Answer:
Alluvial deposit.
Step-by-step explanation:
When the two rivers overflowed, they brought Alluvial deposits that was good for growing because Alluvial deposits comprise of silt, sand, clay, and gravel, as well as organic matter that is necessary for the plant and by using this organic matter, plant produces higher yield. Deltas of Mississippi, Brahmaputra, Nile, Ganges and Huang rivers are the examples of alluvial deposits.