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
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 letterdelta (D).