Why Assam floods: The obvious answer is Brahmaputra, but there's so much more to it

The floodwaters spreading across Assam this week have killed almost a dozen people, displaced over 3.63 lakh and inundated 15 districts from Sivasagar to Tinsukia. A woman leads her livestock to a safer place after flooding in Morigaon district of Assam. (PTI) The Army joined the National Disaster Response Force (NDRF), State Disaster Response Force (SDRF) and fire and emergency services in relief operations in upper Assam as the Brahmaputra and its tributaries remained above danger levels. The Assam State Disaster Management Authority (ASDMA) put the official death toll at 10. Sivasagar was the worst-affected district, with nearly 1.58 lakh people displaced. Rail services were disrupted, with floodwaters inundating tracks and forcing trains to be diverted, cancelled or short-terminated. Tea gardens too have been submerged, crops damaged and landslides reported near Guwahati. But this is not an uncommon crisis in Assam. Nearly 40% of the state is flood-prone, according to the Rashtriya Barh Ayog, (the National Flood Commission established by the Government in 1976). Annual floods have struck the state for decades, with particularly severe disasters in 2018, 2020, 2021, 2022 and 2024. The question of why requires looking not just at the rain, but at the river and Assam’s topography. ALSO READ | Flash floods wreak havoc in Assam, Arunachal; over 22,000 people affected, railway bridge collapses

NDRF officials conduct a rescue operation in a flood-affected area in Assam. (HT Print)

A river that floods Assam sits in the floodplain of the Brahmaputra, one of the world’s largest rivers that carries vast amounts of sediment. The river begins on the Tibetan plateau, flows through Arunachal Pradesh and Assam, and is joined by more than 50 tributaries before eventually reaching the Bay of Bengal. Every monsoon, enormous volumes of water arrive from rainfall, Himalayan snowmelt and those tributaries cascading down from the hills. According to the Assam government, Brahmaputra’s average annual discharge is about 20,000 cumec. For reference, one cumec is roughly equal to 1,000 litres of water flowing per second. Geography makes flooding easy here, but experts say rainfall is only one part of the story. The scale and frequency of flooding in Assam is also shaped by what has been done, and not done, over many decades. A river slowed by its own sediment Dhrubajyoti Sahariah, a geography professor and floodplain researcher at Gauhati University, told HT the fundamental problem is the speed and load of water being carried by the river. After Pasighat, the river’s slope flattens all the way to Dhubri. As the water slows, it loses the energy to carry the sediment it has picked up in the mountains. ALSO READ | Amit Shah assures Centre’s support to floods-affected Assam, Arunachal Pradesh “It’s one of the highest sediment-carrying rivers in the world,” Sahariah said. The result is that enormous quantities of sand and silt are deposited across the floodplain each year. Sahariah said the volume of sediment now coming down from the hills is increasing, and this wasn’t just a natural trend. Deforestation steering the problem A key reason for increasing sedimentation, which increases the risks of landslides, is severe deforestation, Sahariah said. Districts such as Lakhimpur, Biswanath and Dhemaji are particularly vulnerable, drained by rivers including the Siang, Jiadhal, Subansiri, Ranganadi and Dikrong, which flow from the hills composed of loose, sandy material that is inherently prone to erosion and landslides. “A slight rain, high rain can trigger excessive landslide, excessive soil erosion. But it has increased due to deforestation,” he told HT. Sahariah believes flood management has focused too heavily in downstream areas — on embankments and structural interventions. “Contrary to working on the downstream, we should work on upstream, especially in the soil conservation measure,” he said. He also pointed to reports of illegally cut logs accumulating in dam reservoirs as evidence of the scale of deforestation in the hills.estation upstream.

People wade through floodwaters and push two-wheelers along the submerged Jorhat-Sivasagar National Highway near Titabor, in Jorhat, Assam. (PTI)

Why Assam gets so much rain Assam’s location makes it one of India’s naturally wettest regions, where the monsoon arrives with considerable force. Assam is among India's wettest states, receiving an average annual rainfall of around 2,800-3,000 mm, with nearly 70% of it falling during the southwest monsoon (June-September). While states such as Goa, Meghalaya, Sikkim and Arunachal Pradesh typically receive higher annual rainfall, Assam consistently ranks among the country's top five to seven rainiest states, as per the India Meteorological Department (IMD). “Monsoon rainfall in Assam is due to the combined influence of moisture incursion from the Bay of Bengal due to south-westerly winds at the lower levels of the atmosphere, favourable topography, active monsoon synoptic systems like cyclonic circulation, and presence of monsoon trough over the northeast region,” Sunit Das, senior scientist at the IMD, told HT. Sahariah also pointed out that in the longer term, climate crisis was affecting the intensity of rain, and consequently, the intensity of flooding too. What climate change is doing, he said, is amplifying the peaks. “The flood peaks... are higher... greater devastation and extreme events are on the rise,” he said.

A man guides cattle to a safer place after floodwaters submerged low-lying areas, in Morigaon, Assam. (PTI)