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| discharge_round = -2 | discharge_imperial = 33220 | discharge_max_imperial = 420000 | discharge_min_imperial = 4200 | discharge1_location = | discharge1_imperial = | discharge1_note = | map = Willametterivermap.jpg | map_size = | map_caption = A map of the Willamette River, its drainage basin, major tributaries and major cities | map1 = | map1_size = | map1_caption = | map1_locator = | commons = Willamette River }} The Willamette River ( ) 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 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 was thus the destination of many 19th-century pioneers traveling west along the Oregon Trail. The river was an important transportation route during this time, 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 it on more than 50 bridges. Since 1900, more than 15 large dams and many smaller ones have been built in the Willamette's drainage basin, and 13 of them are managed by the U.S. Army Corps of Engineers (USACE). The dams are used primarily to produce hydroelectricity, to maintain reservoirs for recreation, and to divert water into deeper, narrower channels in order 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. ==Course== (詳細は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 to the Columbia River. The river's two most significant course deviations occur at Newberg, where it turns sharply east, and about 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 deep and varies in width from (although the river broadens to in some of its lower reaches),〔〔 enters the Columbia about from the larger river's mouth on the Pacific Ocean. The channel forms the primary navigational conduit for Portland's harbor and riverside industrial areas. The smaller Multnomah Channel, a distributary, is long, about wide, and deep. It ends about further downstream on the Columbia, near St. Helens in Columbia County.〔 Relevant map quadrangles include Eugene East, Coburg, Junction City, Harrisburg, Peoria, Riverside, Lewisburg, Albany, Monmouth, Salem West, Mission Bottom, Dayton, St. Paul, Newberg, Sherwood, Canby, Lake Oswego, Portland, Linnton, Sauvie Island, and Saint Helens.〕 Proposals have been made for deepening the Multnomah Channel to in conjunction with roughly 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.〔Laenen and Dunnette, p. 24〕 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; 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 above sea level, the main stem descends between source and mouth, or about 2.3 feet per mile (0.4 m per km).〔 Source and mouth elevation derived from Google Earth search using GNIS source and mouth coordinates.〕 The gradient is slightly steeper from the source to Albany than it is from Albany to Oregon City.〔Benke, ''et al.'', p. 617〕 At Willamette Falls, between West Linn and Oregon City, the river plunges about .〔 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 .〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Willamette River」の詳細全文を読む スポンサード リンク
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