vers look like they will flow forever. But not so. Not even for the world’s mightiest river, the Amazon, which carries a fifth of all the world’s water that drains from the land to the sea.

Hundreds of millions of years ago, the Amazon flowed west into the Pacific Ocean. Then movements in the Earth’s crust pushed up the Andes mountains and blocked its route. So one day, the Amazon turned round. First, it backed up to form a vast freshwater lake; then it forced its way east, flowing into the Atlantic. To this day, its fish are more like those in Pacific than Atlantic rivers.

Modern engineers have tried to repeat that amazing feat. In 1900, American engineers decided to reverse the Chicago River, which carried the sewage from the city of the same name into Lake Michigan. It was causing epidemics of typhoid and cholera. So they forced the river into a canal that went south to join the Mississippi. It took the sewage on a 2 000-kilometre journey to the Gulf of Mexico. But it stopped the epidemics.

More usually, engineers have just emptied rivers – sometimes with disastrous results. The old Soviet Union took most of the water out of two great rivers to irrigate cotton fields. They were so successful that the Aral Sea, once the world’s fourth largest inland sea, has all but dried up. Fishing boats are still stranded in ports

that are now 80 kilometres from the sea. Several other great rivers no longer reach the sea all year round because we have taken so much water out. They include the Nile in Egypt, the Indus in Pakistan, the Yellow River in China and the Colorado, which is now only a trickle when it crosses the border from the United States of America into Mexico.

Now engineers want to connect up rivers so they are more like the water mains in a city than a natural river system. First up, China is diverting part of the flow of the River Yangtze a thousand kilometres north into the dry Yellow River. And India wants to connect up more than 30 rivers to take water from the wet north to the dry south.

PHOTO : Mbandanyi Viateur/UNEP/B