A scientifically justifiable Total Maximum Daily Load (TMDL) for a waterbody can only be developed based on a quantitative understanding of the system. In practice, water quality modeling offers a feasible tool to establish this quantitative understanding. A water quality model that is customized for a specific waterbody can simulate the major physical, chemical, and biological processes that occur in the system, and thus provide quantitative relationships between the water quality response and external forcing functions. A customized modeling framework was developed to support determination of nutrient TMDLs for Wissahickon Creek and its tributaries. The nutrient TMDLs are presented in the report 'Nutrient and Siltation TMDL Development for Wissahickon Creek and its Tributaries' (USEPA, 2003). This report is intended to accompany the TMDL report and This report is intended to accomprovide a more detailed discussion on the models used for the nutrient TMDL analysis, including assumptions, parameters, and references.