This chapter examines the critical link between upstream environmental health in the Himalayas and the resilience of the downstream Ganga–Brahmaputra-Meghna (GBM) delta, the world’s largest and most populous. Using the 2013 Kedarnath disaster in Rudraprayag, Uttarakhand, as a critical case study, it argues that conventional, engineering-centric disaster management paradigms are inadequate for fragile mountain ecosystems. The research posits that a paradigm shift towards Nature-Based Solutions (NbS) is imperative for building socio-ecological resilience. Employing a descriptive-analytical methodology, comparative case study analysis, and policy framework analysis based on secondary data, this chapter proposes a multi-scalar portfolio of context-specific NbS. These solutions, organized along a “source-to-slope-to-stream” continuum, include catchment reforestation, springshed management, slope bio-engineering, and the restoration of riparian buffers and wetlands. The study demonstrates how these interventions not only mitigate local flash flood and landslide risks but also align with and contribute to India’s commitments under the Sendai Framework for Disaster Risk Reduction and Sustainable Development Goals 11, 13, and 15. Ultimately, the chapter concludes that investing in the resilience of Himalayan headwaters is not merely a local concern but a strategic imperative for the long-term security of the entire river basin. By stabilizing slopes, regulating hydrological regimes, and managing sediment loads at the source, NbS provides a sustainable pathway to enhance the resilience of tens of millions living in the Gangetic plains and delta, reinforcing the indivisible “source-to-sea” connection.