Made of up Adams, Benton, Douglas, Franklin, Grant, Klickitat, Lincoln, Walla Walla, and Yakima counties, this is one of the larger regions of the state. The descriptor Columbia River Plateau is a bit confusing in terms of describing this region of Washington that stretches across the southern border of the state, and up into eastern-central part of the state because the term plateau is very general. A plateau is a large flat area that is elevated in contrast to the area around it. Technically the geologic formation called the Columbia Plateau extends south into Oregon and east into Idaho, even though it is broken up by the Columbia River and Palouse mountain range. This plateau was formed by lava over millions of years.

Formation of the Columbia Plateau

During late Miocene and early Pliocene times, one of the largest flood basalts ever to appear on the earth’s surface engulfed about 63,000 square miles (160,000 km²) of the Pacific Northwest, forming a large igneous province. Over a period of perhaps 10 to 15 million years, lava flow after lava flow poured out, ultimately accumulating to a thickness of more than 6,000 feet (1.8 km). As the molten rock came to the surface, the Earth’s crust gradually sank into the space left by the rising lava. 

The subsidence of the crust produced the large plateau—a large, slightly depressed lava plain. The ancient Columbia River was forced into its present course by the northwesterly advancing lava. The lava, as it flowed over the area, first filled the stream valleys, forming dams that in turn caused lakes. Entities found in these lake beds include fossil leaf impressions, petrified wood, fossil insects, and bones of vertebrate animals.

Features of the Columbia River Plateau Region

The Columbia Basin Plateau Region is marked by the Washington side of the Columbia River Gorge, the Yakima Valley, and Mount Adams. These features help to define the region.

Columbia River Gorge

The Columbia River plays an important part in the history of Washington. From indigenous people who depended on the river for fishing and transportation to early European settlers who used the river as a way to transport goods and people inland from the Pacific Ocean, to the contemporary generation of hydro-electricity, the Columbia River has both shaped and been shaped by humankind. The Columbia River Gorge, which cuts through the Columbia River Plateau and Southwestern regions of the state, has its own fascinating geological history which begins at least forty million years ago. The following text is from the US Forest Service.

The natural story of the Columbia River Gorge begins with the fiery volcanoes of the Cascade Mountain Range. Over the centuries these volcanoes left lava and mudflows up to two miles thick. Remnants of these flows can still be seen in the cliffs of the Gorge. The Columbia River cut a deep canyon through the lava, ash and mud.

The greatest force in the Gorge’s creation was flooding. About 15,000 years ago, near the end of the last Ice Age, gigantic floods up to 1,200 feet deep swept down the river corridor and scoured its cliffs, leaving its tributary streams hanging high above the river’s bed. Today, many streams cascade down the cliffs creating one of the world’s greatest concentrations of waterfalls.

The walls of the Gorge were further altered by massive landslides.

Basalt is a black, fine-grained rock, and it makes up a lot of the bedrock (or solid rock level under the soil) in Washington. Basalt is a result of years of volcanic action, as it is a result of cooling lava.

These Basalt Cliffs show how thick the lava flows were in the formation of the Columbia River Plateau.

Yakima Valley

The Yakima Valley, in particular, is important in contemporary Washington, because of the rich soil used in cultivation of grapes and other crops. The soil of the Columbia Basin Plateau region is a major reason for the thriving agricultural industry in Washington State. A combination of volcanic activity and ice age flooding over millions of years left mineral rich sediment, which is the soil that helps to make the Yakima Valley so valuable for growing crops.

The Rain Shadow Effect

While the soil and warm summers are useful for growing crops such as apples, grapes, hops, cereal grains, and a variety of vegetables the Yakima Valley, indeed most of the Columbia River Plateau region must be manually irrigated because the natural climate does not include a lot of natural irrigation. In other words, the Columbia River Plateau is in a rain shadow. A rain shadow happens when a mountain range blocks rain causing weather on one side of the mountains. In the case of Washington, the Cascade Mountain Range acts as a barrier so that rainy weather is trapped on the western side of the state, while the other side of the mountains has less rain.

This picture of the rain shadow effect shows how the rain falls on the leeward (windy) side of the mountain. The mountain itself causes a barrier, making the clouds rise higher and higher, cooling the air before it can pass to the dry side of the mountain.

Mount Adams

About 900,000 years ago volcanic activity in what is now Yakima County began to form Mount Adams. However, Mt. Adams went through several cataclysmic changes to become the second highest peak in Washington State, behind Mount Ranier. Now the mountain towers 12,281 feet above sea level and is considered an active volcano, even though it last erupted more than a thousand years ago.

Mount Adams is an important source of water in the Yakima Valley. Between the twelve named glaciers and other ice features on Mt. Adams, as well as groundwater springs that flow into rivers, and distribute water throughout the region.

This image shows the twelve named glaciers on Mount Adams. The glaciers are: Adams, Klickitat, Lyman, White Salmon, Pinnacle, Avalanche, Lava, Wilson, Rusk, Mazama, Gotchen, and Crescent Glacier