Willamette River

The Willamette River (/wɪˈlæmɪt/ ^ⓘ wil-AM-it) is a major tributary of the Columbia River, accounting for 12 to 15 percent of the Columbia's flow. The Willamette's main stem is 187 miles (301 km) long, lying entirely in northwestern Oregon in the United States. Flowing northward between the Oregon Coast Range and the Cascade Range, the river and its tributaries form the Willamette Valley, a basin that contains two-thirds of Oregon's population, including the state capital, Salem, and the state's largest city, Portland, which surrounds the Willamette's mouth at the Columbia.

Originally created by plate tectonics about 35 million years ago and subsequently altered by volcanism and erosion, the river's drainage basin was significantly modified by the Missoula Floods at the end of the most recent ice age. Humans began living in the watershed over 10,000 years ago. There were once many tribal villages along the lower river and in the area around its mouth on the Columbia. Indigenous peoples lived throughout the upper reaches of the basin as well.

Rich with sediments deposited by flooding and fed by prolific rainfall on the western side of the Cascades, the Willamette Valley is one of the most fertile agricultural regions in North America, and it was thus the destination of many 19th-century pioneers traveling west along the Oregon Trail. The river was an important transportation route in the 19th century, although Willamette Falls, just upstream from Portland, was a major barrier to boat traffic. In the 21st century, major highways follow the river, and roads cross the main stem on approximately 30 different bridges. More than half a dozen bridges not open to motorized vehicles provide separate crossings for bicycles and pedestrians, mostly in the Eugene area, and several others are exclusively for rail traffic. There are also ferries that convey cars, trucks, motorcycles, bicycles, and pedestrians across the river for a fare and provided river conditions permit. They are the Buena Vista Ferry between Marion County and Polk County south of Independence and Salem, the Wheatland Ferry between Marion County and Polk County north of Salem and Keizer, and Canby Ferry in Clackamas County north of Canby.

Since 1900, more than 15 large dams and many smaller ones have been built in the Willamette's drainage basin, 13 of which are operated by the U.S. Army Corps of Engineers (USACE). The dams are used primarily to produce hydroelectricity, to maintain reservoirs for recreation, and to prevent flooding. The river and its tributaries support 60 fish species, including many species of salmon and trout; this is despite the dams, other alterations, and pollution (especially on the river's lower reaches). Part of the Willamette Floodplain was established as a National Natural Landmark in 1987, and the river was named as one of 14 American Heritage Rivers in 1998.

The upper tributaries of the Willamette originate in the mountains south and southeast of Eugene, Oregon. Formed by the confluence of the Middle Fork Willamette River and the Coast Fork Willamette River near Springfield, the main stem Willamette meanders generally north for 187 miles (301 km) to the Columbia River. The river's two most significant course deviations occur at Newberg, where it turns sharply east, and about 18 miles (29 km) downstream from Newberg, where it turns north again. Near its mouth north of downtown Portland, the river splits into two channels that flow around Sauvie Island. Used for navigation purposes, these channels are managed by the U.S. federal government. The main channel, which is the primary navigational conduit for Portland's harbor and riverside industrial areas, is 40 feet (12 m) deep and varies in width from 600 to 1,900 feet (180 to 580 m), although the river broadens to 2,000 feet (610 m) in some of its lower reaches. This channel enters the Columbia about 101 miles (163 km) from the Columbia's mouth on the Pacific Ocean. The smaller Multnomah Channel, a distributary, is 21 miles (34 km) long, about 600 feet (180 m) wide, and 40 feet (12 m) deep. It ends about 14.5 miles (23.3 km) farther downstream on the Columbia, near St. Helens in Columbia County.

Proposals have been made for deepening the Multnomah Channel to 43 feet (13 m) in conjunction with roughly 103.5 miles (166.6 km) of tandem-maintained navigation on the Columbia River. Between the 1850s and the 1960s, channel-straightening and flood control projects, as well as agricultural and urban encroachment, cut the length of the river between the McKenzie River confluence and Harrisburg by 65 percent. Similarly, the river was shortened by 40 percent in the stretch between Harrisburg and Albany.

Interstate 5 and three branches of Oregon Route 99 are the two major highways that follow the river for its entire length. Communities along the main stem include Springfield and Eugene in Lane County; Harrisburg in Linn County; Corvallis in Benton County; Albany in Linn and Benton counties; Independence in Polk County; Salem in Marion County; Newberg in Yamhill County; Oregon City, West Linn, Milwaukie, and Lake Oswego in Clackamas County; and Portland in Multnomah and Washington counties. Significant tributaries from source to mouth include the Middle and Coast forks and the McKenzie, Long Tom, Marys, Calapooia, Santiam, Luckiamute, Yamhill, Molalla, Tualatin, and Clackamas rivers.

Beginning at 438 feet (134 m) above sea level, the main stem descends 428 feet (130 m) between source and mouth, or about 2.3 feet per mile (0.4 m per km). The gradient is slightly steeper from the source to Albany than it is from Albany to Oregon City. At Willamette Falls, between West Linn and Oregon City, the river plunges about 40 feet (12 m). For the rest of its course, the river is extremely low-gradient and is affected by Pacific Ocean tidal effects from the Columbia. The main stem of the Willamette varies in width from about 330 to 660 feet (100 to 200 m).