In this work, extensive numerical simulations have been performed to assess the hydrodynamic and morphodynamic behavior of a river jet debouching in a large quiescent water body. A refined three-dimensional grid has been used to capture the transition zone between a stable jet and an unstable meandering jet. The model results show that the stability number S, which is a function of friction and river mouth aspect ratio, and the mouth Reynolds number are the two parameters that describe the stable/unstable character of the jet. From a morphodynamic point of view, a stable jet always tends to form a mouth bar. However, a decrease of the stability number together with jet instability increase the delivery of sediments to the jet margins, favoring the formation of subaerial levees and elongated channels. Frictional effects play a major role to set the distance at which the mouth bar becomes stagnant. The importance of the stability number in setting depositional patterns at the river mouth is larger than other variables (i.e., momentum of the jet and potential vorticity) and therefore should be considered in the design of restoration schemes for deltaic land.