The dam will radically alter the nature of downstream water courses, significantly impacting the local people. Investigations into the project in Indonesia have brought to light poor planning and financial mismanagement, whilst overinflated prices for generation indicate possible corruption. It will produce electricity during periods of peak demand, typically between 6pm and midnight. During the day, the river will be blocked and the reservoir above the dam will gradually fill up, to be released later through the tunnel and turbines to generate electricity. Downstream communities, which normally experience drought and flood cycles a few times a year, will now have to learn to cope with them on a daily basis.

The dam’s construction has cost the lives of 18 construction workers and local people, following a series of landslides and tunnel collapses. Campaigners report a pattern of intimidation against those who challenge this project in Indonesia, including the death of WALHI lawyer Golfrid Siregar under suspicious circumstances in 2019. In 2023, a public discussion on the dam was violently disrupted, prompting condemnation by free speech and press rights groups.

Flawed impact assessment The environmental impact assessment (EIA) is deeply problematic. While it recognizes the existence of the many of the problems listed below, including the presence of orangutans and other endangered species, it fails to address the full impacts the dam will have on wildlife, or other problems such as mitigation measures for the impacts on downstream communities.

Risk of extinction The Batang Toru forest is home to the entire remaining population of Tapanuli orangutans, the most endangered great ape species in the world. The new project poses a very real threat of extinction for this entire species. Many more endangered species will be severely impacted, such as Sumatran tigers, sun bears, agile gibbons and pangolins.

Threats to fisheries and river ecology The dam will split the river in two, preventing fish and other species from migrating along the river. This will have serious impacts on fish like jurung, an extremely important and highly valued source of food and income for local communities.

Threats from pollution Despite being billed as a ‘green’ project, like all large hydroelectric projects the Batang Toru dam will emit significant quantities of greenhouse gases, contributing to climate change. These emissions originate from the manufacture of the massive amounts of concrete and other materials needed for construction. They are also emitted by decomposing vegetation in the flooded reservoir above the dam, especially methane, which as a greenhouse gas is 30 times more potent than carbon dioxide. Other dam reservoirs have also led to increased concentration of methylmercury in the aquatic food chain, which is known to cause brain and nervous system damage.

Risk of earthquakes Batang Toru lies in the middle of an earthquake hotspot: the region has the highest density of earthquakes in mainland Sumatra. There is also mounting evidence that reservoirs can themselves trigger earthquakes, due to the increased pressure created by large bodies of water. If an earthquake were to strike and rupture the dam when the reservoir was full the consequences for those living downstream would be catastrophic (Damming Evidence report 2018).