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.
By a line running from the sources of the river Ural to the Tarbagatai range (thus separating the steppes of the Irtysh basin from those of the Aral and Balkash basins), thence along the Chinese frontier as far as the S.E.
This wide area is naturally subdivided into West Siberia (basins of the Ob and the Irtysh) and East Siberia (the remainder of the region).
Aral-Caspian depression to the lowlands of the Tobol, Irtysh and Ob, and thence towards the lower parts of the Yenisei and the Lena.
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.
M.) and the innumerable smaller lakes which surround it being but relatively insignificant remains of the former lacustrine basins; while at the confluence of the Irtysh and the Ob impassable marshes stretch over many thousands of square miles.
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.
Agriculturists, tanners, merchants and mollahs (priests) were called from Turkestan, and small principalities sprang up on the Irtysh and the Ob.
Yermak was drowned in the Irtysh in 1584 and the Cossacks abandoned Siberia.
From this region started an important trade route eastward by the Thyssagetae among the southern Urals, the Iyrcae on the Tobol and Irtysh to the Kirgiz steppe, where dwelt other Scyths, regarded as colonists of those in Europe: then by the Argippaei in the Altai and the Issedones in the Tarym basin, to the one-eyed Arimaspi on the borders of China, who stole their gold from the watchful griffins, and who marched with goat-footed men and Hyperboreans reaching to the sea.
Away from Lake Aral, though they do not cross the Aral-Irtysh waterdivide, so that this sea will not probably have been at that time connected with the Arctic, as some have supposed.
It extends from the river Irtysh and the Dzungarian depression (46°-47° N.) northwards to the Siberian railway and to the Sayan mountains.
The Ek-tagh or Mongolian Altai, which separates the Kobdo basin on the north from the Irtysh basin on the south, is a true border-range, in that it rises in a steep and lofty escarpment from the Dzungarian depression (1550 to 3000 ft.), but descends on the north by a relatively short slope to the plateau (4000-5500 ft.) of north-western Mongolia.