Geology and ground-water resources of the Walla Walla River basin Washington-Oregon

The Walla Walla River, whose drainage basin of about 1,330 square miles lies astride the Washington-Oregon boundary, drains westward to empty into the Columbia River. The basin slopes from the 5,000-foot crest of the Blue Mountains through a structural and topographic basin to the terraced lands adjoining the Columbia River at an altitude of about 340 feet. The main unit of the topographic basin is the valley plain, commonly called the Walla Walla Valley, which descends from about 1,500 feet at the foot of the mountain slopes to about 500 feet in altitude where the river cuts through the bedrock ridge near Divide. In the Blue Mountains the streams flow in rockbound canyons. Beyond the canyons, near Milton-Freewater and Walla Walla, they pass onto the broad alluvial fans and the terrace lands of the valley.