Ice Age Floods in Washington: A Cybertour

This Cybertour explores the impact of the Ice Age floods on Washington state. It was written and curated by Cassandra Tate, and made possible by a grant from the Peach Foundation. Glenn Drosendahl provided the principal photography; geologist Bruce Bjornstad, author of On the Trail of the Ice Age Floods, served as guide and technical advisor.

Introduction

The sound of the great Ice Age floods would have been terrifying: some 530 cubic miles of water bursting through a wall of ice more than 2,000 feet high; roaring over Eastern Washington at speeds of up to 80 miles an hour; drilling deep crevices into ancient basalt, stripping away topsoil in some areas, piling it up in others, flinging boulders around like ping pong balls. The pressure on the earth’s crust probably triggered earthquakes and landslides. The floodwaters pounded down the Columbia Gorge and into the Pacific with enough force to dig channels into the ocean floor. Then a new ice dam formed, impounded more water, and eventually collapsed, unleashing new floods, in a process that was repeated again and again until finally, about 13,000 years ago, the most monumental cycle of hydraulic engineering on earth came to an end.

The very size of the floods made it difficult to recognize their impact. Iconoclastic geologist J Harlen Bretz (a former Seattle high school science teacher) first proposed in 1923 that the arid “scablands” of Eastern Washington had been carved out by a rapid, massive flood -- not, as most of his contemporaries believed, by the ordinary processes of erosion by wind and water, over millions of years.

His ideas were ridiculed for decades. Aerial photographs in the 1950s vindicated Bretz to some degree by revealing the outlines of flood features that were too large to be easily identified from the ground. But it was not until the 1970s that satellite photos, taken from a vantage point 570 miles overhead, provided enough of a perspective to convince the last of the skeptics.

Even Bretz underestimated the scale and frequency of the floods. Geologists now believe that the Northwest was pummeled by a hundred or more “megafloods” during the last Ice Age alone, beginning about 20,000 years ago. There were at least a dozen earlier periods of major glaciation, stretching back for two million years, and flooding may have occurred during each of them.

The primary source of the more recent floods was Glacial Lake Missoula, a huge inland sea created when a lobe of the Cordilleran Ice Sheet crept south from Canada and blocked the drainage of the Clark Fork River in Idaho. Water backed up behind the dam into western Montana, eventually covering some 3,000 square miles of land to a depth of up to 2,000 feet.

Ice makes a flimsy dam. Studies of ice-jammed lakes in Alaska and elsewhere have shown that when the level of water behind an ice dam reaches a certain point, the ice floats, allowing water to escape from underneath, first in a trickle, then in a torrent. That’s one explanation for the failure of the dam holding back Lake Missoula. Another is that highly pressurized water at the base of the dam permeated cracks in the ice. The cracks widened into tunnels at an exponential rate, and the dam caved in on itself.

Once the dam was breached, Lake Missoula was uncorked as explosively as a bottle of fiercely shaken champagne. An estimated two trillion tons of muddy, ice-laden water -- a volume equal to 10 times the combined flow of all the rivers in the world today -- tore through the remnants of the dam and thundered over parts of four states on a 630-mile journey to the sea. The lake drained in two or three days. Almost as soon as it was emptied, ice blocked its outlet again and the lake began refilling, in a cycle believed to have been repeated every few dozen years or so.

The Ice Age floods rearranged the dirt, rocks, streambeds, and contours of 16,000 square miles of the Pacific Northwest. They pared land down to bedrock in places, built it up in others; rerouted rivers; scoured out coulees and potholes; and steepened and widened the Columbia Gorge. Enormous boulders, encased in ice, bobbed along on the floodwaters, ending up on hills and in flatlands hundreds of miles from their origins, a puzzle for the farmers who would someday plow around them.

Temporary lakes formed behind natural constrictions along the course of the Lower Columbia River at Wallula Gap, Rowena Gap, and Kalama. Flood-borne sediments settled out of those lakes, becoming part of the rich topsoil that supports the farms and orchards of Yakima, Walla Walla, and the Willamette Valley today.

The sediment is also the basis for the quality of wines produced in the Mid-Columbia Basin. Wine grapes require soil that is both fast-draining and water-retentive, characteristics provided by the fine-grained sand and silt in the flood deposits. Wheat grows tall in the areas of the Palouse that the floods skipped over, leaving intact the deep layers of windblown silt and soil that once covered most of the Plateau.

Thousands of years after the last drop drained away, the geologic legacy of the floods continues to define the Northwest, from the way the land looks to the uses humans have found for it.

HistoryLink.org Map by Marie McCaffrey

HistoryLink.org map by Marie McCaffrey

Historylink.org map by Marie McCaffrey

This animation depicts the general pathway of Ice Age floods in Washington state; the Cordilleran Ice Sheet is shown in turquoise.

