The Grand Coulee

Channeled
Scablands

Back to
Coulees and
Canyons

Grand
Coulee
Dam

Ocean
Canyons

Carving Basalt

The Grand Coulee is one of the most striking geological features in Washington. This huge channel cut by flowing water in an area where there is no river was a mystery to geologists like J Harlan Bretz who studied it in the early 1920's. The idea of a catastrophic flood was controversial at the time because most geologists thought changes in the Earth were gradual, described as the Uniformitarian Principle.

The map at right shows the area of Grand Coulee. Click on the image or here for a larger view (WA State Parks map)

USBR aerial view of Grand Coulee Dam When an ice dam blocked the Columbia River here, Glacial Lake Columbia was formed. This is where Grand Coulee Dam was built, also. When the ice dam holding Glacial Lake Missoula broke, floodwaters could not get past this ice dam and flowed south creating the Grand Coulee. Banks Lake reservoir partially fills the coulee. This USBR aerial view of the area shows where the ice age floodwaters traveled to create the Grand Coulee. Steamboat Rock (upper left of the photo) divides the channel. Steamboat Rock is a huge island of layered basalt in the channel of the eastern end of the coulee. There is a state park around the island.

Steamboat Rock

Steamboat Rock.

Basalt Cliffs
Layers of basalt on the west side cliffs. Find out how these layers were formed on the Basalt Lava page.

Click image or here for a detail view of the cliffs.

Talus slopes with huge boulders After the floods carved the basalt the walls of the coulee were clear of rocks and debris. Rock debris that has fallen from the sides of a cliff or steep slope is called talus. Through erosion by wind and ice the walls have built up talus slopes.

The picture at right is of some huge boulders eroded from the east side cliffs. The larger boulders are as big as a van or small truck.

The view below is from Banks Lake Dam. The earthen dam was built to hold water for irrigation. The water is pumped in from Lake Roosevelt behind Grand Coulee Dam.

Banks Lake fills most of the Lower Grand Coulee

Banks Lake fills most of the Lower Grand Coulee.
You can see the nearly vertical walls of the Grand Coulee.

At the lower end of Banks Lake which fills the coulee is Dry Falls, which is over 3 miles long and drops about 400 feet. By comparison Niagara Falls is 1 mile wide with a drop of 165 feet. You can see water in the plunge pools carved by the falling floodwaters. Click image or here for Dry Falls. (Jay Cousins Photo)

Geology Terms

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

Basalt: Volcanic rock caused by partial melting of the Earth's crust.
Catastrophic Principle: A concept proposing that all geological features were formed by sudden, catastrophic events like volcanoes, earthquakes, floods and even asteroid hits. Most geologists today believe that certain catastrophic events may form features but most are formed by slow processes that happened in the past just as they do today (uniformitarianism). Also, Catastrophism.
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.
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.
Talus: Rock debris that has fallen from the sides of a cliff or steep slope.
Uniformitarian Principle: A concept proposing that all geological features were formed just as features are formed today. Sudden events like volcanoes, earthquakes, floods and asteroid hits are not to be considered when describing how features are formed. Also, Uniformitarianism.

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