The present work is focussed on 1970/71 climatic jump of the whole "Río de la Plata" basin (García & Vargas, 1998, but mostly on the Parana river basin, upstream of Paraguay river outlet, as well as the Uruguay river basin; all of them belonging to the "Rio de la Plata" basin. The obtained results from the streamflow series in the rivers were above expectation in the last decades. The mean streamflow during 1971/1994 period are, on average, a 34% higher than the mean in the period 1931/1970 (García, 2000). As regard precipitation, it is not possible to accurately quantify the differences between the rainfall previous to and after 1970. With the scarce available data covering the period 1931/1994, we can arrive to the conclusion that the precipitation mean after 1970, upstream the Paraguay river outlet, is on average a 14% higher than the precipitation mean previous to that year (1931/1970); while in the Uruguay river basin, the increase is even higher than in the other two regions, reaching a 20% difference as regard the period 1931/1970. Considering the analysed facts, we could observe evidence of an important change in the land use taking place mainly after 1970, which coincides with the streamflow increase; and also in the same period, a precipitation increase undoubtedly producing a streamflow increase. The effect of those alterations on the land use could be permanent, but the effect of precipitation could be temporary. Then, the question is: which is the importance of each effect on the mean discharges? Anyway, whichever is the answer to that question, it can only affect the results of the present study in a quantitative way but not qualitatively.