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The Paraná River has been increasing its annual flow during the last 30 years. The relative contribution 2727 of its major tributaries (i.e., the upper Paraná and Paraguay rivers) is uneven in as much as the Paraguay is 2828 increasing its annual discharge at a higher pace than the Paraná does. Contrastingly, the upper Paraná has 2929 been increasing significantly its flow during the second half of the year (i.e., historical low water period) 3030 whereas the Paraguay River has been amplifying its flow throughout the hydrological year. The variability 3131 of d 32 18O measured in the Paraná River middle reach tends to follow Paraguays relative contribution to Paranás total discharge. A simple model built on the basis of the mean d 33 18O signature of rainfall (Global Network of Isotopes in Precipitation) shows significant coherency with the relative contribution time ser- 34d 32 18O measured in the Paraná River middle reach tends to follow Paraguays relative contribution to Paranás total discharge. A simple model built on the basis of the mean d 33 18O signature of rainfall (Global Network of Isotopes in Precipitation) shows significant coherency with the relative contribution time ser- 34d 33 18O signature of rainfall (Global Network of Isotopes in Precipitation) shows significant coherency with the relative contribution time ser- 3434 ies during non-El Niño periods but it is necessary to invert the mean isotopic values in precipitation dur- 3535 ing the occurrence of a major ENSO event (i.e., assume that d 36 18O in upper Paraná River water becomes more negative than usual) to improve the resemblance to the observed variability. 37d 36 18O in upper Paraná River water becomes more negative than usual) to improve the resemblance to the observed variability. 3737 2010 Published by Elsevier B.V.2010 Published by Elsevier B.V.