Definitely a stone. There are a couple of species that this could be depending on what river. I see your name and will ASSuME the Poudre or Thompson. Both host some stone but not a substantial hatch like on the Colorado or Gunnison. In actuality the small black winter stone is one of the best hatches up by the hatchery on the Poudre in March. Your exoskeleton is WAY to big for that specie. I would agree with it being a golden but don't overlook the Skwala if indeed there is a hatch. I have never seen a skwala hatch on the Poudre but that doesn't mean much. Way to early for a Goldens.