The nexus of the Bihar flood is the Kosi River's immense alluvial fan, extending some 185 km from the river's exit from the Himalayas and foothills in Nepal, down to its confluence with the Ganges in Bihar. The laws of geology and physics cause rivers to course back and forth across such fans in ephemeral channels lasting decades at most, but for flood control reasons man-made embankments endeavored to contain the Koshi within a channel along its fan's western edge. On 18 August, heavy monsoon rains caused the river to break out of this channel. It started flowing through an old channel much further east, inundating cropland and towns and villages with 1.2 million inhabitants, mostly in Bihar.