Deltas form where a river carrying sediment reaches a larger body of water, such as an ocean, sea, estuary, lake, reservoir, or (more rarely) another river that cannot carry away the supplied sediment.
What are Deltas
Rivers are constantly eroding and transporting sediment, such as sand, mud, silt, and gravel, from their headwaters to their mouths. This sediment is carried by the river's flowing water, and the amount of sediment that can be transported depends on the river's velocity and discharge.
So, When a river reaches a larger body of water, such as an ocean or lake, the velocity of the river's flow suddenly decreases.