Work on a major hydroelectric dam in the Himalayas has been stopped after one of India's most eminent scientists came close to dying on the 38th day of a fast, in protest against the harnessing of a tributary of the sacred river Ganges.

Professor AD Agarwal, 77, former dean of the Indian Institute of Technology Delhi at Kanpur, last week called off his second fast in a year against Himalayan dam projects, after the Indian government agreed to speed up its inquiry into how electricity could be generated without the flow of the water being impeded. The free-running of the river is a crucial element of its sacred status.

"The water ... is not ordinary water to a Hindu. It is a matter of the life and death of Hindu faith," Agarwal said, before his fast began in January.

The 600MW Loharinag-Pala project is one of several hundred major dams and barrages planned or now being constructed by India, Pakistan, Nepal and Bhutan for the foothills of the Himalayas. Together they are expected to provide 150,000MW of electricity for countries in which power cuts are frequent and demand is growing fast. But experts argue the dams will have profound effects on the environment and culture of the region, directly affecting the lives of millions of people.

According to a recent report from the NGO International Rivers, the dams will fundamentally transform the landscape, ecology and economy of the region and will displace hundreds of thousands of people. Shripad Dharmadhikary, one of South Asia's leading water and energy experts who authored the report, said: "Damming and diversion of rivers [in the Himalayas] will severely disrupt downstream flows, impacting agriculture and fisheries and threatening livelihoods of entire populations."

Tomorrow – designated International Day of Action for Rivers – the NGO is organising a global campaign against dams.

Dharmadhikary's report said the dams are being planned and carried out with hardly any environmental assessment of individual or cumulative impacts. "If all the planned capacity expansion materialises, the Himalayan region could have the highest concentration of dams in the world. The dams' reservoirs, tunnels, transmission lines and related works will destroy thousands of houses, rivers, forests, spiritual sites and even parts of the highest highway in the world, the Karakoram highway."

In addition, it warns that climate change could reduce the amount of electricity that the dams are planned to generate. This is because increased melting of glaciers is causing more silt to be washed down the mountains, reducing the capacity of the dams. "The impact of global warming is already being felt much more in the Himalayas than in other parts of the world. This is resulting in the accelerated melting of glaciers and the depletion of the massive water store of the region. There are real fears the snow-covered mountains [will turn] into bare, rocky mountains. As glaciers melt, water in the rivers will rise, and dams will be subjected to much higher flows, raising concerns of dam safety," it says.

In the past few years, Pakistan, India, Bhutan and Nepal have all prepared plans for a massive programme of dam building in the region. Bhutan, one of the most remote but pristine countries in the world, is planning to expand its hydroelectricity capacity by about 10,000 MW - the equivalent of at least five British nuclear power stations - in the next 10 years. Among the projects planned for the near future in Bhutan are the giant 1,095 MW Punatsangchu-I and the 600 MW Mangdechhu projects. Nepal is planning to install hydropower capacity of 22,000 MW in the coming years and while some of the dams would be to meet its own acute needs for reliable electricity, the majority would be for export to India.

Pakistan, with more than 150 million people, expects electricity needs to rise nearly 30% in the next five years. Hydropower is the cheapest form of energy production and the country has plans to add 10,000MW, through five major projects, by 2016, with a further 14 projects, totalling about 21,000MW, under study for construction by 2025. The biggest would be the $12bn, 4,500MW Diamer-Bhasha dam, which would impound nearly 15% of the river Indus and form a lake 100km upstream of the dam's site.

"Degradation of the natural surroundings and a massive influx of migrant workers will have grave implications for the culture and identity of local people, who are often distinct ethnic groups small in numbers", said Dharmadhikary.

The study did not consider dam-building on the Chinese side of the Himalayas. China recently announced plans to build 59 reservoirs, but its stated aim was to store water from its shrinking glaciers.