Where do Rocky Mountain oysters come from if there isn't any large bodies of water running thru the Rocky mtns?

I love seafood and experiened the most tender, melt in your mouth oysters last night at some restaurant my boyfriend took me to, but I'm a bit confused. They were called Rocky mountain oysters but I didn't know there was some large body of water in the Rocky mountains where oysters could live. None of the maps online seem to indicate any large body of water lays in the Rocky mountains, so where did these delicious, melt in my mouth oysters come from? I'd love to know so I can get some more for cooking at home. Oh, and they were kinda salty so I guess they were not fresh water oysters. Thanks!