The Puelo River is a watershed shared between Chile and Argentina with a mean annual streamflow of 644 m(3) s(-1). It has a high ecologic and economic importance, including introduced farmed salmon, tourism, sports fishing and projected hydroelectricity. Using Austrocedrus chilensis and Pilgerodendron uviferum tree-ring records we reconstructed summer-fall (December-May) Puelo River streamflow, which is the first of such reconstructions developed in the Pacific domain of South America. The reconstruction goes back to 1599 and has an adjusted r(2) of 0.42. Spectral analysis of the reconstructed streamflow shows a dominant 84-year cycle which explains 25.1% of the total temporal variability. The Puelo River summer-fall streamflow shows a significant correlation (P > 0.95, 1943-2002) with hydrological records throughout a vast geographic range within the Valdivian eco-region (35 to 46 degrees S). Seasonal Puelo River interannual streamflow variability is related to large-scale oceanic and atmospheric circulation features. Summer-fall streamflows showed a significant negative correlation with the Antarctic Oscillation (AAO), whereas winter-spring anomalies appear to be positively connected with sea surface temperature variations in the tropical Pacific. In general, above- and below-average discharges in winter-spring are related to El Nino and La Nina events, respectively. The temporal patterns of the observed and reconstructed records of the Puelo River streamflow show a general decreasing trend in the 1943-1999 period. Projected circulation changes for the next decades in the Southern Hemisphere would decrease summer-fall Puelo River streamflows with significant impacts on salmon production, tourism and hydropower generation.