What Is A Geological Delta

A delta is located at the mouth of a river.

A delta is a low, nearly flat area constructed of sediment deposited at a river’s mouth, which is where the river ends at the shore of an ocean or large lake.

Why a “Delta”?

The name “delta” refers to the feature’s characteristic triangular shape when viewed from above. A capital letter Delta in the Greek alphabet also has a triangular shape.

Well-Known Deltas

In the United States, the best-known delta is that of the Mississippi River, which underlies most of southern Louisiana. Other well-known North American deltas include the Rio Grande (United States and Mexico) and the Mackenzie River (Alaska and Canada). The largest in the world are the Amazon River (Brazil) and the Ganges-Brahmaputra (India).

How Deltas Form

Deltas form where streams and rivers enter large bodies of water. The flowing water of a river allows it to move sediment, but when a river encounters the relatively still waters of a lake or ocean lack, the sediment stops moving and settles to the bottom. Wherever the action of waves and tidal currents cannot keep up with the amount of sediment dropped by a river, a delta forms.

Deltas and Subsidence

The famous above-ground cemeteries of New Orleans are necessary because the city lies below sea level.

Year after year, rivers drop millions of tons of sediment at their mouths. The enormous weight of this pile of sediment compresses the older sediments beneath it, and the total can cause the Earth’s crust to bend as well. As a result, large deltas subside, or sink, through time. Normally the river simply keeps the top of the delta very near sea level, but if human activity prevents the deposition of sediments on the delta top, the entire delta can sink. This explains why the city of New Orleans lies mostly below sea level.

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