Columbia Plateau is a broad, mostly flat region in the northwestern United States . It forms a hook-shaped area that covers parts of Washington, Oregon, western and southern Idaho, and northern Nevada . The Columbia Plateau is bordered on the north by the Columbia River, on the west by the Cascade Mountains, on the south by the Great Basin, and on the east by the Rocky Mountains. The region ranges in elevation from about 400 feet (120 meters) to as high as 9,800 feet (3,000 meters) above sea level. It has an area of about 81,000 square miles (210,000 square kilometers).
The Columbia Plateau is divided into five sections: (1) the Walla Walla Plateau, (2) the Blue Mountain section, (3) the Payette section, (4) the Snake River Plain, and (5) the Harney section. The Walla Walla Plateau, in the north and west of the region, consists of mostly flat land and small hills. The Columbia and Snake rivers meet there, cutting gorges through the plateau. The Blue Mountain section forms the center of the region. The Payette and Snake River Plain sections form the south central and southeastern portions of the Columbia Plateau. They comprise a low-lying plain strewn with volcanic features. In the southwest corner lies the Harney section, a desert basin. The Cascade Mountains create a rain shadow across the region, blocking much of the moisture coming from the Pacific Ocean. As a result, vegetation on the Columbia Plateau is sparse, consisting mostly of shrubs and grasses. Trees grow at higher altitudes.
Much of the Columbia Plateau is covered with layers of basalt, a dark volcanic rock. Between about 16 million and 15 million years ago, massive eruptions of lava covered the region. Smaller flows occurred until about 6 million years ago. The lava flows covered the area around the Blue Mountains, which had formed over 100 million years earlier. Scientists believe that a volcanic hot spot now beneath the Yellowstone caldera complex may have contributed to the release of the lava. A hot spot is an underground upwelling of hot rock in Earth’s mantle that creates magma (molten rock beneath the ground). Over the next several million years, the North American tectonic plate tracked west over the hot spot in a shallow arc. Occasional eruptions from the hot spot contributed to the formation of the Snake River Plain in southern Idaho.
People have lived on the Columbia Plateau for about 10,000 years. When the Lewis and Clark expedition reached the region in 1805, it was home to many Native American groups, such as the Cayuse, Nez Perce, Umatilla, and Yakama. Today, the Columbia Plateau is home to mostly small populations spread out across the region. Small towns dot the plateau. Only Boise, Idaho, and Spokane, Washington, have populations of over 200,000. Ranching and agriculture are the region’s main industries. In the north, the Columbia River has one of the most developed hydroelectric power systems in the world. More than 400 dams along the river generate electricity. 
