As with many things, the answer lies in Ancient Greece.

A delta is an area of low, flat land on which a river splits and spreads out into several distributaries, or branches, before entering the ocean or sea. The river forms deposits of soil and sand, often called sediment or alluvium. And, in many cases, the shape of a delta resembles a triangle.

The oldest surviving reference to this area of a river as a delta is from the writings of the famous Greek historian Herodotus, who used the word delta because he thought that the lay of the land around the area where the Nile River enters the Mediterranean Sea (above) vaguely resembled the Greek letter Delta, the capital version of which resembles a triangle.

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