It is traditionally a valley. In some cases it can refer to a dry river bed that contain water only during times of heavy rain. The wadi is very widely found in arabic to ponyms were streams in a desert environment & generally are dry year round except after rain. The deposition of wadi is rapid because of the sudden loss of the velocity & absorption of the water into the ground.
Why was the Prime Meridian established at Greenwich
In 1675, the Royal Observatory at Greewich, outside of London was created to study the determination of longitude (a vital comonent to oceanic navigation). The United Kingdom and, by association, the United States, used Greenwich as the reference point for longitude.
Thus, by the time of an international conference in 1884 to establish fixed degrees of longitude on the planet, many countries had been using Greenwich as the Prime Meridian for decades so the tradition became internationally accepted
World Largest Coral Reef!