Kosi

The Kosi country around Purnia was shaped by a river with branching channels, islands and changing banks. Francis Buchanan surveyed the district in 1809–1810 and used Nathpur, near the Nepal frontier on the western side of the river, as a temporary headquarters. His account records a landscape whose waterways mattered to trade as well as to the division of territory.

For part of its course, the Kosi separated East India Company territory from that of the Gorkha ruler; even small islands could provoke disputes. Timber floated down the river was purchased by merchants based near its banks at Dimiya and Matiyari. The river thus carried useful materials into the plains while complicating claims to land and boundaries. Buchanan's descriptions preserve a local view of these relationships before later engineering works attempted to confine the river.