Human encroachment, pollution, overfishing and dam-building have killed one third of fish species in the Yellow River, China's second-longest waterway. Its increasingly desperate plight is also threatening economic growth.

The mighty Yellow River once made its away along 3,395 miles through nine provinces, supplying water to more than 150 million people and watering 15 per cent of China's scarce agricultural land.

Where once the river teemed with many different types of fish, it now is a graveyard. "The Yellow River used to be host to more than 150 species of fish, but a third of them are now extinct, including precious ones," an official from the Agriculture Ministry told the People's Daily newspaper.

The basin was the cradle of Chinese civilisation more than 5,000 years ago, but the river's fate is closely linked to China's future because without water, economic development in the north of the country cannot continue at its current breakneck pace.

The river runs from the Qinghai-Tibet plateau in the west across the parched northern provinces of China, through the flood plains of Shaanxi, where it passes through the coal district picking up hefty quantities of pollutants, and into Henan and Shandong provinces.

The Yellow River was known as "China's sorrow" because it would regularly burst its banks and flood surrounding farmland. It is sometimes called the world's muddiest river because of the amount of silt it carries.

These days environmental degradation means the river often runs dry before it reaches the sea at the Gulf of Bohai. Its flow hit historic lows for 10 months last year. Fishing catches have fallen by 40 per cent.

"It can be mainly blamed on hydropower projects that block fish migration routes, declining water flow caused by scarce rainfall, overfishing and severe pollution," the ministry official told the newspaper.

What fish there are in the river are often inedible. In November, parts of eastern China banned the sale of turbot after carcinogenic residues were discovered inside some of the species.

Last month engineers diverted water from the Yellow River into Baiyangdian Lake - the "pearl of north China" and the largest freshwater lake in the northern region - as part of efforts to restore the river's ecological functions.

Recent years have seen a frenzy of dam-building in China as the country seeks to shift away from dirty, expensive coal-fired power plants towards hydroelectricity. The Three Gorges dam on the Yangtze, which opened last year, is the world's biggest. Hundreds of smaller projects are being built on other rivers, including the Yellow River. Dams also help to check flooding during the rainy seasons.

Controlling the flooding has long been a problem for the Chinese. At some points along its course, the Yellow River's bed is 15 metres higher than the surrounding fields because of the constant building and rebuilding of dykes along its route. Chinese history celebrates a man called Da Yu, who mobilised villagers after a flood to build a dyke and drainage canals, before sinking an ox in the river to tame the flow.

Government experts acknowledge the harm that dams can do to the environment but say that the economic benefits and the environmentally positive aspects of hydropower outweigh the negative impact caused to the surrounding land.

As you travel along China's rivers, you come across factories, seemingly at random, pumping out poisonous clouds and filling the water with pollutants. Pollution now means that more than two thirds of the water is undrinkable in the Yellow River.

The government says it has tried to curb pollution by investing in waste treatment plants and closing major polluting plants along the river.

This week the Agriculture Ministry set up an integrated fishing resources committee to save the river from further problems by introducing protection plans and saying it would tackle serious incidents.

The Yangtze, too, is having problems with fish. Last month it was feared that the the white-fin dolphin had become extinct.