The Los Angeles County Sheriff's Department says the drowning happened shortly before 9 a.m. Friday in the 1500 block of Pass and Covina Road.
According to a preliminary investigation, the mother, 25, was bathing the child along with his two siblings, a 1-year-old girl and 2-year-old boy. The mother left the three children unattended for 10 to 15 minutes. When she returned, she found the 8-month-old face down in the water not breathing.
The child was taken to a local hospital, where he was pronounced dead.
Los Angeles County Department of Children and Family Services have been notified of the incident. Authorities say the other two children are in protective custody.