What is a delta?

A delta, as defined by the USGS (U.S. Geological Survey), is the fan-shaped area at the mouth, or lower end, of a river, formed by eroded material that has been carried downstream and dropped in quantities that can not be carried off by tides or currents.
Boggs defines a delta as any deposit, subaerialor subaqueous, formed by fluvial sediments that build into a standing body of water.