Reconstructing the magnitude and timing of catastrophic drainage of proglacial lakes during Late Quaternary deglaciation of palaeo-ice sheets is important for understanding rates of landscape change and regional freshwater forcing of ocean currents. Here, we provide the first detailed reconstruction of catastrophic glacial lake outburst flooding from the Southern Hemisphere, focusing on the Río Baker catchment in central Patagonia. Firstly, we review the evidence for palaeolake drainage events in the Baker catchment showing the basin witnessed a number of events during Late Quaternary deglaciation. These floods helped re-establish the Pacific drainage pathway of the Río Baker by 12.6-11.7 ka, after millennia blocked by ice during the last glaciation. Secondly, we analyse the landform record and flow hydraulics of a catastrophic flood in Patagonia, focusing on the final, Early Holocene, drainage event of Lago General Carrera/Buenos Aires. Using two independent methods, a one-dimensional hydraulic model and dam break simulation, we estimate a minimum peak discharge of 110,000 m3s-1 (0.1 Sv), large enough to form megaflood-type landforms. Using the flow hydrograph, we constrain timing of landform genesis. Eddy bars at the top of the water column (45-65 m above the valley floor) were inundated over 2-36 hours following the breach; while tractive bars, formed beneath ~20 m of water at sites of valley expansion, were flooded for ~100 hours. Raised deltas and palaeoshorelines provide geomorphic evidence for palaeolake drainage events across Patagonia indicating the likely role that catastrophic floods played in continental-scale palaeohydrology and the evolution of fjord ecosystems.