Tidal waves in river estuaries change in size and shape due to factors like river flow and friction. A study used a model to look at how river flow affects a specific type of tidal wave called M4 overtide. They found that M4 overtide gets weaker in some parts of the estuary but stronger in others as river flow increases. The energy of M4 overtide reaches a peak when the river flow is similar to the average tidal flow. The study shows that the interaction between the river and tides causes these changes in M4 overtide, which can help us understand flooding and sediment movement in river estuaries.