A powerful late-November cyclone that swept across South and Southeast Asia has left northern Sumatra facing one of its worst flood crises in recent memory. Torrents of water buried homes, destroyed roads and pushed thousands of residents from their villages. Rivers that once flowed steadily turned into violent walls of water, carrying debris, mud and uprooted trees.
Cyclone Senyar brought intense rainfall, but researchers say this was not simply a natural weather event. The scale of destruction was made far worse by human actions that have weakened the land’s natural resilience. Decades of forest loss, mining and agricultural expansion have damaged the sensitive soil–forest–water system that once protected communities.
Scientists describe healthy soil as one of nature’s most important flood barriers. Rich in organic matter, full of channels created by roots and protected by leaf litter, forest soil absorbs large amounts of water and slows the force of rain. When forests are cleared, these natural defences disappear. Soil becomes compacted, erosion increases, and rainwater rushes over bare ground rather than sinking safely into it.
In North Sumatra, this collapse is already visible along the Batang Toru watershed, a mountainous region known for rare biodiversity and once protected by deep tropical rainforests. Satellite analysis shows that more than one thousand five hundred hectares of forest cover around Batang Toru have been removed since 2010, largely through mining and oil palm plantations. Exposed slopes can no longer hold heavy rainfall, forcing a surge of water downstream during extreme weather.
A similar pattern unfolded further south in West Sumatra. Heavy rainfall around Padang City increased sharply over a single week, overwhelming soil that had already lost much of its natural structure. More than one hundred fifty hectares of upstream forest area had been cleared, damaging the region’s water cycle. Rivers turned brown and yellow, a clear sign of soil erosion being carried into waterways and eventually out to coastal ecosystems.
Researchers warn that storms in damaged watersheds behave differently from storms in healthy forests. In intact ecosystems, trees, roots and soil reduce runoff and limit landslides. In degraded areas, even the same amount of rainfall can turn deadly. Floodwaters travel faster, riverbanks collapse more easily, and sediment spreads destruction far beyond the storm zone.
Scientists argue that disaster planning must rethink old strategies based only on levees, concrete barriers and emergency response. To reduce future risks, they say, communities must rebuild natural defences. This includes protecting remaining forests, restoring degraded soils, planting vegetation along riverbanks, encouraging agroforestry, and treating land cover and soil health as part of flood-risk planning.
Experts call this approach “ecosystem-based adaptation,” which uses nature’s own systems to defend against climate disasters. They say extreme rain will continue, but the damage does not need to. A healthy watershed can absorb shock, limit runoff and protect communities from the worst impacts.
The floods in Sumatra show how environmental decline and climate change are now shaping disasters. Without restoring forests and rebuilding soil resilience, storms like Cyclone Senyar will not remain single tragic events but will likely become frequent, recurring threats for millions of people living downstream.