Investigating the 2021 cross-border flooding in Washington, USA, and British Columbia, Canada and associated influencing factors using multi-type remote sensing

In November 2021, intense atmospheric rivers impacted the Pacific Northwest, including Washington, USA, and British Columbia, Canada, and caused heavy rainfalls, multiple river spillovers, and widespread flooding. One major spillover occurred from the Nooksack River near the City of Everson, Washington. The floodwaters then flowed northeast and crossed the US–Canada border into British Columbia, causing extensive damage and economic losses in both regions. Mapping and visualizing the spatial patterns, temporal evolution, and influencing factors of the event—made possible by remote sensing—can greatly enhance the understanding of both the event and the regional flood regime. This research collected multi-temporal Sentinel-1 radar and PlanetScope optical images, air photos, and high-resolution LiDAR DEM data to map inundation extents and visualize flood progression. Associated influencing factors, including natural (e.g. terrain and weather) and human-related (e.g. dikes and canals), were investigated. The results show that floodwaters persisted substantially longer and inundated a much larger area in British Columbia compared to Washington (e.g. 20 vs. 5 days; 55.78 vs. 17.37 km²), highlighting the critical need for cross-border collaboration and coordinated flood management. The inundation maps bring valuable insights to policymakers and communities to better understand the regional flood regimes and develop more effective flood management strategies.