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

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Land-Management Concerns

Feature Types of Interest

During the study, earthflows and debris flows were the two feature types that routinely supplied large amounts of material to streams.

Earthflows

Earthflows are fine grained and clay rich, with 70–90 percent of the clay fraction composed of smectite clays (Hulse and others, 2002). Smectite clays have the finest particle-size distribution of naturally occurring clay minerals (less than 0.05 µm) and are a major component of persistent turbidity (Bates and others, 1998; Hulse and others, 2002). Originating in unstable soils generated from eroded volcanic rocks, such as those in the Western Cascades and Foothills, smectite clays are abundant in the upper North Santiam River basin. Smectite clays and other amorphous material, such as kaolinite, commonly calve off or are eroded away from earthflows, especially during high flows. Although the sediment loads may not be large, they can produce persistent high turbidity because the clay particles remain in suspension for prolonged periods.

U.S. Forest Service researchers have mapped 274 active earthflows in the Willamette National Forest area of the North Santiam River basin (Shank, 2004). Of these active earthflows, 252 were nonnested—not contained in another earthflow—and account for 1.4 percent (6.6 mi²) of forest land. Stabilized earthflows, estimated to be hundreds to thousands of years old, have been mapped as well, and account for an additional 7.3 percent (35.1 mi²). Thus, nearly 10 percent of the forest has been affected by earthflows during the last few thousand years. Of the 252 nonnested active earthflows, 86 intersect streams and are considered active sediment-producing sources. The 13 mi of stream adjacent to these sediment-producing earthflows amount to 2.3 percent of the total drainage density of the Willamette National Forest within the North Santiam River basin.

Debris Flows

In addition to earthflows, debris flows also are known sediment sources. For example, following the flood of 1996, the Oregon Department of Geology and Mineral Industries mapped the location of 568 “rapidly moving landslides,” or debris flows, which occurred in Marion and Linn Counties (Hofmeister, 2000; Hofmeister and others, 2002). Of those debris flows, 216 originated in the North Santiam River basin, equating to a density of about 1 debris flow per every 3 mi². The steep slopes and accumulated sediments of the Western Cascades (see section, “Geology”) further aid in the abundance and frequency of debris flows. Since the flood of 1996, several debris flows have occurred in the basin. For example, during the same January 2003 storm that mobilized a debris flow in French Creek (see section, “January 30–February 1, 2003—French Creek,” a second debris flow mobilized in the Blowout Creek subbasin. The debris flow uprooted vegetation and carried it downslope, eventually blocking, and then expelling, a culvert beneath Forest Road 10 (fig. 35). This debris flow and road-failure sequence was converse to the road failure near Ivy Creek (see section, “December 17-19, 2001—Blowout Creek”), as the debris caused the road damage, rather than originating from road material. No sediment values or turbidity measurements exist for this failure because it occurred downstream of the Blowout monitoring station.

Source Areas of Interest

Each subbasin in the North Santiam River basin has source areas that contribute to turbidity; however, the two drainage basins of greatest interest are those of the Little North Santiam River in the lower basin and Blowout Creek in the upper basin, upstream of Detroit Dam and reservoir. Both subbasins produce, on average, the first- and second-largest clay-water volume in the North Santiam River basin (table 11) and therefore greatly affect the water quality in the basin.

The most immediate concern for drinking-water quality is turbid water from the Little North Santiam River. Not only does the Little North Santiam River transport the greatest amount of clay-water, but the turbid water also greatly affects water treatment because the unregulated streamflow enters only a few miles upstream of the treatment facility. Even though the highest recorded turbidity values from the Little North Santiam River have been in the low hundreds (unlike those in the upper basin, which can reach into the thousands), the short travel time can quickly overwhelm the facility. Therefore, scientists are closely examining known sources in the subbasin, such as the Evans Creek landslide and the numerous road failures along North Fork Road.

Blowout Creek is a concern because it transports the second-largest amount of clay-water in the North Santiam River basin. This is in large part because the subbasin has the largest density of mapped active and stabilized earthflows, with 1.8 mi² of actively moving terrain (7 percent of the subbasin) and 9.9 mi² of stabilized earthflows (38 percent of the subbasin; Shank, 2004). Even though Detroit Lake normally serves as a settling basin for most suspended clay from upstream drainage basins, such as Blowout Creek, and prevents its passage to the lower North Santiam River mainstem, during basinwide very high-streamflow events like that of 1996, a large volume of turbid water from upstream can pass through Detroit Dam and affect the operation of the City of Salem water-treatment facility (Hulse and others, 2002).

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