What is measured is the threat to the average raft of flipping. The rapids of the canyon itself are the guideposts, 1 being flatwater and 10 being Lava Falls Rapid. The scale is usually given in a range such as 6 - 8 which is more of a threat than, say 3 - 8. The range takes river level and line options into account. If the rapids were rated according to the international scale probably none of them would rate higher than class III, but that information is rather useless to people trying to get an idea of the degree of difficulty they might encounter on the canyon of the specific rapids.

This is all just my understanding of the issue, someone else might have a different perspective.