All language subtitles for Killer Floods 720p PBS Nova HDTV MVGroup

NARRATOR:
Our planet is capable
of unleashing extreme chaos.

Volcanoes,

earthquakes,

hurricanes,

and floods

can cause untold devastation.

(people yelling)

We may think we've seen
the worst

Mother Nature can throw at us,

but scientists struggling
to understand these disasters

are discovering evidence
that even more extreme events

have struck in the past.

JEAN-CHRISTOPHE KOMOROWSKI:
So this is about 13 times
more powerful

than the Pompeii eruption.

NARRATOR:
They're uncovering clues

that the worst catastrophes
in history

could strike again.

¶ ¶

Thousands of years ago,
floods of unimaginable violence.

This water came up 800 feet.

That's huge.

NARRATOR:
Floods powerful enough

to blast through miles
of solid rock

in just hours.

But how?

VIC BAKER:
Everything in this landscape
was screaming

in terms of its signs or clues

that this was made
by catastrophic flooding.

(water rushing)

NARRATOR:
The clues to some of the biggest
floods ever are here,

carved in mysterious
rock formations,

buried beneath the waves,

or hidden in plain sight,

all around the world.

¶ ¶

Now, scientists find new clues

to understand
our volatile Earth...

¶ ¶

(small rocks falling)

And unravel the secrets
of "Killer Floods,"

right now, on "NOVA."

¶ ¶

(water rushing)

¶ ¶

NARRATOR:
Floods.

Events of such violence,

they turn oceans, rivers,
and lakes

into devastating walls of water.

On average, around the world,
these powerful surges

kill 25,000 people every year.

In 2004, a deadly tsunami
hits Southeast Asia,

leaving over 200,000 people
dead,

and $10 billion worth of damage
in its wake.

More recently, in 2017,

Hurricane Harvey
slams into Houston.

Heavy rains cause catastrophic
flooding,

killing more than 70,

and leaving tens of thousands
homeless.

That same year,

a third of Bangladesh
is submerged by flooding

which extended
throughout South Asia,

including Nepal, India,
and Pakistan.

The flooding, caused by
an especially strong monsoon,

is thought to be the most severe
in the last hundred years.

But could Mother Nature
have unleashed floods

that were even bigger
and more destructive

in the past?

That's what a series
of discoveries is suggesting.

Scientists are unearthing

what looks like
the scars of cataclysmic floods

that dug deep into the rock,

reshaping the surface
of the Earth itself.

MIKAEL ATTAL:
It completely changed

the face of the landscape.

BAKER:
No one has ever witnessed
anything

even close in scale.

NARRATOR:
Across the world,

three far-flung locations
share an eerie similarity.

In the United States,
16,000 square miles

of dry canyons
and bizarre rock formations

cover the Northwest.

(water rushing)

In Iceland,
a 300-foot-deep gorge

appears to have been ripped out
in an instant.

And off the coast of Britain,

a network of mysterious canyons
carved deep into the sea bed

could reveal how this channel

first separated
what is now Britain from France.

Far from eroding gradually,

there's evidence
that vast deluges

tore out these landscapes in
the geological blink of an eye.

But what could have triggered
such killer floods?

And could one strike again?

(water rushing)

¶ ¶

The trail of clues starts here,

on the plains
of Washington state.

A flat expanse, stretching
for hundreds of miles,

until suddenly,

the landscape changes.

Flat fields give way
to sheer gorges,

some almost a thousand feet
deep.

Rock islands rise to the height
of 30-story buildings,

while in other places,

strange round depressions,
like gargantuan potholes,

plunge 50 feet.

These are the Scablands,

named by settlers who thought
the formations

resembled scabs or wounds
on the rocky terrain.

¶ ¶

Located over a hundred miles
east of Seattle,

this mysterious landscape
covers an area

around 16,000 square miles.

(car running in distance)

For over a century,

geologists have been trying
to understand

what forces created
the Scablands.

BAKER:
When you encounter a landscape,

it's not unlike a detective
encountering a crime scene.

In the case of this landscape,

there are features
that act like clues.

¶ ¶

NARRATOR:
But spotting those clues
takes a trained eye.

(helicopter whirring)

¶ ¶

BAKER:
You can't really get
a sense of this area

unless you get up high.

