A mouth bar is a bar in a river that is typically created in the middle of a channel in a river delta. It is created by a positive feedback between mid-channel deposition and flow divergence. As the flow diverges near the ocean, sediment settles out in the channel and creates an incipient mouth bar. As flow is routed around the incipient bar, additional sediment is deposited on the incipient bar. This continued process results in the formation of a full-fledged mouth bar, which causes the channel to bifurcate. This continued process leads to the characteristic fractal tree pattern found in some prograding river-dominated deltas.
^Edmonds, D. A.; Slingerland, R. L. (2007). "Mechanics of river mouth bar formation: Implications for the morphodynamics of delta distributary networks". Journal of Geophysical Research. 112: F02034. Bibcode:2007JGRF..11202034E. doi:10.1029/2006JF000574.