This technical report assesses the string of events that triggered Nepal's Melamchi River floods in 2021 and highlights threats to the catchment area that provides vital drinking water to Kathmandu's fast-growing population. The report explains how floods damaged key water supply infrastructure and analyses risks posed by earthquakes, glacial lakes, and landslides, as climate change complicates conditions in the fragile Himalayas. It shows why a comprehensive catchment strategy centered on disaster risk reduction and early warning systems, plus measures to stabilize the river and control erosion, can help protect Melamchi's critical infrastructure from future events and boost the capital's water security.