¶ ¶

This part of the Scablands
is, like, 30, 40 miles across.

¶ ¶

It's on a mega scale.

¶ ¶

NARRATOR:
It's the kind of scale that
first led geologists

to suspect that the Scablands
had formed slowly,

eroded over millions of years
by rivers, wind, or ice.

(ice grinding)

During past ice ages,
as temperatures plummeted,

giant ice sheets and glaciers

carved deep valleys
through solid rock.

¶ ¶

Like these, in Glacier
National Park, in Montana.

¶ ¶

And rivers,
scouring rock over eons,

helped carve some of the most
dramatic landscapes on Earth,

like the Grand Canyon
in Colorado.

¶ ¶

But mapping sediments
left behind by the ice sheet

when it melted 12,000 years ago,

shows the ice only made it

to the northern edge
of the Scablands.

¶ ¶

And Vic Baker's bird's eye view
reveals

that fast-flowing water
was the culprit.

¶ ¶

The clue?

This curved canyon.

BAKER:
Looking at this from the air,

you can see that the shape
is like a horseshoe,

which is what forms
in waterfalls.

NARRATOR:
Niagara Falls
and many big waterfalls

have a similar horseshoe shape.

¶ ¶

For that reason, this canyon
is called the Dry Falls.

¶ ¶

But stretching
three and a half miles,

this formation is five times
the span of Niagara

and twice as tall.

BAKER:
The cliffs behind me
are 400 feet high.

Niagara Falls would fit
just within the alcove here.

There's a similar-sized alcove

and there's an even bigger one

that extends many miles
to the east.

¶ ¶

NARRATOR:
These are the most extensive
falls, wet or dry, known today.

And in a valley below,

Vic finds another clue

that vast amounts of water
once flowed here.

¶ ¶

Features that look
like sinkholes, or potholes,

often found along the bottom
of turbulent rivers.

But these potholes

are super-sized.

¶ ¶

BAKER:
You can see it's maybe
50 feet deep or so.

¶ ¶

Potholes you see
in a normal river

are about the size of a person.

¶ ¶

Whereas this would hold
multiple elephants.

¶ ¶

NARRATOR:
To Vic, all the evidence
points to flowing water

on a massive scale.

But there is no water
flowing here now.

¶ ¶

Today, the largest rivers
in the region

are the Snake and the Columbia.

¶ ¶

But could they
have played a role?

BAKER:
The Columbia River lies
about 30 miles to the north

and isn't big enough
to make this kind of feature.

¶ ¶

NARRATOR:
The Snake River, as well,
is simply too small

to carve out potholes
and waterfalls on this scale.

To get such an immense volume
of water so fast,

we need something spectacular
to happen.

NARRATOR:
So where did the water
come from?

To answer this question,

scientists are looking
at a distant landscape

3,500 miles away.

¶ ¶

Iceland.

¶ ¶

This island, on the edge
of the Arctic Circle,

is a land of fire and ice.

¶ ¶

(water rushing)

In its northeast corner
are scars on the landscape

that bear a striking resemblance
to the Washington Scablands.

¶ ¶

Sheer cliffs over 300 feet high.

¶ ¶

A towering rock island.

¶ ¶

This is the Ásbyrgi Canyon.

¶ ¶

And like scientists
in the Scablands,

geomorphologist Mikael Attal

wants to understand
how it was created.

¶ ¶

This really looks
like a dry waterfall.

¶ ¶

It's as if
there was a big waterfall here

and it's not there anymore.

¶ ¶

NARRATOR:
Its so similar to the Dry Falls
in the Scablands,

Mikael suspects they were formed
in the same way.

¶ ¶

But how long did it take?

¶ ¶

To find out, he's using
a relatively new technique

called surface-exposure dating.

¶ ¶

Earth's surface
is constantly bombarded

by cosmic rays from outer space.

Rocks buried in the Earth
are sheltered from these rays,

but as soon as the rocks are
exposed, like in these cliffs,

cosmic rays collide with atoms
at their surface.

(water rushing)

The force of these collisions

knocks neutrons and protons
out of the atoms

and changes the elements
in the rocks.

This leads to the formation
of new elements,

including a rare form of helium.

These rare helium atoms
build up over time,

at a predictable rate,

so by measuring
their concentration,

it's possible to determine
how long

the rock has been exposed.

