Grand Coulee (landscape)

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Grand Coulee
The Grand Coulee, below the Dry Falls. The stratification of the periodic lava flows is visible.

The Grand Coulee, below the Dry Falls . The stratification of the periodic lava flows is visible.

location Washington , USA
Geographical location 47 ° 37 '12 "  N , 119 ° 18' 27"  W Coordinates: 47 ° 37 '12 "  N , 119 ° 18' 27"  W.
Setup date 1965
Legal basis National Natural Landmark
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Looking north in the Grand Coulee
Basalt hardy in the Grand Coulee
Part of the Grand Coulee was dammed and filled with water as part of the Columbia Basin Project

The Grand Coulee is an ancient river bed in Washington State . This National Natural Landmark extends 60 miles (approximately 100 km) southwest from Grand Coulee Dam to Soap Lake and is divided into the Upper and Lower Grand Coulees by the Dry Falls .

geology

The Grand Coulee is part of the Columbia River Plateau . The area has granite as bedrock, which was formed 40 ... 60 million years ago deep in the earth's crust. Over millions of years the land was raised and lowered, so that some smaller mountains and finally an inland lake emerged.

From about 10… 18 million years ago a series of volcanic eruptions began to fill the inland lake with lava in the Grand Ronde Rift near the borders of today's states of Idaho, Oregon, Washington and Montana. In some places, the layers of volcanic basalt are 6,600 ft (about 2,000 m) thick. In other places the granite of the earlier mountains still comes to the surface. Many animal species animated the area at that time, including camels, horses and rhinos.

During the Pleistocene , about two million years ago, the area experienced a period of glaciation . Large parts of northern North America were repeatedly covered by ice sheets from the glaciers, which at times were more than 10,000 ft (approx. 3,000 m) thick. Periodic climate changes led to corresponding advances and retreats of the ice.

About 18,000 years ago, a large ice finger pushed into what is now Idaho and formed an ice dam at the site of what is now Lake Pend Oreille . He dammed the outflow of the Clark Fork River , creating a huge lake that stretched into the mountain valleys of western Montana. As the lake deepened, the ice began to flow. Occasionally, gaps opened up and widened, so that the dam finally broke. The 2,100 km³ of Lake Missoula was released within 48 hours - a gigantic flood poured out, ten times the amount of all rivers in the world today.

This mass of water and ice, which piled up about 2,000 ft (600 m) before the dam breach, flowed through the Columbia Basin at a speed of up to 105 km / h. The Flood washed away the soil, dug deep canyons and moved up to 210 km³ of earth; it left areas of bare desert.

Such a cycle was repeated several times for more than 2,500 years. The majority of the moving earth created new landscapes, but some of it was also washed into the Pacific. In the Willamette Valley in Oregon, south to Eugene, the cataclysmic floods deposited fertile soil and icebergs left numerous boulders from Montana and Canada. In what is now Portland, the water was up to 400 ft (122 m) deep. A canyon 200 ft (61 m) deep was dug into the boundary of the continental shelf. The net-like structure can be observed from space. Piles of rubble from the height of forty-story buildings were left behind; Boulders the size of small houses and many tons in weight were scattered in the landscape.

The scratches of the glaciers can still be seen in the exposed granite rocks and numerous erratic boulders can be found in the higher areas northwest of the Grand Coulee.

Early theories suggested that the glaciers diverted the Columbia River into what is now the Grand Coulee and that normal runoff caused the observed erosions. In 1910, Joseph T. Pardee described a large ice age lake, Glacial Lake Missoula, an ice reservoir up to 1,970 ft (600 m) deep in northwest Montana, and in 1940 he reported his discovery that gigantic dunes (50 ft (15 m) high and 200 ft (61 m) to 500 ft (152 m) apart) formed the sea bed. In the 1920s, J Harlen Bretz took a closer look at the landscape and continued Pardee's theory of dam breaks and the resulting massive ice age floods emanating from Lake Missoula .

In the Channeled Scablands , Dry Falls (south of Banks Lake ), one of the largest waterfalls ever known, is an excellent example.

It is possible that people were witnesses and victims of the immense violence of the Ice Age floods. Archaeological studies date the first human presence to the end of the Ice Age, but the raging floods apparently created a cleared landscape, so the question remains, who, if any, should have survived. With the end of the last ice advance, the Columbia River got its current course. The river bed is about 660 ft (200 m) below the Grand Coulee. The walls of the Coulee are 1,300 ft (396 m) in height.

Upper Coulee

The Grand Coulee is the longest and deepest canyon in eastern Washington. Its unique features include the lower level at the entrance compared to the exit and the widest and highest dry cliff in the middle. It was created through the process of erosion on a waterfall that causes the terrain to recede; the former waterfall was twice as high as the existing Dry Falls.

The Grand Coulee consists of two canyons with an open pool in the middle. The Upper Coulee is filled by Banks Lake and is 25 mi (40 km) long with 800 ft (244 m) to 900 ft (274 m) high walls. It connects to the Columbia River at Grand Coulee Dam and runs south through the surrounding highlands. The entrance to the Coulee is 650 ft (200 m) above the Columbia. It was once the beginning of the course of the Ice Age Columbia River. The Okanogan lobe of the Wisconsin ice sheet extended southward over the course of the Columbia River to the southern plateau, creating an ice dam. This dam held back the waters of the Columbia in Glacial Lake Columbia and later diverted water into what is now eastern Washington during the Missoula Floods, creating the Channeled Scablands .

The river did not find an existing valley in the Grand Coulee, so dug its own bed across the watershed, creating the Upper Coulee. The plateau is not level, but is characterized by folds of the Columbia River Basalt Group . The diverted water from the lake hit a monocline in the northwest , a steep bulge 1,000 ft (approx. 300 m) high. Lake Columbia rose above the mountain range on the higher side of the monocline. The new river should tumble over the edge, 800 ft (about 200 m) down into a wide plain where Coulee City and Dry Falls State Park are now .

