The double river-systems of the Volga and Kama, the Ob and Irtysh, the Angara and Yenisei, the Lena and Vitim on the Arctic slope, and the Amur and Sungari on the Pacific slope, are instances.
Chelyabinsk was linked by a transverse line with the middle Urals railway, which connects Perm, the head of navigation in the Volga basin, with Tyumen, the head of navigation on the Ob and Irtysh, passing through Ekaterinburg and other mining centres of the middle Urals.
This wide area is naturally subdivided into West Siberia (basins of the Ob and the Irtysh) and East Siberia (the remainder of the region).
Examples are the Ob and the Irtysh, the Yenisei and the Angara (itself a double river formed by the Angara and the Lower Tunguzka), the Lena and the Vitim, the Argun and the Shilka, while the Amur in its turn receives a tributary as large as itself - the Sungari.
In 1900, one hundred and thirty private and several crown steamers plied on the Ob-Irtysh river system as far as Semipalatinsk on the Irtysh, Biysk on the Ob, and Achinsk on the Chulym.