This week, Ralph Grizzle is sailing aboard Viking River Cruises’ elegant new Viking Odin, one of six Viking Longships that entered service this year, and part of a larger order that will see ten brand-new Longships enter service in 2013, followed by an additional eight in 2014. But for all their technological innovations and passenger comforts, they’re still subject to that most unpredictable factor in European river cruising: water levels.

Ralph and the guests aboard Viking Odin (and any other ships trying to get to or from the Main-Danube Canal) are currently stopped, their progress blocked by a bridge that rests just two inches too low for their ship to clear. The problem, though, isn’t the bridge: It’s the level of water that has pushed Viking Odin past her clearance level. Water levels and European river cruising sometimes don’t mix.

The root cause? Plenty of recent snow and rain that has melted and run off into the Danube, raising the height of the river.

Along the waterways of Europe, the physical dimensions of river cruise ships are heavily restricted. They must have a shallow draft to accommodate for low river levels, while their height is restricted by the numerous bridges they will encounter along their way. A river cruise ship on the Main or Danube can basically only be three decks in height, with the provision that the uppermost deck, or Sun Deck, can collapse entirely down to deck level to clear the lowest bridges. This is accomplished by removing the deck chairs, collapsing the railings, and lowering the navigation bridge into its recessed space in the hull, leaving enough room for the navigating officer to pop his head out of a “sun-screen” cut into the roof.

In Ralph’s case, they’ve done all of this and more, filling all available ballast tanks to increase Viking Odin’s draft by some six feet. Yet as of his last posting, the ship is still two inches too tall.

To illustrate how quickly conditions can change, I sailed aboard Viking Freya just two weeks ago, on the exact run Ralph is travelling from Budapest to Nuremberg. On my sailing, the talk was low water conditions: Guesta on the lowest accommodations deck could hear rocks and pebbles clinking against the hull, caught up in the turbulent water by the ship passing close overhead. Along the shoreline, the water levels past Melk, Austria were noticeably lower than during my last cruise up the Danube in October. When you see a jetty end five feet above the current water level, you know the levels are low.

Still, the water levels weren’t low enough to impede our passage, and we were able to dock in Nuremberg as scheduled.

So what will happen to Ralph, and what could happen to you if the ship should be unable to meet its itinerary due to water levels being either too high or too low?

A couple of things can be done: In the case of low water levels, guests (and even crew) can be taken off the ship to reduce the weight. If this is successful, the ship sails on to the next available port, and guests can typically re-board the ship.

In the case of high water levels, the options are somewhat more limited. After increasing ballast to the available load limit, there is little that can be done. In the past, several river cruise lines have had success in offloading guests from one ship and bussing them to the next available ship that the company has along the waterway (remember: There are likely people stuck on the other side of the bridge or lock you’re waiting to traverse).

In some cases, the cruise line will spring for hotels and operate the rest of the tour as a land-based bus tour. It’s not ideal, but it allows you to salvage your vacation in the best way possible. Uniworld did this for its guests today on River Duchess, Ralph tells me.

In the most extreme cases, guests will be disembarked and the voyage cancelled.

Chances are good that your river cruise will go off without a hitch. But it is always good to expect the unexpected, and more importantly, to know that these situations can happen, and are entirely out of the control of the cruise line. Because of the unpredictability of water levels, when something happens, the crew and staff find out at the same time as their guests. And because of how quickly water levels can change, companies may be understandably reluctant to pull the trigger on any wide-ranging alterations to future voyages.

What if you’re bound for Viking Odin this coming weekend? Not to worry: Unless you hear differently from the cruise line, plan to arrive at Nuremberg airport as scheduled. If there have been any last-minute changes to the ship’s docking locations, the Viking representative there will notify you about them. But also check back as Ralph is posting a story later today to let us know the outcome of his story.

The best advice we can offer: Go with the flow, even when the flow may not be cooperating.