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The majority of formulae for predicting in-channel mixing by longitudinal dispersion are based on empirical evidence from medium to large rivers, whereas small creeks and streams are under-represented despite their hydrological and ecological importance. In this study, twenty-six formulae for predicting the longitudinal dispersion coefficient (K_x) were evaluated for their applicability to small channels using field measurements and hydraulic modeling. Predicted values for K_x, following guidelines recommended in the original publications for the formulae, were compared to measured dispersion coefficients obtained from sodium-chloride plumes injected into two small channels (a concrete-lined, trapezoidal channel and a small, natural stream) based on fitting the Hayami solution to the one-dimensional advection-dispersion equation (ADE) to each plume. The predicted K_x coefficients from the formulae were also utilized to create model-simulated plumes, which were compared to those measured well downstream of the point of injection. The findings demonstrate that the predictive accuracy of the twenty-six formulae was extremely variable; none were able to predict the dispersion process in the small channels with better than ± 50% accuracy. These results show that “universal” formulae are plagued with a large degree of uncertainty and should be used with caution when applied to small channels, although more robust predictions are possible with some formulae if site-specific data are available for calibration.