Why a port begins
Ports begin life for a variety of reasons, but trade always comes first. Whether it is Roman soldiers posted to another corner of the empire, Burgundy wine flowing in from France, Welsh coal going to fuel South America`s railways, or Brent crude oil for refining, people or goods need to be moved in our out of the country. Only then is a port born.
Next to trade, a port needs enough water. Ships should be able to come and go for as much of the twenty-four hours as possible. Southampton has a big advantage in having four high tides each day rather than the usual two. A port also needs sheltered water for loading and unloading. Ports up rivers can usually offer this. Harbours right on the coast need breakwaters [definition] to give protection from waves.
Impression of Southampton (1250AD)
Communications are also important. Unless passengers or goods begin or end their journey right in the port, they need to move inland. This is why many ports are built on or near rivers. Before the railways, water offered the easiest way to move goods. As Britain industrialised, first canals, then railways and finally better roads were built. Many of these connected ports with the industries or communities they served. Ports often encouraged these developments, as their trade benefited from better communications.
Exactly where a port was situated might well depend on river crossings. For instance, London was the furthest point east on the River Thames where existing technology allowed a bridge to be built.
Expanding and growing with its region
Ports themselves stimulate the growth of local industries and communities. Industries that need to import raw materials or export their products will often set up near a port to reduce their transport costs. Flour millers would often build their mills near ports importing grain. Many steel works were built near to facilities for bringing in iron ore. A more recent example is the oil industry. All major refineries have port facilities to import crude and ship out refined products. These industries attract people who want jobs, and the local community grows. With people and trade moving, communications improve. With more industry, more people and better transport, the port itself develops.