Residents of the Kathmandu Valley in Nepal face growing water insecurities: too much at times and not enough at others coupled with constant worry about water quality and access. In this rapidly changing city, ancient water infrastructure is a critical mediator of urban water security. Drawing on six years of mixed methods research, I explain the vital role of water heritage for Kathmandu’s water security and urban development by critiquing three common assumptions about water heritage. First, so-called traditional water infrastructure is not a temporary alternative. Second, water heritage is not a living museum. Third, water heritage is not a panacea for modern woes. This article concludes with reflections on the potentials for Kathmandu’s water heritage to address persistent urban insecurities and inequities.