The authors developed a framework of empirically grounded curricular goals for water-science literacy and documented the challenges that students face in achieving these goals. Water-related environmental science literacy requires an understanding of connected natural and human-engineered systems at multiple scales ranging from atomic-molecular (changes of state and solutions) to large (watersheds, aquifers, and human water-purification and distribution systems). The authors' assessments of students from upper elementary school through high school suggest that virtually all students have some important understandings of water on which educators can build. Yet, the authors found that most students do not systematically trace water and other materials through systems and do not account for invisible aspects of water systems at the atomic-molecular and landscape scales. The results revealed a contrast between students' informal accounts of water in environmental systems and scientific accounts of these systems. The authors discuss curricular implications and the importance of helping students develop a richer understanding of water systems at multiple scales. (Contains 10 figures and 1 table.)