Observing the tidal pulse of rivers from wide-swath satellite altimetry | Nature

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Subjects

- Hydrology

- Physical oceanography

Abstract

The characteristic tides of coastal rivers influence the distribution of estuarine and wetland habitats1, the extent of fresh drinking water2, carbon and nitrogen cycles3,4, and sediment export to the ocean5. Despite the importance of riverine tides, their range is generally unknown over most of the world’s rivers because the propagation of tidal waves in channels is complex, gauging stations are scarce and conventional nadir altimetry6 has historically been too sparse for use in rivers. Here we use data from the recently launched Surface Water and Ocean Topography (SWOT) satellite to quantify tidal dynamics across 3,172 coastal rivers. Capitalizing on the wide swath coverage of SWOT, we show that over 165,000 river kilometres are influenced by tides. More than 700 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 river systems. Natural and artificial obstacles, such as dams, limit tidal propagation in an estimated 16% of all tidal rivers. The tidal dataset opens new possibilities for monitoring and modelling changes in estuarine habitats, fresh drinking water for coastal cities and riverine carbon budgets7 across annual to decadal timescales in response to sea-level rise8, megadrought9, intensifying water extraction and river regulation10.

Main

The importance of tides is embodied in the origins of the word ‘estuary’, which comes from the Latin word ‘aestuarium’, meaning the place of the tide. Along coastlines, where tides are typically magnified11, they profoundly affect navigation, commerce, coastal flooding, water properties and sediment transport12. Tides impact the flooding of rivers and, thus, influence the extent of their floodplain, which has cascading effects on biogeochemical and ecological processes13. Flood cycles, modulated by tides and sea-level rises, are a key factor in the dieback or persistence of saltmarsh systems14. Tidal flooding also accelerates both nitrification and denitrification in soils, leading to variable effects on nutrient levels and coastal water quality15,16. Tides enhance the mixing of saline and freshwater along river channels, which affects water security in coastal cities and food security for agricultural deltas that use river water for irrigation17. Moreover, tides are a critical factor in compound coastal flooding, as demonstrated by the devastating combination of storm surge and high tide that hit New York City during Hurricane Sandy in 201218. Despite their importance, the extent of tides in coastal rivers is poorly known on a global scale because tidal propagation depends on the unique morphologic and hydrologic conditions of each river as well as the magnitude of the tidal signal at the river mouth19. The relative importance of these controls on tidal extent—indeed, the extent itself—has not been unravelled on a global scale, primarily due to limitations of observational coverage.

The combination of numerical modelling and satellite altimetry20 now gives us the ability to predict tidal elevations anywhere in the open ocean with accuracies approaching 1 cm (ref. 21). However, coastal regions and connected inland water bodies remain the Achilles heel for tidal prediction6. The challenge largely stems from two constraints of traditional nadir altimetry: land contamination of radar returns and inadequate density of satellite overpasses. Moreover, nonlinear effects of the shallow water in rivers and estuaries drive complex tidal behaviours22,23, which are best described with physics-based models. Yet adequately resolved models exist only for a handful of locations, owing to the scarcity of bathymetric and water-level data to constrain models and the complexity of the dynamics, which vary from river to river19,24,25.

The highly anticipated Surface Water and Ocean Topography (SWOT) satellite provides two-dimensional swath observations of both ocean and inland water surfaces26. SWOT has the potential to revolutionize the scientific understanding of water-related physical processes27 and to open new avenues of scientific research across the land–ocean–aquatic continuum28,29. Early studies have shown that SWOT can improve the accuracy of tidal estimates in coastal regions, and they have hinted at the potential of using these data to study tidal processes within inland water bodies, including rivers30,31.

The question then becomes, can SWOT be used to pioneer a deeper understanding of tidal dynamics within coastal rivers worldwide? If so, could these insights support the creation of the first-ever global atlas of tidal extent within rivers? By exploiting the observational capabilities of SWOT at the land–ocean interface, we address both these questions and produce a global atlas of tidal rivers, which could radically expand our understanding of the fundamental tidal force t