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
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.
Climate
Precipitation in the Walla Walla basin varies greatly. Along the Columbia River, it can reach as low as per year, while it can average per year in the heights of the Blue Mountains. The average rainfall across the whole basin is below per year. The Cascade Range to the west of the Columbia Plateau creates a wide rain shadow that reaches the Blue Mountains, creating a semi-arid climate in the lowlands across the basin. Temperatures regularly exceed during the summer, and reach below in the winter.
Snow storms during the winter and the resulting snowmelt in the spring have historically been the greatest source of fresh water for the subbasin, with precipitation and streamflow dropping greatly during the summer. Diversion of water for agricultural uses has exacerbated the summer streamflow decline. Warmer temperatures due to climate change is predicted to lessen snowfall and push melting to earlier in the year, worsening existing streamflow and water temperature issues. Thunderstorms occur on average 11 days per year in the watershed, mainly in the summers. They often produce significantly increased streamflow, deposing sediment into the water.
Hydrology The dominant source of water in the Walla Walla basin varies depending on the time of year. During the early winter, runoff from precipitation dominates. Snowmelt forms the main source of water during the spring and early summer, while groundwater discharge is the dominant source during the summer and the coldest period of the winter. The flow in the basin is the highest during the winter and spring, and declines significantly during the summer due to both decreased rainfall and increased demand for irrigation.
The mainstem of the Walla Walla River runs for from the confluence of its forks to the Columbia. The South Fork is long, while the North Fork is . The total length of the river, from its mouth to the headwaters of the South Fork, is about . +Major Walla Walla tributariesTributaryDrainage areaAnnual dischargeLengthConfluence South Fork Walla Walla River139,000 acre-feet (0.171 km3)North Fork Walla Walla River39,200 acre-feet (0.048 km3)Couse Creek8,620 acre-feet (0.011 km3) Mill Creek180,300 acre-feet (0.222 km3) Dry Creek15,490 acre-feet (0.019 km3) Pine Creek9,917 acre-feet (0.012 km3) Touchet River39,200 acre-feet (0.048 km3) Walla Walla River462,000 acre-feet (0.57 km3) (mainstem)
At a measuring station near the confluence with the Touchet River, the flow rate of a 10-year flood (i.e., the largest flood expected in a ten-year period) of the Walla Walla River was estimated to be per second. A 100-year flood would be expected to reach per second, while a 500-year flood would be expected to hit per second. The 100-year flood mark was exceeded in February 2020, when the river reached a high of per second. Between 1865 and 2001, 26 major floods occurred in the Walla Walla basin.
Groundwater Many smaller streams run out of the Blue Mountains through narrow bedrock canyons, creating alluvial fans. Much of the groundwater in the area is stored in aquifers within pockets of gravel alluvium which accumulates in the central valley and the deeper fractures within the Columbia River Basalt Group (a large basalt formation underneath the valley).
The alluvial aquifer in the Walla Walla Valley stretches up to , both discharging water into the river through streams and springs in some areas, and receiving groundwater recharge from it in others. Wells tapping the aquifer supply water to rural areas and farms. A layer of low-permeability Pleistocene-era clay underlies much of the gravel aquifer, forming a confining unit which prevents the aquifer water from descending deeper. The transmissivity of the aquifer (the rate at which water flows) ranges between per day.
The basalt aquifer reaches the surface in some areas, especially around the headwaters of rivers, providing a source of water for cities, farms, and industry. The groundwater in the aquifer is estimated to be between 5,000 and 15,000 years old, potentially due to high recharge rates during the major flood events, or from distant sources of water recharge in the Blue Mountains. The basalt aquifer contains numerous thin sheets of water-bearing rock, with one well able to tap multiple layers at once. These layers are slow to recharge, and water use in some areas has drained them significantly. The transmissivity of the basalt aquifers range from under per day.
Primarily due to agricultural overuse, groundwater levels in the Walla Walla basin have been declining since the 1940s. In some areas, the level has declined as much as . The municipal governments of both Walla Walla and Milton-Freewater have investigated using the basalt aquifer as a means of water storage, injecting water into it during the high winter flows to be used later.
Water quality
Surveys by the Washington State Department of Ecology and the Oregon Department of Environmental Quality have monitored the river for pollution. Water temperatures were found to have reached unacceptable levels on both the Washington and Oregon portions of the river. By 2021, the daily quantity of chlorinated pesticides and fecal coliform breached guidelines in the Washington portion, as did the pH of the water. Wastewater treatment plants are present in Dayton, College Place, Walla Walla, and Waitsburg.
By the 2020s, water temperature became the most consistent water quality problem across the Walla Walla watershed. Human modification of the basin through channel straightening has led to the breakdown of hyporheic exchange (interactions between surface water and groundwater at a streambed) and decreased the cooling caused by pooling and wood obstructions. Additionally, human modifications have removed riverside sources of shade and decreased total stream flow, allowing the water to heat up faster. The forks and the upper portion of the Walla Walla mainstem have cool temperatures, with temperatures increasing once the river enters its leveed portion near Milton-Freewater.
Geology
The Columbia River Basalt Group, a large igneous province consisting of expansive basalt formations dating to the Miocene, stretches across the Columbia Basin and into eastern Oregon and southern Idaho. A series of basalt formations from this group were formed from around 15.5 million to 8.5 million years ago. These formations range from tens of feet to hundreds of feet thick. The tops and bottoms of each lava flow in the formation are permeable and often fractured, allowing for groundwater to flow.