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EAST ASIA.
basin limited by the Trans-Himalayan range ; for here the average
rainfall is at
least 13 feet, and the natural discharge may be taken at from 400 to 500 gallons
per square mile. A basin from 8,000 to 12,000 square miles in extent would suffice
to supply such a quantity, and the unexplored tract separating
the Tsangbo and
Lower Dihong valleys is large enough to contain a basin of this size by including
in it that of the Lopra-ko-chu, which flows to the west between the Himalaya and
Trans-Himalaya, and whose lower course is still unexplored.
On the other hand, the comparative
fluvial discharge, as approximately indicated
for the Tsangbo and accurately for the Irawady, would seem to justify
the Chinese
map reproduced by D'Anville, which represents the Burman river as the continua-
tion of the Tsangbo. At Bhamo the Irawady discharges during the floods over
1,000,000 cubic feet, and its average volume at this place is about two- thirds of the
river in the delta ; that is, scarcely less than 315,000 square feet. No doubt, during
the dry season from November to June, the discharge of the Lower Irawady may
fall to 70,000 and even 47,000 cubic feet per second ; but during that period the
river receives no rain-water, and diminishes by evaporation as it approaches the
sea. The excessive discharge at Bhamo, where the annual rainfall is far less than
in the Brahmaputra basin, can be explained only by assuming a large area of
drainage. Yet on most maps the Irawady basin is strictly limited by an amphitheatre
of hills on the northern frontier of Burmah. Wilcox and Burlton may
doubtless have seen a torrent near its source in Burmese territory ; but it does not
follow that this was the true Irawady, although so named by them. These
explorers themselves heard reports of a great eastern stream belonging to the same
basin, but they made no attempt to reach it.
In any case, before coming to a definite conclusion, it will be wise at least first
to see whether the blocks of wood or stems of trees numbered by order of the
Indian Topographic Department and thrown into the Tsangbo in Tibet will reach
the plains of Bengal or of Burmah. Meanwhile it is to be hoped that the route
from Assam to Tibet may soon be reopened, and that explorers may have free
access from the plains to the uplands through the intervening forests, swamps, and
hills.
HEAD-WATERS OF THE GREAT INDO-CHINESE RIVERS.
North of the Tsangbo depression the Tibetan tableland has been cut into innu-
merable side valleys by the running waters. The southern trade winds from the
Bay of Bengal easily reach the Khachi plateaux through wide openings
in the
Himalayas. Hence the eastern slopes of these uplands receive a copious rainfall
from the Indian Ocean. Whilst the arid soil, the rarefied atmosphere, sultry heats
in summer, and intense winter cold render the plateaux almost inaccessible, the
ravine lands are equally difficult to traverse, owing to their rugged character, their
steep escarpments, fierce torrents, dense forests, and the wild tribes inhabiting the
clearings. Most of this region depends officially on Tibet, and administrative
centres are here established as in the other provinces.
Nevertheless several groups
of tribes are practically independent, No organized army has hitherto been able