River Floods of Nepal — Koshi 2008, 1993, 2017 Terai & Melamchi 2021

Nepal receives roughly 80 percent of its annual rainfall in the June–September summer monsoon, and its 6,000-plus rivers and streams must carry that water — and enormous loads of Himalayan sediment — down some of the steepest river gradients on Earth. Hydrologists group the country's rivers into three classes: the great snow-fed systems (Koshi, Gandaki, Karnali), the medium rivers rising in the Mahabharat range (Bagmati, Kamala, West Rapti, Babai, Kankai), and the small, flashy rivers born in the fragile Chure (Siwalik) hills. The big rivers deliver huge sustained discharges, the Mahabharat rivers respond violently to cloudbursts, and the Chure streams can rise from a trickle to a torrent within an hour.

When these rivers leave the hills and hit the flat Terai plain, they slow abruptly, drop their sediment, raise their own beds and swing sideways across their floodplains. This behaviour earned two rivers of the Nepal–India plains their grim nicknames: the Koshi (Saptakoshi) is called the 'Sorrow of Bihar' for the devastation its shifting course has caused downstream in India, and the West Rapti has historically been called the 'Sorrow of Gorakhpur' for its floods in Uttar Pradesh. Within Nepal the same rivers, together with the Bagmati, Kamala, Babai and Kankai, are responsible for most recorded flood deaths and losses.

Floods are consistently among Nepal's deadliest hazards: records kept by the Department of Hydrology and Meteorology (DHM) and the National Disaster Risk Reduction and Management Authority (NDRRMA), through its BIPAD portal, show flood and landslide events killing dozens to hundreds of people almost every monsoon. The events profiled below are the benchmarks against which every new monsoon is measured.

On 18 August 2008 (Bhadra 2065 BS), the eastern afflux embankment of the Koshi River gave way at Paschim Kusaha in Sunsari district, about 12 kilometres upstream of the Koshi Barrage. The breach was not caused by an extraordinary flood: the river was carrying roughly 144,000 cusecs (about 4,100 m³/s) at the time — well under a sixth of the 950,000-cusec design flood the embankments were built to contain. Decades of sediment deposition had raised the riverbed between the embankments, and poor maintenance of the spurs protecting the eastern bank allowed the river to eat through. Once the embankment failed, the Koshi avulsed eastward into an old channel it had abandoned more than a century earlier.

Inside Nepal, the government counted about 53,800 people (11,572 households) affected in Sunsari district, with tens of thousands displaced from Shreepur, Haripur and Paschim Kusaha and the East–West (Mahendra) Highway severed for months. Downstream in Bihar, India, the disaster was catastrophic: more than three million people were affected across many districts, and official death tolls ran into the hundreds, with some estimates exceeding 500. The breach was closed only in January 2009, when engineers succeeded in turning the river back into its embanked channel.

The 2008 event was at least the eighth recorded breach of the Koshi embankment system since it was completed in the early 1960s under the 1954 Koshi Agreement between Nepal and India. The river's highest recorded flow — about 913,000 cusecs (roughly 25,800 m³/s) in 1968 — remains the benchmark flood; in late September 2024 the barrage passed its largest discharge since 1968, around 661,000 cusecs, without a breach on the Nepal side.

The disaster most often cited as the biggest flood in Nepal by death toll struck central Nepal on 19–21 July 1993 (Shrawan 2050 BS). A stalled monsoon system unleashed an extreme cloudburst over the Kulekhani, Bagmati and East Rapti catchments south-west of Kathmandu: the Tistung station recorded about 540 millimetres of rain in 24 hours, with intensities up to 65–70 millimetres per hour — among the highest ever measured in Nepal. Rivers draining the Mahabharat range rose simultaneously, and hillsides saturated by the deluge collapsed in hundreds of debris flows and landslides.

Research reviews of the event put the death toll at about 1,336 people; the government's situation report of 26 July 1993 already listed 886 dead, 182 injured and 859 missing while search operations were still under way. In the Kulekhani watershed alone, more than 300 landslides were mapped, and a single debris flow buried the village of Phedigaun, killing 62 people. Total property losses were estimated at around NPR 4.9 billion at 1993 prices.

The flood crippled national infrastructure. The Bagmati barrage at Karmaiya in Sarlahi and its irrigation system were severely damaged, the intake of the Kulekhani hydropower system — then the country's only seasonal-storage plant — was destroyed and its reservoir choked with sediment, and washed-out bridges on the Tribhuvan and Prithvi highway corridors cut Kathmandu's road links for weeks. The catastrophe pushed water-induced disaster management up the national agenda and strengthened the institutions that later became the Department of Water Induced Disaster Prevention.

