Today I'm cheating just a little. The story I chose features a lake, not a river.
Hans Dudeldee from The Turnip Princess collection of Franz Xaver Von Schönweth's stories is a cracker. I almost wish I had read that one before the Red Silk Ribbon (that I illustrated for Inktober). Ah well.
This tale however is not the 'Fisherman and his wife' variant that the Grimm's collected of the same name, while there are some similarities this Hans Dudeldee encounters a frog in a castle having a perfumed bath!
Again, I can't see a version of this on the web yet.
The image shows Hans being transported to the island castle by a magical fish with golden scales and that is where he meets the frog having a bath.
Again, let me know if you want to hear the tale in full and I will do my best to summarise it.