Record salt tide stretches into 21st day

Shanghai is suffering from its longest salt tide on record, which has compromised the tap water of about 2 million city residents, local media reported Sunday.

The annual salt tide, in its 21st day on Sunday, occurs when an influx of saltwater from the East China Sea flows into the mouth of the Yangtze River, where one of the city's primary reservoirs draws its water.

Shanghai sometimes suffers a fresh water shortage during the Yangtze River's dry season from October to March, when the flow from the river's upper reaches diminishes.

The city's water authority said that this year's salt tide is expected to last until the beginning of March, according to a report in the Oriental Morning Post.

The longest salt tide that the city experienced before this year was in February 2004, when the phenomenon lasted nine days and 19 hours.

This year's salt tide has primarily affected the Chenhang Reservoir in Baoshan district, which draws water from the Yangtze River, the report said. The reservoir supplies five water plants in the city's northern districts, including Baoshan, Putuo and Jiading.

To ensure the residential water supply in these districts, authorities have replaced 750,000 cubic meters of water that is usually supplied each day by the Chenhang Reservoir with water from the Qingcaosha Reservoir, the report said. The Chenhang Reservoir provides the city with 1.6 million cubic meters of water each day on average.

The Office of State Flood Control and Drought Relief Headquarters agreed Friday to allow the Three Gorges Reservoir to release water into the river to help replenish the lower reaches of the Yangtze River and ease the effects of the salt tide.

"These measures may help to ameliorate the situation, but the amount of water released doesn't compare to the amount of water that naturally flowed down the river to lower reaches before the Three Gorges Reservoir was built," said Li Nansheng, a professor of hydraulic engineering from the College of Civil Engineering at Tongji University.

Li said the salt tide is a natural phenomenon that occurs every year. At its peak, it will affect people's daily lives, and there is little that can be done to stop it. "Nature is resilient, so it has been hard to observe any direct effect on the local ecosystem so far," Li said.