Mapping Our City is a two-year project in which middle school teachers and students in Boston explore the uses of Geographic Information Systems (GIS) in project-based science, environmental education, and geography. The project is funded by the National Science Foundation and is being field tested in three Boston middle school science classrooms. This paper is a progress report on how those middle school students are creating and using spatial data to analyze an urban river. Questions explored relate to the ways that GIS can enhance student learning, which aspects of student learning and classroom practice are most affected by GIS, and the ways students use modified technology in the classroom. One participating school focused upon is an independent, high-achieving school for girls, where weekly classes were held with one group of 15 and one group of 30 sixth graders. Key findings address the use of maps to focus student attention on spatial aspects of the data, using GIS to help students discover relationships among spatial variables and to organize and store their data, making GIS technology easier to use, and having third-party data as a motivational tool. (DDR)