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Suspended sediment behavior in a coastal dry-summer subtropical catchment: Effects of hydrologic preconditions
Citation:
Gray, A., J. Warrick, G. Pasternack, E. Watson, AND M. Goni. Suspended sediment behavior in a coastal dry-summer subtropical catchment: Effects of hydrologic preconditions. Geomorphology. Elsevier BV, AMSTERDAM, Netherlands, 214:485-501, (2014).
Impact/Purpose:
Small mountainous watersheds in tectonically active regions contribute disproportionately to sediment flux to the global ocean . The purpose of this study was to identify the primary controls on sediment transport from a small mountainous watershed (the Salinas River), on California's central coast, including factors such as land use changes, fires, and retention behind reservoirs. In this manuscript, we explore the role of antecedent conditions in structuring river basin response to precipitation forcings, using analysis of residual error terms based on regression analysis of flow-suspended sediment concentration relationships (Q-css). We find that the sediment transport from the river basin varies based on hydrologic preconditions. These results have significant implications for understanding sediment transport, and also for water quality studies. Downstream areas have active sediment management plans (primarily for beach nourishment), and numerous ecotoxicological studies have found poor aquatic water quality based on the presence of agricultural chemicals. This study may inform future water quality sampling patterns, and will help inform beach maintenance for the Monterey Bay region.
Description:
Variation in fluvial suspended sediment–discharge behavior is generally thought to be the product of changes in processes governing the delivery of sediment and water to the channel. The objective of this study was to infer sediment supply dynamics from the response of suspended sediment behavior to antecedent hydrologic factors. The Salinas River (California) is seasonally active,moderately sized, and potentially susceptible to lasting impacts of hydrologic event history because of aridity, high discharge variability, and in-channel terminating flows. Forty-five years of suspended sediment data from the lower Salinas and 80 years of hydrologic data were used to construct hydrologic descriptors of basin preconditioning and to test the effects of these preconditions on suspended sediment behavior. Hydrologic precondition factors — including change in mean daily discharge and increasing elapsed time since the last moderate discharge event (~10–20 times mean discharge (Qmean)) — were found to have significant positive effects on discharge-corrected, fine suspended-sediment concentrations. Conversely, increased elapsed time since the last low discharge event (~0.1–0.4 times Qmean), and the sum of low flow conditions over interannual time scales were found to cause significant negative trends in fine suspended sediment concentration residuals. Suspended sand concentrations are suppressed by increased elapsed time after threshold discharges of ~0.1–2 and 5–100 times Qmean, and increased low to no flow days over time scales from 1 to 2000 days. Current and previous year water yield and precipitation magnitudes correlate positively with sand concentration. Addition of fine sediment from lower Salinas hillslope or channel sources on the rising limb of the hydrograph is the major mechanism behind an overall positive hysteretic pattern, which was forensically supported by the annual occurrence of in-channel suspended sediment deposition by early season, channel terminating flows and by the flushing function ofmoderate hydrologic events found in this study. The importance of hillslope and/or channel fine sediment contributions proximal to the lower Salinas are further highlighted by the lack of control exerted by upper subbasin water provenance on fine suspended sediment concentration, while sand behavior is differentiated by upper basin water provenance. Investigation of suspension of bed-sized sediment showed that the channel bed could exert significant effects on fine and sand-sized suspended sediment dynamics, but this mediation for fine sediment was most likely small in terms of decadal-scale sediment budgets. The magnitude of the effects of hydrologic variables on sediment dynamics remains uncertain, but the factors identified here may play a significant role in water quality, if not long-term sediment flux to the ocean.
