Patagonia’s Chubut River: Overview of the Main Hydrological and Geochemical Features

Abstract

The Chubut River’s mean annual discharge is ~1.1 km³ (~35 m³ s⁻¹), contributes ~2% to Patagonia’s total freshwater discharge, and ranks far behind the mighty Negro River (~32 km³ y⁻¹). In a semiarid scenario, the river has a mountainous active basin, low runoff (<3.5 mm y⁻¹), and a scanty specific water yield (1.1 L s⁻¹ km⁻²). The seasonal Kendall trend test shows that discharges during the low-water months (Jan.–Mar., May) have been significantly decreasing during the last decades. Ca²⁺–HCO₃⁻ are the governing ions in the headwaters but the composition gradually shifts to a Na⁺-type toward the lowermost reaches. Numerous Andean glaciers suggest that subglacial oxidation of pyrite may be an active solute-supplying mechanism. Silicate hydrolysis and limestone dissolution—implied by non-radiogenic ⁸⁷Sr/⁸⁶Sr ratios—are the processes ruling chemical weathering. The Chubut is a mesotrophic river, with a moderate organic load (mean TOC ~290 µmol L⁻¹, and mean yield ~10.5 mmol m⁻² y⁻¹; ~60% accounted for by DOC). Suspended sediment yield at Los Altares (~14 T km² y⁻¹) and in the lowermost reach (~25 T km² y⁻¹) indicate a relatively low denudation. The alteration index of riverbed sediments (mean CIA ≈ 55) suggests scarce weathering; REE spider diagrams of sediments shows a signature compatible with continental island arcs.