Fig. 1: Geographic and climatic setting of the Río Iruya study area.

a Major modern South American climate and topographic components including modern mean annual rainfall derived from the Tropical Rainfall Monitoring Mission (TRMM) 2B31 dataset⁷, the South American low-level jet (SALLJ), and the internally-drained, high-elevation Altiplano and Puna plateaus (see Supplementary Fig. 1 for detailed climatic and topographic characteristics of the study area). The white star locates the study site in a and b. b Plot of modern peak mean annual rainfall (±2σ envelope) (blue) extracted from 115, 50-km-wide by 1000-km-long, orogen-perpendicular swath profiles along the Andes (see⁷) indicating a transitional zone (gray) between ~18– 25°S latitude. Linearly-interpolated mean annual rainfall derived from CLIMBER-2 model results (magenta), extracted from a longitudinal swath spanning 95 − 115°W at 10° latitude intervals from 10°N to 35°S in the model (see Methods for discussion of the geographic offset of the model), shows maximum precipitation values from a precession maximum at 2.670 Ma and minimum values from a precession minima at 2.487 Ma. Magenta percentages indicate the maximum percent increase in mean annual precipitation between the plotted precession minimum and maximum values, with the mean differences in parentheses. c Overview of the Río Iruya watershed geography and Eastern Cordillera topography with predominant thrust faults labeled. Locations of modern cosmogenic radionuclide (CRN) erosion-rate samples (IR-3M and IR-6M) are shown as white stars. d Close-up of the Río Iruya canyon section showing the pre- and post-diversion Río Iruya flow directions (hashed and solid white arrows, respectively), CRN paleo-erosion rate sample locations (white circles) with select stratigraphic ages (white numbers), and total estimated incision in the canyon over the ~150 years post diversion (blue-red color gradient).