Forest disturbance by wildfire or forest harvesting primarily increases low flows in the south-west Rocky Mountains, Alberta, Canada.

Extensive historic research shows large scale forest disturbance (wildfire or forest harvest) in snow dominated mountain environments can increase overall streamflow with variable effects on peak discharges during the melt freshet or in response to precipitation events; however, flow responses to disturbance can vary substantially across hydro-climatic-geologic regions. While the focus of much research has been on the upper or higher end of flow regime, effects on low flows have received less research attention. Here we summarize 21 years of streamflow response to two major historic wildfires (2003 Lost Creek wildfire; post-hoc burned:reference comparisons, 5 catchments 2004-2014], 2017 Kenow Mountain wildfire; post-hoc burned:reference comparisons, 7 catchments 2018-2024] and a long-term study of 3 different forest harvest strategies [clear-cut, strip-shelterwood, partial-cut]; before:after / control:impact [BACI] 8 nested logged/reference catchments, 2009-2024) on seasonal streamflow regimes.Despite large variation in disturbance footprint and catchment scales across studies, 2 large wildfires and 3 differing forest harvest strategies all produced surprisingly similar hydrologic responses; 19-32% increases in annual water yield, advance in the seasonal timing of the snowmelt freshet, and similar increases in fall-winter-early spring baseflow discharges. Most notably, while no meaningful impacts on higher storm discharges were evident after wildfires or logging, large 52-110% increases in lower flows comprising the lowest 1/3 of annual discharges were the clearest, and most consistent hydrologic response to both wildfires and logging in this region where surface-water flow regimes are strongly governed by groundwater inputs. While previous studies report highly variable hydrologic responses to both wildfires or logging depending on disturbance severity, % catchment disturbed, and catchment scales, these findings are novel in showing notable uniformity in a) lack of meaningful impacts to the higher end (i.e. peakflows/stormflows) of the flow regime, with b) concurrent increases in fall-winter-early spring baseflows supporting the notion that enhanced groundwater recharge is likely a key mechanism regulating hydrologic response to forest disturbance in this region. These findings provide important insights in assessing vulnerability of both aquatic ecosystems and human infrastructure to large scale forest disturbance in the context of shifting climates in this Canadian hydro-climatic-geologic region.