What the International Scale of River Difficulty has to say:
Class IV: Advanced
Intense, powerful but predictable rapids requiring precise boat handling in turbulent water. Depending on the character of the river, it may feature large, unavoidable waves and holes or constricted passages demanding fast maneuvers under pressure. A fast, reliable eddy turn may be needed to initiate maneuvers, scout rapids, or rest. Rapids may require “must” moves above dangerous hazards. Scouting may be necessary the first time down. Risk of injury to swimmers is moderate to high, and water conditions may make self-rescue difficult. Group assistance for rescue is often essential but requires practiced skills. A strong eskimo roll is highly recommended. Rapids that are at the lower or upper end of this difficulty range are designated “Class IV-” or “Class IV+” respectively.
........and what the rio say's
Class IV: Advanced
Water is generally lots colder that Class III. Intense, powerful but predictable rapids requiring precise swimming in turbulent water. Swimming may require "must" moves above dangerous hazards. "Must" moves are downgraded to "strongly recommended" after they are missed. Sensation of disbelief experienced while about to swim large drops. Frantic swimming towards shore is alternated with frantic swimming away from shore to avoid strainers. Rocks are clung to with death grip. Paddle is completely forgotten. One shoe is removed. Hydraulic pressure permanently removes waterproof box with all the really important stuff. Paddle partners running along stream look genuinely concerned while lofting throw ropes 20 feet behind swimmer. Paddle partners stare slack-jawed and point in amazement at boat which is finally pinned by major feature. Climbing up river bank involves inverted tree. One of those spring loaded pins that attaches watch to wristband is missing. Contact lenses are moved to rear of eyeballs.
Class IV: Advanced
Intense, powerful but predictable rapids requiring precise boat handling in turbulent water. Depending on the character of the river, it may feature large, unavoidable waves and holes or constricted passages demanding fast maneuvers under pressure. A fast, reliable eddy turn may be needed to initiate maneuvers, scout rapids, or rest. Rapids may require “must” moves above dangerous hazards. Scouting may be necessary the first time down. Risk of injury to swimmers is moderate to high, and water conditions may make self-rescue difficult. Group assistance for rescue is often essential but requires practiced skills. A strong eskimo roll is highly recommended. Rapids that are at the lower or upper end of this difficulty range are designated “Class IV-” or “Class IV+” respectively.
........and what the rio say's
Class IV: Advanced
Water is generally lots colder that Class III. Intense, powerful but predictable rapids requiring precise swimming in turbulent water. Swimming may require "must" moves above dangerous hazards. "Must" moves are downgraded to "strongly recommended" after they are missed. Sensation of disbelief experienced while about to swim large drops. Frantic swimming towards shore is alternated with frantic swimming away from shore to avoid strainers. Rocks are clung to with death grip. Paddle is completely forgotten. One shoe is removed. Hydraulic pressure permanently removes waterproof box with all the really important stuff. Paddle partners running along stream look genuinely concerned while lofting throw ropes 20 feet behind swimmer. Paddle partners stare slack-jawed and point in amazement at boat which is finally pinned by major feature. Climbing up river bank involves inverted tree. One of those spring loaded pins that attaches watch to wristband is missing. Contact lenses are moved to rear of eyeballs.
