Delta Life Cycle
A delta will either eventually reach a state of equilibrium between river deposits and tidal forces or will enter a cycle of progradation and transgression in which it grows and recedes based on the cyclical nature of the river system (i.e. floods, spring river runs, etc.).
Stagnate – This is the term for a delta that has reached equilibrium.
Prograde – A prograde delta is one in which the depositing of silt outweighs the tidal force dragging it away. These deltas grow outward into the large body of water until equilibrium is achieved.
Aggrade – An aggradation occurs when the delta starts to accumulate biomass. This can be from plant growth or from the collection of decaying organic material collected from the tides. These deltas grow upwards instead of outwards.
Transgression – A delta in transgression is receding from the sea. The tidal force is stronger than the river and thus pulls away more silt than is being deposited.
Where a river delta begins and where it ends are always approximations. To determine what is a river delta and what is not would require consistent monitoring of silt deposits. Because this is impractical, the area that is considered the beginning of a delta usually is determined by the coastline surrounding the river while the end of the delta is determined by the reach of the sediment deposits at low tide.
Of course, these are just estimates and the lower delta plain almost always extends past the low tide region, but there is no way to determine where it ends and the sea floor begins. There is simply a transitional area. In any case, the important thing is that the river delta is a transitional space between the river and the larger body of water.