Salt tide is a phenomenon in which the lower course of a river

River

A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...

, with its low altitude

Altitude

Altitude or height is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context...

 with respect to the sea level

Sea level

Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...

, becomes salty

Seawater

Seawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% . This means that every kilogram of seawater has approximately of dissolved salts . The average density of seawater at the ocean surface is 1.025 g/ml...

 when the discharge of the river is low during dry season, usually worsened by the result of astronomical high tide.

The lower course Xijiang (West River) in Guangdong

Guangdong

Guangdong is a province on the South China Sea coast of the People's Republic of China. The province was previously often written with the alternative English name Kwangtung Province...

, China

China

Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...

 was periodically affected and has been widely reported since 2004, for bringing shortage of fresh water supply to the western part of the Pearl River Delta

Pearl River Delta

The Pearl River Delta , Zhujiang Delta or Zhusanjiao in Guangdong province, People's Republic of China is the low-lying area surrounding the Pearl River estuary where the Pearl River flows into the South China Sea...

. The salinity level of tap water at Zhuhai

Zhuhai

Zhuhai is a prefecture-level city on the southern coast of Guangdong province in the People's Republic of China. Located in the Pearl River Delta, Zhuhai borders Jiangmen to the northwest, Zhongshan to the north, and Macau to the south. Zhuhai was one of the original Special Economic Zones...

 was reported to be as high as 800 mg per litre in late February 2006, more than 3 times higher than the World Health Organisation standard of 250 mg.