Final answer:
A delta is formed at the mouth of a river where it meets the ocean because sediment is deposited when the river slows down.
Step-by-step explanation:
A delta is formed where a river meets an ocean because sediment is deposited as the river loses speed. When rivers carry high amounts of silt and enter into bodies of water with minimal wave action and currents, the sediments settle and accumulate, creating deltas.
These low-elevation areas are primarily composed of sand and mud. The depositional process leading to delta formation is a result of the river water slowing down and spreading out, leading to the dropping of the sediments it carries.