Abstract: Computer simulation models are used extensively for the development of total maximum daily loads (TMDLs). Specifically, the Hydrological Simulation Program-FORTRAN (HSPF) is used in Virginia for the development of TMDLs for bacteria impairments. HSPF estimates discharge from a reach using function tables (FTABLES). The FTABLE relates stream stage, surface area, and volume to discharge from a reach. In this study, five FTABLE estimation methods were assessed by comparing their effect on various simulation outputs. Four “field-based” methods used detailed cross-sectional data collected via site surveys. A fifth “digital-based” method used digital elevation data in combination with the Natural Resources Conservation Service Regional Hydraulic Geometry Curves. Sets of FTABLEs created using each method were used in simulations of instream bacteria concentration for a Virginia watershed. Several statistics relating to instream bacteria including long-term average concentration, die-off, and the violation rate of Virginia’s bacteria criterion were compared. The pair-wise Student’s t-test was used for the comparison. The HSPF simulations that used FTABLES estimated from digitally based data consistently produced significantly higher long-term average instream fecal bacteria concentrations, significantly lower instream fecal bacteria die-off, which is related to differences in residence time in the streams, and significantly higher water quality criterion violation rates.