This paper describes the results of an action research project undertaken as a partnership between Macquarie University and Geography teachers from an independent school in regional New South Wales (NSW), Australia. The project focused on the teaching of river landforms and processes, a component of the Biophysical Interactions topic in the NSW Stage 6 Geography syllabus. The aim of the research was to provide four teachers with feedback about depth and accuracy of students' content knowledge, the teachers' knowledge of common student conceptions, and the extent to which the school's fieldwork program promoted cognitive disequilibrium and constructive confusion, affective states required for deep conceptual change. This feedback was used as a prompt for professional reflection and to stimulate conversations about improvements that could be made to the teachers' knowledge and practice. The findings suggest that this form of action research can be an effective tool for enhancing teachers' pedagogical content knowledge (PCK) including their knowledge of evidence-based assessment practices in Geography.