North Santiam River Basin Turbidity and Suspended-Sediment Study

Project Summary

Background

The lower mainstem North Santiam River, downstream of Detroit Lake and Big Cliff Reservoir, is the primary source of drinking water for residents of the City of Salem, Oregon, and the surrounding communities—an area with a combined population of more than 177,000. Because river water is used for consumption, many local agencies monitor any changes to water quality in the basin. For example, turbidity, the cloudiness of water caused by suspended particles, became a major concern for the City of Salem water-treatment facility in February 1996, when heavy rainfall and melting snow flooded streams to greater than 50-year levels. The high flows and accumulated rainfall saturated the landscape, mobilized landslides, and accelerated erosion throughout the North Santiam River basin. The resulting high turbidity in the North Santiam River forced the water-treatment facility to close its intakes for 8 days. Much of the colloidal material remained suspended for months after the event, which necessitated costly pretreatment to meet drinking-water standards. At the time, guidelines for drinking water stated that treated water must not exceed 1 NTU (Nephelometric Turbidity Unit) and not exceed 0.3 NTU in 95 percent of daily samples in any month.

After the turbidity-induced problems of 1996, the City of Salem determined that it needed real-time water-quality monitoring for the North Santiam River basin. In 1998, the City of Salem entered into a cooperative agreement with the U.S. Geological Survey to establish a near real-time, continuous streamflow and water-quality monitoring network in the basin. The network alerts water-treatment-facility operators of high-turbidity events and provides data that help identify sources of sediment and persistent turbidity—information that may help land managers minimize the effect of high-turbidity events in the future [excerpt, USGS SIR 2007-5178].

Pamelia Creek entering the North Santiam River during October 21, 2003 storm, looking downstream from confluence.

Automatic pumping samples collected at the North Santiam River below Boulder Creek (BCKO) water-quality monitoring station during October 21, 2003 storm.

Objectives

1. Establish a network of near real-time streamflow and water-quality stations to monitor short-term (daily, monthly, storm-to-storm, and seasonal) and long-term (year-to-year and decadal) temporal changes in stream discharge, stream temperature, specific conductance, pH, and turbidity.

2. Estimate suspended-sediment loads for the major subbasins and mainstem North Santiam River using correlations developed between real-time turbidity readings and suspended-sediment concentrations.

3. Identify the relative contribution of turbid water (both short-term and long-term persistent turbidity) from major subbasins and other primary sources to Detroit Lake and the North Santiam River at and above the City of Salem's water intake.

4. Establish an early warning system to monitor high streamflow and turbidity events in the North Santiam River Basin that may affect the operation of the City of Salem's water treatment plant.

5. Define the spatial extent of suspended-sediment loads and turbidity in the North Santiam River Basin relative to geology, land use, and topographic features.

Relevance and Benefits

The real-time streamflow and water-quality network designed in this project meets long-term USGS data collection goals and helps minimize loss of life and property by providing data for flood response and emergency management of the Salem WTP. Data from this project will benefit drinking water quality and endangered species recovery in the basin.

Approach

Multiparameter water-quality monitors are installed and telemetered at 10 sites, three at inputs to Detroit Lake and seven below the lake, and made available in real-time via USGS and U.S. Army Corps of Engineers websites. Daily streamflow (discharge), stage, water temperature, pH, specific conductance, and turbidity data will be processed and published for the 10 sites every water year. Suspended-sediment samples will be collected at all sites during different flow conditions. Mean daily and annual estimates of suspended-sediment load for all sites will be computed using regression equations derived from the instantaneous turbidity readings and suspended-sediment concentration data, and will be updated annually. Experiments are conducted analyzing turbidity changes over time from storm-event or high-flow samples collected concurrently with the fluvial sediment samples. Estimates of relative persistent turbidity from these tests, correlated to specific sediment size classes and settling rates, will be compared for each site and sampling period. Study results will identify the status and trends of suspended-sediment load and the relative contributions of persistent turbidity from the different subbasins. Hydrologic, landscape, and geomorphic features also will be used to further characterize the subbasins and explain the overall sediment-flux conditions in the North Santiam River Basin.