Explanation:

- A gorge is a narrow valley between hills or mountains, typically with steep rocky walls and a stream running through it. It’s not related to river mouths or water mixing.

- An archipelago is a chain or cluster of islands. This term does not refer to the mixing of fresh and saline water.

- A fold is a geological term referring to the layers of the Earth's crust that have been bent or folded. It doesn't relate to river mouths or estuarine environments.

- An estuary is a tidal mouth of a river where fresh water from the river mixes with saline water from the sea. It is a critical ecosystem and nursery ground for many marine species.

Correct Answer: Estuary