Well, there's a non-linearity to runoff. Mostly you fill the alpine soil
gradually until saturation; then runoff. But you can also hypothesize
conditions -- similar to the South Platte River in '69 -- when a very
gradual rain for a number of days finally saturates large tracts of the
basin. It is also possible to choke the soil, so to speak; as is the case
with most flashfloods off the reservation. you get rainfall which excedes
the rate of possible infiltration, though it never approaches its maximum
amount the soil could have held with gradual charging.
>
>Now, what about its return interval? Our statistics indicated
>1400 years. But, if that were true, we would expect to see
>evidence of several 800k cfs flows in the canyon with sufficient
>organic material buried under them to allow for carbon dating.
>Is there any evidence of this?
Well there is distinct evidence of the flood of Oct. 1911 along the San
Juan: the high sandbars at 8-foot rapid and certain driftwoods about 45
feet above the river. There is evidence of the flood of 1884 in Cataract:
an obscured talus cut line about 20 - 25 feet above the 70,000 cfs level.
It's of some interest, I think, that you find very little driftwood from
the 1884 level. Perhaps the extreme activity of Cataract, in the geologic
sense, is responsible; those salt-footed slump blocks are constantly
encroaching on the river. Perhaps, on the other hand, big wood doesn't
get put down at the highest levels, and the small organic detritus (pine
needles, etc) has a short life and quick decay. Perhaps, of course, the
evidence isn' there because it simply hasn't happened at the interval
suggested by the statistical exercise.
It is said that driftwood from 1884 was found in Stanton Cave. In Grand
Canyon, the area that interests me as potential evidence is the cemented
breccia in the vicinity of the Little Colorado. Hard to cement, without
postulating a source of interstitial salts. I had previously ascribed this
-- for purpose of argument with Yeamans -- to saturation by highly
saturated or even supersaturated waters backed up by the lava ponding in
the vicinity of present Lava Falls. It still stikes me that this is a
better explanation than postulating an extreme floodflow. (It is also true
that cemented colluvium is found in places, like the Dolores near Bedrock,
where this explanation isn't possible.) I guess I'll argue that cleanup in
the canyons is quicker than we often think, and we may not be finding the
organic detritus from the non-Harding flood because it doesn't last. (Get
that dam drained, and we'll have Glen Canyon back in no time, geologically.)
>
>So, is it possible. Decide for yourself. In my mind, it's
>definitely not impossible, but it might not play out like a
>normal year. For example, it might be the result of a very wet
>fall followed by extraordinary snows and then an extraordinarily
>wet summer. This would imply a flatter hydrograph and thus
>lower peak flows than estimate by Perry.
Wet fall: soil recharge and saturation. Not really much influence on a
hydrograph.
Extraordinary snows: yup. Huge depths, and cloaking the Rockies and the
foothills to depths and down to elevations not very imaginable now. But
still not much runoff.
Extraordinarily wet summer: Warm rain on isothermic snow banks is a
partial explanation for the Christmas floods on the Eel in '64. This
condition has the potential to put a hydrograph right over all previous
scales; the Eel peaked at 750,000, rising from a few hundred. Even if you
have normal spring weather melting out 4 times normal snow water, every
bit of summer precipitation will set spikes on the hydrograph. There just
isn't any place else for the water to go: not into the earth, and not back
into the sky. I estimated the peak at 2X mean monthly discharge,
mathematically; it nearly always goes twice as high as it averages. A
good sharp rain over much of the basin, on top of very high spring flows,
could peak it at 4 or 5 times the monthly average for those 3 months.
In any case, it would
>be an _extreme_ event, probably rarer than a 1400-year return
>interval.
It would be extreme. But you know, it DID flow 300,000 in 1884, and this
predicts a peak of perhaps 600,000. The other thing making its
probability shorten, of course, is the effect of abusive or at least
homocentric land practices all over the upper basin. The Eel floods had
much to do with weather, but the one fork of the Eel that had not been
logged remained only bankful. The forks that had been logged experienced 3
or 4 10-year floods, 2 100 year floods, and a 1000 year flood, in the
decade around '64. These, said the lumber companies, were acts of God.
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