Water for more than two million people in Shanghai was once again threatened by an invasion of salt tide last week at the Yangtze River estuary. This is the season for salt tides.

Disruption of supply and tainted water were averted, however, by emergency measures -- cutting Yangtze supply and replenishing it from local reservoirs so customers were not affected.

The annual salt tide problem is expected to be solved when a much bigger reservoir at the mouth of the Yangtze River -- the Qingcaosha Reservoir is put into use next year.

The new reservoir means a change in Shanghai's main water source from the Huangpu River to the Yangtze River, which generally has better water quality.

A new reservoir is by no means the long-term answer to the city's chronic lack of clean water.

Yangtze River water now accounts for about 20 percent of the city's total supply, while the Huangpu River contributes 80 percent.

The new Qingcaosha Reservoir, to be completed by year's end, is expected to supply 7.19 million cubic meters of tap water daily for about 10 million people, over half of the city, as Shanghai Daily reported early last month.

"Due to its huge storage capacity, it will hardly be affected by the salt tide," water expert Zhu Jianrong to Shanghai Daily. Zhu is professor of the State's Key Laboratory of Estuarine and Coastal Research at East China Normal University.

The reservoir capacity will ensure a normal water supply for up to 68 days without drawing from the Yangtze River - nearly 10 times the capacity of the reservoir currently in use at the mouth of the Yangtze River that supplies water for Shanghai.

However, the new reservoir with limited water inflow and outflow could also mean an outbreak of polluting blue-green algae, expert Li Jianhua told Shanghai Daily. He is deputy director of the Key Laboratory of the Yangtze Water Environment (Ministry of Education), Tongji University.

"With a single inlet and outlet the water flow would greatly slow down," he said, providing conditions for possible algae growth.

Another problem is silt.

"The water from Yangtze River carries huge amounts of silts, and its accumulation will soon reduce storage capacity of the Qingcaosha Reservoir," warned Wang Tianhou, professor of Life Sciences of East China Normal University.

As the new reservoir is far from the city center, the constant pumping of water requires huge consumption of energy, and construction of pipelines could cause damage to the natural environment.

Shanghai in the lower reaches of the Yangtze River is vulnerable to sudden pollution accidents in the upper reaches of the river.