
Lake Agassiz - henron
https://en.wikipedia.org/wiki/Lake_Agassiz
======
niftich
Lake Agassiz and Lake Missoula were glacier-dammed lakes that periodically
overflowed the lowest sill of their basins to cause massive floods. Lake
Agassiz is thought to be responsible for the deeply incised Minnesota River
valley.

The shifting of glaciers and the ice sheet altered the effective relief of the
basins over time, so simulating these lakes today takes a lot of work.

In the Great Basin, away from glaciers, Lake Bonneville and Lake Lahontan were
the largest lakes that formed during a colder, wetter time, as endorheic
valleys filled with water and repeatedly overflowed the lowest sill of their
basins. Lake Bonneville is thought to have overflowed the outer edge of Great
Basin itself at Red Rock Pass near Downey, Idaho; eroding the gap and
releasing a huge flood into the Snake River basin.

Because a moving ice sheet wasn't a factor in their case, DEM shading can be
used to approximate their overflows. This doesn't account for isostatic
rebound or tilt in terrain, but gives a visually enlightening approximation,
and lets one interactively explore how these outflows may have worked.

One can shade a topo map of the Great Basin at 4785 feet -- today's elevation
of Red Rock Pass -- to approximate how Lake Bonneville's floods may have
worked. Or, shade at these key sill elevations, in feet, to see how Lake
Lahontan may have outgrown one valley after another: 3878, 3933, 3976, 4154,
4180, 4301, 4386.

Example with Caltopo at 3976 feet:
[https://caltopo.com/map.html#ll=40.28729,-118.02612&z=8&b=t&...](https://caltopo.com/map.html#ll=40.28729,-118.02612&z=8&b=t&o=r&n=0.25&a=sc_e0-3976fcFF0000-0000FF&cl=%7B%22cfglayers%22%3A%5B%7B%22id%22%3A%22179ac9d4-1494-4e74-ad1a-6b78626556cd%22%2C%22geometry%22%3Anull%2C%22properties%22%3A%7B%22title%22%3A%22%22%2C%22alias%22%3A%22sc_e0-3976fcFF0000-0000FF%22%2C%22class%22%3A%22ConfiguredLayer%22%7D%7D%5D%7D)

~~~
arethuza
That reminds me (on a much smaller scale) of the Parallel Roads of Glen Roy:

[https://en.wikipedia.org/wiki/Glen_Roy#The_Parallel_Roads_of...](https://en.wikipedia.org/wiki/Glen_Roy#The_Parallel_Roads_of_Glen_Roy)

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kqr
> The last major shift in drainage occurred around 8,200 years ago. The
> melting of remaining Hudson Bay ice caused Lake Agassiz to drain nearly
> completely. This final drainage of Lake Agassiz has been associated with an
> estimated 0.8 to 2.8 m (2.6 to 9.2 ft) rise in global sea levels. [...] A
> recent study by Turney and Brown links the 8,500-years ago drainage to the
> expansion of agriculture from east to west across Europe; they suggest that
> this may also account for various flood myths of prehistoric cultures,
> including the Biblical flood narrative.

Reading things like this is very humbling. Humankind has been good at keeping
records of things for the last, what? 300 years? Nearly by accident, we have
successfully retained some accounts from the ancient Greeks, which were
produced not even 3000 years ago. So 3000 years is a long-ass time, and really
stretches our capabilities of preserving records. And this was after we
invented some forms of writing, as far as I understand it.

Yet historians have reason to believe that in the 5000 something years before
that, humans told stories about actual great floods, passing them down one
generation at a time, to a point where they still form part of our cultural
heritage today? That is absolutely mind-boggling to me. Obviously, I can't
tell whether or not that is actually the case, but that there's even a
possibility that it is the case is just beyond me.

Of course, for the generations living around 8,500 years ago, even just a 3 ft
flood must have really been something if it happened all over the world at
once – even if it takes multiple generations for it to reach peak levels. That
I can sort of get my head around. Obviously, you talk about that with your
contemporaries. But that it would have been significant enough that it would
somehow make it into folklore that two hundred generations later, they still
talk about it, that just... no words.

