Ocean tides travel across more than 175,000 kilometers of river channels worldwide. While most people expect tides to remain confined to sea coasts, ocean pulses climb far inland through river mouths. Rising sea swells force water upstream against downhill river flows, raising and lowering channel surfaces twice each day.
Gravitational tugs on ocean basins push tidal waves into low-elevation estuaries. The incoming ocean volume acts like water pumped into a narrow pipe, slowing down as channel beds rise. Channel width, bottom slope, and mouth water volume decide how deep the ocean wave travels inland. Built structures alter this flow, as dams and physical barriers halt tidal progression in eighteen percent of all tidal rivers.
Researchers tracked these global movements by analyzing wide-swath radar data collected by the Surface Water and Ocean Topography satellite. The team matched satellite water elevation profiles against local river gauges and coastal tide stations to verify channel heights. Their mapped data confirmed that 725 million people live along these tidally active river corridors.
This global dataset allows scientists to track and model changes in drinking water supplies for coastal cities facing sea level rise and severe droughts. Resource managers can now evaluate how water extraction and river dams alter wetland habitats and carbon movement over annual and decadal timescales.