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In the vast, arid landscape of Eastern Washington
lie the traces of an ancient disaster.
Outside the city of Spokane, massive scour
marks run through the rocky ground, creating
a strange terrain known as the scablands.
A bit to the west, a channel has been carved
into the Earth that’s as deep as a forty-story
building.
Elsewhere, miles of rolling hills run across
Washington, Montana, and Idaho, resembling
enormous ripples up to 15 meters high.
These features are all the lingering remains
of an epic geological mystery that took nearly
half a century to solve.
Now, every great mystery requires a great
detective, and geologist J Harlen Bretz was
a great detective indeed.
He researched these strange features in the
early 1900’s and soon concluded that features
like these could only have been made by water.
A lot of it.
Running fast.
But he also knew that a flow of water that
could transform the land so drastically would
had to have been unimaginably huge.
It must’ve been a flood, of almost biblical
proportions.
When Bretz presented this hypothesis to fellow
geologists in 1927, he was met with ... skepticism,
to put it lightly.
But ultimately, his research would reveal
one of the most powerful and bizarre episodes
in recent geologic history.
And as a result, it would revolutionize the
way geologists understand the world today.
Because, Bretz was right: This landscape was
the result of flooding.
But not just a single flood.
Instead it was dozens of major, devastating
floods that took place over the course of
more than 7,000 years, forever changing the
landscape of the Pacific Northwest.
What Bretz had discovered was evidence of
floods that can only be described in one word:
catastrophic.
When Bretz first began studying the weird
landscape of the Northwest in the 1920s, there
was a certain school of thought that most
geologists followed.
It was known as uniformitarianism, the idea
that the present is the key to understanding
the past.
In this view, all rocks, landforms, and other
geological features can only have been created
by processes that we can observe today.
And except for the occasional volcanic eruption,
or river overflowing its banks, all modern
processes are gradual, like erosion.
So to these geologists, the scablands of Washington
could only have been created by glaciers,
and the ripples must be deposits of what the
glaciers had slowly scraped away.
Because, the effects of glaciers had been
observed around the world.
And through the lens of uniformitarianism,
they seemed to most closely resemble the features
that Bretz was studying.
But Bretz had studied glacial geology, too,
and he knew what glaciers could do.
And to him, the features he saw just didn't
fit.
Instead, they looked like scaled-up versions
of what happens after a big flood.
For Bretz, the most obvious evidence of flooding
was the shape of the canyons in the Scablands
and other places.
These canyons, also called coulees, have flat
bottoms and steep, vertical walls - very different
from the U shape of valleys that are carved
by glaciers, or the V-shaped valleys made
by rivers.
One especially large coulee, called Dry Falls,
appeared to have formed a massive waterfall
over 100 meters tall and 3 and a half kilometers
wide; that’s twice as tall, and five times
wider, than Niagara falls!
But water doesn't just remove things; it also
deposits things.
And Bretz saw that the landscape was littered
with boulders weighing up to 200 tons, having
tumbled miles away from their sources, like
pebbles on a beach.
He also noted massive ripples in the earth,
and gravel bars up to 90 meters high, all
typical of deposits made by powerfully flowing
water.
Finally, Bretz knew that these features couldn't
be related to glaciers, because of what was
missing: the huge ridges of deposited sand
and gravel called moraines, which form
around advancing glaciers.
Only one tiny moraine was found in the scablands,
not nearly enough evidence for the giant glaciers
that would have been required to carve features
this big.
But despite all of this evidence, other scientists
weren’t convinced that this strange landscape
was shaped by an epic flood.
They argued that humans had never seen a flood
anywhere near as big as the one Bretz proposed,
so they were reluctant to believe that such
a thing was even possible.
Uniformitarianism explained a great deal about
geology, and epic floods just didn't fit into
it.
What giant floods did fit into was the geological
mindset that Uniformitarianism had replaced:
An older school of thought known as catastrophism.
Catastrophism was an idea put forward in the
early 1800s by French scientist Georges Cuvier.
This theory explained all geologic
formations as evidence of large, sudden, unpredictable
events -- often events that were referred
to in the bible -- like celestial impacts,
enormous volcanic eruptions, and ... massive
floods.
So no matter how good his evidence was, Bretz’s
hypothesis seemed extremely outdated.
And there was still one really big question
that Bretz couldn’t answer.
If all this flooding really happened, then
where’d the water come from?
And this was something that puzzled Bretz himself.
He initially thought that the water had come
from some melting glacier.
But he couldn’t explain how the glacier
had melted fast enough to produce so much
water all at once.
It turns out, Bretz was looking in the wrong
place.
But someone else knew where the water came from.
This half of the mystery was solved by Joseph
T. Pardee, a geologist with the U.S. Geological
Survey.
Pardee had attended a conference where Bretz
presented his hypothesis about the Ice Age
megaflood, and watched as Bretz defended his
claim against a room full of skeptics.
And more than 10 years earlier, Pardee had
been working in Western Montana, where I am
now!, and where he’d found evidence of an
enormous, Ice Age lake that had since disappeared.
His main piece of evidence?
Distinctive lines he saw high on the hillsides.
These lines form small benches, much like
the shorelines of a reservoir.
So Pardee figured that these ancient shorelines
were formed by an ancient lake whose source
was the Clark Fork River, which still flows
today through the valley below.
