Chester County officials say the boy and a 25-year-old man were walking along a spillway at the dam Wednesday afternoon when the water level rose and swept them about 100 yards downstream.
Emergency Management director Eddie Murphy says about 60 people from Chester, Fairfield and Lancaster counties worked for about two hours in the water rescue.
The man was found quickly and was taken to Springs Memorial Hospital in Lancaster.
The boy was found stranded on a rock hanging onto a tree. He was taken to Carolinas Medical Center in Charlotte for treatment of symptoms of hypothermia.
The dog also was rescued.
The names of those involved were not released.