La Niña influences Amazon flooding

Work recently published in Nature announces a significant correlation between sediment deposition in two Bolivian rivers, which flow into one of the principal tributaries of the Amazon, and climatic events of the ENSO (El Niño Southern Oscillation) type. It is the fruit of a research partnership between the IRD, the Bolivian Meteorology and Hydrology Centre in La Paz and scientists from the universities of Washington and California. The results represent a major advance in the study of the Amazon Basin’s hydrology and geochemistry dynamics since the beginning of the XXth century (the HYBAM programme). They further more particularly knowledge of the impact of climatic variability on the processes controlling the sediment transport and deposition between the Andes and the Atlantic Ocean. They also provide useful clues for predicting strong flooding events in this region.

The Amazon Basin in figures…

– 6 million km_ -representing 5% of the Earth’s exposed land surface,
– 7 countries: Brazil, Peru, Bolivia, Colombia, Ecuador, Venezuela, French Guiana – annual average discharge rate of the Amazon: 209 000 m3/second
– average annual input of 6 600 billion m3 of freshwater and 600 to 800 million tonnes of sediment into the Atlantic (15% of all freshwater input into the oceans)

The HYBAM programme (Hydrology and Geochemistry of the Amazon Basin)

The Amazon Basin is a highly complex system to study. Its expanse is enormous, it includes a great variety of landforms and the different rivers contributing to the system are diverse in size and source. Research requires a substantial array of technical means and human resources. To ensure their availability, the IRD signed in 1994, in the context of a convention it had concluded with the Brazil National Council for Scientific and Technological Development (CNPq), a cooperation agreement with the University of Brasilia and the National Electrical Energy Agency (ANEEL, Agence Nationale d’Énergie Électrique), the initiator of the HYBAM programme. Its main objective: improve on knowledge about the hydrology and geochemistry of the Amazon Basin acquired since the beginning of the XXth century. Other major goals are to assess the input the Amazon feeds into the ocean and to determine the impact of climatic oscillations, such as El Niño, on the dynamics of the basin’s hydrological and sedimentary system.

About 40 researchers and engineers (half of whom are partners from the countries of the South) are currently involved in this multidisciplinary programme: climatologists, hydrologists, geochemists and remote sensing and modelling specialists.

The HYBAM project has accomplished around 30 or so campaigns in the Amazon region. These, combined with monitoring data obtained from the HYBAM reference network of hydrological and geochemical stations deployed in Bolivia since 1983 and in Brazil in 1995, have yielded precise measurements of the water, sediment and geochemical inputs contributed by the Amazon’s main tributaries. This year, 2003, has seen this network, now accorded the official designation of Environment Research Observatory (ERO), set up on these rivers, with joint IRD-INSU-Ministry of Research finance. This HYBAM ORE is based on a permanent network of 4 hydrological stations installed on the Andean piedmont in Bolivia, Peru and Ecuador, 6 on the Brazilian rivers, and 2 on major rivers in French Guiana. The objective of this ORE is to assess the fluxes of water and materials in the Amazon Basin and, importantly, the impact of climatic variability and human activity on such fluxes.

Results already acquired have shown the importance of the system’s flood zones on transport of water, sediments and associated chemical elements. In order to understand processes which might interfere with these flows, the HYBAM project is focussing on wetlands in Brazil, lakes called “varzeas” and in particular on the Curuai varzea near the town of Obidos. Determination of the input/output balance of this varzea has allowed the development of working models of such Amazonian lakes which are connected to the main river rising in the Andes.

In addition, satellite remote-sensing techniques are currently employed extensively by the HYBAM project. They can be used as much for determining water levels by radar altimetry (Topex, Jason), as for measuring the down-gradient of rivers on the plain (DGPS), or defining the surface concentrations of matter in suspension in the flood-lakes and the main river streams (Landsat and ENVISAT).

Amazon flooding and climatic variability One of the HYBAM programme’s main lines of study concerns the physical erosion and chemical leaching processes along with the mechanisms of transport and deposition of matter from the Andes down to the Atlantic Ocean.

The research of Laurence Maurice Bourgoin, geochemist at the IRD, and his American colleagues was conducted on the flood-plain on two Bolivian rivers rising from the Andes, which in turn flow into one of the Amazon’s main tributaries. The northern part of the eastern Bolivian cordillera is at present a zone of heavy rainfall and steep valley slopes. Soil and rock erosion of these drainage basins is therefore intense. The quantification of sediment accumulations followed by dating (from the 210Pb content) of sediment beds were the first stages of the work. Subsequent examination of these data in relation to climatological measurements and the development (by the University of Washington) of a new geochronological model revealed that the deposition rates of major sediment accumulations in the flood-plain were not constant in time but episodic, and in the Andes strongly linked to climatic events such as La Niña, the cold phase of the ENSO cycle.

A major advance for the HYBAM programme By calling into question the 210Pb based geochronological dating models hitherto used, this research represents an important advance for knowledge of the sedimentation processes in the plain and of their frequency. That will open the way, among other things, for work on other climatic indices to determine, by region, the influence of these climatic factors on the hydrology and sedimentology of the Amazon Basin, and for more detailed study of a climatic hiatus that occurred in the deposits in the 1970s, observed in one of the sub-basins.

In terms of flood prediction, attention is drawn to the years for which a La Niña is forecast and more especially to those which are preceded by a strong El Niño event.