From 11 to 14 August 2017 (late Shrawan 2074 BS), incessant rainfall across the length of the Terai and the Chure hills produced the most extensive flooding Nepal had seen in decades. The National Planning Commission's Post Flood Recovery Needs Assessment (PFRNA) records that 35 of the country's 77 districts were flooded, 18 of them severely, with about 80 percent of the land inundated in substantial parts of the Terai. Rivers from the Kankai and Koshi tributaries in the east through the Kamala, Bagmati and West Rapti to the Babai and Mohana in the west all exceeded danger levels within the same three-day window.

The PFRNA counted 134 deaths in the 18 most affected districts, with Rautahat, Morang, Jhapa and Sarlahi suffering the most fatalities, and around 1.7 million people affected. More than 190,000 houses were destroyed or damaged, Biratnagar's airport and much of the city were submerged, and standing paddy across the grain belt was wiped out weeks before harvest. Total damage and loss was assessed at US$584.7 million, with recovery needs estimated above US$700 million.

The 2017 floods were also a turning point for warning systems: DHM river gauges and mass SMS alerts gave many communities hours of lead time, and the disaster accelerated the creation of the federal institutions — including the NDRRMA in 2019 — that now anchor Nepal's flood response.

On 15 June 2021 (Asar 1, 2078 BS), a wall of sediment-laden water swept down the Melamchi River in Sindhupalchok district, burying much of Melamchi Bazaar and settlements up-valley in Helambu. About two dozen people were killed or never found — compiled reports list 21 confirmed dead and three missing — while preliminary assessments recorded some 260 households destroyed and around 600 people displaced. A second surge on 31 July 2021 hit Kiul, Chanaute and other riverside settlements again.

The Melamchi flood 2021 was not a simple rainstorm flood but what the International Centre for Integrated Mountain Development (ICIMOD) calls a cascading hazard. Intense rain high in the catchment mobilised an enormous store of glacial and lake sediments at Bhemathang — studies using digital terrain models computed on the order of 17 million cubic metres of fresh deposition there — and the failure of landslide dams sent successive pulses of debris, boulders and timber down a river corridor already destabilised by the 2015 Gorkha earthquake. Some investigations also point to the breaching of a small glacial lake (Pemdan) in the headwaters as a contributing trigger.

The flood buried the just-completed headworks of the Melamchi Water Supply Project, which had begun delivering water to Kathmandu only in March 2021, causing damage initially estimated at over NPR 1 billion and interrupting the capital's long-awaited supply. The event is now Nepal's textbook case of compound mountain risk and is studied alongside glacial lake outburst floods (GLOFs), covered in depth on this site's climate page (/climate) and in the GLOF mitigation explainer in these topics.

The West Rapti, which gathers the waters of Rolpa, Pyuthan and Dang before crossing Banke into India, is Nepal's classic recurrent flooder. Lacking high glaciers and snowfields, its flow swings from under 20 m³/s in the pre-monsoon to monsoon torrents; its maximum recorded flood is 7,390 m³/s at Jalkundi on 10 September 1981. Repeated inundation of the plains around Gorakhpur in India earned it the nickname 'Sorrow of Gorakhpur', while inside Nepal its floods regularly strike Banke, alongside the neighbouring Babai and Karnali floodplains of Bardiya and Kailali.

Major western Terai floods were recorded in 1981, 2006, 2008, 2013, 2014 and 2017. The August 2014 event was the deadliest of these: cloudbursts over the Bheri, Babai and West Rapti basins killed more than a hundred people across Surkhet, Banke, Bardiya and Dang and displaced tens of thousands. The 2017 Terai floods again inundated large parts of Banke and Bardiya, and the basin remains a priority zone in Red Cross and government early-action protocols.

The West Rapti basin is also where Nepal pioneered community-based flood early warning: DHM telemetry gauges upstream feed danger-level alerts to downstream communities by siren and SMS, buying the few hours of lead time the river's geography allows. The model has since been replicated on the Karnali, Babai, Koshi and Kankai, and is credited with sharply reducing deaths in warned communities.

Nepal's benchmark flood disasters show three distinct failure modes: engineering failure on an embanked giant river (Koshi 2008), extreme cloudburst over mid-hill catchments (1993, and the September 2024 Kathmandu–Bagmati floods that killed 246 people by the NDRRMA's count), and basin-wide monsoon saturation of the Terai (2017). The 2021 Melamchi disaster added a fourth, distinctly Himalayan mode — the sediment-driven mountain cascade — that overlaps with the GLOF risk documented on this site's /climate page.

For live and historical data, two official windows matter. The DHM flood forecasting service (hydrology.gov.np) publishes real-time river levels against warning and danger thresholds and issues monsoon bulletins and SMS alerts, while the NDRRMA's BIPAD portal (bipadportal.gov.np) records flood incidents, deaths and losses by district and date. Together they are the authoritative starting points for verifying any Nepal flood figure.