The bears were discovered Saturday by two cyclists who heard howling and what sounded like animals in distress at Marshall Canyon Trail in La Verne. They found the bears trapped and called for help.
As of 6 p.m., Los Angeles County firefighters said one of the cubs had managed to get out of the cistern on its own. The mother bear and the other cub remained trapped.
Crews drained the water tank to prevent the bears from drowning. An Urban Search and Rescue team and California Fish and Wildlife officials also responded to the scene.
The family that recorded video of the scene described what they saw.
"I just stuck my head in the hole. One was all tucked up on the top. There's like a little pole. They were just scared to death," Tyler Christensen said.
"I think the bears got very lucky because it would not be easy to hear that," Bradney Boli said.
Because of the location and concerns for the animals' safety, officials are urging the public to avoid the area while rescue efforts are underway.