Planform change of the Río Chubut (~42°S, ~70°W, Argentina) in response to climate drivers in the southern Andes

Abstract

The Río Chubut (southern Argentina) displays complex geomorphological and sedimentological responses to long-term variations in hydrology and sediment supply. A 75 km study reach located 65 km downstream of the former ice margin and proglacial lake (Lago Epuyen) at the foothills of the Andes shows that the river has previously undergone large-scale planform changes from a braided to a meandering regime. The new luminescence chronology presented here constrains the timing of changes in fluvial characteristics in the Chubut valley, including the waning and abandonment of a braid plain dated at four locations, the minimum age for the shift to the modern meandering regime, and the timespan of activity for an alluvial fan. Sedimentology, geomorphology and geochronology findings show that the Río Chubut maintained a braided planform during the last deglaciation until the water supply to the headwaters in the Andes decreased because the mid-latitude storm tracks and westerlies in the Southern Hemisphere weakened and shifted southwards causing drier conditions across Patagonia. This encouraged a time-transgressive shift in the Río Chubut between 12.3 ± 1.0 ka and 9.4 ± 0.8 ka from a braided planform to the meandering system active today. These findings highlight that modern braided rivers (e.g., the Rakaia River, New Zealand) that are experiencing a reduction in discharge could be vulnerable to similar large-scale planform changes with the potential to impact upon unique habitats and ecosystems.