Estuarine salt-plug induced by freshwater pulses from the inner shelf

Abstract

Estuarine salt plugs can cause water quality problems and are usually attributed to hypersaline conditions in low-inflow estuaries, where 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 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. 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, under minimum river discharge, and least evident in summer. Wavelet coherence analysis showed an annual relationship between EOF1 weights and both, river discharge and along-channel wind-stress. This study shows evidence of a salt-plug development by buoyant water pulses from outside an estuary.