The causes of Thursday's crashes in Riverside and San Diego counties remained under investigation but in each case it was either raining or the roadways were slick from downpours, authorities said.
One bus was heading from Los Angeles south to the Valley View Casino & Hotel in San Diego County when it overturned on Interstate 15 near Fallbrook.
A woman was partially flung through a window and died at the scene, and 19 others had injuries, six of them serious, California Highway Patrol Officer Jim Bettancourt said. The woman was identified Friday by the San Diego County medical examiner as 64-year-old Tayde Murguia of Los Angeles.
The driver told authorities that he was traveling about 55 mph when the bus began to fishtail.
"He lost control a little bit, tried to overcorrect," and the bus spun around and landed on its side, Bettancourt said.
The driver was among those with minor injuries, he said.
About 50 miles away, 30 people suffered minor to moderate injuries when a tour bus coming from the Pala Casino Spa & Resort in San Diego County overturned on northbound I-15, authorities said.
That crash occurred in the Corona area of Riverside County, east of Los Angeles.
The driver told investigators he was going 65 mph, which was too fast for a large bus on a wet road, CHP officer Travis Monks said in a statement.
Also in Riverside County, a crash injured more than a dozen people Thursday morning on State Route 74 near Perris.
The county Riverside Transit Agency bus slid sideways and hit a car traveling in the opposite direction, the CHP said. A tow truck behind the Pontiac Bonneville also hit the car.
Fire officials said 14 people were taken to hospitals, including the car's driver, whom the CHP described as a 48-year-old woman who suffered major injuries.
Riverside Transit Agency spokesman Brad Weaver said 15 people were injured, including the bus driver who suffered minor injuries and wasn't taken to a hospital.