But while the river is constrained as to the recurrence level its post-dam
discharges can reach, the side drainages are not. They have the same
probability curve they always had (barring suspected but unexamined
climatic effects of excessive populations relying excessively on
technology). Thus, a 1000-yr flood on a side drainage is as likely now as
ever it was, but a thousand-year flood below the dam is not. It should be
noted, of course, that calculating recurrence intervals is a 'new' activity
for humans, and as such is predicated on a ludicrously short experience
base; what are now thought 1000-yr floods will be gliding down into the
500-yr recurrence interval range, to the 100 year range over the next
couple hundred years of data-gathering. So you have statistically
unconstrained side canyons and a river that can no longer match them.
What the Grand Canyon will do is become a series of stairsteps.
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