Believe it or not, it is possible to have an Idaho-based clam bake. Some of Idaho's rivers are home to edible freshwater clams and mussels.
Clams are the easiest to spot. Look for the same signs that you would look for on a saltwater beach: holes in the sand. Then dig. The problem with local clams is that they are not very big. Most are the size of a fingernail or smaller so finding enough of them to make a meal takes time.
And Idaho clams are not exactly the Manila Bay clams we are accustomed to. Our clams taste like mud. Being bi-valves, they are basically big filters, and what they filter greatly accents their flavor. So, if you pull a pile of clams out of a mud bog ... you can see where I'm going with this. But if you get them out of a nice, clean, sandy bar, they can taste great. Also, they don't have the same saltiness that ocean clams have, so you will need to salt them more than you would ocean clams.
Idaho rivers also contain freshwater mussels. I have found piles of them while snorkeling in the Boise River right in town. They are bigger than the locally found clams and you end up getting a lot more bang for your buck with them, but the clams are easier to find.