Alluvial plains are formed by gradual accumulation of silt brought down by rivers. As a river leaves the mountains, the water spreads out and fast speed is arrested. The sluggish water deposits the material brought by it forming the plains. They mostly deposit the eroded materials along the foot of hills forming Piedmont Alluvial plain, like Bhabhar in India. Plains are also formed by the flooded rivers and are known as flood plains.
Examples of fertile alluvial plains are Indo-Gangetic plains, Mississippi plain, etc.