River delta slope offshore, waves will make river deltas smoother. Waves can also be responsible for carrying sediments away from the river delta, causing the delta to retreat. For deltas that form further upriver in an estuary, there are complex yet quantifiable linkages between winds, tides, river discharge, and delta water levels. Erosion is also an important control in tide-dominated deltas, such as the Ganges Delta, which may be mainly submarine, with prominent sandbars and ridges. This tends to produce a "dendritic" structure. Tidal deltas behave differently from a river- and wave-dominated deltas, which tend to have a few main distributaries. Once
Pearl River Delta Economic Zone 2020. Six cities surround the geographic delta, with a seventh city over the ocean, comprising a total land of roughly 29,000 square kilometers (11,200 square miles). This figure does not include inland Huizhou and Zhaoqing city which amalgamates with the area, nor the separately governed territories of Hong Kong and Macau. The Pearl River Delta Economic Zone encompasses nearly 7000 square kilometers, 0.07 percent of China, with a population of 22 million people, 3.1 percent of the 2010 population of mainland China. A report by the World Bank described this zone as the third largest urban area in the world.