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

Abstract

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 influenced 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 dynamics in a large river system prone to spring flooding and low waterflow events in summer.