Not to rain on your pond but...mine doesn't collect from a watershed. I have a berm completely around mine, no water in or out. Mine is small and lined but not all ponds get there water from runoff. Does mine prevent water from moving downstream? Yes, the amount that would have fallen on that small parcel of land. Also, I have read and been told that it was illegal to have a rain catchment system in AZ. I thought that was the oddest thing so I started calling local county governments throughout the state and was told "Nope, we actually encourage rain catchment." Soooo...check with your local government to verify your information. I know this comes as a shock to many, as Abe Lincoln once said "Not all you read on the internet is true!"