The Lhasa River, also called Kyi Chu (, ), is a northern tributary of the Yarlung Tsangpo River in the south of the Tibet Autonomous Region of China. The Yarlung Tsangpo is the upper section of the Brahmaputra River. The Lhasa River is subject to flooding with the summer monsoon rains, and structures have been built to control the floods. In its lower reaches the river valley is an important agricultural area. The city of Lhasa, named after the river, lies on the river. There are two large hydroelectric power stations on the river, the Zhikong Hydro Power Station (100 MW) and the Pangduo Hydro Power Station (160 MW)
Basin
The Lhasa River drains an area of , and is the largest tributary of the middle section of the Yaluzangbu River. The average altitude of the basin is around . The basin has complex geology and is tectonically active. Earthquakes are common. The river basin is the center of Tibet politically, economically and culturally. As of 1990 the population was 329,700, of whom 208,700 were farmers. 88% of the people were ethnic Tibetans.
The climate is semi-arid monsoon, with a low average temperature of . Average annual precipitation is , with 85% falling in the June–September period. Annual runoff is . Water quality is good, with little discharge of sewage and minimal chemical pesticides and fertilizers. The basin has about of farmland, of which is irrigated. There are of grasslands that may be used for pasturage, and of forest.
Course
The Lhasa River is the longest of the Yarlung Tsangpo tributaries. It flows through the south of the Tibet Autonomous Region of China, and is a left tributary of the Yarlung Tsangpo. It is about long. The river forms where three smaller rivers converge. These are the Phak Chu, the Phongdolha Chu which flows from Damxung County and the Reting Tsangpo, which rises beyond the Reting Monastery. The highest tributary rises at around on the southern slope of the Nyenchen Tanglha Mountains. In its upper reaches the river flows southeast through a deep valley.
Lower down the river valley is flatter and changes its direction to the southwest, The river expands to a width of . Major tributaries in the lower reaches include the Pengbo River and the Duilong River. At its mouth the Lhasa Valley is about wide. It enters the Tsangpo at a point where that river makes a sharp turn to the south, and which therefore seems to be a continuation of the Lhasa River. The river is navigable from its mouth on the Yarlung Tsangpo up to the city of Lhasa and to altitudes of up to .
Intensive melting of snow and glaciers starts in May, contributing 20–30% of the water. The bulk of the water is supplied by the summer monsoon rains, which fall from July to September. There are floods in the summer from July to September, with about 17% of the annual runoff flowing in September. In winter the river has low water, and sometimes freezes. Total flow is about , with average flow about .
Fish population
The Lhasa River has mean monthly water temperatures that range from about . Average water temperatures are . There are abundant species of fish that are adapted to the clear and cool water. The cool temperatures, with the upstream reaches above freezing over at times, has the effect of delaying growth of fish such as Oxygymnocypris stewartii. The Tibetan plateau has thin air and clouds, which combined with the clear water gives strong illumination from a full moon. This may affect the fishes' circadian rhythm. In 2009 it was reported that fish caught in the lower reaches of the river had been decreasing slightly in size, probably due to increasing demand and construction of hydroelectric dams.
Settlements
The river valley is one of the main agricultural areas of Tibet. The city of Lhasa lies on its shore, and there are many Buddhist monasteries in the valley. Riwo Ganden Nampar Gyelpé Ling, the first fortress of the Gelukpa sect, was built on the left bank in 1409, about upstream from Lhasa. In 1416 the monastery of Chödra Chenpo Penden Drepung was built on the right bank near to the Neudzong fortress. The ruins of the Sne'u rdzong fortress are on the left bank of the river, opposite the Norbulingka summer palace of the Dalai Lamas in Lhasa.
The climate in the central river valleys of Tibet is comparatively mild. They are warm in summer, and even in the coldest months of winter the temperature is above freezing on sunny days. A visitor described the valley around Lhasa in 1889 as follows,