Rivers flowing into the ocean can experience changes in water level due to tides, causing a backwater effect that can lead to flooding. A study looked at how this backwater effect affects rivers under normal flow and flood conditions. Using a computer model, researchers found that the backwater effect from river discharge can extend inland until the riverbed is higher than sea level. In lower areas, tides have a bigger impact, while peak river flows mainly raise water levels in upper regions. This simple model can help predict backwater effects in lowland rivers.