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

DEPETRIS, P.; PASQUINI, A.I.

Environmental Assessment of Patagonia's Water Resources

Springer International Publishing

Año: 2021; p. 127 - 154

The Chubut River´s mean annual discharge is 1.1 km3 (35 m3 s-1), contributes 2 per cent 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 per cent 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.