Introduction

The North Santiam River (fig. 1) is the primary source of drinking water for more than 177,000 people in Salem, Oregon, and those in surrounding communities. The U.S. Geological Survey (USGS), in cooperation with the City of Salem, has monitored water quality in the North Santiam River basin since October 1998 (table 1). Streamflow and water-quality data are recorded, transmitted, and available in real time on the USGS National Water Information System website (http://waterdata.usgs.gov/or/nwis). The network provides advance warning of any suspended-sediment surges that may require a change of operations at the City of Salem’s water treatment facility. The data also are used to compute the annual sediment loads at each of the monitoring stations and the suspended-sediment budget throughout the basin.

The North Santiam River basin encompasses 778 mi² on the western slopes of the Cascade Range. The basin elevation ranges from 10,497 ft at the summit of Mount Jefferson to 217 ft on the Willamette Valley floor. The City of Salem’s drinking water intake is located at the Geren Island water treatment facility near Stayton, Oregon (fig. 1). Two U.S. Army Corps of Engineers dams (Big Cliff Dam at about river mile (RM) 58 and Detroit Dam at about RM 61) regulate the flow of the river. Big Cliff Dam primarily is operated as run-of-river to reregulate the flow from Detroit Dam. The two dams effectively divide the North Santiam River drainage into an “upper” basin (upstream of Detroit Dam) and a “lower” basin (downstream of Big Cliff Dam and additional unregulated tributaries).

Background and Previous Investigations

In February 1996, a week of heavy rainfall and melting snowpack resulted in some of the most severe flooding in the history of northwestern Oregon. The North Santiam River streamflow-gaging stations crested at 50- to 100‑year interval flood events (Cooper, 2005). The increased suspended‑sediment load during the flooding caused the City of Salem to temporarily shut down the intakes to the water treatment facility. In the following weeks, the fluvial sediment from the lower basin largely flushed out as the streamflows receded, but much of the fine-grained sediment from the upper basin was still suspended in Detroit Lake. Because the lake water was gradually released downstream, the turbidity in the lower North Santiam River basin remained elevated for months. The City of Salem was forced to implement emergency procedures to remove the sediment from the river water and maintain the supply of drinking water to the community. Since 1996, improvements at the water treatment facility enable it to handle highly turbid water for extended periods of time.

In an effort to understand the sources and transport of sediment in the basin, the City of Salem initiated the North Santiam River Turbidity and Suspended-Sediment Study in cooperation with the USGS. As part of the study, a continuous real-time instream monitoring network was established in 1998. Originally, three stations were installed in the upper basin. During water year 2008, the network consisted of seven stations monitoring instream water quality and streamflow, and one station (Geren Island) monitoring water quality only (table 1). The monitoring network provides an advance warning system for high turbidity and helps to improve the understanding of sediment transport throughout the basin.

Several USGS publications have documented the North Santiam River turbidity and suspended-sediment study. Uhrich and Bragg (2003) documented the first 2 water years of the monitoring program. The report included a detailed description of the geology, land use, climate, and hydrology of the North Santiam River basin, and also documented the methods of data collection and analysis at the three upper-basin monitoring stations (North Santiam, Breitenbush, and Blowout). Bragg and others (2007) updated the data analysis and reported the suspended-sediment loads for water years 1999–2004 at the three original stations, one additional upper‑basin station (which is no longer in operation), and three lower-basin stations (Niagara, Little North, and Mehama). Sobieszczyk and others (2007) documented the major turbidity events and sediment sources associated with the highest suspended-sediment loads during water years 1999–2004. In addition, Sobieszczyk and others (2008) described a debris flow that occurred in the Milk Creek and Pamelia Creek drainage on the western slope of Mount Jefferson (figs. 1 and 2). The fluvial portion of the debris flow emptied into the upper North Santiam River, resulting in the highest turbidity and suspended-sediment concentration values at any monitoring station for the entire period of study (water years 1999–2008).

Purpose and Scope

This report presents a summary of the suspended‑sediment budget in the North Santiam River basin for water years 2005–08 with the purpose of identifying major sediment source areas that may affect water quality at the City of Salem’s water treatment facility. The annual suspended‑sediment loads are computed at seven monitoring stations and the historic streamflow is examined at two monitoring stations. The analysis methods involve updating the regression models annually for each station and estimating the suspended-sediment load related to the debris flow on Mount Jefferson. The sediment budget computations include the inputs to Detroit Lake, output from Big Cliff Reservoir, and additional contributions from the largest lower-basin tributaries. The combined sediment-trap efficiency of the lakes is calculated, and the seasonal transport of sediment is examined.