Adam Gonzales, 58, was using a zero-turn lawn mower — a mower with a turning radius of zero — last Thursday to cut the grass on a property in Hunt, the Kerr County Sheriff's Office wrote on Facebook.
While cutting the grass, Gonzales, who worked at the property, made "a pass too close to the river's edge" and the lawnmower turned over, investigators said.
Gonzalez fell into the river and was pinned underneath the machine, they added. Because the mower was so large, responding law enforcement officials "could not remove it from his body."
Members of the local fire department's dive team concluded they could not safely recover Gonzales' body until Friday morning when it was light outside, officials added.
Hunt is about 80 miles northwest of San Antonio.