There are many great international river rafting trips, and one of the best is unsurprisingly a Canadian-Alaskan crossover. Hatha Callis, owner of Skeena Valley Expeditions in British Columbia, recommends this relatively calm 10-day trip closer to home. "It runs through the planet's largest UNESCO World Heritage Site, and two national parks and one provincial park. You'll see the world's largest non-polar ice caps, lakes full of icebergs, as well as glacier bears, grizzly bears and bald eagles. And you can drink your evening scotch with 10,000-year-old ice." The Tat also incorporates layover days for great hiking.

The fact that just reaching the Karnali requires an overland trek with porters should give some idea of how remote this big-volume Class IV-V waterway is. The week-long trip flows through a series of narrow, forested canyons and ends in Bardia National Park. Most of the gnarly whitewater occurs in the first few days.

The day-long section at the top of Africa's Zambezi river offers what photographer/guide Leon Werdinger calls "gnar gnar whitewater, with five gnars. If your raft flips you're instructed to stay in the main current, because the eddies along the sides have crocs and hippos—and they do flip rafts there." The overnight stretch farther downriver is mellower, and floats through rural villages.

Martha Gaughen calls the Pacuare's whitewater "fun but never threatening. You can take one, two or three-day trips and combine them with other eco-tours to volcanoes or in the rainforest canopy." Highlights include the chance to see toucans and sloths and swim beneath waterfalls.

Eric Hertz, president of Earth River Expeditions, describes the Yangtze as one of the most culturally interesting river trips in the world. "The scenery is pretty— high, rolling, green hills— but the untouched Tibetan culture is unbelievable. We see monks in red robes in stone villages cut out of cliff walls, amazing temples with paintings, and the people are really welcoming and thrilled to see us. This is one of the all-time great river trips."

An unusual island river trip, this one-day outing on the Upper Navua cuts through a gorge of black lava overgrown with ferns, vines and other jungle flora. The easy Class II whitewater is perfect for family outings. Riverside waterfalls await around nearly every bend.

New Zealand's Shotover River is most often run as a day trip from Queensland in the company of Kiwi guides with famously mellow attitudes. Highlights include a rapid that flows through a tunnel and then pours right into a Class IV drop back out in daylight. Leon Werdinger rates the shuttle drive to the Shotover, on an eroding hillside, as "a Class V—it's the scariest road I've ever been on."

Describing the hardcore whitewater of the Futeleufu, Martha Gaughen quotes a client: "If you swallowed a piece of coal before this trip it would come out as a diamond." The intense, challenging Class V river is often run as a day-trip in combination with such other adventure sports as rappelling and zip lines. The river cuts through a beautiful mountain valley.

Eric Hertz calls the Magpie the second best multi-day whitewater river trip in North America—and that's saying something. Guests are flown in by float-plane for a week of warm-water boating in a pristine area that "looks like Siberia." The Magpie has been run so seldom that the rapids don't even have names. One highlight is camping across from a 100-foot waterfall with a powerful volume of water.

Ten- to 12-day trips on the Tombopata carry travelers from the Andes to the Amazon along a river roiling with Class III-IV rapids. It passes through two national parks. Some expeditions are joined by a naturalist who points out such wildlife as monkeys, tapirs, macaws and even jaguars. Guests stay at lodges and camp out en route.