What would it mean? The river needs to pass about 3/4 of its flow in any
given year in the three months of May, June, and July. This would
continue true, so about 48 MAF, or 16 MAF/month, would pass into Lake
Powell. Now a flow of 16MAF/month is roughly equivalent to 260,000 cfs
on an average daily basis for the month. It is also true that the annual
peak flow would be approximately double the average flow for those months,
or 500000 - 600000 cfs, an amount in the range of a low flood on
the Lower Mississippi. If torrential rains spiked the hydrograph, as in
the flood of the San Juan in October, 1911, the peak could be much
greater than the monthly average flows. Since the spillway capacity is
about 276000, the dam would be overtopped. Fontanelle would fail, but its
contribution to the flow would be minimal. There are no instances that I
can recall of high-arch dams failing, though some gravity dams have been
breached by skip-bombing in WW II. One high-arch dam in Italy wetted the
wrong rock layer, and was overtopped by a landslide-triggered "tsunami."
It held. There was great damage downstream. So it is not clear whether, or
how much of, Flaming Gorge, Powell, and Mead, would be lost. Rupture in
part or whole would contribute a great deal to the flow; Powell and Mead,
for instance, each contain about 25,000,000 AF, so they could augment the
flow by 40% each. At any rate, the power house at Glen Canyon would be an
early loss. Also lost would be the capacity to control the spillways.
Harding doubts that the post-84 modifications to introduce air will fully
obviate the cavitation problems, particularly since the spillways would be
running for 5 - 6 months. So abutment failure again becomes a possibility
at Powell; Hoover seems less vulnerable.
It is also of note that from Santa Fe north there would be essentially no
surface transportation across the Rockies and the Sangre de Christos for
about 18 - 24 months. First, the snow depths would overwhelm the Highway
Department. As they dug through, runoff would overtop the roads, and stay
there. Of note is that in 1884, when Grand Canyon reached 300,000 cfs and
Westwater 125,000, in the Spanish Valley (Moab area) the Colorado was out
of its banks for 2 months. These levels would be much greater than
those and would last longer. When the rivers began to recede, the damage
could not be corrected before the next winter began.
There is one chance in 20 that this will happen in our lifetimes. We
reputedly enter an El Nino period similar to water-year 83, with Powell
off 10 feet from 3700, the spill elevation.
Another implication of interest is that the streamflow trace (I have not
seen it) has to contain periods equivalent to the drought of 1286 - 1300,
which apparently made mainstream dwellers and Utes of the pre-Pueblo
Indians of the 4-Corners country. Powell could be drawn down to the level
of the penstock intakes and remain nearly empty for years; we could again
boat Glen Canyon National Mudbank with inner-tubes.
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