The Kabul river itself is little more than a trickle for most of the year, but swells in summer due to melting snows. 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. The Kunar meets the Kabul near Jalalabad. In spite of the Kunar carrying more water than the Kabul, the river continues as the Kabul River after this confluence, mainly for the political and historical significance of the name.
This river is attested in the Rig Veda, the earliest scripture of Hinduism, under the name Kubhā (many of the rivers of Afghanistan are mentioned in the Rig Veda). The Sanskrit word later changed to Kābul.