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 anabranches formed by 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 that was formed by 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 riverside habitat and the surrounding scrubland has been cleared or modified for agriculture.

Named for the indigenous Walla Walla people, the river valley was settled by European-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.

Course

alt=A small river runs through a wooded area with a golden hill in the background|thumb|South Fork of the Walla Walla River at Harris County Park near [[Milton-Freewater, Oregon]]

The Walla Walla River originates from two streams, the North Fork Walla Walla River and the South Fork Walla Walla River, both with their headwaters in the Blue Mountains of Oregon. They flow west from the mountains and meet to form the mainstem Walla Walla River (a course also known as the Tumalum Branch). The river receives many smaller tributary streams, the largest of which within its upper course in Oregon are Couse Creek and Birch Creek. The river becomes channelized and leveed as it flows north into Milton-Freewater, opening up into a wide valley with rolling hills and floodplains.

Within the Walla Walla Valley is a wide network of anabranches formed by distributaries splitting off from the main channel and rejoining it later in its course. These include the Little Walla Walla River, which flows out of the Walla Walla near Milton-Freewater, Oregon, and flows northwards, splitting into two forks. Both forks rejoining the river through both surface flow and groundwater discharge. The distributaries are fed both by water from the Walla Walla River and from various small spring-fed creeks. Many of the small creeks and waterways in this area have names which differ between Washington, Oregon, and federal records.

The mainstem Walla Walla crosses into Washington state about six miles to its north, entering a broad channel constrained by terraces on either side As it turns west and passes near the city of Walla Walla, the river first takes in Yellowhawk Creek as a tributary, which is fed by water diverted from Mill Creek and from streams in the hills east of the city. It then takes in another Mill Creek distributary, Garrison Creek, before taking in Mill Creek itself several miles later. From around May to October, most of Mill Creek's flow is diverted by flood control structures into Garrison Creek and Yellowhawk Creek.

Near the unincorporated community of Lowden, Washington, the river receives Dry Creek as a tributary, which originates in the Blue Mountains. West of the town of Touchet, at an elevation of about , the river receives another major tributary, the Touchet River. The Touchet emerges from four forks in the Blue Mountains, converging near Dayton, and drains an area of largely agricultural land.

Watershed

The Walla Walla basin covers an area of about . About two-thirds of the watershed lie within Washington, with the other third in Oregon. The Washington portions of the watershed lie within Walla Walla County and Columbia County, while the Oregon portion mostly lies within Umatilla County, Oregon, with very small portions of the upper watershed crossing into Wallowa County and Union County. The highest point within the watershed is Table Rock, which reaches an elevation of above sea level, with the crest of the Blue Mountains averaging around . Below the mountains, the terrain slopes from to at the river's confluence with the Columbia.

Much of the northern portion of the Walla Walla basin lies within the Palouse, a large region of rolling dryland hills which supports extensive agriculture. The river valley created by the Walla Walla and Mill Creek creates large plains which host farms and settlements. About 54% of the basin is agricultural land, 25% is scrubland and grassland, and 17% is forests. Much of the watershed is sparsely populated. There are five communities in the basin with a population of above 1,200 people. Walla Walla, Washington, is the largest city, at about 34,000 people as of 2019. Milton-Freewater is the largest settlement in the Oregon side of the basin, at about 7,000 people as of 2019.

! scope="row" |Mill Creek

|180,300 acre-feet (0.222 km3)

! scope="row" |Dry Creek

|15,490 acre-feet (0.019 km3)

! scope="row" |Pine Creek

|9,917 acre-feet (0.012 km3)

! scope="row" |Touchet River

|39,200 acre-feet (0.048 km3)

! scope="row" |Walla Walla River

|462,000 acre-feet (0.57 km3) Soon after the establishment of the fort, McKenzie dug a set of ditches to irrigate the gardens around the fort; the first irrigation canals constructed in the basin.

alt=A color photo of a wide stretch of wheat fields|thumb|Wheat fields in the Walla Walla Valley, 1941

The growth of the Oregon Trail during the 1840s and 1850s brought large numbers of American settlers into the Pacific Northwest.