~~~
fit2rule
You know what's really mind-boggling: the fact that Australian Aborigines have
successfully kept an accurate record of multiple Geological events for 40,000
years - _by way of oral tradition_.

I think the fact this is unrecognised in educated circles is also
staggering...

~~~
dmurray
Accurate, but maybe not precise.

~~~
mrow84
Precision is bounded by the channel capacity. The advance was an intellectual
one - that there is value in maintaining an accurate record.

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8bitsrule
Agassiz fell and rose multiple times over thousands of years as the ice sheet
retreated and regrew. During that time it found many paths to escape its
confinement, and left behind several beach-lines.

One of the most clearly-written and illustrated professional Agassiz stories
is Wright's lightly-technical 1990 geology paper, available in PDF [0]. A lot
more has been learned in the past 30 years, but by that time its effects in
Minnesota were quite well known.

[0]
[https://conservancy.umn.edu/handle/11299/57272](https://conservancy.umn.edu/handle/11299/57272)
Wright, H.E. Jr (Minnesota Geological Survey, 1990)

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valuearb
Lake Missoula wasn’t as big, but it’s floods were more spectacular.

[https://en.wikipedia.org/wiki/Missoula_floods](https://en.wikipedia.org/wiki/Missoula_floods)

“After Pardee studied the canyon of the Flathead River, he estimated that
flood waters in excess of 45 miles per hour (72 km/h) would be required to
roll the largest of the boulders moved by the flood. He estimated the water
flow was 9 cubic miles per hour (38 km3/h), more than the combined flow of
every river in the world. Estimates place the flow rate at ten times the flow
of all current rivers combined.”

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mkl
There was a similar lake in Siberia:
[https://en.wikipedia.org/wiki/West_Siberian_Glacial_Lake](https://en.wikipedia.org/wiki/West_Siberian_Glacial_Lake)

It likely was so dammed up it overflowed back down to the south, through the
Aral, Caspian, and Black Seas, to the Mediterranean:
[https://folk.uib.no/ngljm/PDF_files/Mangerud_et_al_2004,_QSR...](https://folk.uib.no/ngljm/PDF_files/Mangerud_et_al_2004,_QSR_.pdf)

I'm sure I remember reading a good article about it in National Geographic or
Scientific American or something, but I can't find it.

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kevinskii
Fascinating! Also worth a look is Lake Lahontan, which covered much of
northern CA and southern OR about 12,000 years ago:
[https://en.wikipedia.org/wiki/Lake_Lahontan](https://en.wikipedia.org/wiki/Lake_Lahontan)

~~~
gricardo99
you don't have to go that far back. A little more than a century ago the
California central valley looked very different. Tulare Lake was the largest
fresh water lake west of the great lakes, bigger than Tahoe[1]. Of course
there's also Hetch Hetchy, which supplies San Francisco's water[2]. Countless
other engineering projects have vastly changed the flow of water in the state.

1 -
[https://en.wikipedia.org/wiki/Tulare_Lake](https://en.wikipedia.org/wiki/Tulare_Lake)
2
-[https://en.wikipedia.org/wiki/Hetch_Hetchy](https://en.wikipedia.org/wiki/Hetch_Hetchy)

~~~
onychomys
It's always seemed so bonkers to me that Yosemite National Park was made in
1890 and yet Hetch Hetchy, which was part of the park, was dammed in the early
1920s. I get that it was a different time and all, but come on, you can't go
around destroying part of a NP like that.

~~~
jefftk
Consistent sources of water for cities are really important, and people were
historically more willing to make pragmatic tradeoffs.

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galacticaactual
Interesting reading. As a lifelong alpine climber with intimate knowledge of
glaciers, I always get weirdly sad at the thought of receding ice sheets (no
matter the reason or epoch.)

