Scientists have created the first global map showing how ocean tides influence rivers far inland, revealing a major but overlooked flood risk affecting millions worldwide.
The research, led by Technical University of Munich, shows that tides do not stop at coastlines. Instead, they travel upstream through rivers, increasing flood risks in inland areas.
The study found that over 725 million people live in regions affected by tidal river influence, with more than 175,000 kilometres of rivers impacted globally. In some cases, tidal effects extend hundreds of kilometres inland, such as in the Amazon River.
This new mapping tool helps identify vulnerable regions and track how risks may shift with climate change and rising sea levels. Researchers say the data will be crucial for flood planning, infrastructure design and freshwater management in the future.