The cars collided near where park-goers board the ride.
According to the Orange County Fire Authority, nine people on the ride were injured and one bystander was hurt.
All 10 were hospitalized with minor injuries.
Knott's spokeswoman Jennifer Blazey told the Orange County Register that a train leaving the station didn't make it over the first hill and rolled back into another train.
The cause of the accident was under investigation.
The Associated Press contributed to this report.