Major floods can occur in several different ways. For example, in 1840, an earthquake caused a landslide in the Himalayas, damming the upper course of the River Indus, so that a lake soon formed. The lake was 64 km (40 miles) long and over 300 m (984 ft) deep, and when this natural dam broke, flood water swept down the valley, destroying all in its path.
Other floods occur when artificial dams are breached and when storm winds or tsunamis drive seawater inland. Farmers who cause soil erosion help to make floods occur, because the loose soil gets into rivers and raises the level of their beds until they overflow. This factor was involved in the world’s worst flood. This occurred in 1887 when China’s Hwang Ho burst its banks and drowned 900,000 people.