It's like starting
the stopwatch.

¶ ¶

NARRATOR:
Mikael samples rocks
from all over the Ásbyrgi Canyon

and compares the dates
when they were exposed.

His results revealed
something surprising.

¶ ¶

All the rocks in this area
were exposed at the same time,

meaning that this entire canyon
was carved out all at once.

ATTAL:
This canyon was created
in one event

9,000 years ago.

(wind howling)

NARRATOR:
A slow-moving force,
like a glacier, erosion,

or gradual uplift,

would have exposed the rocks
along the canyon

at different times.

¶ ¶

So it had to be a fast-paced
natural disaster,

like a titanic flood.

(water rushing)

ATTAL:
It would have been a flood
on a scale

far greater than anything

that we have witnessed
in human history.

(water rushing)

NARRATOR:
Thousands of miles away,
scientists in the Scablands

had zeroed in on the same idea.

¶ ¶

BAKER:
Everything in this landscape
was screaming,

in terms of its signs or clues,

that this was made
by catastrophic flooding.

¶ ¶

NARRATOR:
A flood big enough to carve
these vast landscapes

seems impossible,

but flowing water can be
surprisingly powerful.

(man yelling in distance)

The physical impact of a flood

rises with every increase
in volume, speed, or duration,

and it doesn't take a lot
to pack a punch.

A flood just six inches deep

can knock people
right off their feet,

and a flow of just
seven miles an hour

can have the same force
as a tornado.

¶ ¶

(water rushing)

But is it possible to blast
through solid rock?

Could a flood carve out
enormous features like these?

(water rushing)

The most destructive floods,

from Hurricane Katrina in 2005

to floods in Colorado in 2013,

have etched painful memories.

But they have done little
to make a mark

on the underlying bedrock...

¶ ¶

Like we see here in Iceland

and the Scablands.

BAKER:
No one has ever witnessed
anything

even close in scale.

¶ ¶

ATTAL:
We're talking about floods here

that completely changed the face
of the planet.

¶ ¶

NARRATOR:
Floods powerful enough

to carve whole canyons
out of bedrock

are rarely seen.

¶ ¶

But in 2002, one was finally
caught on camera.

(sirens blaring)

On July 4,

after a severe storm
struck central Texas,

Canyon Lake Reservoir flooded,
overtopping its dam.

At its peak,

enough water to fill
an Olympic swimming pool

poured over every two seconds.

And when the floodwaters
subsided,

they revealed a brand-new gorge,

carved into the rock
23 feet deep

and more than a mile long.

This provided proof
that floods

can transform whole landscapes
in a matter of days,

as long as there's enough water
flowing quickly enough

to produce the necessary force.

(water rushing)

(car tires grinding surface)

And in Iceland,

geophysicist Magnús Gudmundsson
thinks he's figured out

how such a massive release
of water could occur.

¶ ¶

He's come a hundred miles
south of Ásbyrgi

to the Vatnajökull ice cap,

the largest glacier in Europe,

similar to glaciers
at the end of the last ice age,

9,000 years ago.

GUDMUNDSSON:
This glacier is the only
possible source of water

to create these floods
that made Ásbyrgi.

¶ ¶

NARRATOR:
In places, it's 3,000 feet deep,

hundreds of cubic miles of water
locked up as ice.

GUDMUNDSSON:
We have all this ice here,

but how does it become a flood?

NARRATOR:
Magnús believes the secret lies

in what's hidden
beneath the ice cap:

seven huge volcanoes.

In 1996, one of these,
the volcano called Grímsvötn,

erupted, triggering the most
catastrophic flood in Iceland

for nearly a century.

GUDMUNDSSON:
From time to time, we have
these very large eruptions

that melt enormous amounts
of ice in a matter of hours.

(water rushing)

NARRATOR:
Almost a cubic mile
of meltwater from the eruption

tore across the landscape

at 16 times the rate
of Niagara Falls,

destroying roads, bridges,
and power lines.

¶ ¶

So today, Magnús is monitoring
the volcano.

¶ ¶

By recording elevation
and movement,

his team has discovered
there is a lake of meltwater

beneath the ice.

GUDMUNDSSON:
You're actually standing on an
ice shelf floating on the lake.

