How is a delta formed

A delta is a landform that forms from deposition of sediment carried by a river as the flow leaves its mouth and enters slower moving or standing water

When a river reaches a lake or the sea the water slows down and loses the power to carry sediment. The sediment is dropped at the mouth of the river. Some rivers drop so much sediment that waves and tides can't carry it all away. It builds up in layersforming a delta.