Gore Canyon is one of the most technical commercially run stretches of river in the country, and it has teeth. You don't want to swim here: the riverbanks, made of rock chunks blasted out of the Rockies to make way for train tracks, are jagged and sharp. The fact that the canyon is only accessible by boat or train—or by foot, if things get really desperate—further ups the consequence factor. Gore Rapid and Tunnel Falls are both solid, technical Class V rapids, and the rest of the nine miles of whitewater is made up of big Class IV holes, waves and drops. Many of the rapids have sneak lines, or ways to avoid the biggest hazards, but even those require precision and must-make moves. It’s not uncommon for boaters to walk around Tunnel.