40 times?

For those of you attending "Dr." Mortenson's talk in Morris, MN today - a popular claim is that the canyon carved by the Toutle River after Mt St Helens is 40 times smaller than the Grand Canyon. The Toutle River canyon, being eroded out in a relatively short period of time, provides a seemingly perfect analogy for the 40 (or even 100) days of the "Flood."

But this is wrong on many, many levels. The simplest is one of geometry. The Toutle River canyon is almost one fortieth the length of the grand canyon (close enough to use that number for now). But, it is also nearly one fortieth as wide, and more than one fortieth as deep. Since we should consider dimensions of volume if talking about size, we are looking at a comparison of something less than 1/(40*40*40)^th the size of the grand canyon. So the Grand Canyon is actually 64,000 times larger. That's 175 years worth of material flux. All of this is predicated on the assumption that erosion of the Toutle River canyon is equal to erosion of the Grand Canyon. This assumption is stupid and false. Only a fool would assume the two systems are equally comparable. But that hasn't stopped people I guess...