Hi Bill & Pat ,
it is very difficult find out with the present water levels if it is alright to have the trip in December,
The waterlevel can changes in days, this all depence on the rain.
The captains are very experienced and it sometimes a couple of inches can tell you to continue the trip yes or no.
The Cooperate Captain on the Danube is an expert as he sailed 10 years on the Danube in all kind of circumstance.
He advised the Captains.
Some Captains sail with acouple of Inches water under the ship ( low water), or have couple of Inches free by the bridge ( high water).
But you never can tell, whe all are depended on the weather.
But in the normal saison you have never problems.