The dolphins -- one adult and one juvenile -- were found drifting in the Bolsa Chica Ecological Reserve in Huntington Beach on Friday morning, the Los Angeles Times reported.
It's unclear how they got there and why they stayed, said Sarah Wilkin, a marine mammal stranding coordinator with the National Oceanic and Atmospheric Administration.
"There are different possibilities," Wilkin said. "They could have been foraging and looking for fish. The animal could have been compromised or disoriented, and was lost, so to speak."
Wilkins said authorities had to act quickly to get the dolphins out of the reserve, because freshwater environments can be damaging and eventually even fatal for dolphins, whose bodies need saltwater to survive.
After a failed attempt to get the dolphins to move out on their own, trained workers mounted paddleboards and guided the animals back to open waters.
"We're hopeful that now that they're back in their natural habitat, they'll stay there -- but you never know," Wilkin said. "So I'm sure people in that area will be on the lookout."