When a wide meander of a river is cut off from the main channel and becomes a separate, curved body of water, it is called an:

An oxbow lake is a curved, U-shaped lake formed when a meander of a river is cut off from the main channel. It gets its name from its shape, which resembles the U-shaped collar (bow) of an ox yoke.

How an oxbow lake forms:

• As a river meanders across a flat floodplain, it erodes the outer banks of its bends and deposits material on the inner banks.
• Over time, the bends become larger and the neck of land between two bends becomes narrower.
• During a flood, the river cuts through the narrow neck and takes a shorter, straighter path.
• Deposits of sediment then seal off the old bend from the new channel.
• The abandoned loop remains as a separate, curved lake: the oxbow lake.

What happens to oxbow lakes:

• Since they no longer receive regular flowing water, they slowly fill up with sediment and vegetation.
• Over many years, they turn into marshes or wetlands and may finally dry up, leaving a curved mark called a meander scar.

Importance of oxbow lakes:

• They are rich wetland habitats for fish, birds and aquatic plants.
• They help store floodwater and recharge groundwater.
• They support fishing and agriculture in surrounding areas.

Oxbow lakes in India:

• They are common on the floodplains of the Ganga, Brahmaputra, Kosi and other rivers of the northern plains.
• In Bihar and Assam, such lakes are locally called "maun" or "beel".
• A large oxbow lake in Bihar is covered in a separate question.

Oxbow lakes in the world:

They are found along the Mississippi river in the USA and many other large meandering rivers.

Why not the other options?

• ✗A lagoon is a coastal body of water separated from the sea by a sand bar or reef.
• ✗A tarn is a small mountain lake formed in a cirque eroded by a glacier.
• ✗A crater lake forms in the crater of a volcano or a meteorite impact.