While winds and flames continued to ravage Los Angeles, small teams began creeping onto the charred soils left in their wake.

Roughly a dozen members of the California Watershed Emergency Response Teams and the United States Forest Service are studying the edges of the Eaton and Palisades fires to determine what patches of land burned most severely. Soon, they’ll issue hazard maps to help people prepare for what comes next: the near-certain threat of floods and landslides that will loom for days, months and even years while the city recovers.

“After a wildfire, the hazard to the public is not over,” said Jeremy Lancaster, California’s state geologist. He and his team spent Wednesday hiking in the steep canyons that flank the San Gabriel and Santa Monica Mountains. When it rains hard enough, the sediment on slopes like these can swiftly tumble downhill onto houses that increasingly push up against the fire-prone foothills.

The two major hazards after a wildfire are flash flooding and post-fire debris flows. While spongy soils typically absorb water, burned soils can become hard packed like concrete, repelling water as a raincoat would. Water then funnels downslope without much, or any, vegetation left after a fire to keep it in check.

Hazard maps use a combination of satellite images and field testing of soils to show where patches of moderately to severely burned soils could make these post-fire risks more likely. Recommendations for emergency services to engineer barricades against the danger accompany the maps.

The Palisades burn map was released on Thursday, and the Eaton map is still under review. Experts said the Palisades fire had mostly low to moderate burn severity, while the Eaton fire was likely to have more moderate to high burn severity.

Debris flows require three ingredients — steep slopes, burned soils and rain — and they’re often more dangerous than floods because the sediment they draw in claws at the landscape, creating a snowball effect that pulls a tumult of trees, vegetation, soil, rocks and anything else in its way.

“A debris flow is like a flood on steroids,” said Jason Kean, a research hydrologist with the United States Geological Survey. “It’s all bulked up with rocks and mud and trees.” While floods often have a longer reach, water churns faster in debris flows, which are less common but more destructive.

After the Thomas fire in 2017, a debris flow in Montecito, Calif., killed 23 people and damaged or destroyed more than 400 homes.

Neither homeowners’ insurance nor federal flood insurance covers the impact to properties of debris flows, which are defined by the Geologic Survey as landslides.

The fire-flood cycle is a long-studied relationship, but scientists say a warming planet has made the post-fire threat more likely. Fires burn bigger and more severely. Rains hit harder and more often. Those changes expand the target area for post-fire hazards, which could increase the size and the frequency of floods and flows.

“The fire scientists are telling us that wildfires are increasing in size and severity,” Dr. Kean said. “From that fact alone, you’re exposing more terrain and making more terrain vulnerable to post-fire problems.”

With a dangerous combination of very steep terrain, lots of sediment, high-fire activity and a lot of people pushed up against the mountains, Los Angeles faces an extreme risk.

“The Los Angeles area and Southern California are the world capital for post-fire debris flows,” Dr. Kean said.

Debris flows are so common in Los Angeles that at the edge of the San Gabriel Mountains, where the Eaton fire burned, the state has carved debris basins to collect waste from major flow events. In Southern California, more than two million people live on alluvial fans, landforms that are conducive to flash floods and debris flows, according to Dr. Lancaster.

The National Weather Service in Los Angeles collaborated with the Geological Survey to start the nation’s first early warning systems for these post-fire hazards in 2005. The geological survey sets a rainfall threshold that could set off landslides, and the Weather Service issues a warning if the rainfall they expect nears or surpasses it.

Jayme Laber, a senior hydrologist with the Los Angeles forecast office, has spent the last two decades issuing such warnings. While there’s still no sign of rain in the seven to 10 day forecast he issued on Wednesday, a garden-variety rainstorm that Angelenos see at least once or twice a year could be enough to kick off the next wave of hazards, which can develop within minutes.

“In a burned area, the kind of rain that would not cause problems would be a really light drizzly rain that just goes on and on,” Mr. Laber said. But at some point this winter, he added, “we’re going to get rainfall that has the potential to cause flash flooding and debris flows in these newly burned areas.”

This video of a small debris flow from 2016, which is close to the site of the current Eaton fire, shows how quickly large objects, like a six-foot boulder, can be sent rushing downhill.

Mr. Laber advised residents to prepare in case of a future evacuation, monitor the forecast and pay attention to local emergency officials if a warning is issued.

UnitedHealth Group reported on Thursday that it earned less than expected this past quarter, citing higher medical costs and pressure on its insurance division at a time when the company is still reeling from the shocking murder of a top executive last month.

