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 river flow is similar to tidal flow. River flow makes the bottom of the estuary more effective at slowing down tides, leading to changes in M4 overtide. This study helps us understand how rivers and tides interact in estuaries, which can be important for predicting flooding and sediment movement in these areas.