The Sacramento County Sheriff's Office said in a news release the incident happened around 11:30 a.m. on the San Joaquin River near Isleton, located about 40 miles south of Sacramento.
Officers received a 911 call from a 10-year-old child who explained that her 1-year-old brother fell out of a boat while the children were fishing with their 41-year-old father.
"The child indicated her father dove into the river after the child and related the child was wearing a life vest; however, her father was not," the sheriff's office said.
Officials from the U.S. Coast Guard located the boat as it was drifting in the waterway and rescued the children inside. Coast Guard officials then were able to find the child floating in the river.
"First responders immediately began performing life-saving measures on the child, including CPR, as the child did not initially have a pulse," the sheriff's office said.
The child was transported to a California Highway Patrol helicopter and transported to a Sacramento-area hospital, where he had a pulse on arrival and was listed in serious condition.
"Search crews from the Sheriff’s Office are still combing the area and will continue searching depending on conditions," officials said. "The river in this area is approximately 50 degrees and due to heavy rainfall and runoff in our region, the river is flowing very fast."
Sacramento County Sheriff's Sergeant Shaun Hampton told FOX40 that the river is filled with water that is cold and filled with debris, which makes for a dangerous combination for any person in distress.
Family members identified the man as 41-year-old Moua Lo, and said he jumped in after his son as any parent would.
"He would be the first person to give you his jacket if he sees that you’re in need. He would stop to help anybody," brother-in-law Chong Yang told FOX40.
Friends and family members held a candlelight vigil on Saturday night and were hopeful as agencies continue to search the shoreline.
“The family is really hurt. We’re not giving up," Yang told the television station. "We want to make sure we recover him and hopefully we’ll be able to get that done soon."