Scale of Things

Whitewater is graded according to the International Scale of River Difficulty. Rapids start at class I (easy) — “fast moving water with riffles and small waves …”

The scale moves up through class II (medium), class III (difficult), class IV (very difficult), class V (extremely difficult) and class VI (unraftable). According to americanwhitewater.org, class VI rapids have almost never been run and the “consequences of errors are very severe and rescue may be impossible”.

The Grand Canyon is rated class IV with some class V rapids, along with a lot of class I – III rapids.