Deltas are accumulations of sediments that form where rivers empty into bodies of quieter water. When flowing water enters a lake or ocean basin, its speed slows. The slower water drops its sediments to the bottom of the basin and they accumulate to form a delta.

Deltas are important areas for agriculture, fishing, and vital petroleum reserves. Deltas are also areas rich in wildlife, especially migratory birds. The term delta was first applied in 450 BC by the Greek historian Herodotus, who thought that sedimentary deposits at the mouth of the Nile River resembled an inverted Greek letter—delta (D).