Characterizing the Concentration, Duration, and Frequency of Turbid Events in Tennessee Streams: Potential for Macroinvertebrate Impairment

The impairment of lotic systems due to siltation is one of the most common factors leading to a stream being placed on the 303d list. Once a stream reach is placed on the 303d list, a state’s environmental regulatory agency must then develop sediment total maximum daily loads (TMDLs). However, a deficiency exists in available methods for assessing biotic response to siltation, creating the inability to set TMDLs functionally related to cause of impairment. Water quality sondes can collect voluminous amounts of turbidity data and stage data at intervals that can be used to characterize concentration, duration, and frequency (CDF) of flows with elevated turbidities. Data were collected from 10 streams located in both the Interior Plateau (ER 71) and the Ridge and Valley (ER 67) ecoregions of Tennessee. Utilizing a Poisson arrival approach, sediment transport flux was analyzed stochastically by observing the frequency and duration of recorded turbid events over designated threshold levels for a 6-month period. Turbidity measurements converted into concentrations of suspended sediment and characterized through CDF curves allowed comparison between biotic community structure and episodic fluxes of suspended sediment transport. CDF curves identified a strong influence on the duration of turbid events due to contributing basin scale. The significant results of a combination of bivariate regressions and a Welch’s test of means suggested that the frequency of elevated sediment concentrations explained the most variance in macroinvertebrate response to siltation. The CDF methodology appears to be a practical means for distinguishing suspended sediment flux behavior resulting in biological impairment.