Crystalling

Earl Perry (earlp@ihs.com)
Thu, 25 Sep 1997 17:11:35 -0600


Speaking with a certain simplification, to remove a rapid or rework it, you
need a flow in the main river of at least the recurrence interval of the
flashflood that caused or exacerbated the rapid. 100:100, 1000:1000. Now
with the interpolation of the dam, you can't get more than about an annual
flood (ca 90000 cfs) from the dam until the full spillways kick in, when
the discharge will be 276000. It is of interest that a flow of 300000 has
been observed in modern times, and another of about 220000; this indicates
that the dam was deliberately designed to be unable to pass flows that were
not just projected statistically, but actually observed -- a measure of
incompetence on the designers' parts which could be called incredible if it
were not a fact. Flows from the dam, being uncharged with sediment, will
attack fines, but being relatively low on the recurrence curve, will not be
able to start, and then transport, the very large sediments. Removal of
the fines, of course, leaves the boulders wearing in to each other, and
actually makes them less susceptible of motion. Bed armoring as applied to
rapids.

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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