Flood rings production modulated by river regulation in Eastern Boreal Canada

In eastern boreal Canada, in the absence of long gauge records, changes in tree-ring anatomy of periodically flooded trees have allowed reconstruction of spring floods in natural rivers. This study analyzes the effect of regulation on the flood rings (FR) occurrence and on ring widths in Fraxinus nigra trees growing at 5 sites distributed along the Driftwood River floodplain to determine if a flood reconstruction using FR could be done in regulated rivers. Driftwood River was regulated by a dam in 1917 that was replaced at the same site in 1953. Ring width revealed little to no evidence of the impact of regulation, unlike the FR. Prior to 1917, high frequencies of well-defined FR were recorded during known floods, as indicated by significant correlations with reconstructed discharge of the nearby Harricana River. FR frequencies and intensities after 1917, and mostly after 1953, gradually decreased and were composed of weakly-defined FR, some corresponding to known floods, other years likely reflecting dam management. Strength of the correlations with Harricana River discharge also gradually decrease starting after 1917. At each site, trees in the upper floodplain recorded less frequent FR following 1917 and 1953 than shoreline trees, indicating that water level regulation limited floodplains’ flooding. Compared to downstream of the dam, upstream sites recorded more FR in the post-dam period, highlighting the importance of considering the position of the site in the river continuum and relative to flood exposure during sampling. The results showed that sampling trees in multiple riparian stands and hydrological contexts at a far distance from dams could help disentangle the flooding signal from the dam management signal.