Tryon Creek


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 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 and Oregon Route 99W near Multnomah Village and flows southeast for 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 4.56 or river kilometer 7.34. Shortly thereafter, the stream passes under Route 99W 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 to RM 3.28. At the end of the Marshall Cascade, it receives Owl Creek. 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.
Image:Tryon Creek at Obie's Bridge.jpg|thumb|left|alt= A stream no more than wide meanders through a second-growth forest. | Tryon Creek at Obie's Bridge Park Creek enters from the right at RM 1.80. 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 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 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.

Discharge

Since 2002, the USGS has monitored the flow of Tryon Creek at a station from the mouth. The average flow between 2002 and 2011 was. This is from a drainage area of, about 97 percent of the total Tryon Creek watershed. The maximum flow recorded during this period was on December 9, 2010. The minimum was on September 4, 5, and 12, 2002.

Geology

One hundred and fifty million years ago, Oregon did not exist. Not until plate tectonics separated North America from Europe and North Africa and pushed it westward did the continent acquire, bit by bit, what became the Pacific Northwest. Over many millions of years, the continent collided with and incorporated islands, reefs, and other exotic terranes. Part of the last major exotic terrane acquired by the North American Plate during the Eocene lies under the Tryon Creek watershed. The terrane consisted of a chain of seamounts that by 34 million years ago was being uplifted to become the Oregon Coast Range and the Tualatin Mountains. The easternmost exposure of the basalts of this terrane is in Waverly Heights, near Milwaukie, across the Willamette River from Tryon Creek, and this formation underlies most of Tryon Creek State Park.
Between 15 and 16 million years ago, in the Middle Miocene, eruptions of Columbia River basalts from volcanic vents in eastern Oregon and Washington flowed across much of northern Oregon, sometimes reaching the Pacific Ocean. Although these basalts have been mapped in the West Hills under Marquam Hill, Hoyt Arboretum, and the steepest slopes of Forest Park, they flowed around but did not completely cover the Waverly Hills Formation in the Tryon Creek watershed.
Starting about 3 million years ago and continuing at least through the late Pleistocene, extensional faulting of the Earth's crust led to eruption of small volcanoes in the Boring volcanic field. This field extended roughly from Portland and Tualatin on the west to Battle Ground, Washington, on the north to Sandy and Boring on the east. Two of these volcanoes, Mount Sylvania and Cook's Butte, are in the Tryon Creek watershed. The Mount Sylvania eruptions included ash plumes and lava flows that covered some of the Waverly Heights Formation and Columbia River basalts.
About 15,000 years ago, cataclysmic ice age events known as the Missoula Floods or Bretz Floods originating in the Clark Fork region of northern Idaho inundated the Columbia River basin many times. These floods deposited huge amounts of debris and sediment and created new floodplains in the Willamette Valley. Over long stretches of time between the great floods, dry winds deposited silt. At elevations above in the Tryon Creek watershed, wind-blown silt covers the lava, while at lower elevations sand and gravel cover the bedrock.

Watershed

The Tryon Creek watershed covers about or. Of this, about 80 percent is within the Portland city limits, and the remaining 20 percent is split among the jurisdictions of the city of Lake Oswego, and the counties of Multnomah and Clackamas. These four overlap with the jurisdiction of the State of Oregon, which owns the natural area. Watersheds that border the Tryon Creek watershed are Fanno Creek to the west and northwest, Stephens Creek to the north, the Willamette River to the east and Oswego Lake to the south. More than of surface streets, including parts of Interstate 5, Oregon Route 99W, Oregon Route 43, Boones Ferry Road, Taylors Ferry Road, and Terwilliger Boulevard, run through the watershed.
About of precipitation, almost entirely rain, fall on the watershed each year. Summers are dry, and most of the precipitation occurs between October and May. Elevations within the drainage basin vary from Mount Sylvania, which rises to above sea level in the West Hills, to where the creek enters the Willamette River. Between 60 and 75 percent of the slopes in the watershed exceed a 30-percent grade and are especially steep near the headwaters. Impervious surfaces cover about 24 percent of the basin. These surfaces and the relatively impermeable silt and clay soils that underlie the area contribute to rapid runoff and low baseflows in the creek and its tributaries. The total length of surface streams in the drainage basin is about, while another run through culverts or pipes. Although major flooding in 1996 caused landslides and severe damage to stream beds and banks along Tryon Creek and its tributaries, it caused no significant property damage in the watershed.
In 2000, the population of the Tryon Creek watershed within Portland was about 18,000. In 2005, Portland's Bureau of Environmental Services described land-use zoning in the watershed as follows: single-family residential, 55 percent; parks and open space, 14 percent; multi-family residential, 5 percent; commercial, 3 percent, and insufficient data, 2 percent. To reach 100 percent, BES listed but did not differentiate by zoning type the roughly 20 percent of the watershed that lies beyond the Portland city limits. Metro, the regional government for the Portland metropolitan area, says that parks and natural areas cover about 21 percent of the total watershed, while single-family housing dominates most of the rest.