Comparison of recent major spring flood events in the Saint John River basin

Abstract

The flood-prone Saint John River (SJR) traverses provincial and international borders as it travels from its source in northern Maine to its mouth in southern New Brunswick (NB). In 2008, NB experienced its worst spring flood in 35 years, which was followed by more major spring flooding in 2018 and 2019. As part of the Saint John River Experiment on Cold Season Storms (SAJESS), the objectives of this project are to identify the sequence of events that led to these floods, and to compare these to the 2021 season, in which no major spring flooding occurred. Relying largely upon evaluated reanalysis and hydrometric data, numerous atmospheric, surface, and hydrological variables are examined at various spatial and temporal scales. There are commonalities and differences between flood years as well as between flood years and the non-flood year. When averaged across the upper basin, flood years show consistency in terms of positive winter and spring precipitation anomalies, positive snow water equivalent (SWE) anomalies, and steep increases in April cumulative runoff. However, they show inconsistency in terms of ice jams and positive spring total precipitation anomalies when averaged over the full basin. A comparison of the conditions between flood and non-flood years also reveals commonalities, such as northeastward-moving storms affecting the region and positive winter total precipitation anomalies when averaged over the full basin. There are also differences, such as the early snowmelt and early timing of peak flow and water level in the nonflood year. As well, rain-on-snow events were a prominent feature of the three flood years but not the non-flood year—not because there were no rainstorms, but because there was low SWE when they occurred, due to the early snowmelt. Overall, the concurrence, or lack thereof, of key meteorological-related conditions is a critical issue affecting the likelihood of flooding.