Flow and Sediment Flux Asymmetry in a Branching Channel Delta

River deltas form complex branching patterns that distribute sediment to the coastal sea. The routing and storage of this sediment in deltas are poorly understood. We present results of a 1‐month study of the sediment and water transport through two branches of the Wax Lake Delta on the coast of Louisiana. The two channels maintained near‐equal total partitioning of flow and sediment discharge. East Pass was narrower and had higher tidally averaged velocities, lower tidal velocity fluctuations, more sediment flux, and less alluvial bed cover than Main Pass. We connected these differences to small differences in the geometry of the channels and feedbacks between these differences. East Pass was slightly shorter than Main Pass, leading to a steeper mean free surface profile, yielding higher subtidal currents and advective sediment fluxes. Main Pass received the tides from Atchafalaya Bay earlier than East Pass, allowing tides to reach their junction earlier from Main Pass. This led to Main Pass capturing tidal prism from East Pass and higher tidal velocity fluctuations in Main Pass. These fluctuations caused larger flow convergences in Main Pass, pumping sediment out of East Pass on tidal timescales. Since East Pass had higher mean velocities and lower tidal fluctuations, it spent less time in flow regimes that would allow the formation of alluvial bed cover. By explaining these asymmetries with physical arguments, we showed how geometric differences drive asymmetries in branching systems. Barrier island inlets, coastal lagoons, and other systems may display similar behavior. Plain Language Summary This study explores water and sediment transport through the Wax Lake Delta into the coastal ocean due to geometric differences between the delta's channels. Sediment partitioning into different channels determines deposition and, ultimately, delta form. The two channels studied here, wider Main Pass and narrower East Pass, partitioned water and sediment nearly equally. East Pass was shorter than Main Pass, however, and was therefore the preferred pathway for flows per width to the coastal ocean. Conversely, Main Pass was the preferred pathway for the tide to fill the delta because Main Pass received the tide earlier than East Pass. Within the delta, flood tides from Main Pass converged with river flow, pumping sediment out of East Pass on ebb tides. Since East Pass had higher river velocities and weaker tides, water velocities were rarely low enough to allow sedi