On April 20, 2017, a flood from the Barun River, Makalu-Barun National Park, eastern Nepal formed a 2–3-km-long lake at its confluence with the Arun River as a result of blockage by debris. Although the lake drained spontaneously the next day, it caused nationwide concern and triggered emergency responses. We identified the primary flood trigger as a massive rockfall from the northwest face of Saldim Peak (6388 m) which fell approximately 570 m down to the unnamed glacier above Langmale glacial lake, causing a massive dust cloud and hurricane-force winds. The impact also precipitated an avalanche, carrying blocks of rock and ice up to 5 m in diameter that plummeted a further 630 m down into Langmale glacial lake, triggering a glacial lake outburst flood (GLOF). The flood carved steep canyons, scoured the river’s riparian zone free of vegetation, and deposited sediment, debris, and boulders throughout much of the river channel from the settlement of Langmale to the settlement of Yangle Kharka about 6.5 km downstream. Peak discharge was estimated at 4400 ± 1800 m3 s−1, and total flood volume was estimated at 1.3 × 106 m3 of water. This study highlights the importance of conducting integrated field studies of recent catastrophic events as soon as possible after they occur, in order to best understand the complexity of their triggering mechanisms, resultant impacts, and risk reduction management options.