Giant Floods and Channeled Scablands
When Ice Age glaciers blocked the Columbia River, the Spokane River and the Clark Fork river in Montana they formed Glacial Lake Columbia, Glacial Lake Spokane and Glacial Lake Missoula. The evidence of these lakes can be seen in the lakebeds visible in various road cuts in eastern Washington and lap marks on hills and gravel deposits in Montana. Glacial Lake Missoula, the largest lake, covered some 3000 square miles and was about 2000 feet deep.
|"J Harlen Bretz in numerous publications between 1923 and 1969 drew attention to the large scale flood origin of the Channeled Scabland region on the Columbia Plateau of southeastern Washginton. His early papers found little favor and the flood hypothesis won general acceptance only after a contest with a variety of hypotheses more in accord with uniformitarian principles. This contest lasted nearly forty years. The principal obstacle to acceptance of Bretz's hypothesis was the lack of any evident source for the immense quantities of water required, an objection only removed when the volume and rate of emptying of ice-dammed Lake Missoula was shown to be adequate (Pardee, 1942). Two of Bretz's later papers clearly illustrate the erosional and depositional features that resulted from Lake Missoula's outbreak flood." [Field Guide 2]|
At some time the ice dam blocking the Clark Fork broke, sending nearly 500 cubic miles of water across northern Idaho's Rathdrum Prairie and into eastern Washington. At 45 miles an hour and 10 times the flow of all the rivers in the world today, the raging floodwaters swept into the Spokane Valley and out across the loess-covered basalt plateau. Turbulent, powerful currents eroded the basalt, tearing out whole columns, eroding the loess and scouring the landscape.
Separated into three huge flows up to 600 feet deep the torrents carved the 20 mile-wide Cheney-Palouse Tract, the 14 mile-wide Crab Creek Channel and the 50 mile long Grand Coulee. As you can see on the map at right the waters created numerous cross channels. The flow is from upper right to lower left. The darker blue areas are where the rivers are today. Click on image for a detail view.
Blocked by the Horse Heaven Hills on the west and the Blue Mountains to the south the water raced to the Walula Gap where the Columbia River heads west to the Pacific Ocean. Unable to flow out the narrow gap as fast as it came in the flood waters were pushed back and reversed the flow of the Snake River all the way past Lewiston, Idaho. Temporary lakes formed in the scablands and silt, sand, and gravel settled out of the water. Eventually the water drained down to the Pacific and along the way flooded Oregon's Willamette Valley. Portland, Oregon, would have been under 400 feet of water at this time.
Geological evidence indicates that 89 or more such floods happened over the years as the ice dam blocked the Clark Fork, broke and re-blocked the river's flow.
There is some evidence that Moses Coulee to the west of Grand Coulee was formed before Grand Coulee as an earlier ice dam blocked the Columbia River and a flood channel was carved just east of Wenatchee, Washington.
In this LANDSAT satellite view of the area you can plainly see the darker channels and lighter islands of the channeled scablands. The very dark areas are various lakes and rivers.
So where did all the loess, dirt, sand, gravel and silt end up? Some of the material was deposited in the Willamette Valley in Oregon. Ice rafts even brought very large boulders downstream and as the rafts melted left them in the valley. Core samples from the Escanaba Trough just off the coast of California show that there were at least 12 major floods that were large enough to carry material down the Cascadia Channel and into the trough. Other deposits have been found in the Astoria Channel. See map at right.
Fresh water is usually less dense than sea water and stays on the surface where rivers flow into the ocean. When the river water is very cold and is carrying lots of material, as when the Ice Age floods occurred, the fresh water pushes down and under the sea water and follows channels on the ocean floor. Geologists call these flows turbidity currents. At the time of the floods the oceans were about 100 meters lower due to the amount of water frozen in glaciers and ice sheets covering the land. This allowed the turbidity currents to flow into the off shore channels more freely. It is thought that the majority of the material removed from eastern Washington is now in these channels.
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Here are some basic terms used on this page. Find more geology terms in the Glossary.
- Volcanic rock caused by partial melting of the Earth's crust.
- Catastrophic Principle
- A concept proposing that all geological features were formed by sudden, catastrophic events like volcanoes, earthquakes, floods and even asteroid hits. Most geologists today believe that certain catastrophic events may form features but most are formed by slow processes that happened in the past just as they do today (uniformitarianism). Also, Catastrophism.
- Channeled Scabland
- Area in Washington state where huge floods made channels in a large, deep basalt flow. Named by J Harlan Bretz during the 1920's in various publications. See also Glacial Lake Missoula.
- The study of the changes in landforms due to volcanoes,
earthquakes, weather, floods, etc.
- Coarse-grained igneous rock usually without obvious bands or markings.
- Ice Age
- A period in Earth's history when much of the continents are covered with ice sheets and glaciers.
- Uniformitarian Principle
- A concept proposing that all geological features were formed just as features are formed today. Sudden events like volcanoes, earthquakes, floods and asteroid hits are not to be considered when describing how features are formed. Also, Uniformitarianism.
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