¶ ¶

NARRATOR:
Magnús now believes
that 9,000 years ago...

(volcano erupting)

...a giant eruption
under the ice cap

unleashed a colossal flood

powerful enough
to carve the Ásbyrgi Canyon.

¶ ¶

So could a chain of events
like this have also triggered

a flood massive enough to carve
the Scablands in North America?

(water rushing)

¶ ¶

Just a hundred miles
west of the Scablands

are many active volcanoes,

notably Mount St. Helens,

partially covered in ice.

¶ ¶

(erupting loudly)

In 1980,
Mount St. Helens erupted,

releasing an enormous amount
of heat,

enough to melt the ice
around its crater

and trigger dramatic floods.

¶ ¶

But even if all the ice on this
volcano had suddenly melted...

(water rushing)

It would not be enough

to carve out rock
over a 16,000-square-mile area.

¶ ¶

But what about in the past?

To find out if ancient ice
were to blame,

Vic Baker needs to find out
when the landscape was created

with the technique
of surface-exposure dating

used in Iceland.

(tapping)

Taking samples from all over
the Scablands,

he discovers most of the rocks

were exposed within a few
thousand years of each other,

and one date in particular
stands out.

¶ ¶

BAKER:
Many of the dates we get

are in the range
of about 16,000 years ago.

¶ ¶

NARRATOR:
Although ice sheets covered
much of North America

16,000 years ago,

geologists believe
the ice stopped short

of the volcanoes
of Washington state.

¶ ¶

The trail of clues
seemed to dry up...

¶ ¶

...until a surprising discovery

200 miles east of the Scablands,
in Missoula, Montana.

¶ ¶

Here, geologist Larry Smith
is heading up into the hills,

where he sees a series
of horizontal lines

a thousand feet
above the valley floor.

SMITH:
They look very much

like they'd have been cut
into the hillside

by waves beating
against the rock.

These lines are clearly
lake shorelines,

and show that an immense
body of water temporarily filled

these now-dry valleys
of western Montana.

¶ ¶

NARRATOR:
Tracing these ancient shorelines
for hundreds of miles,

geologists have calculated that
these valleys were once filled

by a body of water
larger than Lake Ontario.

¶ ¶

SMITH:
When there was a lake here
at 4,200 feet,

we would have had a beach
right here in front of us,

or a shoreline,

and extending all the way across
to the other side of the valley,

with a thousand feet of water

over what is now
the city of Missoula.

NARRATOR:
Geologists call it
Glacial Lake Missoula.

And despite being 200 miles away
from the Scablands,

Larry suspects it held
enough water

to tear through the area.

¶ ¶

That is a vast amount of water,

and if this lake drained
very rapidly,

it would be fundamental

to carving the channel
in Scabland.

¶ ¶

NARRATOR:
But today, there is
no lake here

because the valley
is open-ended.

¶ ¶

So where are the formations
that held the water in place?

¶ ¶

SMITH:
So the question is, is

where, when, and how did
a dam form to create this lake?

¶ ¶

NARRATOR:
Searching for clues,

Larry travels back down
to Clark Fork,

at the narrow end of the valley.

He sees no signs of landslides
or rock falls

that could have dammed the lake
in the past.

But on the bare rocks, he spots
some tell-tale markings.

SMITH:
You see scratches
within the rock,

geologically it is impossible

to smooth off rock and
scratch it without glacial ice.

¶ ¶

NARRATOR:
As the glacier moves,
rocks embedded within it

scratch the bedrock
like sandpaper.

So these scratches are evidence
that during the last ice age,

a glacier moved
across this valley.

And by mapping where rocks
have been scratched,

geologists have discovered
that the Clark Fork River Valley

was once blocked
by a giant finger of ice

23 miles wide
and half a mile deep.

Larry Smith believes this
ice dam created Lake Missoula.

SMITH:
It blocked the drainage

of the Clark Fork River.

The water had nowhere else
to go,

so backed up a lake behind this
large glacier in this valley.

¶ ¶

NARRATOR:
All the evidence points
to a massive reservoir of water

held in place
by a giant dam of ice,

a lake large enough

to have carved out the canyons
of the Scablands...

(water lapping gently)

...if it were released
in one catastrophic event.

