The tour bus was about 40 miles south of the park Saturday when the bus driver lost control of the vehicle for unknown reasons, NBC News reported.
The bus rolled onto an embankment, landed back on the road, then careened off the road down toward a ravine, before finally stopping when it hit a tree, said Sgt. Edward Greene of the California Highway Patrol.
"If the tree wasn't there to stop the bus, it would have continued down the ravine," he added.
Several of the passengers -- most of whom were elderly -- were tossed around inside the bus during the incident, receiving minor to moderate injuries, Greene said.
They were taken to Community Regional Medical Center in Fresno, Calif., Clovis Community Medical Center and Madera Community Hospital, The Fresno Bee reported.
The driver was the only person not injured in the accident, the Bee said.
Notable deaths of 2014 [PHOTOS]