Integrated upstream and downstream thinking to mitigate water security challenges from Peruvian glacier retreat Acceleration of glacial melt has severe implications for water-food-energy security and inter-connected livelihoods of vulnerable populations in river basins fed by glaciers. For example, in the Ancash Region of Peru, glacial melt from the Andean Mountains provides up to 67% of dry season water supply going up to 91% during extreme drought (annual average 19%). Rapid retreat of glaciers in the Cordillera Blanca has already had notable impact on that supply, with evidence to suggest the majority of rivers now exhibit decreasing dry-season discharge i.e. have reached and passed 'peak water'. Challenges associated with a reduced supply of water to downstream agriculture, industry and hydropower generation are exacerbated by enhanced sediment and contaminant flux in extreme wet season floods.  Climate change impacts compromise ecosystem service provision at times of both augmented low and high flow. While low flows and water supply are being increasingly impacted by the huge loss of water storage in shrinking glaciers, ENSO-related extreme events are leading to catastrophic delivery of excess water and sediment during high flows which compromise water and environmental quality downstream. Climate change is driving a hydrological regime of extremes with no advantage at either end: from supply and quality issues at low flow to more water than the system can handle at high flow, compromising water and soil quality downstream. Understanding the changing dynamics of glacial melt, hydrology and regional climate change is crucial in order for the design of infrastructure solutions and planning to be effective and resilient.  Responsible, efficient and sustainable water use is necessary in national and transboundary watersheds, to ensure adequate supply and mitigate emerging quality problems. In order to achieve this consultancies and advisory organisations require high quality robust scientific evidence to underpin their design decisions for watershed management. This entails moving from (inefficient) sectorial management of water to a more integrated and holistic approach that takes into account the need for conserving ecosystems services. Indeed, while the Peruvian Congress passed a historic Ecosystem Services law in 2014 to take a holistic approach to tackling these challenges, implementation of integrated action to achieve Sustainable Development Goals has been hampered by a lack of evidence of glacial-fed watershed processes and function. While studies to date have been conducted in the Cordillera Blanca in relation to dynamics of glacial retreat, associated natural disaster risk, hydrology and past glaciations we do not have a sufficiently holistic and integrated knowledge of the wider impacts of glacial melt on current and future ecosystem service provision which is hampered by complexity of human-environment feedbacks, a knowledge base essential for mitigation of future uncertainty and risks.  We propose that a basin-wide understanding of water, sediment and contaminant budgets within Peruvian glacial-fed basins is required to bring policy change for socio-economic benefits through (a) offsetting storage lost from shrinking glaciers through augmentation of mountain ecosystem service provision for landscape water retention and (b) providing the foundation for adaptive management strategies to support and enhance livelihoods under threat from high flows and downstream environmental quality consequences. This research is essential for the design of large-scale energy infrastructure, such as hydropower in glacier-fed regions. Likewise, bringing back and maintaining a balance between sustainable livelihoods and the environment is critical to build community resilience to environmental change. The Newton Fund builds research and innovation partnerships with developing countries across the world to promote the economic development and social welfare of the partner countries.