CONTINUOUS TURBIDITY MONITORING IN STREAMS OF NORTHWESTERN CALIFORNIA

Abstract

Overview. Redwood Sciences Laboratory, a field office of the USDA Forest Service, Pacific Southwest Research Station has developed and refined methods and instrumentation to monitor turbidity and suspended sediment in streams of northern California since 1996. Currently we operate 21 stations and have provided assistance in the installation of 6 gaging stations for agencies, municipalities, universities, and citizens groups. These installations employ a method called Turbidity Threshold Sampling (TTS), an automated data collection and sampling system in which a data logger employs real-time turbidity to control a pumping sampler (Lewis and Eads, 2001; Lewis, 1996). It is common in streams and rivers for most of the annual suspended sediment to be transported during a few, large rainstorm events. Automated data collection is essential to effectively capture such events. TTS was designed to permit accurate determination of suspended sediment loads by establishing a relation between suspended sediment concentration (SSC) and turbidity for each sampling period with significant sediment transport. It does so by collecting pumped suspended sediment samples when pre-selected turbidity conditions, or thresholds, are satisfied. During analysis the relations are applied to the nearly continuous turbidity data for the respective sampling periods to produce a continuous record of estimated SSC (Lewis, 2002). The product of discharge and estimated SSC is then integrated to obtain accurate suspended sediment yields. Additional benefits of TTS are (1) it provides samples that can be used to determine whether turbidity spikes resulted from fouling or actual sediment transport, and (2) the continuous record of turbidity is useful for revealing the timing of erosion events, assessing impacts on beneficial uses, and enforcing water quality regulations.