Effects of forestry on summertime low flows and physical fish habitat in snowmelt-dominant headwater catchments of the Pacific Northwest Gronsdahl, Stefan; Moore, R. Daniel; Rosenfeld, Jordan; McCleary, Rich; Winkler, Rita
Periods of summertime low flows are often critical for fish. This study quantified the impacts of forest clearcutting on summertime low flows and fish habitat and how they evolved through time in two snowmelt-dominant headwater catchments in the southern interior of British Columbia, Canada. A paired-catchment analysis was applied to July-September water yield, the number of days each year with flow less than 10% of mean annual discharge, and daily streamflow for each calendar day. The post-harvest time series were divided into treatment periods of approximately 6-10 years, which were analyzed independently to evaluate how the effects of forestry changed through time. An instream flow assessment using a physical habitat simulation (PHABSIM)-style approach was used to relate streamflow to the availability of physical habitat for resident rainbow trout. About two decades after the onset of logging and as the extent of logging increased to approximately 50% of the catchments, reductions in daily summertime low flows became more significant for the JulySeptember yield (43%) and for the analysis by calendar day (11-68%). Reductions in summertime low flows were most pronounced in the catchment with the longest post-harvest time series. Based on the temporal patterns of response, we hypothesize that the delayed reductions in late-summer flow represent the combined effects of a persistent advance in snowmelt timing in combination with at least a partial recovery of transpiration and interception loss from the regenerating forests. These results indicate that asymptotic hydrological recovery as time progresses follow logging is not suitable for understanding the impacts of forest harvesting on summertime low-flows. Additionally, these reductions in streamflow corresponded to persistent decreases in modelled fish habitat availability that typically ranged from 20-50% during the summer low-flow period in one of the catchments, suggesting that forest harvest may have substantial delayed effects on rearing salmonids in headwater streams.
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