Extreme flooding in large southamerican rivers: hydrological and biogeochemical impacts
by
Pedro Depetris
CIGeS/FCEFyN, Universidad Nacional de Córdoba, Argentina

Rivers deviate, often with discernible frequency, from their most recurring hydrological functioning and produce extreme high and low water stages. Clearly, high floods are the most damaging events in rivers because they generate loss of lives and goods, and impair essential services in the affected communities. The historical record shows that, in terms of losses, flooding can be compared with other disastrous natural hazards known for their catastrophic characteristics (e.g., earthquakes): the 1887 flooding of the Yellow River (China) produced over 900,000 casualties. Extreme flooding is catastrophic in human terms and also alters the biogeochemical functioning of the natural riverine system.

Along with the world's largest river (i.e., the Amazon), South America holds other very large fluvial systems (Orinoco, Paraná, Magdalena, etc.), whose hydrological dynamics are linked to mechanisms that frequently determine extreme high or low stages. The hydrological effect of the ENSO is the best known low-frequency mechanism that affects South American runoff.

In 1982-83, the Paraná River was subjected to an ENSO-triggered exceptional flood (the strongest on record) that lasted for almost 20 months. During such event, the mean flow was over 75% higher than the long-term mean discharge (~15,500 m3s-1). The sediment load, however, was lower then (53 106 t y-1) than during the period that preceded the flooding (66 106 t y-1).

During the flood, the CO2 partial pressure reached uppermost values (16,000 ppmv) when the stage was on the descending limb of the hydrograph. Clearly, CO2-rich water was flushed out of the flood valley and injected into the main stem. Also, DOC increased 2.7-fold during the flooding. POC was largely autochthonous in nature (13C of POC  -20 to -25‰). High POC:PN ratios (20 – 25) in the suspended load indicated that when water was flushed out of the flood valley, there was a significant contribution of vascular plants debris.

Date received: February 10, 2005

Copyright © 2005 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # caod-76.