In relatively recent geologic times, less than forty million years ago, the Indian subcontinent crashed into the Eurasian tectonic plate. As the South Asian plate began to subduct under the Eurasian plate, it pushed up the Himalayas, the Plateau of Tibet, and folded the ranges of mountains to the east of the Plateau of Tibet. The Indian plate is still converging on the Eurasian plate at a little over three-quarters of an inch per year, deforming the boundary and raising the Himalaya Range. This rate is faster than fingernails grow.
The Plateau of Tibet and its surrounding mountains have rarely been treated as an ecological zone because studies, especially studies of rivers, are usually country-specific.1 As an ecological zone, the enormous barrier of the Himalayas and their eastern extensions ecologically divide the region into two parts. The southern slopes receive rain on the lower slopes and snow on the tops of the mountains. This rain and snow generate rivers that flow east, south, and west off the mountains. These same Himalayas, however, form an enormous barrier to moisture-laden clouds. North of the Himalayas is a rain-shadow that extends from Tibet through the Takla Makan and Gobi Deserts. Over millennia, the rain shadow has become increasingly drier.
Twelve major rivers originate on this uplifted plateau. **They provide fresh water for somewhat less than half of the current human population.** Starting in China and moving in a clockwise direction around Asia, these rivers include the Yellow, Yangtze, Red, Mekong, Salween, Irrawaddy, Brahmaputra, Ganges, Helmand, Amu Darya, and Sir Darya. Ten are more than 1,000 miles long.
Syr Darya–1,913 miles
Amu Darya–1,630 miles
Several include the largest drainage basins on earth.
Ganges and Brahmaputra–668,000 square miles
Yangtze–454,000 square miles
Yellow–378,000 square miles
Indus–371,000 square miles
Mekong–313,000 square miles2