Observing the pulse of tidal rivers: A first global analysis from wide-swath satellite altimetry

Coastal rivers are critical arteries between interior lands and oceans that connect ecosystems, people, and commerce. Their characteristic tides influence the distribution of estuarine and wetland habitats ¹ and the extent of fresh drinking water along densely populated deltas ² . Tidal flooding also regulates carbon and nitrogen fluxes to the atmosphere ³ , carbon storage in wetlands ⁴ , and sediment export to the ocean ⁵ . The rise and fall of tides further influences the navigability of coastal rivers for fishing, tourism, and shipping industries. Despite the importance of riverine tides, their range is generally unknown over most of the world’s rivers because tidal wave propagation in channels is complex, and gauging stations are scarce. Conventional nadir altimetry has been used to characterize tides in the open ocean for decades ^6,7 . However, limits in spatial coverage have prevented its use in rivers. We overcome this limitation by using the recently launched Surface Water and Ocean Topography (SWOT) satellite, which provides wide-swath water level measurements with unprecedented coverage and accuracy. Tidal constituents are estimated based on the SWOT River Single-Pass Vector Data Product (RiverSP) and validated against in situ river and tide gauges. We show that over 175,000 river kilometers are influenced by tides globally, and 725 million people live near and depend on these coastal transition zones. River size, slope, and tidal range at the river mouth influence the extent to which tides propagate within global river systems. Natural and manmade obstacles, such as dams, limit tidal propagation in an estimated 18% of all tidal rivers. This global dataset opens new possibilities to monitor and model changes in estuarine habitats, fresh drinking water for coastal cities, and riverine carbon budgets ⁸ across annual to decadal timescales in response to sea level rise ⁹ , megadroughts ¹⁰ , intensifying water extraction, and river regulation ¹¹ .