The gruesome discovery was made around 5:30 p.m. near Irwindale Avenue and Foothill Boulevard.
Police received a call from someone who came across the body in some brush near the riverbed. The head was found a short distance away from the body. According to the Los Angeles County Sheriff's Department, the body is that of a male adult.
Sheriff's homicide detectives are helping Irwindale police with the investigation.
Anyone with information was asked to contact the Los Angeles County Sheriff's Department's Homicide Bureau at (323) 890-5500.