Alluvial plains are flat, fertile areas formed by the deposition of sediment from rivers and streams. These sediments, which include sand, silt, and clay, are carried by water and deposited when the water slows down, creating rich soil ideal for agriculture. These plains are often found near river deltas and floodplains, where water flows over the land. They support diverse ecosystems and are crucial for farming, as they provide essential nutrients for crops. Notable examples of alluvial plains include the Indus River Plain and the Mississippi River Delta.