In densely developed urban watersheds, limited open spaces are available to construct structural Best Management Practices (BMPs); therefore implementation plans to meet Total Maximum Daily Loads (TMDLs) typically include recommendations for non-structural BMPs. Nonstructural BMPs reduce pollutant loads by either reducing or removing the source of a pollutant or by capturing built-up pollutants at or near their source before they can be washed off the land surface into the stormwater drainage system and downstream waterbodies. This paper presents an innovative analytical approach for estimating pollutant load reduction from non-structural BMPs by simulating buildup and washoff processes in urban watersheds. The non-structural BMPs included in the metals TMDL implementation plan for the Los Angeles River (LAR) target brake pads through vehicle product replacement, roadway sediment through street sweeping, and rooftop runoff through downspout disconnection. Historical flow gage date were used to estimate the buildup of metals from controllable sources prior to a storm event, as a function of preceding dry days, and wash-off of accumulated pollutant from the watershed surface, as a function of runoff event depth. Results showed that a large portion of total copper, total lead, and total zinc loads could be attributed to the three sources evaluated in this study, as can be seen in the box and whisker plots. Thus, implementation of non-structural BMPs is an important component in the implementation plan for the LAR metals TMDL.