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Open-File Report O-11-06, Stream Channels of the Tualatin Valley and Lower Willamette River, Clackamas, Multnomah, Washington, and Yamhill Counties, Oregon, scale 1:36,000, by Daniel Coe.
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Lidar-based map; actual size 60 x 52 inches, scale 1:36,000.
Also see northern Willamette Valley stream channels and central Willamette Valley stream channels maps.
This map depicts streams of the Tualatin Valley and the Lower Willamette River watershed in northwestern Oregon. The Tualatin Valley is bound to the south by the Chehalem Mountains, to the north and east by the Tualatin Mountains, and to the west by the Coast Range. Cities in this region include Portland, Beaverton, Tualatin, Hillsboro, Tigard, Forest Grove, and Lake Oswego. The eastern part of the valley is heavily populated; the western portion maintains a more rural character. All stream flow in the valley drains to the Tualatin River, which flows eastward from the Coast Range to the Willamette River near Oregon City.
Much of the valley is covered by sediments left behind by Pleistocene age floods known as the Missoula Floods. Flooding occurred between 15,000 and 12,700 years before present (O'Connor and others, 2001). Evidence of the Missoula floods is seen in many places on the map and includes features like Sullivan's Gulch in Portland, Oswego Lake, and low-lying former channels adjacent to the Willamette River and Tualatin River (Allison, 1978). In addition, northwest of Sherwood and King City along the Tualatin River corridor there is distinct fluvial scarring where the floods flowed into, and subsequently out of, the valley.
To emphasize stream morphology in the Tualatin Valley, a gradient fading from cool blues and greens to warm tans and oranges was applied to the map to represent the relative elevation of land in feet above adjacent water surfaces. Dark blue areas of the map indicate elevations at or near water surface elevation and thus include both water and land areas. The color gradient was applied to digital elevation models (DEMs) that were created using 3-foot resolution bare-earth lidar data. Transparent shaded relief and shaded slope layers derived from lidar were also draped on top of the DEMs. First-return lidar data, also known as highest-hit lidar data, were used to create a DEM that was applied to the cities of the region. This technique allows viewers to see man-made structures in the areas closest in elevation to the streams within city limits. Land that is more than 100 feet above adjacent water surfaces was partially masked in order to emphasize the areas closer to streams. An elevation cross section represented by letters and arrows on the map is located below the main map. This cross section represents both actual and relative elevations and has been vertically exaggerated to better visually represent physical features of the landscape.
Allison, I. S., 1978, Late Pleistocene sediments and floods in the Willamette Valley: Portland, Oreg.: Oregon Department of Geology and Mineral Industries, Ore Bin, v. 40, no. 12, p . 193-202.
O'Connor, J. E., Sarna-Wojcicki, A., Wozniak, K. C., Polette, D. J., and Fleck, R. J., 2001, Origin, ext<>nt, and thickness of Quaternary geologic units in the Willamette Valley, Oregon U.S. Geological Survey Professional Paper 1620, 51 p.