Why are deltas formed?

Answer 1

Basic geologic erosion. It rains in the mountains and the water runs downhill until it reaches the ocean, it carries with it boulders, rocks, gravel, sand, soil, plants, debris and trees in varying amounts dependent on the force of its flow.

Answer 2

Deltas are formed in the estuaries of rivers. This is where the river meets the sea/ocean/lake. The sediment (alluvium) is deposited because the water is no longer able to carry such an amount of load as it no longer has the energy. This results in deposition. The deposition builds up in the river basin, making it shallow, so the river becomes shallow, and often spills. The spillage causes several channels to form. The channels branch out, forming a triangle, like the Greek letter Delta, hence Delta.