Set out some decoys and see if they come in: that will answer your question. Alternatively, get in there sometime during the day and see if they are on the water. I don't know what your swamp is like, but possibly it could have food that the ducks would not find in the free running water of the river. Also, swamp has negative connotations for me. I might refer to a puddle of water grown up with reeds a slough. Swamp associates in my mind with stagnant water and rotten vegetation and bad smells. I'm not sure that is inherent in the word "swamp" or just my peculiar associations with the word.