Forest-cover-loss control on year-round river flow dynamics in the upper Saint John River (Wolastoq) basin, Northeastern North America from 2001 to 2019

Forest-cover removal has led to substantial changes in stream and river flow dynamics globally. The objective of this study was to characterize the daily-to-seasonal forcing of discharge rates in a large, international river in northeastern North America. During a 19-year period (i.e., 2001-2019), forest-cover in the upper Saint John River (Wolastoq) basin was routinely removed as part of large-scale, commercial forestry operations. Annual mean forest-cover removal ranged from 0.39 to 1.35% of basin area, with an obvious increasing trend of 7.7% per year. The long-term annual mean discharge rate at the basin outlet was 457.5 m3 s-1. An analysis based on extreme gradient boosting revealed that daily discharge rates during the snowmelt season were largely influ-enced by (i) cumulative snow degree-days, as proxy of springtime northeasterly advection of warm-moist air from southerly positions and affiliated increases in snowmelt, (ii) cumulative, on-the-ground snow water equivalent (SWE), and (iii) extent of within-basin, annual forest-cover removal. In contrast, daily discharge rates during the warm period of the year (i.e., June-October) was controlled primarily by warm-season degree-days and annual forest-cover removal. Spring flood maxima increased by 2.1% per year with annual forest-cover loss, whereas mid-summer discharge rates tended to drop. We used wavelet analysis to investigate the role of related environmental variables in controlling river flow dynamics across the time-frequency domain. Annually, the time lag between peak discharge rate and SWE significantly decreased with increasing annual forest-cover removal, suggesting an accelerated melting season. This study has important implications for river flow dy-namics in a large river system prone to spring flooding and low waterflow events in summer.

Automated summary

Forest-cover removal globally has caused significant changes in stream and river flow dynamics. This study focused on characterizing the discharge rates in a large, international river in northeastern North America at daily-to-seasonal scales. Over a 19-year period, forest-cover in the upper Saint John River basin was routinely removed, with an increasing trend of 7.7% per year. The study found that daily discharge rates were influenced by snowmelt, snow water equivalent, and forest-cover removal.