Historylink.org animation by Marie McCaffrey from maps prepared by Steven I. Dutch

Drumheller Channels, Columbia National Wildlife Refuge, July 2007

Photo by Glenn Drosendahl

The Ephrata Fan, a flood bar pocked with boulders, July 2007

Photo by Glenn Drosendahl

Ice-rafted granite boulder, Quincy Basin, near Othello, July 2007

Photo by Glenn Drosendahl

Ice Age flood deposits at Hangman (Latah) Creek, near Spokane, 1994

Photo by Steven I. Dutch

Construction site yields mammoth tusk at Moxee, near Yakima, 2001

Courtesy Yakima Valley Museum

Frenchman Coulee, classified by geologists as a dual coulee and cataract system, July 2007

Photo by Glenn Drosendahl

Potholes Coulee, near Quincy, July 2007

Photo by Glenn Drosendahl

Giant current ripples at West Bar, July 2007

Photo by Glenn Drosendahl

Plunge pool at Dry Falls, July 2007

Photo by Glenn Drosendahl

Shallow caves above Lake Lenore, Lower Grand Coulee, July 2007

Photo by Glenn Drosendahl

Columbia River Gorge looking west from North Bonneville, Washington, December 2005

Photo by Glenn Drosendahl

Lighthouse at Cape Disappointment, December 2005

Photo by Glenn Drosendahl

Palouse Canyon near Palouse Falls, June 2005

Photo by Glenn Drosendahl

Wallula Gap, south of the confluence of the Columbia and Walla Walla Rivers, 2003

Photo by Glenn Drosendahl

Dry Falls, with seep lakes in the distance, July 2007

Photo by Glenn Drosendahl

Cape Disappointment at the mouth of the Columbia, n.d.

Courtesy Washington State Historical Society

Vineyard workers near Wallula, 2003

Photo by Glenn Drosendahl

Palouse Falls, Palouse Falls State Park, June 2005

Photo by Glenn Drosendahl

Columbia River Gorge, 1913

Photo by A. H. Barnes, Courtesy UW Special Collections (Image No. Barnes 1699)

Basalt cliffs above Lake Lenore, Grant County, July 2007

Photo by Glenn Drosendahl

Dry Falls, remnant of what was once the world’s largest waterfall, July 2007

Photo by Glenn Drosendahl

J Harlen Bretz (1882-1981), ca. 1949

Courtesy National Park Service

Archaeologists prepare to excavate mammoth tusk, Moxee, 2001

Courtesy Yakima Valley Museum

Participants in a geology field trip pose on a boulder deposited by Ice Age floods near Ephrata, September 2002

Photo by Ross A. Beyer, copyrighted 2002, used with permission

Flood-borne boulders at the mouth of Frenchman Coulee, 2003

Photo by Steven I. Dutch

Flood contoured hills south of Spokane, 1994

Photo by Steven I. Dutch

Flood-borne granite boulders on Frenchman Hills, Grant County, July 2007

Photo by Glenn Drosendahl

Seep lakes at Drumheller Channels, July 2007

Photo by Glenn Drosendahl

Head of a cataract, Potholes Coulee, 2003

Photo by Steven I. Dutch

University of Arizona faculty members and students study current ripples at West Bar, September 2002

Drumheller Channels, near Othello, July 2007

Photo by Glenn Drosendahl

One of several small ponds impounded at Potholes Coulee, 2003

Photo by Steven I. Dutch

Quincy Basin, looking east, 2003

Photo by Steven I. Dutch

Floodwaters plucked boulders from these basalt cliffs near Soap Lake, July 2007

Photo by Glenn Drosendahl

Dropstone (boulder that dropped out of an ice age flood) at Hangman Creek, 1994

Photo by Steven I. Dutch

Basalt columns, known locally as The Feathers, Frenchman Coulee, 2003

Photo by Steven I. Dutch

Mammoth tusk before excavation, Moxee, 2001

Courtesy Yakima Valley Museum

Overlook at Dry Falls, near Coulee City, July 2007

Photo by Glenn Drosendahl

Mouth of the Columbia River, 1929

Photo by Brubaker Aerial Survey, Courtesy National Archives, Pacific Alaska Region (Neg. RG77, Portland District L-224)

Monocline (folded basalt) behind Lake Lenore, Grant County, July 2007

Photo by Glenn Drosendahl

Wallula Gap, 1930s

Postcard

Beacon Rock, 2003

Photo by Glenn Drosendahl

Basalt cliffs in the Lower Grand Coulee, July 2007

Photo by Glenn Drosendahl

Palouse wheat fields, Columbia County, October 2003

HistoryLink.org Photo by Priscilla Long

View of Hangman (Latah) Creek from U.S. 195, near Spokane, 1994

Photo by Steven I. Dutch

Backwater flood deposits at Burlingame Gulch, near Walla Walla, 1994

Photo by Steven I. Dutch

Satellite view of the Channeled Scablands, August 1974

Courtesy National Park Service

Basalt outcropping near Dry Falls, July 2007

Photo by Glenn Drosendahl

Basalt bounders on the Ephrata Fan, July 2007

Photo by Glenn Drosendahl

Geologist Bruce Bjornstad and Historylink.org historian Cassandra Tate with ice-rafted boulders, Quincy Basin, July 2007

Photo by Glenn Drosendahl

Palouse wheat farm on Eagar Road, north of Dayton, Columbia County, October 2003

HistoryLink.org Photo by Priscilla Long

Ice Age Floods in Washington: A Cybertour

Related Topics: