The estuary of the Pearl River has been struck by a serious salt tide, threatening the drinking water of hundreds of thousands of residents in south China's river delta region.

It's only the latest crisis to strike the Pearl River, and experts say urgent measures are needed to preserve water quality and end erosion and silting.

The environmental crisis, which began on Tuesday, is not restricted to the Pearl River. The Pearl has a drainage area of 453,690 square kilometers, mostly in Yunnan and Guizhou provinces, Guangxi Zhuang Autonomous Region, Guangdong, Hunan and Jiangxi provinces.

Earlier this year, the Pearl River Valley was awash in similar crises: The assault of salt tide on the Pearl River Delta last spring, the most serious flooding in 100 years in summer, prolonged drought.

It also experienced the toxic spill on December 15 on the Beijiang River, one of the Pearl River's tributaries and also a major source of drinking water for northern Guangdong.

Moreover, the chlorine content in drinking water sources in cities in the Pearl River Delta, such as Zhuhai and Zhongshan, has kept rising as a result of lesser flow of fresh water from the upper reaches of the Pearl River.

Yue Zhongming, director of the Pearl River Water Resources Committee with the Chinese Ministry of Water Resources, said the cause of the environmental crises and natural disasters was an ecological imbalance characterized by worsening soil erosion in the upper reaches of the Pearl River.

Ample water flows from the upper reaches used to help dilute polluted water in the Pearl River Delta, but now, 55 percent of the territory in the upper reaches of the river are made of limestone, said Yue.

So whenever there is rain, water is not so easily conserved, instead, rainwater, together with plenty of silt, flow into the mainstream of the Pearl River, endangering eco-security at the river's middle and lower reaches.

Some water resources experts said the Pearl River water system's capabilities to combat natural disasters were deteriorating greatly due to a decrease in vegetation. They warned that the Pearl River would go the way of the Yellow River, considered the muddiest waterway in the world, if effective measures are not taken to tackle the problems.

Yue also cited the growing number of unsustainable economic development projects in the entire valley as another reason for a drop in the quality in the river water.

Over 18.2 billion tons of waste and sewage water were discharged in the Pearl River Valley in 2004.

(Xinhua News Agency December 31, 2005)