Raw sewage, dead dogs, fish and industrial waste flowing north across the Mexican border via the New River are creating a major health hazard for residents of the agricultural Imperial Valley on the U.S. side.

Valley residents, led by the county health director, say the refusal of the Mexican government to clean up its portion of the river and the reluctance of U.S. officials to exert pressure on Mexico is breeding hostillity along the "Friendship Border" designated by former president Nixon.

"You could find a river as dirty as this, but I challenge anyone to find one dirtier," says Imperial County's health director, Dr. Lee Cottrell.

As he spoke from the first bridge on the Calexico side of the border, a glob of human feces and six cockroaches the size of field mice floated by.

For more than 20 years residents of Imperial Valley, California's major lettuce-growing area, have complained of waste from Mexicali residents, but now U.S. scientists are confirming reports that the river carries a serious threat of hepatitis, typhoid, salmonella and encephalitis. Organisms growing in the river bottom slime are moving up along the banks.

Cottrell said a boy who frequently played in the New River was recently admitted to a hospital in a comacaused, the doctor believes, by a viral infection. A San Diego woman who returned from a visit to the area ended up with a case of encephalitis, which is carried by mosquitoes known to breed along the New River banks.

After months of complaints and pressure from Rep. Clair Burgener (R-Calif.) and Sen. Alan Cranston (D-Calif.), the state Water Resources Control Board finally agreed to prepare signs warning of the river's contamination. Fifty signs went up on bridges on the U.S. side of the border in mid-November. But Imperial County requested at least 150 signs to properly warn residents and visitors.

From Mexicali, the New River flows north 62 miles to the Salton Sea southeast of Palm Springs. The main sewage treatment plant in Mexiali has been out of operation at least since July, so wastes are flowing untreated into the New River. In addition to carrying human wastes, the river courses through Mexicali's major dump and serves as an outlet for waste from hundreds of homes and businesses.

U.S. Boundrary and Water Commissioner Joseph Friedkin said today that, as of early last week, 90 to 95 percent of Mexicali's sewage going into the municipal system was being into the municipal system was being treated in settling pounds. Friedkin said joint inspections by Mexican and American engineers are conducted weekly.

On Friday two government biologists made an unofficial visit to three sites along the river in Mexicali and verified that human and industrial wastes were flowing from the main sewage plant into the milky white river.

"It is disgusting. I don't know where I have ever seen anything like it," said a U.S. Geological Survey bacteriologist, James Setmire.

Friedkin, asked about the biologists' Friday findings, said today that the pumping stations must have been temporarily out of order.

In the detailed reports Setmire submits to his agency and others, he says, he has been instructed by his superiors to delete all references to Mexico or Mexicali, because of political overones.

"As far a we are concerned the official position is that all this pollution appears by magic on our side of the border," he said.

Friedkin, who plans to meet with county officials and state water officials Wednesday to review the situation, said, "The Mexican authorities have never denied their responsibility to clean up the sewage."