What is the importance of the Moldau River in the novel, The Amazing Adventures of Kavalier & Clay?

The Moldau River is the central river in the city of Prague, Czechoslovakia. There Joe (Josef) and his brother Thomas, attempt to pull off a demonstration of Joe's ability to escape from being tied up in a sack in the river. Although, Joe escapes, he and his brother nearly drown and are miraculously saved by their teacher, Bernard Kornblum.