Spread throughout Washington, Oregon, Idaho and the Canadian province of British Columbia, the Columbia Plateau is the main geographic feature of the interior Columbia River Basin. Within an area composed mainly of mountains and steep valleys, the Columbia Plateau is characterized by flat land and low hills that are occasionally cut by the river canyons of the Columbia River and is tributaries.
The Columbia River Basin is a triangular area measuring approximately 250 miles on each side. On its fringes the elevation of the plateau is about 4,000 feet (1,219 m) above sea level, and at its lowest point it is about 400 feet (122 m). The plateau was formed somewhere between 6 million and 16 million years ago. Its northern border is marked by the Columbia River and the mouth of the Okanagon River. On its south, the plateau extends to Oregon. On the east, the plateau includes the Camas Prairie of central Idaho and on the west extends all the way to the foothills of the Cascade Mountains in Washington.
Most of the plateau is underlain by lava deposits, some as thick as 10,000 feet in some places. Sedimentary rock is embedded within the lava, which is primarily basalt. Older rocks can be seen in outcrops in the Blue and Wallowa mountains, while younger lavas, cinder cones and volcanic ash are characteristics found to the south of the plateau. Older, decayed lavas occur in the northern part of the basin. Coulees — dry river canyons — and scablands — eroded basalt surfaces — were both carved by glacial melting and are among the region’s geologic features.
The first mention of the Columbia River Plateau is in the journals of famed American explorers Lewis and Clark. In the entry for September 18, 1805, Clark wrote, “from the top of a high part of the mountains . . . I had a view of an emence Plain and leavel country to the SW. & West.” In the journal entry for the following day, he noted that the sight of the plain “greatly relieved the spirits of the party” after the difficulty the group experienced traversing the Bitterroot Mountains.
Although humans have lived along the Columbia River for more than 10,000 years, modern engineering since the Industrial Revolution has altered the region’s environment. In fact, the Columbia River Plateau is home to the most hydroelectrically developed river system in the world, with more than 400 dams located along the Columbia River. More than 21 million kw of electricity is generated by these dams, which include the Rock Island Dam, the Bonneville Dam and the massive Grand Coulee Dam.