Delta Vs Estuary
The flow rate of rivers substantially decreases as they approach bodies of water that are still, such as lakes or the ocean. Additionally, silt is deposited as flows slow down. Except for grains that are small enough to remain suspended, almost all sediment transported by a river is thus dumped near the river's mouth. The deposited sediment may be reworked by lacustrine and marine processes and distributed along shorelines.
At the mouths of rivers, deltas developed that transported enough sediment to expand outward. Estuaries, on the other hand, are found where lake or ocean waters overflow into a river valley. Where the sediment carried by the river is collected is the primary distinction between the two. It gathers in the river basin for an estuary and lakeward or seaward of the typical shoreline for deltas. The equilibrium between the flow of sediment and variations in sea/lake level determines whether a particular river-sea/lake interaction is an estuary or a delta.
Deltas need a significant buildup of sediment at the river mouth, which might occur when the river contributes a lot of material, the sea or lake's transport operations are relatively sluggish, and/or the sea or lake level is gradually declining. Each of these scenarios has the river extending more into the body of still water over time. When the sediment supply is limited, large volumes of sediment are carried away from the shore by waves and storms, and/or the sea level or lake level is rising faster than silt is collecting, river valleys flood, forming estuaries.
Similar sedimentary rocks made up of fluvial, shallow marine facies, and shorelines are deposited in deltas and estuaries. The relationships between subenvironments and their vertical order across time, however, differ.