I like the idea of some small unobtrusive signs (like your fictional depiction) on / near bridges / roads crossing 'major' rivers or streams. They would not have to be on / near every stream crossing, but an occasional 'point of reference' would be helpful for Park visitors (and some fishermen not very familiar with the Park). I am not a big fan of anecdotal support for 'new ideas' like the signs, but it seems like at least once every time I've been near a pull-off or along a road in my last few trips to the Park a visitor has unexpectedly hollered out the question 'what river is this?' while I'm in the midst of a cast, mend or while trying to land a trout (once). A few signs won't stop those types of interruptions, but, again, would be helpful for visitors not familiar with the Park.