A harbor is a body of water sheltered by natural or artificial barriers. Harbors can provide safe anchorage and permit the transfer of cargo and passengers between ships and the shore. A harbor is deep enough to keep ships from touching bottom and should give ships and boats enough room to turn and pass each other.
Dredging keeps shipping channels deep and free of silt. Dredging is the process of removing sand and sediment from the bed of a body of water. This deepens and often cleans the body of water. The earthen material dredged from harbors can be used for nearby facilities, like larger beaches or stronger breakwaters (seawalls).
Most harbors are natural. They are located along many types of coastline. They occur in fjords, coves, and lagoons. They also occur along lakeshores and in estuaries, where rivers empty into larger bodies of water. The harbors in North America's Great Lakes, including Toronto, Ontario, Canada (Lake Ontario), and Chicago, Illinois, United States, (Lake Michigan), remain some of the busiest for industrial ship traffic. Iron, steel, and timber are some of the raw materials shipped from manufacturing sites in the U.S. and Canada.
America's New York City has one of the world's finest natural harbors. The harbor has deep water, a small tidal range, and moderate currents. A small tidal range means that the water level is fairly consistent. There is little difference between high tide and low tide. Moderate currents mean movement of the water is predictable. This makes it easy for ships to maneuver, load and unload their cargo. Other cities with outstanding natural harbors are San Francisco, California, United States; Rio de Janeiro, Brazil; and Sydney, Australia.
There are artificial harbors as well as natural ones. Breakwaters, huge walls of concrete, steel, and wire, are the most important element of artificial harbors. Breakwaters protect the harbor from storms and reduce the tidal range. The seabed in protected harbors is more likely to remain stable, although sediment from human activity is likely to accrete, or build up.
The harbor at Chennai, India, (formerly called Madras) relies on a series of artificial breakwaters. It is considered one of the finest artificial harbors in the world. Construction of the harbor began in the mid-1800s, and continued until the mid-1900s. Now, the busy harbor imports and exports such cargo as oil, cars, and consumer goods like clothes and software. The Chennai harbor also loads and unloads thousands of tourists, from throughout India, Australia, and the tropical islands of the Indian Ocean (like Maldives and Seychelles.)
Like Chennai, many harbors may serve as ports (artificial structures where ships load and unload cargo). For this reason, they are often vital to trade. When they function as ports, harbors often have artificial structures such as docks or jetties, as well as lighthouses, buoys, and other aids to navigation. The large size of modern vessels requires that harbors have deep ship channels.
Harbors have played an important role in civilization ever since people began using boats and ships at sea. Some 2,000 years ago, for instance, the Roman leader in what was then Palestine created a magnificent harbor at his city Caesarea Maritima. The ruins of this harbor, called Sebastos, are located on the Mediterranean Sea in present-day Israel. The Sebastos harbor relied heavily on breakwaters constructed from a unique form of concrete: a type of volcanic ash that hardened when mixed with seawater. These breakwaters at Sebastos were called moles.
Sebastos set a standard for future harbors. Most harbors were not improved until the mid-1800s. As commerce increased and ships grew bigger, enlarging and deepening harbors became necessary. Modern harbors range from small enclosures to huge commercial ports.
Harbors can be one of the most polluted ocean ecosystems. Human activity from both land and sea contribute to the pollutants. Because harbors are partially enclosed, the pollution has nowhere to go. It builds up in both the seawater and the sediment below. One source of pollution is ship discharge. This discharge can be anything from sewage and wastewater (used for cleaning) to chemical materials used for packing cargo. The cargo itself can break and spill into the water, releasing plastics, metals, and other toxic materials into the environment. Harbors often have to be dredged to clean up the accumulated waste and clear the channel for ships to pass through.