Big Bend National Park visitors. On one side, Mexico. On the other, the United States.
The hike is rated moderate, because of steep climbs in the trail and steps along the way. The distance is 1.7 miles roundtrip and is one of the "must do" experiences for hikers visiting the park.
This National Parks Service page talks about Santa Elena as a river trip. Visit Big Bend has this about the Canyon, and their list of outfitters for river trips is here.