Question

how is delta formed ​

Answer 1

the river brought the sediments with them and deposited at the mouth of river or any place where the velocity is low and this forms the delta at that place

Answer 2

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