Around 1880, the Tsangpo River was still a mystery. Was this river, which flows from west to east through Tibet, perhaps the same river that flows into the Bay of Bengal under the name of Brahmaputra, as Nain Singh thought? To find this out, the colonial government of India sent a Sikkimese pundit named Kinthup into Tibet, together with a Chinese lama, as whose servant he would act. They were to throw logs into the Tsangpo, fifty logs a day for ten days. Along the Brahmaputra, surveyors would be on the lookout for these logs. However, the lama proved unreliable. He wasted time on flirting with various women and eating and drinking with his colleagues, and then sold Kinthup to a Tibetan lama. After seven months in slavery, Kinthup managed to escape, and travelled east along the Tsangpo. His master nearly captured him, but he fled into a Buddhist monastery, and the head lama bought him from his previous owner.

Despite all, Kinthup was still dedicated to his task, and after a few months he asked permission to make a pilgrimage, and used his leave to cut and mark the logs. He did not throw them in the water yet - it was eighteen months since he had left India, and he realized that no one would be looking for the logs any more. So Kinthup returned to the monastery, some time later asked for permission to make a pilgrimage again, and went to Lhasa, where he had a fellow Sikkimese bring a letter to the survey authorities to tell about his fate, and announce when he would be throwing the logs into the river. Kinthup returned to the monastery, and on his next leave threw the logs into the river as announced. Only then he returned to India, but there disappointment awaited him. His letter had not reached India, and his report of his travels was not believed.

Kinthup left the survey and became a tailor. Only many years later did geographers realize that his reports and his story were completely correct - and that the Tsangpo and the Brahmaputra were indeed the same river.