Aspen and the Roaring Fork Valley Feature

The key to understanding white-water rafting is the rating system. Rivers are rated from Class I, with small waves where you really don't need to paddle to avoid anything, to Class VI, which is almost impossible to run and a mistake can be fatal. To confuse matters, rivers change classes depending on how fast they are flowing (measured in cubic feet of water per second). May and June are peak rafting seasons for those who want the adrenaline rush of fighting spring runoff. By mid-August many rivers are little more than lazy float trips.

The Roaring Fork, a free-flowing river (no dams), is regularly considered a Class III. Because it runs away from major highways through the heart of ranch country, you're liable to see more wildlife than on the Colorado; in June your guides may point out a nest full of croaking bald eaglets. Cemetery Rapids, a half-mile churning stretch of white water, is the most exciting run.

The stretch of the mighty Colorado that runs through the steep, spectacular walls of Glenwood Canyon alongside I-70 is divided into two sections: the rough and tumble Shoshone below the dam, and the wider, mellower regions beyond. During the peak runoff season, the Shoshone is considered a Class IV river, with aptly named rapids like Maneater and Baptism. The lower Colorado still has some exciting stretches, including Maintenance Shack, a Class III rapid that can flip a large raft. By July and August you can hit the same rapid sideways or backward and barely get wet. The lower stretches of the Colorado pass by several hidden, and not-so-hidden shallow hot springs. If you'd like to warm up in them, ask your guides.