http://en.wikipedia.org/wiki/Chinese_dragon#Ruler_of_weather_and_water

Chinese dragons are strongly associated with water in popular belief. They are believed to be the rulers of moving bodies of water, such as waterfalls, rivers, or seas. They can show themselves as water spouts (tornado or twister over water). In this capacity as the rulers of water and weather, the dragon is more anthropomorphic in form, often depicted as a humanoid, dressed in a king's costume, but with a dragon head wearing a king's headdress.

There are four major Dragon Kings, representing each of the four seas: the East Sea (corresponding to the East China Sea), the South Sea (corresponding to the South China Sea), the West Sea (sometimes seen as the Indian Ocean and beyond), and the North Sea (sometimes seen as Lake Baikal).

Because of this association, they are seen as "in charge" of water-related weather phenomena. In premodern times, many Chinese villages (especially those close to rivers and seas) had temples dedicated to their local "dragon king". In times of drought or flooding, it was customary for the local gentry and government officials to lead the community in offering sacrifices and conducting other religious rites to appease the dragon, either to ask for rain or a cessation thereof.

The King of Wu-Yue in the Five Dynasties and Ten Kingdoms period was often known as the "Dragon King" or the "Sea Dragon King" because of his extensive hydro-engineering schemes which "tamed" the sea.