Ocean Canyons

Channeled
Scablands

Grand
Coulee

Grand
Coulee
Dam

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Coulees and
Canyons

Flooding the Ocean

The basalt flows on the Columbia Plateau were so massive that some of the lava flowed down the Columbia river channel and out to the ocean. Haystack Rock (left) near Astoria, Oregon, is an example of this. Millions of years of activity by wind and waves have eroded the rock. The Columbia River has formed a delta (a fan-shaped network of channels) under the ocean as it has deposited silt and sand picked up along its journey from Canada. Floodwaters from Glacial Lake Missoula also entered the Pacific Ocean here.

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 may be in these channels.

Cascadia Channel and the Escanaba Trough Where else did all the loess (fine dust), 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.

Geology Terms

Here are some basic terms used on this page. Find more geology terms in the Glossary.

Bar: A mound of gravel and sand deposited by flowing water. Bretz and other geologists identified many large bars in the Channeled Scablands.
Basalt: Volcanic rock caused by partial melting of the Earth's crust.
Channel: The deepest part of a river or bay.
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 Channeled Scablands.
Erosion: Lifting and removal of rock, dirt, sand and the like caused by wind, water, or glacial ice.

Gravel: Rounded rock fragments larger than sand.
Ice Age: A period in Earth's history when much of the continents are covered with ice sheets and glaciers.
Jokulhlaups: A flood created when a body of water held by a glacial dam breaks through the confining walls. The Lake Missoula Floods were jokulhlaups.
Pleistocene: The period of geologic time that began about two or three million years ago and ended approximately 8,000 years ago.
Sediment: Collection of sand, silt, gravel and organic material that sinks to the bottom of a river, lake or ocean. Some or all of these materials may be present.

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