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Scientific Investigations Report 2007–5178

Scientific Investigations Report 2007–5178

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The lower mainstem North Santiam River, downstream of Detroit Lake and Big Cliff Reservoir, is the primary source of drinking water for residents of the City of Salem, Oregon, and the surrounding communities—an area with a combined population of more than 177,000. Because river water is used for consumption, many local agencies monitor any changes to water quality in the basin. For example, turbidity, the cloudiness of water caused by suspended particles, became a major concern for the City of Salem water-treatment facility in February 1996, when heavy rainfall and melting snow flooded streams to greater than 50-year levels (Cooper, 2005). The high flows and accumulated rainfall saturated the landscape, mobilized landslides, and accelerated erosion throughout the North Santiam River basin (U.S. General Accounting Office, 1998). The resulting high turbidity in the North Santiam River forced the water-treatment facility to close its intakes for 8 days (Uhrich and Bragg, 2003). Much of the colloidal material remained suspended for months after the event, which necessitated costly pretreatment to meet drinking-water standards. At the time, guidelines for drinking water stated that treated water must not exceed 1 NTU (Nephelometric Turbidity Unit, see section, “Definition and Measurement of Turbidity,”) and not exceed 0.3 NTU in 95 percent of daily samples in any month (Hulse and others, 2002; U.S. Environmental Protection Agency, 2002).

Because of the geologic characteristics of the North Santiam River basin (see section, “Geology”), turbidity-causing sediments are easily mobilized by storm runoff. If the eroded sediments are clay-rich, the particles remain suspended for long periods, resulting in persistent turbidity (see section, “Description of Persistent Turbidity and Clay-Water”). The City of Salem water-treatment facility can directly treat water that has a low turbidity value (less than 10 NTU); more turbid water (as high as 50 NTU) can be treated by several alternative pretreatment and distribution systems (Tim Sherman, City of Salem Public Works Department, written commun., 2006). Advance warning of high-turbidity events is needed to prepare alternative treatment processes or, when turbidity is too high, for shutting down facility operations.

After the turbidity-induced problems of 1996, the City of Salem determined that it needed real-time water-quality monitoring for the North Santiam River basin. In 1998, the City of Salem entered into a cooperative agreement with the U.S. Geological Survey (USGS) to establish a near real-time, continuous streamflow and water-quality monitoring network in the basin. The network alerts water-treatment-facility operators of high-turbidity events and provides data that help identify sources of sediment and persistent turbidity—information that may help land managers minimize the effect of high-turbidity events in the future. In general, high-turbidity events were rare: during the entire period of record, less than 1 percent of all instantaneous turbidity values were greater than 100 FNU.

Program History and Monitoring Locations

Between October 1998 and June 2001, water-quality instruments were installed at eight stations, either in conjunction with existing streamflow-gaging stations or at new sites. The network has eight monitoring stations (table 1), four upstream of Detroit Lake (upper basin) and four downstream (lower basin) (fig. 1), which measure water temperature, specific conductance, pH, and turbidity by using YSI multiparameter datasondes (YSI Incorporated, 2007). Instantaneous streamflow also is measured at these locations—excluding Geren Island. Data are logged every 15 minutes and transmitted as many as eight times a day. Data from these water-quality monitoring stations are accessible from the USGS North Santiam Project website at

Purpose and Scope

The purpose of this report is to describe the major turbidity events that occurred in the North Santiam River basin during water years 1999–2004 (October 1998 through September 2004). Major turbidity events are defined for this report as periods when turbidity values were greater than 250 FNU (Formazin Nephelometric Units, see section, “Definition and Measurement of Turbidity”) and the source of the sediment responsible for the elevated turbidity was known. The major turbidity events at five water-quality monitoring stations on unregulated streams are shown in figure 2. The three monitoring stations on the mainstem of the North Santiam River downstream of Detroit and Big Cliff Dams are not included because the dams alter the hydrologic conditions.

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