Normally very coarse load is carried by streams flowing over mountain slopes. This load becomes too heavy for the streams to be carried over gentler gradients and gets dumped and spread as a broad low to high cone shaped deposit called alluvial fan.
An alluvial fan is a fan- or cone-shaped deposit of sediment crossed and built up by streams. If a fan is built up by debris flows it is properly called a debris cone or colluvial fan.
As a stream’s gradient decreases, it drops coarse-grained material. This reduces the capacity of the channel and forces it to change direction and gradually build up a slightly mounded or shallow conical fan shape. The deposits are usually poorly sorted.
In Nepal the Koshi River has built a megafan covering some 15,000 km2 below its exit from Himalayan foothills onto the nearly level plains where the river traverses into India before joining the Ganges.
All along the interface between the Indo-Gangetic Plain and the Himalaya in India, Pakistan, Nepal and Bhutan the outermost, lowest Siwalik foothills are built of poorly consolidated sedimentary rocks that have eroded into a wide, continuous alluvial apron called Bhabar in Hindi and Nepali.
Despite overpopulation on the plains, this bhabar zone is highly malarial and has remained largely uninhabited.
Alluvial fans are subject to flooding and are very dangerous as they cause the flooding to carry a huge sediment load. Take the example of Koshi river.
In the case of the Koshi River, the huge sediment load and megafan’s slightly convex transverse surface conspire against engineering efforts to contain peak flows inside manmade embankments. In August 2008 high monsoon flows breached the embankment, diverting most of the river into an unprotected ancient channel and across surrounding lands with high population density. Over a million people were rendered homeless, about a thousand lost their lives and thousands of hectares of crops were destroyed.
The Koshi is known as the Sorrow of Bihar for contributing disproportionately to India’s death tolls in flooding, which exceed those of all countries except Bangladesh.