Sheffield Creek Trail is a 12.4 mile moderately trafficked out and back trail located near Moran, WY that features a river and is rated as moderate. The trail is primarily used for hiking, walking, nature trips, and birding and is accessible from April until October.
In the National Forest but you must go through Yellowstone to get there. Some bushwhacking is involved but the canyon is amazing and well worth the effort. There are bears, bring your spray.