Hans Nicholas Jong, Jakarta – Best known as the home of the world's rarest great ape, the mountainous Batang Toru forest landscape on the island of Sumatra has become a test case for whether Indonesia can enforce environmental law in a region where mining, energy and plantation projects overlap with fragile ecosystems.
In late November 2025, a rare tropical cyclone, Senyar, swept across this part of northern Sumatra, bringing extreme rainfall that triggered flash floods and landslides in the provinces of Aceh, North Sumatra and West Sumatra. The disaster killed at least 1,178 people and displaced around 1 million others, according to government figures, making it one of Indonesia's deadliest natural disasters in recent history.
While the storm provided the immediate trigger, climatologists and environmental researchers say the scale of the destruction can't be attributed to extreme weather alone. They point also to decades of deforestation, land clearing and landscape alteration that have weakened natural buffers across Sumatra's upland watersheds.
"Extreme weather was only the initial trigger," Erma Yulihastin, a climate researcher at Indonesia's National Research and Innovation Agency (BRIN), said at a recent public discussion in Jakarta on disaster risk. "The destructive impact was shaped by weakened environmental buffers upstream."
The government appears to have acknowledged this, with Environment Minister Hanif Faisol Nurofiq announcing on Dec. 23, 2025, an investigation into eight companies operating in the Batang Toru watershed, to assess whether their activities may have contributed to the floods and landslides. The ministry also ordered all eight companies to cease operations while environmental audits are conducted.
The move marked one of the first times Indonesian regulators have announced watershed-wide audits and sanctions spanning multiple sectors, signaling official recognition of Batang Toru as an ecologically critical and highly stressed landscape rather than an isolated disaster site.
A fragile landscape under pressure
Batang Toru is home to some of Sumatra's most intact remaining forests and the only known population of the critically endangered Tapanuli orangutan (Pongo tapanuliensis). Despite its ecological importance, much of the area has been licensed out for extractive and infrastructure projects, including a gold mine, a large hydropower plant, and an industrial pulpwood plantation.
When Cyclone Senyar unleashed torrential rains and mudslides, Batang Toru was among the regions hit the hardest. Landslides wiped out large swaths of forest and orangutan habitat, raising concerns that extreme weather may have pushed the already small Tapanuli orangutan population closer to collapse.
At least 257 people were reported to have lost their lives in and around Batang Toru as of early January, according to local authorities.
As the government probes the eight companies operating in the watershed, multiple investigations suggest that extensive forest clearance has destabilized slopes and altered hydrological systems, worsening the impacts of the extreme rainfall.
Toba Pulp Lestari
One of the companies under investigation is PT Toba Pulp Lestari (TPL), a major pulpwood producer supplying a mill that manufactures cellulose fibers used in fabrics sold to consumers in the United States and Europe.
A new investigation by the nonprofit groups Earthsight and Auriga Nusantara documents large-scale clearing of natural upland forest within TPL's Aek Raja concession in North Tapanuli.
According to the report, 758 hectares (1,873 acres) of natural forest were cleared there between 2021 and late 2025, along with another 125 hectares (309 acres) outside the concession boundaries – a total area nearly three times the size of New York City's Central Park.
The groups say the clearing accelerated in the weeks leading up to the disaster, with satellite imagery showing steep slopes stripped of forest cover shortly before landslides occurred nearby.
Much of the cleared land lies in areas zoned as limited production or protected forest, where clearing is restricted or prohibited under Indonesian forestry regulations. Logging on very steep slopes is also banned due to the high risk of landslides.
TPL has denied responsibility for the floods and landslides, saying its operations are legal and don't involve logging natural forest.
In its response to Earthsight and Auriga, the company cited evidence from its estate in South Tapanuli and from flood sites in the Nabirong River watershed and in the Sipirok, a tributary of the Batang Toru River.
Earthsight and Auriga say they're not claiming that forest clearance in the Aek Raja block caused flooding in the Sipirok or Nabirong watershed, which lie in different parts of the larger basin. Instead, the groups argue that clearing in Aek Raja contributed to landslides and flooding elsewhere across the wider disaster-affected region.
Satellite images from 1 December, in its immediate aftermath, show that landslides occurred next to the most recent logging activity.
TPL has acknowledged that Indonesian authorities designated 11,315 hectares (27,960 acres), about a quarter of the Aek Raja estate, as protected forest. Maps reviewed by Earthsight and Auriga indicate that some of the recently cleared areas fall within that zone.
The company's own environmental assessment published in 2024 also identified parts of the estate as important for erosion control and as habitat for threatened species.
During a December 2025 field survey, the NGOs documented heavy machinery and stacks of logs near newly cleared areas. The logs lacked the legality markings that are mandatory under Indonesia's timber legality system (SVLK), which the groups say suggests illegal harvesting.
TPL has previously argued that some deforestation within the Aek Raja block was carried out by unauthorized third parties. However, Earthsight and Auriga say satellite imagery shows patterns consistent with industrial plantation development, including engineered roads and bridges, rather than small-scale illegal logging.
