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Tryon Creek is a tributary of the Willamette River in the U.S. state of Oregon. Part of the drainage basin of the Columbia River, its watershed covers about in Multnomah and Clackamas counties. The stream flows southeast from the Tualatin Mountains (West Hills) through the Multnomah Village neighborhood of Portland and the Tryon Creek State Natural Area to the Willamette in the city of Lake Oswego. Parks and open spaces cover about 21 percent of the watershed, while single-family homes dominate most of the remainder. The largest of the parks is the state natural area, which straddles the border between the two cities and counties. The bedrock under the watershed includes part of the last exotic terrane, a chain of seamounts, acquired by the North American Plate as it moved west during the Eocene. Known as the Waverly Hills Formation, it lies buried under ash and lava from later volcanic eruptions, sediments from flooding and erosion, and layers of wind-blown silt. Two dormant volcanoes from the Boring Lava Field are in the Tryon Creek watershed. Named for mid-19th century settler, Socrates Hotchkiss Tryon, Sr., the creek ran through forests of cedar and fir that were later logged by the Oregon Iron Company and others through the mid-20th century. Efforts to establish a large park in the watershed began in the 1950s and succeeded in 1975 when the state park was formally established. As of 2005, about 37 percent of the watershed was wooded and supported more than 60 species of birds as well as small mammals, amphibians, and fish. At the same time, the human population was about 18,000. == Course == Tryon Creek begins slightly north of Interstate 5 (I-5) and Oregon Route 99W near Multnomah Village and flows southeast for 〔Although other sources give the creek's length as , Chapter 10 of ''Fanno and Tryon Creeks Watershed Management Plan'' by the Portland Bureau of Environmental Services gives a precise length of . The plan describes inflow above that point as a "headwaters complex" that "includes all small upper tributaries above RM 4.85." By convention, stream lengths do not include tributary lengths.〕 through Portland residential neighborhoods as well as Marshall Park and the Tryon Creek State Natural Area to its confluence with the Willamette River in the city of Lake Oswego. Not far from its source, the main stem runs through three closely spaced culverts with a combined length of then flows on the surface before entering another culvert, long under Southwest 30th Avenue at river mile (RM) 4.56 or river kilometer (RK) 7.34. Shortly thereafter, the stream passes under Route 99W (Barbur Boulevard) and I-5 through another culvert, long. Tryon Creek receives Falling Creek from the right at from the mouth. Quail and Burlingame creeks enter from the right shortly thereafter. At Marshall Park, the stream passes through a series of rock pools and steps known as the Marshall Cascade from RM 3.48 (RK 5.60) to RM 3.28 (RK 5.28). Arnold Creek, which is Tryon Creek's largest tributary, enters from the right from the mouth. Almost immediately, the creek enters a culvert, about long, that passes under Southwest Boones Ferry Road.〔〔〔 Shortly thereafter, Tryon Creek enters the Tryon Creek State Natural Area. To the stream's right is the North Creek Trail. Soon the creek receives Fourth Avenue Creek from the left. At this point, the Fourth Avenue Trail runs parallel to the creek along the left bank. This trail soon merges with the Lewis and Clark Trail, which also runs parallel to the creek along the left bank. Then Tryon Creek passes under the High Bridge, the uppermost of five footbridges that span the main stem within the park. High Bridge carries the Middle Creek Trail as well as a linear horse trail connecting the park's North and South horse loops. Below this bridge, the creek flows for about before passing under the Beaver Bridge. In this stretch, the Middle Creek Trail parallels the stream along the right bank but crosses to the left bank at the bridge. Shortly thereafter, the creek passes under Obie's Bridge, which carries the Old Man Trail.〔〔〔 Park Creek enters from the right at RM 1.80 (RK 2.90). Slightly below the confluence, the creek passes under the Red Fox Bridge, which carries the Red Fox Trail. Thereafter, the creek runs roughly parallel to the South Creek Trail, which is on the right for about . The creek receives Red Fox Creek from the right and Palatine Hill Creek from the left, then flows out of Portland and Multnomah County and into the city of Lake Oswego and Clackamas County. The stream then passes under the Iron Mountain Bridge, which carries the Iron Mountain Trail. Just below the bridge, Nettle Creek enters from the right at from the mouth, and shortly thereafter the creek passes a United States Geological Survey (USGS) stream gauge, which is on the right. The creek leaves the Tryon Creek State Natural Area just before entering a culvert under Oregon Route 43 at RM 0.24 (RK 0.39) and soon thereafter a set of Union Pacific Railroad tracks. Below this, Tryon Creek forms the boundary between the city of Lake Oswego and Briarwood, an unincorporated Clackamas County community, which is to the left. The Tryon Creek Wastewater Treatment Plant is on the right during this last stretch before the creek enters the Willamette River about upstream from its confluence with the Columbia River.〔〔〔 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「Tryon Creek」の詳細全文を読む スポンサード リンク
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