Erosion by the waterfall
The Upper Grand Coulee began as a 200 m high cascade just north of what is now Coulee City. As the water flow eroded the surface, it eventually formed a waterfall. The waterfall continued the erosion into its hinterland (northward), creating the canyon. When the waterfall crossed the ridge to Lake Columbia, i. H. reaching the pre-glacial Columbia Valley, it finally disappeared, leaving the elongated notch. Today, water from Lake Roosevelt is being pumped 280 ft (85 m) from Grand Coulee Dam into Banks Lake, which acts as a compensating reservoir and provides water for irrigation.

The existence of the waterfall is evident from the lintel basin immediately south of Coulee City at its former beginning. This contains at least 300 ft (approx. 100 m) of rubble that is below the present surface. The river above the falls was shallow and much wider than the gorge. Therefore, it expanded beyond the actual waterfall, creating several lateral falls. These existed until the retreat of the main fall drained them of water. Northrup Canyon in Steamboat Rock State Park contains a dry waterfall that is as wide as Niagara Falls and three times as high. The Steamboat Rock , 880 ft (268 m) high and 2.6 square kilometers in area, is now as isolated collection, but most times he created two waterfalls. When the falls passed Steamboat Rock to the north, they came to a ganite base near the basalt rivers. Granite lacks the vertical structures of basalt and resists erosion from the waterfall. The granite structures remained as hills in the wide base of the Coulee. Some gravel bars are visible along Washington State Route 155 . They show countercurrents and eddies on the back of the rock structures.

Lower Coulee

The Dry Falls are the start of the Lower Grand Coulee. The Great Cataract formed the boundary between the Upper and Lower Coulee. The Lower Coulee stretches along the monocline to Soap Lake, where the canyon ends and the water drained into the Quincy Basin. The Quincy Basin is filled with erosion debris and mud from the Coulee. The Lower Coulee also created its own course through the plain. Evidence of this can be found in the sloping debris visible on the Hogback Islands in Lake Lenore and along Washington State Route 17 from Dry Falls to Park Lake. Numerous canyons acted as a distribution system for the amounts of water that flowed from the Upper Coulee. The distribution begins in the rugged basin below the Dry Falls and extends 15 mi (24 km) before reaching the Quincy Basin. One of the cataracts (Unnamed Coulee) was 150 ft (46 m) high and had three individual drains over a distance of about a mile (1.6 km). There was no actual river bed, the water came in on a wide plain. The debris deposits of the Quincy Basin represent only a third or even a quarter of the estimated 45.85 km³ of the land masses removed from the Grand Coulee and other connected coulees (Dry, Long Lake, Jasper, Lenore and Unnamed). Most of the rubble was transported beyond the Quincy Basin .

Todays use

The area around the Grand Coulee is a steppe with an average annual rainfall of less than 300 millimeters. Lakes Park, Blue, Alkali, Lenore and Soap are in the Lower Grand Coulee. Until recently, the Upper Coulee was largely dry.

The Columbia Basin Project changed this from 1952 by using the former river bed as a network to distribute water for irrigation purposes. The Upper Grand Coulee was dammed and converted into Banks Lake. The lake is filled by pumps from Lake Roosevelt (the reservoir behind the Grand Coulee Dam) and forms the first section of a 100 mi (161 km) long irrigation system. Canals, hoists and other dams are in use throughout the Columbia Basin and supply more than 600,000 acres (approximately 240,000 hectares) of agricultural land.

The water has turned the Upper Coulee and the surrounding landscape into a refuge for flora and fauna, e.g. B. for bald eagles , transformed. Recreation is a useful side effect and includes several lakes, mineral springs, hunting and fishing, and all kinds of water sports. The Sun Lakes State Park and Steamboat Rock State Park are the Grand Coulee. On the other hand, however, the flooding of the lake has destroyed a large area of ​​natural habitats and traditional hunting grounds, as well as displacing indigenous peoples .

See also

literature

  • J. Harlen Bretz: The Channeled Scabland of Eastern Washington . In: Geographical Review . tape 18 , no. 3 , July 1928, p. 446-477 , doi : 10.2307 / 208027 , JSTOR : 208027 (English).

Web links

Commons : Grand Coulee  - collection of images, videos and audio files

Individual evidence

  1. ^ Grand Coulee . In: nps.gov . National Park Service .  ( Page no longer available , search in web archives )@1@ 2Template: Dead Link / www.nature.nps.gov
  2. ↑ David Alt: Glacial Lake Missoula. And its humongous floods . Mountain Press Pub, Missoula, Mont. 2001, ISBN 0-87842-415-6 (English).
  3. ↑ Bruce Bjornstad: On the trail of the Ice Age floods. A geological field guide to the mid-Columbia basin . Keokee Books, Sandpoint, Idaho 2006, ISBN 1-879628-27-9 (English).
  4. ^ J Harlen Bretz: The Channeled Scablands of the Columbia Plateau . In: The Journal of Geology . tape 31 , no. 8 , 1923, pp. 617-649 , doi : 10.1086 / 623053 , JSTOR : 30066357 (English).
  5. ↑ Ted and Marge Mueller: Fire, Faults & Floods. A road & trail guide exploring the origins of the Columbia River basin . University of Idaho Press, Moscow, Idaho 1997, ISBN 0-89301-206-8 (English).
  6. ↑ a b c d e f g h i j k l Washington’s Channeled Scabland. In: Bulletin No. 45; J Harlen Bretz: Division of Mines and Geology, Department of Conservation, State of Washington; April 15, 1959.