The shape of the delta is determined by the sediments as well as the strength of the river against the tides, waves and currents in the ocean. When a large amount of silt is carried by distributaries to the delta, fluvial deposition is strong, resulting in a bird-foot delta. The Mississippi delta is an example. Where there are strong currents and tides that don't allow the distributary channels to deposit more silt in the delta, an arcuate delta is formed, such as the Nile River delta. When the silt is spread in both directions forming sandbars parallel to the direction of the tide, a cuspate delta is formed such as the Mekong delta.
As more sediment is carried to the delta, especially during floods, the main river is overwhelmed by the sediment load. This enormous amount of sediment reduces the speed of the river, splitting it into distributaries that slowly deposit the fluvial in the delta. Sometimes sandy deposits might block distributaries, making them inactive, while other distributaries will remain active and branch off across the surface of the delta.
As the river meets the stationary water of a sea or ocean, its speed reduces, causing the heavy articles to drop out while fine articles are carried further into the water. These finer articles are deposited as silt on the delta, creating a succession of beds.The finer materials made of silt and clay form the bottom beds, while gravel and sand deposited close to the river mouth form steep wedges referred to as foreset beds. Coarser gravel and sand are deposited on the delta surface to form the topset beds. This continuous deposition of material transported by the river contributes to the delta outgrowth as it advances farther into the water. These delta outgrowths have sometimes led to the splitting of a lake. For example, the Bassenthwaite and Derwent lakes located in the English Lake District were initially one lake that was separated by the River Derwent delta flats.
Deltas form fertile areas suitable for crop cultivation such as the Mississippi delta in Louisiana. Inland delta outgrowth is enhanced by deposition of more silt load by the distributaries. The soils deposited on the delta are fertile since they are swept downstream from different areas during floods.