A study looked at how river flow affects tides in the Guadiana Estuary. They used a computer model to simulate tides and river water in the estuary. The model matched real observations well. They found that tides in the lower estuary are mostly influenced by the ocean, while tides in the upper estuary are more affected by the river. Even a small river flow of 100 m3/s can change the tides in the whole estuary. This shows that even weak river flows can have a big impact on tides in rock-bound estuaries.