Are Johnson Creek freshwater mussels safe to eat?

I'm not sure which Johnson Creek to which you're referring.  Mussels are filter feeder and tend to bioaccumulate heavy metals and other contaminants.  If you are going to each mussels from Idaho waters, it is a good idea to flush them with freshwater prior to cooking. As a rule of thumb, if I don't want to drink the water out of a stream because of contaminat risk, I'd be hesitant to eat the mussels from that water source.   F08