(water rushing)

¶ ¶

(birds chirping,
insects buzzing)

The idea that there
had once been a lake here

that had suddenly drained

also explains one of the other
striking features

of the valley floor--

giant ripples.

¶ ¶

SMITH:
These straight crested hills
are current ripples

that show water flowing
from where we're standing

off to the distance.

¶ ¶

NARRATOR:
Ripples like this
are made by flowing water,

like the tide moving in and out
on a beach.

The faster the flow of water,

the larger
and more widely spaced

the ripples become.

Here, they're giant things

that are spaced
hundreds of feet apart

and they're tens of feet high.

NARRATOR:
These ripples are so high,
the lake water that created them

must have poured
through this valley

at speeds of up to 80 miles
an hour.

¶ ¶

(water rushing)

It's evidence that Lake Missoula

was unleashed rapidly
in a massive flood.

¶ ¶

But that means the 23-mile-wide
ice dam holding it in place

must have suddenly given way.

(ice crumbling)

How could an ice dam
of this scale

fail so catastrophically?

¶ ¶

The exposure dates of the rocks
in the Scablands

reveal the flood occurred well
before the end of the Ice Age.

This rules out gradual melting
from a warming climate.

¶ ¶

Could looking at modern
dam failures hold a clue?

¶ ¶

MAN (over radio):
You people down the stream
better get out.

NARRATOR:
In 1976,

the newly constructed
300-foot-high Teton Dam,

in Idaho,

failed,

unleashing almost
80 billion gallons of water.

¶ ¶

Investigators discovered

that water had seeped
under the earth-filled dam,

eroding it from below.

Larry Smith believes water
seeping under the ice dam

also caused the catastrophic
release of Lake Missoula.

¶ ¶

SMITH:
At the bottom
of this 2,000-foot-deep lake,

the water pressures are immense,

and any small cracks in the ice
will get penetrated

by that high-pressure water.

In doing so, that'll expand
that crack network

to form tunnels under the ice.

(water flowing)

NARRATOR:
Lake water began draining
through these tunnels

at a faster and faster rate...

(water rushing)

...until the whole ice dam
suddenly collapsed.

(cracking loudly)

(water rushing)

SMITH:
It falls within minutes
to hours,

with a cascade of water
coming through the area.

(wind howling)

NARRATOR:
All signs point to Lake Missoula

being the source
of a catastrophic flood.

¶ ¶

Still, how likely is it
that floodwater could travel

hundreds of miles southwest
to the Scablands

with enough power to carve out
solid rock

and transform
the entire landscape?

¶ ¶

¶ ¶

Roger Denlinger studies
fluid dynamics.

He's taken the volume
of ancient Lake Missoula

and 3D maps of the Scablands

to build a computer model

that will predict
where the ancient flood

would have traveled.

Effectively, you're just pouring
water over the landscape.

This is simply water flowing
over the Earth's surface,

and it's going to always head
in the direction

that it sees as downhill.

¶ ¶

NARRATOR:
Roger's model
will also determine

the depth of the water.

And this color bar shows
the flood's erosive power.

¶ ¶

If flow lines in the model
turn red,

Roger knows the water
was flowing with enough force

to carve solid rock.

At this point,
we're going to break the dam.

¶ ¶

(ice cracks sharply)

(water rushing)

NARRATOR:
The moment it's released
from the ice dam,

the lake water rushes
southwest,

toward what we know today

as the eroded landscape
of the Scablands.

¶ ¶

And not only that,

the places that the model
has highlighted in red,

where the power of the flood
is greatest,

exactly match the location

of the most dramatically
transformed landscapes today:

¶ ¶

the Dry Falls;

¶ ¶

the rock islands,

¶ ¶

and the sheer gorges.

¶ ¶

We get damage to the surface
in exactly the areas

that we see today.

¶ ¶

NARRATOR:
Roger's model
supports the theory

that a giant flood
from Lake Missoula

carved this landscape,

and reveals that the waters
reached unimaginable heights.

This water came up 800 feet--
that's huge.

(water crashing)

BAKER:
Most people think of floods
by watching the TV

and they see the water rising
in a river,

and they see a house
going underwater,

maybe there's a person
on top of the house.

Think of water hundreds of feet
above the house,

that's the difference
in the scale of this flooding.

NARRATOR:
Bringing all the evidence
together,

scientists can now unpack
the catastrophic flood

blow by blow.

