Rising 260.5 metres (855 ft) above the Bhagirathi River at New Tehri, in the Tehri Garhwal district of Uttarakhand, Tehri Dam is the tallest dam in India and the 13th-tallest in the world. It is a rock and earth-fill embankment dam, begun in 1978 and completed in 2006, that holds a reservoir of 3.54 km³ covering 52 km² in the Himalayan foothills. It generates 1,000 MW of hydropower, with a further 1,000 MW pumped-storage plant alongside that Wikipedia describes as nearing completion, and it supplies irrigation water to 270,000 hectares and drinking water to Delhi, Uttar Pradesh and Uttarakhand. Because the Bhagirathi is the headstream Hindus regard as the source of the Ganges, and because the reservoir drowned the old town of Tehri, it is also one of the most contested dams in India.
The best way to understand the scheme is to follow the water: down the Bhagirathi from its glacier, across the lake, over the crest of the dam, into the power station, and on downstream to the second dam at Koteshwar and the confluence where the Ganges begins.
Upstream: Gaumukh, Uttarkashi and the Road to Chinyalisaur
The Bhagirathi begins at Gaumukh, the snout of the Gangotri Glacier, and reaches the temple town of Gangotri, where the Kedar Ganga joins it at about 3,049 metres. From there it drops through a deep gorge to Bhaironghati, where the Jadh Ganga comes in, then crosses the Bhagirathi Granite near Harsil and opens into a wider valley near Jhala. At Uttarkashi, about 1,158 metres above sea level, the Asi Ganga joins. Below Uttarkashi the river runs on past Dharasu and Chinyalisaur towards what was once the old town of Tehri.
Including its tributaries, the Bhagirathi system measures about 456.5 km, shorter than the 664.5 km of the Alaknanda. That is why hydrologists treat the Alaknanda as the true source stream of the Ganges, while Hindu tradition gives that honour to the Bhagirathi, named after the king Bhagiratha who, in the legend, brought the goddess Ganga down from heaven. There are 18 dams along the Bhagirathi either operating, under construction or planned. Tehri is the largest by far.
On Tehri Lake, Above the Drowned Town of Ganesh Prayag
The reservoir fills two valleys. The main arm follows the Bhagirathi; the second follows the Bhilangna, its biggest tributary, which rises at the foot of the Khatling Glacier at about 3,717 metres, roughly 50 km south of Gaumukh, and gathers the Bal Ganga at Ghansali before reaching the lake. The two rivers used to meet at about 755 metres beside the old town of Tehri, a place formerly known as Ganesh Prayag. That confluence, and the whole town, now lies under the water.
At 52 km², Tehri Lake is the largest lake in Uttarakhand, and it has become a base for boating, kayaking, jet skis, water-skiing and parasailing. The people who lived on its floor mostly did not stay to see that. More than 100,000 people were relocated from the area, and resettlement claims turned into long legal battles that helped delay the project. Opposition began early. Virendra Dutt Saklani, a lawyer, founded the Anti-Tehri Dam Struggle Committee, and from the 1980s until 2004 the movement was led by Sunderlal Bahuguna (1927–2021), who was born at Maroda near Tehri and was already known for the Chipko forest movement of the 1970s.
New Tehri on the Ridge and the Chamba Junction
The people of the old town were moved up the hillside to a planned replacement, New Tehri, which is now the administrative headquarters of Tehri Garhwal district. It had 24,014 residents at the 2011 census and looks down over the reservoir that covers its predecessor. The Chandrabadani Devi temple is nearby. Most visitors arrive by road on National Highway 34: New Tehri is about 99 km from Dehradun, which has the nearest airport, and about 75 km from Rishikesh, which has the nearest railway station.
Many travellers pass through Chamba, a small hill town at about 1,600 metres where the roads from Mussoorie and from Rishikesh meet before running down to the reservoir and New Tehri. It had 7,771 people in 2011 and lies within reach of Dhanaulti, Kanatal and the Surkanda Devi temple, which makes the lake a stop on a wider circuit of the Garhwal hills rather than a destination on its own.
