Santos said he blamed climate change for the wall of water that knocked out power in half the province of Putumayo, where Mocoa is located, and wiped out the city's supply of fresh water.
Muddy water and debris quickly surged through the city's streets, toppling homes, ripping trees from their roots, lifting cars and trucks and carrying them downstream. Witnesses told the AP that they felt buildings vibrate, and that many did not have enough time to retreat to roofs or higher ground.
President Juan Manuel Santos said he would travel to Mocoa to supervise rescue and assistance efforts in the heavily forested region.
He blamed climate change for triggering the avalanche, saying that the accumulated rainfall in one night was nearly half the amount Mocoa normally receives in the entire month of March.
"A big portion of the many houses were just taken by the avalanche, above all people were warned with enough time and they were able to get out, but houses in 17 neighbourhoods have basically been erased".
The AP spoke with Herman Granados, an anesthesiologist at the local hospital, who had worked throughout the night on victims.
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In 2015, torrential rains in northwest Colombia caused a landslide that killed more than 80 people as mud rushed into homes and bridges, officials said.
Santos says: "These rains are increasingly more intense, so we have to be ready". They remember how the late Hugo Chavez within months of taking office in 1999 had to confront the nation's worst-ever natural disaster, in which thousands of people perished in deadly mudslides near the capital.
"We have sent a team of 150 people to make our response effective and machinery began work immediately", Carlos Ivan Marquez, head of the national disaster unit, said in a statement.
Colombian soldiers carry a victim, in Mocoa, Colombia.
The Colombia Red Cross said 220 people were missing and 400 were injured in the incident.