Around 16,000 years ago,

the vast ice dam holding back
Lake Missoula failed,

suddenly unleashing
500 cubic miles of water.

(cracking loudly)

(water rushing)

SMITH:
It was equivalent in volume

to ten times all the rivers
of the world's natural flow.

NARRATOR:
The raging torrent
tears across Washington state,

ripping out billions
of tons of rock

from the once-flat landscape.

BAKER:
There would be blocks of ice,

there would be boulders,
there'd be roiling water,

the sound would be overwhelming.

NARRATOR:
In a matter of hours,

the flood reaches
the Pacific Ocean,

carrying with it 1,200
cubic miles of rock and earth,

violently torn
from the Scablands.

¶ ¶

SMITH:
To do all this landscape change

within a few days to a few weeks

is just mind-expanding.

¶ ¶

BAKER:
Even Hollywood disaster movies
do not compare

to what would have happened

as this flood came across
the landscape.

(water rushing)

¶ ¶

NARRATOR:
This dramatic event entirely
reshaped the landscape.

But there is one final twist
in the tale.

¶ ¶

Further research,
based on core samples

drilled out from the floor
of the Pacific Ocean,

suggests that the Scablands
are a product not of one,

but of many floods.

The evidence reveals
that during the Ice Age,

beginning around
20,000 years ago,

repeated floods
tore across the landscape...

¶ ¶

...as the giant ice dam
repeatedly broke,

reformed, and then broke again.

¶ ¶

BAKER:
The circumstances that created

this immense volume of water

produced multiple floods.

(water rushing)

¶ ¶

NARRATOR:
Decades of geological
detective work

show that the scarred
and eroded landscapes

of Washington state,
as well as Iceland,

both bear the fingerprints
of mega-floods.

And now, this discovery
is helping scientists unravel

a mystery in another part
of the world.

¶ ¶

Thousands of miles away
is the channel that separates

what is now England from France.

¶ ¶

Today, it links the North Sea
in the east

to the Atlantic Ocean
in the west.

Called the English Channel,

it's the busiest shipping lane
in the world.

¶ ¶

And towering more than
350 feet above it,

on the south coast of England,

are the White Cliffs of Dover.

¶ ¶

Geologists, like James Lawrence,

now think
these iconic chalk cliffs

hold an extraordinary secret,

and he's going over the edge
to hunt for the evidence.

¶ ¶

Because these cliffs
look almost identical to cliffs

on the other side
of the channel,

on the northern coast of France.

(seagulls squawking)

LAWRENCE:
People don't realize

that if I was to go over
to France,

we could find similar
chalk cliffs.

NARRATOR:
This chalk formed
100 million years ago,

when this whole area
was covered by a tropical sea.

(waves lapping)

The ancient sea teemed
with microscopic organisms.

When they died,
their calcium-rich skeletons

fell to the sea bed.

Over time,
these built up in thick layers

and were compressed into chalk,

a kind of limestone.

LAWRENCE:
We are getting
exactly the same rocks

which have been deposited
in exactly the same environment

on this side of the channel,

and on the French side
of the channel.

(waves lapping)

NARRATOR:
And James is discovering

that the connection between
the cliffs in France and England

goes beyond the chalk itself.

¶ ¶

Embedded in the white chalk
are a series of horizontal bands

of a dark rock called flint.

Here I have a fantastic band
of flint.

¶ ¶

NARRATOR:
Flint, a form of the
mineral quartz,

is formed by changes
in ocean chemistry.

¶ ¶

But these changes occur
only occasionally,

resulting in these
distinctive dark bands.

¶ ¶

LAWRENCE:
These flint bands

are continuous
throughout the chalk.

¶ ¶

NARRATOR
This band of flint
runs through the entire cliff,

and there are dozens
running horizontally...

¶ ¶

...each one at a different level
in the chalk.

Taken together, these parallel
bands of dark flint

form a unique geological
fingerprint in the white cliff.

¶ ¶

What's extraordinary is that
the same geological fingerprint

is visible on the other side
of the channel.

(seagulls squawking)

So the chalk and the flint
in these cliffs forms a bar code

and is exactly the same
as the chalk and the flint

in the cliffs in France.

NARRATOR:
The spacing and levels of the
flint layers perfectly align.

