The Lhasa River begins where three smaller waterways converge: the Phak Chu, the Phongdolha Chu flowing from Damxung County, and the Reting Tsangpo, which rises beyond the ancient Reting Monastery. Its highest tributary originates at 5,290 meters on the southern slope of the Nyenchen Tanglha Mountains. In its upper reaches, the river cuts southeast through deep valleys before the terrain opens and the current swings southwest, widening to 150 meters. By the time it meets the Yarlung Tsangpo, the Lhasa Valley is three miles across, and the junction is so aligned that the tributary appears to be the continuation of the main river rather than a branch. The basin drains 32,471 square kilometers at an average altitude of 4,500 meters -- a vast, tectonically active landscape where earthquakes are common and 85 percent of the year's rain falls between June and September.

The river valley became Tibet's spiritual corridor. In 1409, the first fortress of the Gelukpa sect, Riwo Ganden Nampar Gyelpe Ling, was built on the left bank about 35 miles upstream from Lhasa. Seven years later, in 1416, the monastery of Chodra Chenpo Penden Drepung rose on the right bank near the Neudzong fortress. Across from the Norbulingka, the ruins of the Sne'u rdzong fortress still mark the left bank. William Montgomery McGovern, who disguised himself to enter Lhasa in 1924, described the Yutok-sampa -- the Turquoise Bridge -- as a roofed structure with walls spanning what had once been the main river channel but had become a stagnant canal. The landscape has always been shifting here, the river reworking its channels, the people rebuilding alongside it. As late as 1984, kowas -- yak-hide boats similar to coracles -- were still ferrying passengers across.

The Lhasa River runs on two calendars. From May onward, snowmelt and glacial melt contribute 20 to 30 percent of its flow, a steady pulse that announces the warming season. Then the summer monsoon arrives, and between July and September, the river transforms. Floods surge through the valley, with September alone carrying 17 percent of the annual runoff. In winter, flow drops to a trickle and sections freeze entirely. Average water temperature hovers around 7.5 degrees Celsius, cold enough to slow the growth of fish like Oxygymnocypris stewartii but clear enough that on the Tibetan plateau, where thin air and sparse clouds combine with transparent water, the full moon's light penetrates deep enough to potentially alter the fishes' circadian rhythms. By 2009, researchers noted that fish in the lower reaches were shrinking slightly in size, a quiet signal of increasing demand and the disruption caused by hydroelectric dams upstream.

Before the Austrian engineer Peter Aufschnaiter designed a dam to protect the Norbulingka, the river's relationship with Lhasa was defined by annual negotiation -- the monsoon surged, the city adapted. That relationship has been fundamentally reengineered. The Zhikong Hydro Power Station, built in Maizhokunggar County beginning in 2003, impounds 225 million cubic meters of water behind a dam at 12,660 feet and generates 100 megawatts. The Pangduo project in Lhunzhub County, called the "Tibetan Three Gorges," impounds over a billion cubic meters at 13,390 feet, generates 160 megawatts, and is designed to irrigate 435 square kilometers. Together these projects represent over 5.9 billion yuan in investment and have transformed the Lhasa River from a seasonal force to be accommodated into an industrial resource to be managed. The river basin's total hydropower potential is estimated at 2.56 million kilowatts. By 1989, 83 small and medium hydroelectric plants already dotted its tributaries. What Harrer's dyke once held back, concrete now controls.