Revenues for UnitedHealth Group amounted to $100.8 billion for the fourth quarter, below what analysts had predicted but still 6.8 percent higher than in the same quarter the year before. The company’s full-year revenue for 2024 rose to $400.3 billion. For UnitedHealthcare, the insurance division, full-year revenue increased to $298.2 billion, up 6 percent from 2023.

The results were the company’s first since Brian Thompson, the chief executive of UnitedHealthcare, was gunned down in front of a Midtown Manhattan hotel.

The murder unleashed public outrage aimed at big health insurers, over lack of access to health care and denials of coverage and insurance claims.

Some shareholders have urged UnitedHealth to issue a report on its practices that “limit or delay access to health care.”

Andrew Witty, UnitedHealth Group’s chief executive, said on a call with analysts on Thursday that frustrations about claims, including delays in receiving care and coverage, were “key areas for us to work hard at to improve.”

A successor to Mr. Thompson has not been named yet. Mr. Witty did not share details about filling the post, nor did he directly address the recent shareholder campaign.

But he and other executives discussed the loss of Mr. Thompson at the top of the call.

“He devoted his time to helping make the health system work better for all of the people we’re privileged to serve,” Mr. Witty said.

UnitedHealth’s results, which disappointed Wall Street, in many ways reflected broader trends and lingering issues for the industry. For several quarters, U.S. health insurers have taken hits to their earnings from high medical expenses and a tightening of government payment policies.

John Rex, the company’s chief financial officer, pointed to cutbacks in government rates in the payment system for Medicare Advantage program, the private insurance arm of the federal coverage for people 65 and over. UnitedHealth has substantial business in these Medicare private plans.

Medicare Advantage performance has declined throughout the industry recently, partly because of regulatory changes meant to prevent overcharging and following increased health spending among some older populations.

Mr. Witty also said there were costs associated with changes in Medicaid, the federal-state insurance program for the poor.

The company’s medical cost ratio, a measure of the cost of providing care, came in higher than expected in the most recent quarter, which could add fuel to investors’ concerns that increased costs for delivery of care might linger, said John Boylan, an analyst at Edward Jones, an investment firm.

UnitedHealth, however, kept its full-year guidance for 2025 intact, unaltered by recent pressure. Analysts at Morgan Stanley said in a research note that the company had set “reasonably prudent targets” for this year.

“Overall, our view is that United is well positioned to navigate the evolving health care landscape due to its diversified business model,” Mr. Boylan said.

UnitedHealth’s stock fell 6 percent on Thursday as investors digested the weaker-than-expected results. UnitedHealth’s results, often seen as a bellwether for performance across the industry, pushed down shares of its rivals, including CVS Health, which is the parent of the insurer Aetna.

UnitedHealth Group also owns Optum Rx, one of the country’s largest pharmacy benefit managers, which employers and government programs hire to oversee their prescription-drug benefits.

Optum Rx has faced scrutiny from regulators over concerns that it has raised drug prices, prioritizing its own interests above those of patients, employers and taxpayers. Just this week, the Federal Trade Commission released a report detailing how P.B.M.s could be inflating drug costs.

The agency criticized Optum Rx and two other major benefit managers — CVS Health’s Caremark and Cigna’s Express Scripts — for raising prices on generic drugs for cancer, heart disease and other illnesses as high as 1,000 percent of national average costs.

Mr. Witty, the UnitedHealth Group chief executive, defended Optum’s practices, stressing that 98 percent of the rebates were passed to customers. By 2028, he said, all rebates would be passed on. Drug prices in the United States, Mr. Witty argued, are “de novo set too high relative to any other price in the world,” and shifted the blame to drug companies.

“The P.B.M. acts on behalf of the ultimate payer — the employer, the union, the state,” Mr. Witty told analysts.

Mr. Witty did not address investigations by the Justice Department or lawsuits seeking to block its proposed acquisition of Amedsys, a large home care and hospice company.

Beyond rising medical costs and increasing use of health care services, UnitedHealth executives pointed to the widespread ransomware attack in 2024 that weighed on the company’s full-year profits. The cyberattack forced the shutdown of the company’s sprawling billing and payment system, Change Healthcare. The company has estimated that the data breach of health and privacy information affected more than 100 million people, and said this week that a review of personal information involved in the incident was “substantially complete.”

Luigi Mangione, 26, has been charged with multiple state and federal counts of murder as well as weapons and stalking offenses. He has pleaded not guilty.

UnitedHealth and the police have said that neither he nor his parents had medical insurance through UnitedHealth.