• Introduction
• Development of the Idea
• Three Processes
  • Erosion
  • Transport
  • Deposition
• Examples of the Channeled Scablands and related Flood Effects
  • Coulees & Waterfalls
    • Grand Coulee
    • Dry Falls
    • Palouse Falls
  • Spokane
  • Backwards Flows
  • Columbia River Gorge
  • Lake Oswego
  • Willamette Valley
• Web Links
• Glossary Terms

Introduction

The glacial Lake Missoula outburst floods had far-reaching effects on the Pacific Northwest, including:

• creating the Channeled Scablands of eastern Washington
• excavating the Grand Coulee and creating the great escarpment of Dry Falls in north central Washington
• creating Palouse Falls in southeastern Washington
• scouring out and steepening the sides of the Columbia River Gorge
• eroding channels and creating Lake Oswego in what is now Portland, Oregon
• creating a temporary lake that backed up into the Willamette River valley south of Portland
• depositing boulders from Montana and the Canadian Rockies as far as the Willamette River valley and Pacific Ocean
• creating the Spokane River aquifer that supplies water to the people of Spokane

Development of the Idea

It took many geologists a long time to recognize and accept the effects of the glacial Lake Missoula floods on the Pacific Northwest because the scale of the erosion and deposition caused by these events is so large. The geologist J Harlan Bretz first proposed a flood origin for the Channeled Scablands in the 1920s, but it was not until the 1950s and 1960s that his idea was fully accepted.

A big step toward confirming the flood origin of the Channeled Scablands took place when Jack Pardee, a geologist based in Montana, came forward with evidence of the existence of glacial Lake Missoula and its abrupt drainage. Pardee's work provided a plausible source for the giant floods proposed by Bretz.

Since the days of Bretz and Pardee (both now deceased), other geologists have shown that Lake Missoula burst through its glacial ice dam many times, causing repeated floods late in the last ice age, between about 15,000 and 13,000 years BP.

Three Processes

To understand the effects of the glacial Lake Missoula floods, think of three processes:

• erosion
• transport
• deposition

Erosion

The huge volumes of glacial Lake Missoula water that discharged suddenly across the Columbia Plateau had tremendous erosive power. Most of the Columbia Plateau was covered by loess--hills of loose, wind-blown silt. The loess had been deposited on top of the Columbia River Basalts-layer after layer of solidified basalt flows, each flow having vertical, separate columns in its bottom layer (the colonnade) and irregular, tight-fitting fractures in its upper layer (the entablature).

Erosion by the floods stripped away the loess first. Then it went to work on the basalt layers. The entablatures of the basalt flows are tougher and more resistant to erosion, so they held at first, but in many places erosion wore down through the entablatures and into the colonnades. The huge flows of surging water had an easier time eroding the loosely joined basalt columns of the colonnade, and excavated them rapidly. This excavation undermined the entablature, causing repeated collapses of big pieces of entablature.

The powerful sudden discharges of Lake Missoula plucked out the colonnades and undermined the entablatures of the Columbia River Basalts, eroding the deep, steep-sided coulees we see today in the Channeled Scablands.

Transport

Huge pieces of basalt and other eroded rock, some bigger than a car, were carried in the main floodways for some distance adjacent to areas where the currents in the torrential flow were focused to the highest rates and volumes of flow. Even in places where the strength of the flow was not enough to move boulders, large volumes of sand and gravel were moved along by the flow of the water, mainly at the base of the flow. Above the base, within the floodwaters, finer sediments were carried along in suspension.

Deposition

At places where the floods emptied from narrower flow channels into wider channels, or in slow eddies on the inside of curve, or where the floods spread out onto open plains, the flowing water slowed down and the larger pieces of sediment were dropped. Today in the Channeled Scablands, large chunks of basalt stand near the mouths of coulees that open onto broad basins, marking places where the flood slowed down.

In many locations at the base of the flowing water, deposits of sand and gravel were transported and worked into sand and gravel bars. The flowing water built ripples (wave-like forms) on the surface of the sand and gravel deposits. These bars and ripples built by the floods must be seen from an airplane to be fully appreciated, because they are so much larger than the sandbars and ripples that form in a typical river channel or beneath an average river flood.

Examples of the Channeled Scablands and related Flood Effects

Coulees & Waterfalls of the Channeled Scablands

A system of coulees and less deeply eroded flood channels extends from Idaho across much of the Columbia Plateau of eastern Washington and down the Columbia River to the Portland area. The main flood drainage system of the Channeled Scablands is so large that it is best seen in images from satellites. Lakes filled some of the channels, and in some places artificial reservoirs were created in what had been dry channels.

