Great Lake Missoula Floods and the Channeled Scablands of Eastern Washington

An ice dam blocked the Clark Fork River in Montana

• The resulting lake was up to 2,000 feet deep and backed water up to over 200 miles.

• It held more water than Lake Erie and Lake Ontario combined!

What it would have looked like on a winter day at the ice dam

• The extent of the lake as it flood numerous mountain valleys

Ice Dam

Missoula

Old Lake Shores of Glacial Lake Missoula in Montana

Each line is a beach or shoreline of the lake level at that time

Glacial Lake Missoula

The Great Floods

Several times between 13,000-15,000 years ago, the ice dam retreated, freeing the entire volume of Lake Missoula to

flood out suddenly.

Jökulhlaups – Icelandic name for a Glacial Outburst Flood Event

As the water raced across the landscape…• It scoured the landscape, eroding canyons and valleys

• It rolled and rounded large boulders on the river bed for miles

• It carried large boulders on ice bergs hundreds of miles

• It backed up against hills to form large temporary lakes

Channeled Scablands• All across Eastern Washington are a series of channels and coulees where

the floods spread out and raced through, scouring rock on the cliff faces.

Channeled Scablands

Coulee – A deep, steeply sided ravine carved by ice age floods

Stream Capture

A Large Coulee

Eroded Landscape of the Floods

A Glacial Erratic – A rock deposited by floating on an glacial ice berg

Glacial Erratics

Dry Falls with rounded “river” rocks the size of boulders

A rounded “river rock” of basalt carried by the floods

Giant Ripple Marks

Giant Ripple Marks

Dry Falls• This was the largest waterfall in the world during the floods.

Water flowed over these falls at 65 mph and contained more water than all of the worlds rivers combined

• The water would have flowed so deep the 400 foot drop would have been almost unnoticeable.

Temporary Lakes• As the floods reached constrictions – the water would have

backed up and flooded areas for days or weeks until it could all get through.

Wallula Gap on the Columbia River• This narrow opening would have held back flood

waters to form large lakes in Eastern WA.

Lake Condon

• Water backing up as it tries to go through the Columbia River Gorge

A flood some 400 feet high raged through the Columbia River Gorge

Imagine the sound…You are standing at the western edge of the Columbia Gorge. You look to see a volume of water greater than all of the world’s rivers combined rushing through the gorge toward you…

Hanging Waterfalls

• The river tore through the rocks on the hillsides, leaving streams flowing down to the Columbia River hanging on the cliff sides.

Flooding the Willamette Valley• As the flood waters hit the low Willamette Valley, it spread

out and went up the valley all the way to Eugene (over 100 miles from Portland) to form a backwater pond.

Long-term Impacts of the Floods

• The floods deposited hundreds of feet of nutrient-rich sediments in the areas where the floods back up into temporary lakes – such as Lake Lewis in Eastern Washington and the Lake Allison in the Willamette Valley.

• Today, these are among the best growing regions for vegetables and fruit.

And Radioactive Rocks…• Radon is a radioactive gas formed by the decay of radioactive

elements like Uranium and Radium.

• Radon is the 2nd leading cause of lung cancer after smoking.

• Western Washington/Oregon does not naturally contain radioactive rocks. But, the northern Rocky Mountains do.

• The Missoula Floods moved those rocks down and deposited them in the Portland, OR area, resulting in a lot more radon than other areas nearby.