Where the Water Begins: A Field Guide to Six Streams Rising in the Knuckles Mountain Range, Sri Lanka

The Knuckles mountain range in central Sri Lanka is one of the wettest parts of the Mahaweli basin, and the streams that rise on its slopes feed the Victoria, Randenigala, Moragahakanda and Kalu Ganga reservoirs on which irrigation, hydropower and drinking water for a large part of the country depend. Walkers in the range meet these streams every day, yet there is no single account of where each one starts, what it passes and where its water goes. This field guide maps six streams that rise in the Knuckles: the Heen Ganga, Hulu Ganga, Hasalaka Oya, Kalu Ganga, Thelgamu Oya and Dankanda Oya, the upper course of the Amban Ganga. Each catchment was delineated on the 30 m Copernicus digital elevation model, after priority-flood conditioning and D8 flow routing, down to the point where the stream reaches the Mahaweli Ganga, the Amban Ganga or a reservoir. Together they drain 761.9 km² and 87% of the mapped Knuckles Conservation Forest. Main stems, traced as the longest flow paths, run 8.6 to 32.0 km and fall 1187 to 1706 m. For each stream the guide gives the source with coordinates, the villages, waterfalls, viewpoints and road crossings along the way, land cover and biomass, and the onward journey of 162 to 229 km to the sea at Koddiyar Bay. Thematic maps show that 62% of the channel length is expected to flow all year and that most waterfalls lie on the steepest reaches. Each catchment has a large map with its summits, villages, waterfalls and viewpoints numbered, a map of its waterways by Strahler order, and three-dimensional views from four sides. The discussion links the streams to flood safety for walkers and to the canals built and being built to carry Knuckles water to the northern dry zone.