Why must a delta receive more sediment than coastal processes can remove?

A delta forms at the mouth of a river only when the volume of sediment delivered to the coast exceeds the capacity of marine processes—such as waves, tides, and currents—to erode or redistribute that material.12

The shoreline acts as a dynamic "battleground" between the river's sediment supply and the energy of the ocean. If coastal processes are stronger than the rate of sediment delivery, the material is dispersed, preventing the accumulation necessary to build a deltaic landform. Conversely, when sediment supply is dominant, the excess material accumulates at the river mouth, allowing the delta to prograde (build forward) into the body of water. This growth is essential for maintaining the delta's unique stratigraphic sequence, where coarse deposits build forward while finer materials are carried further seaward.31