Race against a drying river: what happens when the water drops on a cruise

What really happens when you're on a luxury river-cruise ship and the river begins to run dry? I didn't imagine I would be racing against time during a languid journey around Europe's Rivers and Castles on board AmaWaterways' lovely 162-passenger AmaCerto.

But when we leave our embarkation port of Nuremberg earlier than expected, and Captain Daniel Langius introduces himself over a welcome cocktail in the Lark Lounge, he muses, "Why are we already sailing?"

Since only he knows the answer, he has to wait for himself to respond. "The level of the Rhine drops down," he says. "And at this moment, every hour counts for us. The office called me this afternoon and said we have to start sailing now, otherwise maybe we cannot reach the Moselle."

I don't have enough knowledge about cruising to take this seriously. I assume it's just something the captain says on ships, like the small print on the itinerary that warns the schedule might change "due to navigational circumstances beyond our control".

Besides, Langius is a funny guy.

"I'm not like most captains," he says. "People don't want to know my name or where I'm from. They just want to know how tall I am." Apparently, he measures two metres, and was born in the Netherlands, "where all the tall people come from".

Europe's Rivers and Castles itinerary is an absorbing seven-night voyage, heading west along the Main-Danube Canal to the Main, the Rhine and the Moselle on a relaxed, comfortable ship.

I love to simply sit near AmaCerto's prow, looking out on the river, and I remain largely oblivious to the growing sense of urgency. By my third night on board I'm utterly relaxed, but I'm about to learn what happens when the small print grows large.

More experienced passengers have been googling our problem. We are slowly approaching the shallowest navigable section of the Rhine, the Middle Rhine Gorge, which is the first stretch to close to shipping when water levels plunge during the "low-water season".

It's not the low-water season, but levels are dangerously low at the chokepoint of Kaub near the city of Koblenz. During an ordinary summer, the navigable water depth at Kaub sits around 208 centimetres, fed by Alpine snowmelt and seasonal rain. Today it's about 62 centimetres, below the critical threshold of 78 centimetres.

If it falls below about 30 centimetres, even a vessel with AmaCerto's low draft cannot safely continue. And it's falling all the same.

Google explains that if we are stuck at one side of Kaub, AmaWaterways will have to execute a "ship swap" with a sister vessel travelling west to east. Our luggage will be unloaded and carried onto buses, and we will have to take an improvised shore excursion while our property is delivered to the new - preferably identical - ship stranded at the other end. We will then be dropped off at the new ship, too.

One passenger from the US grumbles that the main reason he took a cruise was so as not to have to pack and unpack his bags. But Captain Langius was born on a ship on which his own father was captain. He has been sailing since he was three years old.

At 1.30am on the fourth morning of the cruise, he makes the decision to cancel all the next day-and-a-half's shore excursions and race past Kaub to Koblenz. It's disappointing but kind of exciting, and we are promised daylight views of parts of Germany we would otherwise have sailed through overnight. The first significant bonus sight is the city of Frankfurt, which was heavily bombed during the Second World War.

Our cruise manager Veronika Kracikova once lived in Frankfurt, and she tries to talk up the city's more pleasing aspects, such as the Sachsenhausen area, but she cannot hold back from saying what she means. "On the riverbank, you can see residential buildings from the 1950s and some of them are not very nice looking," she says. "Frankfurt needed to rebuild quickly, and aesthetics was definitely not the main criteria."

She points to the "beautiful panorama" of the Skyscraper Quarter of financial institutions, although she admits that the old European Central Bank is "actually quite ugly ... but it has a nice Euro sign at the bottom".