River avulsion is a complex multistage process, yet the causes of river avulsions are not well understood. Using four decades of satellite, climatic, and discharge data, we show that on the Magdalena River, Colombia, the frequency of crevasse splay and avulsion triggering increases during La Niña climatic events. Field investigation of an ongoing partial avulsion event that started in 2007 shows that it occurred due to the combined effects of climate and tectonic forcing. The avulsion event started with the formation of a crevasse splay, followed by maintenance and expansion of one crevasse channel, in-filling of a lake, and development of a network of new channels. Based on topographic, bathymetric, and sedimentologic data, we determined that the location of the avulsion point is related to changes in the longitudinal profile caused by a fault; while on a reach scale, it is linked to the location of the channel’s thalweg. We also show that while there is enough gradient to drive the avulsion, because cross-valley slope is larger than down-valley slope, levee height is not higher than channel depth, a criterion that has been proposed to be a necessary factor for avulsion. We show that the avulsion was triggered by frequent and high-discharge events related to the 2007–2008 and 2010–2011 extreme La Niña events. Our field data suggest a trade-off between avulsion setup and trigger, and that the necessary normalized superelevation needed to drive avulsion may decrease with an increase in the magnitude of the trigger. Moreover, our results provide insights into how global-scale climatic processes trigger avulsion, which is useful for hazard mitigation and for the interpretation of avulsive deposits preserved in the stratigraphic record.