are there some sort of riverwalk police or riverwalk authority that might have a lost and found? Could it be in a bush or maybe taken a swim? if its the latter, they empty the RW once a year and fish out all the drunks and cameras. maybe they will find your phone then.

Last part is me just being a smartass, but I wonder if there is a lost and found. There are some "freakish folks" that occassionally return cell phones. I have 3 to my credit so far. Seattle, Colorado, and DC.

Good luck!