Glacial Lake Missoula

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Speaker: This is Missoula, and the campus
of the University of Montana.

A terrific setting in the Rocky
Mountains, and ground zero for

much of the water for the Ice Age
floods of the Pacific Northwest.

Let's tell the story in a nutshell, and
then explore old shorelines, high energy

gravel deposits, and delicate silt beds
that all tell the incredible story of

Glacial Lake Missoula.

During the Ice Age, the valleys of Western
Montana were filled with 1,000 feet of

fresh water, Glacial Lake Missoula,
formed due to an ice dam in

Northern Idaho.

The Purcel Trench Lobe, that blocked the
Clark Fork River and its tributaries,

across the boarder in Montana.

The ice dam area, which we know
today as Lake Ponderay, was 2,000 feet

high, 30 miles long, and sealed off a
mountain valley, creating a backup of

lake water 200 miles to the east.

Like filling a bathtub with the drain
plugged.

A massive lake with long fjord-like
arms.

A southern arm that sat in the
Bitterroot Valley to Hamilton below

Trapper Peak.

An eastern arm, to Drummond.

A northern arm, into the Mission Valley,
and the Mission range.

As the water deepened behind the dam,
the pressure built against the ice sheet.

Eventually, the ice was no match for the
massive volume of water in the lake.

The dam failed quickly.

The lake drained quickly, just a few
days to drain and rush over the floors

of the Clark Fork River, and Flathead
River Valley.

The water barreled over Eastern
Washington, leaving deep cuts in

the desert, and moving tons of
rock from the Rocky Mountains

into Washington and Oregon.

And that was one Missoula flood.

But it happened again at least twice,
probably dozens of times, possibly as

many as 100 times.

The Purcell ice dam reformed, another
glacial Lake Missoula, and a new Ice Age

flood burst through Idaho when the lake
reached a critical depth.

Rinse, and repeat.

The floods took different routes based on
their size and local conditions.

In the Channeled Scablands of Eastern
Washington, thick deposits of loess,

wind-blown silt, were swept away.

A surprising amount of basalt bedrock
was removed by the Missoula floods,

leaving impressive box-shaped canyons,
like the Grand Coulee, with dry falls.

Fields of giant current ripples, huge
potholes drilled into the bedrock.

My God, how big were these floods?

Regardless of size, each floods put on its
brakes at Wallula Gap.

As the water funneled through the narrow
gateway, to the Columbia River Gorge.

That was Lake Lewis in Southern Washington,
a brief delay before the now dirty brown

water continued on through the
Columbia River Gorge, and on to

the Pacific Ocean.

Okay, that's the story.

It's almost impossible to believe, right?

What can we find in Western Montana to
prove that Glacial Lake Missoula

really existed?

Let's start with the obvious.

Ancient shorelines, strandlines, benches
on the hillsides created by wind blowing

across the surface of the old lake tells
us the water was 1,000 feet deep here.

But there's not just old old shoreline,
there are dozens of them.

Different lake levels.

For the Montana Valleys that had Glacial
Lake Missoula in them, the old shorelines

are best seen on Northwest facing slopes.

Like the hillside above the University of
Montana.

The big M.

Above campus.

620 feet above the town of Missoula.

This is only two thirds the way to the
top, the high water mark.

The highest strand line is more than 300
feet above us.

Hiking up this slope, you might expect
real obvious notches, benches, cuts, dug

into this slope, but they're subtle.

These old shorelines are more obvious
from a distance than hiking right on

top of them.

T.C Chamberlin was the first geologist to
note these faint watermarks in 1886.

He had read reports describing Scotland's
parallel roads of Glenroy, and correctly

interpreted the elevated shorelines here
in Montana.

Each Glacial Lake Missoula strandline
was created by lake waves eroding

into the hillsides.

But shorelines are also places of
deposition.

Beach gravels have been found.

Little beach berms.

So far, no preserved organic carbon,
or other dateable materials have been

found at the old shorelines.

So telling a decent story here is
difficult.

Missoula, the big M above campus, the
strandline's on the hillside, even though

we don't have specific dates, most
geologists agree that the highest

strandline is the oldest lake.

That the strandlines get younger as
you go down the hill.

The thinking is, if there was a young lake
up here, and then you drain the lake,

wouldn't you wipe out all these older
strandlines?

That's the thinking.

Older, highest.

Younger, lowest.

Yes, that's the thinking, but without
dateable material, even the most

basic questions remain.

