The principal natural feature is the Tapi River. Unlike the rest of the Deccan, whose rivers rise in the Western Ghats and flow eastward to the Bay of Bengal, the Tapi flows westward from headwaters in southern Madhya Pradesh to empty into the Arabian Sea. The Tapi receives thirteen principal tributaries in its course through Khandesh. None of these rivers is navigable, and the Tapi flows in a deep bed which historically made it difficult to use for irrigation. Most of Khandesh lies south of the Tapi and is drained by its tributaries: the Girna, Bori and Panjhra. The alluvial plain north of the Tapi contains some of the richest tracts in Khandesh, and the land rises towards the Satpuda hills. In the centre and east, the country is level, save for some low ranges of barren hills. To the north and west, the plain rises into rugged hills, thickly wooded, and inhabited by the tribal Bhil people.