Forest Harvesting Impacts on Attributes of the Flow Regime in Snowmelt Regions

Recent studies have revealed how forest cover removal can alter the frequency and magnitude of annual peak flows. However, to fully understand the impacts of land cover changes on stream channels, it is necessary to examine how multiple attributes of the flow regime are affected. Changes in the frequency and duration of peaks over threshold (PoT) discharge for sediment mobilization has the potential to alter the dynamics of bedload mobility and channel form in alluvial gravel-bed streams. A meta-analysis investigation of changes in the duration and number of PoT in four snowmelt catchments of western North America contributes towards a comprehensive understanding of the influence of harvesting on the snowmelt flow regime. Analysis results reveal that harvesting causes the duration and number of PoT of snowmelt peak flows to increase and that these increases are generally greater for larger flood quantiles. Such a response indicates that, following harvesting, the hydrograph in all four catchments has become more responsive during the freshet period. In addition harvesting has resulted in increases in the total volume (i.e. duration) of flood peaks so that discharge remains elevated above specific thresholds for longer periods of time. The meta-analysis results suggest that physical basin characteristics including percentage of alpine area, slope aspect, and gradient, elevation and watershed size all play a role in catchment scale response to harvesting-related increases in flood duration and PoT. In snow environments alterations to the flow regime due to forest removal are likely to persist for many decades until sufficient forest regeneration restores stand level processes of snow accumulation and snowmelt.