The "Big River"—that’s what Rio Grande means. So if you’re headed that way, get ready for exactly what the name describes. This is where Texas’s white water rafting soars to the top of its game. Opportunities abound here, but if you want to fasten your metaphorical seat belt for a Class IV rapid, splash through Santa Elena Canyon, with its 1,500-foot high sheers. Rock Slide Rapids, as giddy as it sounds, takes you to another dimension. Sign up with an outfitter to safely explore the area, options from half-day tours to camping excursions.