Tibet Geography(1)
   
 

The Tibetan plateau lies between the Himalayan range to the south and the Taklamakan plain to the north Tibet, located in central Asia, is an autonomous region of China. Tibet is often called "the roof of the world," comprising table-lands averaging over 4950 meters above the sea with peaks at 6000 to 7500 meters and includes Mount Everest. It is bounded on the north by and east by Xinjiang, Qinghai, and Sichuan, on the west by the Kashmir Region of India and on the south by Nepal, Bangladesh and Bhutan. The entire Tibet sits atop the southern part of a geological structure known as the Tibetan Plateau.

Tibet may be geographically divided into two parts, the "lake region" in the west and north-west, and the "river region", which spreads out on three sides of the former on the east, south, and west. Both regions receive limited amounts of rainfall as they lie in the rain shadow of the Himalayas, however the region names are useful in contrasting their hydrological structures, and also in contrasting their different cultural uses which is nomadic in the lake region and agricultural in the river region.

The lake region extends from the Pangong Tso (Tso = lake) in Ladakh, near the source of the Indus River, to the sources of the Salween, the Mekong and the Yangtse. This region is called the Chang Tang (Byang sang) or 'Northern Plateau' by the people of Tibet. It is some 700 miles broad, and covers an area about equal to that of France. Due to its great distance from the ocean it is extremely arid and possesses no river outlet. The mountain ranges are spread out, rounded, disconnected, separated by flat valleys relatively of little depth. The country is dotted over with large and small lakes, generally salt or alkaline, and intersected by streams, and the soil is boggy and covered with tussocks of grass, thus resembling the Siberian tundra. Salt and fresh-water lakes are intermingled. The lakes are generally without outlet, or have only a small effluent. The deposits consist of soda, potash, borax and common salt. The lake region is noted for a vast number of hot springs, which are widely distributed between the Himalayas and 34¡ã N., but are most numerous to the west of Tengri (north-west of Lhasa). So intense is the cold in Tibet that these springs are sometimes represented by columns of ice, the nearly boiling water having frozen in the act of ejection.

The river region comprises the upper courses of the Brahmaputra, the Salween, the Yangtse, the Mekong, and the Yellow River. Amidst the mountains there are many narrow valleys. Whereas the lake region is an arid and wind-swept desert, the river region is characterized by fertile mountain valleys. The valleys of Lhasa, Shigatse, Gyantse and the Brahmaputra are covered with good soil and groves of trees, are well irrigated, and richly cultivated.

Gongga Shan (also called Minya Konga). Photo by Ben Foster

On the drive west from Chengdu, a city of ten million at the west edge of the Sichuan Basin, the road climbs about 14,000' to a pass just north of Gongga Shan. At nearly 24,800', it is the highest mountain outside the Himalayas, and on a clear day there is a spectacular view of it just south of the highway at Xinduqiao. The regional increase in elevation from the Sichuan Basin to the Tibetan Plateau is one of the steepest on our planet. This dramatic scarp is the result of rapid eastward thrusting of Proterozoic, Paleozoic and Mesozoic rocks that underly eastern Tibet over Mesozoic sediments that underly the Sichuan Basin.
West of Gongga Shan, in the high country around the town of Litang (between the Yalong and the Yangtze), the town of Markham (between the Yangzte and the Mekong) and the town of Zogong (between the Mekong and the Salween), Precambrian schists are overlain by relatively flat lying late Paleozoic carbonates (Carboniferous and Permian) and Mesozoic clastic sediments (Triassic and Jurassic), and the topography is more typical of the Tibetan Plateau. High peaks in this area are glaciated, and evidence of Quaternary glaciation at elevations as low as 12,000 feet is common. High valleys have glacial lakes behind terminal moraines in U shaped valleys but at lower elevations they become steep, deep and narrow canyons. Where these steep canyons flatten out in places such as the town of Bangda (just east of the Yangtze), they are filled with gravels and boulders eroded from glacial moraines.
For the first 50 miles or so below Changdu (Qamdo on some maps, in far northeastern Tibet), the Mekong flows through a broad valley that is filled with glacial gravels so most rapids in this stretch are gravel bar riffles with an average gradient of about 12 feet per mile. Rocks here are Mesozoic clastic sediments that become increasingly folded and faulted as one progresses downstream (falling off the Tibetan Plateau), and in places there are spectacular views of folded quartzite beds.

Photo by Lui Li

After the confluence of the Jing He and Chaya Jiang, the Mekong enters a canyon that is initally cut in late Paleozoic carbonates, but near the end of the road intrusive rocks appear at river level and within another ten miles or so the canyon walls are primarily Precambrian schist. The gradient doubles here, giving it the river enough energy to scour smaller gravels, so rapids are formed where the river pours over larger boulders deposited by landslides and by flash floods down steep side canyons. Major faults and fold axes trend north to northwest, generally parallel to the river. There is a zone of forest from about 11,500' to the tree line at 13,000', but the canyon slopes below 11,500 feet are barren. It rains and snows at higher elevations, but the lower canyons are a desert. We were struck by the strong similarity between the Mekong gorge and the inner gorge of the Grand Canyon , which is also cut through Precambrian schist in a desert environment.