How does a river delta form at the mouth of a river?

A delta forms at the mouth of a river when the volume of sediment delivered by the river exceeds the amount that coastal processes—such as waves, tides, and currents—can remove. This accumulation typically occurs on a gently sloping continental shelf, where the river’s velocity decreases as it enters a body of standing water, causing it to deposit its sediment load.123

The development of a delta involves a dynamic balance between fluvial sediment supply and marine energy. As sediment accumulates, the delta progrades, or builds seaward. This process creates a distinct stratigraphic sequence:452
* Topset beds: Horizontal layers deposited in subaerial environments like marshes and channels.2
* Foreset beds: Inclined layers that form the delta front, where coarser sediment settles as the river meets the sea.2
* Bottomset beds: Fine-grained clays deposited in the deeper, seaward prodelta zone.2

Over time, deltaic growth is highly variable; rivers may shift their channels through crevasses to form new lobes, while abandoned lobes are eroded by marine processes. The final shape of the delta depends on the interaction between the river's discharge and the energy of the coastal environment.615