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

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Introduction

The North Santiam River drains 778 mi² of land from its headwaters in the Cascade Range to its confluence with the South Santiam River 12 mi south of Salem, Oregon (fig. 1). The altitude of the North Santiam River basin ranges from nearly 10,500 ft at the Mount Jefferson summit to 460 ft at the Willamette Valley floor. Like other Western Cascade rivers, the North Santiam River originates in the mountains as glacial runoff, snowmelt, and ground-water outflow. As the river runs its course, tributaries such as Boulder Creek, Rock Creek, and the Little North Santiam River flow into the mainstem.

The North Santiam River is regulated by two dams, both constructed in 1953 by the U.S. Army Corps of Engineers. Detroit Dam is the larger of the two and farther upstream, and has a drainage area of 437 mi². Detroit Dam impounds Detroit Lake, providing flood control, pollution abatement, power generation, irrigation, and recreation. Water-surface altitudes on Detroit Lake vary by season and hydrologic conditions. The maximum pool altitude is 1,574 ft, full pool is 1,569 ft, and minimum conservation pool is 1,425 ft (U.S. Army Corps of Engineers, 1953). Storage capacity for Detroit Lake ranges from 455,000 acre-ft in the summer to 115,000 acre-ft in the winter. Big Cliff Dam is 3 mi downstream of Detroit Dam. The water-surface altitude of Big Cliff Lake ranges from 1,210 to 1,180 ft as it regulates water releases from Detroit Dam.

The North Santiam River basin is prone to occasionally severe erosion resulting from steep slopes, a dense network of tributaries, and periods of intense rainfall. Notably, a storm front with unusually mild temperatures and humid air moved through the region during February 1996, resulting in 12 –15 in. of rainfall in 4 days (Oregon Climate Service, 2005). The intense rainfall, coupled with rapidly melting snowpack, caused peak streamflows with recurrence intervals of 50–100 years at many streamflow-gaging stations (Cooper, 2005). The flooding triggered Federal disaster declarations for all affected counties. Because the North Santiam River supplies drinking water for the city of Salem and neighboring communities, the turbid flood water caused numerous financial problems. The city of Salem’s Geren Island water-treatment facility closed for 8 days because of its inability to process the sediment-rich water. Costs for this closure and other storm-related problems totaled $1.1 million to the water utility (Hulse and others, 2002).

The financial burden and environmental consequences of the 1996 flooding prompted the city of Salem to coordinate with State and Federal agencies, including the U.S. Geological Survey (USGS), to establish a water-quality monitoring network (fig. 1). The North Santiam River Turbidity and Suspended-Sediment Study was initiated in 1998 to investigate the sources and transport of sediment causing high turbidity. Uhrich and Bragg (2003) detailed the first 2 years of monitoring in the North Santiam River basin, from October 1998 to September 2000. The report included streamflow and water-quality data for three monitoring stations in the upper basin (upstream of Detroit Dam) and established a technique for estimating suspended-sediment loads. Since September 2000, five additional stations have been added to the monitoring network, including four stations in the lower basin (downstream of Detroit Dam). The ongoing water-quality monitoring and additional research are jointly funded by the city of Salem and the USGS.

Purpose and Scope

This report provides additional data to update and expand the analysis presented in the previous report (Uhrich and Bragg, 2003). The regression models relating turbidity and suspended-sediment concentration for the three existing upper-basin monitoring stations are refined by inclusion of additional data. Regression models for turbidity and suspended-sediment concentrations also have been established for four additional monitoring stations. Annual suspended-sediment loads and yields for water years 1999–2004, when both streamflow and turbidity data are available, are estimated for the seven monitoring stations. The streamflow data for several long-term gaging stations are analyzed to provide historical context for the high-flow and high-turbidity events. The highest turbidity events are then identified and discussed in relation to the annual suspended-sediment loads.

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