Floodplains:
When the river flows through a flat valley floor, it deposits most of the sediments it carries. Therefore, the river bed is raised. When it rains heavily, the river will overflow its banks and flood the surrounding low-lying land which forms the floodplain. These floodplains are also fertile areas used for cultivating crops.

http://en.wikipedia.org/wiki/File:Floodislewight.jpg

-Gracia-