What did the annual floods leave behind

The annual floods are very useful because they leave behind lots of sediments, material, debris ...

When there's floodings, the water drags with it tons and tons of all sorts of sediments, material, debris, which after the water is retraced in it's river bed is left on the soil that has been flooded. So the soil gets lot of useful depositions which make her very fertile every year and extremely suitable for growing plants, this is the way in which alluvial soils are formed.