BILL MOYERS: The most valuable and the most imperiled resource in the world is fresh water.

PRISCILLA NYEWU (WORKING FOR WATER): Water vapor comes up from the sea into the sky and there it forms clouds that get thick with rain which falls to the ground. We drink it, we make electricity out of it, we even make our homes from it. Right.

BILL MOYERS: Water is for everything we do - but water is not forever. There is now half the fresh water per person than there was in 1960. In twenty-five years, we will have half of what we have today.

They are already running out of water in South Africa - where the population is growing faster than the water supply.

Five years ago the government decided it was time to tackle this threat to life itself. They put 40,000 people to work preserving water. .

NYEWU: [in English] Working for Water. [in Xhosa] What are we working for? We are working for water. The people are doing that.

MOYERS: For centuries, European settlers planted trees in this treeless landscape. Pine, eucalyptus, acacia - these trees drank rain water that should have gone to rivers such as the Palmiet winding here through a nature preserve. The thirsty trees had to go.

This eucalyptus drinks twelve gallons of water every day. Now it drinks none. That's the essence of Working for Water. Forty thousand people saving water by destroying invasive alien plants. It's that simple.

JACKIE VAN DER MERWE: That is a pine preister and up there that is a Port Jackson, that is part of the acacias. OK, so that must be coming out, and that pine must be coming out . . .

MOYERS: Jackie van der Merwe scouts the preserve for invasive plants.

JACKIE VAN DER MERWE: Sometimes I am walking forty kilometers, maybe more, maybe less, up down, up down, over rivers. It's just the nature. It's just good, man. And I love my job.

MOYERS: When Jackie has located invasive alien trees, cutter-slashers move in to kill them . . . no matter where they are . . .

It may take a helicopter, four men, climbing ropes and a chain saw to remove three pines. But if they weren't removed, the hillside would be covered with pine within twenty years - and the Palmiet River flow reduced even more. Because of this work, just the opposite has happened.

BRIAN VAN WILGEN: In areas which have become invaded by these alien plants, where we clear them and people have lived in those areas for 20 or 30 years, they will tell us - you know, we haven't seen the stream flowing for - for 20 or 30 years, and as soon as you clear the trees, the water starts flowing again.

NYEWU: We are taking away all of the trees that drink our water.