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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5178

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Description of North Santiam River Basin

The North Santiam River originates on the western slope of the Cascade Range and flows 98 mi westward before reaching its confluence with the South Santiam River, southeast of Salem. Streamflow is regulated by two U.S. Army Corps of Engineer dams, Detroit and Big Cliff Dams. The North Santiam River basin includes parts of Linn, Marion, and Clackamas Counties in northwestern Oregon (fig. 1). The study area extends from Mount Jefferson and the High Cascades in the east to Geren Island near Stayton in the west. The study area drains 688 mi², omitting the 90 mi² area downstream of the gaging station at Geren Island. Altitudes in the basin range from 10,500 ft at the Mount Jefferson summit to just more than 500 ft along the Willamette River valley floor in the east.

Land Cover

The U.S. Forest Service (USFS) manages most of the land in the North Santiam River basin, with the Willamette National Forest covering 69 percent and Mt. Hood National Forest covering an additional 1.5 percent (fig. 3). The rest of the basin is private (18 percent), State owned (7 percent, managed by the Oregon Department of Forestry), Federally owned (4 percent, managed by the Bureau of Land Management), and tribal land in the Warm Springs Indian Reservation (less than 1 percent). With roughly 75 percent of the basin forested (U.S. Geological Survey, 2005), timber harvesting and recreation are the primary land uses; however, low-altitude valley bottoms sustain agriculture and residential development (fig. 4).

Geology

The geology of the North Santiam River basin (Walker and MacLeod, 1991; Sherrod and Smith, 2000) is mostly volcanic, with three distinct regions: High Cascades, Western Cascades and Foothills, and Willamette Valley(fig. 5). High Cascade geology is relatively young (Pliocene to Holocene in age), less eroded, and currently volcanically active. Bedrock consists mostly of basalts of Tertiary and Quaternary age, with outcrops of andesite and dacite, mafic vent complexes, and basaltic and andesitic ejecta. Surficial geology includes glacial outwash and related glaciofluvial deposits with ages younger than 140,000 years. Because of the young, less-weathered bedrock and the minimal amount of surface sediments, less sediment is transported from the upper basin than from elsewhere in the North Santiam River basin. However, isolated, glacially derived turbidity events can occur when landslides or glacial outwash release sediment into streams.

The Western Cascades are older, heavily eroded volcanic and sedimentary rocks of Pliocene and Eocene age—primarily Tertiary-age andesites with minor Tertiary-age intrusives and dacites. These heavily eroded materials mobilize during rainfall and cloud mid- to lower-basin streams. Other rock types in the Western Cascades include older basaltic and basaltic andesite lava flows and breccias. Lower, in the Foothills, undifferentiated Miocene-age flows and clastic rocks, Quaternary-age terrace deposits and lag gravels, and landslide and debris flow deposits are common. It is this mid-altitude, central basin region in which most of the landslides occur, with several documented historic landslides, earthflows, and road failures (U.S. Forest Service, 1998; U.S. Forest Service, 1999a; U.S. Forest Service, 1999b; Hofmeister, 2000; Sherrod and Smith, 2000; Shank, 2004).

The geology of the Willamette River Valley is dominated by Columbia River basalts of mid-Tertiary age and sedimentary rocks of mid- to late-Tertiary age. Quaternary-age alluvium and colluvium deposits are present on the valley floor. In the lower valley, surface erosion and remnant landslide material (stabilized landslides) supply sediment to streams. The gradual and low-gradient slopes are stable and much less prone to erosion and landsliding than the mid-altitude Western Cascades and Foothills.

Climate

The North Santiam River basin typically is warm and dry in the summer and cool and rainy in the winter, with snowfall at high altitudes. Mean annual precipitation at rain gages in the basin ranges from 40 in. at Salem to 90 in. at Detroit Dam (9 in. of snow), with greater snowfall at higher altitudes—91 in. at Marion Forks (Oregon Climate Service, 2006). About one-half of the precipitation in the High Cascades seeps into the ground-water system, with the amount of infiltration decreasing at lower altitudes (Sherrod and others 1996; Uhrich and Bragg, 2003). Rainfall is the primary catalyst for dislodging sediment for increasing turbidity in streams in the basin.

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