The earth and sand deposited there are called "sediments" and they build up to form what is called a "delta"; a triangular wedge shaped like the Ancient Greek letter "D" - hence "delta".
The soil deposited at the mouth of a river is called silt.
Delta
A Delta is formed
Delta
The river runs however long, all the while picking up sediment, sand, rocks, etc. and continue doing this for the river's length, and at the very end of the river (mouth), all of this sediment is deposited, leaivng sand at the river mouth.
That is called a river 'delta'.
an alluvial fan
The soil that is deposited by a river to form landforms such as river bars and river deltas is called silt. A river delta is the name of the landform that is created the mouth of a river.
Sand is typically deposited in river systems at locations where the water velocity decreases, such as in river bends, along the inside of meanders, and in floodplains. These areas allow for sediment to settle out of the water column as the flow slows down. Additionally, sand can accumulate at the mouth of a river where it meets a larger body of water, creating deltas.
The name given to soil deposited at the mouth of a river is "alluvium." Alluvium consists of sediments such as silt, sand, and gravel that are carried by the river's current and deposited when the flow slows down as it enters a larger body of water, such as an ocean or a lake. This process of deposition forms features like river deltas and floodplains, which are rich in nutrients and fertile for agriculture.
sand bars
Well heavy earth material like sand slows down at the mouth on the river. It slows down at the mouth because at the source of the river is the fastest but as it is moving down the plain it is slowing down because of friction. Also because the eroded sand that is in the water is making the water heavier which means it will slow it down