The ruling by a court in San Francisco earlier this month approved a plan to reline part of the All-American Canal with concrete, stopping accidental run-off from the waterway that has benefited Mexican farmers since it opened in 1942.

The court ordered the refurbishment of around a quarter of the 82-mile (132-km) conduit to proceed "without delay," in an overhaul that is set to take up to two years to complete at a cost of some US$250 million.

The San Diego County Water Authority says the project is needed to recover some 22 billion gallons (83.5 million cubic meters) of water lost through the leaky canal bed each year that local consumers in the water-strapped area have already been billed for.

Mexican authorities, environmentalists and farmers are furious at the planned relining, which they say could harm a fragile wetland environment and leave many villages in the Mexicali Valley south of the border high and dry.

"When they reline the canal, this area will dry out completely," said farmer Alfredo Mendez, pointing to the green wheat fields cut through with irrigation channels near Hechicera village just south of the waterway. "A lot of people are going to get hurt," he added.

ENVIRONMENTAL DAMAGE

Tensions over water have long simmered on the sun-baked US-Mexico border where farmers and city dwellers on either side of the 2,000-mile (3,200-km) line compete for a scarce commodity.

To the east, consumers spar for the murky waters of the Rio Grande as it flows to the Gulf of Mexico. In the west they vie for the Colorado River, which is reduced to barely a trickle by the time it reaches its delta in the Sea of Cortez in Mexico.

The project to reline the canal -- which carries water from the Colorado -- was opposed by a Mexican community group and two US environmental organizations presenting a complex array of objections including claims of violations of US laws such as the Endangered Species Act.

City hall in the Baja California state capital, Mexicali, has agreed to bankroll a three-million-peso (US$270,000) war chest to continue to fight the relining.

Mexican President Felipe Calderon has warned that damage from the project would be "terrible for Mexico." During a recent visit to the affected area, he said the revamp would be counter-productive for the United States because it could drive more Mexicans north to seek jobs at a time when the world's largest economy is eager to stem illegal immigration.

Mexican authorities say stopping the seepage will lower the water table south of the border, forcing dozens of villages and towns to bore deeper wells through salty soils to meet their domestic and agricultural needs.

The relining process will also dry out the fragile ecosystem of the Mesa de Andrade wetlands near the US-Mexico border, which is home to endangered species such as the desert pupfish minnow and a secretive bird, the Yuma clapper rail.

"The natural environment knows no borders, and this project will hurt it," said Karl Flessa, a professor at the University of Arizona who is familiar with the area's ecology.

"The amount of habitat available to these endangered species will be diminished," he added.

REPAIRING A LEAKING HOSE

California is the last of seven states to take their cut from the finite resources of the Colorado River, and water is so precious that authorities seek to conserve every last drop.

For the San Diego County Water Authority, the plan to reline the canal is simply a conservation measure that will save water equivalent to the needs of a city of 500,000 people.

Officials say most will be funneled west to provide for new subdivisions and to meet the needs of farmers tending high-value crops like avocados and cut flowers in the county.

The remainder will go to settle outstanding water debts with American Indian tribes.

Water authority legal counsel Dan Hentschke said he has sympathy for the people in Mexico who have been using water that's been leaking out over the years, he is quite clear as to the rights of the relining project.

"We are just fixing a leaking hose that is carrying water that California has already paid for," Hentschke said.