one editor reported the ship could only go as far as Vilshofen instead of Nuremberg. Passengers were taken to Nuremberg by bus so they could still visit the destination. Departures and transfers took place out of Vilshofen, which is a regular departure port for AmaWaterways.

An editor scheduled to sail on Scenic Amber, departing tomorrow from Budapest, has been moved to Scenic Jasper instead. Scenic and Emerald Waterways both have guarantees that protect passengers if ship swaps aren't timely or itinerary delays get out of hand.

And finally, an Amsterdam to Budapest cruise on Crystal Mahler has been changed to a round-trip Amsterdam cruise that takes in the Moselle and Dutch and Belgian waterways, as opposed to the Rhine south of Koblenz and Danube. The line has been working with passengers to switch hotels and flights, both rerouting people back home from Amsterdam or getting them to Budapest for any post-cruise plans. New tours are also being developed. Each passenger is also being offered a $1,000 future credit toward any Crystal River Cruise booked before Aug. 31, 2019.

"While it's never our intention to change our itineraries, the safety of our guests and crew continues to be of the utmost importance," the line said in a statement. "We thank our guests for their understanding"  

For more on how river lines handle weather events, read How Wind and Water Levels Could Ruin Your River Cruise and What to Expect on a River Cruise: Low River Water Levels, High River Water Level and Weather Issues.

--By Chris Gray Faust, Managing Editor