The Hudson River Watershed contains a variety of geologic, topographic, climatic, and hydrologic features and a diversity of land-use patterns--making it an ideal model for studying human impact on the coastal environment. In this article, the authors present the Hudson River Plume (HRP), a problem-based online module that explores nonpoint-source pollution in the Hudson River ecosystem. The HRP is appropriate for use in high school biology and environmental, Earth, and marine science courses. (Contains 4 figures and 8 online resources.)