Best chances for success on nearly all is to use terrestrial patterns.
Here’s another great small stream to try. It requires a bit of walking , four or so miles minimum, and a Yellowstone Park fishing license, but the rewards of fishing Yellowstone Park’s Boundary Creek are great scenery, solitude, and the chance for large cutthroat-rainbow hybrids. Its another classic meadow stream in about the size of extreme upper Slough Creek, Duck Creek, and Flat Creek. Access it from Bechler River Ranger station from which well maintained trails will take you to either the lower (Bechler Meadows Trail) or the upper (Boundary Creek Trail) reaches of the creek in Bechler Meadows. As with any small stream, it is overshadowed in reputation by its larger neighbors, in this case Bechler and Fall rivers. But the size that fish range to here is comparable to the two rivers. To fool them requires more stealth and a near perfect presentation, whereas the numerous smaller trout here are aggressive and will take just about any pattern in small and medium sizes. During this time of year terrestrial patterns are by far the best way to encounter large trout here. Especially in the lower reaches, tricos will be numerous. So morning visits should include their life cycle patterns. As midday approaches begin switching to your favorite hopper and ant patterns. Take a camera, insect repellent and a water purifier, and in case of thundershowers, a raincoat.