Tidal waves in estuaries change in size and shape due to factors like river flow and friction. A study used a simplified model to look at how river flow affects a specific type of tidal wave called M4 overtide. They found that M4 overtide gets smaller in some parts of the estuary but bigger 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 river flow affects M4 overtide by increasing bottom stress and energy transfer between different types of tides. These findings help us understand how rivers and estuaries interact, which can be important for predicting flooding and sediment movement in these areas.