Deltas form where rivers deposit their sediment, typically in response to a change in slope and/or entry into a body of water (which is pretty much a change of slope to 0). They usually take the form of a pie-shaped wedge because as the silt is dropped in the river channel, it raises the river bottom, forcing the river to the side, where it creates a a new channel. This process repeates over and over, so the river moves back and forth, depositing new sediment to form the alluvial fan.
Deltas often have more than one distributary because the sediment drop is liable to clog the one channel and split it into multiples. Deltas change relatively rapidly over time, so a map from 100 years ago is unlikely to reflect current realities.

Estuaries are places where a freshwater river meets the tides of a sea. As such, they are usually found on or near deltas. The mixing of fresh and salt water creates a habitat that works well for many plants and animals. They are often marshy (grassy wetlands) areas, but some have more woody plants as well (the woody plants and trees moving them over to the swamp side of the spectrum).