The deltas are formed at the mouths of the rivers right before they enter a large water body, like ocean, sea, or lake. The rivers carry lot of material with them, and they do not have any problem carrying it while they are moving faster, but when they are slowing down significantly, they lose the force required to carry the material, so the material starts to deposit on the river banks. As the material deposits, it makes it hard for all of the river's water to pass through its river bed that is becoming partially blocked, so the river starts to branch out, and seen from above it takes a triangular shape if the peripheries are outlined, or as a tree that is branching out.
