May be a long shot but can always check with the rangers office up there. Maybe someobody found it, and turned it in. I mean, what can you do with one fin? lol
Googled parvin lake ranger and this appears to be the number for their area.
Best of luck.
I accidentally left my size 12 Korkers Mossy Oak Camo Wading boots at the Williams Fork South parking lot just off Country Road 330. I loved these boots and am hoping for a miracle. Please contact me if you found them! Thanks.