In West Texas, the Rio Grande forms the U.S.-Mexico border, a wild land of drug cartels, desert disputes, and illegal crossings. It's also home to Big Bend National Park, 800,000 acres of beautiful, remote desert, mountains, and 118 curling miles of that river border. A two-day float on it through Santa Elena Canyon is an ideal way to get a real taste of the park's splendid solitude and scenery.

Big Bend River Tours in Terlingua offers trips by canoe and raft along more than 200 miles of it. The best 22 miles – for views, wildness, and hiking – is the float through Santa Elena. On the first day, guides take you through rugged desert and mountains to the mouth of the canyon, a 10-mile slash through limestone layers of a massive uplift in the Mesa de Anguila Mountains. Night is spent by a campfire on a sand beach while the guides prepare a Dutch oven supper, wine included (otherwise, BYOB, cans, and plastic bottles only). There's no need for a tent – the dry air is perfect for an open sleeping bag – and darkness falls hard out here with no man-made light sources. Stargazing is not only a must, it's all but inescapable.

Most of the river is generally Class II (unless it's flooding), but on day two you hit Rock Slide Rapid, a short but hairy Class IV maze of house-sized boulders choking the water into narrow, fast-flowing channels. At times during the day, the canyon walls, which top out at 1,500 feet, come within 30 feet of each other. Stops along the way include hikes into side canyons seen by few humans and dips in quiet pools – all, at least officially, on the U.S. side of the border.

You'll wrap up a long, otherwise slow day that afternoon along a sandy shore that's constantly rearranged by flash floods. The crew loads up and drives you across the park back to Terlingua, Texas – a mining ghost town famous for its annual chili cook-off – just in time for enchiladas that pack heat as well as sweet, cold, prickly pear margaritas at the Starlight Theater Restaurant. Spring for a shower and bed at La Posada Milagro Guesthouse, built of stacked rock ruins next door ($195).

Conditions are best mid-July through October. [$310 each (plus tax and fees) for 1–3 people, meals included; 800-545-4240,]