¶ ¶

To James Lawrence, this raises
an extraordinary possibility.

So what we know from this
evidence is that a chalk ridge

once connected England
and France.

NARRATOR:
These flint layers tell us

that hundreds of thousands
of years ago,

a ridge of chalk
almost seven miles wide

once extended 21 miles
across the channel,

joining what is now Britain
to the European continent.

¶ ¶

LAWRENCE:
So it's quite incredible

to think that there
would have been a land mass

stretching across the sea.

(seagulls calling, wind blowing)

NARRATOR:
But this discovery
raises a brand-new mystery.

(chalk pieces falling)

Somehow, the cliffs
between England and France

have been separated over time.

¶ ¶

NARRATOR:
If Britain and France
were once joined,

what force separated them and
turned Britain into an island?

¶ ¶

Control, Maverick.

¶ ¶

NARRATOR:
While exploring the sea bed

of the English Channel,

geologist Jenny Collier
finds a telling clue.

¶ ¶

Four, five, six--
ten meters

in a split second.

We've got a really steep
drop-off in the topography

and it's the edge
of a really unusual landform.

NARRATOR:
Using sonar to measure
the depth of the channel,

Jenny is surprised to find what
appears to be a steep canyon

carved into solid bedrock.

¶ ¶

Sonar works by firing
sound waves at the sea bed.

The deeper the water,
the longer it takes the sound

to make the round trip.

¶ ¶

Jenny expected the channel floor
to be flat,

but the sonar has revealed
something far more dramatic.

COLLIER:
We've discovered

just an extraordinary
geological event,

right in the middle
of the straits.

¶ ¶

NARRATOR:
To learn more about
this major geological find,

she and her colleagues
took on a massive task.

¶ ¶

Using a more advanced
sonar system,

they are mapping
53 square miles of the channel,

to an accuracy of four inches.

(sonar beeping)

What this reveals
is a strange picture

of channels, rock islands,
and valleys,

carved nearly 300 feet down,
into the rock of the sea bed.

¶ ¶

I mean, we haven't got anything
like this in Europe.

There's really only one place
that has all of these features.

¶ ¶

NARRATOR:
Without the water, the landscape
beneath the English Channel

appears to have steep valleys
and islands carved into it.

It looks eerily similar
to the channeled Scablands

of Washington state.

But was this underwater
landscape

also created by a mega-flood?

COLLIER:
The only way you can dig out
islands into solid bedrock

is to have extreme water flows,

and that basically pointed us
towards,

this was yet another
catastrophic flood terrain.

(water rushing)

NARRATOR:
What the Scablands revealed is
that carving solid rock

requires a huge reservoir
of water to be trapped,

then released in a single
cataclysmic event.

¶ ¶

But today, the English Channel
flows between two open seas.

So how could a large enough
volume of water

have built up
to cause a mega-flood?

¶ ¶

Geologist Phil Gibbard
believes he has an answer.

And the evidence lies 120 miles
north of the English Channel,

on the coast of the North Sea,

at the bottom of these cliffs.

GIBBARD:
What we've got here

is a glacial deposit

which is from about
450,000 years ago.

¶ ¶

NARRATOR:
Deep, fine-grained deposits
like this

were laid down
across Northern Europe

as giant ice sheets
ground over rocks.

450,000 years ago, England
was in the grip of an ice age.

(ice grinding)

Ice sheets, hundreds of miles
across and a mile high,

reached down from Scandinavia.

They would have dammed the
northern edge of the North Sea.

To the south,
the intact ridge of chalk

between what is now
France and England

formed a natural dam.

Phil believes that meltwater

from the ice sheets and rivers
pouring into the North Sea

had nowhere to go.

A vast amount of water built up
behind the chalk ridge.

He sees the evidence
for this ice age reservoir

in the sea cliffs,

as thin horizontal layers
of silt.

GIBBARD:
The sediments are horizontal,
as you see.

That horizontality
can only be produced

in a lake situation,
a standing-water situation.

NARRATOR:
And not in a turbulent area,
like an ocean.

Phil has discovered
similar-looking formations

in other places
around the North Sea,

some a hundred feet
above sea level today.

¶ ¶

GIBBARD:
So this was a massive lake
on the scale

of the Great Lakes
in North America,

and this lake provides
the only possible source

for the mega-flood
that formed the Dover Straits.

