Dynamic processes in the coastal waters play an important role in regulating marine pollution distribution caused by riverine inputs and are relevant for coastal management. Here we investigate the coastal water properties from field measurements and modeling hydrodynamic processes in the northeastern coast of Sumatra. The present study found that the river discharges affect a low salinity of 28-29 PSU in the surface waters along near the coastal line. The river discharge might influence by strong La Niña with high rainfall in December 2010. However, we suggest that the effect of tidal mixing is stronger than the freshwater discharges, resulted in vertically well-mixed coastal waters in the region. The observed tidal range of 200 cm indicates a strong tidal mixing in the waters. The tidal elevation contributes more than 70% of the total measured sea elevation. The tidal current signal is also dominant (77%), in which the flow pattern simulations show no significant differences among tide and wind-tide driven currents.