The city is experiencing an early salt tide this year due to the reduced flow of the Yangtze River, local media reported Wednesday.

The salt tide is influenced by the tide in the East China Sea and the amount of water coming from the upper reaches of the Yangtze River. A higher sea level caused by climate change also causes more frequent occurrence of the salt tide, said Li Nansheng, a professor at the College of Civil Engineering at TongJi University.

The amount of water in the Yangtze River's upper reaches in September and October was down 30 percent to 40 percent from the same period in previous years.

The Three Gorges Reservoir began to siphon water from the river earlier than usual this year, reducing the flow to the river's lower reaches, according to the report.

Shanghai's salt tide usually starts in December and ends in March or April of the following year, but this year's first salt tide occurred on October 26, the report said. The second lasted from November 7 to Saturday. The third began on Wednesday.

A salt tide can pollute the drinking water if it occurs near a reservoir and it can also hurt the fish in the river, Li told the Global Times.

The Qingcaosha Reservoir, which provides 70 percent of the city's water, has stopped taking in water from the Yangtze River because of the salt tide.

The city's water authority has taken measures to prevent the salt tide from polluting the city's drinking water.

The authority has monitoring equipment in the Yangtze River that can predict a salt tide in advance.

This gives it enough time to add more water to the reservoir before the tide comes in, said Chen Guoguang, a professor at the Shanghai Municipal Water Supply Distribution and Monitoring Center.

There is currently enough water in the reservoir to supply the city for 20 days, Chen told Shanghai Television Station.