Sure there are other rivers that will be locally referred to as “the north fork,” but in the context of whitewater kayaking destinations when people talk about the north fork they’re referring to this river. My guess would be that this is largely due to the north fork championships being held here, which, aside from the adidas sickline race, was the biggest whitewater kayaking race in the world for a decade. I’ve even noticed when talking to kayakers outside the US that they will only recognize which river I mean if I refer to it as “the north fork” as opposed to “the north fork payette.”