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Scientific Investigations Report 2007–5178

Scientific Investigations Report 2007–5178

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From October 1998 to September 2004 (water years 1999–2004), turbidity exceeded 250 Formazin Nephelometric Units on several occasions in streams in the North Santiam River basin in Oregon. This study, done in cooperation with the City of Salem, Oregon, linked eight of these periods of high turbidity to likely source areas, defining them as major turbidity events. For each of these major turbidity events, the study report describes the conditions, such as turbidity and streamflow; calculations, such as suspended-sediment load and clay-water volume; likely source areas, such as landslides or glacial outwash; and any follow-up investigations.

Data from each major turbidity event illustrated that different processes drove sediment loading at different times. The most common catalyst was precipitation, as six of the eight major turbidity events corresponded to a particular storm system. The remaining two events were caused by snowmelt-driven erosion from sediments originating in the Mount Jefferson Wilderness Area. Each subbasin within the North Santiam River basin was capable of producing high turbidity, as demonstrated by the diversity of events in this report; although, generally, the Little North Santiam River and Blowout Creek subbasins produced larger volumes of sediment and clay-water during precipitation events. The largest major turbidity event in this study was produced by the Breitenbush River subbasin in November 1999. Large suspended-sediment loads commonly were caused by more than one source in a subbasin, such as multiple landslides or earthflows. In most of the multiple-source events, it was impossible to determine every source, although at least one likely source always was identified. For some events, such as the road failure near Ivy Creek, field observations were able to directly link discrete source areas with suspended-sediment load and clay-water volume calculations.

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