A dictionary definition of a flood is 'a rising and overflowing of a body of water on to normally dry land. Some have argued that annual inundation of the floodplains of the great Himalayan rivers is not flooding in the strictest sense, since it occurs as a normal phenomenon and, therefore, at the specific time of year, the land is not 'normally' dry. Known as mountain floods, these are classified according to the nature of the inundation, its duration, the suddenness of the onset, or its potential for dislocation of life. The great variation in seasonal flow of the Himalayan rivers is an inevitable natural process regardless of any changes in the pattern of human land use. Intense and prolonged rainfalls do not generally generate floods in the mountains.
Many Himalayan rivers originate from glaciers.
Bhutan, China, Nepal, and Pakistan have suffered a number of GLOF events in the past. Earthquakes also trigger such processes. The resultant flow of water and debris may be several orders of magnitude greater than normal summer high flow. Due to the young geology of the Himalayas and instability of their slopes, the region is prone to recurrent and often devastating landslides. Such landslides and debris flow, released by torrential rain or seismic activity, may cause ephemeral dams across river courses and result in the impoundment of immense volumes of water.