Spatial and temporal structure of the M2 tide in Rivers and Estuaries as observed from SWOT

Understanding how ocean tides propagate within estuaries and rivers is important for quantifying upstream tidally influenced water level variability and flood risk under sea level rise. Conventional observations from tide gauges provide accurate tidal estimates, but are spatially sparse and confined to fixed locations, but have limited ability to resolve lateral and cross-channel structure. In this study, we use high-resolution wide-swath altimetry (100m L2 HR Raster) from the Surface Water and Ocean Topography (SWOT) Satellite mission to directly observe the two-dimensional spatial structure of the M2 tidal variability across estuarine and riverine systems. We characterize how tidal signals evolve spatially as they propagate inland, including regions of amplification, reduced wave speed and spatial gradients coincident with variations in channel geometry. The results reveal coherent spatial structures that are not captured by along-channel or point-based analysis and identify locations where tidal-river interactions may increase sensitivity to background sea level changes and compound flooding. We identify localized regions where the tidal wave exhibits a slowdown in propagation concurrent with an increase in amplitude, indicating spatially confined zones with a convergence of tidal energy. These regions represent sensitive locations where long wavelength disturbances, such as storm surges, may amplify as they propagate inland.