delta, delt-; Δ, δ
(Greek: triangular; fourth letter; Δ, δ, of the Greek alphabet)
2. An object shaped like a triangle.
3. Usually a triangular alluvial or sedimental deposit of sand and earth at the mouth of a river.
4. A similar deposit at the mouth of a tidal inlet, caused by tidal currents.
A Greek letter sits at the mouth of many rivers. Noticing the resemblance between the island formed by sediment at the mouth of a river; such as, the Nile and the triangular shape of their letter delta, the Greeks gave the name "delta" to such an island.
The English language incorporated this sense from Greek, although the word delta appeared first in English as the name of the letter, in a work written possibly around 1200 A.D.
The sense of "alluvial deposit" was not recorded until 1555 A.D., when delta was used with reference to the "Nile River delta".
A decrease in the velocity of the flowing is decreased in speed and lowers the stream's capacity, causing its contents to be deposited as a delta flat.