Bison, Fumaroles & Geysers by Gregory "Slobirdr" Smith Via Flickr: Bison grazing as they pass the the confluence of the Little Firehole and Firehole rivers. Fumaroles emit mixtures of steam and gases (why you might experience that rotten egg smell...) and usually have brightly colored rocks associated with the steam vent. They are caused by channels from below passing through water tables...








