The three diagrams illustrate the process by which a river delta develops.
Overall, the formation of delta is natural, river delta develops from the transportation of sediment downstream to the formation of a network of channels and islands at the river mouth. This material is subsequently deposited at the river mouth, where it accumulates and causes the river to divide into smaller channels, eventually creating a delta.
In the first stage, velocity slows dramatically when a river meets a large body of water. As a result, sediment consisting of sand, silt and clay is carried downstream by the moving water. In the second stage, because of the decrease in velocity, sediments are deposited. Consequently, it can no longer transport all the sediment it carries, and this material is deposited at the river mouth, forming a layer of sediment in the water.
By the last stage, over time, the sediments build up, forming a delta. The deposited material gradually accumulate, so finally build up the new delta around the river mouth. Eventually, the sediment deposition make a larger, roughly triangular area or landform extending into the sea.