Suspended-Sediment Budget

The results of the annual suspended-sediment budget computations are presented in table 5. During water years 2005–08, the upper North Santiam River was the largest contributor of suspended sediment to Detroit Lake. The 4-year SSL total at North Santiam was dominated by sediment transported during the Mount Jefferson debris flow and the subsequent storm in water year 2007. The Breitenbush River was the second largest sediment contributor in the upper basin followed by Blowout Creek.

The Little North Santiam River was the largest contributor of suspended sediment to the North Santiam River at Geren Island for water years 2005–08. The highest SSL and second-highest annual percentage at Little North were related to the peak winter storms during water year 2007. (The highest annual proportion for Little North was artificially inflated because of the lack of data for Rock in water year 2005.) The output from Detroit Lake (Niagara) was the second largest sediment contributor in the lower basin followed by Rock Creek.

The computed trap efficiencies of Detroit Lake (table 6) were consistent during water years 2005–07, despite the variation in annual mean streamflow. In water year 2008, the record snowpack in the Cascade Range (more than 200 percent of the 30-year average; Natural Resources Conservation Service, 2009) increased the streamflow and suspended-sediment input to Detroit Lake into the late spring. The lake level was kept below full pool elevation by increasing the outflow from Detroit Dam throughout May and June 2008. This movement of water through the lake prevented the suspended sediment from settling and resulted in a trap efficiency value lower than the other 3 water years. The increased streamflow and SSL at Niagara in the spring and summer of water year 2008 also resulted in the highest proportion (39 percent) of the lower basin suspended-sediment budget for water years 2005–08 at that station. SSL inputs to the lake were higher during water years 2006 and 2007, but the suspended sediment had more time to settle, resulting in lower percentage contributions at Niagara to the lower basin sediment budget.

The monthly suspended-sediment budget results are presented in figure 10. Late fall and early winter were the dominant seasons for sediment transport during water years 2005–08. About 90 percent of the sediment input to Detroit Lake and 80 percent of the sediment output from Big Cliff Reservoir was in November, December, and January during the 4 years. November and December were the highest months for input to the lake, while January was the highest for output. This demonstrates the capacity of Detroit Lake to store suspended sediment and delay its release downstream. During these same months, 87 percent of the total SSL was transported past the City of Salem’s Geren Island water treatment facility.