According to Chinese documents they came from the Selenga; but most Orientalists identify them with the Uighurs.
The system is pierced by the Bel-kem or upper Yenisei, and in 1 06°, at its eastern extremity, it terminates above the depression of the Selenga-Orkhon valley.
In the same way the Kentei (or Gentei) Mountains, as they are called, to the north of Urga, and the Yablonoi Mountains of Transbaikalia, separate the higher terrace of north-west Mongolia (drained by the tributaries of the Selenga) from the lower terrace of the Gobi, which is drained by the upper tributaries of the Onon and the Kerulen, both belonging to the basin of the Amur.
Besides the rivers just mentioned, there are others belonging to the basin of the Yenisei (Khua-or Khi-khem, Bei-khem and Bornkemchik); while yet others belong to the Selenga, a river formed by the junction of the Eder with the Telghir.
"the Selenga receives the Orkhon, at the head of which remarkable inscriptions were discovered in the end of the 19th century, and cleverly deciphered.