A river delta is a landform that is created at the mouth of a river, where the river flows into an ocean, estuary, lake, sea, or reservoir. These deltas are built from the deposition of the sediment that is carried by the river as the flow exit’s the mouth of the river. Over a long period of time, this deposition constructs the distinctive geographic pattern of a river delta.
The creation of a delta is made up of three core forms: the bottomset, topset, and foreset/frontset.
Bottomset beds are formed from the suspended sediment that settles out of the water as it flows into the body of water and loses its energy. The suspended load is transported out the farthest into the body of water than all the other types of sediment creating a turbidite. These bottomset beds are laid down in a level position, making layers that are made up of smaller grains.
The foreset beds are then built over the bottomset beds as the main delta formation advances. The foreset beds are made of the bed load that the river is moving along which is created by the formation of larger sediments that roll along the main channel. When it reaches the perimeter of the form, the bed load rolls over the edge, and it builds up in steep angled layers over the top of the bottomset beds. The angle of the outermost perimeter of the delta is formed by the sediments relaxed angle. As the foresets remain building up outwards, which make up most of the delta, they pile up and miniature landslides happen. This slope is formed in this manner as the bedload continues to be placed and the delta moves outwards.
The topset beds then settle on the foresets, and are flat layers of smaller sediment size that are formed as the main channel of the river shifts elsewhere and the bigger particles of the bed load are no longer being deposited. As the channels move across the crest of the delta, the suspended load settles out in flat beds covering the top. The topset bed is subdivided into two districts: the upper delta plain and the lower delta plain. The upper delta plain isn’t affected by the tide, while the margin with the lower delta plain is distinct due to the upper limit of tidal influence.
Image Caption: NASA image of Nile Delta. Credit: ChongDae/Wikipedia