A delta is formed at the meeting point of sea water and river water. Why?

River water is the negatively charged colloidal solution of clay and sands, whereas sea water contains a number of electrolytes. At the meeting point of sea water and river water, the electrolytes present in sea water neutralize and coagulate the colloidal solution of clay and sand resulting in their deposition leading to the formation of delta.

Formation of delta. Image taken from Wikipedia under Creative Commons license.