Southwest of Saint Regis, the St Joe River is a scenic freestone river with huge westslope cutthroat trout. This catch-and-release river starts at St Joe Lake and runs 50 miles through forest. Easy access is provided on the lower 33 miles by a parallel road (St Joe River Road aka FS218). Upstream of Spruce Tree Campground, the only access is via a 17 mile long hiking trail that parallels the river. The road can get pretty crowded, so think about hiking in.
Overall, St Joe River is the 4th most popular fly fishing river of all 17 fly fishing spots in Idaho.