Rivers debouching into the sea are subjected to tidal variation at the river mouth. Tide- river interaction can cause variations in discharge by giving an additional gradient in water level which known as a backwater effect and somehow can cause serious flood event. The main purposes for this paper are to determine the effect of backwater on the river under steady flow and flood condition. Hydrodynamic model was believed has the capabilities to simulate the behaviour of tide and flood coming from the river upstream. Numerical simulation of backwater effect due to riverine discharge into the open sea shows that significant backwater effect can be observed landward up to the point where the channel bed rises above the mean sea level. Tidal effect dominates in this lower reaches, whereas riverine peak discharge tends only to increase the water level in the upper reaches. The simple, idealized tide-river model reproduces the hydraulic of low land river well and can be extended to investigate the full range of tide and river flow to generalize the effect for backwater prediction.