The low-frequency response of the upper Paraná River basin is investigated by means of singular spectral analysis (SSA) applied to precipitation rates and runoff contributions of different sub-basins, upstream of the Posadas gauging station. The temporal structure of Posadas discharges is characterized by a trend after 1970 and a modulated signal with a period of T ≈ 9 years that produces a maximum in amplitude around 1983. ENSO-range signals (T ≈ 3-5 years) contribute to fit the highest peaks in the streamflow series. Moderate changes in mean annual precipitation rates are reflected in large increases in mean runoff contributions for each subbasin. Changes in relative mean annual runoff responses before and after the 1970s, downstream of Sao Simao, cannot be only explained by increases in the corresponding mean annual precipitation rates. It is more probable that such changes arise as a consequence of some anthropogenic impact than a climatic one. In particular, the outstanding increase in runoff contribution, after 1970, for the drainage area between Itaipú and Jupiá of around 96% is mainly responsible for a positive trend in Paraná flow. From the detailed analysis of each sub-catchment, we can observe that not all the signals present in precipitation are reflected in runoff contributions and vice versa.