The disaster took place around 3 am local time (800 GMT) in the town of Salgar, about 60 miles southwest of Medellin.
Survivors told local radio they awoke to sound of a loud rumble and neighbors' shouts of, "The river! The river!" Many were barely able to gather their loved and get out of the way before an avalanche of heavy rock and mud overtook the homes bordering the steep Libordiana ravine.
Disaster relief authorities and police helicopters were rushing to the town on orders of President Juan Manuel Santos to mount a search and rescue operation.
He said the flooding had destroyed the town's aqueduct and led to flooding even in less hazardous parts of the town. He called on authorities to send water and food supplies to stem the emergency.
The town of 18,000 lies amid one of Colombia's major coffee-growing regions. Former President Alvaro Uribe, who spent much of his childhood in Salgar, announced on Twitter he was heading to the town to accompany the victims.