How Deforestation Turned Indonesia’s Monsoon Into Deadly Floods

Heavy monsoon rains and an unusual tropical storm triggered the floods that killed hundreds in Indonesia. But experts say widespread forest loss upstream turned intense rainfall into catastrophic flash floods and landslides, sending torrents of mud and timber into villages and along coastlines.

Why forests matter

Forests act like a sponge: the canopy intercepts rainfall and roots bind soil on slopes. When forest is removed, rainwater runs off faster into rivers and steep, denuded slopes become prone to collapse. "Forests upstream act as a protective barrier, a bit like a sponge," said David Gaveau, founder of The TreeMap. "The canopy captures some of the rain before it reaches the ground. The roots also help stabilise the soil. When the forest is cleared upstream, rainwater runs off rapidly into rivers creating flash floods."

Drivers of forest loss

Indonesia consistently ranks among the countries with the largest annual forest loss. Over recent decades, large tracts of rainforest have been cleared for mining, plantations, hydropower projects and fires. Analysis by The TreeMap's Nusantara Atlas shows that more than 240,000 hectares of primary forest were lost in 2024 alone.

Local campaigners point to specific developments in hard-hit areas. "There are seven companies operating along the upstream region," said Uli Arta Siagian, forest and plantation campaign manager for Walhi. "There is a gold mine that has already cleared around 300 hectares of forest cover... the Batang Toru Hydropower Plant has caused the loss of 350 hectares of forest." Large-scale conversion to palm oil plantations and other commercial uses has also increased vulnerability to flash flooding.

Consequences and visible effects

Deforestation raises the risk of floods in multiple ways: faster runoff, destabilised slopes, and increased sediment washing into rivers. Sediment raises riverbeds and reduces channel capacity, so waterways can no longer absorb sudden, intense downpours. In the aftermath, communities found huge logs and timber washed ashore or trapped in river channels — a stark sign of trees lost upstream.

Government response and calls for action

Indonesian leaders and environmentalists have urged stronger protection and restoration. President Prabowo Subianto said, "We must truly prevent deforestation and forest destruction," calling forest protection crucial. Forestry Minister Raja Juli Antoni described the disaster as an opportunity to reassess policy, warning that "the pendulum between the economy and ecology seems to have swung too far towards the economy and needs to be pulled back to the centre."

Experts stress two priorities: prevent further deforestation and invest in large-scale restoration and watershed protection to reduce future flood risk. "Prevent deforestation, avoid it, and also carry out restoration," said Herry Purnomo of CIFOR-ICRAF.

Bottom line: while extreme weather triggered the floods, upstream forest loss magnified their impact. Stronger forest protection, careful land-use planning and restoration of degraded watersheds are key to reducing the risk of similar disasters.