["The Geographical Journal, Volume 192, Issue 2, June 2026. ", "\nShort Abstract\nOne of the world's great rivers, the Brahmaputra, avulsed—changed course—significantly sometime between the dates of 1765 and 1830. These are the dates of surveys by James Rennell (grey) and Richard Wilcox (black), both under the direction of the East India Company; no other surveys between these dates can refine the estimate of the timing. However, archival material enables a piecing together of evidence suggesting that the trigger for the avulsion was a major flood associated with a severe monsoon in 1787–1788.\n\nABSTRACT\nThe Brahmaputra River is one of the world's largest rivers, entering the Bengal Basin from its extensive Himalayan catchment, flowing south through Bangladesh, and joining the Ganges to traverse a complex fan‐delta and enter the Bay of Bengal. When Major James Rennell surveyed the river to establish the ‘Route Surveys’ for the East India Company between 1765 and 1777, it followed a different course, turning to the South‐East into the Sylhet Basin, and joining the Meghna. Some time after this, it changed its course—it avulsed—and a re‐survey was undertaken by Captain Richard Wilcox in 1828–1830 which first showed its present‐day southerly route. However, there is uncertainty about when, why, and how this major adjustment occurred, and these questions can only be answered with recourse to archival materials. This paper accordingly pieces together information in the writings of travellers and East India Company officials in the region in the period between 1787 and 1854; evidence of boundary changes in 1806; archival material indicating that the East India Company was concerned enough about declining flow in the ‘Old’ Brahmaputra to consider engineering works by 1845; and contemporary evidence of the widespread impact of severe monsoon floods in 1787; to conclude that the avulsion probably took place rather earlier than often assumed. And intriguingly, it seems that although Rennell left India in 1777, in the first edition of his Memoir published after those floods had occurred (1788), he inserted a brief reference to the Brahmaputra changing its course.\n"]