Bravo! Great to hear from you.

Thanks for the reference. I guess my sense of "huge floods" is colored by what occurred on the Columbia River (check out Cataclysms on the Columbia: A Layman's Guide to the Features Produced by the Catastrophic Bretz Floods in the Pacific Northwest" by John Eliot Allen and Marjorie Burns) Glacial dams hoding back huge lakes burst as cimate warmed, emptying a lake in a single day.

You don't need something that big to move large boulders. So I appreciate Ranney's point.

Another possibe contributer is the occasional volcanic eruption in the canyon, the lava of which blocks the river for several months until the lake overflows and rapidly erodes a channel around the lava dam and majorly floods downstream.