Is each level a different Glacial
Lake Missoula?

Or is this one lake with periodic
lowering?

Or a combination of the two
somehow?

Without dates for each shoreline, it's
still unclear.

The highest strandline is at 4,200 feet
elevation.

On a steep hillslope, exposed to more than
10,000 years of thunderstorms, it's pretty

amazing how little eroded these
strandlines are.

At its maximum, Glacial Lake Missoula had
a surface area of 3,000 square miles.

The northern shorelines of the lake
sometimes had an ice margin.

Ice calved off into the lake.

Icebergs with big rocks in them that set
sail for various destinations in the lake.

Large boulders show where the big
rocks fell off their ice rafts.

Drop stones, back when the water of
Glacial Lake Missoula was relatively

calm and quiet.

Up north, impressive white lakebeds were
laid down close to the ice margin.

Rock flour, silts created from the
grinding power of the ice sheet to

the north.

Wildlife and dusty white deposits
everywhere.

The drain was plugged.

The lake was big.

And the white silts collected on the
quiet lake floor.

But there are also deposits that speak
of tremendous high energy events.

Mud is usually at the bottom of the lakes,
right?

Swim in your favorite lake, that means
dark mud is oozing up between

your toes!

But at the bottom of much of Glacial
Lake Missoula, deposits of rocks, not

mud, dominates on the valley floors
below the strandlines, why?

I'll bet you know why, right?

High energy water is recorded in these
valley bottoms, that's what the rocks

are telling us!

But when Glacial Lake Missoula was here,
it wasn't high energy, it was low energy,

the water's just sitting there, and layers
of mud and silt are being deposited

at the bottom.

But when we break the ice down, that
water starts moving.

Quickly, fast enough to erode all of those
soft beds at the bottom, and in their

place, a big batch of river gravels were
brought in from elsewhere and sit

at the bottom.

Deposited during the high velocity
flooding.

Rocks the size of my fist, or my head, or
even bigger!

So when we look down the guts of the
Clark Fork River Valley, it's high energy

river gravels in the bottom instead of
the mud!

Okay, make another ice dam, make another
lake.

[writing]

Lay down more silts and muds.

That's fine.

But when we break that ice down, the
water's on the move, and we erase those,

and bring in more river gravels.

All totaled, we have more than 300
feet of big flood deposited gravels at

the bottom of the Clark Fork River Valley.

How many floods does this represent?

Under the tranquil scene of trees and
flowers, the marbles from high

energy floods.

The fastest water probably peaked in
the first few hours during the ice

dam collapse.

The high energy gravels are piled
thick in places, where the water slowed.

Right after being shot through narrow
valley bottlenecks.

A giant flood bar at Tarkio, hundreds of
feet high, and more than a mile long, is

composed of fist-sized rocks.

In other places, water speeds were fast
enough to pluck car-sized boulders from

very hard bedrock.

Not enough to convince you?

Still not sure that Glacial Lake Missoula
drained in a hurry?

Well, how about these?

Giant current ripples.

Formed on the lake floor as the lake
drained quickly.

At Camas Prairie, individual ripples are
35 feet high, and spaced 100 feet apart.

Cobbles of river gravel shaped into these
impressive forms that developed under

more than 200 feet of water moving up
to 16 miles per hour.

Joseph Pardee was the first to study
these more than 75 years ago.

Four sets of ripples sit below four
separate spillways above Camas Prairie.

Western Montana's bedrock is different
than Washington's flood-scoured basalt.

Glacial Lake Missoula sat in sedimentary
bedrock created more than 1 billion

years ago.

It's pretty easy to visualize where the
lake rushed out of Montana, today's

Clark Fork River flows in the direction
that the Missoula floods flowed.

Rugged, vertical-walled canyons like
Eddy Narrows were particularly

energetic spots for the flood water.

Pockets of flood gravels remained stranded
high and dry inside canyons.

Gulch fills help show the depth and the
speed of the water as it ran the

gauntlet down stream to Idaho,
Washington, and beyond.

The deepest Glacial Lake Missoula was
just a few hundred feet shy of spilling

over the Bitterroot Mountains at Look
Out Pass, where Interstate 90 crosses

the Idaho/Montana state line.

Instead, the lake drained through
the Bitterroots using existing river

valleys to the north.

And once into Idaho, the flood swung to
the Southwest, over Spokane, and the

broad openness of Eastern Washington.

So much field evidence for Glacial
Lake Missoula is visible from I-90

between St. Regis, and Missoula.

