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.
Its principal tributaries from the south are the Sungari, which the Chinese consider to be the true head-river of the Amur, and the Usuri; from the north it receives the Oldoi, Zeya, Bureya, Kur, Gorin and Amgun.
From the Khingan ranges to the Pacific, south of the Amur, stretch the rich districts of Manchuria, a province which connects Russia with the Korea by a series of valleys formed by the Sungari and its affluents - a land of hill and plain, forest and swamp, possessing a delightful climate, and vast undeveloped agricultural resources.
In its course through Eastern Manchuria it forms the watershed of the Sungari, Usuri and other rivers, and in the south that of the Ya-lu and many smaller streams. it also forms the eastern boundary of the great plain of Liao-tung.
The three principal rivers of Manchuria are the Sungari,Mutankiang and Usuri already mentioned.