Explanation

the widening of a river when it opens into the sea is known as an estuary.

Estuaries are vital ecosystems that support diverse life forms

Examples are San Francisco.

Additional Information:

Meander:

It is a series of curves, bends, and loops that turn in the channel of a river 

Gorge:

It is a narrow valley between hills and mountains and a stream flows through it

Creek:

It is a stream tributary of a river