Across the Crest: 575 Metres of Rock and Earth Fill
Driving onto the dam, you are on a road along the top of an artificial hill rather than a concrete wall. The crest is 575 metres long and 20 metres wide, but the base spreads 1,128 metres from upstream toe to downstream toe. Instead of concrete, an embankment dam like this uses an impervious core flanked by shells of rock and earth fill, and its sloping faces carry the load of the water down into the valley floor.
What lies beneath the crest is the reason for much of the controversy. Tehri sits in the Central Himalayan Seismic Gap, a major geological fault zone. In October 1991 a magnitude 6.8 earthquake struck with its epicentre 53 km from the dam. The dam’s supporters say the complex is designed to withstand a magnitude 8.4 earthquake; some seismologists argue that earthquakes of magnitude 8.5 or more are possible here. Towns downstream with a combined population of close to half a million people would lie in the path of any dam break.
The project also took a long time. A preliminary investigation was completed in 1961, and in 1972 the design settled on a 600 MW power plant. Work started in 1978, and the Soviet Union provided technical and financial help from 1986 until political upheaval there cut it short. In 1988 the Tehri Hydro Development Corporation (THDC) was formed to manage the project, with 75 per cent of the funding from the central government and 25 per cent from Uttar Pradesh, which paid for the irrigation component. The dam was finally completed in 2006, at a total cost of about US$1 billion.
Tehri Power Station, Pumped Storage and the Northern Grid
Below the crest, water drops through the Tehri power station, whose 1,000 MW of conventional hydro capacity generates about 3,000 GWh a year. Beside it, the Tehri pumped-storage plant, described as nearing completion, adds another 1,000 MW in four 250 MW variable-speed units. When electricity is cheap and plentiful, they pump water from a lower reservoir back up into Tehri Lake; at peak hours they run it down again as turbines. The variable-speed design lets the plant stay efficient as the levels of both reservoirs rise and fall.
The plant is dispatched by the Northern Regional Load Dispatch Centre, and its power goes to Uttar Pradesh, Uttarakhand, Punjab, Delhi, Haryana, Jammu and Kashmir, Chandigarh, Rajasthan and Himachal Pradesh. Electricity is only part of the job. The water that passes through the turbines is also counted on for farms and cities further down: the complex is meant to irrigate 270,000 hectares and to stabilise irrigation on a further 600,000 hectares across the plains, and to deliver about 1.2 million m³ (270 million imperial gallons) of drinking water a day to the industrial areas of Delhi, Uttar Pradesh and Uttarakhand. Ownership changed in November 2019, when the Government of India approved the takeover of THDC by the state-owned NTPC Limited.
Downstream to Koteshwar and the Confluence at Devprayag
About 22 km below Tehri the river meets its second wall. Koteshwar Dam is a concrete gravity dam 97.5 metres tall and 300 metres long. It regulates the water leaving Tehri for irrigation and forms the lower reservoir of the pumped-storage plant, and it has its own 400 MW run-of-the-river power station of four 100 MW units, commissioned between March 2011 and March 2012. Its four radial spillway gates can release 13,240 m³ of water per second at flood level. Floods in September 2010 swamped the construction site before it was finished.
The Koteshwar reservoir is small beside Tehri Lake: it holds 88.9 million m³, of which 35 million m³ is usable, over a surface of 29 km² at a full level of 612.5 metres above sea level. That usable slice matters, because the power station can draw the lake down by as much as 30 metres, and the pumped-storage plant upstream depends on having water here to lift back up the hill.
Below the two dams, the river that pilgrims know has changed. Since the reservoir began filling in 2005, the Bhagirathi’s flow has at times fallen from a normal 1,000 cubic feet per second to about 200, and at some points in the year the river stops flowing. Many Hindus see this as damage to the sanctity of the Ganges; officials say normal flows return when the reservoir is full. The river then runs on to Devprayag, where it joins the Alaknanda and takes the name Ganga.
From there the water continues to the plains and on to the Bay of Bengal, a journey traced in our Ganges River guide. To see how Tehri’s 260.5 metres compares with other giants, look at our list of the tallest dams in the world, or visit the 226-metre Bhakra Dam on the Sutlej, a concrete dam in the Himalayan foothills of Himachal Pradesh.