The Bridge to Bridge (Puente a Puente) section of the Futaleufu River is the most commonly paddled stretch because it has tons of great Class IV/IV+ rapids (one Class V) and easy river access. It's best described as running all the hardest rapids on the Grand Canyon (and a few more difficult ones) in a day.
If you visit the Futaleufu River do this section first so you can get accustomed to big, pushy water. Even though most of the rapids are rated at Class IV, the consequences of a swim can be pretty bad because your boat can float way downstream as you swim to shore. If you're an experienced rafter or kayaker you should consider hiring an outfitter to show you the way down the first few times and provide safety.
Mile 0: Put-in at the Zapata Hanging Bridge.
Mile 0.1: Entrada (IV+) is a long rapid that will give you a sense of what the Futaleufu is all about. Below here are Lower Entrada (IV), Pillow (IV+), S-Turn (III), and Cara del Indio (IV).
Mile 2.2: Mundaca Rapid (V-) the most difficult rapid on this section. After Mundaca there are many great Class III and IV rapids including Last Wave is a Rock (IV), Cazuela (IV), and Tiburon (IV).
Mile 4.3: Take-out at the Futaleufu Bridge on the left. Make sure you know where take-out is because once you're at the bridge you're committed to running Mas-o-menos (V-) and Casa de Piedra (V).
Option: There are two Class V rapids downstream of the Futaleufu Bridge.
Mile 4.35: Mas-o-menos (V-) is a big, long rapid just past the Futaleufu Bridge.
Mile 5.3: Casa de Piedra (V) is the biggest rapid on the lower part of the Futaleufu. Like all Futaleufu rapids, the line varies greatly depending on river levels, so most people scout on the right side.