The wide, funnel-shaped mouth of a river, where fresh river water mixes with salt sea water and tides are felt, is called an:

An estuary is the tidal mouth of a river, where the river widens as it meets the sea, and where fresh water from the river mixes with salt water from the sea.

Features of estuaries:

• The water is brackish, a mixture of fresh and salt water.
• Tides move in and out of the estuary, and their effect can be felt far upstream.
• Estuaries usually form where the coast has sunk or the sea level has risen, drowning the lower part of a river valley.
• They also form where strong tides and currents carry away sediment, preventing the formation of a delta.

Estuary and delta:

A delta forms when a river deposits more sediment at its mouth than the sea can remove. It builds new land and has many branching channels.

An estuary forms when the sediment is carried away by tides and currents, so the river mouth stays open and funnel-shaped.

In India, most east-flowing rivers (Mahanadi, Godavari, Krishna, Kaveri) form deltas, while the west-flowing Narmada and Tapi form estuaries.

Importance of estuaries:

• They are among the most productive ecosystems, with rich nutrients that support fish, shellfish and birds.
• Many fish breed in estuaries, so they are important nurseries.
• Mangrove forests often grow in estuaries.
• They provide sheltered sites for ports; many large ports and cities, such as London (Thames), Kolkata (Hooghly) and Bharuch (Narmada), are located on estuaries.

Famous estuaries:

The Thames estuary (England), the St. Lawrence estuary (Canada) and the Chesapeake Bay (USA).

Threats:

Pollution, land reclamation, dredging and changes in river flow due to dams threaten estuaries.

Why not the other options?

• ✗A delta is a landform built up by deposits at a river's mouth.
• ✗A lagoon is a shallow coastal body of water separated from the sea by a sand bar or reef.
• ✗A fjord is a long, deep, narrow sea inlet formed by glacial erosion.