Its largest tributary is the Kunar River , which starts out as the Mastuj River , flowing from the Chiantar glacier in Chitral , Pakistan and once it flows south into Afghanistan it is met by the Bashgal river flowing from Nurestan . Its largest tributary is the Kunar river , which starts out as the Mastuj River , flowing from the Chiantar glacier in Chitral , Pakistan and after it flows south into Afghanistan it is met by the Bashgal river flowing from Nurestan .