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Scientific Investigations Report 2010–5038

Suspended-Sediment Budget for the North Santiam River Basin, Oregon, Water Years 2005–08

Summary and Conclusions

In cooperation with the City of Salem, the U.S. Geological Survey investigated the sources and transport of sediment in the North Santiam River basin during water years 2005–08. Seven monitoring stations were in operation throughout the basin, providing continuous streamflow and instream turbidity data. Newly collected cross-sectional and point samples were added to previously published analyses in order to verify and update the regression models relating turbidity and suspended-sediment concentration. These models were used to estimate continuous suspended-sediment concentration records and annual suspended-sediment loads at each of the monitoring stations.

The upper North Santiam River was determined to have a sediment source that required a separate regression model for estimating suspended-sediment concentration. During the summer and early fall, the melting glaciers and snowfields on Mount Jefferson transported sediment from high on the exposed mountain slopes, resulting in a distinct suspended‑sediment concentration for any given turbidity. The two regression models for the upper North Santiam monitoring station produced better suspended-sediment concentration estimates than the previously published single regression model.

During water years 2005–08, the basin was exposed to a variety of environmental conditions, producing a wide range of suspended-sediment transport results. The low annual streamflow and lack of major storms during water year 2005 demonstrated how little sediment could be transported through the basin. A major storm during January 2006 produced the highest peak streamflows in the upper basin during the 4 years, resulting in the highest suspended-sediment loads at two of the three upper-basin stations. The upper North Santiam River had its highest sediment load following the debris flow on Mount Jefferson during November 2006 (water year 2007). This event and the subsequent storm contributed to the largest annual suspended-sediment load computed for any of the monitoring stations, as well as the largest annual suspended‑sediment load to pass through Detroit Lake and Big Cliff Reservoir during the period of study. Peak streamflows in the lower basin were highest in association with the same storm of water year 2007, producing the highest suspended‑sediment loads at all lower-basin stations. The annual mean streamflows in water year 2008 were the highest for the 4 years, but the peak streamflows were only slightly higher than during water year 2005. The suspended-sediment loads across the basin were similarly low during water year 2008. This indicated that suspended-sediment loads correlate better to peak streamflows, not mean streamflows, in the North Santiam River basin.

Water years 2005–08 presented examples of a wide range of sediment transport conditions in the North Santiam River basin. The 4-year total sediment budget provides a more balanced assessment. The upper basin tributaries contributed nearly 240,000 tons of sediment to Detroit Lake during the 4 years. However, only 14 percent of that sediment load was transported through the lake into the lower basin. Nearly 160,000 tons of sediment were transported by the North Santiam River as computed at Geren Island during the 4 years. Despite the massive debris flow in the upper basin, it was precipitation-driven, high-flow events in the lower basin that contributed the greatest sediment loads to the source water for the City of Salem’s water treatment facility.

For additional information contact:
Oregon Water Science Center Director,
U.S. Geological Survey,
2130 SW 5th Avenue
Portland, Oregon 97201
http://or.water.usgs.gov

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