A harbor or harbour (see spelling differences), also called a haven, is a body of water where ships, boats, and barges seek shelter from stormy weather, or are stored for future use. The term harbor, referring primarily to a sheltered body of water, is often used interchangeably with port, which is a man-made facility built for loading and unloading vessels and dropping off and picking up passengers. Ports usually include one or more harbors. Alexandria Port is an example of a port with two harbors. Harbors may be natural or artificial. An artificial harbor can have deliberately constructed breakwaters, sea walls, or jettys or they can be constructed by dredging, which requires maintenance by further periodic dredging. An example of an artificial harbor is Long Beach Harbor, California, United States which was an array of salt marshes and tidal flats too shallow for modern merchant ships before it was first dredged in the early 20th century.