Walla Walla River

The Walla Walla River is a tributary of the Columbia River which runs through portions of eastern Washington and Oregon in the United States. Originating in the Blue Mountains, its two forks merge south of the town of Milton-Freewater, before continuing north into a wide river valley and floodplains as it approaches the Washington border. It receives Mill Creek as a tributary near the city of Walla Walla, alongside various distributary channels which split off from the Walla Walla and rejoin it later on. It flows west and receives the Touchet River later in its course, meeting the Columbia River at a delta on Lake Wallula.

The river receives groundwater discharge from an alluvium aquifer which formed during flooding at the end of the last ice age, as well as from a complex of basalt aquifers in the Columbia River Basalt Group, which formed during large volcanic eruptions during the Miocene. The terrain of the watershed includes upland conifer forests as well as large areas of scrubland; however, much of the area's scrubland and riparian habitat has been cleared or modified for agriculture.

Named for the indigenous Walla Walla people, the river valley was settled by Euro-American colonists during the 19th century and began to host a large agricultural community. Mechanised agriculture and irrigation systems in the 20th century greatly expanded the area's productivity, increasing demand for water. The river was channelised, leveed, and dammed; this curtailed floods, disrupted salmon spawning, and caused the river to dry up during the summers for much of the 20th century.