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Lake Agassiz (wikipedia.org)
53 points by henron on May 5, 2020 | hide | past | favorite | 21 comments

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&...

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


You can hike along the prehistoric shoreline of Lake Lahontan in Nevada, which at this point is relatively high up the mountains above the current valley floors. Many stretches of the prehistoric shoreline are marked by prodigious quantities of neolithic rubbish left by the peoples that lived along Lake Lahontan. It requires little effort to find artifacts in that part of the country, they are everywhere.

> 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.

I’ve been thinking lately that these long stretches of time are less long than we think when we’re young.

A person who lives a long but relatively normal life might live until 70 to 90, and for the healthy and or affluent that’s been true basically forever.

Thinking about this in terms of human lifespans shrinks time a lot. I have been told stories about my great great grandparents, so even those experiences are at the fading edge of recently passed down history.

But anyway, back to thinking in terms of lifespans...

It’s only been 1 lifespan since world war 2, and only a generation (1/3 of a lifespan) more since world war 1.

It’s only been 2 lifespans since the civil war. 3 since the Napoleonic Wars. 6 since the renaissance, Columbus, and the fall of Constantinople (separate events but close in time). About 20 lifespans since the Roman Empire.

When you imagine that you yourself will live 70-90 years or more, and the changes you will see and have seen so far, and how quickly that time passes... it’s interesting to think how recently history happened.

At least for me, with this measuring stick, it only starts to sound “a long time ago” again when thinking about things like the first pyramids (2570 BC), about 57 lifespans ago. And it’s interesting to note that that far back, there were only about 14 million people on the earth.

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...

This is incredible. Absolutely stunning. Thank you for bringing it up and the link below.

Accurate, but maybe not precise.

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

Actually, pretty good for what they described happening to the land around them:


It is difficult to square a 3- to 9-foot rise with the Biblical flood narrative, or with other flood narratives. Not all of those narratives come from seashore dwellers; still less from those who dwelt in very flat seashore areas.

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 Wright, H.E. Jr (Minnesota Geological Survey, 1990)

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


“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.”

There was a similar lake in Siberia: 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...

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.

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

Looks like you misremembered or misread the map? "Lake Lahontan was a large endorheic Pleistocene lake of modern northwestern Nevada that extended into northeastern California and southern Oregon." It was mostly in what's now Nevada, and there's a clearer map here: https://en.wikipedia.org/wiki/Lake_Bonneville#/media/File:La...

Perhaps there are American Indian oral histories of this lake? It would have covered the Black Rock desert to a depth of 500 feet.

“Archaeological evidence along the shore indicates the existence of the lake coincided roughly with the first appearance of humans in the region. ”

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 2 -https://en.wikipedia.org/wiki/Hetch_Hetchy

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.

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

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.)

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