Here's why:

* Estuaries are coastal inlets where freshwater from rivers mixes with saltwater from the ocean.

* The mixing of these waters creates a unique environment with fluctuating salinity levels and strong tidal currents.

* This dynamic interaction between the tide and river current is a defining characteristic of estuaries.

Other terms related to this phenomenon include:

* Tidal river: A river that is affected by tides and experiences a noticeable fluctuation in its flow.

* Tidal bore: A wave that travels up a river, often occurring when a strong tide enters a narrow estuary.

Let me know if you have any other questions!