Low Impact Developments (LIDs) are devices or techniques used to mitigate the effects of stormwater runoff and associated pollutant loadings. The impacts of LIDs were investigated by developing Hydrologic Response Units (HRUs) for the Lake Simcoe Watershed which were defined as areas that contain similar runoff generating mechanisms. These runoff generating mechanisms are hydrological characteristics such as soil type and slope. By identifying HRUs, the responses of large drainage areas that are composed of hydrologically homogeneous units are evaluated in a more efficient manner. As a result, similar opportunities for the potential implementation of LIDs can be identified. After the development of the HRUs, hydrological modelling was performed using US EPA SWMM in order to investigate the reductions in stormwater runoff and pollutant loadings as a result of LID implementation. Seven types of LIDs were modelled on individual properties or lots in addition to seventeen combinations of LIDs which led to the development of HRU performance curves. The runoff and pollutant loadings generated by the areas examined were modelled both for existing conditions and for the implementation of LIDs and LID combinations. The results quantified the potential of widespread implementation of LIDs to reduce both runoff and pollutant loadings. At the watershed level, for example, the soakaway pit and bioretention cell LID combination produced an overall reduction in runoff of 14 % and an overall reduction in total phosphorus of 19%.