The wide part of a river where it goes into the sea is called an estuary.

Here's a bit more about estuaries:

* Formation: Estuaries form where rivers meet the sea, creating a transition zone between freshwater and saltwater environments.

* Characteristics: They are typically characterized by:

* Wide, funnel-shaped mouths: This allows for the mixing of freshwater and saltwater.

* Tidal influence: The rise and fall of tides create currents that move water in and out of the estuary.

* Brackish water: The water in estuaries is a mix of fresh and salt water, making it brackish.

* High biological productivity: Estuaries are rich in nutrients and support a wide variety of plant and animal life.

* Examples: Famous estuaries include the Chesapeake Bay, the San Francisco Bay, and the Thames Estuary.

Let me know if you'd like to know more about estuaries!