Not only do their high capital costs and long lead times before they start generating revenue make them a challenge to finance, but there is also the risk of significant environmental impacts and the displacement of communities.

But as the world grapples with climate change and the urgent need for cleaner energy sources, some huge new hydroelectric power megaprojects are finding their way back to the drawing board.

2025 has seen a surge of renewed interest in ambitious projects. Below, we explore four of the biggest hydroelectric dam projects either currently in the planning phase or moving closer to a construction start:

1) Medog Hydropower Project (Tibet, China)

The entrance to the Yarlung Tsangpo Grand Canyon (Image: Biao via AdobeStock - stock.adobe.com)

China has approved the construction of what is set to be the world鈥檚 largest hydropower dam, the Medog Hydropower Project, located on the lower reaches of the Yarlung Tsangpo River in Tibet. If completed, it could generate three times as much electricity as the Three Gorges Dam, producing an estimated 300 billion kilowatt-hours annually.

Originally announced in late 2020, the project will require at least four 20km-long tunnels to be drilled through Namcha Barwa Mountain to divert the river鈥檚 flow, at an estimated cost of US$137 billion.

However, concerns about its impact on the region鈥檚 delicate ecosystem and its potential displacement of Tibetan communities have sparked global criticism. While the Chinese government has not disclosed the number of people who may be displaced, comparisons to the Three Gorges Dam鈥攚hich displaced 1.4 million people鈥攊ndicate the scale of the impact.

There are also geopolitical concerns. The Yarlung Tsangpo River, which becomes the Brahmaputra in India and Bangladesh, is a crucial water source for millions. Some analysts, including the Australian think tank Lowy Institute, have warned that the dam could give China the ability to control water flows into neighbouring countries.

2) Siang Hydroelectric Project (India)

The Siang river valley in the mountains of Arunachal Pradesh, India (Image: Cyril Redor via AdobeStock - stock.adobe.com)

As a response to China鈥檚 Medog project, India has proposed its own massive hydroelectric dam on the Siang River鈥擨ndia鈥檚 portion of the Yarlung Tsangpo. The 11,000MW Siang Hydroelectric Project would have a reservoir capable of storing nine billion cubic metres of water. Beyond energy production, the dam aims to act as a buffer against potential sudden water releases from China鈥檚 upstream projects. India鈥檚 hope is that it would reduce flood risks in the states of Arunachal Pradesh and Assam, as well as in Bangladesh.

While the Indian government has authorised a pre-feasibility survey, the project has been met with protests from local communities, who fear displacement and ecological damage.

The cost of the project has been estimated at US$13.2 billion.

3) Rogun Dam (Tajikistan)

Rogun Dam, Tajikistan (Image: Webuild)

Tajikistan鈥檚 Rogun Dam, once fully completed, will become the tallest dam in the world at 335 meters.

The project has been in development for decades but recently received a boost with a $6.3 billion funding commitment from the World Bank to fund its completion. The dam is being constructed by Italian firm Webuild.

Once finished - completion is currently projected for 2033 - the Rogun Dam will have an installed capacity of 3,600 MW. However, revenue generation from electricity sales is not expected to begin until 2039, illustrating the long timelines often associated with large-scale hydropower projects. The dam鈥檚 completion also raises concerns over water resource management in Central Asia, particularly for downstream nations like Uzbekistan.

4) Grand Inga Dam (Democratic Republic of Congo)

The Grand Inga Dam is one of the most ambitious hydroelectric projects in history, decades in the making, but has struggled to get off the ground.

Planned as a series of six dams on the Congo River, the Grand Inga complex could generate 40,000 MW of power鈥攎ore than twice that of the Three Gorges Dam鈥攑otentially transforming Africa鈥檚 energy landscape.

The Democratic Republic of the Congo (DRC) already has two operational dams, Inga 1 and Inga 2, built in the 1970s and 1980s. However, these facilities are poorly maintained and operate at only 80% capacity.

The construction of the 4,800MW Inga 3, was originally slated to begin in 2018, but it has faced repeated delays. But late last year, the World Bank announced it had resumed talks with the Congolese government regarding potential funding, reigniting hopes for the project鈥檚 future, having withdrawn funding in 2016.

The head of DRC鈥檚 public electricity company Snel said in November 2024 that if work on Inga 3 started in 2026, it could start generating electricity by 2032 from just two turbines. These could then help finance further tubines for the dam.

The future of the project remains uncertain, however.