That would be called silt. Are you studying ancient Mesopotamia or Egypt, by any chance?
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It is called alluvial soil.
Alluvial Soil
Silt
sediment
The fertile soil of the Harappan civilization was primarily fed by the rich silt deposits from the Indus River. The periodic flooding of the river deposited nutrient-rich sediments along its banks, creating ideal conditions for agriculture.
loess
silt
The yearly event was the flooding of the Nile River. As the river flooded, it deposited nutrient-rich silt onto the surrounding land, providing fertile soil for Egyptian farmers to grow their crops.
The fertile soil that the Harappan civilization farmed was fed by rich silt from the Indus River. The annual flooding of the river deposited nutrient-rich silt onto the surrounding floodplains, creating ideal conditions for agriculture. This allowed the Harappans to thrive and develop a sophisticated urban society.
Because of the yearly inundation of the river Nile that deposited silt-rich soil on Egypt's farm lands.
The soil that is deposited is called silt. The landform that is often created by deposited silt at the mouth of a river is called a river delta.
Soil rich in minerals deposited by flooding rivers is called silt.
When the floods went down it left thick rich mud (black silt) which was excellent soil to plant in.Silt deposited by annual floods along the Nile River created the rich and fertile soil that could be cultivated.
The fine soil that was deposited at the mouth of a river in Egypt was called silt.
Due to the nutrient-rich soil and ample supply of water.
Silt deposited by annual floods along the Nile River that created the rich and fertile soil that could be cultivated.