Water surface elevation across the river-ocean interface from SWOT satellite

River deltas are home to more than 400 million people worldwide and are critical centers for industry and ecosystems. Many low-lying Asian Mega-deltas such as the Mekong Delta and the Ganges-Brahmaputra-Meghna (also known as the Bengal Delta) are increasingly exposed to compound coastal–river flooding, driven by tropical cyclones, storm surge, extreme river discharge, land subsistence, higher tides and sea-level rise. Reliable integrated coastal–river hydraulic models are needed to predict these events and support contingency planning, but model accuracy is often limited by sparse and discontinuous observations across the river–estuary–coastal transition zone. Developing reliable data sets for these environments is challenging because (i) coastal and estuarine water levels vary strongly in space and time, (ii) delta morphology is complex (floodplains, siltation, and human-made structures), and (iii) continuous observations capturing the interaction between river and ocean processes are generally lacking.

While in-situ gauges provide time-series data at a few monitoring locations, they cannot resolve the spatial structure of water levels across the delta. Previous satellite altimetry missions increased the spatial coverage of water surface elevation (WSE) observations but were typically limited to point-based observations (e.g., Sentinel-3) or cross-section tracks (e.g., ICESat-2). SWOT (Surface Water and Ocean Topography) is the first mission to provide two-dimensional WSE observations over both inland waters and the ocean. In this study, we explored the SWOT L2 HR Raster product at 100 m resolution over the Mekong (Vietnam) and Meghna (Bangladesh) rivers to generate continuous river-to-ocean WSE datasets. The raster product is sampled along river centerlines from upstream reaches, through the estuary, and tens of kilometers into the ocean, to generate 1D river-ocean WSE profiles. Assuming along-channel hydraulic continuity in the WSE, we remove outliers and fill gaps in the data to obtain consistent profiles.

These observations reveal key hydrodynamic features that are difficult to resolve with conventional monitoring, including the along-channel spatial variability of tidal propagation and damping, including zones of strong tidal influence, and changes in the tidal signal during high-discharge periods, when river flow alters tidal penetration and water-level gradients. By providing coincidence two-dimensional snapshots of river and coastal water levels, SWOT enables a consistent characterization of how ocean forcing and river discharge interact across the delta. The resulting coastal–river datasets are a foundation for validating integrated coastal–river hydraulic models and improving simulations of WSE along the river–ocean continuum, strengthening compound coastal flood modeling and climate-impact assessments in deltaic environments.