The lingering drought that has plagued central China for months triggered salt tides in the coastal city of Shanghai on the lower reaches of the Yangtze River.

Shanghai has been invaded by salty tides seven times since the end of last year, a spokesman with the Shanghai Water Authority (SWA) said at a press conference Wednesday.

The latest salt tide lasted for seven days as of Wednesday and is likely to continue, said Meng Mingqun, director of the water supply department of SWA.

A salt tide is a phenomenon in which the lower course of a river, with its low altitude with respect to sea level, becomes salty when the discharge of the river is low during dry season.

Though a salt tide is not strange to Shanghai, especially in winter and spring when the water level on the Yangtze River is relatively low, it is rare that the phenomenon persists into the summer season, said Meng.

At the survey point of the Chenxing reservoir, salt concentrations have reached 676 milligrams per liter by Wednesday, according to the Shanghai Water Authority.

The lingering drought in the areas of the Yangtze River's middle and lower reaches is one of the major reasons behind the salt tide, said Zhao Pingwei, deputy director of the Shanghai Water Supply Monitoring Center.

Zhao said the salt tide might continue into June, but the situation could become less severe.

Sources with the SWA said Shanghai's fresh water supply has not been affected.

The drought has caused problems in parts of the country as lakes have dried and farm production halted.

Parts of east China's Jiangsu Province have witnessed the worst drought in 60 years.

The flow of the Huaihe River, one of China's major waterways, was cut and the water level at Hongze Lake, China's fourth largest fresh water lake, has reached a record low of 12.14 meters on average.

Authorities have decided to discharge more water from the Three Gorges Dam on the Yangtze River to relieve the downstream drought.

Larger discharges will begin on Wednesday, according to the China Three Gorges Corporation, the company in charge of operating the dam.