Atrato River https://en.wikipedia.org/wiki/Atrato_River https://www.stockholmresilience.org...-river-colombia-using-sar-interferometry.html https://erdc-library.erdc.dren.mil/jspui/handle/11681/1811Where in Columbia and what route would it take? I would imagine it would be like the Nicaragua canal, nice idea but lots of obstacles to it being built.
At least the Nicaragua route is decently flat. If you go up the Atrato, you're going to have to cut through the coastal range to get to the Pacific. Even that valleys look to be around 200m above sea--level - and that part of Colombia is very remote and unsettled, even today.