How the delta formed?

Answer 1

I assume you mean river deltas. They are an accumulation of soil deposits from the river. The river picks up certain amounts of soil as the water travels down its length and once it empties into a lake or sea the soil falls on the same spot. This eventually builds up a small mound of soil.

BETTER, MORE EASIER TO UNDERSTAND ANSWER:

Deltas are formed when rivers pull dirt and silt from their banks into the current. When the river reaches the ocean it deposits the silt. Over time the silt builds up to create little Islands at the mouth of the river called a Delta.