Hydropower development
Another firm facing scrutiny is PT North Sumatera Hydro Energy (NSHE), which is building a large hydropower plant on the Batang Toru River. The Chinese-backed project has drawn controversy for years due to its location in steep, landslide-prone terrain that overlaps with the only remaining natural corridor linking separate Tapanuli orangutan populations.
Since 2017, construction of the project has cleared more than 535 hectares (1,322 acres) of forest, according to environmental groups, including slopes known to be geologically unstable.
Indonesian NGO Satya Bumi has identified logs left along riverbanks that it suspects were carried downstream during the floods, contributing to debris flows seen in videos circulating on social media.
Forests along rivers and hillsides function as critical hydrological buffers, absorbing rainfall and stabilizing soil. Removing them increases surface runoff and erosion, particularly during intense storms.
Dense forest vegetation acts like a sponge, absorbing rainfall into the soil and slowing the flow of water into rivers, said Hatma Suryatmojo, a forest hydrology and watershed conservation researcher at Gadjah Mada University on the island of Java.
Research on tropical forests on the islands of Borneo and Sumatra shows that forests can intercept 15-35% of rainfall in the canopy, while undisturbed soil can absorb up to 55%, leaving only 10-20% as surface runoff.
When forests are cleared, upstream watershed areas lose their ability to regulate and absorb heavy rainfall, Hatma said. As a result, rainwater rapidly becomes surface runoff, increasing erosion and the risk of landslides and flash floods.
NSHE has rejected any link between its project and the disaster, arguing that hydropower facilities depend on stable watersheds and therefore have an incentive to protect upstream environments.
"[C]onservation is indispensable for the continuity of our operations," NSHE spokesman Arie Dedy said as quoted by local media.
The Ministry of Energy and Mineral Resources has echoed that position.
"Hydropower plants must protect soil fertility and carry out reforestation in the upstream area because they need to maintain water discharge. So it's not accurate to say hydropower causes flooding," Eniya Listiani Dewi, the ministry's director-general of new add renewable energy, said as quoted by local media.
Mining watchdog Jatam disputes the claim, noting that the project is being built in steep mountain forests crossed by the seismically active Sumatra Fault. In such terrain, it says, excavation, tunneling and road construction can increase slope instability rather than reduce disaster risk.
"In such a landscape, calling a hydropower project a 'guardian of soil fertility in the upstream watershed' denies basic disaster science: the combination of deforestation, steep topography, and an active fault is a classic recipe for deadly flash floods and landslides," Jatam said.
The group also points out that multiple landslides during construction have already killed workers at the site.
"This series of incidents shows that even long before entering full operation, the project has already destabilized slopes and hydrology, repeatedly costing lives in the same area," Jatam said.
Critics argue that claims of environmental stewardship ring hollow in a policy landscape that systematically underfunds disaster prevention.
Research by the Jakarta-based Center of Economic and Law Studies (CELIOS) shows that Indonesia allocates only about 0.03% of its state budget to disaster management, a figure that has declined over time.
"When disasters happen, the state is structurally unprepared," said CELIOS researcher Nailul Huda. "Recovery itself can take decades."
The Martabe gold mine
Questions have also been raised about the role of the Martabe gold mine, operated by PT Agincourt Resources, which is also under investigation by the Ministry of Environment.
The mine covers more than 600 hectares (nearly 1,500 acres) of steep, erosion-prone terrain upstream of several villages. Environmental group Satya Bumi says clearing at these elevations can accelerate runoff and increase landslide risk during intense rainfall.
Studies also suggest that extractive industries systematically increase disaster risk in surrounding communities, with one in two mining-dependent villages more likely to experience flooding, compared to one in four elsewhere.
Agincourt has denied contributing to flooding in the Aek Garoga River, saying its operations don't drain into the Nabirong watershed, a part of the Batang Toru ecosystem.
However, several areas hit by flash floods fall within Agincourt's concession boundaries, according to maps reviewed by activists.
They also point out that the company has long carried out riverbank "conservation work" along the Aek Garoga and Aek Ngadol, raising questions about its claim of having no hydrological connection to those rivers.
Satellite analysis by Satya Bumi suggests additional land clearing upslope of the mine may have contributed to surges of mud and debris into the Aek Garoga River system.
The group says some of that clearing appears to have been carried out by palm oil firm PT Sago Nauli inside Agincourt's concession area.
There's no publicly available record of a permit transfer or shared management arrangement between the two companies. If none exists, responsibility would remain with Agincourt as the concession holder, Satya Bumi said.
Agincourt has not ruled out possible impacts in parts of the Batang Toru watershed where sections of its mining footprint lie.
Scientists warn that once forest cover is lost in steep tropical watersheds, recovery can take decades, particularly in regions with fragile soils and intensifying rainfall.
With environmental audits and investigations now underway, the effectiveness of Indonesia's response will depend on whether enforcement measures are guided by scientific findings rather than political or economic pressure.