Thinking of taking a river cruise in Europe? Don’t just look at the calendar or the weather report — ask about the water levels on that specific river.

This summer, water levels dropped precipitously on major waterways like the Rhine and the Danube. As a result, there hasn’t been enough water depth for ships to operate normally.

Cruises either travel midway down a river, dock, and bus passengers dozens of miles to another ship where the water is higher, or cancel the trip altogether.

The hope is that water levels will return to near-normal by the fall and winter, when popular Christmas market cruises are scheduled. In the meantime, always request a water-level report from the cruise line before you book or depart.