delta
A fan-shaped deposit of sediment at the mouth of a river where it enters a lake, a sea, or the ocean — the river's life's work, spread out at the end. As the current slows and loses its ability to carry sediment, the material drops: the heaviest first, the finest last, building a flat, branching, ever-extending platform of new land. Deltas are among the most fertile landscapes on earth and among the most threatened by rising seas.
Etymology
Greek delta, the fourth letter of the alphabet (Δ), whose triangular shape resembles the plan view of a river mouth. Herodotus first applied the name to the Nile delta in the 5th century BC.
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