At the exit for 9 Mile Road, one more
very important study site.

These are famous silty beds west of
Missoula.

Partly because we're still debating the
significance of them.

These are rhythmites, there's 40 of
them here.

With that zebra striping, what's the
story?

Why are these delicate silts still here if
this was a place where high energy

floodwater was cruising through.?

These repetitive layers of silt and mud
contain details with important clues.

But debate continues in what these
layers are telling us about Glacial

Lake Missoula's history.

Even the terms are confusing.

Rhythmites, varve's, are they the same
thing?

Not here at 9 Mile.

Interstate 90 from the freeway, you can
see the rhythmites.

Dark, light, dark, light, from the freeway
those are the zebra stripes.

But within one dark zebra stripe: varve's.

At a tinier scale.

Dark, light, dark, light.

Those are annual patterns.

Dark/light couplet, that's one
winter/summer pattern.

Varve's, rhythmites.

Many geologists see the more than 500
varve couplets here as annual layers.

Like counting tree rings in the mud.

But not everybody agrees that these
tiny layers are annual.

Why not seasonal storms, they say.

Or occasional debris flows into the
bottom of the lake?

But the tiny layers are so clean, some
people say.

Not a root, not a leaf, not a twig, not
a gopher hole, no tracks, no cut and

fill gullies, very little organic carbon.

At least everybody can agree the dark
zebra stripes, the dark rhythmites, were

deposited at the bottom of Glacial Lake
Missoula.

The dark bands are mud, the light bands
are silt, and the rhythmites get thinner

and thinner toward the top.

But that's it for agreement.

What do the light colored silty
rhythmites really tell us?

Do they record lake drainings?

Or lake fillings?

The 9 Mile rhythmites sit on top
of the high energy gravels that we

talked about earlier.

It's looking like the coarse gravels, not
these delicate rhythmites, are the

record for the truly huge floods, the big
draining's of Glacial Lake Missoula.

But how many big floods?

It's pretty tough to tease out individual
huge floods from a big pile of marbles.

With so much water speed, it seems
unlikely that these soft beds would

survive, especially since they sit in the
areas most deeply scoured

canyon stretches.

Are these beds at 9 Mile from the last,
and smallest Glacial Lake Missoula?

Almost an afterthought in the Ice Age
flood story?

A progressively smaller Lake Missoula
toward the end of the Ice Age is

consistent with these rhythmites that
progressively thin-up section, and have

decreasing numbers of varve's per
zebra stripe as you head up the slope.

And that agrees with the strandlines
getting lower and lower with time

above Missoula.

Each successive thinning ice dam existed
for less years resulting in a lower

ancient shoreline.

Flood magnitudes must have decreased
through time.

But did each dam collapse completely
with each big flood?

Or was there a slower release of water
that somehow tunneled through the

ice sheet?

Did each Glacial Lake Missoula drain
completely?

Are partial lake drains even possible?

It's tempting to correlate the rhythmites
of Glacial Lake Missoula with Northern

Washington's Glacial Lake Columbia, and
Southern Washington's Lake Lewis.

Are these the same beds?

Is each rhythmite from a major Missoula
flood from Montana?

Was each flood from Montana?

Are there other potential sources of
water to the north?

Bed for bed correlation is almost
impossible due to differences in the

character in the sediments.

The varved muds at the bottom of Glacial
Lake Columbia show many, many years of

lake water due to the Columbia River being
blocked by the Okanogan ice sheet, a

different plug.

A different bathtub.

And at the bottom of Lake Lewis in
Southern Washington, no varve's at all.

The lake down there lasted just a few days
at a time.

That bathtub had an open drain, Wallula Gap.

Emerging dates seem to suggest that some
of the huge floods struck earlier in the

Ice Age, we just don't have enough
dates to tell a more complete story.

Not yet, anyway.

Answers will come from the next generation
of field geologists, new dates are

trickling in.

Surface exposure dating techniques are
being used now on basalt bedrock and

Coulee walls, and on the surfaces of
erratic's, sitting in Washington's

channeled scablands.

As more dates emerge from across the
Ice Age floods country, some of the

mysteries that remain will be solved about
Glacial Lake Missoula.

Ideally, with new techniques used by
future geologists, new dates will come

from the floor of the old lake, and maybe
even from the strandlines up high

above Missoula.

Glacial Lake Missoula: where it all began,
and where unsolved mysteries still remain.

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