L CENTRO, Calif. The dying season began early here this year, with four bloated bodies found floating in the All-American Canal on March 14. The victims, young men ages 19 and 20, had made their way from Chiapas, in southernmost Mexico, before drowning in the canal's churning currents just 35 yards from United States land.

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For the Imperial County Sheriff's Department, it was an ominous sign. The dead usually start showing up in multiples in high summer, when the desert becomes an inferno and the canal, roiling beneath a calm veneer, lures migrants looking for a quick way across and relief from the killing sun. If bodies were washing up in groups in March, what would the summer be like?

The answer, so far, is grim. Even though deaths along the Mexican border have declined over all as the slumping American economy has attracted fewer migrants, the toll is reaching record rates in the most remote and dangerous outposts. To avoid the stepped-up border patrols in populated areas, the most desperate migrants cross in the more unguarded and desolate deserts of Arizona and eastern California. June was the deadliest month ever for the southwest border, with 67 migrants dying, mostly in the unrelenting heat of the United States Border Patrol's Tucson sector, a barely habitable land that covers most of southern Arizona.

Here in the mountainous El Centro sector, which includes the vast Imperial Desert, 52 migrants have died since Oct. 1. The sheriff's department believes the deaths could outpace last year's record of 95.

"It seems quiet, but we're finding more multiples bodies in threes, fours and fives," said Gary Hayes, a deputy coroner in the department. "They're really trying to avoid detection, so they're going to more and more remote areas."

The rising toll in these barren regions is the more remarkable because illegal immigration from Mexico has fallen 29 percent, largely because of the faltering United States economy and tighter security, and border deaths in general are down 20 percent.

Experts warn that the deadliest months are to come. August, traditionally, is the cruelest. They also note that the statistics do not include people who die in Mexico. (The Mexican government counted 22 migrants who died inside its border in June. It counts only Mexicans and not migrants who pass through from Central America or elsewhere.)

The deaths are full of suffering. People have suffocated in airless trucks, died in vehicle crashes, been struck by lightning or drowned. Most often, though, they are felled by heatstroke or dehydration. Some carry no identification and, in a tragic irony, end up where they wanted to be, in the United States but in anonymous pauper's graves. Other migrants, not counted by the Border Patrol, never make it across.

Migrant advocacy organizations blame the Border Patrol for the mounting deaths, saying that its decision to focus its policing on border cities has driven migrant traffic to the most severe terrain, with the most extreme climates, winter and summer. The policy, which began as Operation Gatekeeper in 1994, added officers, enhanced surveillance equipment and put up physical barriers like concrete walls, as well as introducing other measures, at the San Diego border. The strategy was then expanded to Arizona and Texas.

Since the operation began, about 2,000 migrants have died trying to cross into this country, according to the Mexican government, with an average of more than one a day in the last two years. The shift has also made expensive smugglers called coyotes indispensable. Possibly hundreds of migrants have died because they have been abandoned by these smugglers, or because they have been led by people who themselves could not manage a brutal landscape, their advocates say.

"Once the deaths started happening by the dozens in the mountains east of San Diego," the federal government "never rethought its strategy," said Claudia Smith, border project director for the California Rural Legal Assistance Foundation, in San Diego. "The Border Patrol, as planned, went on to push them into the deserts," she said, "where the risk increased exponentially."

The Border Patrol denies that its policies are responsible for the increase in deaths and has no plans to change its strategy. It counters that it is doing everything it can to deter migrants from passing through the desert, including adding medically trained search and trauma teams to rescue migrants, helicopter patrols in treacherous areas and several rescue beacons in the desert that send an electronic distress signal with the push of a button. It has also mounted a public service campaign in Mexico and Central America, using celebrities to do television and radio advertisements to warn would-be migrants of the dangers of trying an illegal border crossing.

"Our primary mission is to protect our nation's borders," said Mario Villarreal, a spokesman for the Border Patrol in Washington, D.C., adding that unscrupulous smugglers, charging between $1,000 and $2,000 a person or more, are to blame for persuading would-be border crossers to make the dangerous trek.

Crossing the border illegally has always come with risks. Before Operation Gatekeeper, most traffic entered via cities like San Diego and El Paso, where migrants became targets for muggers and the like.

A study released in July by the Public Policy Institute of California, a research organization in San Francisco, found that the Gatekeeper strategy, which costs more than $2 billion a year, has done little to significantly diminish illegal immigration. It actually increased after the border buildup. Economic opportunities in the United States and Mexico, the study found, have a stronger effect on migration than does the number of agents at the border.

Migrants risking their lives in the extremes of the desert tend to come from the poorest states in Mexico, like Chiapas and Oaxaca, where the economy is in collapse and whole villages have been vacated by working-age men, and, increasingly, women. At Casa Madre de Asunto in Tijuana, a safe house for migrant women that is run by a Catholic nun, a dozen or more women at a time have either migrated to the city from the south or have stopped by on their way to cross the border.

Recently, a 31-year-old woman from Oaxaca said she and her brother, a cousin and a friend, who were resting at a safe house for men next door, had driven for two days and two nights to make it to Tijuana and planned to make a two-day trek through mountains with a "coyote" who was taking them into Los Angeles, charging them $1,500 each.

"That's $1,000 less than he charges others," she said, "because he knows us." She had tried to make the trek months before, she said, but lost her nerve after feeling faint and dehydrated and turned back after a day.

She said she hoped to become a maid or work in a store and make some money to send to her mother. "I'm coming for a better life for all of us," she said.

Days later, a woman at the safe house said she assumed the group had made it across; they had not heard from the Oaxaca woman.

When smugglers are caught, said Mr. Villarreal of the Border Patrol, efforts are made to prosecute them. In a recent incident in Dallas, where two men were found dead in a stifling 54-foot-long truck that had transported 40 illegal immigrants from El Paso, a nine-hour trip, two truckers have been charged with murder.

"This is good work' but we're not done," Mr. Villarreal said. "The main message we still want to get out is that it is dangerous to try to cross along the southwest border."

Here in the El Centro sector, five suspected illegal immigrants died of heat exposure in mid-July in an area of the Imperial Desert that resembles a moonscape. "It's almost totally devoid of plant life," Mr. Hayes, the deputy coroner, said.

Spotted by a military aircraft, the bodies could not retrieved for a day because of the terrain. Mr. Hayes said satellite equipment was needed to mark the positions of the bodies.