Why does delta often form where a river and an ocean meet?

A delta is a landform where the mouth of a river flows into an ocean, sea, desert, estuary, lake or another river . It builds up sediment outwards into the flat area which the river 's flow encounters (as a deltaic deposit) transported by the water and set down as the currents slow. Deltaic deposits of larger, heavily-laden rivers are characterized by the main channel dividing amongst often substantial land masses into multiple streams known as distributaries. These divide and come together again to form a maze of active and inactive channels. This hydrogeologic formation is known as an alluvial fan. The deposit at the mouth of a river is usually roughly triangular in shape. The triangular shape and the increased width at the base are due to blocking of the river mouth, with resulting continual formation of distributaries at angles to the original course. These distributaries start out flowing fairly fast, but slow as more sediment is deposited and ultimately, the water flows elsewhere. This change in flow affects the particle size in the suspended and bed loads, the size of the particles decrease as the flow slows and the larger particles are deposited. This deposition goes on continually in a cyclic fashion, creating alternating sediment beds of coarse and fine grain deposits. Herodotus the great historian used this term for the Nile river delta because the sediment deposit at its mouth had the shape of upper-case Greek letter Delta : Δ.