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 may 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 mandatory but often difficult.
Swims are dangerous, and rescue is difficult even for experts.
A very reliable eskimo roll, proper equipment, extensive experience, and practiced rescue skills are essential for survival. These runs often exemplify the extremes of difficulty, unpredictability and danger. The consequences of errors are very severe and rescue may be impossible. For teams of experts only, at favorable water levels, after close personal inspection and taking all precautions.