Canada and the world are faced with unprecedented water-related challenges which impact our ability to meet many of the UN sustainable development goals. Climate warming and human actions are altering precipitation patterns, reducing snow levels, accelerating glacier melting, intensifying floods, and increasing risk of droughts, while pollution from population growth and industrialization is degrading water systems. Canada has some of the world's highest rates of climate warming along with associated extreme weather; together they impact infrastructure, institutions, ecosystems, and human health. Half of the world's population and all of Canada are dependent upon water from rapidly warming cold regions. More are impacted by the effects of rapid and often poorly regulated development on the sources of their freshwater supply. With such unprecedented change, it is clear that the historical patterns of water availability are no longer a reliable guide for the future. Adaptation to these changes requires new science to understand the changing earth system (changing climate, land, water, and ecosystems and their interactions); new modeling tools that precisely capture these interconnected forces and their societal implications; new monitoring systems with greater capacity to warn of critical environmental changes; and more effective mechanisms to translate new scientific knowledge into societal action. This poses a grand challenge for how water science can address sustainable development goals: "How can we best forecast, prepare for and manage water futures in the face of dramatically increasing risks?"