Grand Coulee
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The Grand Coulee is an ancient river bed in the U.S. state of Washington. It stretches for about sixty miles southwest of the Grand Coulee Dam ending at Soap Lake.
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[edit] Geological history
The Grand Coulee is part of the Columbia River Plateau. This area has underlying granite bedrock, formed deep in the Earth's crust 40 to 60 million years ago. The land periodically uplifted and subsided over millions of years giving rise to some small mountains and, eventually, an inland sea.
About 10 to 15 million years ago volcanic eruptions from the Cascade Mountains began to fill the inland sea with lava. In some places the volcanic basalt is 6000 feet (2 km) thick. In other areas granite from the earlier mountains is still exposed.
Two million years ago the Pleistocene epoch began and Ice age glaciers invaded the area. They scoured the Columbia River Plateau, reaching as far south as highlands above the Grand Coulee. In some areas north of the Grand Coulee they were as much as 3 km (10000 feet) thick. Grooves in the exposed granite bedrock are still visible in the area from the movement of glaciers and numerous glacial erratics in the elevated areas to the Northwest of the coulee.
Although early theories suggested that glaciers diverted the Columbia River into what became the Grand Coulee and that normal flows caused the erosion observed, geologists now agree that it was created by glacial diversion combined with massive glacial floods from Lake Missoula. This was a glacier dammed lake with water up to 2000 feet (600 m) deep. The dam periodically gave way, creating the Missoula Floods and sending huge amounts of water through the Columbia Basin as recently as 18 thousand years ago. These floods created the Channeled Scablands which are characterized by unique erosion features. The Dry Falls are an example (at the bottom of Banks Lake and visible in the image at right).[1][2][3][4]
With the end of the last ice age the Columbia settled into its present course. The river bed is about 600 feet (200 m) below the Grand Coulee.
[edit] Modern Uses
The area surrounding the Grand Coulee is a shrub-steppe with an average annual rainfall less than ten inches. Until recently, the Coulee was dry.
The Columbia Basin Project changed this in 1952, using the ancient river bed as an irrigation distribution network. Part of the Coulee was dammed and turned into Banks Lake. The lake is filled by pumps from the Grand Coulee Dam and forms the first leg of a hundred mile (160 km) system. Canals, siphons and more dams are used throughout the Coulee and surrounding area, supplying over 600,000 acres (2,400 km²) of farm land.
Water has turned the area into a haven for wildlife, including Bald Eagles. Recreation is a side benefit of the project and includes several lakes, mineral springs, hunting and fishing, and water sports of all kinds.
[edit] External links
- The Geologic Story of the Columbia Basin, BPA site
- 3D images of the Grand Coulee, USGS site
- WDFW - Wildlife of Eastside Shrubland and Grassland Habitats
- University of Washington Libraries Digital Collections – Lawrence Denny Lindsley Photographs Includes 146 images (ca. 1938 – 1958) of the Columbia River, Eastern Washington and the Grand Coulee region.
[edit] References
- ^ Alt, David (2001). Glacial Lake Missoula & its Humongous Floods. Mountain Press Publishing Company. ISBN 0-87842-415-6.
- ^ Bjornstad, Bruce (2006). On the Trail of the Ice Age Floods: A Geological Guide to the Mid-Columbia Basin. Keokee Books; San Point, Idaho. ISBN 978-1-879628-27-4.
- ^ J Harlen Bretz, (1923), The Channeled Scabland of the Columbia Plateau. Journal of Geology, v.31, p.617-649
- ^ Mueller, Ted and Marge (1997). Fire, Faults & Floods. University of Idaho Press, Moscow, Idaho. ISBN 0-89301-206-8.
| Glacial Lake Missoula | Missoula Floods | Channeled scablands | Grand Coulee | Dry Falls | Drumheller Channels | Columbia River Plateau | Wallula Gap | Touchet Formation | Lake Lewis | Columbia River Gorge | Columbia River Basalt Group | Palouse Falls | Sims Corner Eskers and Kames | Moses Coulee | Withrow Moraine | Crab Creek | Corfu Slide
