Drops may contain large, unavoidable waves and holes or steep, congested chutes with complex, demanding routes. Rapids may continue for long distances between pools, demanding a high level of fitness. What eddies exist may be small, turbulent, or difficult to reach. At the high end of the scale, several of these factors may be combined. Scouting is Recommended but may be difficult.
Swims are dangerous, and rescue is often difficult even for experts.
A very reliable Eskimo roll, proper equipment, extensive experience, and practiced rescue skills are essential. Each of these levels is an order of magnitude more difficult than the last. Increasing difficulty from class 5.0 to class 5.1 is a similar order of magnitude as increasing from class IV to Class 5.0. These runs have almost never been attempted and often exemplify the extremes of difficulty, unpredictability and danger. The consequences of errors are very severe and rescue may be impossible.