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The debate over isostatic uplift versus discharge as drivers of Quaternary river incision is explored here through geospatial analysis of a ~250‐m‐relief terrace sequence from the Río Pinturas (Argentine Patagonia). The geomorphic setting of the Cañadon Caracoles reach allows evaluation of discharge as a driver of terrace incision because advancing ice during Pleistocene glaciations blocked Pacific drainage and routed meltwater from an expanded ~104‐km2 catchment to the Atlantic through the dryland steppe of the study area. Twenty‐two terrace levels, some assigned to previously dated ice margins [Marine Isotope Stage (MIS) 32–36, MIS 16, MIS 8 and MIS 2], were identified. Average net incision since 800 ka (~0.34 m ka−1) exceeded regional uplift rates. The MIS 2 terraces, with ante quem and post quem age constraint on the timing of terrace formation, show that terrace incision was episodic and faster still during a transitional warming climate. Glacier recession and proglacial lake formation at ~18 ka led to rapid incision of ~11.7 m ka−1 over a few millennia. River capture and negligible flow from ~15.0 ka caused fan‐dammed lake formation on the valley floor and vertical stability during MIS 1. The Pinturas terraces demonstrate rapid incision can be driven by discharge and sediment dynamics. [ABSTRACT FROM AUTHOR]