When river water is no longer flowing downhill, the water slows down. This makes sediment drop to the bottom. Sediment deposited where a river flows into an ocean or lake builds up and a landform called a delta is formed.
A delta doesn't always form near an ocean, but yes, it often does. It forms from deposition of sediment. Sediment is the process of wearing or erosion of a natural object. The water slows down as it nears the ocean.
