An artificial harbor usually has purpose-built breakwaters, and dredging is also used in the construction of artificial harbors. Natural harbors require maintenance through periodic depth measurements and, if necessary, further periodic dredging.[4] An example of an artificial harbor is Long Beach Harbor, California, USA, which was an array of salt marshes and tidal banks too shallow for modern merchant ships. At the beginning of the 20th century, dredging works were carried out here.[5]