Inside Yellowstone National Park, take the Natural Bridge Trail for a 1.5-mile walk to view a 51-foot natural bridge. Access to the top of the bridge is prohibited, however the views from below are well worth the trip. The trail usually opens in early summer.
In Montana, visit the Makoshika Natural Bridge near Glendive. This impressive 50-foot sandstone span can be reached after a short walk on the Cap Rock Nature Trail.
In southern Idaho, Wind Tunnel Arch is a natural bridge that spans 33 feet. It is located in Jackknife Canyon, in the Lemhi Mountains north of Howe. From here take an easy side trip to Craters of the Moon National Monument, which also has several natural arches of note.