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Swot helps in assessing the tides in estuaries and rivers

Image of the Month - February 2026

Global river tide classification map with detailed insets with selected coastal areas. Rivers are divided into four different classes depending if a tidal effect has been detected or not ("tide", "likely tide", "likely no tide", and "no tides" - only the first three classes are plotted). Red dots mark major cities. (from [Hart-Davis, 2026], credit DGFI-TUM)
Global river tide classification map with detailed insets with selected coastal areas. Rivers are divided into four different classes depending if a tidal effect has been detected or not ("tide", "likely tide", "likely no tide", and "no tides" - only the first three classes are plotted). Red dots mark major cities. (from [Hart-Davis, 2026], credit DGFI-TUM)
Summed M2 and O1 tidal amplitudes, estimated from Swot elevations within the Gironde Estuary and upstream river networks. Stars with black borders along the trajectory of the rivers represent the summed M2 and O1 amplitude calculated from in situ river gauge observations. The white squares mark the location of dams. (from [Hart-Davis, 2026], credit DGFI-TUM)
Summed M2 and O1 tidal amplitudes, estimated from Swot elevations within the Gironde Estuary and upstream river networks. Stars with black borders along the trajectory of the rivers represent the summed M2 and O1 amplitude calculated from in situ river gauge observations. The white squares mark the location of dams. (from [Hart-Davis, 2026], credit DGFI-TUM)
Tidal probability used to derive the tidal classification in the Gironde Estuary and upstream. Pinks areas are the major cities (from [Hart-Davis, 2026], credit DGFI-TUM)
Tidal probability used to derive the tidal classification in the Gironde Estuary and upstream. Pinks areas are the major cities (from [Hart-Davis, 2026], credit DGFI-TUM)

 
Tides are fundamental in coastal dynamics. However, their propagation up into rivers remains one of the least observed elements of the tidal system. In the meantime, tides up an estuary and a river can have impacts on flood extent and dynamics, ecosystems by salinizing the fresh waters, among others. With Swot, estuaries and rivers' heights are measured regularly in two dimensions, thus enabling to assess how far, and with what amplitude, tides are penetrating into rivers. The most impressive (but rare) effect of such penetration is tidal bores (See Image of the Month, Feb. 2025), but even without them tides do get into the estuaries and up rivers.

A study led by Michael Hart-Davis from the Deutsches Geodätisches Forschungsinstitut at the Technical University of Munich (DGFI-TUM) proposes a global characterization of tidal rivers wider than 30-90 m around Earth, using Swot river-dedicated products. The analysis reveals that tides extend for hundreds of kilometers in many rivers, even reaching about 900 km in the Amazon system - median being 161 km for the largest rivers, 13 km for the smallest. Another conclusion is that tidal extent depends also on slope and tidal amplitude at the river mouth. However, human-made infrastructures such as dams sharply decrease the tidal wave propagation up river.

In addition to coastal studies, Swot thus enables to link tides from the open ocean into rivers. This should have an impact on both oceanography in the estuaries and hydrology.

See also:

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References:

  • Hart-Davis M., Scherer D., Schwatke C., Pavelsky T., Sawyer A., Ray R., Dettmering D, Seitz F.. Observing the pulse of tidal rivers: A first global analysis from the SWOT satellite mission. In review. https://doi.org/10.21203/rs.3.rs-6825606/v1
  • Hart-Davis M. , O. Andersen, R. Ray, M. Lippert, E. Zaron, R. Arildsen, A. Bonaduce, C. Schwatke, D. Dettmering, Tides in complex coastal and polar regions, Swot Science Team meeting, Arcachon, France, Oct. 2025
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