It is mainly watered by the river Aar, with its affluents, the Kander (left), the Saane or Sarine (left) and the Emme (right); the Aar forms the two lakes of Brienz and Thun.
Those of the eastern slope, on the other hand, such as the Musi, Jambi, Indragiri, Kampar, Siak, Rokan, Panei, Bila and Asahan, are longer, and with many of their affluents are navigable in their middle and lower courses over considerable stretches for craft drawing 6 to 10 ft.
This section is divided by the Middengebergte or middle chain into a northern half watered by the Ombilin or upper Indragiri with its affluents, and a southern half traversed by the Batang Hari or upper Jambi.
South of the volcanoes the northern affluents of the Ombilin - Sumpur, Sello and Sinamar - flow through valleys parallel to one another in a north-west to south-east direction.
Unlike the northern, the southern affluents of the Ombilin do not follow longitudinal valleys hemmed in by the Barisan range and ranges of slate, limestone and sandstone.