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.
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 the Grand Coulee. In some areas they were 10000 feet (3 km) thick. Grooves in the exposed granite bedrock are still visible in the area.
The Grand Coulee was formed during this period (starting about 2 million years ago) by water erosion. Some theories believe that glaciers diverted the Columbia River into what became the Grand Coulee.
Others believe it was created by massive water flows 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).
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.
Modern Uses
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The area surrounding the Grand Coulee is a desert, its average annual rainfall is 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.
External links
- The Geologic Story of the Columbia Basin, BPA site (http://www.bpa.gov/Power/pl/columbia/4-geology.htm)
- 3D images of the Grand Coulee, USGS site (http://3dparks.wr.usgs.gov/grandcoulee/html/thumbs.htm)