This giant lake came to be known as Glacial
Lake Missoula, named after the town -- which is also my
hometown! -- where Pardee saw those lines.
But a reservoir requires a dam, and a lake
this size would’ve needed a big one.
So what had dammed the river to form the lake,
and what happened to the dam?
To find out, Pardee followed Lake Missoula’s
shorelines for miles to the west, into the
panhandle of Idaho, at which point the lines
… just disappeared.
But where they ended, he found something else:
big, U-shaped valleys and glacial moraines
-- both evidence of glaciers in the area.
So, the evidence suggested that a glacier
had blocked the river to form the lake.
Judging by the landforms around it, it must’ve
been about 50 kilometers wide and more 600
meters tall.
And the reason it didn’t exist anymore was
just because it was made of ice.
So with his missing dam now found, Pardee
had a new question to answer: Where’d all
the water go?
By some accounts, Pardee had already suspected
that the scablands that Bretz described were
created by the drainage of his lake.
But it took more than a decade for Pardee
to publish the evidence that linked his lake
to Bretz’s flood.
On a mountain pass in northern Washington,
for example, he found massive scour marks.
In the river valleys of western Montana, he
recorded large bars of debris that had been
carried there by currents.
And in Montana and Idaho, he studied enormous
rippling dunes made of gravel.
All of these strange features were consistent
with evidence of flooding.
And they were all downstream of where the
ice dam would have been.
So Pardee concluded that, periodically, too
much water built up behind the ice dam that
held back Glacial Lake Missoula, until it
ruptured.
After all, ice is less dense than water.
So when the water level in Lake Missoula got
high enough, it would’ve caused the dam
to float upward.
And as the water began to rush out underneath,
the enormous pressure would cause the dam
to break.
Then, by most estimates, about 2500 cubic
kilometers of water -- enough to fill half
of Lake Michigan -- broke free.
The water formed massive waves as it rushed
away from Lake Missoula to the west.
Along the way, it lifted giant boulders, carved
the steep cliffs and rolling hills of Bretz’s
scablands, and helped shape the vast Columbia
River Gorge that today forms the boundary
between Washington and Oregon.
In 1942, Pardee finally wrote up all of this
evidence, detailing what happened to the missing
lake, and connecting it to the massive floods
that Bretz had postulated.
And in the decades after these two intrepid
detectives did their work, other
geologists used newer techniques to establish
that these floods actually happened many,
many times.
One of the clearest pieces of evidence is
in the remains of the bed of Lake Missoula
itself.
The dark and light bands of sediment on the
floor of the lake, known as varves, are like
an archive of the years when the lake was
full of water.
Dark varves correspond to winter deposits,
and light ones to summer.
But some of these layers are interrupted by
beds of gravel -- gravel that was deposited
by rapidly moving floodwater.
So the number of varves that appear between
the layers of gravel tells us that these catastrophic
floods happened every 20 to 60 years.
And scientists have even been able to track
down multiple lines of evidence to estimate
when they happened.
Over the years, geologists have studied flood
deposits in the ocean, where the Columbia
River empties into the sea.
They’ve studied the sediments in rocky outcrops,
and the chemistry of the giant boulders found
along the path of the flood.
And together these clues suggest that Glacial
Lake Missoula flooded many times within a
span of 7,000 years, from around 20,900 to
13,500 years ago.
But as freaking massive as the Lake
Missoula floods were,
they weren’t the only megafloods
that happened.
And they definitely weren't the biggest.
For that, let’s hear from Stefan Chin at
SciShow, where they’re talking about the
biggest Ice Age flood of them all!
There was an even bigger glacial lake in central
North America, called Lake Agassiz, and it
was at least 8 times the size of Lake Missoula.
Because it was so big, its megafloods were
even more devastating.
The drainage from this lake was so enormous
that it disrupted ocean currents.
And that in turn may have caused a climate
cooling event 13,000 years ago that’s at
least partly responsible for the extinction
of mammoths and other Ice Age megafauna in
North America.
When you’re done here, head on over to SciShow
to learn all about Lake Agassiz and how its
floods may have changed the climate of the
entire planet.
The Floods from Lake Missoula didn't change
the climate, but they did change the world
in other ways.
For one thing, they fundamentally changed
the landscape of the Pacific Northwest.
But perhaps more importantly, they also changed
how we understand geology, all over the world,
to this day.
By 1965, with lots of evidence and finally
a source for his floods, Bretz's research
was accepted and became part of a new understanding
of the processes of the Earth, both big and
small.
So, today, geologists understand that, although
a lot of geology is slow and small, sometimes
our world is shaped by huge, catastrophic
things.
This new framework is sometimes called neo-catastrophism,
or modern uniformitarianism.
But for me, probably the most fascinating
part of all of this is that there may actually
have been people around to witness these gigantic
floods!
The oldest evidence for humans in the Pacific
Northwest is about 15,000 years old, old enough
that the last of the Missoula floods may have
been seen by human eyes.
As a person who lives in Montana in the 21st
century, all I can do is look up at those
lines on the hills around my city, and imagine
what it must have felt like to witness one
of those floods that changed 
the world.
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Thanks to our friends and colleagues at SciShow who work just down the hall from us.
For taking part in this epic ice age collaboration
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big thanks to our current Eontologists, Jake
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