Formation and disappearance of salt plug in response to changes in river discharge in multi-channel tropical estuary

A salt plug or salinity maximum zone associated with evaporation and outwelling of salt from tidal salt flats at low tides has been observed in a single channel tropical estuary. In this study, a salt plug was identified in response to changes in river discharge in multi-channel tropical estuary having different tidal phases upstream. The longitudinal and time series depth profile of water temperature, salinity and density measured with conductivity-temperature-depth recorder were used to investigate the formation and disappearance of salt plug in the multi-channel Pasur river estuary (PRE). The longitudinal transects were obtained during spring and neap tides in dry summer, wet monsoon and dry winter seasons in 2014. The time series data were also taken during spring and neap tides in dry summer and wet monsoon seasons in 2014. The longitudinal and time series data of salinity and water density showed a persistence of salt plug in Pasur river estuary in dry summer and winter seasons. An export of high saline water from Shibsa river estuary (SRE) to PRE through Chunkhuri channel interconnected at about 70 km upstream from coastal entrance occurred due to arriving of tidal waters about half an hour early near the entrance of Chunkhuri channel in Shibsa river estuary than that in PRE. The tidal range is also higher in SRE than that of PRE. As a result, the earlier tidal water of SRE pushed the saline water from SRE to PRE in dry summer and winter seasons due to relatively low water level in PRE. The salt plug persisted for seven months (December~June) in dry summer and winter seasons, and its relative position changed with the magnitude of freshwater discharge. In contrast, the high river discharge (> 400 m³s^-1) flushed out this salt plug from PRE to Bay of Bengal during wet season and hindered the export of saline water from SRE to PRE due to relatively high water level in PRE, even though the saline water existed in Chunkhuri channel compared to PRE. As a result, PRE shows the circulation of a typical estuary in wet season, when salinity decrease upstream from downstream.