Estuarine salt-plug induced by a freshwater pulse from the inner shelf

Estuarine salt-plugs can potentially cause water quality problems and are usually attributed to hypersaline conditions in low-inflow estuaries, when evaporation rates exceed or match freshwater input rates. However, salt-plugs may also develop in typical estuaries, in brackish waters, by an additional freshwater influence from the estuary’s mouth. In this study, month-long and years-long data showed the formation of a salt-plug generated by a freshwater pulse from the ocean into Barataria Bay, in the Mississippi River Delta. Month-long currents showed an inverse estuarine circulation at Barataria Pass with inflow in the surface and outflow underneath. A dimensionless analysis showed that the dynamics governing the inverse circulation was related to a density gradient reversal, which was related to a salt-plug. However, tidal stresses dominated over inverse density gradients in driving the residual circulation during tropical tides. Empirical Orthogonal Functions applied to nine years of salinity values along the estuary showed that mode 1 (EOF1) represented the salt-plug structure and explained 86% of the variability. The salt-plug structure was most pronounced during winter and least evident in summer. Wavelet coherence analysis showed an annual relationship between EOF1 weights and both, river discharge and along-channel wind-stress. During winter, river discharge had annual minima and southward wind-stress had annual maxima. This study shows evidence of a salt-plug development by freshwater pulses coming from the outside of an estuary.