An inspection of the original study that developed the rating equations for Parshall flumes indicates that the calibration tests were not conducted for discharges as low as is currently recommended for the flumes' use. A numerical model was developed to predict the effect of fluid viscosity on the depth‐discharge relation. An experimental investigation for several flume sizes indicates that the original rating equations and data overpredict the discharge at flow rates that are less than about 15% of the maximum rated discharge for the flume. The discrepancy can be as much as 25% for the range of flows for which the flume is recommended to be used. The numerical model successfully matches the experimental data for the flume sizes studied. Alternative rating equations are developed for use at low discharges.