Suspended sediment transport at a tidal river divergence

This study investigated suspended sediment routing through a diverging tidal river channel, a complex landform largely ignored in the geomorphic literature. The lack of research is largely due to the assumption that suspended sediment 'goes with the flow', meaning that a calculation of the ratio of discharge divided between downstream branches will yield a similar ratio for suspended sediment concentrations (SSC) by proxy. The goal of this study was to determine: (1) if the 'go with the flow' assumption is legitimate and (2) if non-uniform distributions of cross-channel SSC driven by secondary flows and channel bathymetry play a role in suspended sediment routing through a tidal river divergence. These questions were investigated at a divergence on the Sacramento River at Georgiana Slough in the Sacramento-San Joaquin River Delta in central California over a range of flow and tidal conditions. Instrumentation was boat-based and consisted primarily of an Acoustic Doppler Current Profiler and Optical Backscatterance Sensors to measure flow and suspended sediment across six channel cross-sections. Fixed tide gages were also installed in each branch of the divergence. The data show that channel bathymetry and secondary flows contributed to preferential transport of suspended sediment into the larger downstream branch, but that the total effect was small (∼3%). During periods of low-flow coupled with flood tide, significant variations in SSC between the downstream branches occurred for a short period of time, due to a reversal of flow direction on the larger downstream branch (lower Sacramento River). Different tidal signatures of the downstream branches resulted in a seaward directed water slope in Georgiana Slough, giving it the capacity to receive the flows of both the mainstem and lower Sacramento River. This condition was brief (∼1 hr) and total suspended sediment discharge was minimal due to small flow velocities. A modeling exercise showed that the effects of secondary flows and non-uniform cross-channel distribution of SSC were overwhelmed by the greater magnitude downstream velocity field. Resource managers interested in suspended sediment availability for tidal marsh reconstruction can use the 'go with the flow' assumption with a large degree of certainty.