Rainfall generated stormflow response to clearcutting a boreal forest: peak flow comparison with 50 world-wide basin studies

Increase in bankfull peak flows and a reduction of lag and base times of the storm hydrographs were the only change in stormflow characteristics observed after harvesting balsam fir stands over 85% of the area of basin 7A (122 ha) at Montmorency Forest (Quebec, Canada). The maximum peak flow increase by 63% and occurred when harvesting had reached 61% of the basin area. For the five-year period after harvesting 85% of the basin area, the maximum increase of bankfull flow was 57% while the average change derived from the regression between the treatment and control basin during the pre-harvest period was 54%. These peak flow changes were compared with results from harvesting effects on bankfull peak flow from 50 paired watershed studies. The maximum increase in peak flow of 63% (basin 7A) was at the upper end of published results for harvesting 45-70% of a basin area while a 54% peak flow increase corresponded to the average value (49%) of published results for the 70-100% harvesting intensity. The relatively high peak flow response of basin 7A was mainly attributed to the connections of skid trails and road ditches with two branches of the stream. Considered globally, the results from watershed studies indicate that logging should not cover more than 50% of a basin area to minimise the occurrence of peak flow increases above 50%, which are deemed to affect stream morphology significantly.