The black dog was clinging to a concrete ledge in the Los Angeles River, with water appearing to rise.
Fire officials tried to rescue the dog by throwing down a float ring, but the effort did not work.
Crews were spreading out on the banks of the river and a nearby bridge, planning another rescue attempt.
The drama was being carried live on television.
[Updated at 12:35 p.m.: A rescuer hoisted down from a chopper rescued the dog, who was taken away in an ambulance.]