Grand Coulee

Grand Coulee in north central Washington is the largest coulee in the Channeled Scablands. It was created partly by the glacial Lake Missoula floods channeling through it. Grand Coulee grew from a smaller channel that had been originally eroded by the Columbia River. Before the glacial Lake Missoula outburst floods, the Columbia River was dammed by the Okanogan lobe of the Cordilleran ice sheet. The Columbia got around the ice dam by eroding the diversionary channel, which later grew into the Grand Coulee.

Dry Falls

The arid, steep cliffs of Dry Falls are near the south end of Grand Coulee. These once mighty waterfalls were formed when the glacially diverted Columbia River began to erode a thick layer of resistant basalt, forming a steep precipice in the channel of the river. The glacial Lake Missoula floods dramatically enhanced the erosive power, carving an even larger cataract. When the Okanogan lobe retreated, removing its dam of ice, the Columbia River went back to its old channel and the Grand Coulee and Dry Falls were left high and dry.

Palouse Falls

In what is now southeastern Washington, the floods eroded new channels through the basalt layers. The Palouse River subsequently occupied one of these flood channels. The river still flows over scenic Palouse Falls, a waterfall created initially by the glacial Lake Missoula floods.

Spokane and its aquifer

The glacial Lake Missoula floods enlarged the system of valleys from Coeur d'Alene, Idaho, west through Spokane. The city of Spokane is built in the path of one of the main flood outlets from Idaho into the Channeled Scablands. The floods filled the Spokane valley floor with sand and gravel deposits. Water from the floods, water from the nearby melting glacier and water from large amounts of precipitation (late Pleistocene lake levels indicate higher precipitation rates at that time than today), filled the sand and gravel, creating an aquifer. An aquifer is a plentiful deposit of underground water that can be tapped with wells for human use. This aquifer, left by the glacial Lake Missoula floods, provides water that the city of Spokane, its residents, and its industries rely on today.

Backwards flows of many floods

The Columbia Plateau forms a basin into which rivers from surrounding high areas drain. Water that flows into the basin drains into the Columbia River. The Columbia River is funneled through the Wallula Gap and continues through the Cascade Range into the Pacific.

During the glacial Lake Missoula floods, the huge volumes of water that surged into the Columbia basin backed up behind Wallula Gap and raced up the river valleys that emptied into the basin. These backwards flows created deposits of sand and gravel. The sand and gravel were overlaid by layers of silt and clay from the water that was left standing and slowly subsided after the initial surge. Cross-beds and other sedimentary structure in the sand and gravel show that the currents flowed in the up-river direction, consistent with the backwards flows created by the giant floods. These deposits repeat over and over again, one layer on top of another, indicating that there were many glacial Lake Missoula outburst floods.

Columbia River Gorge

The glacial Lake Missoula floods swept through the gorge where the Columbia River cuts through the Cascade Range. They filled the gorge to hundreds of feet deep, eroded and steepened its sides, and left behind the steep-walled scenery of the Columbia River Gorge today.

Lake Oswego

In the area that is now the city of Portland, Oregon, the glacial Lake Missoula floodwaters spilled out of the Columbia River Gorge and across the flatter land around the lower Willamette River. The floods eroded several new channels as they crossed the area. One of these flood channels remained filled with water and became Lake Oswego, in what is now the city of Portland.

Willamette Valley

The glacial Lake Missoula floods backed up into the Willamette River Valley, south of Portland. The floods carried icebergs that had been pieces of the glacial ice dam, which had impounded Lake Missoula. As the temporary lake in the Willamette Valley gradually receded, the icebergs gradually melted. Boulders carried in the icebergs fell to the ground. That is how large boulders known to have originated in Montana and the Canadian Rockies came to rest in western Oregon.

Web Links

Map of lobes of Cordilleran Ice Sheet http://www.uidaho.edu/igs/iafi/iafidesc.html

Details from the U.S.Geological Survey about ice sheets, glacial Lake Missoula, and the floods that formed the Channeled Scablands: http://vulcan.wr.usgs.gov/Glossary/Glaciers/IceSheets/framework.html

Glossary terms that appear on this page: erosion; transport; deposition; loess; basalt; colonnade; entablature; coulee; ice sheet; cross-bedded; sedimentary structure