Effects of forestry on summertime low flows and physical fish habitat in snowmelt‐dominant headwater catchments of the Pacific Northwest

Abstract

Periods of summertime low flows are often critical for fish. This study quantified the impacts of forest clear-cutting 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 postharvest time series were divided into treatment periods of approximately 6-10 years, which were analysed independently to evaluate how the effects of forestry changed through time. An instream flow assessment using a physical habitat simulation-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 July-September 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 postharvest time series. On the basis of 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 following 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% to 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. K E Y W O R D Sfish, forestry, headwater, hydrology, instream flow assessment, logging, low flows, snowmeltdominant

Approaching four decades of forest watershed research at Upper Penticton Creek, British Columbia: A synthesis

“…Onset Hobo ® and Tidbit Jowett et al, 2016) to quantify relations between weighted usable area (WUA) and discharge for both rainbow trout fry and parr (Gronsdahl et al, 2019).…”

“…During the pre-treatment period, streamflow peaked on average in mid-May in 240 and 241 Creeks, and approximately 1 week later in 242 Creek. Pre-and post-logging flows are detailed in andGronsdahl et al (2019).Data are publicly available through Environment Canada at https:// wateroffice.ec.gc.ca/search/historical_e.html.During the first 7 years following final logging in the 241 Creek watershed(2008)(2009)(2010)(2011)(2012)(2013)(2014), only small changes in total annual yield were observed (5%, p ≤ 0.05), reflecting the relatively dry climate and the small differences in pre-melt snow accumulation between the pinedominated forest and clearcuts (12% on average). However, the seasonal timing of flows changed substantially.…”

Fifty years of runoff response to conversion of old‐growth forest to planted forest in the H. J. Andrews Forest, Oregon, USA

“…These observations are supported by detailed long‐term studies in these watersheds documenting vegetation basal area, species composition, and post‐clearcut succession (Halpern & Lutz, 2013; Lutz & Halpern, 2006), mechanistic studies of transpiration in conifers and deciduous trees (Moore et al, 2011), and models of hydrologic and climate effects on carbon assimilation (Emmingham & Waring, 1977). Several recent studies have documented the progressive development of deficits in summer daily flow in watersheds with 30 to 50‐year‐old forests (Gronsdahl et al, 2019; Perry & Jones, 2017; Segura et al, 2020). Collectively these findings imply that planted forests increasingly deplete deep soil moisture or prevent soil moisture recharge, especially in the relatively dry summer and fall in this marine west‐coast climate.…”

River management response to multi‐decade changes in timing of reservoir inflows, Columbia River Basin, USA

“…Young, dense forests use more water during the dry summer period compared to mature or old‐growth forest (Jones & Post, 2004). A growing body of work indicates that increased intensive forest management in the CRB may have contributed to long‐term reductions in low flows (Gronsdahl, Moore, Rosenfeld, McCleary, & Winkler, 2019; T. D. Perry & Jones, 2017; Segura et al, 2020). Insect outbreaks and low‐severity wildfire may not produce increases in streamflow that could otherwise mitigate declining trends (Goeking & Tarboton, 2020; Hallema et al, 2018).…”

“…Young, dense forests use more water during the dry summer period compared to mature or old-growth forest (Jones & Post, 2004). A growing body of work indicates that increased intensive forest management in the CRB may have contributed to longterm reductions in low flows (Gronsdahl, Moore, Rosenfeld, McCleary, & Winkler, 2019;T. D. Perry & Jones, 2017;Segura et al, 2020).…”