This article analyzes the influence which they have the wooded masses in the water production of the river basin of the Purapel river, located in the central zone of Chile. This river basin has a surface of 264., km2 and it has been affected by processes of vegetational substitution, happening of native forests a massive forestation, mainly of Pinus radiata (D.Don). The made multitemporary analysis, showed that the vegetal cover I vary of 0 per cent of forest plantations in 1957, 51,7 per cent for year 1997. The methodology studied the use of the ground in 1957, 1978 and 1997; the precipitations were analyzed and the flow and hidrogramas of hour in hour studied; the variation of the water reserves was analyzed on monthly scale and annual, the runoff coefficients of monthly and annual were analyzed, the maximum flow, and finally, their relation with the vegetal cover and the water production studied between years 1960 and 2000. The results allow to conclude that for the river basin of the Purapel river, differences in the observed hydrology behavior in the 40 studied years do not exist. The oscillations observed in the hydrologyc variables would have to the amounts and happened intensities of precipitation and to the high intermonthly variability. Only the monthly reserves show a different behavior, since they tend to be increased in 1979/2000 period. It would have to important precipitations fallen in the decade of the 80, and not directly to a change of the type of vegetal cover.