While the Los Angeles area wildfires have been contained, the danger to foothill communities persists.

Nearly 170,000 people live in Altadena, Pasadena, and Sierra Madre at the foot of the San Gabriel Mountains and many are potentially in the path of debris flows.

Heavy rainfall in mid-February triggered warnings and a flash flood watch for areas burned by the Eaton, Palisades, Franklin, and Bridge fires.

Crews from the county's Public Works Department worked to stem catastrophic flows of mud, debris, and boulders that now threaten these fire-stricken communities.

These stormwater crews must ensure that the county's network of dams, debris basins, channels, and storm drains are up to the task.

Emergency teams from multiple agencies have cleared out flood basins beneath the Eaton fire's 14,000-acre burn scar.

They distributed sandbags and placed long concrete barriers to redirect potential flows.

The power of debris flows lies largely in their ingredients.

They often hold more sediment and rock than water.

The highest danger of a debris flow occurs immediately after a fire, but it can persist for years.

California has a history of such catastrophic events.

The most recent devastation was in 2018 when a debris flow in Montecito after the Thomas Fire killed 23 people.

As meteorologists watch incoming weather patterns, more heavy rain could trigger more flood watches.

For residents living in or near a fresh burn area, that means it's time to prepare for a possible evacuation.

Jayme Laber, a senior hydrologist with the National Weather Service's Weather Forecast Office in Oxnard, said, ''That's their heads-up to make preparations.

That's the time to start thinking about making an evacuation call, not when it's already raining.''

For CalMatters, I'm Alistair Bland reporting with Julie Cart.