NARRATOR:
Could this enormous reservoir,

a glacial lake,

have suddenly drained
to form the dramatic features

on the bed
of the English Channel?

And if so, how?

¶ ¶

COLLIER:
In order to carve
these features,

this rock ridge must have failed
very, very rapidly.

¶ ¶

NARRATOR:
But what could have caused this?

¶ ¶

How could the giant ridge
of solid rock

between France and Britain

have given way
so catastrophically?

(waves lapping)

A clue lies in the way chalk
reacts to water.

¶ ¶

LAWRENCE:
Having a glacial lake
in contact with a chalk ridge

would have saturated the chalk,
making it much weaker

and much more likely to fail.

NARRATOR:
When water soaks into chalk
and saturates it,

the chalk can lose
half its strength,

making it far more likely
to fail.

One of the problems
with the chalk being so weak

is that it will often lead
to cliff collapses,

like the one we can see
behind us.

¶ ¶

NARRATOR:
Every year, thousands of tons
of rain and wave-soaked chalk

collapse into the channel,

dramatically eroding
the coastline.

¶ ¶

Many geologists now believe
that during a previous ice age

almost a half million years ago,

water from the North Sea
reservoir

soaked the chalk ridge,
fatally weakening it.

¶ ¶

Once the lake was deep enough,

water began pouring over the top
of the ridge in a waterfall,

rapidly eroding
the waterlogged chalk.

(water rushing)

We'd have had initially
a small stream of water

coming over
the top of the rock ridge,

that would have
catastrophically crumbled,

with large amounts of rock
being removed

and more and more water
flooding through,

just running away with itself.

NARRATOR:
From the shape of the features
on the sonar,

Jenny estimates that
the floodwaters raced through

at a rate of about
264 million gallons a second.

¶ ¶

That's almost 60 times the flow
rate of the Mississippi River.

(water rushing)

You would have seen
a tidal wave overtopping

and washing a giant gorge
into that landscape.

NARRATOR:
The deluge crashed on,

breaking through the chalk ridge

linking today's
Britain and France,

before finally reaching
the Atlantic Ocean.

It was this cataclysmic flow
that created the English Channel

and began the process of erosion

that led to what's now Britain

becoming an island
for the first time.

¶ ¶

The clues in Iceland,

the English Channel,

and the Channeled Scablands
of Washington state

reveal that floods bigger
and more devastating

than anything we see today

have torn across and
helped shape Earth's surface.

BAKER:
These giant mega-floods
totally shaped a landscape

in a matter of days or weeks.

¶ ¶

NARRATOR:
But the question is:

could a flood on this scale
happen again?

(ice cracking softly)

The one thing all these
mega-floods have in common

is that they involve
huge volumes of ice melting

and being released

in one sudden burst.

(crashing loudly)

In Iceland,
a volcano beneath the ice sheet

could trigger a mega-flood
at any moment.

(volcano erupting)

¶ ¶

Fortunately, very few people

live in the Icelandic
flood zone,

and the huge volume of ice
needed to create glacial lakes,

on the scale of the ones
that carved the English Channel

and the Scablands,

can only build up
during ice ages.

(whirring)

¶ ¶

But there is one region
on Earth today

where stores of melting ice

still pose a major flood risk
to millions:

ice- and snow-covered mountains.

¶ ¶

SMITH:
Wherever you have glaciers,

you have a lot of water.

Wherever you have glaciers
in a mountain,

you have the high likelihood of
making a glacially dammed lake,

and those glacially dammed lakes
are unstable

and could drain
catastrophically.

¶ ¶

BAKER:
We're not going to get, today,

releases of water
like Lake Missoula,

that was 2,000 feet deep.

But we can get glacial lakes
that are a hundred feet deep,

and these will produce

really dangerous
and spectacular floods.

¶ ¶

NARRATOR:
Today, floods in populated areas
wreak untold devastation.

Faced with Hurricane Harvey's
impact on Houston

and the floods in Bangladesh,

it may seem that floods
could not get any worse.

But the vast floods of the past
carved huge features

into the very bedrock
of continents.

Those scars are a stark reminder

of just how destructive
floods can be.

¶ ¶

This "NOVA" program
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