Rescuers climbed down a steep and rocky path through trees that led to the base of the creek, where they threw ropes out to the stranded group, said Chuck Singleton, captain of Community Rescue Service Inc.
Meanwhile, a Hagerstown Fire Department crew parked an engine on the bridge and extended the ladder out over the water where Firefighter Kevin Eichelberger, who was sitting on the end, tossed a safety line to the boat below.
With the ropes, rescuers below made their way into the cold water to hip level, about half way to the boat, and pulled each person parallel to the dam to safety.
The entire rescue took about 15 minutes.
Samuel Richard Long, 27, of Hagerstown, his 7-year-old son, and 14-year-old nephew placed the motorless boat into the creek about 10 feet downstream from the dam with the intent of fishing from it, Mash said.
They had planned to rely on using oars to power the boat but they couldn't overcome the strong current, she said.
"The power is unbelievable. The force of the water is very strong," Mash said.
The third adult rescued, a 24-year-old Hagerstown man, was a fisherman who saw the accident from his spot downstream and who jumped in to try to save them, she said.
The fisherman apparently went under the water at the dam when he reached the group, but was pulled up by one of the males who had been in the boat, Singleton said.
All four were wrapped in blankets and taken to Washington County Hospital where they were treated for hypothermia and released, said John Costopoulos, hospital spokesman.
The three in the boat were wearing a flotation device, Mash said.
Ray Reese, who was having a party at his home which sits next to the creek, said he heard someone yelling and ran across the street to where a fisherman said people were drowning.
As Reese called 911, one of the members of a band playing at Reese's party jumped into the creek but had to turn around before he got to the boat because the water was too cold.
"When the water was around his chest it was so cold he said he felt constricted," Reese said.
Since Reese didn't have any rope, he and others started tying electrical cords together to be used as a safety line before rescuers arrived on the scene.
Dams like the one below Mount Aetna Road are commonly referred to as "drowning machines" because the force of the water will keep pulling objects back into the dam, said Richard Hopkins, Hagerstown Fire Department battalion chief.
"It could have caught the boat and sucked them under, constantly rotating and churning them around," he said.
Rescue officials and Hagerstown Police, who responded for crowd control, said they do not remember having to perform a rescue at that dam before.