River mouth
River mouth bars form because the cross-sectional area of the expanding sediment-laden outflow increases, and consequently, the sediment transport rate down the jet centerline decreases basinward as flow progresses from confined to unconfined. More specifically, four stages of the river mouth bar formation are: (1) Turbulent jet, expanding into a shallow and sloping basin, first creates parallel subaqueous levees extending basinward and starting a river mouth bar basinward of the levee tips due to the decrease in jet momentum flux and resulting the high sedimentation rate in this region; (2) The subaqueous levees extend basinward and the river mouth bar aggrades and progrades since its presence causes flow acceleration on streamlines over the bar, and subsequently, this acceleration changes the sediment transport gradient over the bar triggering erosion on the upstream bar face and deposition in the downstream bar wake; (3) River mouth bar progradation stops and it stagnates when depth over the bar is shallow enough to create a fluid pressure on the upstream side of the bar forcing flow around the bar, and consequently decreasing velocity and shear stress over the bar top; (4) Finally, as the levees continue to grow and spread due to the presence of the bar, increased water and sediment discharges around the bar cause widening and creation of a classic triangular river mouth bar in plan view.