Phosphorus input from stormwater runoff is the leading cause of eutrophication in the Charles River and the target of a new federal regulatory program to dramatically reduce nutrient loadings from three municipalities in the upper watershed. The integration of nonstructural best management practices (BMPs), which target phosphorus at or near its source, into municipal nutrient management plans could provide substantial cost savings over a solely structural approach. However, phosphorus sources in urban environments are poorly understood, and treatment strategies hard to evaluate and incorporate into regulatory statues. A literature-based loading model developed in this study suggests that municipal programs could substantially reduce phosphorus loadings by addressing nutrient input from various nonpoint sources, particularly from dog waste, lawn runoff, and leaf litter. The implementation of creative community-based social marketing programs could offer cost-effective means to reduce loadings from these and other sources that lie largely beyond the scope of regulatory controls; however, more evidence is needed to evaluate their performance.