Do I or can I use an emerger dry fly on a freestone river?
Perhaps you can I suppose. However, absent of any actual hatch or emergence there really is no need to use an emerger dry fly. Trout in freestone rivers have huge search images and are likely to be looking to the surface to actively feed when there is no hatch, hoping a morsel might just happen by. Freestone rivers flowing in the mountains have hatches but they tend to be minimal by comparison to a tailwater river. Many freestone rivers are close to being sterile. It is not uncommon for freestone rivers to have incredible hatches. The Teton, Bitterroot, Big Wood rivers are freestone rivers flowing through huge valleys where its waters become very fertile. This fertile water creates huge hatches. We think of freestone rivers as rivers flowing in the mountains and I’m assuming that is the direction of the question. During a hatch on a freestone river all eyes are on the surface and using an emerger pattern is the most logical thing to do. Another way of looking at it is freestone river trout are always looking up for food on the surface. Your typical emerger pattern will float just under the surface. To be effective dry fly fishing a freestone river during a non-hatch scenario your fly should be on the surface. Failure to float your fly on the surface means that your success rate will suffer. Logically, you would not want to use an emerger pattern on a freestone river because of this. “If it aint a floatin you aint a catchin”