All 22 passengers were rescued after being stuck on the ride, although some were trapped for almost three hours. One car on the coaster was dangling about 20 feet in the air.
"It looked like a pine tree fell and it fell across the track of the Ninja ride," Los Angeles County Fire Inspector Rick Flores told KTLA. "The car came around the corner and it hit the tree."
Two of the injured passengers were taken to the hospital as a precaution.
"We were riding on the Jet Stream when we saw the Ninja go into the trees," said witness Brandy Chambers. "It was a lot of leaf noise and then cracking noises...and a lot of screaming."
The ride, which can reach speeds of up to 55 mph, will remain closed until an inspection can be completed.
"We were going across one turn and all of a sudden a loud noise happened," roller coaster rider Jeremy Ead told CBS Los Angeles. "I ducked down just in time. A hard branch hit me in the head. I was there bleeding from my head."