The Willamette River (pronounced ) is a tributary of the Columbia River. The name derives from a similar Clackamas Indian village name. The river is long, lying entirely in northwestern Oregon in the United States. Flowing northward between the Oregon Coast Range and Cascade Range, the river and its tributaries form a basin called the Willamette Valley containing 70 percent of Oregon's population, including its largest city Portland, which sits along both sides of the river near its mouth on the Columbia. Its lush valley is fed by prolific rainfall on the western side of the Cascades, forming one of the most fertile agricultural regions of North America that was the destination for many if not most of the immigrants along the Oregon Trail. The river was an important transportation route throughout much of the early history of the state, furnishing a means of conveying the vast timber and agricultural resources of the state to the outside world.
The Willamette rises in three separate forks in the mountains south and southeast of Eugene, at the southern end of the Willamette Valley. The Middle Fork and North Fork rise on the western side of the Cascades between Three Sisters south to Diamond Peak. The Middle Fork receives the North Fork northwest of Oakridge and flows northwest through the mountains to the southern end of the Willamette Valley. The Coast Fork rises in the lower mountains south of Cottage Grove, flowing north to join the Middle Fork southeast of Eugene.
From Eugene, the combined river flows north-northwest across the plain of the southern Willamette Valley to Corvallis, then follows a zigzag course past Albany and around the isolated hills in the central valley, passing west of downtown Salem. From Salem it flows north in a meandering course across the northwest plain of the valley, reaching the hills at Newberg, where it turns sharply east-northeast along the hills, passing through an opening in the hills at Oregon City, the location of the Willamette Falls and the de facto head of navigation. From Oregon City it flows northwest, past Lake Oswego and Milwaukie on the south edge of Portland, then passing between east and west Portland, where it is spanned by a series of urban bridges. Downstream of downtown Portland it flows northwest through the industrial port area of Portland Harbor, then splits into two channels around Sauvie Island that both hook around to enter the Columbia from the west. The main channel enters on the north edge of Portland, and the smaller Multnomah Channel enters about to the north-northwest at St. Helens.
The river's many tributaries drain the surrounding valley as well as portions of the Cascades and the Coastal Range. Downstream from the confluence of its forks, it is joined by McKenzie on the north side of Eugene, and by the Long Tom River from the southwest approximately south of Corvallis. It is joined by the Marys from the west at Corvallis, and the Calapooia from the southeast northeast of Corvallis, and it is joined by the Santiam from the east and the Luckiamute from the west within of each other approximately north of Albany. It is joined by the Yamhill from the west at Dayton, by the Molalla from the southeast near Canby, by the Tualatin from the west at West Linn, and by the Clackamas from the southeast at Gladstone.
The river forms part of the boundary of the following counties: Benton, Linn, Polk, Marion, Yamhill and Clackamas. Tributaries of the Willamette River also drain some or all of Lane, Washington and Multnomah counties.
Although riverboats navigated the upstream part of the Willamette into the first decades of the 20th century, currently there is little commercial traffic on the river above the Willamette Falls. The Willamette Falls Locks allow boat traffic, primarily recreational vessels, around the falls. The river is crossed by three ferries along its route in the Willamette Valley. The three ferries are located (from south to north) at Buena Vista, Wheatland, and Canby. The only locks on the river are located at Oregon City.
The Willamette River is prone to periodic floods. The great winter flood of 1861 destroyed several towns, including Linn City, Champoeg, and Scottsburg. The flood of 1894 seems to be the first flood well documented with photographs: one such famous picture depicts men thigh-deep in water in downtown Portland, pointing shotguns at decoy ducks as they float by in the flood waters. The great Vanport Flood of 1948 wiped Oregon's then second-largest city off the map. It was a WWII-era project city, built to temporarily house shipyard workers. The city stood near the confluence of the Willamette and Columbia Rivers, a spot later occupied by Portland International Raceway. Other notable floods include events in 1899, 1964, and the Willamette Valley Flood of 1996.
The river below Willamette Falls (RM 26.5) is subject to semidiurnal tides caused by the Pacific Ocean, but also with the effects of dams on the Columbia River and Willamette river basin which are regulated for hydroelectric power generation. Reverse flow is regularly observed above Ross Island at (river mile 15).
The lower river flow rate varies considerably by season and due to weather. Warm winter rains, for example, cause extensive mountain snow melt which significantly raises the river level and flow rate. The Willamette Valley Flood of 1996 was an extreme combination of saturated ground, snow melt and heavy rain. The maximum flow rate is not known, however 283,000 cubic feet per second (8010 m³/s) was estimated on January 18, 1974 at river stage 23.84 feet (7.27 m). The peak river stage (measured at the Morrison Bridge gauge in downtown Portland) on February 9, 1996, was 27.74 feet (8.46 m). Flood stage is 18.0 feet (5.5 m). The other extreme occurred during a drought on July 10, 1978, at 4,200 cu ft/s (120 m³/s). The Willamette's mean discharge rate is approximately .
The Superfund site is downstream from most of the river however and lies less than twelve miles from where the Willamette ends at the Columbia river. Further upstream the pressing environmental issues have been mainly variations in pH and dissolved oxygen. In the Portland Metropolitan Area, these issues are exacerbated by sewer overflow events during periods of high rainfall. The city has embarked on expanding the sewer system in order to minimize these events through construction of the Big Pipe Project part of the river renaissance project.
Even further upstream however, the Willamette is not heavily polluted and is used by communities, such as the City of Tigard, for drinking water. The major contaminants are from agricultural runoff.
The project consisted primarily of two large pipes on either side of the river. The project was part of a larger effort by the Bureau of Environmental Services to reduce CSOs through a series of combined efforts, which had already netted results of a 53 percent reduction in CSO volume by 2003.
The west side pipe, which is approximately in diameter and travels from Southwest Clay Street to the Swan Island Pumping Station, was completed in 2006. The pipe connects to the Southwest Parallel Interceptor, another pipe project approximately in diameter, at Southwest Clay Street, which continues south for several more miles, covering the Portland Metro South Waterfront area.
As of June 2008, the east side pipe is under construction and is slated for completion in December 2011. The pipe, like its cousin, will connect to the Swan Island Pumping Station but will extend a full south down the east bank. The east side pipe, serving a much larger segment of population, is wide and will be able to hold more than 83 million gallons (310 million L) of storm water and sewage.
Together the pipes and other CSO projects will provide a 94 percent reduction in CSO volume by 2011, dramatically reducing one of the largest pollutants of the Willamette River.