Students' thinking should serve as the foundation of effective science curriculum and instruction. To promote science learning, particularly in the geosciences, teachers must attend to students' existing ideas about natural phenomena through the use of ''high-leverage'' instructional practices such as formative assessment. Elementary teachers need support to learn to implement formative assessment practices effectively. However, few studies have explored relationships between elementary teachers' content knowledge and formative assessment practices, analysis of students' thinking, and instructional decision making. To begin to address this gap in the literature, we conducted a convergent parallel mixed methods study to examine how elementary teachers employ formative assessment practices to scaffold elementary students' learning about interactions between water and the geosphere, which is a core, unifying concept in the Earth sciences. This research is embedded within a multiyear professional development program designed to support elementary teachers (grades 3-5) to learn to employ formative assessment in their classrooms. Study findings show teachers' own knowledge of geoscience disciplinary content is unrelated to their formative assessment practices. They also highlight the importance of the elementary science curriculum materials teachers use in framing disciplinary concepts in ways that influence how teachers evaluate student artifacts and engage in follow-up instruction based.