Grand Coulee - Geological History

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 Ma (million years ago). The land periodically uplifted and subsided over millions of years giving rise to some small mountains and, eventually, an inland sea.

From about 10 to 18 Million years ago, a series of volcanic eruptions from the Grand Ronde Rift, near the Idaho/Oregon/Washington border began to fill the inland sea with lava. In some places the volcanic basalt is 2,000 metres (6,600 ft) thick. In other areas granite from the earlier mountains is still exposed. Many animals roamed the area including camel, horse and rhinoceros.

Between two million years ago the Pleistocene epoch, glaciation took place in the area. Large parts of northern North America were repeatedly covered with glacial ice sheets, at times reaching over 10,000 feet in thickness. Periodic climate changes resulted in corresponding advances and retreats of ice.

About 18,000 years ago a large finger of ice advanced into present-day Idaho, forming an ice dam at what is now Lake Pend Oreille. It blocked the Clark Fork River drainage, thus creating an enormous lake reaching far back into mountain valleys of western Montana. As the lake deepened, the ice began to float. Leaks likely developed and enlarged, causing the dam to fail. Suddenly 500 cubic miles of Lake Missoula, 10 times the combined flow of all the rivers in the world, were released in 48 hours.

This mass of water and ice, towering 2,000 feet (610 m) thick near the ice dam before release, flowed across the Columbia Basin, moving at speeds of up to 65 miles per hour. The deluge stripped away soil, cut deep canyons and carved out 50 cubic miles (210 km3) of earth, leaving behind areas of stark scabland.

Over nearly 2500 years the cycle was repeated many times. Most of the displaced soil created new landforms, but some was carried far out into the Pacific Ocean. In Oregon's Willamette Valley, as far south as Eugene, the cataclysmic flood waters deposited fertile soil and icebergs left numerous boulders from as far away as Montana and Canada. At present day Portland, the water measured 400 feet (120 m) deep. A canyon 200 feet (61 m) deep is carved into the far edge of the continental shelf. The web-like formation can be seen from space. Mountains of gravel as tall as 40-story buildings were left behind; boulders the size of small houses and weighing many tons were strewn about the landscape.

Grooves in the exposed granite bedrock are still visible in the area from the movement of glaciers, and numerous erratics are found in the elevated areas to the northwest of the coulee.

Early theories suggested that glaciers diverted the Columbia River into what became the Grand Coulee and that normal flows caused the erosion observed. In 1910 Joseph T. Pardee described a great Ice Age lake, "Glacial Lake Missoula", a glacier dammed lake with water up to 600 metres (2,000 ft) deep, in northwest Montana and in 1940 he reported his discovery that giant ripple marks 50 feet (15 m) high and 200–500 feet apart had formed the lake bed. In the 1920s J Harlen Bretz looked deeper into the landscape and put forth his theory of the dam breaches and massive glacial floods from Lake Missoula.

Of the Channeled Scablands, Dry Falls, one of the largest waterfalls ever known, is an excellent example (south of Banks Lake and visible in the image below).

It is probable that humans were witnesses, and victims, of the immense power of the Ice Age Floods. Archeological records date human presence back to nearly the end of the Ice Age, but the raging torrents erased the land of clear evidence, leaving us to question who, if anyone, may have survived. With the end of the last glacial advance, the Columbia settled into its present course. The river bed is about 200 metres (660 ft) below the Grand Coulee. Walls of the coulee reach 400 metres (1,300 ft) in height.