That is a 'delta'
A Delta is where a large river enters the sea which has a low tidal range.
Because the sea floods it twice a day.
The Ganges-Brahmaputra Delta in Bangladesh and India is the largest delta in Asia. It is formed by the confluence of the Ganges, Brahmaputra, and Meghna rivers, covering a vast area of tidal plains and wetlands.
A river which arrives at the sea which has a high tidal range will not form a delta. This is because the change in the tidal area will wash the sediment brought down by the river away. It is the build up of sediment brought down by the river into a relatively tideless sea which forms the delta.
Flood plain, alluvial plain, tidal plain, firth, inlet, bayou, estuary, slough, and river mouth are all terms that are synonymous with 'delta'.
A major river which flows into a sea with a small tidal range will form a Delta. I think it is arguable whether the Amazon has a delta, at all.
The river tees is not a delta as the north sea (the sea it drains into) is tidal, meaning that the tides wash away the sediment that cause deltas.
The geographic definitions of a delta includes areas of water action such as an alluvial deposit at the end of a river as it empties into the sea, and a collection of minerals at the mouth of a tidal inlet. Since there are no permanent rivers on Antarctica that empty into the sea -- 98% of the continent is covered with an ice sheet, and the tidal inlets to open lagoons are non-existent, there is no occurrence of a delta on the continent.
The Congo River has an estuary, an expanding tidal mouth.
The River Severn is technically neither a delta nor an estuary. The River Severn is a tidal river that flows into the Bristol Channel. It has characteristics of both a river and an estuary, where the freshwater river meets the saltwater of the sea.
An arcuate delta is a fan-shaped deposit of sediment that forms a curved or crescent shape at the mouth of a river where it enters a body of water. It is typically found in areas with low wave energy and strong tidal currents, resulting in the sediment being distributed in a specific curved shape. Examples of arcuate deltas include the Nile Delta and the Ganges-Brahmaputra Delta.