The chemical, physical and biological condition of global waterways – together termed “water quality” – is of paramount concern for ecological and human health, as well as for commerce, food supply, and conflict management. The goal of this session is to provide a forum for sharing cutting edge methods, new synthesis products, and new models about hydrology, connectivity and water quality. We also seek to explore new tools and new data sets that reveal spatial and temporal patterns in water quality at the scales of catchments and river networks. Together, the emerging capacity for remote sensing of water quality, the explosion in synthesis opportunities from data archives, the development of novel sensors for enhancing the precision and resolution of water quality measurements, and continued improvements in the specificity and parsimony of water quality models enable new understanding of where, when, and why water quality varies, and new theory to explain these patterns.