Although uncertainty analysis has been discussed in recent water quality modeling literature, much of the work has assumed that all input variables and parameters are mutually independent. The objective of the paper is to evaluate the importance of correlation among the model inputs in the study of model output uncertainty. The model used for demonstrating the influence of input variable correlation is the Streeter-Phelps dissolved oxygen equation. The model forms the basis of many of the water quality models currently in use and the relationships between model inputs and output state variables are well understood.