HYPERPYCNAL RIVER PLUME PROCESSES – INSIGHTS FROM THE SMALL, SEMI-ARID RIVERS OF SOUTHERN CALIFORNIA

The rivers of southern California deliver the majority of their sediment load to the Pacific Ocean load during large, infrequent events. These events are especially turbid with suspended-sediment concentrations commonly exceeding 10's of g/l. Further, we calculate that >50% of the historical sediment discharge from southern California was discharged at concentrations exceeding the theoretical negative buoyancy (i.e., hyperpycnal) threshold of 40 g/l. Dispersal processes and fate of this river sediment in the coastal ocean is important not only for the understanding of strata formation, but also for evaluating the mixing and fate of pollutants from these rivers. Here we report on observations of two sediment transport events on the inner continental shelf (10m water depth) occurring immediately after discharge events from the Santa Clara River. These observations reveal that highly concentrated fluid muds existed in the lower 20-50 cm of the water column immediately following discharge events measured by acoustic and optical sensors. These observations of fluid muds coincided with decreased bottom salinity and offshore bottom currents of 5-10 cm/s. These offshore bottom currents were strongest in the bottom 20-50 cm of the water column and deviated strongly from the typical logarithmic bottom-boundary current profiles expected if derived purely from overlying currents. Because the first of these events occurred when the river sediment concentrations (~10 g/l) were below hyperpycnal thresholds (40 g/l), we suggest that river sediment under such conditions can rapidly settle from buoyant plumes at rates of >2 cm/sec, consistent with the laboratory work of McCool and Parsons (2004). Thus, we propose that river sediment dispersal exhibits a continuum of processes based on the suspended-sediment concentration (particle settling, flocculation, convective sedimentation, and buoyancy-related phenomena), rather than a simple hyper/hypopycnal model.