By Nick Carey

EAST HANNIBAL, Illinois (Reuters) - Toiling day and night, thousands wage a war against water with sand, bulldozers and even their bare hands.

Atop a section of a 54-mile-long levee overlooking the rushing Mississippi River, Russ Koeller surveyed this week's handiwork: a ridge of sand covered with plastic and sandbags guarding this Illinois hamlet and land growing with crops worth $150 million (76 million pounds) at harvest.

"Things could get very serious, very quickly," he said of the streams of water that frequently form on the levee's gently sloping backside, washing the sand away.

The rushing current also scours the base of the levee, threatening to undermine it.

At the point where Koeller stood, the river broke through in 1993 in the last great flood along the Mississippi, flooding 46,000 acres (18,620 hectares) of prime farmland.

"With commodity prices where they are, you could say that's around $1,000 per acre. So if this levee were to go, you're talking about $150 million in damages to farm land alone," Russ Koeller said.

Prices of corn and other commodities have soared because millions of acres of farmland have been ruined by the worst Midwest flooding in 15 years.

Every few hours around the clock some 75 bulldozers are driven up and down the line, reinforcing any perceived weak points with fresh sand. Some 5,000 volunteers -- and around 400 prison inmates -- do patchwork. They have raised the levee to a height of 35 feet (11 metres), hopefully beyond the river's crest.