They form at the end of river channels, towards the river mouth. They occur where the river is losing energy, and slowing down, as a result it can't continue to carry all of its sediment. The sediment gets deposited, and the river splits into smaller, riverlets where the water flows into the ocean. The most famous delta is the River Ganges delta in Bangladesh. There are various different types of delta, and their names usually reflect their shape, for example - bird foot deltas.
