More than 250 people were confirmed dead and many others missing as heavy rain battered the region for the second week.
Colombian President Juan Manuel Santos declared a state of emergency and promised recovery efforts would continue until all missing people are accounted for.
Rescue teams reported shortages of drinking water, sanitation and dry elevation areas as they struggled to transport survivors to safety.
Meteorological forecasts said Colombia could be in for a long haul -- with mounting casualties, massive infrastructural damage and risk of disease -- as it lay in the path of El Nino conditions that could last through February.
Officials said that, in addition to the increasing loss of lives, material damage could exceed $5 billion. Many bridges and roads were washed away as the floods advanced.
In one positive outcome of the national emergency, Ecuador restored diplomatic relations it severed in an angry response to a Colombian cross-border raid aimed at a FARC guerrilla outpost in March 2008.
The crisis over the Colombian military operation abated when Santos succeeded Alvaro Uribe as president Aug. 7. But it was made worse when Colombian intelligence was found to be eavesdropping on conversations of President Rafael Correa.
Despite those rows, Correa arrived in Cali in the Valle del Cauca region to offer help and inspect flood damage with Santos. The chill in bilateral relations caused by the eavesdropping scandal dissipated as the two leaders pledged to pool resources to deal with the floods.
After an appeal from Santos, the U.S. Army Corps of Engineers sent experts to offer advice on how to repair a breached levee that threatened to flood other areas and displace thousands more of inhabitants in northern Atlantico state.
The U.S. Agency for International Development contributed more than $1.3 million to relief efforts and offered use of several helicopters and other aircraft normally deployed in counternarcotics operations. Colombian and U.S. forces are collaborating in a multimillion-dollar operation to stem the flow of cocaine and other drugs from Colombia and neighboring countries to North America.
Officials said they weren't certain how many people were missing and unaccounted for but the total of those made homeless was running into tens of thousands.
Added to that is problem of tens of thousands of school children left without schools, which have been damaged or washed away.