Flood-Rings Production Modulated by River Regulation in Eastern Boreal Canada.

Alexandre F Nolin,Yves Bergeron,Jacques C Tardif,France Conciatori

doi:10.3389/fpls.2021.757280

Abstract

In northeastern boreal Canada, the long-term perspective on spring flooding is hampered by the absence of long gage records. Changes in the tree-ring anatomy of periodically flooded trees have allowed the reconstruction of historical floods in unregulated hydrological systems. In regulated rivers, the study of flood rings could recover past flood history, assuming that the effects of hydrological regulation on their production can be understood. This study analyzes the effect of regulation on the flood-ring occurrence (visual intensity and relative frequency) and on ring widths in Fraxinus nigra trees growing at five sites distributed along the Driftwood River floodplain. Driftwood River was regulated by a dam in 1917 that was replaced at the same location in 1953. Ring width revealed little, to no evidence, of the impact of river regulation, in contrast to the flood rings. Prior to 1917, high relative frequencies of well-defined flood rings were recorded during known flood years, as indicated by significant correlations with reconstructed spring discharge of the nearby Harricana River. After the construction and the replacement of the dam, relative frequencies of flood rings and their intensities gradually decreased. Flood-ring relative frequencies after 1917, and particularly after 1953, were mostly composed of weakly defined (less distinct) flood rings with some corresponding to known flood years and others likely reflecting dam management. The strength of the correlations with the instrumental Harricana River discharge also gradually decrease starting after 1917. Compared with upper floodplain trees, shoreline trees at each site recorded flood rings less frequently following the construction of the first but especially of the second dam, indicating that water level regulation limited flooding in the floodplains. Compared with the downstream site to the dam, the upstream ones recorded significantly more flood rings in the postdam period, reemphasizing the importance of considering the position of the site along with the river continuum and site conditions in relation to flood exposure. The results demonstrated that sampling trees in multiple riparian stands and along with various hydrological contexts at a far distance of the dams could help disentangle the flooding signal from the dam management signal.

Highlights

Over the last decades, extreme floods and droughts have become more recurrent and severe in boreal eastern Canada (Buttle et al, 2016; Bush and Lemmen, 2019; Aygün et al, 2020)
Most gage stations in remote northern rivers were installed in the 1920s following the construction of water regulation systems or hydroelectric facilities, which reduced the availability of natural records (Pellerin, 2019; Nolin et al, 2021b)
In F. nigra, flood rings are characterized by a noticeable increase in earlywood vessel number which is accompanied by a noticeable decrease of earlywood vessels area

Summary

Introduction

Extreme floods and droughts have become more recurrent and severe in boreal eastern Canada (Buttle et al, 2016; Bush and Lemmen, 2019; Aygün et al, 2020) Understanding if these recent trends are part of the natural hydrological variability is critical and studies remain complicated due to the lack of long instrumental hydrological series (Mortsch et al, 2015; Bush and Lemmen, 2019; Pellerin, 2019). Most gage stations in remote northern rivers were installed in the 1920s following the construction of water regulation systems or hydroelectric facilities, which reduced the availability of natural records (Pellerin, 2019; Nolin et al, 2021b) Biological proxies such as tree rings have provided hydrological time series in areas that lack instrumental records and have allowed dendrohydrologists to extend existing instrumental records when available. The impact of dam regulation on riparian forests could alter their ability to provide proxy (natural) hydrological data when trees have coped with altered hydrological regimes

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