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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5187

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Suspended-Sediment Loads

The effects of the varied hydrologic conditions from year to year were reflected in the SSL estimates. On the basis of the streamflow analysis, annual SSL was highest during water years 1999 and 2000 and lowest during water year 2001 for most of the monitoring stations (fig. 12). In addition to the annual SSL, the highest consecutive 3-day SSL was calculated to demonstrate the effect of a single short-term event at each station for each water year.

SSL was calculated at the monitoring stations only when both turbidity and streamflow data were available. Therefore, SSL was not estimated for the lower-basin stations (Niagara, Little North, and Mehama) for water years 1999–2000 or for French for water years 1999–2001. Water-quality instruments had not yet been installed at these stations, and turbidity data were not available.

Water Year 1999

Total SSL was substantial during water year 1999. Blowout had its highest annual SSL and both North Santiam and Breitenbush had their second highest SSL for the period of study. Annual mean streamflow data indicated that water year 1999 was the highest annual streamflow year for water years 1999–2004. The highest 3-day SSL occurred during December 27–29, 1998, when the basin received 7–8 in. of rain (Oregon Climate Service, 2005). Peak streamflow during this storm was classified as a 2–5 year recurrence flood at several of the gaging stations (Cooper, 2005). The 14,100 tons of sediment transported past Blowout during the December storm were more than double the 3-day SSL at that station for any of the other water years (fig. 12).

Water Year 2000

SSL was highest during water year 2000 for the period of study at both North Santiam and Breitenbush. Although the annual streamflows were lower than in water year 1999, the maximum instantaneous flows were much higher at the three upper-basin gaging stations. Seven to 10 in. of rain fell in the basin during November 25-27, 1999, resulting in the largest precipitation-driven event for the period of study (Oregon Climate Service, 2005). The resulting 10–25-year recurrence interval flood produced the highest streamflow and SSL for the water year (fig. 12; Cooper, 2005). The 22,200 tons at North Santiam was the highest 3-day SSL at the station for water years 1999–2004. The greatest SSL at any of the monitoring stations for the entire period of study occurred at Breitenbush during this storm. An estimated 69,800 tons of sediment was transported past the station in 3 days.

Water Year 2001

Water year 2001 was a severe drought year. The highest SSL and streamflow occurred at different times during the winter and spring in conjunction with storms producing less than 4 in. of rain over 3 days (Oregon Climate Service, 2005). Annual mean flows and the peak streamflows were less than the 5^th percentile for the four gaging stations for which data was analyzed. The highest SSL recorded at Blowout was during December 22–24, 2000, and at Breitenbush during April 30–May 2, 2001. The highest SSL at North Santiam was not related to either of these storms. As discussed in section, “Data Analysis,” an event in the upper North Santiam River subbasin on October 1, 2000, caused extremely turbid water at the monitoring station. The resulting 1,700 tons of sediment was the highest 3-day SSL for any of the stations for the water year. SSL was first estimated at the lower-basin monitoring stations for water year 2001. The highest SSL at Niagara occurred December 23–25, 2000 (following the highest SSL at Blowout). The highest SSL values at Little North and Mehama were recorded during the storm on April 30–May 2, 2001. The annual SSL values for all six monitoring stations were the lowest for each during the 6 years of study (fig. 12).

Water Year 2002

A spring storm produced the highest SSL totals for water year 2002. The highest 3-day SSL for three of the upper-basin stations (North Santiam, Breitenbush, and French) occurred during April 13–15, 2002, when 2 to 3 in. of rain fell in 3 days (Oregon Climate Service, 2005). Little North and Mehama also had their highest SSL during this storm. The annual SSL of 35,200 tons at Mehama was the highest for the period of study (fig. 12). The peak streamflows at the gaging stations were near average (between 25^th and 75^th percentile) for all stations during this storm. Blowout recorded increased SSL during the April storm, but its highest SSL occurred during December 17–19, 2001. As discussed in section, “Data Analysis,”, debris flow and a road failure in the Blowout Creek subbasin produced increased turbidity and SSL at the station. The maximum SSL values at Niagara occurred during December 21–23, 2001, and April 14–16, 2002, in the days following high SSL values in the upper basin.

Water Year 2003

During water year 2003, several storms produced increased SSL totals for the North Santiam monitoring stations. The highest SSL values for most of the upper-basin stations occurred January 30–February 1, 2003, during a storm that generated 5–7 in. of rain in 3 days (Oregon Climate Service, 2005). This storm also resulted in the highest 3-day SSL totals at Little North and Mehama (fig. 12). The peak streamflow analysis indicates that the storm was considered average for three of the four gaging stations. The January-February storm resulted in increased SSL at Blowout, but the highest SSL at the station occurred during March 21–23, 2003. Niagara had its highest SSL values following both the January and March storms.

Water Year 2004

Annual mean and maximum instantaneous streamflows for water year 2004 were below-average for most of the North Santiam River basin monitoring stations. As in October 2000 (and discussed in section “Data Analysis”), the highest SSL at North Santiam resulted from an event originating high in the subbasin. For the other upper-basin stations, one of two storms produced the highest 3-day SSL. The first storm occurred December 13–15, 2003, with 4–5 in. of rain in 3 days (Oregon Climate Service, 2005). The highest 3-day SSL values at Breitenbush and Blowout were recorded during this storm. The next storm occurred during January 28–30, 2004, and generated 3–5 in. of rain in 3 days (Oregon Climate Service, 2005). This storm produced the highest SSL at French. The three lower–basin stations also recorded their highest 3-day SSL in relation to the January storm.

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