In churning water with a lot of air, their movement is probably pretty chaotic. Afterwards whether they tend to float, or are weighted, they could get hung up o

In churning water with a lot of air, their movement is probably pretty chaotic. Afterwards whether they tend to float, or are weighted, they could get hung up on any place where the channel isn't smooth. There could be an enormous subterranean cavern that would hold trainfuls of ping-pong balls somewhere along the water's path, too.