The Kabul River is little more than a trickle for most of the year, but swells in summer due to melting snows in the Hindu Kush Range. Its largest tributary is the Kunar River, which starts out as the Mastuj River, flowing from the Chiantar glacier in Chitral, Pakistan and after flowing 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.
The word Kubhā (कुभा ) which is the ancient name of the river is both a Sanskrit and Avestan word. Many of the rivers of Pakistan and Afghanistan are mentioned in the Rig Veda. The Sanskrit word later changed to Kābul.
The Kubha is the modern Kabul river which flows into the Indus a little above Attock and receives at Prang the joint flow of its tributaries the Swat (Swastu) and Gauri 
In the older parts of the Rigved the Indian people appear to be settled on the north western border of India, in the Punjab and even beyond the Punjab on the borders of the Kubha river the Kowpher in Kabul. The gradual diffusion of these people from this point towards the east, beyond the Saraswati and Hindustan as far as the Ganges, can be traced almost step by step in the later portions of the Vedic writings.
^The History and Culture of the Indian People : The Vedic age. By Ramesh Chandra Majumdar, Achut Dattatrya Pusalker, A. K. Majumdar, Dilip Kumar Ghose, Bharatiya Vidya Bhavan, Vishvanath Govind Dighe Published by Bharatiya Vidya Bhavan, 1962 Page 247
^Original Sanskrit Texts on the Origin and History of the People of India By John Muir page 352