The Kosi's heavy sediment load makes the river very mobile along its 200km alluvial fan after it breaks through the mountains in Chhatara.
But new embankments built in Bihar in the past 60 years has raised the river bed above the surrounding land. In Nepal, the Kosi barrage dams up not just water, but also silt. the breach of the east embankment on 18 August was exacerbated by sand and boulder mining of the nearby Kosi bed that made it easier for the river to change course and scour the levee. Once it was breached, the Kosi started flowing down a channel it had abandoned in 1867.