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Exiting Dang Valley and its district, the Babai enters Salyan District and flows between sub-ranges of the Siwalik Hills along their west-northwest axis. Sharada Khola drains about half of Salyan's larger Middle Hills region before cutting through the Mahabharat Range and joining the Babai from the right. Salyan was another Baise principality before unification. About 20 kilometres (12 mi) beyond this confluence, the Babai crosses into Bardiya District and enters Bardiya National Park. The river continues another 30 kilometres (19 mi) west-northwest until the enclosing Siwalik hills fall away and the Outer Terai begins. At this point the river crosses Nepal's main east–west Mahendra Highway and exits the national park.
On the Outer Terai the Babai is finally free to gradually bend left toward the main inclination of the Indo-Gangetic Plain. The river flows south some 40 kilometres (25 mi) and enters India's Uttar Pradesh state. The Babai continues about 50 kilometres (31 mi) (straight line) south from the border before joining the much larger Ghaghara from the left at about 35 kilometres (22 mi) west-northwest of Bahraich. This confluence is about 10 kilometres (6 mi) upstream of the Sharda (Mahakali) confluence from the right.
In Nepal the catchment of the Babai is bordered by that of the Rapti on the north, east and south; and by the main Ghaghra catchment on the west until their confluence. In India the Rapti takes a more easterly course, joining the Ghagra some 285 kilometres (177 mi) southeast of the Babai's confluence.