A 1000-year history of large floods in the Upper Ganga catchment, central Himalaya, India

Determining the frequency, magnitude and causes of large floods over long periods in the flood-prone Himalaya is important for estimating the likelihood of future floods. A thousand year record (with some information from 2600 years ago) of the frequency and some estimates of velocities and discharges of large floods has been reconstructed in the Upper Ganga catchment, India, using written reports, litho-stratigraphy and sedimentology, and dated by optical and radiocarbon methods. In the Upper Ganga catchment rainfall triggers large landslides that dam rivers and release large amounts of water when they burst, thereby amplifying the effects of rainfall. The large floods in the catchment may be the result of landslide dam bursts rather than glacial lake bursts, and these are likely to continue and possibly worsen as the monsoon intensifies over the next century. However preliminary information suggests that the recent devastating flood of June 2013 was the result of heavy rainfall not landslide dam bursts. The frequency record is non-random and shows a high frequency between AD 1000 and AD 1300 (omitting uncertainties), then a low frequency until a cluster of floods occurred about 200 years ago, then increased frequency. This temporal pattern is like but not identical with that in Peninsular India, and both appear to be the result of variations in the monsoon.