GSCE: How is a delta formed? (5 marks)

A delta is formed in the lower course of a river when the river reaches the sea, or another still body or water such as a large lake. The river slows down and loses energy as there is now no limiting chanel for the river to flow through. The competence of the water is decreased, as is its velocity. A lower competence level means that the river cannot hold as much sedimentary material in its flow. Therefore, it has to deposit some of the rocks and sediments that it has carried downstream. The deposition begins with large, impermeable rocks and finishes with silt and sand further downstream. These depositions create smaller tributaries or channels for the river to flow through. There is a positive feedback loop as deposited sediments create lower energy environments, facilitating lower flow velocities and therefore more deposition.