The amount of water and material in the river increases as more tributaries join in. The bed widens, its slope becomes gentler, and the water flows much more slowly as the river flows out of the mountains. As a slow-moving river cannot keep heavy material moving, piles of gravel form along the banks. At this stage, the river carries its load of sand, mud, and small stones suspended in water. It also begins to swing from side to side, cutting into some banks and drifting away from others. As there are no obstacles such as rapids, the river's course becomes smoother and more regular.

A floodplain is a wide and flat plain. It is built  up by alluvium laid down on the river banks during repeated flooding.

When there is  a flood, the river overflows its banks. When the flood subsides, there is a decrease in the volume of water and the river starts to deposit its load. Most of the alluvium is deposited on the banks close to the river. With repeated flooding, the materials that are deposited on the river banks accumulate to form floodplains.

The path of the middle river is always changing as it cuts sideways into the land and starts to deposit its load. Loops, called meanders, and oxbow lakes are typical features of this part of the river. During a storm, meanders stop river water flowing easily. This causes water to build up in places, and may lead to flooding.

Formation of Meanders

Picture: Corbis