This paper discusses a full-fledged uncertainty analysis implementation using the Guide to the Expression of Uncertainty in Measurement (GUM) as framework. Currently, there are few examples of rigorous GUM implementations in hydrometry. This work fills this gap by demonstrating the use of the GUM framework to estimate the uncertainty of open-channel discharge measurements conducted with the velocity-area method using acoustic doppler velocimeters. The paper first presents the GUM protocol and the steps leading to discharge measurement followed by evaluations of Type A uncertainty from customized experiments, Type B uncertainty informed by prior experiments and engineering judgment, and the total discharge uncertainty. Finally, the uncertainty analysis (UA) implications and practical usage for data management and improvement of measurement processes are discussed. While the detail of the analysis can be further increased, the main role of this paper is to illustrate a step-by-step implementation of GUM procedures applied to natural-scale measurements using a GUM-compliant software.