Plateau, also known as the Columbia Basalt Plain, is the prominent geographic
feature of the interior Columbia River Basin. The plateau formed between 6
million and 16 million years ago as the result of successive flows of basalt.
Its northern border is marked by the Columbia River and the mouth of the
Okanagon River. On the south, the plain includes the drainage of the Deschutes
River in Oregon. On the east the plateau includes the Camas Prairie of central
Idaho, and on the west it runs to the foothills of the Cascade Mountains. The
Columbia River makes a looping S-curve through the central Washington part of
the Columbia Plateau.
a river basin that is mostly mountains and steep valleys, the Columbia Plateau
is an area of flat land and gently rolling hills cut by the sometimes steep
river canyons of the Columbia and its tributaries. It is a roughly
triangular-shaped area about 250 miles on a side. On its fringes the elevation
of the plateau is about 4,000 feet (1,219 meters) above sea level, and at its
lowest point it is about 400 feet (122 meters). The Columbia Plateau is the
major agricultural region of the interior Columbia River Basin and includes the
cities of Ellensburg, Wenatchee, Yakima, Moses Lake, Walla Walla, Pullman, and
the Tri-Cities of Washington (Pasco, Kennewick, Richland); Pendleton, Umatilla,
and The Dalles, Oregon; and Moscow and Lewiston, Idaho.
journals of Lewis and Clark briefly note the Plateau. On Sept. 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.” After
their difficult crossing of the Bitterroot Mountains, the sight of the plain
“greatly relieved the spirits of the party,” he wrote in the journals for the