Abstract
Freshwater input to estuaries may be greatly altered by the river barrages required to meet human needs for drinking water and irrigation and prevent salt water intrusion. Prior studies have examined the salt plugs associated with evaporation and salt outwelling from tidal salt flats in single-channel estuaries. In this work, we discovered a new type of salt plug formation in the multi-channel Pasur River Estuary (PRE) caused by decreasing river discharges resulting from an upstream barrage. The formation of a salt plug in response to changes in river discharge was investigated using a conductivity-temperature-depth (CTD) recorder during spring and neap tides in the dry and wet seasons in 2014. An exportation of saline water from the Shibsa River Estuary (SRE) to the PRE through the Chunkhuri Channel occurred during the dry season, and a salt plug was created and persisted from December to June near Chalna in the PRE. A discharge-induced, relatively high water level in the PRE during the wet season exerted hydrostatic pressure towards the SRE from the PRE and thereby prevented the intrusion of salt water from the SRE to the PRE.
Highlights
The natural variability of river inflows to estuaries has been greatly modified by dam construction for irrigation, drinking water and energy production
We investigated the influence of the discharge-induced relative water level variation between the Pasur River Estuary (PRE) and the Shibsa River Estuary (SRE)
The salinity section in December implies an initiation of salt water intrusion from the SRE to the PRE through the Chunkhuri Channel during the transition from the wet season to the dry season when the monthly median river discharge was 21 m3 s−1
Summary
The natural variability of river inflows to estuaries has been greatly modified by dam construction for irrigation, drinking water and energy production. Dry season, evaporation exceeding precipitation results in a salinity maximum zone near the mouth of tropical estuaries that is referred to as a salt plug[5]. The Pasur River Estuary (PRE) is a complex, tropical multi-channel estuarine system connected by the Chunkhuri, Pankhali and Batiaghata Channels to the Shibsa River Estuary (SRE) upstream (Fig. 1b,c). The salt water intrusion has extended as far as ~164 km from the estuarine mouth (at Hiron Point) to a head at Lohagara, Narail, during the spring tide in the dry season (March 2014), with a salinity of 0.1919. Previous studies have typically omitted hydrographic details, such as longitudinal and time-series depth profiles of water temperature, salinity, and density, which are necessary for understanding the hydrodynamics and causes of salt water intrusion upstream through this complex multi-channel estuarine ecosystem
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