Human activity in the catchments of rivers and lakes produces significant influence on the formation of water resources. About 65% of the Lake Onega catchment area is occupied by forest, so forest management plays an important role in the formation of runoff from it. The characteristics of the stand growing in the catchment and areas designated for clear cutting were determined from published data. The definitions are given for the concepts of old-growth (primary) and secondary forest. It is remarked that the temporal variability of runoff from old-growth forests is controlled by meteorological conditions only, whereas runoff from secondary forests significantly depends on anthropogenic factors. It is proposed to estimate the anthropogenic component in runoff as the difference between the actual flow and the flow that would have come from the same catchment area had it been covered in old-growth forest. It is noted that there is high similarity in the formation of the water balance between old-growth and mature secondary forests. Our calculations show that owing to greater evaporation actual runoff from a secondary forest is lower than runoff from mature and overmature forests in the same catchment. This situation is due to stand rejuvenation and a higher rate of processes in a young stand. The area where removal of the stand by clear cuttings is possible in the catchment of Lake Onega is about 244 km². To estimate how clear cutting would modify the runoff a previously developed technique was used. The resultant data show that the total increase in annual runoff from the catchment in the first year after clear cutting may reach 49.4 million m³.