Effects of logging on summertime low flows and fish habitat in small, snowmelt-dominant catchments of the Pacific Northwest

Reductions in forest cover due to logging and natural disturbance often result in an earlier peak flow and an increase in annual yields. Most studies that consider the hydrological impacts of forest harvesting have focused on these changes to peak flows and annual yields, and not summertime low flows, even though summertime low flows are critical for instream rearing fish. This study specifically considered how forestry has impacted summertime low flows and fish habitat through time in three catchments in the southern interior of BC, two with an area of about 5 km², and one with an area of 36 km². To assess the hydrological impacts of logging, a paired-catchment analysis was applied to each day-of-year, as well as the July 1-September 30 yield. The post-harvest time series was divided into treatment periods, each about 6-10 years. To quantify the effects on of altered flows on fish habitat, an eco-hydraulic instream flow habitat assessment using a physical habitat simulation (PHABSIM)-style approach was coupled to the results of the paired-catchment analysis. In the two catchments with the longest post-harvest time series, beginning approximately two decades after the onset of harvesting, summertime low flows were consistently less than the predictions from the pre-harvest regression. Reductions in summertime low flows were less pronounced in the third catchment, where there are only 10 years of post-harvest data. The eco-hydraulic habitat modelling indicated that these reductions in streamflow often corresponded to reductions in fish habitat that typically ranged from about 20-50%. The delay between forest harvesting and reductions in streamflow is speculated to be due to regenerating forests that produces greater transpiration loss than the original, mature forests. These results begin to fill an important gap in our knowledge about longer-term impacts of logging on summertime low-flows and fish habitat in small snowmelt dominated hydrological systems of the Pacific Northwest.Arts, Faculty ofGeography, Department ofGraduat