What happened was the cub got wet. And, being unable to climb back up, he was now in serious danger.
Mom, however, didn't miss a beat.
"I thought that baby was going to drown but Mama climbed that fence and went right down to the water," Sommerfeldt told The Dodo. "I was rather amazed how fast she reacted when she noticed one of her babies gone."
The mother bear then swam to the closest shore, carrying the cub on her back. After placing him on dry land, she went back in the water, headed to where her two other cubs were waiting, and encouraged them to jump in the water, too. She then carried them, one by one, on her back to safety as well.
Here's a video of the brave mother completing that heroic task: