Rainfall peak flow response to clearcutting 50% of three small watersheds in a boreal forest, Montmorency Forest, Québec

The scientific literature suggests that a 50% increase in bankfull discharge can significantly modified stream morphology and that such increases could be avoided by clearcutting less than 50% of the watershed area. Also hypothesized is that clearcutting near the stream network could have greater effect on peak flow than harvesting a comparable area further away. Following a 5-year calibration period, three small watersheds (<50 ha) within the “Ruisseau des Eaux-Volées” Experimental Watershed (REVEW) at Montmorency Forest (Quebec, Canada) were harvested over 50% of their areas. Regeneration was protected and logging was carried out when snow cover was present to prevent soil perturbation. A neighbouring watershed was maintained undisturbed as control. During the two summers following logging and for all treated watersheds, analysis of regression lines indicated non-significant peak flow increases and bankfull discharge augmentations below the 50% threshold. Proximity of harvested areas to stream network had no significant effect on increase in bankfull discharge. Furthermore, the smallest augmentation was recorded on the watershed where the harvested area was closest to the stream network. For all treated watersheds, one particularly intense rainstorm (resulting from after effects of Hurricane Katrina) significantly increased peak flows above the upper confidence limit of the pre-treatment regression lines. These individual bankfull increases were all above 50% indicating that the 50% cutting threshold may not be applicable to very small watersheds. However, that particular meteorological event was exceptional and given a harvesting rotation of 80 years, the probability for peak flow to augment above 50% was increased by only 3%.