The delineation of floodplains involves, in most circumstances, solving the one-dimensional energy equation. However, uncertainties in the identified floodplain arise from both computational and data uncertainties; data uncertainties are concluded to be generally more significant than computational uncertainties.A method is developed to calculate the uncertainty in floodplain delineation arising from data uncertainties. The proposed method requires only HEC-2 computer output and a small computer program. Application of the method to two case studies and comparison with another uncertainty method suggest that the proposed uncertainty theory is applicable to practical situations within the given constraints. Key words: data uncertainty, floodplain, uncertainty analysis, water profile computation.