What happens to Ludwig in The Silver Sword?

In the story, Ludwig originally belonged to the Bavarian farmer, Kurt Wolff, and his wife, Frau Wolff. Jan takes a liking to Ludwig during the childrens' stay at the farm. By all indications, the affection is returned. The faithful dog even hides in one of the canoes the children use for their escape.

As the children make their way down the river, they have to battle their way through the rapids. All of them manage to survive the danger, but they soon find themselves without the use of their canoes. The children and Ludwig walk to Falkenburg, cross the Danube, and eventually get a ride to Switzerland. Their destination is Lake Constance. Eventually, after stopping to make camp, the children discover that Jan and Ludwig are not in their company. Presumably, Jan has gone back to the Bavarian farm to look for the silver sword that he now says is missing.

Edek, Ruth, and Bronia decide to continue the journey without Jan. Eventually, they meet up with him again when they discover him tied up in the back of a truck; the truck is driven by an American soldier, Joe Wolski, who decides not to take Jan north with him.

The children soon find themselves at a crossroads. In order to get to Appenzell where their parents are, Edek and his siblings have to cross Lake Constance. They try to wait patiently for the Swiss boat that is to take them across but are so excited that they are unaware of the approaching rain. Meanwhile, Edek, sick and disoriented, takes shelter in a small boat. Soon, it rains heavily, and Edek's boat floats away. Ruth and Bronia struggle to save Edek. Meanwhile, Ludwig goes missing in the heavy rainstorm, and Jan's first concern is to find and save Ludwig.

Ruth gets frustrated with Jan when she realizes that Edek is not Jan's first priority. She yells at Jan to go after Ludwig, while she and Bronia try to save Edek. Her words jolt Jan back to reality: he suddenly realizes that he has to choose whether he will help save Edek or Ludwig. The text tells us that he chose to save Edek:

The two girls jumped into the boat. Jan was staring at the dog. He wanted to run after the animal, but Ruth's words had hurt him. He looked back at her brave face, and he knew what he had to do. He had lost Ludwig, but he could not lose Ruth.

Since there are no further mentions of Ludwig after the storm incident, we can deduce that Ludwig has probably drowned.