I am not sure of the answer but it is not a delta since a delta need not be marshy and the question asks about marshy land.
The mouth part of a river is called a delta. It is the large silty area at the mouth of a river at which the river spills into different slow flowing channels.
cataract
The triangular-shaped area of land at the mouth of a river is called a delta. Deltas form from the deposition of sediment carried by the river as it slows down and meets a standing body of water, such as an ocean or lake. This accumulation of sediments creates fertile land that is often rich in biodiversity and is important for agriculture. Famous examples include the Nile Delta and the Mississippi Delta.
To fill a marshy area, first assess the wetland's ecological importance and obtain any necessary permits, as many marshes are protected by environmental regulations. Next, remove any vegetation and dig out excess water if needed. Then, use fill material such as soil, gravel, or other suitable materials to raise the area to the desired level, compacting it as you go to ensure stability. Finally, consider planting native vegetation to help stabilize the soil and prevent erosion.
the flood plain
a delta
Usually at the mouth of a river and is when the mouth spreads into a marshy area due to the deposit of sediment (thus, not every river has a delta). Well-known deltas include those of the Mississippi, Amazon, Nile, Ganges and Mekong.
The area of fertile soil at the mouth of a river is called river delta.
The broadened seaward end or extension of a river is known as the delta. It is in this area that the river will deposit excess mud on its way to the sea. Much of the delta is often very marshy and divided into many small waterways.
It is called a delta.
Nile Delta
The mouth part of a river is called a delta. It is the large silty area at the mouth of a river at which the river spills into different slow flowing channels.
Generally, the mouth of a river is found where it empties into another river, a lake, reservoir or ocean. A large river's mouth can cover a massive area.
cataract
cataract
Lake Alexandrina is the lake at the mouth of the Murray River. It has a surface area of about 570 square km.
The soil deposited at the mouth of a river is called silt.