The Drangme Chhu River is Bhutan’s largest drainage, a spillway for Himalayan snow and ice that rolls into turquoise Class IV and V rapids through sheer granite walls. And it has never been run. But this month a team from Bio Bio Expeditions and its UK based partner, Ultimate Descents, will make a first descent of the Drangme Chhu into Royal Manas National park – an area that has long been closed to tourists.