First-Order Second Moment (FOSM) and Monte Carlo analysis were applied to characterize the uncertainty in selected water levels and velocities simulated by a two-dimensional hydrodynamic model of the Upper St. Lawrence River downstream from Lake Ontario. The analysis utilized an application of the Resource Management Associates' RMA2 model. Both FOSM and Monte Carlo analysis provided similar estimates of uncertainty, with Monte Carlo analysis results being 15% less than FOSM. Based on the findings of this work, the FOSM is preferred. FOSM provides a conservative estimate of the uncertainty and it is simpler to apply than Monte Carlo analysis, requiring less information and fewer model executions. FOSM also provides an immediate indication of the primary contributors to the uncertainty in the output, where Monte Carlo analysis requires additional effort to do the same. Results indicate that the parameter describing bottom resistance using Manning's n contributed more to model uncertainty than other factors investigated. The uncertainty in and sensitivity in Manning's n is large which results in a significant amount of uncertainty in the model outputs is contributed by this parameter. The calculations described in this study show that uncertainty analysis is a practical addition to the two-dimensional hydrodynamic modelling process. It provides insight to the model developer, quantifying how good the model actually is. It also provides a measure of the accuracy of the model for future model developers or clients using hydrodynamic modeling outputs.