Evaluation of Influence of River Inflow Discharge and Tidal Level to Spatial-temporal Distribution of Salinity in Can Gio Area, South of Vietnam by Two Series of Field Observations

This study assesses the influences of river inflow discharge and tidal levels on the spatial–temporal distribution of salinity along the main rivers in Can Gio mangrove forest in the south of Vietnam. Field observations were conducted to collect salinity time series, spatial salinity distribution, and vertical profiles of salinity during two periods in the 2017 and 2018 wet seasons. Period I (2018) experienced a high discharge from upstream rivers as well as at middle tide, while Period II (2017) had a regular discharge from the upstream rivers and at spring tide. The results showed that salinities in Can Gio area changed depending not only on the tidal regime from the East Sea, but also on river discharge resulting from rainfall runoff. Most of the freshwater flows into Can Gio area through Phu Xuan, which is located at the upstream end of the observed area. The salinities at Phu Xuan changed significantly in Period II, with the practical salinity unit (psu) ranging from 1.6 to 6.0, while in Period I salinity only ranged from 0.1 to 0.4 psu. The salinities along the main rivers in the area, Long Tau River and Soai Rap River, changed depending on their respective distances to the sea, i.e. sites closer to the sea had a higher salinity. Conspicuously, the salinities along Thi Vai River were very high, approximately 20psu, even though the distances to the sea were relatively large. This unusual salinity prevalence can be explained by the location and surrounding topography of Thi Vai River, as its upstream inflow is relatively small. Furthermore, this narrow inlet is combined with peculiar topography and a river intersection, causing an unmixing water area.