Dozens of volunteers are shoveling mud out of homes, digging out cars and removing large rocks by hand.
Everyone in the two towns was accounted for and no injuries were reported, officials said.
To the west, a 48-year-old man died in a car that was swept into a rain-swollen creek near Mount Baldy. Coroner's officials identified him on Monday as Joo Hwan Lee of El Segundo.
Residents of Mount Baldy awoke Monday to sunny skies and mud-filled streets. They swapped stories between drying out carpets and shoveling dirt from in front of their homes.
"The stream was a raging black torrent of debris and big logs and muddy, silty water," said Michael Honer, who watched the flood build over an hour from a friend's house up the road. "It was apocalyptic. ... It sounded like a cross between a railroad train and a jet engine."
The white Toyota Prius where the driver died was wedged in Bear Creek among boulders and a log. The windshield was shattered and the car was full of dirt.
The driver had been parked Sunday in Angela Batistelli's driveway when she returned home with groceries. Hikers frequently park there and she asked the driver to move.
Rain was falling hard when she carried some bags up to her house. She saw the Prius down the street; its taillights were surrounded by water and then it was gone in the roar of water filling the canyon.
Batistelli's car, a Toyota Echo, also washed away. It was found sticking straight up, its front end buried in the silt-filled streambed. Her 250 gallon propane tank was torn from the house and carried down the street.
The road leads the way to a hiking trail up Mount San Antonio, known as Mount Baldy. At 10,068 feet, the barren peak is one of the tallest in Southern California and is popular among hikers and skiers.
The creek, which hadn't run in the summer for two years, turned to a gusher of rocks and logs, jumping its banks and surging across the adjacent road. The gorge that had been 5 to 15 feet deep in places was filled to the banks Monday with rocks and silt and level with the road. Only a trickle of water remained.
Gerard Masih drove up to the mountains of Forest Falls for a picnic with his wife and 8-month-old baby to escape the heat at home in Highland. They packed up when the rain started but got stuck in a line of cars when the road was closed.
The family joined other visitors at the community center, but baby Jacob couldn't sleep amid the hubbub of 80 other stranded tourists, so they bundled him up and slept in their car.
"Oh man, the diaper situation. We're down to the last one. We had to just smell the stuff all night," Masih said.