Here's why:

* River Erosion and Transport: Rivers constantly erode their banks and bed, carrying sediment (sand, silt, clay) downstream.

* Flood Events: During floods, the river overflows its banks, spreading water and sediment across the surrounding land.

* Deposition: As the floodwaters recede, the sediment is deposited on the land, forming a flat, fertile area called a floodplain.

So, floodplains are essentially built up over time by the deposition of sediment during repeated flood events. This makes them some of the most fertile agricultural lands in the world.