The historical avulsion of the Tista River, and its relationship to the Brahmaputra: Map and archive evidence from 1750 to 1835

Alluvial fans are constructed when river deposition raises local elevations until the “avulses” to an alternative lower elevation alignment. Avulsions may be triggered by exogenous events, such as floods that locally overtop river’s banks; they then cause extreme flooding on a fan and have major impacts. Rivers draining Himalayas into Gangetic plain behaved historically in this way; for example, Kosi River’s historical westward migration (1731–1963), its recent eastward shift (1980). In fan-delta of Bengal Basin, Brahmaputra tributary, Tista, both undergone avulsions, with some suggestion these been causally linked. Between Rennell survey (1765–77) partial re-survey Wilcox Ommanney (1828–35), River substantially changed course. Rennell’s time it turned south-east Sylhet Basin leaving higher land Madhupur Tract right; 1830, flowed due south left. One hypothesis timing avulsion relates impact flood Tista 1787. This paper examines evidence maps, East India Company archives correspondence between District Collectors about diaries Francis Hamilton Buchanan who travelled along 1807–11. From contemporary sources is possible re-construct changes late 18th century, conclude had little direct influence Brahmaputra, although went through radical rearrangements own.