Eutrophication of Dillon Reservoir (Summit County, Colorado) is a concern, primarily from phosphorus (P) loading. The reservoir, at the mouth of within the Blue River Watershed, is a primary drinking-water reservoir for Denver. Local officials have attributed the P loading to onsite wastewater systems (OWS). The watershed model, SWAT, is used to construct a hydrologic and transport model of the Blue River watershed to better understand the potential influence of various point and nonpoint sources of P in the watershed. The watershed model was calibrated to measured flow rates and the model’s performance with respect to pollutant transport was evaluated by comparing results to measured P concentrations. The hydrogeologic model results are most sensitive to the physical parameters associated with snowmelt, as well as orographic effects on precipitation and evapotranspiration. However, uncertainties in chemical-transport parameters preclude a rigorous assignment of relative contributions of various P sources. Rather, the effort has elucidated what P transport parameters that are most crucial to accurate simulations for this watershed. The model was most sensitive to the P sorption coefficient, the P availability index, and the P enrichment ratio (a measure of P in runoff sediments compared to immobile sediments). Modeling results indicate that OWS are likely not the most significant sources of P to Lake Dillon. Rather, runoff of P-laden into the stream is the more likely cause of P loading.