The boundary between Tibet and China settled by the Manchu Emperor and the Tibetans in 1727 and lasting down to 1910 ran from the Mekong just north of A-t'un-tzu, crossed northward into the Chin-sha Chiang valley and followed the water divide between the Chin-sha and the upper Mekong sources to the Kokonor Territory.
Source: wiktionary
In view of the situation, however, there was little alternative but to withdraw to southern Kweichow and then to veer towards northeastern Yunnan to find a new crossing point on the Chin-sha, the upper course of the Yangtze.
Source: wiktionary
The Yun-Kwei region, a plateau in which virtually no rivers are navigable and all official and commercial transport moved by land, was defined to include the upper reaches of the Hung-shui (a tributary of the West River), of the Wu (a tributary of the Yangtze), and of the Chin-sha (as the Yangtze is known along its upper course) from approximately the point where each becomes unnavigable even for small junks.
Source: wiktionary