Question

Why most of the rivers form distriduteries before flowing into a sea

Answer 1

Because when the rivers enter into the sea the area and amount of water is large and they distributed into many delts.

Answer 2

the river is in its 3rd/last stage before entering the sea. by this time it has very low velocity and gradients. thus, the river loses most of its erosive powers as in the 1st and 2nd course, and flows sluggishly. the river focuses on depositing alluvium. if there is any obstacle in its way to the sea, the river breaks up into distributaries, instead of overcoming the obstacle. these distributaries spread out in a certain patter resulting in the formation of a delta(triangle-shaped landform).