Torrential rains starting from Friday night caused three rivers surrounding Mocoa city, Putumayo province, to overflow -- sending a torrent of mud surging through the city.
The President on Sunday night warned that the toll was likely to rise, Efe news reported.
Santos said 43 children were among the dead, and 22 more were hospitalised.
Several children have been reunited with their parents; many children are in shelters, CNN quoted Santos as saying.
According to the Colombian military at least 400 people have been injured, and over 150 were reported missing.
A state of emergency has been declared in the region.
Authorities have identified about 170 of the dead, according to the President.
"Here we are facing a disaster caused by nature, by climate change," Santos said earlier.
He added that the region received nearly 500 mm of rain in March, which he said is about 80 per cent more than the usual amount for the month.
Power and water supplies to Mocoa have been cut by the disaster, and the hospital system has shut down, local authorities said.
Images on social media showed cars and buses trapped in several feet of mud.
Gabriel Umana, a spokesman for the Colombian Red Cross, told CNN that 300 families were displaced and more than two dozen homes had been flattened.
Many were sound asleep when the river of mud hit their neighbourhoods, and witnesses said the sludge flowed so fast that they had to run for their lives, Umana added.
(This story has not been edited by Social News XYZ staff and is auto-generated from a syndicated feed.)
About VDC
Doraiah Chowdary Vundavally is a Software engineer at VTech . He is the news editor of SocialNews.XYZ and Freelance writer-contributes Telugu and English Columns on Films, Politics, and Gossips. He is the primary contributor for South Cinema Section of SocialNews.XYZ. His mission is to help to develop SocialNews.XYZ into a News website that has no bias or judgement towards any.