Hydrologic modeling outputs are influenced by how a watershed system is represented. Channel routing is a typical example of the mathematical conceptualization of watershed landscape and processes in hydrologic modeling. We investigated the sensitivity of accuracy, equifinality, and uncertainty of SWAT modeling to channel dimensions to demonstrate how a conceptual representation of a watershed system affects streamflow and sediment modeling. Results showed that the amount of uncertainty and equifinality strongly respond to channel dimensions while model performance does not significantly react. No statistically significant linear correlation was found between channel dimensions and the amount of uncertainty and equifinality, indicating non-linear and complicated interactions between them in channel routing simulations. Since the conceptual nature of hydrologic models and parameters accommodate parameter value ranges wide enough to always enable identifying a parameter set providing acceptable performance statistics, the differences between the conceptual representation of a watershed and the reality could be easily covered by adjusting parameter values in calibration. We showed that the equifinality and uncertainty of hydrologic modeling are case-dependent rather than specific to models or regions, suggesting uncertainty analysis results should be transferred or applied to other modeling studies with great caution.