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

Observing the Tidal Pulse of Rivers from Wide-Swath Satellite Altimetry

Overview

• The Surface Water and Ocean Topography (SWOT) satellite enables the first comprehensive global observations of tidal dynamics in coastal rivers.
• SWOT's wide-swath coverage allows quantifying tidal influence across 165,000 km of the world's coastal rivers, where over 700 million people live.
• The dataset reveals the extent to which tides propagate inland, their dissipation by natural and artificial obstacles, and their influence on processes like flooding, saltwater intrusion, and carbon budgets.

Problem & Context

• Tides profoundly affect estuarine and wetland habitats, freshwater supply, biogeochemical cycles, and sediment transport in coastal rivers.
• However, the extent of tidal influence has been poorly known globally due to the complexity of tidal propagation and the sparsity of in-situ observations.
• Conventional satellite altimetry has been too sparse to capture the spatial variability of tides in rivers.

Methodology

• Analyzed SWOT River Single-Pass Vector Data from March 2023 to May 2025 to estimate the amplitudes of the dominant M2 and O1 tidal constituents at each observed river node.
• Validated SWOT tidal estimates against 622 in-situ tide and river gauge stations globally.
• Classified river nodes as tidal, likely tidal, likely not tidal, or not tidal based on a set of criteria including tidal amplitude, signal-to-noise ratio, and river topography.
• Analyzed relationships between tidal extent and river characteristics like Strahler order, slope, and mouth width.
• Identified river obstructions (dams, waterfalls) that limit tidal propagation.

Results

• Over 165,000 km of the world's coastal rivers are influenced by tides, affecting 715 million people living nearby.
• Tidal extent scales with river size (Strahler order) and is also influenced by slope and mouth width.
• 16% of observed tidal rivers have artificial or natural obstructions that limit the tidal extent.
• SWOT tidal data can improve the analysis of historical floods and droughts on tidal rivers, and support modeling of saltwater intrusion and coastal carbon budgets.

Interpretation

• The SWOT dataset provides the first global view of tidal dynamics within rivers, enabling new interdisciplinary research at the land-ocean interface.
• Tidal amplitudes from SWOT can be used to refine analyses of extreme flows, saltwater intrusion, and coastal carbon budgets in tidal rivers.
• The data can be integrated into models to identify at-risk coastal communities and support climate adaptation efforts.

Limitations & Uncertainties

• The current SWOT time series is too short to fully resolve complex tide-river interactions and the dependence of tides on river discharge.
• Smaller first- and second-order streams are undersampled due to SWOT's resolution limits.
• Errors increase with distance upstream from the river mouth, and can be influenced by factors like sea ice at high latitudes.

What Comes Next

• Extending the SWOT time series will enable the derivation of more tidal constituents and the study of non-stationary tidal processes.
• Integrating SWOT tidal data with river discharge, water quality, and other observations will advance understanding of coupled hydrological-oceanographic processes.
• Future satellite missions building on SWOT's capabilities can further refine global tidal estimates in rivers.