Dear Student.
When river water meeets sea water, delta formation takes place,This is because when a river reaches a lake or the sea the water slows down and loses the power to carry sediment. The sediment is dropped at the mouth of the river. Some rivers drop so much sediment that waves and tides can't carry it all away. It builds up in layers forming a delta. Also When river water meets sea water, the lighter fresh water rises up and over the denser salt water. Sea water noses into the estuary beneath the outflowing river water, pushing its way upstream along the bottom.
Regards
Rashmi Sheokand