The article deals with consequences of intensive clear-cuts of old-growth boreal forests within modelled net of small river basins (Straler’s 3–4 order). Resulting from cluster analysis of parameters of relief and rivers, 7 types of small basins play different roles in water and sediment discharge processes were described. Accordingly, consequences of intensive clear-cuts are also different due to the basin types. In basins where discharge accumulation is the main trend, one can predict intensive elevation of ground-water level that will impede reforestation for a long time. Where discharge transit prevails, strong soil erosion is highly expected, and this will bring such negative consequence as eutrophication of rivers and lakes in these basins and also in downstream-situated basins which unload discharge into sea. In the last ones, mostly situated within the national park “Onezhskoe Pomorie”, the rivers are important as water sources for local villages and as places of fish breeding, so the damage will be very sufficient from the eutrophication induced. Conclusion is produced that basins, where discharge-transit process prevails, are key places for nature conservation because of either their strong impact on downstream basins or the largest percentage (42%) of old-growth forests with rare species of flora and fauna localized within such transit basins.