The relationship between light availability and the growth and health of submergent aquatic vegetation (SAV) has been extensively examined. However, most studies have focused on impacts of reduced light associated with degradation of the optical transmissivity of the water column, such as that arising from nutrient and sediment loading. Far fewer studies have been directed at the influence of shading by structures and have primarily focused on small over-water features such as docks as opposed to larger, transportation-associated structures such as bridges. The studies of small over-water structures universally demonstrated significant and persistent reduction in plant density and biomass and in some cases, mortality as a result of structure shading. Here, day-long shading surveys in areas adjacent to and under several over-water bridges of varying orientation and geometry revealed an average 92% reduction in light reaching the water surface in the bridge shadow which, if present for even short periods of time over an SAV community is sufficient to produce significant declines in SAV. Therefore, to provide the ability to predict where and when this shading will occur and, in a form readily accessible to non-specialists, we developed an interactive GIS-based tool that predicts the position of shading by over-water structures through time. The tool's model computes solar angle by geographic location and date and utilizes GIS visualization tools to create a shadow projected onto the water surface from an over-water structure of user-specified height, width, orientation and geographic position, throughout the year. The model correctly forecasts the position of this temporally and spatially dynamic shadow zone with approximately 95% accuracy. In addition, a simple water column light attenuation calculator is provided for the user to then estimate, for any point in the landscape, the total amount of light reaching the seafloor (and the SAV community) based on user-defined water column light attenuation and water depth. With this combination of spatial and temporal modeling of light, the spatial extent and degree of shading impact on the SAV community by an over-water structure can be predicted and quantified. When combined with further consideration of light reaching the seafloor, this provides a defensible basis for informed mitigation planning and alternatives. We will briefly review the history of SAV in response to light and shading, describe the development steps of the model and discuss the remaining challenges of forecasting light fields in an aquatic environment.