"When the rains come, it's going to be pretty bad," Geff Deedrick tells them. Deedrick is a lieutenant in the Los Angeles Sheriff's Department. "We don't want anybody to get killed in the flood."

This winter threatens to put two of California's biggest issues on a collision course.

One is natural: After years of drought, a record El Niño is predicted to bring heavy rains to the state.

The other is man-made: After years of skyrocketing housing costs, a record number of homeless people dwell on the streets -- and in the cement culverts and dry riverbeds that crisscross the region.

Deputies warn L.A.'s homeless of the coming El Niño storms

When it rains in California, roiling torrents race downhill from mountain slopes and valley streets to the Pacific, turning the dry beds and culverts into raging rivers.

CNN recently accompanied Deedrick and a handful of other officers as they took their warning to the underpasses and riverbeds, talking to homeless people and handing out fliers.

It was a journey into a hidden world of misfortune that millions of Californians drive over each day, unaware of its existence.

The exact number of homeless river dwellers is difficult to pin down. But a rough estimate from conversations with law enforcement and social workers in nine Southern California counties puts the number at about 2,000. That includes as many as 800 in Los Angeles County and another 300 in Ventura County.

Homelessness here has grown substantially since the last El Niño, which dumped 30 inches of rain on Los Angeles during the winter of 1997-98, authorities say. In September, the Los Angeles City Council declared a state of emergency on homelessness and called for $100 million to help address the growing crisis.

"This huge homeless population is already so vulnerable, and life becomes even more difficult with the El Niño conditions," said Naomi Goldman, spokeswoman for the Los Angeles Homeless Services Authority.

El Niño is a cyclical rise in surface temperatures in the equatorial Pacific Ocean that influences jet stream patterns and rainfall totals and can cause unusual weather around the world.

Based on current sea surface temperatures, there's a 95% chance El Niño will bring repeated storms to California this winter, says National Weather Service meteorologist Mark Jackson.

"And that means above-average rainfall," he said. Perhaps even record rainfall.

To make sure the riverbed homeless get the message, officers have been taking to the culverts in all-terrain vehicles. But will their warnings be heeded?

The homeless couple Deedrick spoke to under the 60 Freeway didn't start packing. Their spot on the cement along the San Gabriel River features a barbecue and a television powered by a single solar panel.

"Are you willing to go to a shelter?" Deedrick asked.

"No," the man grunted.

"Why not?"

"Because the shelters have rules, and you have to be in at certain times," the man said. "You can't do this. You can't do that."

The officers kept pressing the couple to pack up and leave but stopped short of putting them in handcuffs.

"Our goal is to get them help, transition out of homelessness, it's not to arrest them for now," Deedrick said. "We have time."

El Niño usually peaks from January through March, the National Weather Service says.

Should a potentially lethal storm from El Niño threaten Southern California, Deedrick said the officers would use other means if necessary, including charging homeless people with trespassing and illegal camping.

Some of the homeless said they would leave; others planned to stay put.

After warning the couple, the officers hopped on two ATVs and throttled off to a hut in the deep shade 30 feet below the freeway.

The next person to hear their warning was Deanne, 22 years old with girl-next-door charm despite her rough living conditions. Her house is made of canvas and blankets draped over thick tree branches.

How did Deanne end up here? How do any of them get here? What currents of misfortune bring them to these villages in the shadows?

Rotten luck, they say: lost jobs, severed family ties, drugs -- or all of the above.

Deanne utters the four-letter word that has pushed so many into misery -- meth, short for methamphetamine, crystalline wrecker of lives.

"Too many drugs," Deanne confides. "I was on meth for a while and it went downhill from there."

She seems clean now, lucid enough to recognize that the river could easily swallow up the ramshackle Shangri-La she shares with a boyfriend who ran when we arrived, apparently to hide in nearby bushes.

"I keep all my things in three bags," Deanne says. "Just the other night the water came up pretty high and we had to move to higher ground."

Anthony Cantrelle found higher ground beneath a connector road to the freeway. He spends a lot of time reading books -- including a favorite, "Seabiscuit."

The 60-year-old says he's looking for a more permanent place. The Los Angeles Homeless Services Authority, anticipating El Niño, has opened 10 winter indoor shelters early, including several at armories.

Cantrelle vows he and his homeless neighbors are packed and ready to go on a moment's notice.

"They ain't just gonna sit there and drown," he says.

Social workers from the authority also are patrolling the riverbeds, often close on the boot heels of the deputies.

They explain how just-opened shelters now offer 1,400 additional warm beds to people often sleeping on tattered mats with rolled-up T-shirts for pillows.

"We want to not only get them into the shelters, but help them get off the streets into permanent housing," the authority's Goldman said.

Deputies spoke to the homeless and handed out fliers warning of the potential for floods with El Niño.

Word of El Niño's danger has been passed along the San Gabriel riverbed, where many residents collect cans, copper, bottles -- anything they can haul off and recycle for a few dollars.

The homeless follow their own code of honor.

"Just don't go into other people's camps and take their stuff," Cantrelle explains. "Stay cool, don't go ripping off and stay out of the way."

But it's not just the homeless who seek refuge in the dim wedges between the rivers and overpasses. They serve as well-concealed caverns for criminals, drug dealers and addicts.

A flash catches the eye under a bikeway bridge. It comes from a small disk of burned metal, the remnants of a heroin addict's efforts to cook up a fresh dose.

There aren't any drug users around for deputies to warn about El Niño on this day. If they're here, they may have fled to the shadows at the sight of the law.

The officers say that when they pass through here again, they hope they don't run into Cantrell, Deanne or any of the others making their home in these tenuous riverbed campsites, so susceptible to the ravages of a storm.