Effects of hydrologic event history on suspended-sediment behavior

The suspended-sediment yields of many developed watersheds have decreased with time, and increased urbanization and hydrologic modifications are often identified as contributing mechanisms. Examination of a river system that did not experience these alterations during the period of record, yet displayed high variability in suspended-sediment behavior and a decreasing trend in sediment yield provided an opportunity to evaluate the effects of hydrologic event history. The objectives of this study were to identify the time-dependent behavior of suspended-sediment concentrations at the terminus of the Salinas River, California since the initiation of monitoring in the late 1960's, and determine the hydrologic factors that influenced this behavior. The Salinas is a seasonally active river of moderate size that may be particularly susceptible to the effects of hydrologic event history on suspended-sediment behavior due to the high variability of discharge in this system. Sediment and hydrologic data were obtained from samples collected by the USGS from 1967-2010 and the authors from 2008-2011. Comparisons of chronologically stratified rating relationships between fine suspended-sediment concentration (CSSf) and instantaneous water discharge using ANCOVA revealed that changes in rating curve offset and slope had occurred over time. Suspended-sediment yield from the Salinas had decreased, despite little change in the proportion of urbanized land area and no major dam emplacement during the period of record. To evaluate the potential effects of hydrologic and landscape forcing factors on suspended-sediment behavior, sediment concentration residuals about the rating curves were compared with sample attributes including: hydrograph position and hydrologic routing history, seasonality, basin aridity, major and moderate hydrologic event proximity and effective wildfire burn area. The following hydrologic factors were found to have significant positive effects on discharge corrected CSSf: the ratio of mean daily discharges between the date of the sample and the previous day, and elapsed time between sample date and the last major hydrologic event. Elapsed time between sample date and the last moderate discharge event, and effective burn area were found to have significant, negative effects. Only effective burn area possessed a significant temporal trend, which was positive. As increased burn area is generally associated with an increase in suspended-sediment delivery, the apparent negative response of CSSf to effective burn area and the positive temporal trend of this factor indicate that other temporally trending factors not identified by this study are affecting a negative trend in CSSf over time. The positive effect of daily discharge ratio on discharge corrected CSSf implied that fine suspended sediment in the lower Salinas displayed a generally positive hysteretic behavior, which was supported by the prevalence of positive hysteresis in events with sufficient data density for analysis. Identification of the preferential mobilization of sediment on the rising limb of the hydrograph as the major mechanism of the overall hysteretic pattern is forensically supported by the annual occurrence of in-channel suspended-sediment deposition by early season, channel terminating flows, and the flushing function of major hydrologic events found in this study.