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.