A delta is formed where a river enters the sea, where the river flow velocity drops and much of the sediment carried by the river is deposited at or near the delta front. As most sediment deposition occurs near sea level, ongoing deposition means that deltas tend to grow ('prograde') seaward. How fast they grow depends on how much sediment the river is carrying that can be deposited in the sea; and on the depth of the sea at the delta front. Sediment deposition occurs in river channels and floodplains, in estuaries, on tidal flats, on beaches, and further out on the delta front and the deeper (below wave base) prodelta. Each of these environments produces its own typical sedimentary facies.
Sediment deposition in a delta is very sensitive to sea level change. XXXX